Experimental investigation of noise reduction in supersonic jets due to jet rotation and to nozzle geometry changes

The present work deals with two different methods used for noise reduction in supersonic jets. The first method employed consisted of a swirl chamber, which produced axisymmetric jets with a tangential velocity component. This component was found to affect the structure of the jet cell and the internal oblique shocks formed inside. In the second method, the nozzle exit plane of the cylindrical chamber was perturbed such that the nozzle's exit lip was not lying in one plane perpendicular to the axis of symmetry of the nozzle. These two methods were investigated experimentally via spark Schlieren photography, static, Pitot and sound pressure measurements. Schlieren photography was employed to visualize the flow and shock structure associated with any of the above changes. Static and pitot pressure measurements were carried out to obtain the velocity distribution whereas sound pressure measurements were used to determine the resulting gain in noise reduction. The introduction of a small flow rotation was found to weaken the internal shock strength and to reduce the cell length and screech noise generated. Increasing the flow rotation beyond a certain limit was found to effect the shock structure without any noticeable noise reduction. Sinusoidal perturbation was found to affect the symmetry and strength of the oblique shocks in the supersonic cells. A considerable noise reduction was also noted with perturbation amplitude equal to 12.5% of the nozzle inside diameter. Larger perturbation did not result in a further noise reduction. In the first and second methods, a maximum noise reduction of 12 dB and 8 dB were measured, respectively