Turbulence characteristics of an incompressible free jet under the influence of acoustic disturbances

Abstract

An acoustically excited (St = 0.50), Mach 0.15, axisymmetric free jet operating at Reynolds number of 41,000 was studied in order to determine the transition behavior, the overall turbulence characteristics, and to quantify the contributions of the coherent structure. Axial and radial turbulence intensities and the Reynolds stress were measured. The axial and radial flow fluctuations were ensemble averaged to determine the nature of the fluctuations of the coherent structure. Reynolds stress contours were obtained for the flow field. When combined with the mean velocity distribution, contours of turbulent kinematic eddy viscosity were obtained. These values maximized at 400 times the thermodynamic kinematic eddy viscosity of the air along the center line, downstream of the potential core end. Above a length Reynolds number of 30,000, the transition to a turbulent jet began to occur. Hence, the dimensionless eddy viscosity started to increase until it reached an asymptotic value of 0.00095 at a length Reynolds number of 300,000 which established the fully turbulent region of the jet. The fluctuation levels maximized in the shear layer when the potential core was present and on the jet centerline downstream of the potential core. Maximum turbulence levels of 12% and 11% were obtained for axial and radial turbulence intensities, respectively. The coherent structure maximized in amplitude before the end of the potential core and decayed rapidly. The axial fluctuations maximized at 3% of the jet exit velocity on the centerline of the jet; the radial fluctuations maximized at the same level (3%), but in the shear layer of the jet. Spectral cascades of the axial and radial coherent structure fluctuations were performed which illustrated the spatial dependence of the coherent structure's frequency content. Relative axial phase contours over the entire jet showed that at each axial location the phase of the axial fluctuations remained comparatively constant in the core region of the jet, but varied rapidly within shear layers. In contrast, the relative radial phase distribution changed 180$spcirc$ along the centerline of the jet but remained constant within the shear layers. These relative axial and radial phase distributions were characteristic of a vortex ring structure.