Compressible Turbulence

The structure of weakly compressible grid generated turbulence

Hotwire probe

A decaying compressible nearly homogeneous and nearly isotropic, grid generated turbulent flow has been set-up in a large scale shock tube research facility. Experiments have been performed by using instrumentation with spatial resolution of the order of 7 to 26 Kolmogorov viscous length scales. A variety of turbulence generating grids provided a wide range of turbulence scales with flow Mach numbers ranging from 0.3 to 0.6 and turbulent Reynolds number up to 700. The decay of Mach number fluctuations was found to follow a power law similar to that describing the decay of incompressible isotropic turbulence. It was also found that the decay coefficient and the decay exponent decrease with increasing Mach number while the virtual origin increases with increasing Mach number. A mechanism possibly responsible for these effects appears to be the inherently low growth rate of compressible shear layers emanating from the cylindrical rods of the grid. Measurements of time-dependent, three dimensional vorticity vector were attempted for the first time with a 12 wire miniature probe. This also allowed estimates of dilatation, compressible dissipation and dilatational stretching to be obtained. It was found that the fluctuations of these quantities increase with increasing Mach number of the flow. The time-dependent signals of enstrophy, vortex stretching/tilting vector and dilatational stretching vector found to exhibit a rather strong intermittent behavior which is characterized by bursts of high amplitudes with values up to 8 times their r.m.s. followed by less violent periods of time. Several of these bursts are evident in all signals suggesting the existence of a dynamical flow phenomenon as a common cause.

Probe Sensor Geometry.

Probe Sensor Geometry.

Photograph showing vorticity probe size comparison with toothpick.

Photograph showing vorticity probe size comparison with toothpick.

Close-up view directly into probe.

Close-up view directly into probe.