Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session G21: Flow Instability: Rayleigh-TaylorConvection Instabilities
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Chair: Arindam Banerjee, Lehigh University Room: 706 |
Monday, November 20, 2017 10:35AM - 10:48AM |
G21.00001: Self-similarity in high Atwood number Rayleigh-Taylor experiments Mark Mikhaeil, Prasoon Suchandra, Gokul Pathikonda, Devesh Ranjan Self-similarity is a critical concept in turbulent and mixing flows. In the Rayleigh-Taylor instability, theory and simulations have shown that the flow exhibits properties of self-similarity as the mixing Reynolds number exceeds 20000 and the flow enters the turbulent regime. Here, we present results from the first large Atwood number (0.7) Rayleigh-Taylor experimental campaign for mixing Reynolds number beyond 20000 in an effort to characterize the self-similar nature of the instability. Experiments are performed in a statistically steady gas tunnel facility, allowing for the evaluation of turbulence statistics. A visualization diagnostic is used to study the evolution of the mixing width as the instability grows. This allows for computation of the instability growth rate. For the first time in such a facility, stereoscopic particle image velocimetry is used to resolve three-component velocity information in a plane. Velocity means, fluctuations, and correlations are considered as well as their appropriate scaling. Probability density functions of velocity fields, energy spectra, and higher-order statistics are also presented. The energy budget of the flow is described, including the ratio of the kinetic energy to the released potential energy. [Preview Abstract] |
Monday, November 20, 2017 10:48AM - 11:01AM |
G21.00002: Finite plate thickness effects on the Rayleigh-Taylor instability in elastic-plastic materials Rinosh Polavarapu, Arindam Banerjee The majority of theoretical studies have tackled the Rayleigh-Taylor instability (RTI) problem in solids using an infinitely thick plate. Recent theoretical studies by Piriz et al. (PRE \textbf{95}, 053108, 2017) have explored finite thickness effects. We seek to validate this recent theoretical estimate experimentally using our rotating wheel RTI experiment in an accelerated elastic-plastic material. The test section consists of a container filled with air and mayonnaise (a non-Newtonian emulsion) with an initial perturbation between two materials. The plate thickness effects are studied by varying the depth of the soft-solid. A set of experiments is run by employing different initial conditions with different container dimensions. Additionally, the effect of acceleration rate (driving pressure rise time) on the instability threshold with reference to the finite thickness will also be inspected. Furthermore, the experimental results are compared to the analytical strength models related to finite thickness effects on RTI. [Preview Abstract] |
Monday, November 20, 2017 11:01AM - 11:14AM |
G21.00003: Centrifugally Driven Rayleigh-Taylor Instability Matthew Scase, Richard Hill The instability that develops at the interface between two fluids of differing density due to the rapid rotation of the system may be considered as a limit of high-rotation rate Rayleigh-Taylor instability. Previously the authors have considered the effect of rotation on a gravitationally dominated Rayleigh-Taylor instability and have shown that some growth modes of instability may be suppressed completely by the stabilizing effect of rotation (Phys. Rev. Fluids 2:024801, Sci. Rep. 5:11706). Here we consider the case of very high rotation rates and a negligible gravitational field. The initial condition is of a dense inner cylinder of fluid surrounded by a lighter layer of fluid. As the system is rotated about the generating axis of the cylinder, the dense inner fluid moves away from the axis and the familiar bubbles and spikes of Rayleigh-Taylor instability develop at the interface. The system may be thought of as a “fluid-fluid centrifuge”. By developing a model based on an Orr-Sommerfeld equation, we consider the effects of viscosity, surface tension and interface diffusion on the growth rate and modes of instability. We show that under particular circumstances some modes may be stabilized. [Preview Abstract] |
Monday, November 20, 2017 11:14AM - 11:27AM |
G21.00004: Inviscid linear stability analysis of two fluid columns of different densities subject to gravity Aditya Prathama, Carlos Pantano We investigate the inviscid linear stability of vertical interface between two fluid columns of different densities under the influence of gravity. In this flow arrangement, the two free streams are continuously accelerating, in contrast to the canonical Kelvin-Helmholtz or Rayleigh-Taylor instabilities whose base flows are stationary (or weakly time dependent). In these classical cases, the temporal evolution of the interface can be expressed as Fourier or Laplace solutions in time. This is not possible in our case; instead, we employ the initial value problem method to solve the equations analytically. The results, expressed in terms of the well-known parabolic cylinder function, indicate that the instability grows as the exponential of a quadratic function of time. The analysis shows that in this accelerating Kelvin-Helmholtz configuration, the interface is unconditionally unstable at all wave modes, despite the presence of surface tension. [Preview Abstract] |
Monday, November 20, 2017 11:27AM - 11:40AM |
G21.00005: Applications of Analytical Self-Similar Solutions of Reynolds-Averaged Models for Instability-Induced Turbulent Mixing Tucker Hartland, Oleg Schilling Analytical self-similar solutions to several families of single- and two-scale, eddy viscosity and Reynolds stress turbulence models are presented for Rayleigh--Taylor, Richtmyer--Meshkov, and Kelvin--Helmholtz instability-induced turbulent mixing. The use of algebraic relationships between model coefficients and physical observables (e.g., experimental growth rates) following from the self-similar solutions to calibrate a member of a given family of turbulence models is shown. It is demonstrated numerically that the algebraic relations accurately predict the value and variation of physical outputs of a Reynolds-averaged simulation in flow regimes that are consistent with the simplifying assumptions used to derive the solutions. The use of experimental and numerical simulation data on Reynolds stress anisotropy ratios to calibrate a Reynolds stress model is briefly illustrated. The implications of the analytical solutions for future Reynolds-averaged modeling of hydrodynamic instability-induced mixing are briefly discussed. [Preview Abstract] |
