Mountain Scholar :: Browsing by Author "Venayagamoorthy, Karan, committee member"

Browsing by Author "Venayagamoorthy, Karan, committee member"

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Open Access
Action potential initiation mechanisms: analysis and numerical study
(Colorado State University. Libraries, 2022) Aldohbeyb, Ahmed A., author; Lear, Kevin L., advisor; Vigh, Jozsef, committee member; Prasad, Ashok, committee member; Venayagamoorthy, Karan, committee member
Action potentials (AP) are the unitary elements of information processing in the nervous system. Understanding AP initiation mechanisms is a fundamental step in determining how neurons encode information. However, variation in neuronal response is a characteristic of mammalian neurons, which further complicate the analysis of neuronal firing dynamics. Several studies have associated the variation in AP onset with the type and densities of voltage-gated ion channels, diversity in synaptic inputs, neuron intrinsic properties, cooperative Na+ gating, or AP backpropagation. But the mechanisms that underlie the response variability remain unclear and subject to debate. Even though all these studies tried to answer the same question, the definition of AP onset and rapidity differs between them, highlighting the need for a more systematic and consistent method to quantify AP onset features, and hence analyzing the variation in AP onset. Two novel methods were developed to quantify AP rapidity. The proposed methods have lower relative variation, higher ability to classify neuron types, and higher sensitivity and specificity to voltage-gated Na+ channels parameters than current methods. AP rapidity was used to analyze different factors impacting the AP activation mechanism. However, the prior rapidity quantification methods are subjectively based on the researcher's judgment, which complicates the comparison between different studies. Thus, we proposed a more systematic and consistent method based on the full-width or half-width at half the rising phase peak of the membrane potential's second-time derivative (Vm). First, using an HH-type model, we showed that the peak width methods are sensitive to changes in the Na+ channel parameters and conductance and minimally impacted by changes in the K+ channel parameters compared to the phase slope, the standard quantification method. Second, we compared the peak width methods to the two prior methods, phase slope and error ratio, using recordings from cortical and hippocampal pyramidal neurons, hippocampal PVBCs, and FS cortical neurons found in online databases. The results showed that the new methods have the lowest variation between neurons within a specific type while significantly differentiating several neuron types. Together, the two studies showed that the peak width methods provide another sensitive tool to investigate the mechanisms impacting AP onset dynamics and provide a better tool to study Na+ channels kinetics and AP onset features. A conductance-based model that includes dynamics of ion concentration and cooperative Na+ channels was developed to investigate the mechanisms responsible for observed neuronal response variation. Random response variability has previously been observed in spike trains evoked from individual neurons by the same DC stimulus, but we observed systematic variation. The first APs' in a burst had attributes that were comparable regardless of the stimulus strength, while the subsequent APs' attributes monotonically change during bursts, and the magnitude of change increases with stimulus strength. These two spike train features were observed in three different neuron types (n = 51), indicating a shared mechanism is responsible for the spike train pattern. Various existing computational models fail to replicate the monotonic variation in AP attributes. We proposed incorporating ion concentration dynamics and cooperative gating to account for the missing behavior. A model with dynamic reversal potential but without cooperative Na+ channel gating reproduces the AP attribute's variation during bursts, but not the first APs' attributes. The first APs' attributes were reproduced only in the presence of a fraction of cooperative Na+ channels. Cooperative gating also enhanced the magnitude of modeled variation of some AP attributes to better match the electrophysiological recordings. Therefore, we conclude that changes in ion concentration dynamics could be responsible for the monotonic change in some AP's attributes during normal neuronal firing, and cooperative gating can enhance this effect. Thus, the two mechanisms contribute to the observed variability in neuronal response, especially the variation in AP rapidity.
Open Access
Approaching Arctic-midlatitude dynamics from a two-way feedback perspective
(Colorado State University. Libraries, 2019) McGraw, Marie C., author; Barnes, Elizabeth A., advisor; Randall, David A., committee member; Schumacher, Russ S., committee member; Venayagamoorthy, Karan, committee member
Arctic variability and the variability of the midlatitude circulation are closely intertwined. Although these connections are interrelated and bi-directional, and occur on a variety of timescales, they are not often studied together. Modeling studies generally focus on a single direction of influence--usually, how the midlatitude circulation responds to the Arctic. Studying these relationships in a two-way feedback perspective can offer new insights into these connections, providing information on how they feed back upon each other. This work approaches Arctic-midlatitude dynamics from a two-way feedback perspective, mostly on sub-monthly timescales. Particular emphasis is placed on the influence of midlatitude circulation variability upon the Arctic, as this direction of influence is less-studied than the converse pathway. Reinforcing feedback loops are identified between the North Pacific and North Atlantic jet streams and the Arctic. Variability in both the North Atlantic and North Pacific jet streams drives Arctic variability, which then drives further variability in the jet streams. The circulation variability in many regions, including North America, the east Pacific and Alaska, and Siberia, drives Arctic variability far more than it is driven by Arctic variability. These relationships exhibit substantial regional variability, stressing the important role of an analytical approach that incorporates this spatial heterogeneity. The two-way nature of Arctic-midlatitude connections is also explored in the context of Arctic moisture fluxes. The circulation response to sea ice loss also drives changes in Arctic moisture fluxes, with moisture flux out of the Arctic increasing more than moisture flux into the Arctic. The two-way feedback perspective explored in this research is built around the ideas of causal discovery, particularly Granger causality. Most of these two-way Arctic-midlatitude relationships are considered in the context of added variance explained, or added predictive power. That is, these relationships are characterized by comparing how much an additional predictor improves predictability beyond autocorrelation. Limiting the ability of autocorrelation to color these results emphasizes added variance explained--how much additional variance in the circulation can be explained by Arctic temperature variability, and vice versa? As an example, many recent studies have concluded that warm Arctic temperatures or low sea ice conditions drive a strengthening of high pressures and an increase in cold temperatures over Siberia. However, when memory and autocorrelation are accounted for, it emerges that the circulation variability over Siberia drives a response in the Arctic more than the other way around--results that are in concordance with modeling studies that have also disputed the veracity of the claim of the Arctic driving a strong response in Siberia. Ultimately, this research seeks to offer a different perspective on analyzing climate dynamics, with an emphasis on two-way feedbacks. While targeted climate modeling studies offer great physical insights, and provide substantial opportunities to explore and test physical mechanisms, they are often limited to exploring only one pathway of influence. In reality, these relationships do go in both directions, and a comprehensive understanding of such large-scale interactions between different parts of the atmosphere must ultimately consider the two-way relationships. The causal discovery methods used in much of this research can be used in conjunction with modeling studies to better understand these two-way relationships, and improve interpretation of results. While this research has focused on the relationships between the Arctic and the midlatitude circulation on sub-seasonal timescales, the broad framework and ideas presented within can be more widely applied to many other questions in climate variability studies. Thus, this work has also put a special emphasis on describing and implementing these causality-based methods in a manner that is accessible and interpretable for atmospheric and climate scientists.
