Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session ZC43: Turbulence: BuoyancyDriven and Stratified II 
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Chair: Vishal Kumar, Argonne national laboratory; Miles Couchman, Department of Mathematics and Statistics, York University Room: 207B 
Tuesday, November 21, 2023 12:50PM  1:03PM 
ZC43.00001: Towards generalization of visual anemometry using honami wave theory Roni Goldshmid, Han Liu, Lian Shen, John O Dabiri Honami is a phenomenon in which wavelike motions propagate downstream along the top of homogeneous canopy fields such as cereal crops, alfalfa fields, rice fields, and grasses. These waves occur when a high momentum coherent fluid parcel, also known as a sweep, is accelerated in the streamwise direction and moves towards the canopy, bending over a group of plant stalks. The phase velocity of honami waves can be interpreted as the frozen histories of gust velocity or the convection velocity of the large eddies with which the gusts are associated. The literature to date has focused on identifying the relationship between the wave properties and mean boundary layer statistics above the canopy and concluded that honami speeds are significantly higher than the mean wind speed at the canopy height. Our research examines the ability of honami speeds to identify instantaneous flow properties and turbulent properties. We will present the relationship between gust speeds and honami phase speeds and compare the results of quadrant analysis for both the canopy and the flow. The combination of these relationships can contribute to the advancement of generalization of visual anemometry, which is the process of inferring wind conditions from visual observations of vegetation kinematics without prior knowledge of the flow or observed canopy. 
Tuesday, November 21, 2023 1:03PM  1:16PM 
ZC43.00002: Decoupling smallscale turbulence generation from a largescale domain in direct numerical simulations of RayleighTaylor instabilities Aaron Nelson, Guillaume Blanquart RayleighTaylor (RT) instabilities are buoyantturbulent phenomena of immense practical value in the areas of wildfire modeling, astrophysics, and inertial confinement fusion. Unfortunately, direct numerical simulations (DNS) of the full domain of RT flow, including both reservoirs of heavy and light fluid, may be an inefficient use of computational resources when most turbulence generation is confined to smallscale eddies in the central mixing layer. To isolate these effects in a smaller domain, this study relies on a Reynoldsdecomposition of the governing equations into mean and fluctuations. The fluctuations are transformed into variables verified to be spatially homogeneous by leveraging results of previous DNS of RT instabilities, allowing the use of a 3D periodic box for boundary conditions. The equations are closed by assuming the mean flow to be of a form known from previous DNS, resulting in a set of equations similar to NavierStokes but with a few additional source terms. These terms act to maintain the turbulent kinetic energy and mixture fraction variance, emulating the effect of the largerdomain flow without having to resolve it in a fullscale simulation. The equations are then implemented in the periodic box DNS and the resulting statistics are compared with those found in a full DNS of RT instabilities. 
Tuesday, November 21, 2023 1:16PM  1:29PM 
ZC43.00003: Abstract Withdrawn

Tuesday, November 21, 2023 1:29PM  1:42PM 
ZC43.00004: Understanding the effect of Prandtl number on momentum and scalar mixing rates in neutral and stably stratified flows using gradient field dynamics Andrew D Bragg, Stephen M de Bruyn Kops Recent DNS of stably stratified turbulence show that when increasing the Prandtl number ($Pr$) from 1 to 7, the mean turbulent potential energy dissipation rate (TPEDR) drops dramatically, while the mean turbulent kinetic energy dissipation rate (TKEDR) increases significantly. Through an analysis of the equations governing the fluctuating velocity gradients (FVG) and fluctuating density gradients (FDG) we provide a mechanistic explanation for this surprising behavior. We show that the mean density gradient gives rise to a mechanism that opposes the production of FDG and is connected to the emergence of rampcliffs. An equal but opposite term appears in the FVG equation, corresponding to the contribution from buoyancy, and this is ultimately the reason why the TPEDR reduces while the TKEDR increases with increasing $Pr$. Our analysis predicts that the effects of buoyancy on the FVG become stronger as $Pr$ is increased, which is confirmed by our DNS data. Due to this, the buoyancy Reynolds number does not correctly estimate the impact of buoyancy on FVG when $Pr eq 1$, and we derive a suitable alternative parameter. Finally, an analysis of the filtered gradient equations reveals that the buoyancy term acts as a source for FVG at small scales, but as a sink at large scales. 
