Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session L04: Focus Session: Probing Multiscale Flows by Coarsegraining 
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Chair: Nicholas T. Ouellette, Stanford Room: 203 
Monday, November 25, 2019 1:45PM  1:58PM 
L04.00001: Gaining ``insight" by blurring one's ``sight" Invited Speaker: Hussein Aluie Coarsegraining (CG), which is equivalent to observing flows through “eyeglasses” of varying strength, provides a powerful yet intuitive approach to analyze and understand multiscale flows. This is because it allows for resolving the dynamics simultaneously in scale and in space, at every instant in time, and without requiring assumptions of homogeneity or isotropy, making it ideal to probe complex noncanonical flows that are unsteady, inhomogeneous, anisotropic, and multiphase. With examples from plasma, oceanic, and variabledensity flows, I will illustrate some of the advantages CG has over traditional tools such as Reynolds averaging, structure functions, and Fourier analysis. I will discuss a new method based on CG to extract the spectrum, including that of nonquadratic quantities such as kinetic energy in variable density flows, selfconsistently, which the Fourier spectrum and the wavelet spectrum cannot. I will also discuss the emergence of new invariants in MHD and in compressible turbulence which can be unraveled by the proper scaledecomposition based on CG. Finally, I will present a generalization of CG to spherical domains, allowing for the analysis of scaleprocesses in oceanic flows from satellites. [Preview Abstract] 
Monday, November 25, 2019 1:58PM  2:11PM 
L04.00002: Enhanced Spectral Transfer in Weakly Mixing Regions of a Turbulent Flow Lei Fang, Nicholas Ouellette, Sanjeeva Balasuriya Scaletoscale spectral energy flux is a hallmark of turbulent flows. The geometric alignment of smallscale turbulent stress and large scale rate of strain leads to the net flux of energy from small scales to large scales in 2D turbulence. We have found, however, that the instantaneous alignment between these two tensors is surprisingly weak, and thus that the spectral transport of energy is inefficient. In our experimental work, we have shown that the strain rate is much better aligned with the stress at times in the past, suggesting that the differential advection of the two is responsible for the inefficient spectral transfer. Based on this understanding, we developed a tool to specifically look for weakly mixing regions in the flow based on the linearity this implies, which we term Linear Neighborhoods (LNs). We demonstrate that these LNs are computable in real data using experimental measurements from a 2D turbulent flow. Consistent with our previous results, we find that that the spectral energy flux behaves differently inside the LNs, where the spectral energy flux is more efficient. Our results add additional support to the conjecture that turbulent flows locally tend to transport energy and momentum in space or between scales but not both simultaneously. [Preview Abstract] 
Monday, November 25, 2019 2:11PM  2:24PM 
L04.00003: Baroclinic Energy Transfer in the Ocean Benjamin Storer, Hussein Aluie The role of baroclinicity, which arises from the misalignment of pressure and density gradients, is wellknown in the vorticity equation, yet its role in the kinetic energy budget has never been obvious. Ref. [1] has recently shown that baroclinicity appears naturally in the kinetic energy budget after carrying out the appropriate scale decomposition. Here, we extend the coarsegraining decomposition to study this process within the shallow water model and apply it to numerical simulations as well as satellite data. [1] A. Lees and H. Aluie, Fluids 4, 92 (2019). [Preview Abstract] 
Monday, November 25, 2019 2:24PM  2:37PM 
L04.00004: Eigenframe alignment dominates scaletoscale energy fluxes in turbulence Joseph Ballouz, Nicholas Ouellette In largeeddy simulation, the cascade is cut off at a certain scale and the transfer term to smaller scales is modeled. This transfer term depends on the eigenvalues of the turbulent stress and resolved strainrate tensors and the Euler angles between their eigenframes. In a typical Smagorinsky closure, two assumptions are made: these angles are set to zero and the stress eigenvalues are modeled as scaled version of the stress. We show that it is the Euler angles that have a much larger effect on the transfer in terms of the directionality of the cascade and its efficiency. [Preview Abstract] 