Open Access
Calibration and uncertainty of a head-discharge relationship for overshot gates under field conditions
(Colorado State University. Libraries, 2019) Kutlu, Caner, author; Gates, Timothy K., advisor; Venayagamoorthy, Karan, committee member; Butters, Gregory, committee member
Adjustable overshot gates (pivot weirs) are commonly used to control discharge and water levels in irrigation water delivery networks. The degree to which this control can be achieved depends upon reliable relationships between flow rate and the hydraulic head upstream and downstream of the gate. Moreover, such relationships also can be used for flow measurements. This study aims to develop a head-discharge equation for free flow over a overshot gate, to describe its uncertainty, and to examine the impact of gate submergence on the equation. Previous research on the flow characteristics of overshot gates has been performed primarily in laboratories, with very little investigation of performance in the field. This thesis provides a report of a field study conducted on four Obermeyer-type pneumatically automated overshot gates, which were operated for irrigation water delivery in northern Colorado. Utilizing both classical and amended forms of the sharp-crested weir equation, Buckingham-Pi dimensional analysis, and incomplete self-similarity theory, head-discharge equations for free flow have been developed which are alternately dependent on and independent of the gate inclination angle. To estimate the flow rate, three fully-suppressed Obermeyer-type overshot gates with crest widths of 22 ft, 20 ft, and 15 ft, and respective lengths of 5 ft., 6.3 ft., 6.08 ft , were inspected for eight different inclination angles (α = 22.8°, 23.6°, 29.7°, 32.6°, 34.6°, 35.3°, 38.9°, 40.4°), under free flow conditions. The best-performing equation is of classical form and contains a discharge coefficient dependent on gate inclination angle. It can be used to relate the discharge to upstream hydraulic head with about ± 10 % (standard deviation range of residual error) for free flow conditions. This equation is applicable for inclination angles between 20° and 40° and for flow rates ranging from 20 to 330 ft3/s. To reduce uncertainty of the discharge coefficient and to prevent the misleading consequences of neglecting the velocity head in the approaching flow, the total upstream energy head was employed in the equation. The effect of velocity head was significant for flow estimation. Dependency of the equation on the gate and field characteristics was examined by testing the equation with field data for a different type of overshot gate. Alternate equations were developed which altered the classical form for a sharp-crested weir to include both a coefficient and an exponent that are dependent upon gate inclination angle, and which preserved the classical form and treated the discharge coefficient as a constant independent of gate inclination. Although, satisfactory results were obtained for these alternative forms, inclusion of the angle in the discharge coefficient alone was recommended for higher accuracy of flow rate estimation, particularly for larger overshot gates with inclination angles ranging from about 20° to about 40°. Furthermore, the modular limit of the overshot gates was investigated for a fourth Obermeyer gate with a crest width of 17 ft and a length of 5.8 ft. Up to a submergence ratio of 0.51, the submergence effect was not observed to decrease the flow rate over for the gate. More data for a higher submergence conditions are required to develop a modular limit and a head-discharge equation for submerged flow.
Open Access
Formation of rain layers in the Indian Ocean and their feedbacks to atmospheric convection
(Colorado State University. Libraries, 2023) Shackelford, Kyle T., author; van Leeuwen, Peter Jan, advisor; DeMott, Charlotte, advisor; Maloney, Eric, committee member; Venayagamoorthy, Karan, committee member
Rainfall over the tropical warm pool spanning the Indian and West Pacific Oceans is relatively colder, fresher, and less dense than the near-surface ocean. Thus, under low-to-moderate winds, rainfall can act to stably stratify the upper ocean, forming a rain layer (RL). RLs cool and freshen the ocean surface and shoal ocean mixed layer depth, confining air-sea interaction to a thin, near-surface ocean layer. The shallow, transient nature of RLs has limited their observation, and RL impact on air-sea interaction is not well understood. This two-part thesis aims to address knowledge gaps surrounding 1) RL formation and characteristic traits, and 2) RL feedbacks to the atmosphere. In the first part of this thesis, we examine Indian Ocean RLs and their potential feedbacks to the atmosphere using a 1D ocean model. Initial experiments focus on model validation, and demonstrate that the model is able to effectively replicate upper ocean response to precipitation as revealed by in situ measurements. Following model validation, Indian Ocean RL characteristics are studied by forcing a 2D array of 1D model columns with atmospheric output from an existing convection-permitting simulation. Results from this experiment demonstrate that SST reduction within RLs persists on time scales longer than those of the parent rain event. To evaluate RL feedbacks to the atmosphere, a second 2D array experiment is conducted over the same domain with identical atmospheric forcing except rainfall is set to zero at every time step. Comparison between simulations with and without rain forcing demonstrate that RLs reduce SST through cold rain input to the ocean surface, and maintain and enhance SST reductions through a stable salinity stratification. Through prolonged SST reduction, RLs also enhance spatial SST gradients that have previously been shown to excite atmospheric convection. In the second part of this thesis, RL feedbacks to the Madden-Julian Oscillation (MJO) are studied by conducting regional ocean-atmosphere coupled simulations. Output from two convection-permitting coupled simulations of the November 2011 MJO event, one with rain coupling to the ocean surface and a second without rain coupling, is used to evaluate two potential RL feedback mechanisms. The first feedback is the ''SST gradient effect,'' which refers to RL-enhanced SST gradients imposing low-level patterns of convergence/divergence in the atmospheric boundary layer. The second is the ''SST effect,'' which refers to RL-induced SST perturbations altering turbulent heat fluxes. During the MJO transition from suppressed to enhanced convection, the SST gradient effect and SST effect have opposing feedbacks to convection, as RL-enhanced SST gradients favor convective initiation, while RL-induced SST reduction hinders convection. Comparison of coupled simulations with and without rain coupling to the ocean demonstrates that RL-induced SST reduction has a more substantial impact than enhanced SST gradients during this transitory phase. A delayed pathway in which RLs feedback to the MJO through the SST effect arises from frequent RL presence during the disturbed phase, which isolates subsurface ocean heat from the atmosphere. At the onset of the MJO active phase, westerly wind bursts erode near-surface RLs and release previously trapped subsurface ocean heat to the atmosphere, amplifying the intensity of MJO convection. Between the direct and delayed SST effect, RLs are shown to modify intraseasonal tropical variability.
Open Access
Generalized pressure drop and heat transfer correlations for jet impingement cooling with jet adjacent fluid extraction
(Colorado State University. Libraries, 2022) Hobby, David Ryan, author; Bandhauer, Todd M., advisor; Olsen, Daniel B., committee member; Prawel, David, committee member; Venayagamoorthy, Karan, committee member
Jet impingement technologies offer a promising solution to thermal management challenges across multiple fields and applications. Single jets and conventional impinging arrays have been studied extensively and are broadly recognized for achieving extraordinary local heat transfer coefficients. This, in combination with the versatility of impinging arrays, has facilitated a steady incline in the popularity of jet impingement investigations. However, it is well documented that interactions between adjacent jets in an impinging array have a debilitating effect on thermal performance. Recently, in an attempt to mitigate the jet interference problem, a number of researchers have created innovative jet impingement solutions which eliminate crossflow effects by introducing fluid extraction ports interspersed throughout the impinging array. This novel adaptation on classical impinging arrays has been shown to produce dramatically improved thermal performance and offers an excellent opportunity for future high-performing thermal management devices. The advent of jet-adjacent fluid extraction in impinging arrays presents a promising improvement to impingement cooling technologies. However, there have been very few investigations to quantify these effects. Notably, the current archive of literature is severely lacking in useful, predictive correlations for heat transfer and pressure drop which can reliably describe the performance of such impinging arrays. Steady-state heat transfer and adiabatic pressure drop experiments were conducted using nine unique geometric configurations of a novel jet impingement device developed in this work. This investigation proposes novel empirical correlations for Darcy friction factor and Nusselt number in an impingement array with interspersed fluid extraction ports. The correlations cover a broad range of geometric parameters, including non-dimensional jet array spacing (S/Dj) ranging from 2.7 to 9.1, and non-dimensional jet heights (H/Dj) ranging from 0.31 to 4.4. Experiments included jet Reynolds numbers ranging from 70 to 24,000, incorporating laminar and turbulent flow regimes. Multiple fluids were tested with Prandtl numbers ranging from 0.7 to 21. The correlations presented in this work are the most comprehensive to date for impinging jet arrays with interspersed fluid extraction. Nusselt number was found to be correlated to impinging jet Reynolds number to the power of 0.57. The resulting correlation was able to predict 93% of experimental data within ±25%. During adiabatic pressure drop experiments, multiple laminar-turbulent flow transition regions were identified at various stages in the complex jet impingement flow path. The proposed Darcy friction factor correlation was separated into laminar, turbulent, and transition regions and predicted experimental data with a mean absolute deviation of 20%. The heat transfer and pressure drop correlations proposed in this investigation were used in a follow-on optimization study which targeted an exemplary impingement cooling application. The optimization study applied core experimental findings to a microchip cooling case study and evaluated the effects of geometry, flow, and heat load parameters on cooling efficiency and effectiveness. It was discovered that reducing non-dimensional jet height results in all-around improved cooling performance. Conversely, low non-dimensional jet spacing results in highly efficient but less effective solutions while high non-dimensional jet spacing results in effective but less efficient cooling.
