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 ZC40: Turbulent Mixing |
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Chair: Luminita Danaila, Université de Rouen Room: 355 F |
Tuesday, November 26, 2024 12:50PM - 1:03PM |
ZC40.00001: Application of a Variable-Density Turbulent Mixing Model to a High-Resolution Richtmyer-Meshkov/Reshock Simulation Jon R Baltzer, Daniel Livescu, Forrest W Doss Recently, it was shown that constructing a model for turbulent mixing of a fluid interface leads to a telegraph equation indicating diffusion- and wave-like behaviors of the expanding interface (Doss, Physics Letters A, 430, 2022). Applied to Richtmyer-Meshkov instability in which turbulence decays after a shock passes the interface, this model predicts the dependence of fine-scale mix on initial parameters such as the perturbation spectrum. The model also elucidates the relationship between instability growth rate and fluid mixedness. High-resolution simulations of a shock wave passing a perturbed two-fluid interface (Wong, Baltzer, Livescu & Lele, Physical Review Fluids, 7, 2022) provide detailed statistical profiles and evolutions to compare the model's predictions. These simulations include both an initial shock and reshocking of the interface; conclusions in relation to the model are presented in both regimes. |
Tuesday, November 26, 2024 1:03PM - 1:16PM |
ZC40.00002: Evaluation of joint probability density function descriptions for three-layer Rayleigh-Taylor mixing Kevin Ferguson, Brandon E Morgan Simulations of a three-layer Rayleigh-Taylor mixing problem are presented. The simulations are conducted in a heavy-light-heavy configuration, where one of the two interfaces is unstable, and a heavy-intermediate-light configuration, where both interfaces are unstable. Each of the two configurations are further considered in a high- and low-Reynolds number regime. The joint probability density function (PDF) of species concentration from the simulation data is then compared against model PDFs that have been proposed to describe the species concentration distribution in three-component mixing. PDFs considered include the Dirichlet distribution, which is a multivariate generalization of the beta distribution that has been shown to accurately describe two-component mixing, as well as more complicated PDFs with increased generality. Qualitative and quantitative comparisons of the model PDFs and simulation data as a function of time and problem configuration are presented. Notably, the Dirichlet distribution does not appear to accurately describe the data in general, and more complicated PDFs appear to be necessary. |
Tuesday, November 26, 2024 1:16PM - 1:29PM |
ZC40.00003: Mixing dynamics of a sharp density interface in mean shear free homogeneous isotropic turbulence Arefe Ghazi Nezami, Blair Anne Johnson The interaction of a sharp density interface with homogeneous isotropic turbulence with negligible mean shear is a fundamental problem with broad environmental, geophysical, and industrial relevance. Despite extensive research in this field, critical aspects of this problem have yet to be uncovered. We present laboratory experiments of a two-layer system conducted in a water tank in which a random jet array generates homogeneous isotropic turbulence with negligible mean flow. The turbulent layer, composed of an alcohol-water mixture, overlies a quiescent dense layer of a sugar-water mixture. Experiments are performed under varying density differences and turbulence levels to comprehensively characterize the dynamics. Simultaneous measurements of velocity and density fields, using particle image velocimetry (PIV) and laser-induced fluorescence (LIF), respectively, allow us to track the evolution of the turbulent flow and interface over time. We then quantify mixing efficiencies and their dependence on non-dimensional parameters such as the Richardson and buoyancy Reynolds numbers. Further, we identify distinct mixing scenarios and their relationship to these governing parameters. These findings contribute to a deeper understanding of density interface dynamics in turbulent environments. |
Tuesday, November 26, 2024 1:29PM - 1:42PM |
ZC40.00004: Stochastic modeling of multi-stream mixing based on one-dimensional turbulence Marten Klein, Christian Zenker, Tommy Starick, Heiko Schmidt Measurements of multiple scalar mixing in a turbulent jet show a strong location dependence of the scalar fluctuations and mixing processes. Mixing is quantitatively described by the state space of scalar fluctuations in terms of a joint probability density function (JPDF). The JPDF evolves in the downstream and radial directions and has non-Gaussian shape which is a burden for mixing modeling since factoring into marginal distribution functions is not permissible. Stochastic simulations based on one-dimensional turbulence (ODT) are able to reasonably reproduce the JPDF and its spatial evolution by a parabolic marching problem that circumvents constraints of the underlying elliptic problem. The model reproduces the inertial-advective range (exponent -5/3) and predicts the emergence of the viscous-advective range (exponent -1) at higher wavenumbers as the Schmidt number increases. The model offers full-scale resolution at affordable cost providing means to reasonably capture state-space statistics of turbulent mixing. |
Tuesday, November 26, 2024 1:42PM - 1:55PM |
ZC40.00005: How much Navier-Stokes dynamics is needed to represent turbulent mixing? Shilpa Sajeev, Diego A. Donzis Turbulent mixing plays a key role in many natural and engineering systems and is typically studied as passive scalars advected by a turbulent velocity field. Phenomenology posits, and previous research shows, some degree of universality in mixing regardless of the details of flow geometry or stirring mechanism. However, some quantities characterizing scalar mixing are known to depend on the details of the advecting velocity. Naturally, a question arises on the sensitivity of mixing characteristics to the velocity field. |
