Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session G32: Flow Instability: Kelvin-Helmholtz |
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Chair: Adrian Fraser, University of California, Santa Cruz; John McHugh, University of New Hampshire Room: 240 |
Sunday, November 20, 2022 3:00PM - 3:13PM |
G32.00001: Nonmodal growth in magnetohydrodynamic shear flows with stabilizing magnetic fields Adrian E Fraser, Jeff S Oishi, Alexis K Kaminski Shear flows are common and important in astrophysical and fusion plasmas, where they often drive fluctuations and turbulence. In turn, these motions enhance momentum, energy, and particle transport, crucially affecting the evolution of the system. The parameter boundaries delineating shear-driven fluctuations are often assumed to be adequately provided by normal-mode linear stability analyses. However, such analyses are known to be misleading in many canonical fluid systems, including pipe flow and stratified shear flows. Perturbations can undergo significant nonmodal or non-normal growth, in some cases driving turbulence and mixing even at parameters where linear stability analyses predict no growth. Here, we explore the degree of nonmodal growth when equilibrium magnetic fields aligned with the flow nearly or entirely stabilize shear flows, and show epsilon-pseudospectra that indicate significant amplification in linearly stable regimes for 2D systems. We also present the linear optimal perturbations that maximize fluctuation growth over finite times, and discuss how the growth and optimal perturbation structure varies with Reynolds number and magnetic field strength. |
Sunday, November 20, 2022 3:13PM - 3:26PM |
G32.00002: Dynamics of Variable Density Jets in Crossflow Elijah W Harris, Davi B Souza, Andres Vargas, Stephen Schein, Leonardo Alves, Ann R Karagozian The equidensity jet in crossflow (JICF) is known to demonstrate absolute/global instability characteristics in the upstream shear layer (USL) at relatively low jet-to-crossflow momentum flux ratios, approximately J < 10, and to produce relatively weak, convective instability at larger J values [Megerian, et al., JFM 2007]. Reducing the jet-to-crossflow density ratio S to values well below unity (S < 0.45) has been shown to generally produce global instability, as would be expected when compared with the classic free jet [Getsinger, et al, Exp Fluids 2012]. Yet in gas-phase experiments there can be anomalous trends as one gradually reduces jet density while fixing J and jet Reynolds number [Shoji, et al., JFM 2020]. The present experiments examine the variable density transverse jet in detail, focusing in addition on higher density JICF conditions (1.0 < S < 2.0), which have useful practical applications. Here jet dynamics and USL transition are studied via simultaneous acetone PLIF and PIV as well as hotwire anemometry. Experimental observations of shear layer stability transition are compared with those predicted in linear stability analysis, making use of the counter-current shear layer analogy with curvature and viscous effects [Souza, et al, PR Fluids 2021] but now with variable density conditions. |
Sunday, November 20, 2022 3:26PM - 3:39PM |
G32.00003: Effects of cross-axial perturbations on 3D Kelvin Helmholtz instability growth Mae Sementilli, James Chen Kelvin Helmholtz instability is a phenomenon seen in nature, most notably in ocean waves and cloud formations. The simulation of Kelvin Helmholtz instability has been studied extensively in the 2D plane as well as in axisymmetric flows, analyzing the effects of various fluid conditions and initial conditions. However, as the study of Kelvin Helmholtz instability is expanded to 3D, there are additional variations in the cross-wise direction that affect the instability growth rate. The introduction of cross-axial perturbations in the initial interface initiates additional dynamics in the third direction, which results in complex 3D vorticity profiles and alters the instability amplification at the interface. The purpose of this research is to use direct numerical simulation to reveal the 3-dimensional effects of various initial conditions in Kelvin Helmholtz flow. The main focus of this study is to compare these 2D and 3D simulations to establish how significant the changes in instability growth are with the addition of multidimensional perturbations. |
Sunday, November 20, 2022 3:39PM - 3:52PM |
G32.00004: Role of unsteadiness in shear instability: exhalation flow analog Naijian Shen, Lydia Bourouiba Shear-instabilities at the interface of two fluids, such as the classical Kelvin-Helmholtz (KH) instability, can lead to fluid fragmentation critical in a wide range of physical, environmental, and biological applications. While many insights into such instabilities are derived using the assumption of steady background forcing flow, unsteady impulse flows are ubiquitous in environmental and physiological processes, such as exhalations. Yet, little is understood on the role of unsteadiness in shaping the outcome of the interface's topological change and its destabilization. In this combined theoretical and numerical study, we study the fundamentals of a liquid-air interface exposed to unsteady shear flows that mimic exhalation impulses. Evolution of the perturbed interface is formulated as an impulse-driven initial value problem using both linearized potential flow equations and nonlinear boundary integral methods. We show that the unsteadiness of the forcing can lead to surprising outcomes with the amplitude of the interface's inherent gravity-capillary wave being amplified, up to wave-breaking transition by the imparted unsteady flow, even in the equivalent regime that would be stable under classical linear KH theory. We discuss how it is, in fact, the cumulative history of the unsteady forcing that is key to this amplification and transition. The insights gained are discussed in the context of interface distortion and destabilization relevant for mucosalivary fluid fragmentation. |
Sunday, November 20, 2022 3:52PM - 4:05PM Author not Attending |
G32.00005: The effect of initial perturbations on the merging and mixing of Holmboe instabilities in stratified shear flows Adam Jiankang Yang, Edmund Tedford, Jason Olsthoorn, Gregory A Lawrence Initial perturbations are commonly used in direct numerical simulations to stimulate shear instabilities of stratified fluids. Without these perturbations, the velocity and density fields diffuse molecularly and no instabilities yield. We investigated the sensitivity of wave merging and mixing to initial perturbations in Holmboe instabilities through two- and three-dimensional simulations. Two-dimensional simulations are used to test the sensitivity of wave merging to the amplitude and phase difference between the primary Holmboe and subharmonic components of the perturbation. Three-dimensional simulations are used to investigate the effect of wave merging on mixing. The amplitude has a more significant effect on the merging of Holmboe instabilities compared to the initial phase difference. For a given amplitude of the primary component perturbation, a larger amplitude of subharmonic component perturbation results in an earlier merging event. In three-dimensional simulations, the subharmonic mode is incited if the subharmonic component perturbation is imposed, increasing the amplitude of the Holmboe instability and its associated cumulative mixing. |
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