Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session G28: Flow Instability: Rayleigh-Taylor/Richtmyer-Meshkov II |
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Chair: Praveen Ramaprabhu, University of North Carolina, Charlotte Room: Georgia World Congress Center B316 |
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Monday, November 19, 2018 10:35AM - 10:48AM |
G28.00001: Self-Similar Solutions to Three- and Four-Equation Reynolds-Averaged Models for Rayleigh−Taylor Mixing and Comparisons to Numerical Simulation and Experimental Data Oleg Schilling Analytical self-similar solutions to three- and four-equation mechanical/scalar turbulence models [Schilling and Mueschke, Physical Review E 96, 063111 (2017)], similar to those used in turbulent combustion, are presented for Rayleigh-Taylor mixing. The mechanical turbulence is described by the turbulent kinetic energy, K, and its dissipation rate, ε, and the scalar turbulence is described by a scalar variance, S, and its dissipation rate, χ. The addition of the scalar variables gives a prediction for the molecular mixing parameter, θ, as a function of the model coefficients. The spatiotemporal evolution of the mean and turbulent fields is illustrated. The growth and mixing parameters, ratio of turbulent kinetic energy to released potential energy, and production-to-dissipation ratios are obtained as a function of the model coefficients and are compared to their values from direct numerical simulation of a small Atwood number Rayleigh−Taylor mixing experiment. The agreement between the model and data are quite good, with the four-equation model predictions in better agreement with data than the three-equation model predictions. |
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Monday, November 19, 2018 10:48AM - 11:01AM |
G28.00002: Linear stability analysis of two fluid columns of different densities and viscosities subject to gravity Aditya Heru Prathama, Carlos Pantano We investigate the linear stability of a vertical interface between two fluid columns of different densities and viscosities under the influence of gravity. This work continues from the inviscid analysis presented last year, which showed that the interface was unconditionally unstable at all wave modes, despite the presence of surface tension, and that instability grew as the exponential of a quadratic function of time. Currently, we initially employ the quasi-static approach based on frozen approximation of the base flow and solve the eigenvalue problem. The eigenmodes then act as initial conditions for the initial value problem of the time-dependent base flow. Preliminary results indicate that perturbations: i) grow as the exponential of quadratic function of time at small wavenumbers; ii) grow with rate less than that of i) as wavenumbers increase; iii) decay at large wavenumbers as viscous effects become dominant; iv) are neutrally stable at lower wavenumbers than predicted by eigenvalue analysis. Results will be compared to the asymptotic solutions for validation. The role of varying Reynolds number, density and viscosity ratios will be presented. |
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Monday, November 19, 2018 11:01AM - 11:14AM |
G28.00003: On the effects of variable deceleration periods on Rayleigh-Taylor instability with multiple acceleration reversals Denis Aslangil, Zachary K Farley, Arindam Banerjee, Andrew G.W. Lawrie We will discuss results from our numerical simulations to study the effects of time-varying stabilizing deceleration periods on the Rayleigh-Taylor instability (RTI) when subjected to multiple acceleration reversals at different Atwood numbers (0.05 and 0.5). A massively parallel implicit large eddy simulation code, MOBILE was used to simulate the multiple reversal (accel-decel-accel or ADA) cases; the results were compared with a single reversal (accel-decel or AD) and no reversal (classical constant gravity) case. During the deceleration period, evidence of internal wave-like patterns can be observed inside the mixing layer. The re-acceleration point was varied based on the phase of the internal waves generated in the AD cases. This allowed for a series of ADA cases with varying deceleration periods that highlight possible effects of the wavelike motion during the deceleration phase on the late time growth of the instability after re-acceleration. To better characterize and compare instability growth after re-acceleration, multiple parameters were analyzed, including the integral mixing width, growth rates, and anisotropic tensor for the kinetic energy. |
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Monday, November 19, 2018 11:14AM - 11:27AM |
G28.00004: Suppression of Rayleigh-Taylor turbulence by time-periodic acceleration Guido Boffetta, Marta Magnani, Stefano Musacchio Turbulent convection, in the framework of Rayleigh-Taylor turbulence, is studied in the presence of an alternating, time periodic acceleration. By means of extensive direct numerical simulations of the Boussinesq equations, we discover a new mechanism of relaminarization of turbulence: The alternating acceleration, which initially produces a growing turbulent mixing layer, at longer times suppresses turbulent fluctuation and drives the system toward an asymptotic stationary configuration. Dimensional arguments and linear stability theory are used to predict the width of the mixing layer in the asymptotic state as a function of the period of the acceleration. |
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Monday, November 19, 2018 11:27AM - 11:40AM |
G28.00005: Compressible single-fluid single-mode Rayleigh-Taylor instability Xin Bian, Daniel Livescu, Hussein Aluie We study the single-fluid single-mode Rayleigh-Taylor instability (RTI) growth rate in the fully compressible regime in 2D and 3D using high resolution simulations. The single fluid set-up, where density differences arise from temperature variations, is important for the deceleration stage in Inertial Confinement Fusion. We systematically analyze the effects of perturbation Reynolds number ($Re_p$) and Atwood number ($A$).
