An Analytical Study Of Separated Flows Induced By Shock Wave Boundary Layer Interaction PDF Download

Shockwave boundary layer interactions (SBLI) occur on both internal and external surfaces and adversely affect both the structural and propulsive performance of high-speed flight vehicles operating in the trans/super/hypersonic flow regimes. In the absence of a comprehensive understanding of the flow physics associated with SBLI, the most common approach to mitigating the negative ramifications is structural over-design, often resulting in reduced aero-propulsion efficiencies and excessive cost. SBLI have been the subject of numerous experimental and numerical investigations focusing on simplified two-dimensional (2-D) canonical configurations derived from relatively complicated aircraft/turbomachinery components. A few recent studies have focused on addressing the knowledge gaps by examining component geometries that produce three-dimensional (3-D) SBLI and therefore a closer representation of real-world configurations. The current experimental investigation explores the viscous/inviscid interaction of an incoming supersonic turbulent boundary layer and a single, sharp unswept fin generated shockwave. This kind of SBLI is of keen interest to the high-speed aerodynamics community as the separated flow induces a strong crossflow component, giving rise to a highly 3-D flowfield. Although previous studies on 3-D SBLI have provided a substantial knowledge base, there are still a number of consequential questions pertaining to the flowfield topology and dynamical behavior that remain unanswered. First, what is the effect of Reynolds number on SBLI flow features, in particular, the length scales associated with the shock-induced separation region and its interaction with the shock generator (sharp-fin)? Second, what is the extent of facility dependence on the 3-D SBLI? Which, if any, component(s) of the unsteadiness is inherent to the interaction and which are facility dependent and therefore limit or bias the flowfield? Are the geometric and boundary layer constraints imposed by the size of the facility necessary for numerical simulations to ensure the proper development of scaling parameters as experiments shift from the laboratory scale to flight testing. Finally, how do the spatio-temporal scales associated with SBLI vary with the interaction strength? The main objective of the present experimental study is to answer the posed questions by conducting a detailed flowfield analysis of the sharp fin induced SBLI over a range of Reynolds numbers and interaction strengths. The research methodology involves high-fidelity experiments at the state-of-the-art wind tunnel facilities housed at the Florida Center for Advanced Aero-Propulsion at Florida State University and the data available from previously published literature. Cutting-edge global flowfield diagnostics allow for the full-field reconstruction of both skin friction (mean) and pressure (time-averaged/unsteady) underneath the single fin SBLI as the incoming Mach number (M[infinity] = 2 - 4), fin angle of attack ([alpha]F = 10° - 20°), and unit Reynolds number (Re/m 17 x 106 - 108 x 106) are parametrically varied. Reynolds number sweeps, spanning nearly an order of magnitude, illustrate that the interaction footprint is distinctly affected by the Reynolds number, with the effects being most prominent near the fin/surface junction and the outer edges of the interaction near the freestream boundary. The results indicate that the interaction flowfield becomes less receptive to Reynolds number variations as the Reynolds number continues to increase. This shrinking dependence indicates that there may be a point beyond which any further increases to the Reynolds number produce negligible differences in the flowfield id est Reynolds number independence. Identical surface oil flow and pressure measurements carried out in facilities of different scale/size compare favorably throughout the interaction region with Reynolds number based scaling. However, different incoming boundary layer thicknesses impose limitations on the extent of the inception region and the onset of finite fin effects. When investigating the mean skin friction between different scale facilities, the Reynolds number scaling could not be assessed due to limitations of the available data sets. An angular scaling was applied to enable proper inter-facility comparison between the conical regions of both identically matching and nominally equivalent interaction strength test cases. The results showed trends similar to those seen in the pressure measurements, with skin friction matching well between the facilities across the interaction with minor divergences in the near fin region, where viscous effects become more prominent. Simultaneously sampled high-speed pressure transducers and fast response PSP measurements allowed for a full-field investigation of the flow dynamics. The RMS pressure field highlights regions of increased unsteadiness along the interaction boundary, inviscid shock line and at/upstream of the fin tip vertex. Increased coherence levels indicate a communication mechanism is present between the inviscid shock and the interaction boundary. When compared with studies conducted in a smaller facility, findings of the current work are consistent in both the locations of increased unsteadiness and their respective magnitudes. In addition to illustrating the robustness of these dynamical features between differing size facilities, the current work identifies the presence of elevated levels of low-frequency content. The presence of this low-frequency content has been observed in investigations associated with 2-D SBLI, but has been absent in the 3-D SBLI studies conducted in smaller facilities. The present study has contributed significantly to a better understanding of swept 3-D SBLI, in particular, the role of Reynolds number and the size of facility on the interaction characteristics. The flowfield analysis has discovered the underlying physics associated with the fin induced SBLI. The high-fidelity experimental database generated will be very useful for the validation of numerical tools and the development of flight vehicle design guidelines.