Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session J25: Computational Fluid DynamicsLive

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Sponsoring Units: DFD Chair: Enkeleida Lushi, New Jersey Inst of Tech 
Tuesday, March 16, 2021 3:00PM  3:12PM Live 
J25.00001: Fluid phase interface dynamics of coupled Lattice Boltzmann and molecular dynamics simulations Daniel Cresta, Colin Denniston

Tuesday, March 16, 2021 3:12PM  3:24PM Live 
J25.00002: MLPDE: A Framework for a Machine Learning Enhanced PDE Solver Jaideep Pathak, Mustafa Mustafa, Karthik Kashinath, Emmanuel Motheau, Thorsten Kurth, Marcus Day Simulation of turbulent flows at high Reynolds number is a computationally challenging task relevant to a large number of engineering and scientific applications in diverse fields such as climate science, aerodynamics, and combustion. Turbulent flows are typically modeled by the NavierStokes equations. Direct Numerical Simulation (DNS) of the NavierStokes equations with sufficient numerical resolution to capture all the relevant scales of the turbulent motions can be prohibitively expensive. Simulation at lowerresolution on a coarsegrid introduces significant errors. We introduce a machine learning (ML) technique based on a deep neural network architecture that corrects the numerical errors induced by a coarsegrid simulation of turbulent flows at highReynolds numbers, while simultaneously recovering an estimate of the highresolution fields. Our proposed simulation strategy is a hybrid MLPDE solver that is capable of obtaining a meaningful highresolution solution trajectory while solving the system PDE at a lower resolution. 
Tuesday, March 16, 2021 3:24PM  3:36PM Live 
J25.00003: Numerical and analytical studies of windwave growth Anthony Bonfils, Woosok Moon, Dhrubaditya MITRA, John Scott Wettlaufer In 1957, Miles proposed a linear theory for the growth of windinduced small ripples on water. In the framework of hydrodynamic stability, these ripples are regarded as perturbations of the air flow. There is an energy transfer from the wind to the waves in the critical layer, which is the height where the phase speed is equal to the wind speed. We unify the many approaches of Miles' theory and provide a physical mechanism for the wave growth and we develop a simple method to compute the growth rate for an arbitrary wind profile. With this numerical scheme, we revisit the work of Morland and Saffman (1993) on Squire's theorem for the Miles instability, and study various turbulent boundary layer profiles as wind models. Moreover, for short and long waves we obtain uniform approximations of the perturbation stream function by the method of Matched Asymptotic Expansions, and infer approximate formulae for the growth rate. Finally, we simulate a quasilinear model proposed by Janssen (1982), which takes into account the feedback of growing waves on the wind. 
Tuesday, March 16, 2021 3:36PM  3:48PM Live 
J25.00004: Molecular dynamics simulation study of a polymer droplet motion over an array of spherical nanoparticles Anish Thomas, Nikolai V Priezjev The dynamic behavior of a partially wetting polymer droplet driven over a nanostructured interface is investigated using molecular dynamics simulations. We consider the beadspring model to represent a polymeric liquid that partially wets a rough surface composed of a periodic array of spherical particles. It is found that at sufficiently small values of the external force, the droplet remains pinned at the particles’ surface, while above the threshold, its motion consists of alternating periods of pinning and rapid displacements between neighboring particles. The latter process involves large periodic variation of the advancing and receding contact angles due to attachment and detachment of the contact line. Finally, upon increasing external force, the droplet center of mass is displaced steadily but the oscillation amplitude of the receding contact angle as well as the maximum contact angle hysteresis remain relatively unaffected. 
Tuesday, March 16, 2021 3:48PM  4:00PM Live 
J25.00005: Novel closed nonharmonic solutions for 1D, 2D, and 3Dwave equations with time as a directional angle in generalized (n+1)spherical coordinates Hector Munera Wave equations are ubiquitous: classical physics, fluid dynamics, relativistic quantum theory. Standard solutions use nspatial dimensions (n=1,2,3) plus one independent time t dimension. The method of descent connects solutions for n=3 down to n=2 and n=1, the latter solved by D’Alembert with canonical variables p=x+w and m=xw, where x is position along xaxis, w=Ct and C a relevant speed, see Lessons 1724 in [1]. Instead, we ascend from the onedimensional (1D) wave equation in spherical coordinates, with radial distance 0<r instead of ∞<x<+∞. Time is treated here as a novel orientation angle whose tangent is q=w/r. The 1Dwave equation is thus solved as a superposition of a permanent background potential dependent on r only, plus a new entangled potential dependent on q only; meaning of the rq plane is discussed. Solutions with same structure hold for n>1, with an azimuth angle added in the 2Dplane, plus an additional elevation angle for the 3Dcase. Firsttime explicit closed solutions are reported here for wave equations with n=1,2,3. Our novel solutions are nonharmonic and depend upon two independent variables r and q, plus (n1) spatial angles. Reference: [1] S. J. Fowler, Partial Differential Equations for Scientists and Engineers (Dover, New York 1992). 