Open Access
Heat transfer enhancement in two-phase microchannel heat exchangers for high heat flux electronics
(Colorado State University. Libraries, 2020) Hoke, Jensen, author; Bandhauer, Todd, advisor; Windom, Bret, committee member; Venayagamoorthy, Karan, committee member
Laser diodes are semiconductor devices that emit high intensity light with a small spectral bandwidth when a forward voltage is applied. Laser diodes have a high electrical to light conversion efficiency which can be greater than 50%. These robust, high efficiency laser sources are used in medical and manufacturing fields and, if their power can be increased, show promise in inertial confinement fusion and defense applications. Individual diode emitters are arrayed into bars with a footprint of 1 mm by 10 mm to increase their light output power. These bars are further combined into arrays with the light emitting edges stacked close together. As the spacing in these arrays are reduced to increase brightness, thermal management becomes the limiting factor for each bar. State of the art diode arrays can have heat fluxes exceeding 1 kW cm-2. Effective thermal management strategies are key because the diode's output wavelength, bandwidth, efficiency and lifetime are temperature dependent. Commercially available high powered laser diode arrays are traditionally cooled using a single-phase fluid passing through conduction coupled copper-tungsten channels. These heat exchangers have high thermal resistances which require the coolant to be significantly subcooled before entering the device. High working fluid flow rates are required to reduce thermal gradients in the diode bars and working fluid conditioning is required to reduce corrosion in the cooling plates. Many of these issues can be addressed by cooling the diodes with a two-phase working fluid in a corrosion resistant, silicon microchannel heat exchanger. The high heat transfer coefficients associated with flow boiling, as well as the high surface area to volume ratios in microchannel arrays allow the working fluid temperature to be much closer to that of the diode which reduces the cooling load on a system level. Additionally, as heat is added to a two-phase fluid, there is virtually no change in temperature. Therefore, the working fluid flow rate can be much lower than a comparable single-phase heat exchanger, which reduces pump work. However, using a two-phase working fluid presents its own unique set of challenges. This work presents a novel approach to increasing the effective critical heat flux and reducing thermal resistance in an array of 125 high aspect ratio silicon microchannels (40 µm × 200 µm) subjected to heat fluxes up to 1.27 kW cm-2. R134a is used as the two-phase working fluid and outlet vapor qualities up to 80.7% are reported. The silicon heat exchangers are manufactured using a DRIE MEMS process that allows fine control over feature sizes. The performance of traditional plain walls is compared to a novel sawtooth structuring pattern that increases available heat transfer area by 41% and provides bubble nucleation sites. A 17% decrease in thermal resistance is reported for one of the area enhancement schemes and critical heat flux is increased in both area enhanced parts. A thermal FEA model is used to determine heat transfer coefficients and local heat fluxes within the test section. This model is used to investigate alternate patterning schemes. An adjustment to the Bertsch two-phase heat transfer coefficient is also suggested for smaller microchannels geometries and higher heat fluxes. Examination of the model results show that performance increase observed in the area enhanced test sections is driven by an increase in bubble nucleation sites. The additional area available for heat transfer has little effect because reduction of heat flux at the fluid wall interface reduces two-phase heat transfer coefficients. This effect is driven by the relative importance of nucleate boiling in these small channels.
Open Access
Ignition and combustion of liquid hydrocarbon droplets in premixed fuel/air mixtures at elevated pressures and temperatures
(Colorado State University. Libraries, 2022) Bhoite, Siddhesh Bharat, author; Windom, Bret, advisor; Marchese, Anthony J., advisor; Olsen, Daniel B., committee member; Venayagamoorthy, Karan, committee member
The combustion of two fuels with disparate reactivity such as natural gas and diesel in internal combustion engines has the potential to increase fuel efficiency, reduce fuel costs and reduce pollutant formation in comparison to traditional diesel or spark-ignited engines. However, dual-fuel engines are presently constrained by uncontrolled fast combustion (i.e., engine knock) as well as incomplete combustion and a better understanding of dual-fuel combustion processes is necessary to overcome these challenges. In addition to dual-fuel engines, this work is also motivated by abnormal combustion phenomenon that has been observed in highly boosted, spark ignited, natural gas engines, which is caused by engine lubricant oil droplets entering the cylinder and serving as unwanted ignition sources for the natural gas/air mixture. To study the fundamental combustion processes of ignition and flame propagation in dual-fuel engines and abnormal combustion triggered by lubricant oil droplets, single isolated liquid hydrocarbon droplets were injected into premixed CH4/O2/Inert mixtures at elevated temperatures and pressures. In this research, a rapid compression machine (RCM) was used in combination with a newly developed piezoelectric droplet injection system that is capable of injecting single liquid hydrocarbon droplets of 40 to 500 μm along the stagnation plane of the RCM combustion chamber. A high-speed Schlieren optical setup was used for imaging the combustion events in the chamber. Experiments were conducted for diesel fuel and lubricant oil droplets at various initial diameters (50 μm < do < 500 μm), various CH4/O2/Inert equivalence ratios (0 < ϕ < 1.2) and various compressed temperatures (740 K < Tc < 1000 K). Dual fuel experiments revealed multiple modes of droplet ignition, droplet combustion, and premixed flame propagation, which depend on the initial droplet temperature, droplet diameter, droplet velocity, and stoichiometry of the CH4/O2/N2/Ar mixture. In the case of small droplets, spherical ignition events were observed that transition into spherical non-premixed flames that envelope the droplet, producing an outwardly propagating spherical premixed flame. For larger droplet diameters moving at moderate velocity, the ignition event occurs near the droplet surface on the leeward side of the droplet and subsequently creates a non-premixed flame that envelopes the droplet and a non-spherical premixed flame. For droplets moving with high velocities, the ignition event occurs in the wake of the droplet, multiple diameters from the droplet surface, and creates a flame that propagates toward the droplet. Spherical, outwardly propagating premixed flames were observed for diesel droplet ignition in stoichiometric CH4/O2/N2/Ar mixtures, whereas elongated premixed flames were observed in lean CH4/O2/N2/Ar mixtures. The experiments conducted to understand abnormal combustion caused by lubricant oil droplets provided a valuable dataset of ignition delay periods of various petroleum and ester-based lubricant oils at a wide range of thermodynamic and mixture conditions. In concert with the experiments, a combined analytical droplet evaporation model and computational combustion model were developed that simulate the evaporation, ignition, and combustion processes observed in the experiments. The ignition delay dataset was used to successfully develop and validate a surrogate chemical kinetic mechanism suitable for mimicking the ignition characteristics of different lubricant oils. The experiments also revealed the different thermodynamic and mixture conditions at which the lubricant oil droplets did not show ignition. At compressed pressure of 24 bar and varied compressed temperatures of 740 K < Tc < 900 K in CH4/O2/N2/Ar mixture of ϕ = 0.6, two ester-based oils (EBO3 and EBO4) showed no ignition. The experiments and modeling indicate the minimum and maximum droplet sizes for which ignition will occur, the location and mode of ignition in the vicinity of the liquid droplet and the conditions under which the ignition event will transition into a propagating premixed flame. These experimental observations further enhance our understanding of lubricant oil combustion and provide qualitative information of engine operating conditions which can lower the abnormal combustion occurrence in natural gas engines. The results of this study advance the fundamental understanding of dual fuel combustion and provide the practical knowledge to inform which lubricant oil types and droplet sizes promote or inhibit abnormal combustion in natural gas engines.
