Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session R38: Flow Instability: Computations and Modeling |
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Chair: Fernando Grinstein, Los Alamos National Lab Room: Portland Ballroom 255 |
Tuesday, November 22, 2016 1:30PM - 1:43PM |
R38.00001: Initial Conditions and Modeling for Shock Driven Turbulence. Fernando Grinstein We focus on the simulation of shock-driven material mixing driven by flow instabilities and initial conditions. Beyond complex multi-scale resolution of shocks and variable density turbulence, me must address the equally difficult problem of predicting flow transition promoted by energy deposited at the material interfacial layer during the shock interface interactions. Transition involves unsteady large-scale coherent-structure dynamics which can be captured by LES, but not by URANS based on equilibrium turbulence assumptions and single-point-closure modeling. Such URANS is frequently preferred on the engineering end of computation capabilities for full-scale configurations -- and with reduced 1D/2D dimensionality being also a common aspect. With suitable initialization around each transition -- e.g., reshock, URANS can be used to simulate the subsequent near-equilibrium weakly turbulent flow. We demonstrate 3D state-of-the-art URANS performance in one such flow regime. We simulate the CEA planar shock-tube experiments by Poggi et al. (1998) with an ILES strategy. Laboratory turbulence and mixing data are used to benchmark ILES. In turn, the ILES generated data is used to initialize and as reference to assess state-of-the-art 3D URANS. We find that by prescribing physics-based 3D initial conditions and allowing for 3D flow convection with just enough resolution, the additionally computed dissipation in 3D URANS effectively blends with the modeled dissipation to yield significantly improved statistical predictions. [Preview Abstract] |
Tuesday, November 22, 2016 1:43PM - 1:56PM |
R38.00002: Shock driven multiphase flow with particle evaporation Jeevan Dahal, Jacob McFarland The computational study of the shock driven instability of a multiphase system with particle evaporation is presented. The particle evaporation modifies the evolution of the interface due to the addition of the vapor phase to the gas. The effects can be quantitatively measured by studying various gas parameters like density, temperature, vorticity and particle properties like diameter and temperature. In addition, the size distribution of particles also modifies the development of instability as the larger size particles damp the evolution of interface in comparison to the smaller size particles. The simulation results are presented to study these effects using FLASH developed at the FLASH Center at the University of Chicago. The capabilities of FLASH for particle modeling were extended using the Particle in Cell (PIC) technique for coupling of mass, momentum, and energy between the particle and carrier gas. A seeded cylinder of gas with particles having either a single radius or a distribution of radii was studied. The enstrophy production and destruction mechanisms were explored to understand the reason for change in vorticity with particle size. [Preview Abstract] |
Tuesday, November 22, 2016 1:56PM - 2:09PM |
R38.00003: Application of Self-Similarity Constrained Reynolds-Averaged Turbulence Models to Rayleigh-Taylor and Richtmyer-Meshkov Unstable Turbulent Mixing Tucker A. Hartland, Oleg Schilling Analytical self-similar solutions corresponding to Rayleigh--Taylor, Richtmyer--Meshkov and Kelvin--Helmholtz instability are combined with observed values of the growth parameters in these instabilities to derive coefficient sets for $K$--$\epsilon$ and $K$--$L$--$a$ Reynolds-averaged turbulence models. It is shown that full numerical solutions of the model equations give mixing layer widths, fields, and budgets in good agreement with the corresponding self-similar quantities for small Atwood number. Both models are then applied to Rayleigh--Taylor instability with increasing density contrasts to estimate the Atwood number above which the self-similar solutions become invalid. The models are also applied to a reshocked Richtmyer--Meshkov instability, and the predictions are compared with data. The expressions for the growth parameters obtained from the similarity analysis are used to develop estimates for the sensitivity of their values to changes in important model coefficients. Numerical simulations using these modified coefficient values are then performed to provide bounds on the model predictions associated with uncertainties in these coefficient values. [Preview Abstract] |
