Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session G43: Turbulence: BuoyancyDriven and Stratified I 
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Chair: Anthony Haas, Los Alamos National Laboratory Room: 207B 
Sunday, November 19, 2023 3:00PM  3:13PM 
G43.00001: Entrainment at multiscales: the case of RayleighTaylor turbulence Stefano Brizzolara, JeanPaul Mollicone, Maarten van Reeuwijk, Markus Holzner A partially turbulent flow continuously incorporates irrotational fluid into the turbulent region, a phenomenon known as entrainment. Although locally acting at viscous scales, turbulent entrainment is independent of viscosity from a global point of view: these two apparently contraddictory phenomena are reconciled by taking into account the fractal nature of the turbulent/nonturbulent interface (TNTI), the thin interface separating the turbulent from the irrotational region. In this contribution, we present a new equation for computing the entrainment velocity at multiscales. This is done by defining the local entrainment velocity as the propagation speed of an isosurface of filtered enstrophy relative to the coarsegrained velocity field, and using the filtered enstrophy budget to split the total velocity into its individual components, i.e. viscous, inviscid, baroclinic and subfilter. We test our equation on a dataset of RayleighTaylor turbulence, a temporal mixing layer substained by an unstable buoyancy gradient. 
Sunday, November 19, 2023 3:13PM  3:26PM 
G43.00002: Abstract Withdrawn

Sunday, November 19, 2023 3:26PM  3:39PM 
G43.00003: A Reynoldsaveraged Navier–Stokes closure model for natural convection based on Rayleigh–Prandtl scaling theory DaSol Joo, Donghyun You A new turbulence model has been developed for Reynoldsaveraged NavierStokes (RANS) simulations of buoyancydriven flows to overcome the incapability of existing models for accurately predicting RayleighBénard convection (RBC). In this study, the behavior of the commonly employed kε model for RBC is investigated at very high Rayleigh number (Ra) conditions. The present analysis reveals that the conventional kε model incorrectly predicts the turbulent heat flux as an exponentially growing solution, instead of converging to a correct finite value. To deal with this issue, a new model is proposed, aiming to accurately predict a steadystate heat transfer solution for RBC by applying the Grossmann and Lohse theory, which considers the Nusselt and Reynolds dependence on the Ra and Prandtl (Pr) numbers. The new model algebraically modifies the term in the ε equation that is related to buoyancyinduced turbulent kinetic energy production, allowing the new model to be used in conjunction with the kε model. The proposed modeling methodology for natural convection based on RaPr scaling and dynamical systems theory, is expected to be applicable to other RANS models that include the buoyancy effect on turbulent kinetic energy. 
Sunday, November 19, 2023 3:39PM  3:52PM 
G43.00004: Superultimate transient of turbulent thermal convection between horizontal porous walls Fanyu Meng, Shingo Motoki, Atsushi Shirai, Genta Kawahara Turbulent thermal convection between horizontal porous walls heated from below and cooled from above has been investigated numerically and experimentally. It has been found that at low Rayleigh number Ra, the Nusselt number Nu in the bulk region scales with Ra^{1/3} as in the classical state, whereas at high Ra, Nu scales with Ra^{1/2}, implying the ultimate state in which the vertical heat flux is independent of thermal conductivity, i.e., the socalled conduction anomaly. At low Ra, vertical (wallnormal) fluid motion is not excited in the nearwall region despite wall permeability, so that the classical state can be observed. At high Ra, largescale thermal plumes appear even near the walls from convective instabilities of nearwall thermal conduction layers to significantly intensify the wall heat flux, leading to the ultimate state. In between these two distinct scaling ranges of Ra, we have found superultimate transient in which Nu ~ Ra. This superultimate scaling is considered to be a consequence of full excitation of largescale thermal plumes comparable with those in the ultimate state at high Ra and of less energy dissipation in the flow through porous media than in the ultimate state at high Ra. 
Sunday, November 19, 2023 3:52PM  4:05PM 
G43.00005: Effect of initial conditions on transition in RayleighTaylor and RichtmyerMeshkov instabilities Vincent Laroche, Christopher Pezanosky, Daniel M israel, Filipe Pereira, Anthony P Haas RayleighTaylor (RT) instability is a type of flow consisting of a layer of heavy fluid over light fluid. Small perturbations along the interface separating the two layers cause the unstable system to begin mixing and eventually break down into turbulence. RichtmyerMeshkov (RM) instability, while otherwise similar to RT, has the additional complexity of a shockwave disrupting the interface. These two variabledensity flow instabilities are relevant to a variety of applications such as climate and combustion. Existing research suggests that the transition process in both RT and RM is strongly influenced by initial conditions. We explore this dependence using direct numerical simulation and implicit large eddy simulation in a variety of natural transition scenarios initialized with different combinations of perturbation modes and amplitudes. For analysis, we consider various quantities of interest including mixing layer height, mixture fraction, vorticity, turbulent length scale, and turbulent kinetic energy of the flow. Specifically, we use these metrics to track the amount of time required for transition to occur and to describe the state of the resulting turbulence. Lastly, we discuss the results of this sensitivity analysis on initial conditions and its importance for understanding variability in instabilitydriven turbulence. 
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