MEJORAS DESTINADAS A LAS FAMILAS Y COMUNIDAD.

Instantaneous schlieren photographs display the flowfield of the normal shock wave/boundary layer interaction in Fig. 14. The Mach 1.4 flow is from left to right with the leading edge of the microramp array at x = 0 mm. In the schlieren images, the normal shock wave is positioned at its mean streamwise position for each flow control case. The shock wave shape and position were extracted from points along the shock wave in 300 images (Fig. 15). This information allows an analysis of the change in shock structure when using microramp flow control. The mean normal shock positions are x = 79.2 mm and x =70.0 mm with and without the microramp array, respectively.

Figure 14: Instantaneous schlieren photography of normal shock wave/boundary-layer interaction: (a) wide view without control, (b) zoomed-in view without control, (c) wide view with control, and (d) zoomed-in view with control.

The SWBLI without microramp flow control is presented in Figs. 14a and 14b. A close up view of the interaction is provided in Fig. 14b. The instantaneous schlieren shows that there is no clear evidence of separation due to the SWBLI; however, there is an increase in boundary-layer thickness across the normal shock. The flow is likely incipiently separated, and the rapid thickening of the boundary layer

(a) (b)

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creates a lambda shock structure. A Mach 1.4 SWBLI is usually incipiently separated because the shock strength is not high enough to cause full separation [25]. The schlieren images reveal that the shock is not exactly normal but rather is a near-normal shock with curvature as it approaches the upper wind tunnel wall. The shock wave is approximately normal in the bottom half of the wind tunnel and in the SWBLI region. A weak secondary shock downstream is also present in Fig. 14, showing the transonic nature of the flowfield.

Figures 14c and 14d display instantaneous schlieren photographs with microramp flow control. The boundary layer in the wake of the microramp array apparently thickens in comparison to the incoming boundary layer, and periodic large-scale turbulent structures develop. The increase in the boundary-layer thickness will affect the necessary geometry of a supersonic inlet when compared with traditional inlet designs using a bleed system for boundary-layer control. The leading and trailing edges of the microramp array both produce oblique shock waves. These shock waves are three dimensional, similar to a cone- shaped shock wave. The shock wave at the trailing edge is created because of the necessary turning, or recompression, into the increased boundary-layer height wake following the rapid expansion after the microramps. The angle of the leading- and trailing-edge shock waves with respect to the wall is approximately 48 and 42 deg, respectively, with more fluctuation in the trailing-edge shock angle. The leading-edge shock angle relates well to the 46 deg angle of a shock generated by an ideal cone with equivalent angle of the microramp array in a Mach 1.4 flow.

Comparing the control cases in Fig. 14, it appears that the boundary layer is thicker after the normal shock with the microramp array present in the flowfield. Again, there does not appear to be any evidence of separation with the microramp array. From the schlieren images (Fig. 14) and mean shock shapes (Fig. 15), the extent of the lambda structure in both the streamwise and wall-normal direction has been reduced with the microramp array. This likely indicates an even weaker propensity for separation with the microramp-controlled SWBLI. Figure 15 also clearly indicates that the shock wave becomes more normal with the microramp configuration present. Tracking the bifurcation point through multiple schlieren images helps visualize the decrease in shock fluctuations with the microramp array (Fig. 15b). The standard deviation in shock position is decreased from 4.8 mm to 3.7 mm with flow control. An interesting trend is also visible in the distribution of bifurcation points; as the shock moves downstream, the height of the bifurcation increases, and conversely, as the shock moves upstream the bifurcation height decreases. This trend is present for both control cases.

Figure 16 shows a more detailed look at the wake region of the microramp array. It appears that the primary vortex pair is present immediately downstream of the ramps as coherent vortices. These coherent vortices are slightly elevated from the wind tunnel wall and become less visible within approximately

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20 mm (4δ) of the microramp trailing edge. The loss in visibility is likely due to the creation of the complex vortex structure containing streamwise vortices surrounded with a train of vortex rings, or large- scale hairpins [6, 65]. From the schlieren images, it appears that the instabilities in the wake region create what others have described as Kelvin-Helmhotz (K-H) vortices within 4δ of the microramp trailing edge (Fig. 16) [56]. Comparing the schlieren images between the control cases in the post-shock region (Fig. 14) reveals an increase in periodic large-scale turbulent structures with microramp control. This may give evidence that the K-H vortices persist through the SWBLI. However, this is not clear because the schlieren images integrate the boundary layer structures through the entire test-section span.

Figure 15: (a) Mean shock wave structure and (b) distribution of bifurcation points.

Figure 16: Instantaneous schlieren photography of (a) microramp array and (b) microramp wake region.

(a) (b)

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