ANÁLISIS DEL CENTRO ESCOLAR.

A surface oil-flow visualization photograph of the microramp array is shown in Fig. 17 with the prominent features corresponding well with the previously proposed five-pair vortex model [6]. The streaklines past the trailing edges of each microramp show how the microramp creates two large counter- rotating vortices: one on each side. The darker areas are indicative of the high-momentum/high-shear flow of these vortices along the surface of the wind tunnel. The surface flow visualization also shows how the flow curves over the edges of the front face of the microramp, creating the vortices as this flow separates from the ramp. The formation of the two large vortices is evident from the streaklines left along the wind tunnel surface as highlighted in Fig. 17. Flow separation from the microramp trailing edge is indicated by the oil left along the ramp edges in addition to a large oil deposit at the ramp vertex. Even though the flow appears separated at the trailing edge, the lack of oil downstream of the ramp indicates a high-momentum/shear region created from the vortices along the wall in the wake of the ramp. Additional shear stress measurements downstream of MVG devices are discussed in Appendix A. The high- momentum regions show that the flow near the surface is energized in the wake of the microramp, signifying that it may indeed help with separation created in a SWBLI.

Figure 17: Microramp array surface oil-flow visualization with important flowfield features highlighted. The surface flow measurements also reveal evidence of two additional pairs of secondary vortices. Oil located in the ramp/wall junction denotes a secondary vortex pair that develops along the microramp

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and is marked in Fig. 17. The primary vortex pair induces a secondary vortex pair further downstream of the ramp at approximately x = 16 mm. The formation of this secondary vortex is clarified through a series of surface flow images. The proposed third set of secondary vortices in the five-pair vortex model (Fig. 2a), created along the top trailing edge of the ramp, is not in view with the current measurements [6]. However, a small separation region is present at the ramp leading edge with a horseshoe vortex encasing the high-shear region downstream of the microramp (Fig. 17). Outside of the horseshoe vortex, to either side of the ramp, the flow appears to be relatively unaffected, indicating that an array of microramps is necessary to have a substantial effect on the flow in a supersonic inlet. Each microramp in the array has similar surface flow characteristics, showing that the flow at the microramps is unaffected by neighboring microramps, and the flow appears to be essentially symmetric over each microramp.

The surface oil-flow visualization of the normal shock wave/boundary-layer interaction region is displayed in Fig. 18, both with and without the microramp array. Figure 18 confirms that there is no clear evidence of separation in the SWBLI with either flow control case. One of two possibilities for the lack of separation is that the flow is incipiently separated and the incoming Mach number is not high enough to trigger separation; or the other possibility is that small amounts of separation occur, but the shock wave position fluctuations wash out any evidence of separation in the time-averaged surface oil-flow measurements.

As indicated with the schlieren photography, the surface flow measurements confirm that the shock wave has an average position further upstream when microramps are present in the flowfield. The high- shear/momentum wakes of the microramps are clearly present in the SWBLI region (Fig. 18b). In the SWBLI region, less oil is present in the wake of the ramps when compared to the oil located in the span between ramps. This is an indication that directly downstream of the microramps the propensity for separation at the SWBLI has been reduced, while the gap between ramps experiences similar levels of separation as the no-control case. The surface oil-flow results support the idea that an array of microramps has the ability to break up separation in a SWBLI. These conclusions are more clearly supported when analyzing a series of surface oil-flow images. Downstream of the SWBLI there is still an indication of the microramp wakes with reduced clarity.

One aspect of SWBLIs studied in small-scale facilities requiring discussion is side-wall effects or corner effects. The surface oil-flow results show that the oil moves towards the wind tunnel centerline at the SWBLI. This is a symptom of wind tunnel wall effects. The normal shock wave/boundary-layer interaction creates a small separation zone in the wind tunnel corners. It is this separation that is measured with the surface flow visualization. Past studies have shown evidence that corner effects can affect the flow along the wind tunnel centerline in a complex three-dimensional interaction [87, 88]. Experimental

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investigations have discovered that for a set incoming Mach number, the separation along the centerline of a SWBLI can be eliminated with increased separation in the sidewall region. Conversely, if the separation in corners is reduced, the centerline separation can be enhanced. Burton and Babinsky [87] believe that the primary mechanism behind this to be that the corner separations alter the lambda-foot structure, which in turn influences the adverse pressure gradient imposed at the centerline. Thus, the adverse pressure gradient in the corner region is smeared by compression waves created from a blockage effect of the corner separations. However, the same study also reveals that the corner separation size does not influence the downstream velocity. The blockage effect from the corner separations is negligible in the downstream subsonic region. Therefore, boundary-layer health (H and Cf) measurements from

velocity profiles downstream of the SWBLI are not affected by a change in corner effects between control cases. In the present investigation, even though there is evidence of small corner effects, there is little difference in them between the control cases (Fig. 18). Hence, any conclusions reached in these experiments on the effects of microramps in the control of SWBLIs are not influenced by corner effects but rather only by the microramps themselves.

Figure 18: Surface flow visualization of normal shock wave region: (a) without microramp array and (b) with microramp array.

(a)

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