Parametric study and scaling of jet manipulation using an unsteady minijet

Abstract

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 $C_{m}$ , excitation frequency ratio $f_{e}/f_{0}$ and exit diameter ratio $d/D$ of the minijet to main jet, along with the duty cycle ( $\unicode[STIX]{x1D6FC}$ ) 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 $\unicode[STIX]{x1D6FC}$ . Empirical scaling analysis unveils that, prior to the minijet impingement upon the wall of the nozzle and the generation of turbulence, the relationship $K=g_{1}$ ( $C_{m}$ , $f_{e}/f_{0}$ , $d/D$ , $\unicode[STIX]{x1D6FC}$ ) may be reduced to $K=g_{2}$ ( $\unicode[STIX]{x1D709}$ ), where $g_{1}$ and $g_{2}$ are different functions and the scaling factor $\unicode[STIX]{x1D709}=(\sqrt{MR}/\unicode[STIX]{x1D6FC})(d/D)^{n}$ ( $\sqrt{MR}\equiv C_{m}(D/d)$ is the momentum ratio and $n$ is a constant that depends on $\unicode[STIX]{x1D6FC}$ ) is physically the effective momentum ratio per pulse or effective penetration depth. Discussion is conducted based on $K=g_{2}$ ( $\unicode[STIX]{x1D709}$ ), which provides important insight into the jet control physics.