Monday, November 20, 2017 11:40AM - 11:53AM |
G21.00006: Manipulating Rayleigh--Taylor Growth Using Adjoints Ali Kord, Jesse Capecelatro It has been observed that initial interfacial perturbations affect the growth of Rayleigh--Taylor (RT) instabilities. However, it remains to be seen to what extent the perturbations alter the RT growth rate. Direct numerical simulations (DNS) provide a powerful means for studying the effects of initial conditions (IC) on the growth rate. However, a brute-force approach for identifying optimal initial perturbations is not practical via DNS. In addition, identifying sensitivity of the RT growth to the large number of parameters used in defining the IC is computationally expensive. A discrete adjoint is formulated to measure sensitivities of multi-mode RT growth to ICs in a high-order finite difference framework. The sensitivity is used as a search direction for adjusting the initial perturbations to both maximize and suppress the RT growth rate during its non-linear regime. The modes that contribute the greatest sensitivity are identified, and optimized perturbation energy spectrum are reported. [Preview Abstract] |
Monday, November 20, 2017 11:53AM - 12:06PM |
G21.00007: Numerical investigation on the effects of acceleration reversal times in Rayleigh-Taylor Instability with multiple reversals Zachary Farley, Denis Aslangil, Arindam Banerjee, Andrew G.W. Lawrie An implicit large eddy simulation (ILES) code, MOBILE, is used to explore the growth rate of the mixing layer width of the acceleration-driven Rayleigh-Taylor instability (RTI) under variable acceleration histories. The sets of computations performed consist of a series of accel-decel-accel (ADA) cases in addition to baseline constant acceleration and accel-decel (AD) cases. The ADA cases are a series of varied times for the second acceleration reversal (t$_{\mathrm{2}})$ and show drastic differences in the growth rates. Upon the deceleration phase, the kinetic energy of the flow is shifted into internal wavelike patterns. These waves are evidenced by the examined differences in growth rate in the second acceleration phase for the set of ADA cases. Here, we investigate global parameters that include mixing width, growth rates and the anisotropy tensor for the kinetic energy to better understand the behavior of the growth during the re-acceleration period. [Preview Abstract] |
Monday, November 20, 2017 12:06PM - 12:19PM |
G21.00008: Large-Eddy Simulations of Rayleigh-Taylor Instability in a Convergent Geometry Brandon Morgan, Wolfgang Black, Jacob McFarland Large-eddy simulation (LES) is performed of a Rayleigh-Taylor mixing layer in a convergent geometry. The harmonic content of a multimode initial condition is varied, and effects of the initial condition on linear and non-linear growth rates are analyzed. Simulations are demonstrated to cover several bubble merger generations, and distance from self-similarity is quantified using the metric proposed by Morgan \emph{et al.} [Morgan, B.E., Olson, B.J., White, J.E., and McFarland, J.A., ``Self-similarity of a Rayleigh-Taylor mixing layer at low Atwood number with a multimode initial perturbation,'' \emph{J. Turbul.}, 2017]. Finally, turbulence profiles are compared against LES from a planar mixing layer and against one-dimensional Reynolds-averaged Navier-Stokes simulation. [Preview Abstract] |
Monday, November 20, 2017 12:19PM - 12:32PM |
G21.00009: Multimodal Perturbation Evolution in the Compressible Rayleigh-Taylor Instability Scott Wieland, Scott Reckinger, Peter Hamlington, Daniel Livescu Explorations of the miscible, compressible, and single mode Rayleigh-Taylor instability (RTI) have shown that the initial type and strength of the background stratification can have a wide range of effects on the growth of the RTI, such as exaggerated bubble and spike asymmetries and complete growth suppression. These effects arise, in part, because background stratification significantly alters the vorticity dynamics of the system in comparison to the incompressible regime. In order to further understand the effects of background stratification on RTI growth and dynamics, high fidelity wavelet-based direct numerical simulations (WDNS) have been performed for an initially isothermal background state at a variety of stratification strengths, where the RTI is initiated using a multimodal perturbation at low Atwood number (i.e., 0.04). The formulation of the multimodal perturbation is outlined and the temporal evolution of the system is described for different stratification strengths. Preliminary results show that, in addition to the effects seen in the single mode regime, background stratification dampens high frequency perturbations, resulting in only the lower wavelength perturbations surviving until late times. [Preview Abstract] |
Monday, November 20, 2017 12:32PM - 12:45PM |
G21.00010: Hydrodynamic and diffusive mixing in ICF implosion modeling Alexander Ames, Chris Weber, Andy Cook Inertial confinement fusion requires efficient spherical compression of a deuterium-tritium gas mixture by a shock-driven implosion. The performance of the implosion is limited by several phenomena, including differential acceleration of deuterium and tritium ions, and mixing due to the Richtmyer-Meshkov and Rayleigh-Taylor instabilities. The MIRANDA radiation hydrodynamics code at LLNL has recently incorporated multi-species diffusion and multi-group radiation transport models. This enables modeling of the impact of diffusive mixing on the fuel, as well as investigation of ablative Rayleigh-Taylor instability growth and resultant hydrodynamic mixing using single-group and multiple-group radiation drives. [Preview Abstract] |
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