Axisymmetric jet manipulation using multiple unsteady minijets

A turbulent round jet with a Reynolds number of 8000 based on the minijet exit diameter is experimentally manipulated using 1–6 unsteady radial minijets. Five control parameters are investigated, including the minijet number or configuration (N), the duty cycle (α) of minijet injection, and the mass flow rate, excitation frequency, and diameter ratio (Cm,N, fe/f0, and d/D) of an individual minijet to a main jet. The decay rate (Ke) of the jet-centerline mean velocity under manipulation strongly depends on these five parameters. The typical flow structures or conceptual flow models are proposed for each minijet configuration based on extensive measurements. Three distinct mechanisms behind the highly effective manipulation are identified and discussed from the flow structures. It has been found that the optimal fe/f0 and α, i.e., (fe/f0)opt and αopt at which the maximum Ke or Ke,max occurs, do not vary with N, though optimal Cm,N or (Cm,N)opt does. Empirical scaling analysis performed under (fe/f0)opt reveals that the relationship Ke = g1 (N, Cm,N, α, d/D) may be reduced to Ke/Ke,max = g2 ζ, where g1 and g2 are different functions, and the scaling factor ζ = Cm,N/(Cm,N)opt, representing physically the minijet penetration depth. Both Ke,max and (Cm,N)opt are found to depend on N, α, and d/D. Discussion conducted based on Ke/Ke,max = g2 ζ provides important insight into the jet control physics. It is also discussed how fe/f0 and α are connected to the vortex interaction and, hence, Ke.