Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session L30: Instability: Rayleigh-Taylor II |
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Chair: Daniel Livescu, Los Alamos National Laboratory Room: 408 |
Monday, November 25, 2013 3:35PM - 3:48PM |
L30.00001: Comparison Between Turbulence Model Initialization Approaches for Rayleigh-Taylor and Richtmyer-Meshkov Turbulent Mixing Bertrand Rollin, Nick Denissen, Jon Reisner, Malcolm Andrews Implementation and validation cases of a novel approach to initialization for RANS simulations of interfacial instability induced by mixing are presented. The strategy consists of using an analytical model to compute the instability evolution from the quiescent state, and make use of its prediction to generate initial conditions for the turbulence model. Explicitly, an incompressible inviscid model for Rayleigh-Taylor and Richtmyer-Meshkov instabilities continuously updates the turbulence model variables values in the mixing layer, until the Reynolds number suggests that the flow has become turbulent. Implementation of this procedure is made in three steps: first, the instability model is run alone while the interface is evolved by the hydrocode hosting the turbulence model; second, the turbulence model is started and the turbulence variables updated in accordance with the instability growth model prediction; finally, the Reynolds number suggests that the turbulent mixing regime is reached, causing the instability model to stop and the turbulence model to continue alone. The initialization methodology is applied to Rayleigh-Taylor and Richtmyer-Meshkov problems. Comparisons between simulations using a traditional initialization technique and the new initialization approach are presented and discussed. [Preview Abstract] |
Monday, November 25, 2013 3:48PM - 4:01PM |
L30.00002: Blast-Driven Hydrodynamic Instability Marc T. Henry de Frahan, Eric Johnsen Accurate characterization of mixing from hydrodynamic instabilities, such as Richtmyer-Meshkov, Rayleigh-Taylor, and Kelvin-Helmholtz, is important to many multi-fluid applications, particularly, inertial confinement fusion, supernova collapse, and scramjet combustion. We investigate the dynamics of a perturbed interface between two fluids subjected to a planar blast wave. An initial point source explosion initiates a blast, which can be described as a shock front followed by a rarefaction wave. The interface, therefore, experiences an instantaneous acceleration (a pressure increase) followed by a gradual, time-dependent deceleration (a pressure decrease). The resulting interaction gives rise to Richtmyer-Meshkov and Rayleigh-Taylor growth, depending on the shock strength and blast profile. Using a high-order accurate numerical method that prevents pressure errors at interfaces when simulating variable specific heats ratios, we identify regimes in which one or the other instability dominates. [Preview Abstract] |
Monday, November 25, 2013 4:01PM - 4:14PM |
L30.00003: Temporal Evolution and Scaling of Mixing in Two-dimensional Rayleigh-Taylor Turbulence Quan Zhou We report a high-resolution numerical study of two-dimensional (2D) miscible Rayleigh-Taylor (RT) incompressible turbulence with the Boussinesq approximation. We present results from an ensemble of 100 independent realizations performed at unit Prandtl number and small Atwood number with a spatial resolution of $2048 \times 8193$ grid points and Rayleigh number up to $Ra\sim10^{11}$. Our main focus is on the temporal evolution and the scaling behavior of global quantities and of small-scale turbulence properties. Our results show that the buoyancy force balances the inertial force at all scales below the integral length scale and thus validate the basic force-balance assumption of the Bolgiano-Obukhov scenario in 2D RT turbulence. It is further found that the Kolmogorov dissipation scale $\eta(t)\sim t^{1/8}$, the kinetic-energy dissipation rate $\varepsilon_u(t)\sim t^{-1/2}$, and the thermal dissipation rate $\varepsilon_{\theta}(t)\sim t^{-1}$. All of these scaling properties are in excellent agreement with the theoretical predictions of the Chertkov model [Phys. Rev. Lett. 91, 115001 (2003)]. [Preview Abstract] |
Monday, November 25, 2013 4:14PM - 4:27PM |
L30.00004: Electrohydrodynamically induced mixing in immiscible multilayer flows Radu Cimpeanu, Demetrios Papageorgiou In this study we investigate electrostatic stabilization mechanisms acting on stratified fluids. A classical example shows how an electric field can be used to control and even suppress the Rayleigh-Taylor instability when a heavy fluid lies above lighter fluid. We present a linear stability study, as well as extensive direct numerical simulations via the volume of fluid method to show that when the fluids are dielectrics and an electric field acts horizontally (in the plane of the undisturbed liquid-liquid surface), growth rates and critical stability wavenumbers are reduced thus shifting the instability to longer wavelengths. Agreement between linear theory and direct numerical simulations is shown to be excellent. From a practical perspective, we aim to identify active control protocols in confined geometries that induce time dependent flows in small scale devices without having moving parts. This effect has numerous applications, ranging from mixing phenomena to electric lithography. Two- and three-dimensional computations are carried out and several such protocols are described. [Preview Abstract] |
