Effect of a tab on the aerodynamical development and noise of an underexpanded supersonic jet

Mechanical tabs are often used as a screech-suppressing device in order to study broadband shock-associated noise (BBSAN). In this study, various experimental methods are used to characterise the effect of a tab on the development and noise of an underexpanded supersonic round jet. It is shown that the jet development is considerably modified by the introduction of the tab in that the shock spacing is shortened and the shock-cell pattern loses axisymmetry and strength. Moreover, the broadband shock-associated noise radiated by the tabbed jet is compared to that of a jet in which screech was suppressed by means of a notched nozzle. The peak frequency of BBSAN for the tabbed jet approximately matches that of the notched counterpart, but the amplitude of this noise component was seen to be smaller for the former jet. More importantly, the axisymmetry of the acoustic field is lost when using a tab, resulting in a dependence of measurements on the location of the tab relatively to the microphones. It is concluded that the use of a tab should be avoided to remove screech when studying broadband shock-associated noise.