Open Access
Latent heating and cloud processes in warm fronts
(Colorado State University. Libraries, 2012) Igel, Adele, author; van den Heever, Susan, advisor; Rutledge, Steven, committee member; Venayagamoorthy, Karan, committee member
The results of two studies are presented in this thesis. In the first, an extratropical cyclone that crossed the United States on April 9-11 2009 was successfully simulated at high resolution (3km horizontal grid spacing) using the Colorado State University Regional Atmospheric Modeling System. The sensitivity of the associated warm front to increasing pollution levels was then explored by conducting the same experiment with three different background profiles of cloud-nucleating aerosol concentration. To our knowledge, no study has examined the indirect effects of aerosols on warm fronts. First the budgets of ice, cloud water, and rain in the simulation with the lowest aerosol concentrations were examined. The ice mass was found to be produced in equal amounts through vapor deposition and riming and the melting of ice produced ~75% of the total rain. Conversion of cloud water to rain accounted for the other 25%. When cloud-nucleating aerosol concentrations were increased, significant changes were seen in the budget terms, but total precipitation was relatively constant. Vapor deposition onto ice increased, but riming of cloud water decreased such that there was only a small change in the total ice production and hence there was no significant change in melting. These responses can be understood in terms of a buffering effect in which smaller cloud droplets in the mixed phase region lead to both an enhanced Bergeron process and decreased riming efficiencies with increasing aerosol concentrations. Overall, while large changes were seen in the microphysical structure of the frontal cloud, cloud-nucleating aerosols had little impact on the precipitation production of the warm front. The second study addresses the role of latent heating associated with the warm front by assessing the relative contributions of individual cloud processes to latent heating and frontogenesis in both the horizontal and vertical directions. Condensation and cloud droplet nucleation are the largest sources of latent heat along the frontal surface and together produce rates of horizontal frontogenesis that are of the same order of magnitude as the deformation and tilting terms at midlevels; however near the surface latent heating does not cause strong frontogenesis. In the vertical, frontogenesis caused by these two processes is nearly everywhere higher than frontogenesis caused by dry dynamics, and are the primary mechanisms through which high static stability is found along the frontal surface. The horizontal and vertical components of frontogenesis are combined in a new way to form an expression for the frontal slope tendency. While dynamic processes lead to increases in frontal tilt, latent heating often counteracts this tendency. This indicates that the direct effect of latent heating on the thermal structure of the front is to decrease the slope and in that sense weaken the warm front.
Open Access
Modeling sensible heat flux for vegetated surfaces through an optimized surface aerodynamic temperature approach
(Colorado State University. Libraries, 2019) Costa Filho, Edson, author; Chavez, Jose L., advisor; Ham, Jay M., committee member; Venayagamoorthy, Karan, committee member
Agricultural water management advancements rely on improved methods to accurately determine crop water use. Crop evapotranspiration modeling based on the surface energy balance depends on the accurate estimation of all incoming and outgoing heat fluxes at the surface level. This thesis particularly goal is to improve sensible heat flux estimates for row crops through an optimized aerodynamic surface temperature (To) approach based on remote sensing and weather data. Empirical linear and non-linear To models were developed based on percent cover, surface temperature, air temperature, and a new variable named turbulent mixing row resistance using data collected at the USDA-ARS Research Farm located in Greeley (CO). The experiment took place in two sub-surface drip irrigation corn fields with different irrigation water management practices in 2017-2018. Sensible heat flux were measured using LAS, eddy covariance, aerodynamic profile, and Bowen ratio methods. Remote sensing data were measured on-site using a radiometer. The fields were considered a point in space. Data from Aimes (IA) and Rocky Ford (CO) were used to assess proposed model performances under different locations and in comparison to published To models. The results have indicated that the optimized linear To models performed better than the non-linear and published models approaches, indicating that the introduction of percent cover and the new variable has provided reliable results under different data sets. The linear proposed To approaches improved sensible heat flux estimation, on average, in 33 % and 28 % for the deficit and fully irrigated field at LIRF in comparison to the sensible heat based on published To models. Sensible heat flux modeling results were better for the modeling approaches considering the empirical linear To model than the non-linear approaches for all three data set tested.
Open Access
Numerical simulations of binary mixtures under gravity deposition using the discrete element method
(Colorado State University. Libraries, 2021) Jiang, Chao, author; Heyliger, Paul, advisor; Bareither, Christopher, committee member; Ellingwood, Bruce R., committee member; Venayagamoorthy, Karan, committee member; McGilvray, Kirk, committee member
Binary granular mixtures are frequently used in manufacturing, geotechnical engineering, and construction. Applications for these materials include dams, roads, and railway embankments. The mixing process requires dealing with particles with varying sizes and properties, and the complex composite nature of these mixtures can bring unpredictable results in overall performance. At present, there are no specifications for mixing these materials that can be used to quantify the levels of mixing and give estimates of the overall bulk properties. In this study, the Discrete Element Method (DEM) is used to examine the mechanics of the mixing process and give guidelines on how to achieve a well-mixed aggregate. A comprehensive non-linear visco-elastic damping collision model was developed to better represent the interactions between two dissimilar particles. A general Hertz model was applied for describing the normal force but a refined non-linear spring model was generated to imitate the friction force behavior without having to consider the entire loading history. A transition zone revealing the interactions between static and dynamic friction forces was shown in our numerical results. A moment resistance model was also added to capture the behavior of particle surface asperities and the damping force was calculated using relative motion. An alternative condition was applied to determine the end of a collision. Excellent agreement was found with well-established benchmark solutions and new results are also provided for future comparisons. Using this new DEM model, the mixing process of binary unbonded particles was studied using the effects of the number and position of geometric mixing obstacles and the number of mixing iterations. It was found that the mixing degree can be best quantified by measuring the spatial variation of the volume ratio φv. It was also found that small adjustments in the geometric position of the mixing obstacles could have a significant impact on the final mixing parameters. Surprisingly, the results indicate that two mixing iterations provided almost identical levels of mixing regardless of the number and nature of mixing obstacles. Estimates of the bulk elastic constants were provided and showed a high level of anisotropy as measured by the Poisson ratios for the horizontal versus vertical planes of the control volume. Particle crushing is a typical characteristic of many granular materials and can influence the mixing process, and it is possible to model non-particulate materials by bonding individual spheres together. The particle interactions and possibly impact with mixing barriers can result in the fracture of these solids as the allowable bond strength is exceeded. Therefore, the strength of the bond between individual particles that can be part of the mixing process is a critical parameter. The parallel bond model of Potyondy and Cundall (2004) was extended with the present DEM model was used to study the effects of bond strength on the mixing and mechanical properties of binary mixtures. Three types of particle blocks were studied for this purpose: unbonded, weakly bonded, and strongly bonded particles. The bonded particles result in a wider range of reflection angles as the particles interact with geometric mixers and simultaneously change and improve the level of mixing. Overall, these simulations serve to established specific guidelines and provide a basis for field-level mixing operations. They also provide some levels of expectation for the final mixing and bulk elastic behavior for the final aggregates.
