Experimental visualisation of trapped waves in circular subsonic jets with and without chevrons

In this work, we describe our experimental observations of trapped waves and guided jet waves in schlieren images of jets operating at jet Mach numbers upper M Subscript j Baseline equals 0.9 M j = 0.9 $M_{\!j}=0.9$ and upper M Subscript j Baseline equals 0.983 M j = 0.983 $M_{\!j}=0.983$ produced by circular nozzles. Results from a chevron nozzle are also included as a special case. We establish that: (i) several families of trapped waves are detectable in our experimental images, including higher order axisymmetric and helical modes; (ii) the dispersion relationship of these waves is in agreement with the vortex sheet model of Tam & Hu ( J. Fluid Mech. , 1989, vol. 201, pp. 447–483) and (iii) the introduction of chevrons does not significantly affect the presence or intensity of the first several peaks associated with the trapped wave modes. Finally, we observe that all detected trapped waves have subsonic group velocities with respect to the quiescent stream.