Effects of acceleration on turbulent jets

Abstract

Effects of acceleration on turbulent jets were investigated in a series of flow visualization experiments. Prior to the initiation of acceleration, a steady jet with a Reynolds number of 3000 was established. Three distinct acceleration schemes of linear, quadratic, and exponential were utilized to increase the nozzle exit velocity by an order of magnitude. As the flow accelerated, a discernible ‘‘front’’ was established. The parcels constituting the front were less diluted than the steady jet parcels at the same location. For each acceleration scheme, the temporal evolution of the front position had the same functional form as the nozzle velocity. The front velocity increased linearly with the acceleration rate for the linear and quadratic cases. In comparison 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 though the linear cases had generally larger acceleration rates. A model, based on the scaling of centerline velocity in steady jets, appears to correctly predict the time dependence of the front position.