Visualization of Supersonic Free and Confined Jet using Planar Laser Mie Scattering Technique

Abstract

Planar Laser Mie Scattering (PLMS) imaging technique isemployed in supersonic axisymmetric free jet and supersonic rectangular confined jet to extract out the prominent flow features. Efficient seedingof the flow field is achieved using an in-house seeder unit which employsmodified Laskin Nozzle. For this imaging, flow field is seeded with particles of water or DEHS in case of free jet studies and with acetone for confined jet studies. Using high power laser and high speed camera, flow field is illuminated and captured.A series of simple image processing routines are carried out to extractthe noticeable attributes of the flow. Prominent flow features like Mach disc, shear layerinstability and shock cells are captured in the instantaneous PLMS imagesof a severely under-expanded supersonic axisymmetric free jet of designMach number (MD) 1.367. Qualitative increase in the shock cell spacing(Ls) for increasing Mach number ratio (MR) is observed for time-averagedPLMS imaging of a supersonic axisymmetric free jet (MD = 2.0 & 2.5). Alsoan effective increase in the jet width of an over-expanded jet undergoingscreech is noticed. Major flow features like large scalecoherent structures and terminal potential core instability are seen clearly in the instantaneous PLMS images of a rectangular supersonic confined jet of M_PD = 2.0.Shock cell spacing and mixing layer formation are observed distinctly inthe time-averaged PLMS images. Qualitative increase in Ls for increasingstagnation pressure ratio (SPR) is highlighted along with the increase inthe wavelength of the potential core instability. More details regardingthe experimentation procedure and the underlying physics behind the observed flow features along with the variations encountered for differentoperating conditions are discussed elaborately in this paper.