EFFECTS OF ACCELERATION ON TURBULENT JETS

Effects of acceleration on turbulent jets were investigated in a serie s of flow visualization experiments. Prior to the initiation of accele ration, a steady jet with a Reynolds number of 3000 was established. T hree distinct acceleration schemes of linear, quadratic, and exponenti al were utilized to increase the nozzle exit velocity by an order of m agnitude. As the flow accelerated, a discernible ''front'' was establi shed. The parcels constituting the front were less diluted than the st eady jet parcels at the same location. For each acceleration scheme, t he temporal evolution of the front position had the same functional fo rm as the nozzle velocity. The front velocity increased linearly with the acceleration rate for the linear and quadratic cases. In compariso n with a steady jet, the front's lateral growth rate was reduced by 16 % in the linear case and by 25% in the two nonlinear cases, even thoug h the linear cases had generally larger acceleration rates. A model, b ased on the scaling of centerline velocity in steady jets, appears to correctly predict the time dependence of the front position. (C) 1996 American Institute of Physics.