Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session J27: Flow Instability: Kelvin-Helmholtz |
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Chair: MD Erfanul Alam, North Central College Room: 251 E |
Sunday, November 24, 2024 5:50PM - 6:03PM |
J27.00001: Effect of gravity and surface tension on compressible Kelvin-Helmholtz instability Yasuhide Fukumoto, Rong Zou, Kazuo Matsuura, Nobutaka Taniguchi For an ideal incompressible fluid, an interface of tangential velocity discontinuity necessarily undergoes the Kelvin-Helmholtz instability (KHI), with growth rate proportional to the magnitude of discontinuity in tangential velocity. Compressibility acts to weaken KHI, and even suppresses KHI when the Mach number of velocity discontinuity is greater than √8 (Landau 1944). In this investigation, by adding density jump across the interface of velocity discontinuity, we explore the effect of gravity force and surface tension on the compressible KHI. In case a heavy fluid lies on a light fluid, the problem becomes interaction of KHI with the Rayleigh-Taylor instability of a compressible fluid. In case a light fluid lies on a heavy fluid, the gravity force as well as the surface tension acts as restoring forces. We are concerned with the influence of these restoring forces on the compressible KHI. The results should follow Krein's theory of Hamiltonian spectra. By extending the wave-energy formula of incompressible flows to compressible flows, we identify negative-energy modes and, from this viewpoint, clarify the roles of the gravity and the surface tension in stabilization / destabilization of KHI. |
Sunday, November 24, 2024 6:03PM - 6:16PM |
J27.00002: Dominant Spatiotemporal Shear Layer Modes for Variable Density Jets in Crossflow Andres Vargas, Derik Peroomian, Fernanda Spilotros Costa Cordeiro, Davi B Souza, Leonardo Alves, Ann Renee Karagozian This study explores gaseous jets in crossflow (JICF), focusing on the upstream shear layer (USL) instabilities and their transitions. It builds upon extensive prior experiments [Besnard, et. al., PR Fluids 2022; Harris, et. al., PR Fluids 2023] that examine various flow conditions and nozzle geometries to study regimes of and conditions producing convectively unstable (CU) and absolutely unstable (AU) shear layers. The present experiments explore flowfield dynamics via laser-based particle image velocimetry (PIV) and acetone planar laser-induced fluorescence (PLIF), where flowfield dynamics are extracted using proper orthogonal decomposition (POD), revealing dominant spatiotemporal characteristics. Structures associated with transverse jet vortex roll-up along the USL trajectory produced POD mode coefficients exhibiting sinusoidal patterns reminiscent of wave packets, with an onset that initiates earlier and can propagate upstream as crossflow velocity is increased and the USL instabilities transition from CU to AU. These reduced spatiotemporal variables may be used to model the JICF's complex dynamics as it transitions from CU to AU and further, to globally unstable (GU) regimes, for example, via reduced order modeling using the Counter-Current Shear Layer (CCSL) model [Shoji, et. al., JFM 2020; Souza, et. al., PR Fluids 2021] and/or through the Ginzburg-Landau amplitude equation. |
Sunday, November 24, 2024 6:16PM - 6:29PM |
J27.00003: Influence of porous material on the flow behind backward-facing step - experimental study Lukasz Klotz, Karol Bukowski, Konrad Gumowski We investigate effect of porous insert located upstream of the separation edge of backward-facing step (BFS) in early transitional regime as a function of Reynolds number. This is an example of hydrodynamic system that is a combination of separated shear flow with large amplification potential and porous materials known for efficient flow destabilisation. Spectral analysis reveals that dynamics of backward-facing step is dominated by spectral modes that remain globally coherent along the streamwise direction. We detect two branches of characteristic frequencies in the flow and with Hilbert transform we characterise their spatial support. For low Reynolds numbers, the dynamics of the flow is dominated by lower frequency, whereas for high enough Reynolds numbers cross-over to higher frequencies is observed. Increasing permeability of the porous insert results in decrease in Reynolds number value, at which frequency cross-over occurs. By comparing normalized frequencies on each branch with local stability analysis, we attribute Kelvin-Helmholtz and Tollmien-Schlichting instabilities to upper and lower frequency branches, respectively. Finally, our results show that porous inserts enhance Kelvin-Helmholtz instability and promote transition to oscillator-type dynamics. Specifically, we observe that amplitude of vortical (BFS) structures associated with higher frequency branch follows Landau model prediction for all investigated porous inserts. |
Sunday, November 24, 2024 6:29PM - 6:42PM |
J27.00004: Abstract Withdrawn
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Sunday, November 24, 2024 6:42PM - 6:55PM |
J27.00005: Abstract Withdrawn |
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