Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session L34: Flow Instability: Rayleigh-Taylor II |
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Chair: M. Gad-El-Hak, Virginia Commonwealth University Room: 616 |
Monday, November 25, 2019 1:45PM - 1:58PM |
L34.00001: Effect of Magnetic Fields on the Nonlinear Rayleigh-Taylor Instability Xin Bian, Riccardo Betti, Hussein Aluie The magneto-Rayleigh-Taylor instability (mRTI) plays an important role in inertial fusion schemes, including ICF and magLIF. It is also hypothesized that mRTI is pervasive in the interstellar medium, where it tends to concentrate the plasma into discrete clouds. We investigate numerically the effects of external magnetic fields on RTI during its nonlinear stages in 2D and 3D. We consider magnetic fields oriented in both parallel and perpendicular directions relative to the initial interface. Both magnetic orientations tend to suppress bubble development in 2D but can enhance it in 3D, where we find a non-monotonic dependence on field strength. For example, we observe that a perpendicular magnetic field stronger than a threshold enhances the RTI growth by enhancing the anisotropy of mixing and suppressing horizontal motions, especially at the smallest scales. Moreover, magnetic fields in either direction tend to accentuate the asymmetry between bubbles and spikes [Preview Abstract] |
Monday, November 25, 2019 1:58PM - 2:11PM |
L34.00002: Direct Numerical Simulations of Magnetic Rayleigh-Taylor Instability in ICF Coasting Stage Zhaorui Li, Daniel Livescu The development of hydrodynamic instabilities is generally believed to be one of the main obstacles to achieving economically controlled inertial confinement fusion (ICF). In this study, accurate simulations of 2D magnetic Rayleigh-Taylor instability (RTI) under ICF coasting stage conditions have been conducted with a newly developed high-order two-fluid plasma solver (Li and Livescu, 2019), in which full transport terms, including temperature and magnetic field dependent ion and electron heat fluxes and viscous stresses, are implemented. The numerical results show that, for the initial configuration with small Atwood number (0.33) and high hot-spot temperature (10 keV), the extremely large heat conduction and viscous stresses completely suppress the RTI development. Instead, the late-time RTI and magnetic field can grow to structures close to those seen in previous studies, which relied on numerical transport to regularize the equations, only when unrealistically small transport coefficients are used for calculating heat conductivity and viscosity. Further simulations with realistic transport phenomena are being conducted with more relevant ICF coasting stage conditions (e.g. Weber \textit{et al}. 2014) in which Atwood number is 0.875 and hot-spot temperature is 2.25 keV. [Preview Abstract] |
Monday, November 25, 2019 2:11PM - 2:24PM |
L34.00003: Rayleigh-Taylor instability with sinusoidal acceleration histories Zachary Farley, Denis Aslangil, Arindam Banerjee, Andrew G.W. Lawrie We will discuss the effects of accel-decel-accel (ADA) acceleration histories on the Rayleigh-Taylor instability (RTI) using a sinusoidal acceleration profile. An implicit large eddy simulation technique based massively parallel code was used for the purpose. Majority of the reported studies in scientific literature use an acceleration profile that consists of a series of step-functions. However, it is conjectured that the sinusoidal profile would better represent the transitions between accel and decel phases in real applications. We will present our findings of comparisons between the two sets of acceleration histories. In RTI with variable acceleration studies, a length scale, Z(t) has been commonly used and is defined as the double integration over time of the time variable acceleration, g(t). The flow evolution of the two profiles due to variations in Z(t) will be discussed. In addition, global parameters to measure the growth of the instability and turbulence statistics that characterize the internal mechanics of the dynamically accelerated and decelerated phases of the RTI mixing layer will be discussed. [Preview Abstract] |
Monday, November 25, 2019 2:24PM - 2:37PM |
