Stably stratified turbulence subjected to a constant area vertical expansion

Stably stratified grid turbulence has been passed through a constant-area vertical expansion, in a thermally stratified wind tunnel. The latter part of the test-section expands linearly in the vertical direction while contracting accordingly in the horizontal direction. The resulting reduction in stratification strength interacts with the vortex stretching due to the induced cross-strain, an effect first studied by Townsend in a homogeneous fluid. The expansion is applied to the turbulence at two different states of evolution, by using two different stratification strengths. For the weaker stratification the turbulence is strongly stirring, while for the stronger stratification it is in the restratification regime, as it enters the expansion. Both of these cases show a clear reduction in the normalized buoyancy flux. This reduces the stirring in the former case, while enhancing restratification in the latter. The vertical velocity fluctuations thereby experience qualitatively different effects due to the buoyancy forces, as the restratification adds to w′, whereas in the stirring case the buoyancy forces assist the cross-strain in reducing w′ by extracting kinetic energy from w′. and storing potential energy in the fossilizing density field.