Strained turbulence and low-diffusivity turbulent mixing using high performance computing
Clay, Matthew Paul
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In this thesis, turbulent flows are studied using the method of direct numerical simulation (DNS), whereby exact governing equations are computed without modeling. Beginning with isotropic turbulence and turbulent mixing under axisymmetric contraction, comparisons with experiments are made by directly modeling strain rates from wind tunnel facilities in the DNS. The simulations reproduce key findings from the experiments for the evolution of the one-dimensional component velocity spectra, which are strongly influenced by spectral transfer and pressure-strain mechanisms following the contraction. For simulations of low-diffusivity (i.e., high Schmidt number) turbulent mixing in isotropic turbulence, the increased resolution requirements of the Batchelor scales are addressed by adopting a dual-grid dual-scheme numerical approach. The one-way coupling of the velocity and passive scalar fields, along with their disparate resolution requirements at high Schmidt number, are exploited in the design of the parallel code by computing each field separately in disjoint message passing communicators. Good scalability of the code up to O(10^5) cores on machines at multiple national supercomputer centers is maintained by overlapping communication and computation through extensive use of shared-memory programming, both in homogeneous and heterogeneous (i.e., GPU-accelerated) computing environments. Simulations of passive scalars maintained under a uniform mean gradient in forced isotropic turbulence are conducted. The highest grid resolution employed is 8192^3 (0.5 trillion) for a scalar of Schmidt number 512, which is comparable to salinity mixing in the ocean. The results give strong support to the emergence of Batchelor scaling in the scalar spectrum and an approach toward local isotropy with increasing Schmidt number.