Tuesday, November 21, 2023 1:42PM  1:55PM 
ZC43.00005: Lagrangian irreversibility in rotatingstratified turbulent flows Sebastian Gallon, Alessandro Sozza, Fabio Feraco, Raffaele Marino, Alain J Pumir In homogeneous and isotropic turbulence (HIT), two tracer particles separate faster backward than forward in time, a manifestation of the irreversibility induced by the energy flux from large to small scales. We establish that this property extends to flows of geophysical relevance, with broken isotropy. Specifically, we study turbulence in the presence of both solid body rotation (ROT, Coriolis parameter f) and stable stratification (STRAT, BruntVäisälä frequency N). At the fixed relative strength of ROT and STRAT N/f = 5, we perform a series of direct numerical simulations (DNS) with increasing strength of both ROT and STRAT from flows that are close to HIT to wavedominated flows. 
Tuesday, November 21, 2023 1:55PM  2:08PM 
ZC43.00006: Mixing efficiency in shearless, inhomogeneous, and stably stratified turbulence Ryan Hass, Sanjiva K Lele Large scale simulations of the Earth’s oceans and atmosphere rely on crude parameterizations of turbulent processes using bulk quantities due to the prohibitive mesh resolution required to capture these processes directly. An interesting flow regime useful for evaluating existing models is one in which shearless turbulence, generated in a localized region of space, decays and interacts with background stratification, a scenario common in geophysical settings. 
Tuesday, November 21, 2023 2:08PM  2:21PM 
ZC43.00007: The characteristics of the meandering effect in a stratified wake Xinyi Huang, Jiaqi Li, Xiang Yang In a stratified wake, the nearwake flow is similar to its nonstratified equivalent, and the buoyancy effect grows stronger as the flow develops downstream. The flow starts with initial vortex shedding. Then, the vertical motions get suppressed and large horizontal structures are observed in the late wake. The evolving flow structures bring in meandering behavior in the wake regime. We aim at understanding how the meandering impacts the flow behavior, especially the scaling of the deficit velocity. 
Tuesday, November 21, 2023 2:21PM  2:34PM 
ZC43.00008: Assessment of subgridscale models in largeeddy simulations of decaying rotating stratified turbulence SHAHU NATHU JADHAV S JADHAV, Rahul Agrawal, Abhilash J Chandy We report the results of large eddy simulation (LES) using three subgrid scale models, namely: constant coefficient Smagorinsky, dynamic Smagorinsky, and a nonlinear model, for rotating stratified turbulence in the absence of forcing using largescale isotropic initial condition. The LES results are compared to inhouse direct numerical simulation (DNS) for establishing gridindependence requirements. Three cases with varying ratios of BruntVaisala frequency to the inertial wave frequency, $mathcal{N}/f$, have been chosen to evaluate the performance of LES models. The Reynolds number and N/f are chosen as (a) Run1: Re=3704, N/f=5, (b) Run2: Re=6667, N/f=40 and, (c) Run3: Re=6667, N/f=138. This framework is used to illustrate the relative magnitudes of the stratification and rotation which is observed in geophysical flows. Various quantities including turbulent kinetic energy (tke), turbulent potential energy (tpe), total dissipation, potential & total energy spectra, and their fluxes, are analyzed to understand the predictive capability of the various LES models. Results showed that all the SGS model predictions were very similar to each other with the classical Smagorinsky model displaying the highest deviation in comparison to DNS. The effect of an increase in value of N/f was also seen in the results of LES with an increase in the oscillation observed in the evolution of tke and tpe, and reduction in dissipation, which is exhibited by DNS. The spectral analysis shows that the nonlinear and dynamic Smagorinsky models predict the largescale physics ($kappa < 10$), while the small scales (10< κ< 64) energy is underpredicted. 
Tuesday, November 21, 2023 2:34PM  2:47PM 
ZC43.00009: Linear and nonlinear effects in stably stratified turbulent channel flow Simon Toedtli, Pierluigi Morra, Tamer A Zaki Even small levels of stable stratification significantly alter the dynamics of wallbounded turbulent flows. The observed changes are due to (i) modifications to the linear dynamics and (ii) spectral and wallnormal redistribution of energy in the nonlinear terms. This work examines the two from an inputoutput perspective, at moderate Reynolds number. The linear dynamics are described by the resolvent operator where the effect of stratification is represented by changes in the forcingresponse relation, due to the modified mean flow profiles and bulk Richardson number. As for the nonlinear terms, their statistics are computed from direct numerical simulations, and contrasted for the isothermal and weaklystratified cases. Numerical experiments where we modify the resolvent operator and the nonlinear forcing in isolation differentiate the contributions of the linear and nonlinear effects. The analysis draws attention to the channel center where the influence of stratification is most evident. 
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