Open Access
Optimizing remote sensing data for actual crop evapotranspiration mapping at different resolutions
(Colorado State University. Libraries, 2024) Costa Filho, Edson, author; Chávez, José L., advisor; Venayagamoorthy, Karan, committee member; Niemann, Jeffrey, committee member; Kummerow, Christian, committee member
This study aimed to advance irrigation water management by developing and evaluating a procedure to improve the multispectral data from sub-optimal remote sensing sensors when using the optimal spectral resolution for a given remote sensing (RS) of crop actual evapotranspiration (ETa) algorithm. Data have been collected at three research sites in Colorado under different irrigation systems, soil textures, and vegetation types. The research site in Greeley (CO) has a five-year dataset (2017-2018 and 2020-2022). The fields in Fort Collins and Rocky Ford (CO) have data from 2020 and 2021. Three categories of ETa algorithms were evaluated in the study: The reflectance-based crop coefficient (RBCC) with three different models based on the normalized difference vegetation index (NDVI), soil-adjusted vegetation index (SAVI), and fractional vegetation canopy cover (fc), the one-source simplified surface energy balance (OSEB) based on a surface aerodynamic temperature approach, and the two-source surface energy balance algorithm (TSEB) using two different resistance approaches (parallel and series). All three ETa modeling categories use either just surface reflectance in the visible and invisible light spectrum (e.g., RED, BLUE, GREEN, Near-infrared) or a combination of multispectral and thermal data as inputs to predict crop ETa, alongside local micrometeorological data from nearby agricultural weather stations. A total of six RS of ETa algorithms were evaluated in this study. A total of five RS sensors were evaluated: three spaceborne sensors (e.g., Landsat-8, Sentinel-2, and Planet CubeSat), one proximal device (multispectral radiometer), and an uncrewed aerial vehicle (UAS). The spatial resolution of the RS sensors varied from 30 m to 0.03 m. The accuracy assessment of the crop ETa predictions considered a statistical performance analysis using, among several statistical metrics, the mean bias error (MBE) and root mean square error (RMSE), and compared estimated ETa values from all seven RS ETa algorithms with observed ETa values obtained from the Eddy Covariance Energy Balance System (Greeley and Fort Collins sites) and a weighing lysimeter (Rocky Ford). The study was divided into three stages: a) the evaluation of different remote sensing (RS) pixel spatial resolutions (scales) as inputs on the estimation of different types of data needed for estimating ETa in hourly and daily time frames; b) the development of a calibration protocol and standards for the use of different imagery spatial resolutions (scales) in RS of ETa algorithms. The calibration approach involved a novel two-source pixel decomposition approach for partitioning surface reflectance into soil and vegetation using a non-linear, physically based spectral model, machine-learning regression, and a novel spatial light extinction model (kp); c) the accuracy evaluation of resulting ETa rates from calibrated/standardized data (for each selected RS of ETa algorithms). Results of stage one of the study indicated that depending on the RS of ETa and RS sensor data (spatial and spectral resolutions), the accuracy (MBE ± RMSE) of estimated ETa predictions varied. For the NDVI and fc RBCC ETa algorithms, Sentinel-2 provided the best RS data for predicting daily maize ETa. Errors were 0.21 (5%) ± 0.78 (18%) mm/d and 0.59 (14%) ± 1.07 (25%) mm/d, respectively. For the OSEB algorithm, Planet CubeSat gave the best RS data since it provided the smallest error for hourly maize ETa, -0.02 (-3%) ± 0.07 (13%) mm/h. For the SAVI RBCC model, the MSR data provided the best results since the maize ETa error was -0.13 (-3%) ± 0.67 (16%) mm/d. For the TSEB in series and parallel, the errors when estimating hourly maize ETa were -0.02 (-3%) ± 0.07 (11%) mm/h and -0.02 (-4%) ± 0.09 (14%) mm/h, respectively when using MSR data. For stage two of the study, the best machine learning regression model for a given RS sensor data and RS of the ETa algorithm depended on the surface reflectance composite (plant or bare soil values). The best machine-learning models for adjusting RS data were the regression tree and the Gaussian Process Regression. Regarding the pixel decomposition approach based on the novel spatial light extinction coefficient model, the novel approach provided reliable predictions of kp using the different RS sensor data. The error in predicting kp was -0.01 (-2%) ± 0.05 (10%) when combining all RS sensor data for the two-year data set at LIRF (years 2018 and 2022). For stage three of the study, results showed improvements in the accuracy of crop ETa estimation after adjusting the RS data using the proposed calibration protocol. At the Greeley site, regarding the RBCC RS of ETa algorithm, adjusted data from Planet CubeSat had better performance when estimating daily crop ETa since the error was reduced from 21% to 16% for the fc-input model. For the SAVI-input model, the RS data that performed better was the UAS. Errors were reduced from -0.42 (-11%) ± 0.76 (20%) mm/d to -0.21 (-5%) ± 0.41 (11%) mm/d. For the NDVI-input model, the adjusted UAS data performed better when estimating daily maize ETa. The improved accuracy was 0.32 (8%) ± 0.40 (10%) mm/d. At the Rocky Ford site, for the fc-input model, adjusted RS optical data from the MSR performed better. Daily maize ETa error was reduced from 17% to 15%. For the SAVI-input model, the RS data that performed better was the Landsat-8, with errors being reduced from -1.84 (-28%) ± 2.61 (39%) mm/d to -1.14 (-17%) ± 1.79 (27%) mm/d. The NDVI-based RBCC model had better performance when using adjusted MSR data daily maize ETa. Regarding the OSEB RS of crop ETa approach, at the Greeley site, the OSEB-adjusted data from UAS performed better. Hourly maize ETa error was reduced from 0.11 (19%) mm/h to 0.07 (13%) mm/h for the OSEB algorithm. For the TSEB parallel algorithm, the RS data that had better performance was the Landsat-8/9 since the error was reduced from 0.19 (34%) mm/h to 0.11 (20%) mm/h. For the TSEB series algorithm, the adjusted UAS data performed better. Daily maize ETa errors decreased from 0.10 (18%) mm/h to 0.05 (9%) mm/h. In summary, this study provided an RS calibration approach to support irrigation water management through the development and evaluation of a method for enhancing optical multispectral data sourced from various RS sensors. This study also highlighted the efficacy of machine learning models, like regression tree and Gaussian Process Regression, in adjusting RS data based on surface reflectance composites. Furthermore, a novel pixel decomposition approach utilizing a spatial light extinction model effectively predicted the light extinction coefficient. Overall, this research showcases the potential of RS data adjustments in improving the accuracy of ETa estimates, which is crucial for optimizing irrigation practices in agricultural settings.
