Small Reynolds number nearly isotropic turbulence in a straight duct and a contraction

Abstract

Measurements have been made of nearly isotropic, low Reynolds number, grid-generated turbulence. The decay results agree with the predictions for ’’final period’’ isotropic turbulence decay (in time) by the linear approximation of von Kármán and Howarth, and Batchelor and Townsend. This agreement occurs in spite of the fact that at the smallest turbulence Reynolds number attained, Rλ≈4, the inertial (triple velocity correlation) term in the double velocity correlation equation is not negligible as assumed in the theoretical estimates. As in experiments of Batchelor and Stewart, the turbulence shows appreciable departure from isotropy, indicated by component energy inequality growth downstream. It is also shown by nonzero velocity skewness, not reported heretofore at small Rλ. Measurement of the effect of a secondary contraction on small Reynolds number turbulence indicates that the contraction has less effect than at larger Reynolds numbers.