Boundary Layer Turbulence Control via Acoustically Resonating Porous Surfaces

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Porous surfaces are ubiquitous in aeronautical applications, some of which include: suppression of noise in gas turbines; control of flow instabilities in combustion chambers; enhancement of aerodynamic performance of wings under high-lift configurations, e.g. during landing; turbine blade temperature control via coolant air bleeding through the pores; reduction of aerodynamic heating on the surface of hypersonic (Mach7) vehicles. At the core of all of the aforementioned applications there is the interaction between: (a) the flow evolving over the porous surfaces and (b) the flow trapped within the pore-space of the surfaces themselves. State-of-the-art modeling techniques are unable to simulate the physics under these combined effects. As a result, technological development in flow control over porous surfaces has been limited by the lack of progress in the modeling, and hence understanding, of the fundamental interaction between the features of the porous surfaces and overlying flow dynamics. The goal of the proposed study is to carry out highly controlled experiments and simulations in a canonical flow configuration that will allow direct comparison between the two approaches, assisting in overcoming the aforementioned limitations. The proposed research will have broader impacts on society through direct effects on engineering practice. Development of resonating surfaces will have important technological impacts in aeronautical applications, including delaying separation in high-speed boundary layers and control of trailing edge noise in commercial aircrafts. The majority of fluid machinery applications will also be impacted by the success of this effort, for it provides simple means to enhance turbulent heat-and-mass-transfer and mixing. In addition, this project will work with successful diversity and outreach programs at Purdue, including recruiting initiatives and retention of underrepresented groups with which the PIs collaborate. Porous surfaces interacting with a compressible flow are classically modeled via impedance boundary conditions. The latter are formulated directly in frequency domain, relating the Fourier transforms of the pressure and wall-normal transpiration velocity at the surface. Recent computational advances have allowed for the first time to impose impedance in the time-domain in a fully compressible Navier-Stokes solver by retaining full numerical and physical realizability (respecting causality of wave-propagation), unlocking the key computational strategy needed to unravel the fundamental physics of the coupling between a compressible flow and porous surfaces. This enabling capability provides an original and elegant way to computationally model the effect of porous surfaces on the overlying flow structure. This has allowed the first high-fidelity numerical simulation of a compressible turbulent flow over wall-impedance to be carried out, revealing the fundamental structure a new self-sustaining state of near-wall turbulence altered by acoustic resonance. The proposed work will investigate sound-turbulence interactions by controlling the acoustic impedance at the wall in the canonical setting of turbulent channel flow turbulence at flow conditions reproducible both experimentally and numerically. The proposed effort combines highly-parallel large-eddy simulations with state-of-the-art experiments, which include time-resolved tomographic and highly resolved planar particle-image velocimetry (PIV), with direct estimates of turbulent characteristics and measurement uncertainty. An experimental prototype of a tunable resonating surface will be developed and will be employed to physically demonstrate the feasibility of this process. A new series of numerical investigations will proceed in parallel, supporting analysis and modeling of wave-turbulence interactions and the resulting flow instabilities. Advanced subgrid-scale modeling techniques will be used to investigate the compressible regime where wave modes exhibit stronger coupling with the flow hydrodynamics.