Measurement Results on Smooth Surface

The baseline of the parametric study of the surface roughness elements on fht plate was provided by an investigation of the ft>w properties on the smooth surface. The measured surface static pressure distributions at Re=174000, as well as the numerical simulation results from ITP, are presented in Figure 3. The peak velocity occurs around 45%S0 and the diffusion factor (Dv = (Vmax — V97%s)/Vmax) is about 0.27. The boundary layer separated at about 55%So. At Tu = 0.5%, the few reattaches at about 80%S 0. At Tu = 3.5%, the high level of disturbance in the freestream fbw helps to reattach the sepa­rated fbw earlier and results in a smaller bubble.

Figure 4. Steady boundary layer mea­surements on smooth surface

The surface static pressure distributions under unsteady ft>w conditions were also measured by using the same technique as that in the steady ft>w. Due to the low frequency response of the Scanivalve system, only time-averaged infor­mation can be provided. The measurement results at ф = 0.83 and fr = 0.84 for Re=174000 with Tu=0.5% is also presented in Figure 3. The incoming wakes periodically reduce the separation bubble size. This can be identified by the large change in Cp distribution near 70%S0, where the maximum height of separation bubble is located under the steady condition.

To further study the ft>w patterns, the boundary layer development on the fht plate with a smooth surface was surveyed using hot-wire traverses at 11 different stream-wise locations between 51%S0 and 97%So under both steady and unsteady ft>w conditions. Figure 4 presents the measured velocity con­tours normalized by the exit velocity for Re=174000 with Tu=0.5% under steady ftjw condition. The velocity contour plot suggests that the boundary layer separates at 55%S0 and reattaches around 83%S0. The contour lines of turbulence intensity normalized by the exit velocity and the infection line are

also plotted in the figure. It can be clearly see that the instabilities originate at the infection point.

The selected boundary layer integral parameters for the unsteady condition with ф = 0.83 and fr = 0.84 at Re= 174000 and Tu=0.5% are presented in Figure 5 as S-T diagrams. Four velocity trajectories are also plotted in the figures. The wakes travel at the free stream velocity (V*). The leading edge of the wake-induced transitional/turbulent region travels at about 0.9 V*. The trailing edge of the wake-induced transitional/turbulent fow or the leading edge of the calmed region travels at about 0.5V^ and the trailing edge of the calmed region travels at about 0.3 V*.

The shape factor, H12, presented in Figure 5(a) clearly shows the periodic interaction between the coming wakes and separation bubble. The boundary layer separates at almost the same location as that in the steady fow condi­tions, marked as a dashed line in the figure. The separation bubble is prevented from reestablishing by the wake-induced turbulent region and the calmed re­gion until the trailing edge of the calmed region, along the line A-B marked in the figure. Then the separation bubble between the wakes starts to reestab­lish. The steep slope of the separation reattachment line (marked B-C in the figure) suggests that the bubble recovery rate between wakes is very slow. The momentum thickness 9, shown in Figure 5(b), also clearly shows the periodic effects of wake passing. In the wake-induced turbulent region, there is higher momentum thickness. In the calmed region, the 9 value is lower, which is one of the important features of the calmed region as it corresponds to the loss re­duction. The effect of the calmed region almost persists to the trailing edge.

Between the wakes, the momentum thickness increases as the separation bub­ble reestablishes.

The boundary layer development strongly affects the profile loss. The loss coefficients, calculated based on Equation 2, at different ft>w conditions on smooth surface are presented in Figure 6. The loss coefficients under steady conditions with Tu=0.5% decrease as the Reynolds number increases. Due to the existence of the large separation bubble in the steady conditions, the loss level is high, especially at low Reynolds numbers. Increasing the free stream turbulence intensity (Tu=3.5%) causes the boundary layer to transition earlier and reduces the bubble size. Thus the loss is reduced. However, at lower Reynolds numbers the reduction in losses is not as large as that at higher Reynolds number due to the larger separation bubble and the later transition that occurs at lower Reynolds numbers.

In the unsteady ft>w condition, with f r = 0.84 and Tu=0.5%, the periodic suppression of the separation bubble due to the wake passing and the calmed region following it significantly reduce the losses. At Tu=3.5%, the losses are further reduced at the lower Reynolds numbers. However, at the highest Reynolds number, the loss does not change much. This is because the loss vari­ation is a balance between the positive effect of the bubble reduction and the negative effect of the increase in the turbulent wetted area. At high Reynolds numbers, with high free stream turbulence intensity, the separation bubble is already small under steady conditions. In this case, the negative effect caused by the wake passing is larger than the benefit that arises due to the reduction in the size of the bubble and the calmed region.