Tuesday, November 22, 2016 2:09PM - 2:22PM |
R38.00004: A Comparative Analysis of Reynolds-Averaged Navier-Stokes Model Predictions for Rayleigh-Taylor Instability and Mixing with Constant and Complex Accelerations Oleg Schilling Two-, three- and four-equation, single-velocity, multicomponent Reynolds-averaged Navier--Stokes (RANS) models, based on the turbulent kinetic energy dissipation rate or lengthscale, are used to simulate $At=0.5$ Rayleigh--Taylor turbulent mixing with constant and complex accelerations. The constant acceleration case is inspired by the Cabot and Cook (2006) DNS, and the complex acceleration cases are inspired by the unstable/stable and unstable/neutral cases simulated using DNS (Livescu, Wei \& Petersen 2011) and the unstable/stable/unstable case simulated using ILES (Ramaprabhu, Karkhanis \& Lawrie 2013). The four-equation models couple equations for the mass flux $a$ and negative density--specific volume correlation $b$ to the $K$--$\epsilon$ or $K$--$L$ equations, while the three-equation models use a two-fluid algebraic closure for $b$. The lengthscale-based models are also applied with no buoyancy production in the $L$ equation to explore the consequences of neglecting this term. Predicted mixing widths, turbulence statistics, fields, and turbulent transport equation budgets are compared among these models to identify similarities and differences in the turbulence production, dissipation and diffusion physics represented by the closures used in these models. [Preview Abstract] |
Tuesday, November 22, 2016 2:22PM - 2:35PM |
R38.00005: Adjoint-based approach to Enhancing Mixing in Rayleigh-Taylor Turbulence Ali Kord, Jesse Capecelatro A recently developed adjoint method for multi-component compressible flow is used to measure sensitivity of the mixing rate to initial perturbations in Rayleigh-Taylor (RT) turbulence. Direct numerical simulations (DNS) of RT instabilities are performed at moderate Reynolds numbers. The DNS are used to provide an initial prediction, and the corresponding space-time discrete-exact adjoint provides a sensitivity gradient for a specific quantity of interest (QoI). In this work, a QoI is defined based on the time-integrated scalar field to quantify the mixing rate. Therefore, the adjoint solution is used to measure sensitivity of this QoI to a set of initial perturbations, and inform a gradient-based line search to optimize mixing. We first demonstrate the adjoint approach in the linear regime and compare the optimized initial conditions to the expected values from linear stability analysis. The adjoint method is then used in the high Reynolds number limit where theory is no longer valid. Finally, chaos is known to contaminate the accuracy of the adjoint gradient in turbulent flows when integrated over long time horizons. We assess the influence of chaos on the accuracy of the adjoint gradient to guide the work of future studies on adjoint-based sensitivity of turbulent mixing. [Preview Abstract] |
Tuesday, November 22, 2016 2:35PM - 2:48PM |
R38.00006: Vortical Effects on the Compressible Rayleigh-Taylor Instability Scott Wieland, Daniel Livescu, Oleg V. Vasilyev, Scott J. Reckinger High fidelity wavelet based direct numerical simulations (WDNS) of compressible, miscible, and single mode Rayleigh Taylor instability (RTI) with a stratified background density have been completed in 2 and 3 dimensions. As the instability grows, vorticity dynamics are largely responsible for the self-propagation and growth of the bubble and spike. However, in the presence of a background stratification, the vortex interactions are significantly altered. In the case of low Atwood number RTI, this leads to previously unseen regimes, namely, the exaggeration of bubble and spike asymmetries for a weakly stratified background state and the complete suppression of RTI growth in the strongly stratified scenario. To better understand these results, the vorticity transport equation budget was compared to the simplified scenarios of vortex pairs (2D) and vortex rings (3D) moving in a stratified medium. [Preview Abstract] |
Tuesday, November 22, 2016 2:48PM - 3:01PM |