(Author Not Attending)

L04.00005: Filterwidth and Atwood number effects in filtered homogeneous variable density turbulence Denis Aslangil, Juan Saenz, Daniel Livescu We investigate Atwood number ($A)$ and filter width ($w)$ dependence in filtered DNS of buoyancy driven homogeneous variable density turbulence, where density differences affect mixing and turbulence, and we discuss implications for modeling. We show that statistics and budgets of filtered fields transition smoothly between DNS and RANS fields and budgets, and we discuss these transitions in the context of flow length scales. At small $w$, filtered fields tend to DNS fields and the largescale flow kinetic energy ($k_{l})$ budget tends to the total kinetic energy ($k_{t})$ budget; at large $w$, filtered fields approach RANS fields and $k_{l}$ approaches the mean kinetic energy ($k_{m})$ budget. At intermediate $w$, the $k_{l}$ budget has dissipation and pressuredilatation work terms from the $k_{t}$ budget, a mean pressure gradient term from the $k_{m}$ budget, a production term from both the $k_{t}$ and $k_{m}$ budgets, and work by residual stresses against the filtered shear $e_{s}$, which tends to zero at both limits. Work by mean pressure gradients and by $e_{s}$ exhibit density dependent backscatter: at high $A$, $e_{s}$ backscatter occurs mainly in light fluid. Statistics of filtered fields, normalized by their RANS counterparts, smoothly and monotonically vary between 0 and 1 as $w$ varies from d$x$ to domain size, and the dependence on $w$ is different for different quantities. [Preview Abstract] 
Monday, November 25, 2019 2:50PM  3:03PM 
L04.00006: Datadriven coarsegrained modeling for polymers Shu Wang, Wenxiao Pan We present a datadriven coarsegraining method to establish coarsegrained (CG) modeling for polymers, which conserves both static and dynamic properties of the finegrained (FG) system. The dynamics of the CG system is governed by the generalized Langevin equation (GLE) derived via the MoriZwanzig formalism, by which the CG variables can be directly linked to the statistics of FG data. The effect of unresolved degrees of freedom on the kinetics of polymers can be captured by the nonMarkovian stochastic dynamics in GLE, where the memory kernel is determined from the FG data. To circumvent the difficulty of directly solving the GLE with memory term and colored noise, we exploit the equivalence between the nonMarkovian dynamics and Markovian dynamics in an extended space. To this end, the CG system is supplemented with auxiliary variables that are coupled linearly to the momentum and among themselves, subject to uncorrelated Gaussian white noise. For several different polymer systems in melts or in solution, we demonstrate that the established CG modeling can reproduce both static and dynamic properties of the reference FG system. [Preview Abstract] 
Monday, November 25, 2019 3:03PM  3:16PM 
L04.00007: Is vortex stretching the main cause of the turbulent energy cascade? Andrew Bragg, Maurizio Carbone While the dominant idea is that in 3D turbulence, the energy cascade occurs through the process of vortex stretching, evidence for this is debated. In the framework of the KarmanHowarth equation, we derive a new result for the average flux of kinetic energy between two points in the flow. The result shows that vortex stretching is in fact not the main contributor to the average energy cascade; the main contributor is the selfamplification of the strainrate field. We emphasize the need to correctly distinguish and not conflate the roles of vortex stretching and strainself amplification in order to correctly understand the physics of the cascade, and also resolve a paradox regarding the differing role of vortex stretching on the mechanisms of the energy cascade and energy dissipation rate. Direct numerical simulations are used to confirm the results, as well as provide further results and insights on vortex stretching and strainself amplification at different scales in the flow. Interestingly, the results imply that while vortex stretching plays a subleading role in the average cascade, it may play a leading order role during large fluctuations of the energy cascade about its average behavior. [Preview Abstract] 
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