Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Q11: Transitional Flows and Nonlinear Dynamics IRecordings Available

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Sponsoring Units: DFD Chair: Greg Voth, Wesleyan University Room: McCormick Place W181B 
Wednesday, March 16, 2022 3:00PM  3:12PM Withdrawn 
Q11.00001: A geometric model for helicity evolution in vortex rings Robert Morton, Xinran Zhao, Hridesh Kedia, Nicola Lucarelli, Daniel PeraltaSalas, Carlo Scalo, William T. M. Irvine The helicity of a laminar vortex ring is prescribed by its geometry in the forms of writhe and twist. In viscous fluids, helicity is not conserved, but nonetheless its evolution is naturally characterized by the geometry and topology of the vorticity field. By generating helical vortices using hydrofoils, we are able to measure their helicity and its evolution over a range of Reynolds numbers. Fully resolved DNS with adaptive mesh refinement provide complementary insight. We present an analytic model for helicity evolution in vortex tubes with a natural geometric interpretation and compare its predictions to experiments and simulations. 
Wednesday, March 16, 2022 3:12PM  3:24PM 
Q11.00002: Scaling of the turbulence transition in pipe flow Justin Beroz, A. John Hart, John W Bush Despite significant experimental and theoretical advances, a complete physical picture for the turbulence transition in pipe flow remains elusive. Here we present a theoretical model for a flow disturbance’s critical amplitude to trigger sustained turbulent puffs downstream as a function of the pipe’s Reynolds number Re. Current theory (Trefethen 1993) suggests that flow disturbances excite a spectrum of pseudo modes, which grow transiently in time before exponential decay. These modes may in turn excite a sustained travelling wave packet in the pipe, marking the transition to turbulence. We find this yields a simple scaling for the critical disturbance amplitude u ~ g/Re, where g plays the role of an amplitude response function for the travelling wave. We computed g for several pipe disturbance methods and successfully collapsed the data from corresponding experimental papers onto our scaling; to date, flow obstacles (Nishi 2008) and pulsed fluid injections (Hof 2003 & 2005, Peixinho 2007). We find the travelling wave manifests with the same frequency in the direction of flow regardless of the disturbance method, evidencing the same pathway to turbulence. 
Wednesday, March 16, 2022 3:24PM  3:36PM 
Q11.00003: Dynamic Phase Alignment in NavierStokes Turbulence Lucio M Milanese, Nuno F Loureiro, Stanislav A Boldyrev In NavierStokes turbulence, energy and helicity injected at large scales are subject to a joint direct cascade, with both quantities exhibiting a spectral scaling $\sim k^{5/3}$. A ``na\"ive", dimensional estimate of the spectrum of helicity would, however, yield the scaling $\mathcal{H}(k) \sim k v_{\lambda} \omega_{\lambda} \sim k^{2/3}$, violating conservation of the helicity flux in the inertial range. We demonstrate via direct numerical simulations that this apparent contradiction is revolved because of the existence of a strong scaledependent Fourier phase alignment between velocity and vorticity fluctuations, with the phase alignment angle scaling as $\cos\alpha_k\propto k^{1}$ [L. M. Milanese \textit{et al.}, ``Dynamic Phase Alignment in NavierStokes Turbulence", arXiv:2104.13518]. This strong dependence on scale of $\cos\alpha_k$, termed \textit{dynamic phase alignment}, underpins the spectral scaling of helicity, i.e., $\mathcal{H}(k) \sim k v_{\lambda} \omega_{\lambda} \cos\alpha_k \sim k^{5/3}$. Dynamic phase alignment plays a role in the turbulent dynamics in the presence of two invariants beyond NavierStokes, and it has been shown to underpin the joint direct cascade of energy and (generalized) helicity in a variety of turbulent plasma environments [L. M. Milanese \textit{et al.}, ``Dynamic Phase Alignment in Inertial Alfv\'en Turbulence", Physical Review Letters, 2020]. 
Wednesday, March 16, 2022 3:36PM  3:48PM 
Q11.00004: Measurements of turbulence over a streamwise preferential porous medium Mahiro Morimoto, Yuki Okazaki, Yusuke Kuwata, Kazuhiko Suga This study experimentally examines the possibility of the turbulent drag reduction by an orthotropic porous medium whose streamwise permeability is larger than the wallnormal permeability. 
Wednesday, March 16, 2022 3:48PM  4:00PM 
Q11.00005: Odd transport phenomena from broken symmetries Cory M Hargus, Kranthi K Mandadapu Linear constitutive relations connect fluxes of quantities such as mass, heat, and momentum to corresponding thermodynamic driving forces. Such relations reveal macroscopic consequences of microscopic fluctuations. When timereversal and parity symmetries are broken at the microscale, as is often the case in chiral active matter, new "odd" transport coefficients can emerge at the continuum scale. We discuss two odd transport coefficients in particular: the odd diffusivity, which generates diffusive fluxes perpendicular to concentration gradients, and the odd viscosity, which generates normal stresses perpendicular to shear flow. In both cases, we show that the behavior under time reversal and parity inversion can be understood through deriving GreenKubo relations, which additionally provide an avenue for direct numerical verification in model active matter systems using molecular dynamics simulations. Our work provides steps towards extending the framework of nonequilibrium thermodynamics to systems like active matter, which are inherently out of equilibrium, or for which no equilibrium reference state exists. 
