ORIGIN AND DECAY OF LONGITUDINAL VORTICES IN DEVELOPING FLOW IN A CURVED RECTANGULAR DUCT

Developing turbulent flow in a 90 deg curved duct of rectangular cross -section, and an aspect ratio of 6, was investigated. Mean-velocity an d Reynolds-stress components were measured using a five-hole pressure probe and two-sensor hot-wire probes, respectively, in the boundary la yers on the duct walls to document the pressure-driven secondary motio n and the formation of a longitudinal vortex near the corner on the co nvex wall. Special attention was paid to the three-dimensionality of t he flow exiting the two-dimensional contraction of the wind tunnel in order to provide proper inlet boundary conditions for future computati onal work. The mean velocities and wall shear stresses were measured a t seven sections and turbulence measurement were made at four sections . The data provide insights into the development of three-dimensional turbulent boundary layers under the influence of strong streamwise cur vature, both convex or concave, and attendant pressure gradents, and c learly elucidate the mechanism by which strong pressure-driven seconda ry motion results in a longitudinal vortex.