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Effect of Roughness Type
The shape of the roughness elements also has effect on the transition process. To investigate the inflience of the roughness shape on the loss reduction, different types of roughness elements were tested under steady and unsteady conditions with Tu=0.5%. The tested roughness elements included straight steps, wavy steps and wavy wires.
The measured loss coefficients for the straight step S-2 (k/5* = 0.57) are presented in Figure 11. The loss coefficient for W-7, which has the same diameter as the height of S-2, is also plotted in the figure. Under steady conditions, the step is more effective at reducing the loss than the wire with the same dimensions. This observation is the same as that of Pinson and Wang (2000). This is because that the disturbances generated by the sharp edge of the step are stronger than these after the wire, as shown in Figure 12. This figure shows the RMS profiles measured just after the roughness elements at 51%Sо. However, under the unsteady flow conditions, the difference in the loss coefficients for these two roughness elements is not much. This is again because that the loss coefficient is due to the combined effects of the bubble size reduction and larger turbulent wetted area.
Another two types of roughness elements, a wavy step and a wavy wire as shown in Figure 2, were also tested. Figure 13 presents the measured loss coefficients with the wavy step S-3. Under steady flow conditions, the wavy step is more effective than the straight step S-2 in loss reduction although they have the same heights. To find the reason for this difference, the disturbance levels after S-3 were measured at two different span-wise locations: after the tail and between the tails of the roughness element(see Figure 2). As shown in Figure 14, between the tails the RMS level is similar as that after S-2. However,
there is very high level of turbulence after the tail. There are strong streamwise vortices generated in the region of the tails. These help to trigger early transition and further reduces the losses. However, under unsteady ft>w condition, the stronger streamwise vortices cause more turbulent mixing and weaken the calmed region resulting in a higher loss. The wavy wire, which has a diameter equal to the height of S-3, gave the similar results as that of the wavy step S-3.
Figure 14. Measured RMS after wavy step in steady conditions
