Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session R17: Free Convection and RayleighBenard I 
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Chair: Aaron Lentner, George Washington University Room: 145B 
Monday, November 20, 2023 1:50PM  2:03PM 
R17.00001: Effect of the horizontal length scale on scaling relations in quasistatic magnetoconvection Shujaut H Bader, Xiaojue Zhu The morphology of the flow in RayleighBénard convection stabilized by the effect of a strong external vertical magnetic field is characterized by the columnar structures directed along the field lines. The resulting anisotropy significantly alters the length scale of the flow; the scaling of which remains of vital importance in the scaling relations for the response parameters. Previous studies concerning heat transport scaling in such a system have considered the domain height L as the sole length scale. In the present study, under the quasistatic assumption, we consider the horizontal width of the columnar structures as a relevant length scale and explore the effect of its dependence on the Chandrasekhar number (l/L~ Q^{1/6}) on the scaling of the dimensionless heat transport (Nu), flow velocity (Re), and Ohmic dissipation (ε_{η}). Based on our theoretical analysis, incorporating the effect of the horizontal length scale, the scaling laws derived for the dimensionless heat transfer (Nu ~ Ra/Q) and flow velocity (Re ∝ RaQ^{5/6}) and the Ohmic dissipation (ε_{η}L^{4}/ν^{3} ∝ Ra^{2} Q^{1}) are successfully validated using our high fidelity 2D DNS data spanning an unprecedented range of input parameters. 
Monday, November 20, 2023 2:03PM  2:16PM Author not Attending 
R17.00002: Abstract Withdrawn

Monday, November 20, 2023 2:16PM  2:29PM 
R17.00003: Application of the generalized quasilinear approximation in RayleighBénard convection Rikhi Bose, Xiaojue Zhu In the generalized quasilinear (GQL) approximation (Marston et al., Phys. Rev. Lett., 116, 214501, 2016), a flow field is decomposed into largescale (l) and smallscale (h) components by applying a spectral cutoff filter. In this work, we investigate the applicability of the GQL approximation to the twodimensional planar RayleighBénard convection (RBC). Recently, regimes have been identified in RBC where the flow can attain different turbulent roll states based on initial roll conditions defined in numerical simulations (Wang et al., Phys. Rev. Lett., 125 (7), 074501, 2020). The GQL approximation is tested in this flow regime. A direct numerical simulation (DNS) is performed first starting from an initial temperature field composed of random perturbations superimposed on a linear conductive profile. The GQL simulations are initiated once a statistically stationary state is reached in the DNS. The GQL approximation is able to capture the convection roll states if the lsubspace contains more wavenumbers than a critical limit. Interestingly, the GQL approximation leads to different states depending on the threshold wavenumber segregating the l and h–subspaces. An increase in the threshold wavenumber yields states corresponding to higher number of convection rolls as in the DNS. Additionally, the GQL methodology is applied as an intrusive technique to study the nonlinear triadic scale interactions in the planar RBC system. It is found that the h → l → h scattering is essential for the sustenance of the convection rolls. 
Monday, November 20, 2023 2:29PM  2:42PM 
R17.00004: Significance of coherent structures in augmented heat flux in roughnessaided RayleighBénard convection Krishan Chand, Mukesh Sharma, Arnab Kr. De In the present work, we investigate the effect of conical roughness on heat flux and coherent structures for Ra=10^{8} in a 3D cubic cell filled with air. Two roughness setups are explored, and unlike the uniform case, height of the conical elements varies as 50100% of the maximum roughness height for the irregular case. In addition, we also study their halfvariants (HU and HR), where the rough surface is considered only on the bottom plate. While heat flux increases significantly in the rough cases, roughness weakly impacts the flow strength. The heat flux increases due to emission of a large number of intense thermal plumes, which is confirmed by an increase in plume volume fraction V_{pl}. Larger temperature variance in the rough cases confirms the frequent emission of thermal plumes. A shift in mean temperature in the bulk region is observed for HU and HR cases due to a surge in hot plumes emitting from the rough bottom surface. LSC remains in the diagonal plane in the smooth case, whereas it is found aligned along the Cartesian planes in the rough cases. The strength and orientation of the LSC are quantified by Reynolds number based on angular velocity. In the plane of LSC, we also obtain greater local heat flux in rough cases, which indicates augmented plume morphology. Secondorder structure function of vertical velocity and temperature reveals energetic flow structures in the rough cases, whereas power spectra of temperature fluctuations reveal the existence of BO59 scaling in the bulk region. 
