Separation Bubble Dynamics: A Comprehensive Investigation of Low- and High-Speed Flows using Experiments, Simulations and Stability Theory
Abstract
Issues of low-frequency unsteadiness and localized regions of elevated surface heat transfer are persistent challenges in the design of modern supersonic/hypersonic flight vehicles, typically requiring excessive weight to mitigate against potentially catastrophic failure, limiting vehicle performance. Similar flow features have also been observed at subsonic speeds, suggesting that the root mechanisms are active across a far-greater range of flow environments than has previously been expected. In this work, we propose a synergistic experimental-computational investigation spanning low- and high-speed flows to examine these phenomena across a range of flow states (laminar, transitional, turbulent) with conclusions being critical in improving the state-of-understanding of these flows, aiding vehicle design. A comprehensive coordinated effort between the Turbulence and Flow Control Laboratory (TFCL, PI: Little) and the Computational Fluid Dynamics Laboratory (CFDLab, PI: Fasel) at University of Arizona (UArizona) will be carried out in order to investigate separation bubble dynamics for a wide range of Mach and Reynolds numbers. This unique research project is aimed towards understanding and connecting the dominant physical mechanisms governing the separation bubble dynamics across a wide range of speed regimes (low-speed to hypersonic). Low-frequency unsteadiness in high-speed flows has been observed across a wide range of geometric configurations and is associated with the interaction between a shock wave and low-velocity boundary layer that forms on the surface. When the shock is of sufficient strength, it can promote flow departure from the surface (i.e., separation), causing a recirculating zone of fluid that moves the shock further upstream resulting in large-scale changes in its position. This motion imparts significant unsteady pressure loading to the vehicle at particularly low frequencies that can couple with the vehicle structure, resulting in panel flutter, dynamic unstart of engine intakes and hazardous fatigue loading of the underlying structure. Similar pulsation of recirculated flows has been observed at subsonic speeds, in both laminar and turbulent flows; however, the effect of compressibility on this Ôbubble-breathingÕ mode remains unclear. In addition, the high-temperature environments associated with high-supersonic and hypersonic flight provide further challenges due to localized heating of the vehicle surfaces, particularly near regions of high-energy flow-impingement, as is present in the recirculating flow regions mentioned above. Indeed, coupling between the low-frequency unsteadiness and localized heating has been noted in literature and associated with longitudinal ÔGšrtler-likeÕ vortices present within the interaction, promoting dynamic hot streaks at reattachment. Surface heating streaks have also been observed at low-speeds further indicating a coupling between unsteadiness and streaks for incompressible and compressible flows. The underlying physical mechanisms, and the potential role of secondary instabilities in the development of streamwise ÒhotÓ streaks downstream of the separation bubble will be investigated for a flat plate (low-speed) and a cylinder-flare geometry (high-speed). The experimental test campaigns will rely on various state-of-the art facilities (subsonic, supersonic, hypersonic) located at UArizona and involve characterization of 2D canonical geometries using a wide variety of diagnostic techniques (pressure transducers, infrared thermography, schlieren, particle image velocimetry, etc.). The computational component of this research effort will include Linear Stability Theory and Linearized Navier-Stokes calculations as well as highly-resolved Direct Numerical Simulations (DNS).
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- May 24, 2023
- Source ID
- W911NF2310151
Entities
People
- Jesse C. Little
Organizations
- Army Contracting Command
- Office of the Secretary of Defense
- University of Arizona