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 (alpha) of minijet injection, and the mass flow rate, excitation frequency, and diameter ratio (C-m,C-N, f(e)/f(0), and d/D) of an individual minijet to a main jet. The decay rate (K-e) 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 f(e)/f(0) and alpha, i.e., (f(e)/f(0))(opt) and alpha(opt) at which the maximum Ke or Ke,max occurs, do not vary with N, though optimal C-m,N or (C-m,C-N)opt does. Empirical scaling analysis performed under (f(e)/f(0))(opt) reveals that the relationship K-e = g(1) (N, C-m,(N), alpha, d/D) may be reduced to K-e/K-e,K-max = g(2) zeta, where g(1) and g(2) are different functions, and the scaling factor zeta = C-m,C-N/(C-m,C-N)(opt), representing physically the minijet penetration depth. Both K-e,K-max and (C-m,C-N)(opt) are found to depend on N, alpha, and d/D. Discussion conducted based on K-e/K-e,K-max = g(2) zeta provides important insight into the jet control physics. It is also discussed how f(e)/f(0) and alpha are connected to the vortex interaction and, hence, K-e. Published under an exclusive license by AIP Publishing.

Automated summary

This study experimentally manipulates a turbulent round jet with Reynolds number of 8000, investigating the effect of five control parameters on the decay rate of the jet-centerline mean velocity. Different flow structures for various minijet configurations are proposed, and three distinct mechanisms of highly effective manipulation are identified. The optimal parameters for maximum decay rate occurrence do not vary with the minijet number, but the penetration depth of the minijet does.