MIXING ENHANCEMENT IN AXISYMMETRICAL TURBULENT ISOTHERMAL AND BUOYANTJETS

Measurements of the velocity and concentration in axisymmetric, turbul ent, isothermal and buoyant jets have been performed with laser-Dopple r velocimetry and planar and point laser-induced fluorescence to quant ify the mixing enhancement achieved by periodic forcing when the jet e xit has a fully-developed turbulent pipe flow, a situation less well-s tudied than the case of laminar initial conditions. It was found that forcing at Strouhal numbers around 0.6 enhances mixing in the developi ng region of the jet and this enhancement increased with increasing am plitude of excitation, consistent with results of initially-laminar je ts. The initial turbulence intensity did not have any effect, but an i ncrease in the initial lengthscale of the turbulence, controlled by a perforated plate inside the nozzle, caused faster mixing. In agreement with previous experiments, the initial conditions of the jet did not affect the far-field rate of decay, but the jet-fluid concentration th ere was significantly reduced by forcing due to the increased mixing d uring the early stages of development, an effect that can be described by a smaller virtual origin in decay laws of jet decay. These results are independent of the Froude number because the initial conditions h ave an influence only in the early stages where the flow is still mome ntum dominated.