All three velocity fluctuations and the temperature fluctuation have been measured in a slightly heated turbulent plane jet. Attention is focused on the interaction region of the flow, which is situated between the location where the two mixing layers nominally merge and that which corresponds to approximate self-preservation.For the jet considered here the mixing-layer structures are symmetrical with respect to the centreline, and when they meet in the interaction region the redistribution of turbulence quantities is dramatic. This redistribution is examined in detail. Also examined is the effect of the generation, in the interaction region, of new structures, asymmetric with respect to the centreline, which evolve into the self-preserving flow region downstream.Turbulence parameters, such as the turbulent Prandtl number, probability density functions, skewness and flatness factors, are also presented, primarily to guide computer simulations of this flow. The superposition procedure of Weir, Wood & Bradshaw (1981), which assumes that the turbulence structure of each mixing layer is not significantly altered by the interaction, is not appropriate to the present flow.

The interaction region of a turbulent plane jet

It is shown that measurements of the statistical properties of the concentration distributions of dispersing scalars taken from many different turbulent shear flows have a great number of common features. In particular the same simple relationship between the mean concentration and the mean-square fluctuation is shown to hold in all the flows, and this relationship is derived theoretically from well-known results for the unreal case when there is no molecular diffusion by a natural hypothesis about the effects of molecular diffusion. Application of the hypothesis to the higher moments and shape parameters gives results that agree reasonably well with the data (given the unavoidable experimental errors). The hypothesis should be subjected to further experimental analysis, and could simplify the application of turbulence closures and similar models. Extensions of the ideas to the probability density function of the scalar concentration suggest that it becomes self-similar. A final conclusion is that more attention to experimental errors due to instrument smoothing is highly desirable.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112090002087

M. Elena

Papers

Points of symmetry in turbulent shear flows