R38.00007: Three-Dimensional DSMC Simulations of the Rayleigh-Taylor Instability in Gases T.P. Koehler, M.A. Gallis, J.R. Torczynski, S.J. Plimpton The Direct Simulation Monte Carlo (DSMC) method of molecular gas dynamics is applied to simulate the Rayleigh-Taylor instability (RTI) in atmospheric-pressure monatomic gases (e.g., argon and helium). The computational domain is a 1-mm by 1-mm by 4-mm cuboid uniformly divided into 62.5 billion cubical cells. A total of 1 trillion computational molecules are used, and time steps of 0.1 ns are used. Simulations are performed to quantify the growth of perturbations on an initially flat interface as a function of the Atwood number. The DSMC results reproduce many features of the RTI and are in reasonable agreement with theoretical and empirical models. Consistent with previous work, the DSMC simulations indicate that the growth of the RTI follows a universal behavior. The numbers of bubble-spike pairs that eventually appear agree with theoretical values based on the most unstable wavelength and are independent of the statistical representation of the gas. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, November 22, 2016 3:01PM - 3:14PM |
R38.00008: On new non-modal hydrodynamic stability modes and resulting non-exponential growth rates - a Lie symmetry approach Martin Oberlack, Andreas Nold, Cedric Wilfried Sanjon, Yongqi Wang, Jan Hau Classical hydrodynamic stability theory for laminar shear flows, no matter if considering long-term stability or transient growth, is based on the normal-mode ansatz, or, in other words, on an exponential function in space (stream-wise direction) and time. Recently, it became clear that the normal mode ansatz and the resulting Orr-Sommerfeld equation is based on essentially three fundamental symmetries of the linearized Euler and Navier-Stokes equations: translation in space and time and scaling of the dependent variable. Further, Kelvin-mode of linear shear flows seemed to be an exception in this context as it admits a fourth symmetry resulting in the classical Kelvin mode which is rather different from normal-mode. However, very recently it was discovered that most of the classical canonical shear flows such as linear shear, Couette, plane and round Poiseuille, Taylor-Couette, Lamb-Ossen vortex or asymptotic suction boundary layer admit more symmetries. This, in turn, led to new problem specific non-modal ansatz functions. In contrast to the exponential growth rate in time of the modal-ansatz, the new non-modal ansatz functions usually lead to an algebraic growth or decay rate, while for the asymptotic suction boundary layer a double-exponential growth or decay is observed. [Preview Abstract] |
Tuesday, November 22, 2016 3:14PM - 3:27PM |
R38.00009: Motion of multiple superposed viscous fluids Magnus Vartdal In this study, the initial-value problem arising from small-amplitude disturbances on the interfaces between multiple superposed viscous fluids is analysed. First, linearized governing equations for the evolution of the amplitudes, valid in the general case, are presented. These equations are then used to study the effect of the presence of nearby interfaces on the initial growth-rate of a Rayleigh-Taylor instability. The present work is an extension of the analysis of Prosperetti (Prosperetti, A. (1981). Motion of two superposed viscous fluids. Physics of Fluids (1958-1988), 24(7), 1217-1223) to the multiple interface case. [Preview Abstract] |
Tuesday, November 22, 2016 3:27PM - 3:40PM |
R38.00010: Stabilization of a finite slice in miscible displacement in homogeneous porous media Satyajit Pramanik, Manoranjan Mishra We numerically studied the miscible displacement of a finite slice of variable viscosity and density. The stability of the finite slice depends on different flow parameters, such as displacement velocity $U$, mobility ratio $R,$ and the density contrast. Series of numerical simulations corresponding to different ordered pair ($R$, $U)$ in the parameter space, and a given density contrast reveal six different instability regions. We have shown that independent of the width of the slice, there always exists a region of stable displacement, and below a critical value of the slice width, this stable region increases with decreasing slice width. Further we observe that the viscous fingering (buoyancy-induced instability) at the upper interface induces buoyancy-induced instability (viscous fingering) at the lower interface. Besides the fundamental fluid dynamics understanding, our results can be helpful to model CO2 sequestration and chromatographic separation. [Preview Abstract] |
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