Parametric study and scaling of jet manipulation using an unsteady minijet

A parametric study is conducted for the control of a turbulent jet using a single unsteady minijet. A number of control parameters that influence the decay rate K of the jet centreline mean velocity are investigated, including the mass flow rate ratio Cm, excitation frequency ratio f(e)/f(0) and exit diameter ratio d/D of the minijet to main jet, along with the duty cycle (alpha) of the minijet injection. Extensive hot-wire, particle image velocimetry and flow visualization measurements were performed in the manipulated jet. Various flow structures have been identified, such as the flapping flow, non-flapping flow and that showing a manipulable thrust vector, depending on C-m, f(e)/f(0) and alpha. Empirical scaling analysis unveils that, prior to the minijet impingement upon the wall of the nozzle and the generation of turbulence, the relationship K = g(1) (C-m, f(e)/f(0), d/D, alpha) may be reduced to K = g(2) (xi), where g(1) and g(2) are different functions and the scaling factor xi =(root MR/alpha)(d/D)(n) (root MR equivalent to C-m (D/d) is the momentum ratio and n is a constant that depends on alpha) is physically the effective momentum ratio per pulse or effective penetration depth. Discussion is conducted based on K = g(2) (xi), which provides important insight into the jet control physics.