L34.00004: Temperature-Gradient Effects on Taylor--Couette Flows M. Khirennas, H. Oualli, M. Mekadem, A. Benaiche, T. Azzam, A. Bouabdallah, M. Gad-El-Hak A numerical investigation of a Taylor--Couette flow with a heated rotating inner cylinder and an isothermal stationary outer one. We focus on the effect of temperature gradients on the first instability formation. The calculations are implemented on FLUENT based on the finite-volume method. The basic system geometry is characterized by its height, ratio of inner-to-outer cylinder radii, radial gap length, and aspect ratio. For validation, the present results compare well to the high-order DNS computations of Viazzo \& Poncet ({\it Computers \& Fluids}, vol.\ 101, pp.\ 15--26, 2014). The genesis mechanism of Ekman and Taylor vortices is revisited herein to shed light on the temperature-gradient effects on the flow restructuring. It is concluded that the flow behavior exhibits considerable sensitivity to temperature gradient, leading to strong stabilizing effect. The flow topology is found to shift instantly to the known B\â€™enard convective cell for all flow evolution stages even at the end of the restructuring process. The critical Taylor number is substantially increased according to the superimposed temperature gradient as characterised by Rayleigh number. An increase of 30\% in the critical Taylor number is observed when Rayleigh number is 7,150. [Preview Abstract] |
Monday, November 25, 2019 2:37PM - 2:50PM |
L34.00005: Numerical simulation of droplet formation by Rayleigh-Taylor instability in multiphase corium Raphael Zanella, Herve Henry, Romain Le Tellier, Mathis Plapp During a severe accident in a nuclear reactor, the melting of the core forms a multiphase pool (corium), where the heat transfer at the boundary is affected by the segregation of the metallic and oxidic liquid phases driven by chemical and convective mass fluxes [1]. We use a Cahn-Hilliard pseudo-binary model to describe the uranium/zirconium/oxygen/iron mixture. The diffusion and the convection are governed by the Cahn-Hilliard equation and the Navier-Stokes equations with the buoyancy and capillary forces. Also used for hydrodynamic coarsening [2], the model is solved in 2D and 3D with a spectral code. The initial configuration of a lighter layer of iron-rich fluid above a heavier layer of uranium/zirconium/oxygen mixture is mechanically stable. However, as diffusion progresses, the heavier metallic phase form at the interface. Due to the Rayleigh-Taylor instability, droplets of metallic phase grow and fall into the underneath layer with a fixed frequency. The droplet formation observed in a former experiment of corium stratification transient [3] is well captured. [1] C. Cardon, R. Le Tellier, M. Plapp. CALPHAD 52, 2016. [2] H. Henry, G. Tegze. Phys. Rev. Fluids 3, 2018. [3] Kurchatov Institute. RCW Post-Test Analysis Results (OECD report), 2003 [Preview Abstract] |
Monday, November 25, 2019 2:50PM - 3:03PM |
L34.00006: Direct Numerical Simulations of Combined Rayleigh-Taylor/Shear Flow to Late Times Jon Baltzer, Daniel Livescu Rayleigh-Taylor instability between two fluids of differing densities occurs when the density gradient is misaligned with the pressure gradient. Background shear may also be present in applications such as ICF. Shear itself can also trigger instabilities of the Kelvin-Helmholtz type. Olson \emph{et.~al.}~(Phys. Fluids, 2011) previously simulated combined Rayleigh-Taylor instability and shear, and they found that shear produced complex and non-monotonic changes to the growth rate in the early nonlinear regime, in contrast to the simple increase predicted by linear stability theory. New direct numerical simulations are performed to determine how the interactions of buoyant production and shear affect the structure of turbulence at later times. The density ratio of the fluids is 7. The configuration is similar to Rayleigh-Taylor instability studies, but introducing shear affects transitional structures and continues to significantly change the statistics of density fluctuations and Reynolds stresses at late times. Statistics and budgets associated with this flow are important for variable-density turbulence modeling. [Preview Abstract] |
Monday, November 25, 2019 3:03PM - 3:16PM |
L34.00007: PLIF Measurements and Interface Stretching in the Blast-Driven Instability Benjamin Musci, Samuel Petter, Gokul Pathikonda, Devesh Ranjan The presented work focuses on the preliminary implementation of Planar Laser Induced Fluorescence (PLIF) to study the Blast-Driven Instability (BDI) in cylindrical geometry at the Georgia Tech Shock Tube and Advanced Mixing Laboratory. By using detonators to generate blast waves, a gaseous interface is subject to the combined Richtmyer-Meshkov (RMI) and Rayleigh-Taylor Instabilities (RTI); the two instabilities comprising the BDI. Previous validation of the facility was completed using high speed Mie Scattering and demonstrated faithful reproduction of the phenomena of the BDI. Previously completed Mie Scattering measurements will be used in conjunction with PLIF to measure the degree of inter-facial stretching for bubble-spike pairs in this facility. These efforts will be put toward the implementation of simultaneous PLIF and PIV for full flow field measurements. [Preview Abstract] |
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