Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session P17: Turbulence: Stratification, Rotation and Magnetic Fields (3:10pm  3:55pm CST)Interactive On Demand

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P17.00001: Phase transitions and fluxloop metastable states in rotating turbulence Luca Biferale, Patricio Clark Di Leoni, Alexandros Alexakis, Michele Buzzicotti By using direct numerical simulations of up to a record resolution of 512x512x32768 grid points we discover the existence of a new metastable outofequilibrium state in rotating turbulence. We scan the phase space by varying both the rotation rate and the dimensionless aspect ratio, $\lambda=H/L$, where $L$ and $H$ are the sizes of the domain perpendicular and parallel to the direction of rotation, respectively. We show the existence of three turbulent phases. For small $Ro$ but finite $\lambda$, we have a split cascade where the injected energy is transferred to both large and small scales. For large $\lambda$ and finite $Ro$ there is no inverse cascade and the energy is transferred downscale in Fourier space only. Surprisingly, between these two regimes, a third phase is observed as reported here for the first time. Consequently, for certain intervals of $Ro$ and $\lambda$, energy is no longer accumulated at arbitrarily large scales, rather it stops at some characteristic intermediate lengthscales from where it is then redistributed forward in Fourier space, leading to a fluxloop mechanism where the flow is out of equilibrium with vanishing net flux, and nonvanishing heterochiral and homochiral subfluxes. P. Clark Di Leoni, et al. arXiv preprint arXiv:2002.08784 (2020). [Preview Abstract] 

P17.00002: Turbulent/nonturbulent interface in a turbulent patch arising from a breaking internal wave Takahiro Katagiri, Tomoaki Watanabe, Koji Nagata We perform a direct numerical simulation (DNS) of the flow over a twodimensional hill in a uniformly stratified fluid for investigating the characteristics of a turbulence patch generated by a breaking internal gravity wave. The flow over the twodimensional hill generates the downwind rotor because of separation and reattachment and the rotor is related to the wave breaking. The visualizations of density showed that RayleighTaylor instability occurs by overturning motions, generating the turbulence patch, which remains a quasisteady state for a long time. The turbulence patch has large potential enstrophy, and the potential enstrophy isosurface is used to detect the turbulent/nonturbulent interfacial (TNTI) layer that separates the patch from the outer internal gravity wave region. The flow near the TNTI is investigated with the conditional statistics calculated as functions of the distance from the interface. The conditional average of the potential enstrophy changes more rapidly across the TNTI than the enstrophy because of the vorticity due to the internal gravity wave in the nonturbulent region. The thickness of the TNTI layer divided by the Kolmogorov scale in the patch is almost constant during the quasisteady period. The buoyancy Reynolds number is uniform in the turbulent region far away from the TNTI, gradually decreases to the TNTI, and rapidly drops within the TNTI layer toward the nonturbulent region. [Preview Abstract] 

P17.00003: Transient mixing in turbulent open channel subject to radiative heating Vassili Issaev, Nicholas Williamson, Steve Armfield, Stuart Norris The temporal evolution of initially neutral turbulent open channel flow subject to radiative surface heating through the BeerLambert law is examined through direct numerical simulations. As the flow transitions to stably stratified conditions we observe three distinct regimes in time. Initially the temperature field acts as a passive scalar growing directly in proportion to depth varying heatsource $q(z)$ with turbulence largely unaffected. A second transient period is observed in which the flow begins to move towards local energetic equilibrium. During this regime a strong transient effect is observed whereby the turbulence microstructure of the flow is still relatively unaffected by buoyancy, however the local buoyancy gradient $N^2$ is significantly large. As stable stratification begins to affect the smallest scales of the flow a final temporal regime is observed whereby the flow becomes independent of transient effects and instantaneous mixing can be again be estimated from local measurements of Froude number $Fr = \frac{\epsilon}{Nk}$. [Preview Abstract] 

P17.00004: Influence of Diurnal Heating on a Stratified Open Channel Flow Cindy NguyenDang, Nicholas Williamson, Steven W. Armfield, Stuart E. Norris, Michael P. Kirkpatrick This study employs direct numerical simulation (DNS) to investigate stably stratified open channel flows that are subjected to a timevarying surface radiative forcing whose function is approximated by a diel solar heating cycle. This paper follows from the earlier work of a statistically steady state open channel flow with a constant heat source. The simulations have a Prandtl number $Pr$ of 1 and a friction Reynolds number $Re_{\tau}$ of 400. It’s bulk buoyancy parameter $\lambda$ of 1, defined by the ratio of the channel depth $\delta$ and a bulk Obukhov length scale $\mathscr{L}$, corresponds to strongly stratified conditions and the diurnal timescales, normalised by the wall friction velocity $u_{\tau}$ over $\delta$, cover the range between $t^{*} = 6  12$. Results demonstrate strong mixing and restratification during the ‘night and day’ heating cycle. Numerical results have been found to be in agreement with existing parameterisation and scaling relations for the irreversible flux Richardson number $Ri_{f}^{*}$ and turbulent Froude number $Fr$ and eddy diffusivity $k_{h}$ and local buoyancy Reynolds number $Re_{b}$ at certain positions along the water column. [Preview Abstract] 

P17.00005: Generalized quasilinear simulations of strongly stratified Kolmogorov flow Adhithiya Sivakumar, Gregory Chini Generalized quasilinear (GQL) theory provides selfconsistent approximations for the smallscale dynamics of various flows. The approximation is performed by specifying a threshold wavenumber $\Lambda$ that separates state variables into large and small scales in spectral space and then removing select nonlinear interactions. The resulting equations respect the conservation laws of the original PDEs and enable a systematic homotopy between quasilinearity ($\Lambda = 0$), i.e. the mean field limit, and full nonlinearity ($\Lambda \rightarrow \infty$), i.e. DNS. When $\Lambda > 0$, nonlinear interactions among the large scales and small scale energy transfers via interaction with the large scales are captured. These physical processes are particularly important for shear flows with highly anisotropic structures. Here, we investigate these interactions by performing DNS and GQL simulations of strongly stratified Kolmogorov flow, comparing results across a range of computational domain sizes, buoyancy Reynolds numbers, and threshold wavenumbers. [Preview Abstract] 

P17.00006: Largescale Anisotropy Determines Mixing Regimes of Stablystratified Turbulence Young Yi, Jeffrey Koseff, Ali Mani Using both unsheared and sheared simulations of stablystratified turbulence, we study the effects of largescale anisotropy on the mixing coefficient, which is typically required to estimate the eddy diffusivity of density when using a downgradient model. The unsheared and sheared simulations occupy distinct and very different regions of the Lumley triangle, indicating that stratification and shear introduce different anisotropy effects to the large scales of the flow. We propose and test new scalings for the mixing coefficient of sheared, stablystratified turbulence that explicitly account for the mean shear rate in addition to background stratification. In line with the recent work by Maffioli et al. (2016) and Garanaik and Venayagamoorthy (2019), our work further highlights the predictive capability of turbulence parameters that describe the largescale anisotropy of stablystratified turbulence for estimating the mixing coefficient. [Preview Abstract] 
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