Open Access
Prediction and mitigation strategies for the transient thermal performance of low thermal resistance microchannel evaporators
(Colorado State University. Libraries, 2024) Anderson, Caleb Del, author; Bandhauer, Todd M., advisor; Venayagamoorthy, Karan, committee member; Windom, Bret, committee member; Wise, Daniel, committee member
Microchannel flow boiling heat transfer offers an effective thermal management solution for high heat flux microelectronic devices such as laser diodes. The high heat transfer rates, nearly isothermal flow conditions, high surface area-to-volume ratios, and lower required pumping powers facilitate smaller component systems while more efficiently cooling devices and reducing packaging stresses associated with thermal expansion when compared with single-phase cooling systems. Although much study has been dedicated to optimizing steady state flow boiling performance, the typically highly transient operation of these microelectronic devices leads to unsteady spikes in heat flux and, subsequently, in device temperatures and may potentially exacerbate flow instabilities present at steady state. The low thermal capacitance of the package that often accompanies the low thermal resistance of microchannel evaporators increases the potential for device damage and failure since large temperature swings are more likely. Predicting and mitigating the transient response of a low thermal resistance microchannel evaporator is paramount to practical application as a thermal management technique. In this work, temperature, pressure, and flow visualization measurements during stepped heat loads on two, low thermal resistance, microchannel evaporators revealed the presence of severe vapor backflow, large temperature overshoots, and impacted flow dynamics at the onset of nucleate boiling (ONB) despite the stability and high performance of the device under steady state heating conditions. These overshoots were exacerbated with higher heating rates and reduced subcooling but were generally improved with higher flow rates. Applying a slower heating rate greatly improved the transient thermal response, reducing both peak temperature and vapor backflow. Channel and inlet orifice geometry were found to greatly impact the performance, with smaller channels and smaller orifice-to-channel restriction ratios resulting in intensified vapor backflow and temperature spikes, despite offering improved steady state performance. A computational model embedded in a reduced order design tool was created and validated with the experiments. Two separate models were created due to the different transient conditions observed between the two tested microchannel evaporators. The models allow predictive modeling of these evaporators to determine the impact of the transient heating behavior on microchannel evaporator devices. The effect of incorporating gallium-based, solid-liquid Phase Change Materials (PCMs) was studied semi-empirically by simulating the performance of a virtual test section with predicted properties of a microchannel evaporator combined with gallium and gallium-composite foam PCMs. Properties of the PCMs were estimated and used to predict the test section thermal response under a range of PCM volumes. Models assuming single phase performance were conducted initially and the resulting predicted heat rate to the fluid applied experimentally to the test section heater to determine the temperature response. It was found that the simulated addition of the PCM slightly reduced the ONB temperatures but did not affect the peak temperature experienced by the device. The applied heating rate, however, did not consider the increased thermal resistance to the refrigerant fluid during the transient vapor backflow regime. The effect was most pronounced in the PCMs with the largest exposed surface area and with thermal conductivity-enhanced PCM composites comprised of gallium infiltrated in a copper foam matrix. Additional PCM models utilizing the transient flow boiling model were subsequently run on a series of representative heat load test cases comparing the performance of a gallium-nickel and gallium-copper composite with similar dimensions to the earlier simulations. Key assumptions included the same ONB temperatures and vapor backflow conditions as the baseline cases without PCMs. The models predicted significantly lowered peak device temperatures due to the heat absorption into the PCM during the transient vapor backflow phase. The effect was dependent on the PCM thickness, latent heat, and thermal conductivity, reflecting trade-offs in material. In addition, peak temperature variability observed experimentally across multiple trials at the same nominal testing conditions was greatly reduced with the inclusion of a PCM.
Open Access
Quantifying function in the zebrafish embryonic heart: a study on the role of timed mechanical cues
(Colorado State University. Libraries, 2014) Johnson, Brennan Michael, author; Dasi, Lakshmi, advisor; Garrity, Deborah, advisor; Kisiday, John, committee member; Orton, Christopher, committee member; Venayagamoorthy, Karan, committee member
Congenital heart defects are a relatively common problem, yet the cause is unknown in the large majority of cases. A significant amount of past research has shown that there is a link between altered blood-induced mechanical stress and heart development. However, very little research has been done to examine the effect of altered loading timing. During embryonic development, the heart undergoes a drastic change in morphology from its original valveless tube structure to a complete multi-chambered pump with valves. Blood flow dynamics are consequently altered significantly as well. Given the changes occurring through this period, it is hypothesized that significant and persistent decreases in heart function occur when cardiac loading is altered during certain time windows of early development. The main objectives of this work were to (1) develop a methodology to quantify heart function in the embryonic zebrafish from high-speed bright field images, (2) develop a model for temporary and noninvasive alteration of cardiac loading, and (3) apply the methodology to normal and treated embryos to determine whether certain time windows of altered loading are more impactful than others. Results indicated that altered loading during the tube and early looping stages of development produce persistent changes in heart morphology along with accompanying decreases in cardiac function. Altered loading during late cardiac looping resulted in temporary changes in function which did not persist through the latest time point measured. This work has produced extensive tools for quantifying heart function from high speed images and presents a new model for altered cardiac loading in the zebrafish. Results support the hypothesis that the heart is more sensitive to altered loading during certain windows in development. This provides new insight into how congenital defects may develop and sets the stage for future experiments investigating the effects of altered loading on heart development.
Open Access
Quasidimensional modeling of reacting fuel sprays using detailed chemical kinetics
(Colorado State University. Libraries, 2016) Dobos, Aron Peter, author; Kirkpatrick, Allan T., advisor; Marchese, Anthony, committee member; Gao, Xinfeng, committee member; Venayagamoorthy, Karan, committee member
Since its invention in the late 1800s, the internal combustion engine has been indispensable to society for motive transport at all scales worldwide. Despite growing concern about the environmental damage caused by the pervasive use of these engines, no compelling alternative has yet emerged that matches the internal combustion engine's robustness, versatility, and high power-to-weight ratio. Consequently, as requirements on engine designs continue to increase to meet new emissions and efficiency standards, there is a strong need for computationally efficient and accurate predictive modeling of complex engine combustion processes. This work presents an efficient approach to direct injection engine combustion simulation that uses detailed chemical kinetics with a quasidimensional fuel spray model. Instead of a full multidimensional approach that solves continuity, momentum, energy, and chemistry equations simultaneously over a fine grid, the spatial information is greatly reduced and modeled phenomenologically. The model discretizes the fuel spray into independent parcels that entrain air from the surroundings and account for liquid fuel vaporization. Gas phase species concentrations and heat release in each parcel are calculated by detailed chemical kinetic mechanisms for the fuel under consideration. Comparisons of predicted pressure, heat release, and emissions with data from diesel engine experiments show good agreement overall, and suggest that spray combustion processes can be modeled without calibration of empirical constants at a significantly lower computational cost than with standard multidimensional tools. The new combustion model is also used to investigate spray structure and emissions trends for biodiesel fuels in a compression ignition engine. Results underscore the complex relationships among operational parameters, fuel chemistry, and NOx emissions, and provide further evidence of a link between stoichiometry near the flame lift-off length and formation of NOx. In addition, fuel molecular structure is demonstrated to be a significant factor in NOx emissions, but more robust chemical kinetic mechanisms and soot models for biodiesel are likely needed for improved predictive accuracy in modeling alternative fuels.
