New results from the experimental study of the return to isotropy of the Reynolds-stress tensor in the case where the third invariant of its deviator is positive

The return of a homogeneous anisotropic turbulence to isotropy is investigated experimentally for the case of a positive invariant III of the deviator tensor. A quasi-isotropic turbulence is generated in the wake of a grille of 5.08-cm mesh and 0.34 solidity in a working section 1.2 x 1 m and then encounters a slight contraction at a distance of 18 grid lengths to approach ideal isotropy, a three-dimensional uniform deformation rendering it anisotropic with positive invariant III, and a channel 5.3 m long of section 35 x 99 cm where a return toward isotropy occurs. The results are presented graphically and discussed. Invariant III varies from 0.005 to 0.0015, and the intrinsic initial Reynolds number is calculated as 300. The characteristic time of the return to isotropy is shown to be equal to the characteristic energy-decay time.