On the fluidic behavior of an over-expanded planar plug nozzle under lateral confinement

The present work aims to study the fluidic behavior on lateral confinement by placing sidewalls on the planar plug nozzle through experiments. This study involves two cases of nozzle pressure ratio (NPR = 3, 6), which correspond to over-expanded nozzle operating conditions. Steady-state pressure measurements, together with schlieren and surface oil flow visualization, reveal the presence of over-expansion shock and subsequent interaction and modification of the flow field on the plug surface. The flow remains attached to the plug surface for NPR = 3; whereas for NPR = 6, a separated flow field with a recirculation bubble is observed. Spectral analysis of the unsteady pressure signals illustrates a clear difference between the attached and the separated flow. Besides, other flow features with a distinct temporal mode associated with and without lateral confinement are observed. The absence of lateral confinement reduces the intensity of low-frequency unsteadiness; however, on the contrary, the interaction region is relatively reduced under lateral confinement.