Tuesday, March 16, 2021 4:00PM  4:12PM Live 
J25.00006: Energy conservation equation for onecomponent pseudopotential lattice Boltzmann method Luiz Czelusniak, Alexander Wagner, Luben CabezasGómez The pseudopotential lattice Boltzmann method has become an alternative tool to simulating multiphase systems. As is typically done, the lattice Boltzmann equation LBE is only used to solve the mass and momentum conservation equations. To solve nonisothermal problems an appropriate energy conservation equation ECE is solved using finite differences. When the LBE is coupled with an ECE, simulations often become more unstable and can be difficult to reach high values of the density ratio. Also, most ECE are written as a discrete equation for temperature, not exactly conserving total energy in a periodic domain. This talk focuses on studying the application of an ECE coupled to the pseudopotential method for multiphase simulation. The goals are: evaluate the presence of discretization errors, violation in the conservation of total energy and analyze current methods stability. A smooth oscillation problem is employed to evaluate the ECE. The comparison between numerical and analytical results revealing that the second order of the numerical scheme is not sufficient to properly model the nonisothermal oscillation problem. 
Tuesday, March 16, 2021 4:12PM  4:24PM Live 
J25.00007: Numerical model of the Moving Burning Surface in Highenergy Systems Alexander Kiryushkin, Leonid Minkov Experimental investigation of processes occurring in highenergy systems is complicated and expensive. High temperatures and pressures, transience, and nascent toxic gases make mathematical tools demanding in this domain. Hence, using numerical models is a key component for an efficient design of highenergy systems. 
Tuesday, March 16, 2021 4:24PM  4:36PM Live 
J25.00008: Accellerating Integer Lattice Gasses by using a Sampling Collision Operator Noah Seekins, Alexander Wagner The Integer Lattice Gas provides a solution to one of the major problems that the Boolean Lattice Gas has by ensuring it recovers the proper Boltzmann Distribution as opposed to the FermiDirac Distribution recovered by the Boolean Lattice Gas. This allows the Integer Lattice Gas to fully recover the NavierStokes Equation, something that Boolean Lattice Gasses struggled with. However, the Integer Lattice Gas has, thusfar, been far less computationally efficient than the more commonly used Lattice Boltzmann method, and thus has not been viable for practical utilization. We have created, by sampling from a local equilibrium distribution, a sampling collision operator for the Integer Lattice Gas that allows it to be competative with comparable Fluctuating Lattice Boltzmann Methods. This makes the Integer Lattice Gas a far more viable method, and, although not yet faster than the Lattice Boltzmann method, its stability and nondeterministic nature give several advantages that may make the Integer Lattice Gas a better option for some applications. 
Tuesday, March 16, 2021 4:36PM  4:48PM Not Participating 
J25.00009: Glazing of Doughnuts: NonNormal Instability and Asymmetric Pattern Formation Nicholas White, Sandra Troian The flow of a thin Newtonian film coating a solid substrate with complex curvature is a challenging problem of interest to many fields, especially biophysics. By a combination of multiplescale analysis and center manifold techniques, Roy, Roberts and Simpson (2002) were able to develop a general nonlinear interface equation, whose solutions reveal the mechanism responsible for film thinning near regions of high substrate curvature. Here we examine the perturbative response of this equation by investigating an interfacial instability involving capillary driven flow of a thin film on a toroidal substrate. We coin this system the doughnut glazing problem. Results of a generalized linear stability analysis based on nonnormal growth of a nonuniform liquid layer reveal the migration patterns of growing modes to the interior surface of the torus. Contrary to expectation, as the torii radii approach infinity, the wavelength of the maximally unstable mode does not asymptote to that of the wellknown sinusoidal instablity for a thin film on a solid cylinder (Goren 1962). We demonstrate how the asymmetry inherent in the torus geometry biases the flow to help steer the formation, evolution and growth of unstable nonnormal modes. 
Tuesday, March 16, 2021 4:48PM  5:00PM Live 
J25.00010: Adsorption of grapheneoxide nanoparticle at a watervapour interface: a molecular dynamics investigation Simon Gravelle, Lorenzo Botto Graphene oxides (GO) is used as a particulate surfactant to change the interfacial properties of drops and bubbles. To get insights into the processes governing GO adsorption, we use molecular simulations to simulate GO nanoparticles at the interface between water and vapour. We first extract the free energy profile normal to the interface for a single particle with varying degree of functionalization at the basal plane and varying particle size using umbrella sampling. We found that the affinity of the particle with the interface decreases for increasing degree of functionalization, and that only particles with a degree of functionalization lower than a threshold are interfacially actives. This is in agreement with experimental observations. We then use molecular dynamics to evaluate the behaviour of a single GO particle at the interface and show that, as a consequence of hydrophilic and hydrophobic patches at the particle surface, a GO particle only partially adsorbs to the interface. Finally we evaluate the role of particleparticle interactions on adsorption as well as on the surface tension by exploring the case of multiple GO particles. Implications of our results for drops/bubbles stabilisation will be discussed. 
Tuesday, March 16, 2021 5:00PM  5:12PM On Demand 
J25.00011: Falling films over phasechanging planes Darish Jeswin Dhas S, Srikanth Toppaladoddi, Anubhab Roy We study the interplay between shear, buoyancy, and phase change on the stability of a flow of a liquid over an inclined, phasechanging plane under the influence of gravity. In the absence of buoyancy and phase change, this flow is known to exhibit a longwavelength instability – which was first identified by Yih (Phys. Fluids. 6(3), pp. 321334, 1963) – and a shortwavelength shearmode instability – identified by Kelly (Phys. Fluids. A: Fluid Dynamics 1(5), pp. 819828, 1989). We use a combination of linear stability and asymptotic analyses to study the effects of stratification and phase boundary on these instabilities, and find that they significantly affect the flow dynamics 
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