Jacob McFarland

Computational study of the shock driven instability of a multiphase particle-gas system

This paper considers the interaction of a shock wave with a multiphase particle-gas system which creates an instability similar in some ways to the Richtmyer-Meshkov instability but with a larger parameter space. As this parameter space is large, we only present an introductory survey of the effects of many of these parameters. We highlight the effects of particle-gas coupling, incident shock strength, particle size, effective system density differences, and multiple particle relaxation timeeffects. We focus on dilute flows with mass loading up to 40% and do not attempt to cover all parametric combinations. Instead, we vary one parameter at a time leaving additional parametric combinations for future work. The simulations are run with the Ares code, developed at Lawrence Livermore National Laboratory, which uses a multiphase particulate transport method to model two-way momentum and energy coupling. A brief validation of these models is presented and coupling effects are explored. It is shown that even for small particles, on the order of 1 μm, multi-phase coupling effects are important and diminish the circulation deposition on the interface by up to 25%. These coupling effects are shown to create large temperature deviations from the dusty gas approximation, up to 20% greater, especially at higher shock strengths. It is also found that for a multiphase instability, the vortex sheet deposited at the interface separates into two sheets. Depending on the particle and particle-gas Atwood numbers, the instability may be suppressed or enhanced by the interactions of these two vortex sheets.

Simulations and Analysis of the Reshocked Inclined Interface Richtmyer–Meshkov Instability for Linear and Nonlinear Interface Perturbations

A computational study of the Richtmyer–Meshkov instability (RMI) is presented for an inclined interface perturbation in support of experiments being performed at the Texas A&M shock tube facility. The study is comprised of 2D, viscous, diffusive, compressible simulations performed using the arbitrary Lagrange Eulerian code, ARES, developed at Lawrence Livermore National Laboratory. These simulations were performed to late times after reshock with two initial interface perturbations, in the linear and nonlinear regimes each, prescribed by the interface inclination angle. The interaction of the interface with the reshock wave produced a complex 2D set of compressible wave interactions including expansion waves, which also interacted with the interface. Distinct differences in the interface growth rates prior to reshock were found in previous work. The current work provides in-depth analysis of the vorticity and enstrophy fields to elucidate the physics of reshock for the inclined interface RMI. After reshock, the two cases exhibit some similarities in integral measurements despite their disparate initial conditions but also show different vorticity decay trends, power law decay for the nonlinear and linear decay for the linear perturbation case.

Non-uniform volumetric structures in Richtmyer-Meshkov flows

We perform an integrated study of volumetric structures in Richtmyer-Meshkov (RM) flows induced by moderate shocks. Experiments, theoretical analyses, Smoothed Particle Hydrodynamics simulations, and ARES Arbitrary Lagrange Eulerian simulations are employed to analyze RM evolution for fluids with contrast densities in case of moderately small amplitude initial perturbation at the fluid interface. After the shock passage the dynamics of the fluids is a superposition of the background motion and the interfacial mixing, and only a small part of the shock energy is available for interfacial mixing. We find that in the fluid bulk the flow fields are non-uniform at small scales, and the heterogeneous volumetric structures include reverse jets, shock-focusing effects, and local hot spots with the temperature substantially higher than that in the ambient.

Investigation of the initial perturbation amplitude for the inclined interface Richtmyer?Meshkov instability

A simulation studying the effects of inclination angle and incident shock Mach number on the inclined interface Richtmyer–Meshkov instability is presented. Interface inclination angle is varied from 30° to 85°, with incident shock Mach numbers of 1.5, 2.0 and 2.5 for an air over SF6 interface. The simulations were performed in support of experiments to be performed in the Texas A&M shock tube facility, and were created with the ARES code developed at Lawrence Livermore National Laboratory. The parametric cases are separated by inclination angle into nonlinear and linear initial perturbation cases. A linear initial perturbation is defined as when the interface amplitude over wavelength is less than 0.1. Density, pressure gradient and vorticity plots are presented for a nonlinear and a linear case to highlight the differences in the flow field evolution. It is shown that the nonlinear case contains strong secondary compressible effects which reverberate through the interface until late times, while in the linear case these waves are almost completely absent. The inclined interface scaling method presented in previous work (McFarland et al 2011 Phys. Rev. E 84 026303) is tested for its ability to scale the mixing width growth rate for linear initial perturbation cases. This model was shown in the previous work to collapse data well for varying Mach numbers and nonlinear inclination angles. The scaled data is presented to show that a regime change occurs in the mixing width growth rate near an inclination angle of 80° which corresponds to the transition from a linear to nonlinear initial perturbation.

Self-similarity of a Rayleigh–Taylor mixing layer at low Atwood number with a multimode initial perturbation

ABSTRACT High-fidelity large eddy simulation (LES) of a low-Atwood number (A = 0.05) Rayleigh–Taylor mixing layer is performed using the 10th-order compact difference code Miranda. An initial multimode perturbation spectrum is specified in Fourier space as a function of mesh resolution such that a database of results is obtained in which each successive level of increased grid resolution corresponds approximately to one additional doubling of the mixing layer width, or generation. The database is then analysed to determine approximate requirements for self-similarity, and a new metric is proposed to quantify how far a given simulation is from the limit of self-similarity. It is determined that mixing layer growth reaches a high degree of self-similarity after approximately 4.5 generations. Statistical convergence errors and boundary effects at late time, however, make it impossible to draw similar conclusions regarding the self-similar growth of more sensitive turbulence parameters. Finally, self-similar turbulence profiles from the LES database are compared with one-dimensional simulations using the k-L-a and BHR-2 Reynolds-averaged Navier–Stokes models. The k-L-a model, which is calibrated to reproduce a quadratic turbulence kinetic energy profile for a self-similar mixing layer, is found to be in better agreement with the LES than BHR-2 results.

