Secondary flow in a boundary layer

Abstract

The secondary flow in the developing boundary layer of a square duct is investigated experimentally. Measurements of the horizontal components of secondary flow were made for free stream velocities of 3, 6.1, and 12.2 m/s, corresponding to Reynolds numbers based upon the hydraulic diameter of 3.6 x 105, 7 .2 x 105, and 1.4 x 106 at a ratio of L/Dh of 6.7. Measurements of the developing boundary layer parameters and turbulence quantities in a corner were made for the intermediate Reynolds number. The secondary flow and turbulence distribution in a corner are discussed and compared with the fully developed flow situation. The results indicate the maximum secondary flow velocity is less than 2% of the free stream velocity, and further, that the secondary flow may encompass the entire cross section. The turbulence distribution was found to not differ fundamentally from those found in fully developed flow. A criterion for two dimensionality of wind tunnel flow is examined and a correction for three dimensionality is evaluated from the experimental data.