Repository logo

Browsing by Author "Guzik, Stephen, advisor"

Now showing 1 - 5 of 5
  • Thumbnail Image
    ItemOpen Access
    A novel smoother-based data assimilation method for complex CFD
    (Colorado State University. Libraries, 2024) Hurst, Christopher L., author; Gao, Xinfeng, advisor; Guzik, Stephen, advisor; Troxell, Wade, committee member; van Leeuwen, Peter Jan, committee member
    Accurate computational fluid dynamics (CFD) modeling of turbulent flows is necessary for improving fluid-driven engineering designs. Traditional CFD often falls short of providing truly accurate solutions due to inherent uncertainties stemming from modeling assumptions and the chaotic nature of fluid flow. To overcome these limitations, we propose the integration of data assimilation (DA) techniques into CFD simulations. DA, which incorporates observational data into numerical models, offers a promising avenue to enhance predictability by reducing uncertainties associated with initial conditions and model parameters. This research aims to advance our understanding and application of DA for CFD modeling of highly chaotic dynamical systems. This dissertation makes several novel contributions in DA and CFD: i) A novel DA algorithm, the maximum likelihood ensemble smoother (MLES), has been developed and implemented to provide better model parameter estimation and assimilate time-integrated observations while addressing nonlinearity, ii) Multigrid-in-time techniques are applied to enhance the computational efficiency of the MLES by improving the optimization processes, and iii) The MLES+CFD framework has been validated by classical test problems such as the Lorenz 96 model and the Kuramoto-Sivashinsky equation. The effectiveness of the MLES has been demonstrated through a few test problems featuring chaos, discontinuity, or high dimensionality.
  • Thumbnail Image
    ItemOpen Access
    Computational analysis of aircraft pressure relief doors
    (Colorado State University. Libraries, 2016) Schott, Tyler, author; Gao, Xinfeng, advisor; Guzik, Stephen, advisor; Kirkpatrick, Allan, committee member; Liu, Jiangguo, committee member
    Modern trends in commercial aircraft design have sought to improve fuel efficiency while reducing emissions by operating at higher pressures and temperatures than ever before. Consequently, greater demands are placed on the auxiliary bleed air systems used for a multitude of aircraft operations. The increased role of bleed air systems poses significant challenges for the pressure relief system to ensure the safe and reliable operation of the aircraft. The core compartment pressure relief door (PRD) is an essential component of the pressure relief system which functions to relieve internal pressure in the core casing of a high-bypass turbofan engine during a burst duct over-pressurization event. The successful modeling and analysis of a burst duct event are imperative to the design and development of PRD's to ensure that they will meet the increased demands placed on the pressure relief system. Leveraging high-performance computing coupled with advances in computational analysis, this thesis focuses on a comprehensive computational fluid dynamics (CFD) study to characterize turbulent flow dynamics and quantify the performance of a core compartment PRD across a range of operating conditions and geometric configurations. The CFD analysis was based on a compressible, steady-state, three-dimensional, Reynolds-averaged Navier-Stokes approach. Simulations were analyzed, and results show that variations in freestream conditions, plenum environment, and geometric configurations have a non-linear impact on the discharge, moment, thrust, and surface temperature characteristics. The CFD study revealed that the underlying physics for this behavior is explained by the interaction of vortices, jets, and shockwaves. This thesis research is innovative and provides a comprehensive and detailed analysis of existing and novel PRD geometries over a range of realistic operating conditions representative of a burst duct over-pressurization event. Further, the study provides aircraft manufacturers with valuable insight into the impact that operating conditions and geometric configurations have on PRD performance and how the information can be used to assist future research and development of PRD design.
  • Thumbnail Image
    ItemOpen Access
    Geometry considerations for high-order finite-volume methods on structured grids with adaptive mesh refinement
    (Colorado State University. Libraries, 2022) Overton-Katz, Nathaniel D., author; Guzik, Stephen, advisor; Gao, Xinfeng, advisor; Weinberger, Chris, committee member; Bangerth, Wolfgang, committee member
    Computational fluid dynamics (CFD) is an invaluable tool for engineering design. Meshing complex geometries with accuracy and efficiency is vital to a CFD simulation. In particular, using structured grids with adaptive mesh refinement (AMR) will be invaluable to engineering optimization where automation is critical. For high-order (fourth-order and above) finite volume methods (FVMs), discrete representation of complex geometries adds extra challenges. High-order methods are not trivially extended to complex geometries of engineering interest. To accommodate geometric complexity with structured AMR in the context of high-order FVMs, this work aims to develop three new methods. First, a robust method is developed for bounding high-order interpolations between grid levels when using AMR. High-order interpolation is prone to numerical oscillations which can result in unphysical solutions. To overcome this, localized interpolation bounds are enforced while maintaining solution conservation. This method provides great flexibility in how refinement may be used in engineering applications. Second, a mapped multi-block technique is developed, capable of representing moderately complex geometries with structured grids. This method works with high-order FVMs while still enabling AMR and retaining strict solution conservation. This method interfaces with well-established engineering work flows for grid generation and interpolates generalized curvilinear coordinate transformations for each block. Solutions between blocks are then communicated by a generalized interpolation strategy while maintaining a single-valued flux. Finally, an embedded-boundary technique is developed for high-order FVMs. This method is particularly attractive since it automates mesh generation of any complex geometry. However, the algorithms on the resulting meshes require extra attention to achieve both stable and accurate results near boundaries. This is achieved by performing solution reconstructions using a weighted form of high-order interpolation that accounts for boundary geometry. These methods are verified, validated, and tested by complex configurations such as reacting flows in a bluff-body combustor and Stokes flows with complicated geometries. Results demonstrate the new algorithms are effective for solving complex geometries at high-order accuracy with AMR. This study contributes to advance the geometric capability in CFD for efficient and effective engineering applications.
