Effects Of Initial Perturbations Research Articles

Three-dimensional simulations of reshocked inclined Richtmyer-Meshkov instability: Effects of initial perturbations

A Mach 1.55 shock and subsequent reshock interacting with predominantly single-mode and multi-mode interfaces between nitrogen and carbon dioxide is studied through three-dimensional adaptive mesh refinement simulations using the FLASH code. The simulations are directly compared with the PLIF/PIV experimental measurements to validate the computational code. Development of instability after first shock and chaotic turbulent mixing after reshock are discussed for the entire shock tube domain.

Effects of the initial perturbations on the Rayleigh—Taylor—Kelvin—Helmholtz instability system

In the paper, the effects of initial perturbations on the Rayleigh-Taylor instability (RTI), Kelvin-Helmholtz instability (KHI), and the coupled Rayleigh-Taylor-Kelvin-Helmholtz instability (RTKHI) systems are investigated using a multiple-relaxation-time discrete Boltzmann model. Six different perturbation interfaces are designed to study the effects of the initial perturbations on the instability systems. Based on the mean heat flux strength $D_{3,1}$, the effects of initial interfaces on the coupled RTKHI are examined in detail. The research is focused on two aspects: (i) the main mechanism in the early stage of the RTKHI, (ii) the transition point from KHI-like to RTI-like for the case where the KHI dominates at earlier time and the RTI dominates at later time. It is found that the early main mechanism is related to the shape of the initial interface, which is represented by both the bilateral contact angle $\theta_{1}$ and the middle contact angle $\theta_{2}$. The influence of inverted parabolic and inverted ellipse perturbations ($\theta_{1}<90$) on the transition point of the RTKHI system is greater than that of other interfaces.

Effects of Initial Perturbations in the Early Moments of an Explosive Dispersal of Particles

Recent experiments have shown that when a dense layer of solid particles surrounding a high-energy reactive material is explosively dispersed, the particles cluster locally leading to jetlike patterns. The formation of these coherent structures has yet to be fully understood and is believed to have its origin in the early moments of the explosive dispersal. This paper focuses on the early moments of an explosive dispersal of particles. In particular, the effect of initial perturbations on both the gas and particulate phase is investigated, considering heavy particles with a low initial particle volume fraction. Two-dimensional simulations are carried out, and results suggest that a distinctive heterogeneity in the form of a single wavelength perturbation in the rapidly expanding detonation products does not have a significant impact on the early evolution of neither the gas phase nor the cloud of particles. In contrast, the equivalent distinctive heterogeneity in the initial particle volume fraction distribution lingers for the duration of our simulations. Developing instabilities in the gas phase and at the inner- and outer-most front of the particle bed display a dominant wavelength equal to the wavelength of the initial perturbation in the particle volume fraction.

Effect of initial perturbations on the simulation capability of numerical models

The effect of an initial perturbation on simulation is studied in a series of numerical experiments using a one-dimensional nonlinear advection equation model. It is shown that the capability of the model tends to decline with an increase in initial perturbation. The error in the initial perturbation grows in a nonlinear manner, indicating that the simulating results can be improved only to a limited degree by increasing the accuracy of the initial data.

Accuracy and nonuniqueness aspects of numerical solutions of some natural convection problems

This paper examines the source of machine and mesh sensitivity and the effect of initial perturbations on the onset and development of natural convection in fluids with unstable density stratification due either to thermal or solutal gradients. Numerical simulations are presented for two problems: the liquid phase epitaxial process used to grow semiconducting crystals and the Rayleigh Bénard problem. The simulations are performed by solving the unsteady form of the momentum equations in their stream-function vorticity form coupled with the unsteady scalar (temperature or solute) transport equation. The higher than usual mesh and round-off error sensitivity exhibited by the numerical solutions of these buoyancy-driven flows is investigated by comparing high resolution solutions on different machines and solutions on different meshes. It is shown that flows initiated ‘spontaneously’, without controlled perturbation, are in reality not spontaneous but rather the result of numerical noise. The onset and development of convection in such solutions are delayed and depend on both the computational grid and the machine. It is shown that this dependency is eliminated by imposing proper initial conditions such as controlled initial perturbations large enough to dominate numerical noise.

Initiation and assembly of the plasma in a plasma flow switch

A series of fast opening switch experiments was carried out on the Pegasus pulsed power facility. The first series of experiments characterized the assembly and run down of a 50 mg plasma flow switch operating in the 6 MA regime. The switch plasma arises from the vaporization of a wire array and a barrier foil. Arrays with different numbers of wires were examined. Two-dimensional (2-D) radiation magnetohydrodynamic simulations of the initial vaporization stage of the array plasma and its assembly on the barrier foil are presented, which include the effects of initial perturbations on the evolution of the switch plasma as it moves down a coaxial gun barrel. Sparse wire arrays were observed to perform poorly. A change in the experimental design, motivated by 2-D simulations, led to improved performance. Computational results which lead to these changes are discussed, and the results are compared with experimental data.