Nonlinear interactions in strained axisymmetric high-Reynolds-number turbulence

We present measurements, over a wide range of Reynolds numbers ( 40≤ Rλ ≤ 470 ), of grid-generated turbulence subjected to axisymmetric strain, and of the subsequent evolution of the turbulence after the strain is released. The Reynolds number was varied by the use of both passive and active grids and the strain was produced by a 4:1 area-change axisymmetric contraction placed at various distances from the grid. The time scale ratio of the turbulence to that of the mean strain varied from approximately 10 to 30. The results show reasonable agreement with (linear) rapid distortion theory (RDT) for the velocity variances but, contrary to linear theory, the strained longitudinal, u₁ , spectrum peaked at significantly higher wavenumber than the transverse, u₂ , spectrum. The mismatch in peaks increased with increasing Rλ and at the highest Reynolds number ( Rλ = 470 ) the peak of the strained u₁ -spectrum occurred at a wavenumber 200 times greater than that of the u₂ -spectrum. As the flow relaxed toward isotropy after the contraction, further evidence of the non-locality in the flow field became apparent, with a second peak in the u₂ -spectrum emerging at a similar wavenumber to the high-frequency peak in the u₁ -spectrum. The strain also caused the longitudinal derivative skewness to change sign but as the flow evolved after the contraction the derivative skewness returned to its typical value of - 0.4. We also show that single-point turbulence models are inadequate to describe the relaxation of the turbulence towards an isotropic state in the postcontraction region.