A study of passive scalar turbulence at high Reynolds numbers enabled by exascale computing

Many sources of data in the literature suggest, at least at Schmidt numbers of order unity or higher, that passive scalar fields transported by turbulence tend to be more intermittent than the velocity field, and also show significant departures from local isotropy when a production effect arising from a mean scalar gradient is present. In this talk we present direct numerical simulations at high resolution enabled by the Frontier GPU platform to investigate effects of Reynolds number and resolution to greater detail than in the past. For unity Schmidt number it appears that a grid spacing close to the Kolmogorov scale coupled with Courant number 0.3 for time-stepping can provide sufficiently accurate results for scalar gradient moments up to the fourth order, and for the peak value of normalized scalar dissipation rate, which is higher than that for the energy dissipation. It appears that inadequate resolution reduces the gradient skewness relevant to local anisotropy, in a manner related to ramp and cliff structures in the scalar field.

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