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Abstract:
An efficient approach for extracting three-dimensional local averages in spherical subdomains is proposed and applied to study the intermittency of small-scale velocity and scalar fields in direct numerical simulations of isotropic turbulence. We focus on the inertial-range scaling exponents of locally averaged energy dissipation rate, enstrophy, and scalar dissipation rate corresponding to the mixing of a passive scalar theta in the presence of a uniform mean gradient. The Taylor-scale Reynolds number R-lambda goes up to 1300, and the Schmidt number Sc up to 512 (albeit at smaller R-lambda). The intermittency exponent of the energy dissipation rate is mu approximate to 0.23 +/- 0.02, whereas that of enstrophy is slightly larger; trends with Rk suggest that this will be the case even at extremely large R-lambda. The intermittency exponent of the scalar dissipation rate is mu(theta) approximate to 0.35 for Sc = 1. These findings are in essential agreement with previously reported results in the literature. We additionally obtain results for high Schmidt numbers and show that mu(theta) decreases monotonically with Sc, either as 1/ log Sc or a weak power law, suggesting that mu(theta )-> 0( ) as Sc -> infinity, reaffirming recent results on the breakdown of scalar dissipation anomaly in this limit.
