EXPERIMENTS ON DENSITY-GRADIENT ANISOTROPIES AND SCALAR DISSIPATION OF TURBULENCE IN A STABLY STRATIFIED FLUID

The anisotropic behaviour of density-gradient fluctuations in stably s tratified grid turbulence and the consequences for simplified (isotrop ic) estimates of scalar dissipation rates chi were experimentally stud ied in a thermally stratified wind tunnel at moderate Reynolds numbers (Re-lambda similar or equal to 20). Strong stable stratifications wer e attained, with Brunt-Vaisala frequency N as high as 4 rad s(-1). The correlation method was used to estimate the mean-square cross-stream and streamwise density gradients. Cross-stream gradients were measured using two cold wires. The mean-square vertical gradients were found t o become larger than the streamwise gradients by as much as a factor o f 2.2 for the largest dimensionless buoyancy times (Nt = 7). This corr esponds to a 40% error in the scalar dissipation estimates based on pa rtial derivative theta/partial derivative x alone, and assuming the va lidity of the isotropic relations. Gradient spectral relations show th at this buoyancy-induced anisotropy persists at all length scales. Bet ter closure of the scalar variance balance was attained than in previo usly reported measurements by other researchers. This is attributed to our use of cold-wire temperature sensors having larger length-to-diam eter ratio than used in the previous measurements.