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Robert Rubinstein
Computational AeroSciences Branch, NASA Langley Research Center Hampton, Virginia 23681-2199, USA

Timothy T. Clark
X-Division, X-3, Los Alamos National Laboratory Los Alamos, NM; Northrop Grumman Information Technology, Advanced Technology Division Albuquerque, New Mexico 87106, USA

Li-Shi Luo


Time dependent isotropic turbulence belongs to a class of problems for which modeling assumptions based on Kolmogorov's theory may not be appropriate: the Kolmogorov theory defines a steady state, or perhaps more generally a self-similar state, but during transient evolution, even the smallest scales of motion can well be very far from steady or self-similar. But spectral closures, which make no special assumptions about either small or large scales, are appropriate methods for such problems.
The transient evolution of turbulence driven by statistically steady forcing, the transient evolution of turbulence driven by linearly unstable large-scale modes, and the transition from one steady state to another in turbulence driven by a statistically unsteady force are analyzed using a recently developed spectral closure. Although DNS has an important role in validating spectral closures, because these analyses require considering a large number of test cases, often run to very large numbers of time steps, direct simulation alone is impractical.