Akshay Gowardhan

Simulations of Richtmyer–Meshkov instabilities in planar shock-tube experiments

In the large eddy simulation (LES) approach, large-scale energy-containing structures are resolved, smaller structures are filtered out, and unresolved subgrid effects are modeled. Extensive recent work has demonstrated that predictive under-resolved simulations of the velocity fields in turbulent flows are possible without resorting to explicit subgrid models when using a class of physics-capturing high-resolution finite-volume numerical algorithms. This strategy is denoted as implicit LES (ILES). Tests in fundamental applications ranging from canonical to complex flows indicate that ILES is competitive with conventional LES in the LES realm proper—flows driven by large-scale features. The performance of ILES in the substantially more difficult problem of under-resolved material mixing driven by under-resolved velocity fields and initial conditions is a focus of the present work. Progress in addressing relevant resolution issues in studies of mixing driven by Richtmyer–Meshkov instabilities in planar sho...

The bipolar behavior of the Richtmyer–Meshkov instability

A numerical study of the evolution of the multimode planar Richtmyer-Meshkov instability (RMI) in a light-heavy (air-SF6, Atwood number A = 0.67) configuration involving a Mach number Ma = 1.5 shock is carried out. Our results demonstrate that the initial material interface morphology controls the evolution characteristics of RMI (for fixed A, Ma), and provide a significant basis to develop metrics for transition to turbulence. Depending on initial rms slope of the interface, RMI evolves into linear or nonlinear regimes, with distinctly different flow features and growth rates, turbulence statistics, and material mixing rates. We have called this the bipolar behavior of RMI. Some of our findings are not consistent with heuristic notions of mixing in equilibrium turbulence: more turbulent flow—as measured by spectral bandwidth, can be associated with higher material mixing but, paradoxically, to lower integral measures of turbulent kinetic energy and mixing layer width.

QUIC transport and dispersion modelling of two releases from the Joint Urban 2003 field experiment

An in-depth comparison of plume calculations from the Quick Urban and Industrial Complex (QUIC) dispersion model to street- and roof-level concentration measurements for a daytime and night time release taken during the Joint Urban 2003 field experiment held in downtown Oklahoma City is presented. A number of improvements to the empirical-diagnostic wind solver to better account for high-rise buildings and dense urban areas will be discussed. Traditional plume statistical performance measures reveal that the code is performing as well as computational fluid dynamics models and meets the criteria proposed by Hanna and Chang (2012) for urban dispersion models. Statistics for the day and night release are fairly similar, and model performance drops slightly after the release is turned off in the flushing phase. Scatterplots indicate that rooftop measurements were generally well predicted, although the percentage of outliers (false negatives, large over predictions) was greater as compared to street-level measurements. Using all tracer data within a kilometre of the release, including rooftop samplers, from 39 to 46% of the model-computed concentrations were within a factor of two of the observations when broken up into four cases stratified by day or night and release on or off.

Two classes of Richtmyer-Meshkov instabilities: A detailed statistical look

A single parameter numerical study of the evolution of the multimode planar Richtmyer-Meshkov instability (RMI) in a shocked/reshocked (air-SF6, Atwood number A = 0.67) configuration with a Mach number Ma = 1.5 shock is carried out. Our results demonstrate that the initial material interface morphology (for fixed Ma, A) controls the RMI evolution characteristics. Our discussion focuses on the light-to-heavy configuration with initial A > 0 and heavy-to-light reshock. Depending on the rms slope of the initial interface, ηo, there are two different instabilities: one with the classical RMI trends and another with trends suggesting a very different fluid physics which we study in detail. We use statistical metrics to demonstrate that the two different regimes are characterized by very different and self-consistent fluid physics. The response of the rate of mixing layer growth to increasing ηo is different and opposite in sign in each regime: in the high-ηo class of initial conditions, increasing ηo leads to ...

Implicit large eddy simulation of shock-driven material mixing.

Under-resolved computer simulations are typically unavoidable in practical turbulent flow applications exhibiting extreme geometrical complexity and a broad range of length and time scales. An important unsettled issue is whether filtered-out and subgrid spatial scales can significantly alter the evolution of resolved larger scales of motion and practical flow integral measures. Predictability issues in implicit large eddy simulation of under-resolved mixing of material scalars driven by under-resolved velocity fields and initial conditions are discussed in the context of shock-driven turbulent mixing. The particular focus is on effects of resolved spectral content and interfacial morphology of initial conditions on transitional and late-time turbulent mixing in the fundamental planar shock-tube configuration.

Planar Richtmyer-Meshkov Instabilities and Transition

A numerical study of the evolution of the multimode planar Richtmyer-Meshkov (RM) instability in a light-heavy (Air-SF6, Atwood number A = 0.67) configuration involving a Mach number Ma = 1.5 shock is carried out. Our results demonstrate that the initial material interface morphology controls the evolution RM characteristics, and provide a significant basis to develop metrics for transition to turbulence. Depending on the initial rms slope of the interface, RM evolves into linear or nonlinear regimes, with distinctly different flow features and growth rates, turbulence statistics and material mixing rates. We have called this the bipolar behavior of the RM instability. We demonstrate an important practical consequence of our results: reshock effects on mixing and transition can be emulated at first shock if the initial rms slope is high enough.

Three dimensional simulations of Richtmyer-Meshkov instabilities in shock-tube experiments

In the large eddy simulation (LES) approach large-scale energy-containing structures are resolved, smaller (presumably) more isotropic structures are filtered out, and unresolved subgrid effects are modeled. Extensive recent work has demonstrated that predictive simulations of turbulent velocity fields are possible based on subgrid scale modeling implicitly provided by a class of high-resolution finite-volume algorithms. This strategy is called implicit LES. The extension of the approach to the substantially more difficult problem of material mixing IS addressed, and progress in representative shock-driven turbulent mixing studies is reported.

Analysis of Computational and Laboratory Shocked Gas-Curtain Experiments

A crucial aspect in collaborative laboratory / computational research is that of adequately characterizing and modeling the conditions in the turbulent flow experiments, so that potential sources of discrepancies can be clearly evaluated and controlled. Shock-driven turbulent flow studies involving under-resolved material mixing promoted by under-resolved velocity field fluctuations and insufficiently characterized initial conditions, are particularly difficult. The present exercise illustrates typical challenges, and reports progress in achieving analysis, scalings, and understanding which can be corroborated by joint laboratory experiments and simulations.

On the simulation of shock-driven turbulent mixing in high-Re flows*

In the large eddy simulation (LES) approach, large-scale energy-containing structures are resolved, smaller (presumably) more isotropic structures are filtered out and unresolved subgrid effects are modeled. Extensive recent work has demonstrated that predictive simulations of turbulent velocity fields are possible based on subgrid scale modeling implicitly provided by a class of high-resolution finite-volume algorithms. This strategy is called implicit LES. The extension of the approach to the substantially more difficult problem of material mixing is addressed, and progress in representative shock-driven mixing studies is reported.