Browsing by Author "Williams, R. J. R."

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    Growth of a Richtmyer-Meshkov turbulent layer after reshock
    (2011-09-30T00:00:00Z) Thornber, Ben; Drikakis, Dimitris; Youngs, D. L.; Williams, R. J. R.
    This paper presents a numerical study of a reshocked turbulent mixing layer using high-order accurate Implicit Large-Eddy-Simulations (ILES). Existing theoretical approaches are discussed, and the theory of Youngs (detailed in Ref. 1) is extended to predict the behaviour of a reshocked mixing layer formed initially from a shock interacting with a broadband instability. The theory of Mikaelian2 is also extended to account for molecular mixing in the single-shocked layer prior to reshock. Simulations are conducted for broadband and narrowband initial perturbations and results for the growth rate of the reshocked layer and the decay rate of turbulent kinetic energy show excellent agreement with the extended theoretical approach. Reshock causes a marginal decrease in mixing parameters for the narrowband layer, but a significant increase for the broadband initial perturbation. The layer properties are observed to be very similar post-reshock, however, the growth rate exponent for the mixing layer width is higher in the broadband case, indicating that the reshocked layer still has a dependence (although weakened) on the initial conditions. These results have important implications for Unsteady Reynolds Averaged Navier Stokes modelling of such instabilities.
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    The influence of initial conditions on turbulent mixing due to Richtmyer-Meshkov instability
    (Cambridge University Press, 2010-07-10T00:00:00Z) Thornber, Ben; Drikakis, Dimitris; Youngs, D. L.; Williams, R. J. R.
    This paper investigates the influence of different three-dimensional multi-mode initial conditions on the rate of growth of a mixing layer initiated via a Richtmyer-Meshkov instability through a series of well-controlled numerical experiments. Results are presented for large-eddy simulation of narrowband and broadband perturbations at grid resolutions up to 3 x 10(9) points using two completely different numerical methods, and comparisons are made with theory and experiment. It is shown that the mixing-layer growth is strongly dependent on initial conditions, the narrowband case giving, a power-law exponent theta approximate to 0.26 at low Atwood and theta approximate to 0.3 at high Atwood numbers. The broadband case uses a perturbation power spectrum of the form P(k) proportional to k(-2) with a proposed theoretical growth rate of theta = 2/3. The numerical results confirm this; however, they highlight the necessity of a very fine grid to capture an appropriately broad range of initial scales. In addition, an analysis of the kinetic energy decay rates, fluctuating kinetic energy spectra, plane-averaged volume fraction profiles and mixing parameters is presented for each case.
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    Richtmyer-Meshkov turbulent mixing arising from an inclined material interface with realistic surface perturbations and reshocked flow
    (2011-04-19T00:00:00Z) Hahn, Marco; Drikakis, Dimitris; Youngs, D. L.; Williams, R. J. R.
    This paper presents a numerical study of turbulent mixing due to the interaction of a shock wave with an inclined material interface. The interface between the two gases is modeled by geometrical random multimode perturbations represented by different surface perturbation power spectra with the same standard deviation. Simulations of the Richtmyer-Meshkov instability and associated turbulent mixing have been performed using high-resolution implicit large eddy simulations. Qualitative comparisons with experimental flow visualizations are presented. The key integral properties have been examined for different interface perturbations. It is shown that turbulent mixing is reduced when the initial perturbations are concentrated at short wavelengths. The form of the initial perturbation has strong effects on the development of small-scale flow structures, but this effect is diminished at late times.