Vorticity and velocity alignment in compressible flows: An experimental study of helicity density in turbulence and vortices

A nearly homogeneous and isotropic decaying compressible turbulence and a vortex flow have been investigated experimentally in a large-scale shock tube facility. Rectangular grids of various mesh sizes were used to generate turbulence with Reynolds numbers based on Taylor's microscale ranging from 260 to 1300. Measurements of the three-dimensional velocity and vorticity vectors, the full velocity gradient and rate-of-strain tensors with instrumentation of high temporal and spatial resolution have been carried out. This allowed estimates of dilatation, compressible dissipation and dilatational stretching to be obtained. The time-dependent signals of enstrophy, vortex stretching/tilting vector and helicity were found to exhibit a rather strong intermittent behavior which is characterized by high amplitude bursts with values up to 8 times their r.m.s. within periods of less violent and longer lived events. Several of these bursts are evident in all the signals suggesting the existence of a dynamical flow phenomenon as a common cause. Further interpretation of the results was obtained by considering the decomposition of the velocity field into a solenoidal contribution and a potential part. The effect of the Mach number is to increase the influence of the solenoidal part of the contribution and therefore make the flow more "vortex-dominated".