PLANIFICACIÓN DE MI PROPUESTA

Instantaneous schlieren images of the normal shock position with no control, microramps, ramped vanes, porous plates, and the open cavity were obtained as well as the associated stagnation and static pressures for each image. The mean shock position and standard deviation at each streamwise location were extracted from the schlieren images using the algorithm outlined in Section 3.2.2 over a range of stagnation pressures (134.4 to 148.2 kPa) for each flow control device and also for the case without control. Figure 49 shows an example instantaneous schlieren photograph for each control case at a stagnation pressure that places the normal shock at the entrance of the diffuser except for the open cavity control case (Fig. 49h). In Fig. 49, the abscissa displays the streamwise coordinate subtracted by the diffuser entrance location and normalized by the incoming boundary layer thickness.

Figure 50 details the shock position and associated stagnation pressure for every schlieren image obtained. The clusters of data points are centered on target stagnation pressures with the average stagnation pressure and associated mean shock position represented as a white data point in the center of each cluster. From Fig. 50 we see that, as the mean stagnation pressure increases, the normal shock moves further downstream, as expected. Figure 50 also shows that the open cavity streamwise position data do not cover a large range of shock positions because when the normal shock moves past the first third of the open cavity it becomes an oblique shock spanning the cavity. The normal shock is able to sit on the shear layer generated by the cavity for the first third before becoming an oblique shock.

The schlieren photographs in Fig. 49 show the transonic nature of the flowfield with the presence of a shock train. The shock train exists regardless of control device; however it is not in view of the ramped vane schlieren photographs (Figs. 49c and 49d). The no-control case produces a lambda shock near the diffuser entrance without clear indication of a separated flow. In the shock region the flow is likely incipiently separated, i.e., no separation occurs in the mean but the probability of observing instantaneously reversed flow is significant [34]. The surface oil-flow visualization will reveal that the flow is separated downstream of the normal shock in the diffuser. Though the separation is difficult to observe with schlieren photography due to spanwise integration through the tunnel, the boundary layer does clearly grow downstream of the diffuser entrance, or shoulder.

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Figure 49: Instantaneous schlieren images of control devices: (a) no control, (b) microramps, (c) ramped vanes, h = 0.4δ, (d) ramped vanes, h = 0.6δ, (e) 7.5δ porous plate, (f) 10δ upstream porous plate, (g) 13.5δ porous plate, and (h) open cavity.

(a) (b)

(c) (d)

(e) (f)

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Figure 50: Stagnation pressure vs. normal shock position (a) no control, (b) microramp array, (c) ramped vanes, h = 0.4δ, (d) ramped vanes, h = 0.6δ, (e) 7.5δ porous plate, (f) 10δ upstream porous plate, (g) 13.5δ porous plate, and (h) open cavity.

(a) (b)

(c) (d)

(e) (f)

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Large periodic turbulent structures are generated in the boundary layer with microramp and ramped vane control (Figs. 49b, 49c, and 49d). The ramped vanes appear to generate larger structures compared with the microramps. The largest growth in boundary layer thickness is produced by the h = 0.6δ ramped vanes. These observations suggest that the ramped vanes produce stronger streamwise vortices when compared to the microramp control devices, with the larger ramped vanes injecting the largest amount of vorticity into the boundary layer. Unfortunately, the oblique shock wave reflections generated by the microramp and h = 0.6δ ramped vane control devices intersect the normal shock in the vicinity of the SWBLI when the normal shock is positioned near the diffuser entrance. This unavoidable result is due to the dimensions of the tunnel and recommended positioning of the MVG devices. However, this likely had little effect on the shock stability results which is evident when comparing the observed trends between the two ramped vane cases, since their generated shocks intersect the normal shock at different locations.

The porous plate flow control devices result in an increased lambda structure size (Figs. 49e, 49f, and 49g), with the larger the streamwise extent of the porous cavity, the larger the subsequent lambda structure. When the normal shock is positioned near the diffuser shoulder, the 7.5δ and 13.5δ length porous-plate cavity devices appear to create two lambda structures. One is generated at the leading edge of the cavity, and one is positioned inside the large lambda with a similar size to that observed without flow control. The 10δ length porous-plate cavity device located upstream of the diffuser entrance (Fig. 49f) generates the leading lambda shock from the downstream edge of the cavity. Based on visual inspection, the boundary layers created with the porous plate control devices are very similar to the no control case in terms of thickness and turbulent structure size.

