An experimental study of a rib-roughened rotating U-bend flow

In this paper we report an experimental investigation of turbulent flow through a square-sectioned U-bend of strong curvature, in which the inner and outer walls of the upstream and downstream sections are artificially roughened with square ribs, in a staggered arrangement. The U-bend is either stationary or rotates about an axis parallel to that of curvature, with positive rates, i.e. so that the secondary flows provoked by curvature and rotation are in the same sense. The main objective is to provide CFD validation data for flows which contain most of the flow features encountered in blade-cooling passages, but which are numerically easier to compute, while retaining the modelling challenges provided by a real gas-turbine blade. In earlier investigations we showed that the introduction of ribs in the upstream and downstream sections: (a) raises overall turbulence levels; (b) reduces the size of the separation bubble formed along the inner wall of the U-bend and (c) causes the formation of a large separation bubble along the outer wall, as the flow encounters the first outer-wall rib, after the bend exit. Here we: (a) explore how the location of the first outer-wall rib, after the bend exit, affects the development of the downstream flow and (b) focus on the three-dimensional character of these flows, by providing data along a plane close to the top (flat) wall, in addition to data along the duct symmetry plane. We show that, for both stationary and rotating conditions, as the first outer-wall rib is moved further away from the bend exit, the size of the separation bubble along the outer wall is reduced. The separation bubble along the inner wall, however, increases in size but, overall, turbulence levels are reduced. The flow within and immediately downstream of the U-bend is highly three-dimensional, showing strong variations from the symmetry plane to the top wall. Rotation generates the expected secondary motion in the straight sections that, at the duct centre, convects the faster fluid towards the pressure side, and along the top and bottom (flat) walls, has the opposite effect. Within and downstream of the bend, positive rotation reduces the three-dimensionality of the flow. (C) 1999 Elsevier Science Inc. All rights reserved.