NUMERICAL ANALYSIS OF TURBULENT CROSS STRESSES AND PRESSURE IN THE DEVELOPING REGION OF AN AXISYMMETRIC JET

摘要

A numerical study was conducted of the transition region of an axisymmetric isothermal jet, from 10 to 44 diameters downstream of the nozzle exit. The transition region is of particular interest because if offers an opportunity to examine the relationship among physical quantities in a developing flow. A model for the Reynolds cross-stress terms was developed from empirical correlations of mean velocity. It was then used, along with correlations for RMS velocities, to numerically predict pressure in the developing region. This turbulence model is more accurate than experimental measurements in high Reynolds number, transitional flows because the scales of turbulence in these conditions are too small to resolve experimentally. Numerical results show that close coupling exists between the mean axial velocity and the Reynolds cross-stresses. Similarly, a close relationship exists between the axial and radial RMS velocity fluctuations and the developing pressure gradients in the axial and radial direction. Therefore, when the mean velocities reach fully similar form, similarity of Reynolds cross stresses is also required. The axial and radial fluctuations continue to develop, and asymptotically approach similar forms at rates closely matching the decay of the mean pressure gradients. The information presented here can be used to test turbulence models applied to developing flows.