Monday, November 20, 2023 2:42PM  2:55PM 
R17.00005: How wall modes affect the onset and character of rapidly rotating RayleighBénard convection Robert E Ecke, Xuan Zhang, Olga Shishkina Rotation and buoyancy drive many natural phenomena including atmospheric, oceanic, and inner Earth flows as well as convective processes in planetary atmospheres, in the dynamics of the sun, and in other astrophysical systems. An important experimental, computational, and theoretical system that incorporates many of the features of these more complex systems is rotating RayleighBénard convection. In rapidly rotating RayleighBénard convection, wall modes that precess in the rotating frame form the foundation for the onset of bulk convection especially in convection cells of small diametertoheight ratio Γ. In doing so they reveal fascinating features of both instabilities [1]. Here, we consider direct numerical simulations of a fluid with Prandtl number Pr = 0.9 for Ekman number Ek = 10^{6} over a range of Rayleigh number 3 x 10^{7} ≤ Ra ≤ 5 x 10^{9}. The convection cell geometry is cylindrical with aspect ratio Γ = 1/2. We describe detailed investigations of the complex interplay between wall modes and bulk flows including a subcritical bifurcation to lateral jet instability, contributions to heat transport from both instabilities, the disparate radial localization of temperature and velocity fields, and the evolving nonlinear character of wall modes that survive the onset and development of bulk instability leading to a robust boundary zonal flow (BZF) [2,3]. 
Monday, November 20, 2023 2:55PM  3:08PM 
R17.00006: Characterization of LSC flow regimes in turbulent liquid metal RayleighBenard convection Sven D Eckert, Thomas Wondrak, Max Sieger, Rahul Mitra, Frank Stefani, Felix Schindler, Tobias Vogt This paper reports on an experimental study focusing on the manifestation and dynamics of the largescale circulation (LSC) in turbulent liquid metal convection. The experiments are performed inside a cylinder of aspect ratio Γ = 0.5 filled with the ternary alloy GaInSn, which has a Prandtl number of Pr = 0.03. The largescale flow structures are classified and characterized by means of the contactless inductive flow tomography (CIFT) which enables the full reconstruction of the threedimensional flow structures in the entire convection cell. The analysis reveals that a singleroll structure of the LSC alternates in short succession with doubleroll structures or a threeroll structure. This is accompanied by dramatic fluctuations of the Reynolds number, whose instantaneous values can deviate by more than 50% from the timeaverage value. No coherent oscillations are observed, whereas a correlation analysis indicates a residual contribution of the torsion and sloshing modes. Results of the POD analysis suggest a stabilisation of the singleroll LSC with increasing Ra at the expense of flow structures with multiple rolls. Moreover, the relative lifetime of all identified flow states, measured in units of freefall times, increases with rising Ra. 
Monday, November 20, 2023 3:08PM  3:21PM 
R17.00007: QuasiGeostrophic RayleighB'enard Convection on the tilted fplane Keith A Julien, Abe Ellison, Benjamin Miquel, Michael Calkins, Edgar Knobloch In this presentation, we consider rotationally constrained thermal convection on the tilted fplane. Recent investigations have established the existence of a dual turbulent cascade  a direct enstrophy cascade to small scales and a nonlocal inverse cascade to domain scales. The topology of large scale structures is found to be dependent on both the strength of the thermal forcing and the latitudinal location of the local fplane. Specifically, simulations of quasigeostrophic RayleighBenard convection identify three classes of large scale condensates: large scale vortices, large scale zonal jets, and a regime of bistability capturing both. This is accomplished via novel reformulation that represents the governing fluid equations in a nonorthogonal coordinate system that aligns the upright coordinate with the axis of rotation as opposed to gravity. In this talk we detail the dynamical mechanism driving the inverse cascade with a focus on anisotropies in the stresses that play a critical role. 
Monday, November 20, 2023 3:21PM  3:34PM 
R17.00008: Reversals of the largescale circulation in thermal convection Nick N Moore, Jinzi M Huang The largescale circulation (LSC) associated with thermal convection is known to spontaneously reverse direction. In the atmosphere, reversals can result in a sudden change in wind direction, while in the mantle, reversals may play a role in magnetic dipole shifts. We examine LSC reversals within the context of thermal convection in an annular domain. Through comparison with direct numerical simulations, we show that a lowdimensional dynamical system derived systematically from Galerkin truncation of the governing equations accurately describes a sequence of parameter bifurcations, including the onset of circulatory flow, the appearance of chaotic LSC reversals, and finally a highRayleighnumber state of periodic LSC reversals with smallscale turbulence. When cast in terms of the fluid's angular momentum and center of mass, the model reveals equivalence to a pendulum system with driving term that raises the center of mass above the fulcrum. It is the competition between driving, restoring, and damping that leads to the range of convective states. This physical picture yields accurate predictions for the frequency of regular LSC reversals in the high Rayleighnumber limit and offers a transparent mechanism for reversals. 
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