Wednesday, March 16, 2022 4:00PM  4:12PM 
Q11.00006: ModulationInstability in odd nonlinear waves Sudheesh Srivastava, Gustavo M Monteiro, Sriram Ganeshan Modulation Instability (MI) is an ubiquitous phenomenon; It has been catalogued under various names, and manifests in diverse systems ranging from water waves to lasers. In this work, we study the interplay of MI and parity breaking (in particular odd viscosity) in a hydrodynamic system of surface gravity waves. We derive a generalised nonlinear Schrodinger equation (NLSE) with an oddviscosity dependent stability criterion for amplitude modulation. We find that the parity breaking effects result in different stability criterion for left and right moving modes of NLSE. In the end, we show that similar phenomena can arise in 1D dielectrics through the introduction of paritybreaking terms. We provide two different models as possible nonreciprocal optical platforms that realise the generalised NLSE. 
Wednesday, March 16, 2022 4:12PM  4:24PM 
Q11.00007: Design optimization for Richtmyer—Meshkov instability suppression Dane M Sterbentz, Charles F Jekel, Daniel White, Sylvie Aubry, Jonathan L Belof The Richtmyer—Meshkov instability (RMI) is a phenomenon that occurs at the interface of two substances of different densities due to an impulsive acceleration, such as a shock wave passing through this interface. Under these conditions, the instability can be seen as interface perturbations begin to grow into jets or spikes of one substance that propagate into the other. The control of RMI growth is one major limiting factor for technological challenges such as inertial confinement fusion, which involves using highpressure shock waves to implode a fuel target. RMI growth can lead to asymmetry in the implosion process that significantly reduces the energy yield. This work is at the forefront of understanding RMI and designing for the suppression and control of perturbation growth. We use hydrodynamic simulations of impactor shockwave experiments and design optimization to suppress RMI growth by altering the geometry and other properties of a shocked material target. Our results show that RMI suppression can be achieved by intentionally creating a secondary instability to counteract RMI growth at a perturbed interface. 
Wednesday, March 16, 2022 4:24PM  4:36PM 
Q11.00008: Linear stability of the hydromagnetic pipe flow subject to a transverse magnetic field Yelyzaveta Velizhanina, Bernard C Knaepen Despite decades of research, the problem of developping a fully consistent framework to explain the instability of Poiseuille pipe flow remains challenging. Surprisingly, little attention has been devoted to the linear stability analysis of the circular pipe in the hydromagnetic case and the only thorough numerical study available considers a parallel magnetic field acting on the flow. 
Wednesday, March 16, 2022 4:36PM  4:48PM 
Q11.00009: Particle capture in a model chaotic flow with a moving capture unit Mengying Wang, Julio M Ottino, Richard M Lueptow, Paul B Umbanhowar To investigate the capture of passive scalars in geophysical flows, we computationally study capture in the simpler, but still chaotic, timedependent 2d doublegyre flow. For a range of model parameters, the doublegyre flow consists of a rapidly mixing chaotic region interspersed with nonmixing islands in which particle trajectories are regular. Here we consider a moving capture unit (plant) rather than a fixed one, because the former can potentially capture more material. We restrict plant motion to be along a straight line connecting the centers of the flow’s two gyres. Three prescriptions of plant motion are studied for flow conditions ranging from nonchaotic to fully chaotic: motion based on maximum capture for nonchaotic flow but applied to chaotic flow; a method based on Lagrangian coherent structures (LCS) in which the plant follows a maximum in the finitetime Lyapunov exponent field; and a biologicallyinspired approach that tracks concentration gradients. The capture efficiency depends on flow conditions. The maximum capture approach based on nonchaotic flow is effective when the flow is less chaotic, while the LCS method is better for fully chaotic flow. The biologicallyinspired approach works well when plant motion is not restricted to a straight line. 
Wednesday, March 16, 2022 4:48PM  5:00PM 
Q11.00010: Physicsconstrained datadriven subgridscale parameterization of 2D turbulence in the smalldata regime YIFEI GUAN, Adam Subel, Ashesh K Chattopadhyay, Pedram Hassanzadeh In this work, we develop a datadriven subgridscale (SGS) model for large eddy simulation (LES) of 2D turbulence using a fully convolutional neural network (CNN). In the smalldata regime, the LESCNN generates artificial instabilities and thus leads to unphysical results. We propose four remedies for the CNN to work in the smalldata regime: (1) data augmentation (DA), (2) group equivariant convolution neural network (GCNN), leveraging the rotational equivariance of the SGS term, (3) incorporating a physical constraint on the SGS enstrophy transfer, and (4) variational autoencoder providing stochasticity and uncertainty quantification. The rotational equivariance of SGS terms can be accounted for by either including rotated snapshots in the training data set (DA) or by a GCNN that enforces rotational equivariance as a hard constraint. Additionally, The SGS enstrophy transfer constraint can be implemented in the loss function of the CNN. Stochasticity can be crucial in modeling backscattering (energy transferred from subgrid scales to resolved scales) of SGS terms. A priori and a posteriori analyses show that the proposed approaches enhance the SGS model and allow the datadriven model to work stably and accurately in a smalldata regime. 
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