Experimental Studies on the Limiting Tab

Abstract

Experiments were carried out to study the effect of the tabs with limiting length, termed Rathakrishnan limit, on controlling the mixing and acoustic characteristics of free jets. The limiting tab is found to be efficient in promoting the jet mixing even in the presence of an adverse pressure gradient. Furthermore, the limiting tab is found to bemost efficient in promoting the mixing near the correct expansion rather than at an underexpanded condition, with a favorable pressure gradient at the nozzle exit. The reason behind theworking of the limiting tab in the formof a cross wire is discussed forMach 1.6, 1.79, and 2.0 jets. As high as 50%reduction in the core length is achieved using a cross wire forMach 1.79 jet operatedwith nozzle pressure ratio 5.66. The combination of thewaves in the jet core and their interaction with the detached shock envelop generated by the wire has a strong influence on the overall jet noise and its spectral content. Even though thewire reduced the overall noise of the jet and the reduction is significant formany cases of the present study, it caused an increase of screech amplitude for some cases. Jet noise reduction was found to be sensitive to the wire orientation.