ON THE ADEQUACY OF THE THIN-SHEAR-FLOW EQUATIONS FOR COMPUTING TURBULENT JETS IN STAGNANT SURROUNDINGS

This paper reports that the usual parabolic truncation of the Reynolds equations, used in conjunction with second-moment closure, leads to r ates of growth in the self-similar region of the axisymmetric jet in s tagnant surroundings that are some 1 2 percent greater than when the c omplete (elliptic) equation set is solved. The differences between the Reynolds stress profiles are relatively small, however. A term-by-ter m examination indicates that the omission of the normal-stress terms i n the streamwise momentum equation and of the streamwise diffusion fro m the epsilon equation has the greatest effect on the spreading rate, the former raising the rate of spread by about 4.5 percent and the lat ter by 6 percent. The stress-generation terms associated with streamwi se gradients have little effect on spreading rate but modify the stres s profiles, particularly in the near-axis region. A comparable explora tion for the plane jet shows qualitatively similar behavior, though th e influence of the secondary terms arising from streamwise gradients i s much less for this flow.