Periodic excitation for jet vectoring and enhanced spreading

The effects of periodic excitation on the evolution of a turbulent jet were studied experimentally. A short, wide-angle diffuser was attached to the j et exit, and excitation was introduced at the junction between the jet exit and the diffuser inlet. The introduction of high-amplitude periodic excita tion at the jet exit enhances the mixing and promotes attachment of the jet shear layer to the diffuser wall, Vectoring is achieved by applying the ex citation over a fraction of the circumference of the circular jet, enhancin g its spreading rate on the excited side and its tendency to reattach to th at side. Static-deflection studies demonstrate that the presence of the wid e-angle diffuser increases the effectiveness of the added periodic momentum caused by a favorable interaction between the excitation, the jet shear la yer, and the diffuser wall. This point was further demonstrated by the evol ution of a wave packet that was excited in the jet shear layer. Strong ampl ification of the wave packet was measured with a diffuser attached to the j et exit. The turbulent jet responds quickly (10-20 ms) to step changes in t he level of the excitation input. The response scales with the jet-exit vel ocity and is independent of the Reynolds number. Jet deflection angles were found to be highly sensitive to the relative direction between the excitat ion and the jet how and less sensitive to the excitation frequency. Signifi cant jet deflection angles were obtained for a diffuser length of one-half to two diameters and for diffusers with half-angles greater than 15 deg.