EMPIRICAL CORRELATIONS FOR GRID TURBULENCE

Homogeneous isotropic turbulence (HIT) is approximately generated by passing a uniform flow through a grid followed by a short contraction. The grid flow is slightly heated so that temperature acts as a passive scalar. In the self-similar region of grid flow downstream of the contraction, there is no mean shear and no turbulence production. The amplitudes of velocity and temperature fluctuations simply decay under the effect of viscosity and thermal diffusion. The decay rate is well represented by a power law; this is supported by present measurements in three different grid flows and by previously published data for grid turbulence obtained over different ranges of streamwise distance and/or Reynolds number. From dimensional analysis and the (empirical) power-law correla-tions, basic flow parameters, namely the Kolmogorov/Taylor/ Corrsin microscales and the Reynolds/Péclet numbers, are established as functions of streamwise distance. From this, it is possible to determine the flow parameters for a required grid geometry or initial condition