  • Thumbnail Image
    ItemOpen Access
    Large-eddy simulation of compressible flows using the stretched-vortex model and a fourth-order finite volume scheme on adaptive grids
    (Colorado State University. Libraries, 2022) Walters, Sean, author; Guzik, Stephen, advisor; Gao, Xinfeng, advisor; Randall, David, committee member; Yalin, Azer, committee member
    State-of-the-art engineering workflows are becoming increasingly dependent on accurate large-eddy simulations (LES) of compressible, turbulent flows for off-design conditions. Traditional CFD algorithms for compressible flows rely on numerical stabilization to handle unresolved physics and/or steep gradient flow features such as shockwaves. To reach higher levels of physical-fidelity than previously attainable, more accurate turbulence models must be properly incorporated into existing, high-order CFD codes in a manner that preserves the stability of the underlying algorithm while fully realizing the benefits of the turbulence model. As it stands, casually combining turbulence models and numerical stabilization degrades LES solutions below the level achievable by using numerical stabilization alone. To effectively use high-quality turbulence models and numerical stabilization simultaneously in a fourth-order-accurate finite volume LES algorithm, a new method based on scale separation is developed using adaptive grid technology for the stretched-vortex subgrid-scale (SGS) LES model. This method successfully demonstrates scheme-independent and grid-independent LES results at very-high-Reynolds numbers for the inviscid Taylor-Green vortex, the temporally-evolving double-shear-flow, and decaying, homogeneous turbulence. Furthermore, the method clearly demonstrates quantifiable advantages of high-order accurate numerical methods. Additionally, the stretched-vortex LES wall-model is extended to curvilinear mapped meshes for compressible flow simulations using adaptive mesh refinement. The capabilities of the wall-model combined with the stretched-vortex SGS LES model are demonstrated using the canonical zero-pressure-gradient flat-plate turbulent boundary layer. Finally, the complete algorithm is applied to simulate flow-separation and reattachment over a smooth-ramp, showing high-quality solutions on extremely coarse meshes.
  • Thumbnail Image
    ItemOpen Access
    Numerical algorithms for two-fluid, weakly-compressible flows
    (Colorado State University. Libraries, 2024) Brodin, Erik, author; Guzik, Stephen, advisor; Colella, Phillip, advisor; Gao, Xinfeng, committee member; Troxell, Wade, committee member; Bangerth, Wolfgang, committee member
    A multifluid numerical method is developed for flows of two fluids in a single domain at low Mach numbers. An all-speed formulation of the Navier-Stokes equations governs the dynamics of both fluids and the level-set method defines the interface between them and the domain of each fluid. The algorithm represents velocity and pressure as single valued throughout the whole domain, and fluid dependent variables, density and bulk modulus, only in the domain of their respective fluid. The all-speed equations are not subject to the divergence-free velocity constraint through use of a redundant velocity equation, and are evolved in time using an implicit-explicit additive Runge-Kutta method resulting in a time step constrained only by the bulk fluid velocity. Each fluid is evolved conservatively with respect to the moving interface between them. Due to errors in the evolution in the interface, perturbations in the volume of each fluid, and thereby the density, can develop. A thermodynamically consistent correction is made to the position of the interface to reduce these unphysical perturbations. The algorithm developed here includes three novel contributions: (i) the use of a multifluid all-speed algorithm with a level-set method for evolution of the solution in time, (ii) a multifluid algorithm using the level-set to capture the interface in the weakly compressible regime that is thermodynamically consistent, and (iii) an initialization method for sharp corners in the level-set. Numerical tests have demonstrated that the algorithm exhibits the expected low Mach number behavior, achieves second order-accuracy, and ensures fluid volumes are bounded and convergent.