Measurement Results with Roughness

Even under unsteady fbw conditions, the losses on the smooth fht plate are still high, especially at low Reynolds numbers. This is because that the profile is not an optimal design and there is a large separation bubble between the passing wakes. Ramesh et al. (2001) and Roman et al. (2003) showed that the profile loss of the ultra high lift aerofoil can be further reduced by the combined effects of surface roughness and wake unsteadiness. The current study is aimed at optimizing the roughness parameters, including the roughness size, type and location, so as to reduce the loss of ultra high lift LP turbine blades.

Boundary Layer Behavior with Roughness

The steady velocity contour plots from hot-wire measurements atRe=174000 and Tu=0.5% with the straight step S-2 (k/5* = 0.57) placed at 50%So are presented in Figure 7. Due to the high disturbance level after the roughness element, the separation bubble is much smaller than that on the smooth sur­face in both length and height. The turbulence intensity contour lines indicate that the roughness element does not induce transition immediately but that it hastens the transition process in the separated shear flow.

The measured unsteady boundary layer integral parameters are presented in Figure 8. The shape factor, H, is shown in Figure 8(a). Due to its small size, the recovery of the separation bubble after the wake passing is very quick, which is indicated by the short reattachment line A-B being followed by the almost vertical reattachment line B-C in. Figure 8(b) shows the momentum thickness, 9, at this ft>w condition. Under the wake, at the beginning the mo­mentum thickness is larger than that on the smooth surface due to the effect of the roughness element. However, its growth rate is lower, so that the high­est value at the trailing edge is lower. This is the main contribution to the loss reduction. The momentum thickness associated with the calmed region is similar to that on the smooth surface at first but becomes higher close to the trailing edge. Between the wakes, the momentum thickness after the rough­ness element is slightly higher due to the high disturbance after the roughness. However, near the trailing edge, it is almost at the same level as that on the smooth surface.