As is expected, the open cavity control device generates a shear layer across its streamwise length. For the stagnation pressures for which a shock is able to sit on the cavity shear layer, the lambda shock is surprisingly quite stable. Conversely, the flow downstream of the cavity is highly disturbed by its presence.

Figure 51 displays the stagnation pressure vs. mean shock position, as well as the standard deviation in shock position vs. mean shock position for each of the cases shown in Fig. 50. Figure 51 shows that the mean shock position associated with each stagnation pressure varies depending on the passive control device. In order to concentrate on shock stability around the diffuser shoulder, each control device displayed in Fig. 50 has varying target stagnation pressures and not every target stagnation pressure was run for each control device. Figure 50a displays the shock positions for the no-control case. The ultimate goal for the passive control devices is to decrease the amount that the shock position varies in the streamwise direction for a given target stagnation pressure, thereby increasing the shock position stability. In other words, the goal of the control device is to decrease the width of the data clusters in Fig. 50 for a

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target stagnation pressure. Figures 50b, 50c, 50d and 51b show that the micro-vortex generators (the microramps and ramped vanes) increase shock position stability once the shock has moved into the diffuser, but do not display much of an effect on shock stability upstream of the diffuser entrance. It also shows that the ramped vanes improve the shock position stability in the diffuser to a greater extent than the microramp array. Figures 50e, 50f, 50g, and 51d do not exhibit much variation in shock position stability for the porous plate control devices compared to the no-control case. Figures 50h and 51d show that the shock position stability is improved with the open cavity where the normal shock occurs, but Fig. 49h shows that the flow downstream of the cavity is disrupted.

Figure 51: (a) Stagnation pressure vs. mean normal shock position for micro-vortex generators (b) standard deviation vs. mean normal shock position for micro-vortex generators, (c) stagnation pressure vs. mean normal shock position for recirculation devices, and (d) standard deviation vs. mean normal shock position for recirculation devices (control device location indicated by thick solid lines at bottom of each plot).

(a) (b)

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The larger the slope of the mean position curves in Figs. 51a and 51c the less sensitive the shock position is to stagnation pressure changes. This is due to the fact that a large stagnation pressure change then results in a smaller mean shock position change. Figures 51a and 51c show that upstream of the diffuser shoulder is where the mean shock position is most sensitive to stagnation pressure changes for all of the control cases, and that the shock position becomes less sensitive to stagnation pressure changes once the shock moves into the diffuser, as would be expected in a diverging duct. Figure 52 displays the slopes of linear least-squares curve fits to the data, both upstream and in the diffuser.

Figure 52: Slope of mean shock position vs. stagnation pressure.

Figure 52 shows that the porous plate control devices slightly decrease the mean shock position sensitivity upstream of the diffuser relative to the no-control case with the open cavity decreasing the sensitivity the most by far. In Fig. 51c it appears that the initial slope of the position curve using the porous plate control devices is steep all the way until the shock is located at the midpoint of the porous plate. However, downstream of this point the slope decreases, indicating an increased sensitivity to stagnation pressure changes. The increased sensitivity helps reveal the tendency for the shock to “snap” downstream close to the downstream edge of the cavity with small increases in stagnation pressure confirming a similar trend observed by Orphanides et al. [94]. The micro-vortex generators somewhat increase shock position sensitivity upstream of the diffuser shoulder relative to no-control with the microramps slightly decreasing sensitivity and the ramped vanes again somewhat increasing sensitivity downstream of the diffuser shoulder. The porous plates also somewhat increased mean shock position sensitivity downstream of the diffuser shoulder relative to the no-control case.

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Figures 51b and 51d show the standard deviation of the shock position at each streamwise shock location. It should be noted that the standard deviation in shock position has a relatively large uncertainty due to the variability in shock location between wind tunnel runs. Figure 51b shows that the micro-vortex generators greatly reduce the shock position fluctuations relative to no control once the shock has moved to an average streamwise position inside the diffuser. However, it also shows that the shock position fluctuations upstream of the diffuser entrance are slightly increased, with the h = 0.4δ ramped vanes increasing the fluctuations the least. Figure 51d illustrates that porous plates greatly reduce shock position fluctuations relative to the solid wall on the upstream portion of their respective cavities. Only the 10δ upstream porous plate reduces shock position fluctuations at the diffuser entrance, but only to a small degree. All of the porous plate control devices increase shock position fluctuations relative to no control once the shock moves into the diffuser. Figure 51d also shows that the open cavity greatly reduces shock position fluctuations over the limited range examined.