Open Access
Role of Rossby wave breaking in the variability of large-scale atmospheric transport and mixing
(Colorado State University. Libraries, 2017) Liu, Chengji, author; Barnes, Elizabeth A., advisor; Birner, Thomas, committee member; Kiladis, George N., committee member; Schubert, Wayne H., committee member; Venayagamoorthy, Karan, committee member
We demonstrate that Rossby wave breaking (RWB) plays an important role in both horizontal and vertical large-scale transport/mixing in both observations and idealized general circulation models. In the horizontal direction, RWB contributes to a substantial fraction of transient moisture flux into the Arctic. In the vertical direction, RWB modifies thermal stratification near the tropopause which leads to enhanced mass exchange across the tropopause. In understanding the variability of RWB related transport and mixing, we show that it is essential to separate the two types of RWB – anticyclonic wave breaking (AWB) and cyclonic wave breaking (CWB) – for two fundamental differences between them. The first difference is the opposite relationship between jet positions and their frequencies of occurrence. For both horizontal transport of moisture into the Arctic and vertical mixing of ozone across the tropopause, the robust relationship between jet position and AWB/CWB frequency is of first order importance in explaining the large-scale transport/mixing anomaly patterns influenced by climate variabilities involving jet shifting, such as the El-Nino Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). The second robust difference is the mixing strength exhibited by individual AWB and CWB events. In idealized lifecycle and climate simulations, as well as reanalysis data, CWB consistently exhibits stronger mixing strength than AWB. Combined with the robust relationship between jet variability and AWB/CWB frequency, such a difference is demonstrated to translate into a decrease in total upper troposphere diffusivity as the jet shifts poleward in an idealized climate simulation.
Open Access
Seasonal sensitivity of the eddy-driven jet to tropospheric heating in an idealized atmospheric general circulation model
(Colorado State University. Libraries, 2015) McGraw, Marie C., author; Barnes, Elizabeth, advisor; Birner, Thomas, committee member; Venayagamoorthy, Karan, committee member
A dry dynamical core is used to investigate the seasonal sensitivity of the circulation to two idealized thermal forcings–a tropical upper tropospheric forcing, and a polar lower tropospheric forcing. The circulation is modified using a set of perpetual simulations to simulate each month of the year, while the thermal forcings are held constant. The circu- lation responses to tropical warming and polar warming are studied separately, and then the response to the simultaneously applied forcings is analyzed. Finally, the seasonality of the internal variability of the circulation is explored as a possible mechanism to explain the seasonality of the responses. The primary results of these experiments are: 1) There is a seasonal sensitivity in the circulation response to both the tropical and polar forcings. 2) The jet position response to each forcing is greatest in the transition seasons, and the jet speed response exhibits a seasonal sensitivity to both forcings although the seasonal sensi- tivities are not the same. 3) The circulation response is nonlinear in the transition seasons, but approximately linear in the summer and winter months. 4) The internal variability of the unforced circulation exhibits a seasonal sensitivity that may partly explain the seasonal sensitivity of the forced response. The seasonality of the internal variability of daily MERRA reanalysis data is compared to that of the model, demonstrating that the broad conclusions drawn from this idealized modeling study may be useful for understanding the jet response to anthropogenic forcing.
Open Access
Seasonal to multi-decadal variability of the width of the tropical belt
(Colorado State University. Libraries, 2013) Davis, Nicholas Alexander, author; Birner, Thomas, advisor; Thompson, David, committee member; Venayagamoorthy, Karan, committee member
An expansion of the tropical belt has been extensively reported in observations, reanalyses, and climate model simulations, but there is a great deal of uncertainty in estimates of the rate of widening as different diagnostics give a wide range of results. This study critically examines robust diagnostics for the width of the tropical belt to explore their seasonality, interannual variability, and multi-decadal trends. These diagnostics are motivated by an exploration of two simple models of the Hadley circulation and subtropical jets. The width based on the latitudes of the maximum tropospheric dry bulk static stability, measuring the difference in potential temperature between the tropopause and the surface, is found to be closely coupled to the width based on the subtropical jet cores on all timescales. In contrast, the tropical belt width and Northern Hemisphere edge latitudes based on the latitudes at which the vertically-averaged streamfunction vanishes, a measure of the Hadley circulation's poleward edges, lags those of the other diagnostics by approximately one month. The tropical belt width varies by up to ten degrees latitude among the diagnostics, with trends in the tropical belt width ranging from -0.5 to 2.0 degrees per decade over the 1979-2012 period. Nevertheless, in agreement with previous studies nearly all diagnostics exhibit a widening trend, although the streamfunction diagnostic exhibits a significantly stronger widening than either the jet or dry bulk stability diagnostics. Finally, GPS radio occultation observations are used to assess the ability of the reanalyses to reproduce the tropical belt width, finding that they better situate the latitudes of maximum bulk stability versus those of the subtropical jets.
Open Access
Some efficient open-loop control solution strategies for dynamic optimization problems and control co-design
(Colorado State University. Libraries, 2021) Sundarrajan, Athul Krishna, author; Herber, Daniel R., advisor; Cale, James, committee member; Venayagamoorthy, Karan, committee member
This thesis explores strategies to efficiently solve dynamic optimization (DO) and control codesign (CCD) problems that arise in early-stage system design studies. The task of design optimization of dynamic systems involves identifying optimal values of the physical elements of the system and the inputs to effectively control the dynamic behavior of the system to achieve peak performance. The problem becomes more complex when designing multidisciplinary systems, where the coupling between disciplines must be accounted for to achieve optimal performance. Developing tools and strategies to efficiently and accurately solve these problems is needed. Conventional design practices involve sequentially optimizing the plant parameters and then identifying a control scheme for the given plant design. This sequential design procedure does not often produce system-level optimal solutions. Control co-design or CCD is a design paradigm that seeks to find system-level optimal design through simultaneous optimization of the plant and control variables. In this work, both the plant and controls optimization are framed as a integrated DO problem. We focus on a class of direct methods called direct transcription (DT) to solve these DO problems. We start with a subclass of nonlinear dynamic optimization (NLDO) problems for the first study, namely linear-quadratic dynamic optimization problems (LQDO). For this class of problems, the objective function is quadratic, and the constraints are linear. Highly efficient and accurate computational tools have been developed for solving LQDO problems on account of their linear and quadratic problem elements. Their structure facilities the development of automated solvers. We identify the factors that enable creating these efficient tools and leverage them towards solving NLDO problems. We explore three different strategies to solve NLDO problems using LQDO elements, and analyze the requirements and limits of each approach. Though multiple studies have used one of the methods to solve a given CCD problem, there isa lack of investigations identifying the trade-offs between the nested and simultaneous CCD, two commonly used methods. We build on the results from the first study and solve a detailed active suspension design using both the nested and simultaneous CCD methods. We look at the impact of derivative methods, tolerance, and the number of discretization points on the solution accuracy and computational times. We use the implementation and results from this study to form some heuristics to choose between simultaneous and nested CCD methods. A third study involves CCD of a floating offshore wind turbine using the levelized cost of energy (LCOE) as an objective. The methods and tools developed in the previous studies have been applied toward solving a complex engineering design problem. The results show that the impact of optimal control strategies and the importance of adopting an integrated approach for designing FOWTs to lower the LCOE.
