RAPID COMMUNICATIONS PHYSICAL REVIEW FLUIDS 1, 062401(R) (2016) Parameter governing the far-field features of round jets

This study is inspired by the observation and hypothesis that the spreading and decay behaviors of a jet directly depend on the momentum-mixing mechanism between the jet and surrounding fluid. This mixing behavior is dictated by the kinematic viscosity ν for a laminar jet, which can be dramatically enhanced in a turbulent flow and is represented by the eddy viscosity ε. Similarly, pulsation in a synthetic jet is identified as another mechanism for enhancing mixing, which can be captured by an enhanced eddy viscosity beyond what is observed in a corresponding turbulent continuous jet. To this end, an effective-eddy-viscosity concept is proposed to model any excitation of a jet that could result in enhanced mixing beyond what is predicted by the kinematic viscosity. Our previous study found that ε is actuator dependent and its relationship with the spreading or decay behavior of a jet is not obvious. To remove the actuator dependence, this study performs a dimensional analysis to relate the spreading and decay behaviors to a scaled effective eddy viscosity ε/ √ K (K is the momentum flux). This quantity physically represents a competition between the radial diffusion and the axial convection of the jet axial momentum. The experimental results confirm that ε/ √ K governs the spreading and decay rates of the far field for any round jets.