PLIF investigation of coannular supersonic jets

Abstract

We present experiments on the morphology and evolution of large turbulent eddies in coannular supersonic jets. The study encompassed Mach 1.5, round, perfectly expanded jets composed of air or a mixture of helium and air. A double-exposure planar laser-induced fluorescence (PLIF) system, using gaseous acetone as the tracer molecule, enabled visualization of the turbulent structure of the jet and its evolution a short time later. The convective velocity of the eddies was extracted from the PLIF images by means of twodimensional cross correlations. Eddies in the air jet propagate with a speed approximately 80% of the local centerline velocity and are subsonic with respect to the ambient air. The helium-air jet, which is faster than the air jet, emits Mach waves and exhibits substantial turbulent motions in the azimuthal direction. In the helium-air jet, addition of a Mach 0.82 secondary flow reduces the convective velocity of the primary eddies from 72% to 63% of the primary exit velocity. The speed of the secondary eddies is 44% of the secondary exit velocity. All turbulent motions in this coannular helium-air jet are intrinsically subsonic, leading to elimination of Mach waves and substantial reduction in noise. The results of this study are relevant to mixing, combustion, and jet noise. behavior of fully-turbule nt shear layers suggest that large eddies constitute the dominant instability of the flow [2, 3]. A key aspect of the instability is its phase speed, or convective velocity Uc. In subsonic shear layers, Uc controls the entrainment ratio and influences the growth rate [4]. In high-speed shear layers and jets, the value Uc determines the production of strong noise emission. In compressible mixing layers, large eddies lose their organization but are still ubiquitous [5]. The compressibility of mixing layers is characterized by the Mach number in the frame of reference of the instability, called the convective Mach number. For an instability wave traveling with a velocity Uc between a fast stream (1) and a slow stream (2), the convective Mach number takes the values MC1 = Ui-Uc