Open Access
The role of physical and chemical properties of single and multicomponent liquid fuels on spray processes, flame stability, and emissions
(Colorado State University. Libraries, 2019) Alsulami, Radi Abdulmonem, author; Windom, Bret, advisor; Marchese, Anthony, committee member; Olsen, Daniel, committee member; Venayagamoorthy, Karan, committee member
Ensuring reliable and clean combustion performance of IC engines, such as liquid-fueled gas turbines, is associated to our understanding of the impact of fuel composition and properties, as well as the processes that the liquid fuel experiences, e.g., atomization, vaporization, turbulent mixing, and chemical kinetics, on the combustion efficiency, stability, and emissions. This understanding is a key prerequisite to the development of fuel surrogates and the deployment of alternative jet fuels. Most of the surrogate formulation activities, especially with regard to aviation fuels, have targeted only the gas-phase behavior of the real fuels, often neglecting properties responsible for atomization, vaporization, and fuel/air mixing (i.e., physical properties). In addition, much research has been done to understand the flame stability (e.g., lean blowout limit and flame liftoff height) of gaseous and pre-vaporized fuels. Thus, the optimization of the fuels and the liquid fueled combustion devices, e.g., gas turbines, requires the consideration of the two-phase process and the coupling between the complex physical and chemical processes. This will improve the understanding of the mechanisms that controls flame lean blowout limit and liftoff height of liquid fuels. Therefore, an appropriate surrogates will be formulated and a faster processes to certify the alternative fuels will be achieved. In this work, the flame stability in spray burner, quantified by flame lean blowout liftoff height, for different single, binary, alternative, and conventional fuels were experimentally measured. The flame behavior from the spray burner was compared to the results which was done using gaseous flame platform, e.g., counterflow flame burner, to clearly demonstrate the significant importance of two-phase spray processes (i.e., atomization, vaporization, and turbulent mixing) on flame stability. It was found that the atomization process, which can lead to the variation of the droplet size and distribution, has significant impact on flame stability. This is because any change in the droplet size can enhance/diminish the vaporization and mixing processes, and therefore influence the clean and efficient energy conversion process. In addition, the sensitivity of the fuels properties on flame stability was evaluated to provide an explanation for why certain fuel properties govern flame stability, such as lean blowout and liftoff height. Thus, flame stability mechanisms can be developed. A number of approaches were used in this work to address these issues, such as multiple linear regression analysis, and previously developed correlations. The results indicate the importance of the atomization process (i.e. droplet size) on the vaporization rate and suggest that the liquid fuel fraction entering the flame plays a dominant role in controlling lean blowout limits. Thus, the large droplet and less volatile fuel was the most resistance fuel to flame blowout. The differences in liftoff height was shown to be a result of two-phase flame speed, which accounts for both pre-vaporized fuel reactivity defined by laminar flame speed (SL) and time scales associated with droplet evaporation. The influence of the physical and chemical properties of different jet fuels on spray process and thus on emissions is also investigated. This is done by measuring soot formation using Laser-Induced Incandescence (LII). The trends in spray flame soot formation are compared to the gas-phase Yield Sooting Index (YSI). Results indicate differences in planar soot distributions amongst the fuels and suggest a significant influence of the atomization and the vaporization processes on mixing and the soot formation.
Open Access
Tropical deep convection, entrainment, and dilution during the DYNAMO field campaign
(Colorado State University. Libraries, 2014) Hannah, Walter, author; Maloney, Eric, advisor; Randall, David, committee member; Johnson, Richard, committee member; Venayagamoorthy, Karan, committee member
This dissertation presents a study of outstanding questions in tropical meteorology relating to tropical deep convection, entrainment, and dilution. Much of the discussion in this study will focus on an important convectively-coupled phenomenon in the tropical atmosphere known as the Madden-Julian Oscillation (MJO), which is an eastward propagating atmospheric disturbance over the Indian and West Pacific Oceans that dominates the tropical variability on intraseasonal timescales (30-90 days). The MJO is most active during the Northern Hemisphere winter season and is characterized by alternating periods of enhanced and suppressed convective activity. A field campaign known as the "Dynamics of the MJO" (DYNAMO) was conducted in the boreal winter months from October 2011 through February 2012 to study the initialization of the MJO with in-situ observations. The first part of this study examines hindcast simulations of the first two MJO events during DYNAMO in a general circulation model (GCM). The model used for this is the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM5) version 5, which uses parameterized convection. In these simulations, an entrainment rate parameter is varied to test its effects on the representation of the MJO, following previous studies. Hindcast simulations with CAM5 reveal that the entrainment parameter can improve the representation of the MJO. However, analysis of the column integrated moist static energy (MSE) budget reveals that this improvement is the right answer for the wrong reason. CAM5 incorrectly enhances vertical MSE advection, which compensates for cloud radiative feedbacks that are too weak. A promising theory for the MJOs fundamental dynamics is that of a moisture mode. The gross moist stability (GMS) describes the ratio of advective MSE import to a measure of convective activity. Negative GMS, and specifically the vertical component of GMS (VGMS), is thought to be a necessary condition for the destabilization of a moisture mode. In CAM5, VGMS becomes negative when the entrainment parameter is increased, indicating that the model can more easily destabilize moisture modes. However, this is inconsistent with re-analysis data, which exhibits positive VGMS. The second part of the study examines hindcasts using the super-parameterized version of CAM5 (SP-CAM) that uses embedded cloud-resolving models (CRM) to explicitly simulate convection on the sub-grid scale. SP-CAM was used for these hindcast simulations because previous studies have shown this type of model can reproduce the MJO much better than conventional GCMs. SP-CAM hindcasts yield a more robust MJO representation than CAM5, as expected. SP-CAM has an interesting systematic drift away from the initial conditions that projects well on the Real-time Multivariate MJO index (RMM), which negatively impacts the RMM skill scores. Analysis of the column MSE budget shows that SP-CAM has more realistic cloud-radiative feedbacks when compared to CAM5. SP-CAM also has a bias towards stronger import by vertical MSE advection that is similar CAM5 and inconsistent with re-analysis data. VGMS in SP-CAM is also found to be negative, which is similar to CAM5 and inconsistent with re-analysis data. The results from the first part of this study highlight a paradox surrounding entrainment. Although, previous studies have shown that entrainment rates should be larger than typical values used in parameterizations, increasing the entrainment rate does not make global model simulations more realistic. This prompted a detailed investigation into entrainment processes in high-resolution CRM simulations. A series of simulations are conducted where deep convection is initiated with a warm humid bubble. The bubble simulations are compared to a more realistic field of deep convection driven by forcing derived from the DYNAMO northern sounding array data. Entrainment and detrainment are found to be associated with toroidal circulations, consistent with recent studies. Analysis of the directly measured fractional entrainment rates does support an inverse relationship between entrainment and cloud radius, as is often assumed in simple models of deep convection. A method for quantifying the dilution by entrainment is developed and tested. Dilution and entrainment are generally not synonymous, but dilution is found to have a weak inverse relationship to cloud core radius. Sensitivity experiments show that entrainment and total water dilution are enhanced with environmental humidity is increased, contrary to the assumptions of some parameterizations. More vigorous convection in a more humid environment is better explained by a reduction of buoyancy dilution. An additional sensitivity experiment shows that entrainment and dilution are enhanced when convection is organized by the presence of vertical wind shear. The enhanced dilution is associated with entrainment of drier air on average.