Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session T25: Flow Instability: Geophysical and Kelvin-Helmholtz |
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Chair: Marek Stastna, University of Waterloo Room: 150B |
Monday, November 20, 2023 4:25PM - 4:38PM |
T25.00001: Transient growth in magnetohydrodynamic shear layers Adrian E Fraser, Alexis K Kaminski, Jeff S Oishi Shear flows are ubiquitous in astrophysical and fusion plasmas and can drive turbulent fluctuations that enhance momentum, heat, and particle transport. Normal-mode linear stability analyses are widely used to identify unstable parameter regimes that might drive such turbulence. However, these analyses are known to be misleading in many canonical fluid systems, including pipe flow and stratified shear flows. Even in the absence of linear instability, small-amplitude perturbations can grow significantly due to transient, nonmodal growth mechanisms. In some cases, they can even drive turbulence and significant mixing. |
Monday, November 20, 2023 4:38PM - 4:51PM |
T25.00002: Chemo-Hydrodynamic Kelvin-Helmholtz Instability Manoranjan Mishra, Surya Narayana Maharana Hydrodynamic instabilities, such as the Saffman-Taylor or Rayleigh-Taylor Instability, have long been associated with chemical reactions [1]. Recently, we, for the first time, identified the Kelvin-Helmholtz Instability as a Chemo-Hydrodynamic Instability that can be induced by a simple (A+B→C)-type reaction [2]. This reaction alters the viscosity profile of layered fluids within a channel flow. In our study, two viscosity-matched reactants, A and B, flow axially in a layered manner, producing a more viscous product, C, in a two-dimensional channel. When we perturb this flow with a sine wave, intriguingly, a Kelvin-Helmholtz type pattern forms at one front while the other remains stable. This perturbation amplification arises from oscillatory streamlines exhibiting a phase lock system. Moreover, increasing the log-mobility ratio (R_{c}) intensifies the perturbation amplitude. Furthermore, the location of the interfacial region near the bottom wall results in streamlines developing hump-like structures and becoming out of phase, causing the regular pattern to become irregular. Additionally, at higher Reynolds numbers (Re), we observe the formation of ligament-type waves, resembling patterns seen in earlier experimental observations [3]. In the presentation, we will discuss when the dominance of convection and diffusion affects the flow's stability. |
Monday, November 20, 2023 4:51PM - 5:04PM |
T25.00003: Transitioning Instabilities for Variable Density Jets in Crossflow Andres Vargas, Davi B Souza, Derik Peroomian, Elijah W Harris, Leonardo Alves, Ann R Karagozian This experimental-theoretical study investigates variable-density gaseous jets in crossflow (JICF), focusing on the nature of transitions in the upstream shear layer (USL) and sensitivity to the constituents that make up the jet mixture. Prior studies demonstrate that jet-to-crossflow density ratio (S) and flux momentum ratio (J) influence global/absolute instability transitions in the USL. Building on these results, the present work analyses the JICF flow field using acetone PLIF and stereo PIV measurements with complementary hotwire anemometry along the USL trajectory to identify trends in the critical J values (J_{cr}) associated with USL transition. The proper orthogonal modes of the PIV flow field yield systematic mode transitions, also represented in spectral contour maps of the hotwire-based measurements. Characteristic signatures suggest that the transition can be gradual under slight changes to the crossflow, but for a fixed jet Reynolds number and prescribed S and J values, the jet’s gas constituents can affect J_{cr} and hence affect the transition in the USL instability and its dynamical character. As done in recent equidensity JICF studies, comparisons with linear stability analysis aid in interpreting these results utilizing a counter-current shear layer analogy. |
Monday, November 20, 2023 5:04PM - 5:17PM |
T25.00004: A priori Validation of a Transition Model for Variable Density Flows Anthony P Haas, Daniel M israel Numerous applications, including astrophysical and geophysical flows, as well as Inertial Confinement Fusion, require variable density models for stratified and sheared flows. Although existing models perform well in the fully turbulent regime, they are often unable to model the early stages of the disturbance growth. We assess a new transition model (Israel & Haas, 2023, APS DFD) using linear stability theory (LST) and direct numerical simulation (DNS). The model is an extension of the variable density BHR turbulence model developed by Besnard et al. Besides the conservation equations for mass and momentum, equations for the full Reynolds stress, the turbulence mass flux (a-equation) as well as the density-self-correlation (b-equation) are modified to ensure a physics-based approach to transition. LST and DNS are used to test the model for Rayleigh-Taylor (RT) and Kelvin-Helmholtz (KH) dominated flows. Here we show the results of a priori tests using both LST and DNS results, considering balance equations to get insight into the contributions of various terms (e.g. production, dissipation, transport, etc.), and also to provide useful information for the design and validation of the transition model. |
Monday, November 20, 2023 5:17PM - 5:30PM |
