Effect of launchpad modification on the hydrodynamic and acoustic modes of an impinging jet

Supersonic impinging jets are often characterized by an acoustic feedback loop which results in the generation of high-amplitude impingement tones. For a launch vehicle, this high-amplitude noise environment during lift-off can cause severe loading on the ground structures and the vehicle payload. As a result, the risks due to launch acoustics often fall under the highest risk category for most rocket missions. Over the years, different methods have been tested that target this feedback mechanism to alleviate the acoustic loading during lift-off. These range from the use of water injection in and around the launch pad, to changing the geometry of the launchpad itself. The goal of the present study is to understand the effect of a launchpad modification in the form of the addition of walls with circular cut-outs on the feedback mechanism in a Mach 1.5 impinging jet using a sequence of well-validated detached eddy simulation (DES) databases. The cut-outs are placed between the impingement surface and the nozzle in order to block the bulk of the upstream acoustic waves from reaching the nozzle exit. The DES flow-field is further segregated into its hydrodynamic and acoustic components through the use of Doak’s Momentum Potential Theory (MPT). Using Dynamic Mode Decomposition, it is observed that the hydrodynamic-acoustic interaction at the nozzle exit in the baseline impinging jet results in the generation of a hydrodynamic sinusoidal perturbation in the jet shear layer. It is demonstrated that this forcing mechanism in the initial shear layer can be controlled by using an appropriate cut-out diameter in the modified configuration, ultimately resulting in a disruption of the feedback loop.