On the effects of nozzle lip thickness on the azimuthal mode selection of a supersonic impinging flow

The dependence of azimuthal mode selection on nozzle geometry in an impinging jet is demonstrated for the first time. Impinging jets from thin and infinite lipped nozzles at a nozzle pressure ratio (NPR) of 3:4 and plate spacing of 5:0D are investigated, where D is the nozzle exit diameter. Thoe flows are studied using high resolution particle image velocimetry (PIV). A peak in transverse variance at the shock cell locations is noted for the thin lip nozzle, whereas a large regin of axial variance is observed at the standoff shock for the infinite lip case. This is associated with a change in azimuthal modes between the flows. This difference is confirmed through proper orthogonal decomposition (POD), which suggests a dominant asymmetric helical mode for the thin lip jet and an axisymmetric mode for the infinite lip case. A lower energy, asymmetric helical mode is also present for the infinite lip case. It is concluded that the change in nozzle geometry causes the observable changes in the azimuthal modes, with two explanations for the different mode selection discussed.