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Using direct numerical simulations of homogeneous isotropic turbulence, the temporal evolution of small-scale vorticity and passive scalar structures has been examined. Visualization of small-scale vorticity structures indicates a process where vortex sheets may roll up into intense vortex tubes through a Kelvin–Helmholtz-type instability. The ability of these vortex structures to mix a passive scalar has also been examined. The regions of intense scalar gradient form sheets which are found to occur in regions of persistent straining flow, such as between neighboring vortex structures of approximately equal circulation. The magnitude of vorticity of these structures is usually not large, as persistent straining is most commonly found in connection with regions of moderate vorticity. Scalar gradient sheets are also found between low intensity regions of antiparallel vorticity, which are also observed to produce persistent straining.
Ruetsch et al. (Tue,) studied this question.