Jet flows are prevalent in many practical aerospace systems, and also represent a canonical configuration for understanding basic flow physics. An underexpanded jet, like those common in jet or rocket propulsion systems, will generally have that characteristic “shock diamonds” associated the off-designed pressure conditions. Jet screech is an acoustic disturbance phenomena associated with the interaction between these shocks and the turbulent structures in the jet flow. The resulting interaction creates a feedback loop that producing self-sustained oscillations of the jet flow-field. Understanding and controlling this phenomena has been a topic of interest for several decades, and researchers are still learning the details of the interactions.
In the image below, you can see the effects of changing the nozzle pressure ratio. Not only does the structure of the jet change, but you can see the intense acoustic radiation generated by the jet screech mechanism. Of particular note is the upstream acoustic radiation that is a critical aspect of self-sustainment of jet screech, but also shows that acoustic disturbances can propagate back towards the vehicle. The other video shows the underexpanded jet at a steady nozzle pressure ratio. Note the shock structures within the jet.
While schlieren provides information about the acoustic radiation, it’s path-integrated nature is restrictive for studying the true dynamics of the flow. Therefore, particle image velocimetry was done at 50 kHz to get quantitative maps of flow velocity in a planar slice through the jet.
It is important to understand the frequency associated with the screech phenomena, and this can be found using Fourier analysis. This analysis can be applied to both images and simple voltage recordings from microphone measurements. Both indicate that a 17 kHz tone was present as shown by the sharp peak in the spectra below.