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Chapter 1 Introduction

1.2 Previous Work in SBLIs

1.2.3 Effects of Internal Confinement on SBLIs

As was noted previously, Babinsky et al. [46] investigated the effects of oblique imping- ing/reflected SBLIs in the presence of sidewall and corner boundary layers at Mach 2.5. They observed that the interplay between the corner and floor flows could have the effect of either reducing or enlarging the primary SBLI, reducing the effects to a function of a “inverse viscous aspect ratio,” defined as the boundary layer thickness inbound to the interaction normalized by the wind tunnel width (i.e. the width between the sidewalls), orδ0/W. Figure 1.16 depicts the

regimes of interplay between the primary and corner SBLIs based on δ0/W. In the model of

Babinskyet al.[46], the waves resulting from the corner separation were treated as quasi-conical, propagating outwards from the point of separation along the corner junction. In the just region ahead of the nearby primary SBLI (i.e. the floor flow very near the floor/sidewall junction), these corner waves smear the adverse pressure gradient and reduce the local separation length. The corner waves were postulated to continue propagating through the impinging/reflected oblique shock, with the point of intersection between the corner waves from either side influencing the

Figure 1.16: Impinging/reflected SBLI centerline separation length as a function of inverse viscous aspect ratio, from Babinsky et al. [46].

extent of the primary separation.

From Fig. 1.16 it can be observed that for very small values ofδ0/W, this point of intersection

occurs well downstream of the primary SBLI and therefore does not influence its separation extent. For middling values of δ0/W, the corner waves intersect in the downstream portion of

the primary interaction, actually increasing its separation length. For very large values ofδ0/W

(i.e. small aspect ratio tunnels with thick boundary layers), the corner intersection takes place forward of the primary interaction, potentially decreasing the adverse pressure gradient there and reducing separation length much like what was observed on the periphery of the primary SBLI for lower inverse viscous aspect ratio cases.

Benek et al. [47] conducted a companion Reynolds-averaged Navier-Stokes (RANS) study to Ref. 46, investigating the effects of wind tunnel aspect ratio W/H and the inverse viscous aspect ratio δ0/W on the mean separation characteristics of an incident shock SBLI at Mach

2.9. They observed that the corner effects could both increase and decrease the size of the primary separation along the floor depending on tunnel width, which validated the findings of

Babinskyet al.[46]. In general, the magnitude of the three-dimensionality observed throughout the interaction increased with decreasing tunnel width. An initial increase and then a decrease in separation length with increasing δ0/W was also observed, with a peak separation length

occurring atδ0/W ≈0.06 as shown in Fig. 1.17(a). Larger boundary layer thicknesses resulted

in an effective reduction in the wind tunnel width, with thicker boundary layers inducing more three-dimensionality for the same tunnel aspect ratio. While these trends are consistent with what is suggested by Fig. 1.16, it is not clear from Ref. 47 whether the interplay mechanism between the primary and corner SBLIs is actually the intersection of the corner waves at the floor centerline. The corner separation shock in the work of Beneket al.[47] appears to terminate at the primary separation shock surface (as is shown in Fig. 1.17(b)), rather than propagating through it and intersecting the companion wave originating from the opposite junction. This is consistent with what was observed in the compression ramp LES of Bisek [45]. In Ref. 47, it was further stated that the spanwise flow in the primary separation zone may be another important communication mechanism coupling the primary, corner and sidewall SBLIs.

Wang et al. [23] carried out wall-resolved LES of impinging/reflected SBLIs in rectangu-

(a) (b)

Figure 1.17: Results of RANS simulations of impinging/reflected SBLIs in rectangular channels for different inverse viscous aspect ratios, from Beneket al.[47] (a) centerline separation length as a function of inverse viscous aspect ratio (b) impinging shock structure.

lar channels with different aspect ratios at Mach 2.7. They noted that much of the three- dimensionality observed along the inlet floor at lower aspect ratios was induced by the incident shock wave being modified by its own sidewall separation prior to impinging upon the floor. Depending on the effective aspect ratio of the channel, the sidewall shocks (which propagate toward the mid-span symmetry plane) were observed to modify the impinging shock to differing extents. For very large aspect ratios, a 2-D portion of the impinging shock is preserved. As the aspect ratio decreases, the sidewall shocks affect the incident shock along more of its span, and the resulting floor interaction becomes more 3-D. Eventually, weak compression waves from either sidewall intersect near the center of the domain and the entire incident shock is weak- ened, which was observed to cause a reduction in the separation length along the floor. These trends are consistent with the those suggested by Babinsky et al. [46] with increasing δ0/W,

but the causal mechanism is slightly different. Babinskyet al.[46] do not clarify whether some pre-impingement modification of the incident shock may have taken place, but this does appear to occur to some degree in the RANS simulations of Beneket al. [47], as the incident shock in their work can be seen to be fairly curved before impinging in Fig. 1.17(b).

For wall-bounded compression ramp configurations, far less data exists to determine whether separation length obeys the trends indicated by Fig. 1.16. The aforementioned simulations of Bisek [45] had an inverse viscous aspect ratio ofδ0/W = 0.06, resulting in a centerline separation

length of Lsep/δ0 = 7.2. Poggie & Porter [48] recently conducted LES simulations on a setup

almost identical to that of Bisek [45], except for a thicker effective incoming boundary layer resulting in a higher inverse viscous aspect ratio (δ0/W = 0.12). They noted a smaller centerline

separation length than in Ref. 45, which would seem to support that the interplay between the primary and corner SBLIs behave in a similar manner between compression ramp and impinging/reflected SBLIs in rectangular channels.