PARAMETRIC STUDYOF SURFACE ROUGHNESS ANDWAKE UNSTEADINESS ON A FLAT PLATE WITH LARGE PRESSURE GRADIENT

Abstract

The combined effects of surface roughness and upstream unsteady wakes were investigated on a flat plate subjected to the same pressure distribution as that on the suction surface of an ultra high lift low-pressure (LP) turbine blade. The parametric study included the effects of roughness size, type and location under steady and unsteady flow conditions. The roughness elements included straight wires, straight square steps, wavy steps and wavy wires. The experimental results show that the combined effects of surface roughness and upstream unsteady wake can further reduce the profile losses of the ultra highly loaded LP turbine blades. For each flow condition, there is a height of roughness element, which results in the lowest loss. The step-type roughness elements are more effective at inducing boundary layer transition than the wire-type roughness due to the higher disturbance level generated after the sharp edges. The wavy roughness elements are more effective at inducing transition and further reduce the profile losses under steady flow conditions. However, the strong stream-wise vortexes generated by the wavy elements will increase the loss in unsteady conditions. The optimum location of the roughness element is between the blade suction peak and the separation onset location.