Tuesday, November 26, 2024 1:55PM - 2:08PM |
ZC40.00006: The effect of mean flow pulsation on grid turbulence and passive scalar diffusion Stavros Tavoularis, Dana Duong The characteristics of pulsatile, grid-generated turbulence and turbulent diffusion of a passive scalar injected from a line source downstream of a grid were investigated in a specially designed non-stationary flow wind tunnel. Flow pulsation was generated by oscillating a hinged flap downstream of the blower, which diverted part of the flow through a bypass section. Velocity measurements were taken with hot wire probes, calibrated against a laser Doppler velocimeter, in grid turbulence behind two grids with mesh sizes 51 and 19 mm in both stationary flow with a mean speed of 2.9 m/s and in roughly sinusoidally pulsating flow with a time-averaged velocity of 2.9 m/s, a phase-averaged amplitude of about 0.6 m/s and a frequency of 0.33 Hz. Reported measurements include values of the Reynolds stresses and turbulent kinetic energy, integral length scale, Taylor and Kolmogorov microscales, energy dissipation rate, dissipation parameter and velocity derivative skewness and flatness factors. It was found that pulsation increased the ratio of the time-averaged turbulent kinetic energy and its value in stationary flow and that this ratio increased with downstream distance. Flow pulsation was also found to increase the spread of a passive scalar plume within the grid turbulence test section core, while diminishing scalar diffusion in the boundary layer. |
Tuesday, November 26, 2024 2:08PM - 2:21PM |
ZC40.00007: Entrainment across a density-stratified interface via isotropic turbulence mixing Manikandan Balasubramaniyan, Joseph C Klewicki, Andrew Western, Jimmy Philip The present laboratory experiments are motivated to further our understanding of how turbulence generated by wind on the surface of a river or an estuary leads to entrainment and mixing of a stably stratified water column. The experiments are conducted in a transparent double glass tank, with turbulence produced using an oscillating grid arrangement. The inner and outer tanks have dimensions of 24.5 cm x 24.5 cm x 35 cm and 35 cm x 35 cm x 50 cm, respectively. The main purpose of using an inner tank is to minimize the effect of secondary flows in achieving isotropic turbulence. By varying the grid Reynolds number (Reg = fgS2/υ, where fg is the gird frequency, S is the stroke length, and υ is the kinematic viscosity of water) from 1510 to 3025, we achieve a Taylor Reynolds number of 120 to 150 and a Froude number of 0.09 to 0.14. A stable step density of 1% is established in the inner tank using a double-bucket method, with fresh water above saline water. The instantaneous velocity field is measured using time-resolved Particle Image Velocimetry (PIV), and the results indicate the presence of reasonably isotropic conditions in the constant density fluid region, whereas turbulence becomes anisotropic near the interface. We also find that the decay of vertical velocity follows the expected linear rate of z-1 in the constant-density fluid, but there is a greater decay when entering the denser fluid. |
Tuesday, November 26, 2024 2:21PM - 2:34PM |
ZC40.00008: A Lagrangian view of mixing in stratified shear flows Xingyu Zhou, John R Taylor, Colm-cille P Caulfield We consider numerically the effect of turbulent mixing in stably stratified parallel shear flow where the initial velocity and density have a hyperbolic tangent profile in the vertical coordinate with the same inflection point. The flow can be susceptible to two types of instabilities: Kelvin-Helmholtz instability (KHI) and Holmboe wave instability (HWI). These instabilities lead to two distinct types of mixing: mixing by ‘overturning’ and mixing by ‘scouring’. We examine mixing from a Lagrangian perspective using direct numerical simulations (DNS). Lagrangian particles are tracked in the simulations, and the fluid buoyancy sampled along particle paths provides a Lagrangian measure of mixing. The particles exhibit aggregation in buoyancy space when there is sustained overturning motion within the interface. The root mean square buoyancy for a set of particles that start with the same buoyancy is larger for HWI than KHI for the same bulk Richardson number. Finally, the number of particles starting close to the mid-plane of the interface which experience a change in sign in the local fluid buoyancy and end on the opposite side of the mid-plane is compared for KHI and HWI for several values of the bulk Richardson number. Surprisingly, for HWI with a large bulk Richardson number, more than half of the particles that start near the mid-plane end on the opposite side of the mid-plane. |
Tuesday, November 26, 2024 2:34PM - 2:47PM |
ZC40.00009: Transient shear wave propagation in a solid-liquid coupled system Aaron D'Cruz, Pierre Ricco We present a novel closed-form analytical solution for the response of a forced solid-fluid system. It consists of a wide finite elastic solid located underneath a Newtonian fluid. The bottom surface of the solid is forced horizontally for a finite time period. The system is governed by coupled partial differential equations, describing the shear displacement in the elastic solid and the shear velocity in the fluid. The boundary conditions represent the continuity of shear velocity and shear stress at the solid-fluid boundary, the lower-boundary shear forcing by continuous imposed displacement or shear rate, and the decay of the shear velocity of the fluid at large distance from the solid-liquid interface. Initial conditions of zero displacement and velocity are used. |
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