Consistent with previous studies of the incompressible RTI [Wei and Livescu, Phys. Rev. E (2012), Ramaprabhu et al., Phys. Fluids (2012)], we find that at low to moderate Atwood numbers, the bubble re-accelerates and does not saturate at late times when $Re_p$ is large enough, contrary to potential flow theory prediction. The simulations further show that compared to 2D RT, RTI in 3D develops faster and re-accelerates at smaller $Re_p$ and larger $A$. At high Atwood number, while the bubble does not exhibit a clear re-acceleration regime over the times and $Re_p$ we analyzed, its velocity fluctuates with increasing amplitudes at higher $Re_p$. An analysis of the vorticity dynamics suggests that at high Atwood numbers, a re-acceleration regime would set in at sufficiently high $Re_p$.
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Monday, November 19, 2018 11:40AM - 11:53AM |
G28.00006: Validation of a two-point spectral turbulence model for inhomogeneous flows Ismael Djibrilla Boureima, Nairita Pal, Susan Kurien, Praveen K Ramaprabhu, Timothy T Clark, Andrew Lawrie Implicit Large Eddy simulations (ILES) of an inhomogeneous variable density (VD) flow in the form of Rayleigh-Taylor (RT) turbulence is used to validate and calibrate a spectral turbulence model. The two-point, spectral model is built on the approach laid out in the BHRZ1 method, and computes the time evolution of two-point correlations of the density fluctuations with the momentum and specific volume. For such flows, earlier efforts2 have shown the central role played by the density-specific volume correlation b in the production of mass flux a, which in turn drives the conversion of buoyant potential energy into kinetic energy E. The time evolution of spectral distributions of a(k), b(k), and E(k), where k is the wavenumber, are predicted by the spectral closure model and compared with data from ILES. The RT system is initialized with a multimode perturbation at the interface between a light fluid accelerated into a heavier fluid against gravity. The simulations were performed using the MOBILE software3 at a resolution of 512 x 512 x 2048 zones. 1D.C. Besnard, F. Harlow, R. Rauenzahn and C. Zemach, Theor. Comp. Dyn., 8, 1-35, (1996) 2D. Livescu and J.R. Ristorcelli, J. Fluid Mech., 591, 43-71, (2007) 3A.G.W. Lawrie and S.B. Dalziel, Phys. Fluids, 23, 085109 (2011) |
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Monday, November 19, 2018 11:53AM - 12:06PM |
G28.00007: Gravity-driven multiphase instability Peter V Vorobieff, Patrick J Wayne, Sumanth Reddy Lingampally, Gregory Vigil, Daniel Freelong, C Randall Truman Relatively recently, shock-driven multiphase instability (SDMI) was identified in multiphase flows and shown to have both similarities with and differences from Richtmyer-Meshkov instability (RMI). A multiphase analog of Rayleigh-Taylor instability was described in 2011 for a situation when air sparsely seeded with glycol droplets was placed above a volume of unseeded air, producing an unstably stratified average density distribution that was characterized by an effective Atwood number 0.03. In that case, the evolution of the instability was indistinguishable from single-phase RTI with the same Atwood number, as the presence of the droplets largely acted as an additional contribution to the mean density of the gaseous medium. Our recent experiments investigate a situation when the volume (and mass) fraction of the seeding particles in gas is considerably higher, and the gravity-driven flow is dominated by the particle movement. In this case, we still observe an interfacial instability. We present data characterizing its morphology and growth rate, and discuss the relevant physics. |
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Monday, November 19, 2018 12:06PM - 12:19PM |
