A visualization study of the longitudinal structures of a plane wall-jet

In the present study the flow field near the orifice of a plane wall jet is in focus. Two main flow regions may be distinguished in the wall-jet, i.e. a free shear layer away from the wall and a boundary layer close to the surface. In both of these layers streamwise coherent structures are detected by means of smoke visualization and hot-wire measurements. The structures, which occur naturally, have different spanwise scales and emerge at different distance from the nozzle. Effects of the flow velocity, upstream perturbations and acoustic excitation on the generation and characteristics of the streamwise disturbances are investigated and especially the interaction between the two layers is studied. In order to resolve the complex 3D flow by means of hot-wires a system for accurate automated traversing and data acquisition has been developed. It was found that the value of outlet velocity and the frequency of Kelvin-Helmholts rolls have a clear influence on the size of the structures. Higher outlet velocities and higher frequencies of triggered two-dimensional roll-ups lead the size of longitudinal structures to decrease