SIMULATIONS OF SPATIALLY DEVELOPING 2-DIMENSIONAL SHEAR LAYERS AND JETS

A computational study of spatially evolving two-dimensional free shear flows has been performed using direct numerical simulation of the Nav ier-Stokes equations in order to investigate the ability of these two- dimensional simulations to predict the overall Bow-field quantities of the corresponding three-dimensional ''real'' turbulent flows. The eff ects of inflow forcing on these two-dimensional Bows has also been stu died. Simulations were performed of shear layers, as well as weak (lar ge co-flow and relatively weak shear) and strong (small co-flow and re latively strong shear) jets. Several combinations of discrete forcing with and without a broadband background spectrum were used. Although s patially evolving direct simulations of shear layers have been perform ed in the past, no such simulations of the plane jet have been perform ed to the best of our knowledge. It was found that. in the two-dimensi onal shear layers, external forcing led to a strong increase in the in itial growth of the shear-layer thickness, followed by a region of dec reased growth as in physical experiments. The final downstream growth rate was essentially unaffected by forcing. The mean velocity profile and the naturally evolving growth Fate of the shear layer in the case of broadband forcing compare well with experimental data. However, the total and transverse fluctuation intensities are larger in the two-di mensional simulations with respect to experimental data. In the weak-j et simulations it was found that symmetric forcing completely overwhel ms the natural tendency to transition to the asymmetric jet column mod e downstream. It was observed that two-dimensional simulations of ''st rong'' jets with a low speed co-flow led to a fundamentally different flow with large differences even in mean velocity profiles with respec t to experimental data for planar jets. This was a result of the domin ance of the two-dimensional mechanism of vertex dipole ejection in the flow due to the lack of spanwise instabilities. Experimental studies of planar jets do not show vortex dipole formation and ejection. A thr ee-dimensional ''strong''-jet simulation showed the rapid evolution of three-dimensionality effectively preventing this two-dimensional mech anism, as expected from experimental results.