Temperature-dissipation measurements in a turbulent boundary layer

The three components of the average temperature dissipation have been measured using a pair of parallel cold wires in an approximately self-preserving turbulent boundary layer. The mean square value of θ, x the temperature derivative in the longitudinal direction, is determined mainly by the use of Taylor's hypothesis, following direct verification of this hypothesis at a few locations in the flow. Mean square values of θ, y and θ, z , the temperature derivatives in directions normal to the flow, were estimated mainly from the curvature of spatial temperature autocorrelations. In the outer layer, the measurements indicate that θ²̄,z > θ²̄,y > θ²̄,x , and the resulting distribution for dissipation leads to a good closure of the 1/2θ²̄ budget. In the near-wall region the measurements indicate that θ²̄,y > θ²̄,z > θ²̄,x . The ratios θ²̄,y/θ²̄,x and θ²̄,z/θ²̄,x are as large as 13 and 7 respectively at y + = 12, underlining the strong anisotropy in this region. The behaviour of the turbulent diffusion, estimated by difference, provides reasonable support for the accuracy of the near-wall temperature-dissipation measurements. Using existing data of near-wall distributions of the turbulent energy and of its dissipation rate, the timescale for the turbulent-energy dissipation is found to be approximately equal to that for the temperature dissipation.