Flowfield of a Three-Dimensional Swept-Shock Boundary Layer Interaction at Mach 2

An experimental study is conducted on the interaction of a swept-shock wave with a turbulent boundary layer. The shock wave is generated by a sharp un-swept fin in a Mach 2 flow, where the strength of the interaction is varied from weak to moderate by changing the fin angle of attack fro...

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Bibliographic Details
Other Authors: Arora, Nishul (author)
Format: Others
Published: Florida State University
Online Access:http://purl.flvc.org/fsu/fd/2018_Fall_Arora_fsu_0071E_14877
Summary:An experimental study is conducted on the interaction of a swept-shock wave with a turbulent boundary layer. The shock wave is generated by a sharp un-swept fin in a Mach 2 flow, where the strength of the interaction is varied from weak to moderate by changing the fin angle of attack from 10° to 15°, which corresponds to a normal Mach number of 1.3 and 1.4, respectively. Surface oil-flow visualization is used to study the mean characteristics of the interaction where surface features such as the upstream influence and separation line are identified. By taking advantage of the quasi-conical symmetry of the flowfield, two- and three-component velocity field measurements are acquired in the conical reference frame for the interaction of moderate interaction strength (Mn ~ 1.4) at two locations from the fin apex. Flowfield features such as the λ-shock structure, slip line, and the separation bubble with the reverse flow are clearly visible in the in-plane velocity fields. These results are also examined in the spherical coordinate frame, and good agreement in the spatial location of the critical features is found, providing direct quantitative, experimental evidence of quasi-conical symmetry of this flowfield above the surface. An examination of the velocity field downstream of the rear-foot of the λ-shock shows a region - a 'streamtube,' bounded on one side by the slip line emanating from the triple point - where the flow accelerates to transonic and supersonic speeds. This flow eventually turns towards and impinges upon the flat plate, a phenomenon referred to as an 'impinging jet' in literature and is believed to be the principal cause of the high mean, and unsteady pressures, very high heating and skin friction coefficients near impingement. The out-of-plane velocity fields unveiled the presence of a significant radially outward velocity component distinctly showing the presence of an 'open' separation bubble with a flattened conical vortex, a typical characteristic of a 3-D SBLI flowfield. The interaction dynamics are explored through unsteady surface pressure measurements at strategic locations. The highest unsteadiness is observed near the intermittent separation and underneath the open separation bubble. Further insights into the interaction dynamics is sought by examining the contributions to unsteady pressures in three spectral regimes - low-frequency (Stδ < 0.01), mid-frequency (0.01 < Stδ < 0.2) and high-frequency (Stδ > 0.2) to separate the contributions of each band to the total pressure fluctuations. Mid-frequency fluctuations dominate the current 3-D interaction flowfield, in contrast to 2-D SBLI where low-frequency disturbances are shown to be prominent. The spectral behavior shows no discrete peaks. However, relatively high coherence is observed between the intermittent separation region and underneath the separation bubble at Stδ ~ 0.013. It is plausible that the separation and rear shock are undergoing a low-frequency correlated motion, but the energy in this low-frequency periodic motion is found to be much lower than the mid-frequency unsteadiness that dominates this flowfield. Finally, the interaction is visualized using high-frame-rate conical shadowgraphy (24000 fps), where a shock detection scheme is utilized to identify the primary shock features from the instantaneous conical shadowgraphy images. The PDFs of their mean-subtracted spatial locations revealed that the separation shock undergoes the highest range of motion compared to other two shock features. It is inferred that the smaller extent of the λ-shock is more probable, which is further confirmed by the conditional sampling of the separation and rear shock slope based on the upstream or downstream movement of the separation shock. The response of this interaction to unsteady REM perturbations is also studied using three-component velocity and z-vorticity fields. Various REM configurations are tested (A1, A2, and A123) and it is distinctly seen that the A123 actuation most affected the flowfield. From the out-of-plane velocity fields, a significant impact of the REM induced CVPs on the flowfield inside the separation bubble is observed, whereas the inviscid region appears to remain unaltered. A general trend of increase in near-wall vorticity when compared to the baseline case, upstream of the intermittent separation is seen from the z-vorticity fields. Finally, the path of these induced CVPs in this highly 3-D flowfield became evident when these vorticity fields are visualized in conjunction with the surface flow maps from our earlier work. === A Dissertation submitted to the Department of Mechanical Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy. === Fall Semester 2018. === November 2, 2018. === 3-D SBLI, SBLI, Separated Flows, Sharp fin-generated Interactions, Shock Boundary Layer Interactions, Shock Wave Boundary Layer Interactions === Includes bibliographical references. === Farrukh S. Alvi, Professor Directing Dissertation; Okenwa Okoli, University Representative; Rajan Kumar, Committee Member; Emmanuel G. Collins, Committee Member.