G28.00008: Stability of a cylindrically imploding liquid cavity formed through a honeycomb mesh Jovan Nedic, Justin Huneault, Joerg Zimmermann, Andrew J Higgins A gas-filled cylindrical cavity is created by rotating a liquid to solid body rotation, thus creating a cylindrical liquid shell surrounding the gas-filled cavity. The liquid shell is then radially imploded causing the gas-filled cavity to compress and hence collapse. It has been shown (DFD18-002341) that the angular velocity of the solid body rotation plays a significant role in the stability of the Rayleigh-Taylor driven perturbation growth on the cavity surface. In this talk, an initial perturbation is created by pushing the rotating liquid through a cylindrical honeycomb mesh i.e. one in which the depth of the mesh, d, is several times larger than the mesh size, M, and wall thickness t; as the fluid emerges from the honeycomb mesh, a series of jet interactions occur creating the initial perturbation. We experimentally investigate the effects of initial liquid depth, relative to the honeycomb mesh surface, on the stability of the imploding cavity surface. The cavity surface was measured using high-speed videography for a range of liquid depths ranging between -M (i.e. the liquid started inside the honeycomb) to M (liquid is one mesh length in front), sufficient to capture the change from a rough cavity surface to a smooth interface. |
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Monday, November 19, 2018 12:19PM - 12:32PM |
G28.00009: Rotational Stabilization of a Rayleigh-Taylor Unstable Cylindrical Implosion Justin Huneault, Victoria Suponitsky, David Plant, Andrew J Higgins The collapse of cavities within liquids is of relevance to a number of applications, including magnetized target fusion schemes in which a plasma is compressed by an imploding liquid metal surface. This paper examines the Rayleigh-Taylor (RT) driven growth of perturbations on the surface of an imploding cylindrical liquid shell that compresses a gas-filled cavity. The fast rise in pressure at the point of maximum convergence causes the cavity surface to rapidly decelerate in the direction opposite to the density gradient at the gas-liquid interface, which induces the RT-driven growth of perturbations. This perturbation growth may be suppressed by rotating the liquid shell at a sufficient angular velocity such that the net surface acceleration remains aligned with the interface density gradient throughout the implosion. This paper will examine the effect of fluid rotation on the growth of small and large mode number perturbations using an experimental arrangement that allowed for visualization of the cavity surface. Experiments were performed over a range of initial angular velocities, demonstrating both stabilized and under-stabilized implosions. These results were compared to 3-D simulations of the implosion performed in OpenFOAM. |
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Monday, November 19, 2018 12:32PM - 12:45PM |
G28.00010: Evaporation driven Rayleigh-Taylor instability in aqueous polymer solutions Endre Joachim Mossige, Vineeth Chandran Suja, Meiirbek Islamov, Sam Fox Wheeler, Gerald G. Fuller The Rayleigh-Taylor (RT) instability can occur when a heavy fluid rests on top of lighter fluid and is often observed as plumes of the heavier fluid descending through the lighter fluid. This instability has been well described for immiscible and miscible systems, however few studies have focused on polymer solutions, which are important for many applications. Here we study the RT instability in aqueous polymer solutions containing dextran and polyethylene glycol. By means of optical measurements, we show that unstratified aqueous polymer solutions can spontaneously exhibit the RT instability when exposed to air. We further show that the instability is driven by evaporation of water, which results in the creation of a dense, polymer-rich layer above the bulk fluid. Hence, unlike in previously reported RT instabilities, this evaporation driven instability is sustained continuously as long as evaporation is present. By varying the diffusivity, viscosity and density of our polymer solutions, we further characterize the onset and evolution of the spontaneous RT instability. |
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