Abstract
Isotropic turbulence is closely approximated by stretching a grid flow through a short (1.36:1) secondary contraction. The flow is operated at small values of the Taylor microscale Reynolds number (about 25–55) and is slightly heated just downstream of the grid, so that the temperature serves as a passive scalar and the initial velocity/thermal length-scale ratio is about 1. For the same grid, the contraction reduces the skewness and kurtosis of the thermal fluctuations and their derivative. The thermal fluctuations and their mean dissipation rates follow a power-law rate of decay that depends on the geometry of the grid. Comparison with velocity measurements shows that, for three different grids, the ratio between the temperature and velocity power-law exponents closely matches the velocity/thermal timescale ratio. For the present measurements, the timescale ratio is slightly larger than 1 but does not exceed 1.2, in accordance with the proposal by Corrsin (J Aeronaut Sci 18(6):417–423, 1951b).
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