DNS of the thermal effects of laser energy deposition in isotropic turbulence
TL;DR: In this paper, the effect of energy deposition on an isolated vortex is studied as a first step towards plasma/turbulence interaction using numerical simulations using air as the working fluid and assume local thermodynamic equilibrium.
Abstract: The interaction of a laser-induced plasma with isotropic turbulence is studied using numerical simulations. The simulations use air as the working fluid and assume local thermodynamic equilibrium. The numerical method is fully spectral and uses a shock-capturing scheme in a corrector step. A model problem involving the effect of energy deposition on an isolated vortex is studied as a first step towards plasma/turbulence interaction. Turbulent Reynolds number Reλ = 30 and fluctuation Mach numbers Mt = 0.001 and 0.3 are considered. A tear-drop-shaped shock wave is observed to propagate into the background, and progressively become spherical in time. The turbulence experiences strong compression due to the shock wave and strong expansion in the core. This behaviour is spatially inhomogeneous and non-stationary in time. Statistics are computed as functions of radial distance from the plasma axis and angular distance across the surface of the shock wave. For Mt = 0.001, the shock wave propagates on a much faster time scale compared to the turbulence evolution. At Mt of 0.3, the time scale of the shock wave is comparable to that of the background. For both cases the mean flow is classified into shock formation, shock propagation and subsequent collapse of the plasma core, and the effect of turbulence on each of these phases is studied in detail. The effect of mean vorticity production on the turbulent vorticity field is also discussed. Turbulent kinetic energy budgets are presented to explain the mechanism underlying the transfer of energy between the mean flow and background turbulence.
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11 Jan 2021
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03 Jan 2022
19 Jan 2023
TL;DR: In this paper , the effect of viscosity on the resulting flow field and on drag reduction is studied. And the optimum pulsing frequency for maximum viscous drag reduction was investigated.
Abstract: Pulsed laser energy deposition in viscous supersonic flow over a half-cylinder is studied using numerical simulation. A finite volume based flow solver is developed using the AUSMDV scheme. The effect of viscosity on the resulting flow field and on drag reduction is studied. The optimum pulsing frequency for maximum viscous drag reduction is investigated. Various parameters affecting the flow are identified and their effects are studied.
01 Jan 2023
TL;DR: In this paper , a numerical algorithm for compressible turbulent flows is discussed in which the discrete equations reduce to the incompressible equations at low Mach numbers, and the discretization conserves kinetic energy in the inviscid incompressibility limit.
Abstract: DNS/LES of compressible turbulent flows is challenged by the nonlinear numerical instabilities that arise at high Reynolds number, the existence of supersonic and highly subsonic regions, and unsteady shock waves. This chapter discusses solutions to these challenges from the author's perspective to simulate complex flows with fidelity comparable to that of canonical flows. A numerical algorithm for compressible turbulence is discussed in which the discrete equations reduce to the incompressible equations at low Mach numbers, and the discretization conserves kinetic energy in the inviscid incompressible limit. Illustrative examples are shown along with details of the formulation. A characteristic-filtering methodology for shock capturing is presented for unstructured grids. The filter is implemented in a predictor–corrector formulation and can be combined with several different base schemes. Examples are shown of capturing strong shock waves in a Fourier pseudospectral code, using a conservative, nondissipative predictor step on unstructured grids, and using a non-conservative, nondissipative predictor step. Finally, these ideas are extended to nonideal gases and fluid mixtures, to simulate multiphase cavitating flows on unstructured grids.
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TL;DR: In this paper, the Eulerian time correlation coefficient of turbulent velocities passed through matched narrow-band niters shows a strong dependence on nominal filter frequency (∼ wave-number at these small turbulence levels).
Abstract: Space-time correlation measurements in the roughly isotropic turbulence behind a regular grid spanning a uniform airstream give the simplest Eulerian time correlation if we choose for the upstream probe signal a time delay which just ‘cancels’ the mean flow displacement. The correlation coefficient of turbulent velocities passed through matched narrow-band niters shows a strong dependence on nominal filter frequency (∼ wave-number at these small turbulence levels). With plausible scaling of the time separations, a scaling dependent on both wave-number and time, it is possible to effect a good collapse of the correlation functions corresponding to wave-numbers from 0·5 cm−1, the location of the peak in the three-dimensional spectrum, to 10 cm−1, about half the Kolmogorov wave-number. The spectrally local time-scaling factor is a ‘parallel’ combination of the times characterizing (i) gross strain distortion by larger eddies, (ii) wrinkling distortion by smaller eddies, (iii) convection by larger eddies and (iv) gross rotation by larger eddies.
991 citations
TL;DR: In this paper, an approach which closely maintains the non-dissipative nature of classical fourth or higher-order spatial differencing away from shock waves and steep gradient regions while being capable of accurately capturing discontinuities, steep gradient, and fine scale turbulent structures in a stable and efficient manner is described.
626 citations
TL;DR: In this article, the authors derive a shock capturing tool able to treat turbulence with minimum dissipation out of the shock for a large-eddy simulation (LES) of the interaction.
605 citations
TL;DR: In this paper, the interaction of isotropic quasi-incompressible turbulence with a weak shock wave is investigated, and a linear analysis of the interaction is conducted for comparison with the simulations.
Abstract: Direct numerical simulations are used to investigate the interaction of isotropic quasi-incompressible turbulence with a weak shock wave. A linear analysis of the interaction is conducted for comparison with the simulations. Both the simulations and the analysis show that turbulence is enhanced during the interaction. Turbulent kinetic energy and transverse vorticity components are amplified, and turbulent lengthscales are decreased. It is suggested that the amplification mechanism is primarily linear. Simulations also showed a rapid evolution of turbulent kinetic energy just downstream of the shock, a behavior not reproduced by the linear analysis. Analysis of the budget of the turbulent kinetic energy transport equation shows that this behavior can be attributed to the pressure transport term. Multiple compression peaks were found along the mean streamlines at locations where the local shock thickness had increased significantly.
274 citations
TL;DR: In this article, a shock-capturing scheme was developed to accurately simulate the unsteady interaction of vortical turbulence with shock waves, and an existing controversy between experiments and theoretical predictions on length scale change was thoroughly investigated through the shockcapturing simulation.
Abstract: As an extension of the authors' work on isotropic vortical turbulence interacting with a shock wave (Lee, Lele & Moin 1993), direct numerical simulation and linear analysis are performed for stronger shock waves to investigate the effects of the upstream shock-normal Mach number (M1). A shock-capturing scheme is developed to accurately simulate the unsteady interaction of turbulence with shock waves. Turbulence kinetic energy is amplified across the shock wave, and this amplification tends to saturate beyond M1 = 3.0. An existing controversy between experiments and theoretical predictions on length scale change is thoroughly investigated through the shock-capturing simulation: most turbulence length scales decrease across the shock, while the dissipation length scale (ρq3/e) increases slightly for shock waves with M1<1.65. Fluctuations in thermodynamic variables behind the shock wave are nearly isentropic for M1<1.2, and deviate significantly from isentropy for the stronger shock waves, due to the entropy fluctuation generated through the interaction.
261 citations