Measurements of turbulence quantities and bursting period in developing turbulent boundary layers on the concave and convex walls of a 90° square bend

Abstract

The results of an experimental study of developing turbulent boundary layers on the concave and convex walls of a 90° bend of square cross-section using hot-wire anemometry are presented. The bend has a mean radius to height ratio of R/ H=1.17. The flow has a Reynolds number Re H =3.6×10 5, and at start of the bend the boundary layer thickness to the radius of curvature ratio is δ/ R cc=0.041 and δ/ R cx=0.054 on the concave and convex walls, respectively. Conditional sampling based on the VITA method applied to the streamwise velocity fluctuations is used to obtain the period of bursting events in the concave and convex boundary layers. Three scaling laws, namely, inner scaling, outer scaling, and the mixed scaling, are used to non-dimensionalize the bursting period. Results are presented for the conventional mean turbulence quantities and the period of bursting. Turbulence enhancement on the concave wall and the opposite effect on the convex wall are most pronounced in the outer layer, and the effect on the normal intensity is greater than on the streamwise intensity. On the convex wall, suppression of turbulence shear stress to insignificant levels occurs for normal distances from the wall greater than y/ δ≈0.4. The results of conditional sampling show the opposite effects of the concave and convex walls on the bursting frequency, and that none of the scaling laws results in the collapse of this parameter.