T25.00005: Input-output analysis of two-dimensional supersonic shear layer flow over a thick plate Mitesh Thakor, Yiyang Sun, Mark N Glauser, Datta V Gaitonde We conduct an input-output analysis of a shear layer flow to comprehend its perturbation dynamics and utilize this understanding to design active flow control strategies to reduce flow oscillations. A large eddy simulation has been performed for a flow passing along a relatively thick splitter plate separating an upper Mach 1.23 stream from a lower Mach 1.0 stream. We use the time-averaged flow to construct a linearized input-output operator. Pressure responses from distinct inputs – streamwise velocity, transverse velocity, and pressure, respectively, are characterized. The inputs are constrained to three spatial locations in the proximity of the upper, lower, and trailing surfaces of the plate. Conversely, the output pressure is not spatially constrained. In all cases, large amplifications are captured at the dominant vortex shedding frequency of St = 0.27, which is attributed to the Kelvin-Helmholtz instability in base flow. Moreover, the spatial distribution of the output pressure mode depends on the perturbed frequency, indicating the potential usefulness of control with unsteady actuation. Among all the input-output variable combinations, the largest amplification is induced by the transverse velocity input at the trailing surface, which will be validated through simulations. |
Monday, November 20, 2023 5:30PM - 5:43PM |
T25.00006: Three dimensional effects of Ekman adjustment on instabilities within seamount bottom boundary layers. Arjun Jagannathan, Kaushik Srinivasan, James C McWilliams, Jeroen Molemaker, Andrew Stewart Boundary currents flowing past seamounts precondition the downwelling bottom boundary layer (BBL) along the seamount slopes towards developing one or more type of instability associated with the potential vorticity (PV) reversing sign. We show using idealized submesoscale and BBL-resolving ROMS simulations that a useful characterization of the instabilities can be obtained by analyzing the sources of their turbulence kinetic energy (TKE) and the structure of PV within the BBL. These are in turn found to depend both on the non-dimensional seamount height Nh_{s}/af as well as its lateral aspect ratio β=b/a which measures the along-flow seamount length b to its cross-slope width a. N, h_{s} and f are respectively the constant background stratification, seamount height and Coriolis frequency. Increasing Nh_{s}/af has the effect of reducing the bottom stress through the thermal wind shear induced by sloping isopycnals within the BBL, which opposes the ageostrophic BBL shear. This dynamical mechanism, referred to as Ekman adjustment, suppresses BBL turbulence and dissipation. However the preconditioning due to Ekman adjustment simultaneously renders the BBL susceptible to centrifugal, symmetric and mixed centrifugal-symmetric-gravitational instability modes which equilibrate and dissipate over long downstream distances. The most intense turbulent dissipation in our simulations is seen in the wake of circular seamounts (β=1) , and is identified as arising primarily from a centrifugal mode of instability. It produces dissipation rates over 3.5 times higher than would be expected over a flat bottom, and persists over a downstream distance that is an order of magnitude larger than the seamount length itself. For elongated seamounts, i.e. β>>1, along-slope Ekman adjustment gives rise to a BBL configuration that develops a hybrid centrifugal-symmetric-gravitational mode which is only modestly dissipative relative to a pure centrifugal mode, with normalized, area-averaged dissipation rates around 0.5 for β=8 and β=16, compared to 3.5 for β=1. |
Monday, November 20, 2023 5:43PM - 5:56PM |
T25.00007: On the onset of trapped mountain waves in stably stratified turbulent boundary layers Lucile Pauget, Christophe Millet, Francois Lott Atmospheric boundary layers (ABLs) with weak stratification are relatively well-described by similarity theory. With common strong stratification, similarity theory becomes unreliable and interactions with the mean flow assume a variety of scenarios. In this work, we focus on trapped mountain waves developing in a turbulent ABL, using the Monin-Obukhov similarity theory to define a mixing length model. Solutions are obtained analytically in the outer region, where the inviscid approximation holds, and by solving numerically a sixth order differential equation in the inner region, close to the surface. An expression for the reflection coefficient is derived using matched asymptotic expansions to describe the vertical structure of solutions. It is shown that the response of the ABL to a downward gravity wave strongly depends on the stratification and favors the onset of trapped lee waves for small Richardson numbers. The influence of the roughness is interpreted using a pseudo critical level below the surface. A spatial linear stability analysis is used to demonstrate that coexisting preferential modes dominate downstream the mountain. The vertical structure of these modes depends in a nontrivial way on the horizontal wavenumber. Finally, the implications of these results are discussed for a family of mountain ridges. |
Monday, November 20, 2023 5:56PM - 6:09PM |
T25.00008: Shear current effects on radially propagating internal waves Marek Stastna This talk will report on simulations of radially propagating internal waves in the presence of a shear current. We will demonstrate that as the strength of the background current increases, a substantial region of anomalously high kinetic energy occurs near the plane aligned with the background current. The kinetic energy in this region can surpass that induced by the radially propagating waves. Substantial density overturns are also observed to be associated with this region. For strong enough shear, the radial symmetry of the wave front is completely broken and the anomalous kinetic energy region is observed to have a dominant component below the perturbed pycnocline. The region of anomalous kinetic energy spreads out from the plane of the background current and is, to the best of our knowledge, a completely novel phenomenon Time permitting, these results will be contrasted with classical critical layer theory. |
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