Monday, November 25, 2013 4:27PM - 4:40PM |
L30.00005: The Rayleigh-Taylor Instability driven by an accel-decel-accel profile Praveen Ramaprabhu, Varad Karkhanis, Andrew Lawrie We describe numerical simulations of the miscible Rayleigh-Taylor (RT) instability driven by a complex acceleration history, g(t), with initially destabilizing acceleration, g $>$ 0, an intermediate stage of stabilizing deceleration, g $<$ 0, and subsequent destabilizing acceleration, g $>$ 0. Initial perturbations with both single wave-number and a spectrum of wave-numbers (leading to a turbulent front) have been considered with these acceleration histories. We find in the single-mode case that the instability undergoes a so-called phase inversion during the first acceleration reversal from g $>$ 0 to g $<$ 0. If the zero-crossing of g(t) occurs once the instability growth has reached a state of nonlinear saturation, then hitherto rising bubbles and falling spikes reverse direction and collide, resulting in small-scale structures. For multi-mode perturbations, we find that bubbles and spikes collide during phase inversion, the interfacial region is turbulent, and undergoes a period of enhanced structural breakdown. This is accompanied by a rapid increase in the rate of molecular mixing, and increasing isotropy within the region. During the final stage of g $>$ 0 acceleration, self-similar RT mixing re-emerges, together with a return to anisotropy. [Preview Abstract] |
Monday, November 25, 2013 4:40PM - 4:53PM |
L30.00006: Initial Condition Effects on Turbulent Rayleigh Taylor Instability under Variable Acceleration History Denis Aslangil, Andrew Lawrie, Arindam Banerjee Initial condition effects on Rayleigh Taylor Instability are investigated for various fixed and variable acceleration histories. A massively parallel high resolution code (MOBILE) is used to model incompressible flow using an Implicit Large Eddy simulation technique. The simulations are initialized to understand the effects of spectral index, spectral bandwidth and discrete banded spectra. This study will present both general low-order metrics such as mix widths and growth constants and will compare the results for different acceleration histories and initial conditions. Studies on higher order turbulence parameters such as second order moments, their dissipations, and production--dissipation ratios will also be presented and are important to identify the similarities and differences between the Rayleigh--Taylor turbulence and the more conventional stationary turbulence. [Preview Abstract] |
Monday, November 25, 2013 4:53PM - 5:06PM |
L30.00007: ABSTRACT WITHDRAWN |
Monday, November 25, 2013 5:06PM - 5:19PM |
L30.00008: ABSTRACT WITHDRAWN |
Monday, November 25, 2013 5:19PM - 5:32PM |
L30.00009: Mixing and turbulence generated by the tilted Rayleigh-Taylor instability Daniel Livescu, Tie Wei In most practical applications of Rayleigh-Taylor instability (RTI), the initial interface is not perpendicular to the direction of acceleration. When the degree of tilting of the interface is non-negligible, the resulting mean flow is no longer one-dimensional as is the case with the classical RTI, and the two main turbulence production mechanisms, buoyancy and shear, are both present. The development of the instability can be decomposed into a large overturning motion, which leads to a strengthening of the mean shear, the formation of a large side wall bubble and spike, and the interior mixing layer growth. Results from very large Direct Numerical Simulations are presented of this unique unit problem and used to study the competition between shear and buoyancy production of turbulence and the respective effects on the mixing and turbulence properties. In particular, the development of the mixing layer seems more sensitive to the properties of the initial perturbation than in classical RTI. [Preview Abstract] |
Monday, November 25, 2013 5:32PM - 5:45PM |
L30.00010: The Rayleigh-Taylor instability for a thin film on the inside of a horizontal cylinder Naima Hammoud, Philippe Trinh, Peter Howell, Jonathan Chapman, Howard Stone Thin films on curved surfaces are widely observed in coating and painting processes and wetting problems. We consider a thin film on a curved substrate under the effect of gravitational, viscous, and surface tension forces. When the film is on the underside of the substrate, gravity works as a destabilizing force, and a Rayleigh-Taylor type instability is expected. We consider the stability of a uniform thin film coating the inside of a horizontal circular cylinder. Using asymptotic methods, we find that instabilities are of a transient nature, thus showing that curvature helps stabilize the film. We also find that these ``instabilities'' occur primarily in the angular direction with the axial perturbations only appearing as higher-order corrections. These results seem to agree well with experiments (H. Kim et al., this conference). [Preview Abstract] |
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