A time-domain evaluation of the large-scale flow structure in a turbulent jet

This paper describes an investigation of the large-scale flow processes which occur in turbulent circular jet flows ( Re > 10 5 ). The existence of regular large-scale flow structures at low and moderate Reynolds numbers ( Re < 5 x 10 4 ) has clearly been demonstrated by flow-visualization experi­ments, but visual evidence for order in jet turbulence becomes ambiguous at a Reynolds number around 7 x 10 4 . A new time-domain technique for the study of two-dimensional large-scale flow structures has been developed by Bruun (1977). In this paper this technique is extended to the study of three-dimensional large-scale flow structures by the inclusion of X hot-wire and circumferential eductions. The evaluated large-scale structures in the turbulent jet ( Re = 2 x 10 5 ) are shown to deviate considerably from the axi-symmetric flow structures which occurs at low and moderate Reynolds numbers ( Re < 5 x 10 4 ). We observe a much smaller deformation rate of the semi-regular flow structure in the potential core in the turbulent jet case, and also the circumferential eductions reveal a rapid radial decrease in the circumferential coherence of the related large-scale flow structure in the mixing region. Further-­more, combining these results with the X hot-wire eductions in the mixing region proved that the major contributions to the shear stress uv is caused by circumferentially-narrow tongues of ‘fast moving ejected’ and ‘slow moving entrained ’fluid, similar to the ‘burst’ and ‘sweep’ events observed previously in turbulent wall boundary layers.