Turbulent Boundary-Layer Development Around a Square-Sectioned U-Bend: Measurements and Computation

Abstract

A computational and experimental study is reported of turbulent flow around a square-sectioned U-bend with a mean bend radius equal to 3.375 times the hydraulic diameter (DH): the duct Reynolds number is 58,000. The bend geometry is the same as that for which Chang et al. (1983) have reported extensive LDA data except that in the latter experiment the bend was preceded by some thirty hydraulic diameters of straight ducting (thus the boundary layers filled the duct). In the present case, with the inlet section shortened to only 6 DH, the boundary layer thickness at inlet to the bend was only about 0.15 DH. Despite the thinner boundary layers a strong secondary flow is generated which, by 135° around the bend, appears to have broken down into a chaotic pattern. Computations of the flow using a three-dimensional finite-volume solver employing an algebraic second-moment (ASM) turbulence model are in generally close agreement with the experimental data and suggest that the secondary flow, in fact, breaks down into a system of five eddies on either side of the mid-plane, in place of the classical single vortex structure.