A numerical method for shock driven multiphase flow with evaporating particles

Abstract A numerical method for predicting the interaction of active, phase changing particles in a shock driven flow is presented in this paper. The Particle-in-Cell (PIC) technique was used to couple particles in a Lagrangian coordinate system with a fluid in an Eulerian coordinate system. The Piecewise Parabolic Method (PPM) hydrodynamics solver was used for solving the conservation equations and was modified with mass, momentum, and energy source terms from the particle phase. The method was implemented in the open source hydrodynamics software FLASH, developed at the University of Chicago. A simple validation of the methods is accomplished by comparing velocity and temperature histories from a single particle simulation with the analytical solution. Furthermore, simple single particle parcel simulations were run at two different sizes to study the effect of particle size on vorticity deposition in a shock-driven multiphase instability. Large particles were found to have lower enstrophy production at early times and higher enstrophy dissipation at late times due to the advection of the particle vorticity source term through the carrier gas. A 2D shock-driven instability of a circular perturbation is studied in simulations and compared to previous experimental data as further validation of the numerical methods. The effect of the particle size distribution and particle evaporation is examined further for this case. The results show that larger particles reduce the vorticity deposition, while particle evaporation increases it. It is also shown that for a distribution of particles sizes the vorticity deposition is decreased compared to single particle size case at the mean diameter.

Investigation of Buoyancy Effects on Heat Transfer Characteristics of Supercritical Carbon Dioxide in Heating Mode

Experiments were performed to investigate the effects of buoyancy on heat transfer characteristics of supercritical carbon dioxide in heating mode. Turbulent flows with Reynolds numbers up to 60,000, at operating pressures of 7.5, 8.1, and 10.2 MPa, were tested in a round tube. Local heat transfer coefficients were obtained from measured wall temperatures over a large set of experimental parameters that varied inlet temperature from 20 to 55°C, mass flux from 150 to 350 kg/m2s, and a maximum heat flux of 65 kW/m2. Horizontal, upward, and downward flows were tested to investigate the unusual heat transfer characteristics due to the effect of buoyancy and flow acceleration caused by large variation in density. In the case of upward flow, severe localized deterioration in heat transfer was observed due to reduction in the turbulent shear stress and is characterized by a sharp increase in wall temperature. In the case of downward flow, turbulent shear stress is enhanced by buoyancy forces, leading to an enhancement in heat transfer. In the case of horizontal flow, flow stratification occurred, leading to a circumferential variation in wall temperature. Thermocouples mounted 180° apart on the tube revealed that the wall temperatures on the top side are significantly higher than the bottom side of the tube. Buoyancy factor calculations for all the test cases indicated that buoyancy effects cannot be ignored even for horizontal flow at Reynolds numbers as high as 20,000. Experimentally determined Nusselt numbers are compared to existing correlations available in the literature. Existing correlations predicted the experimental data within ±30%, with maximum deviation around the pseudocritical point.

Simulations and Experimental Investigation of the Inclined Interface Richtmyer-Meshkov Instability

The Richtmyer-Meshkov instability (RMI) [1, 2], forms when a shock wave interacts with a misaligned density gradient created by a fluid interface. This instability can be viewed as the result of baroclinic vorticity (eq. 1) generated at the interface from the misalignment of the pressure and density gradients.

Simulations and Analysis of Shock Accelerated Inhomogenous Flows With and Without Reshock

A computational study of the Richtmyer-Meshkov instability is presented for an inclined interface perturbation in support of experiments being performed at the Texas A&M shock tube facility. The study is comprised of simulations performed using the Arbitrary Lagrange Eulerian (ALE) code called ARES. These simulations were performed to late times after reshock with varying parameters including inclination angle, and incident shock Mach number. The inclination angle was varied over a wide range which provided initial interface perturbations in both the linear and non-linear regimes. Recent work have shown a distinct difference between the linear and nonlinear interface perturbations growth using a newly developed inclined interface scaling model. This work is extended here by examining the vorticity distribution for these two cases and focusing on the conditions before and after reshock. One linear and one non-linear interface perturbation case are examined qualitatively through plots of the vorticity, and density fields. The total circulation and circulation production rates for these cases are plotted as a function of time. The circulation is shown to double after reshock for the non-linear case, while for the linear case it increases by approximately the same amount as the non-linear case but from near zero just before reshock. The mixing width, and mix mass growth rates are also examined for each case both before and after reshock.© 2012 ASME

COLLISION DYNAMICS OF EXCITED SODIUM MOLECULES

Collision cross section for transfer of anisotropy arising from collisions between electronically excited sodium dimer and ground level argon atoms has been examined. The experimental method is based on a polarization spectroscopy using a sophisticated resonant cw-pump-stimulated emission probe technique. Measurement of polarization from analysis of the emitted light is a very powerful method gaining information about the inelastic collision process between the electronically excited molecules and other collision partners. From the measurement, anisotropy-dependent polarization spectra of the Na2 with Ar has been investigated. a