Experimental characterization of the velocity field of a coaxial jet configuration

Abstract The mean and fluctuating flow fields of a coaxial jet configuration, with ratios between inner and outer diameters and velocities D i /D o = 0.485 and U i /U o = 0.67 , turbulent exit boundary layers, and high turbulence levels in the exit cores are studied by means of LDA and hot-wire measurements, using an experimental facility designed for the purpose. The profiles of the mean axial velocity, of the axial and radial turbulence intensities, and of the shear stress are described for the initial and intermediate zones of the near field. The lenghts of the potential cores are in good agreement with available data obtained with lower exit turbulence levels and laminar boundary layers. It is shown that the prevailing frequencies of the flow fluctuations may be better recognized if the hot-wire spectra of both the axial and the radial velocities are analyzed. These frequencies show significant radial differences in the initial and intermediate mixing zones but become progressively more uniform with increasing distance from the exit. In the near-exit region of the inner mixing layer evidence is also found of a probable alternate vortex shedding from the inner duct wall. This phenomenon is shoen to exist for U i /U o ≥ 0.44 , with a constant Strouhal number St t = 0.24 , based on the thickness of the wall and on the average velocity of the two streams.