Effects of moderate Reynolds numbers on subsonic round jets with highly disturbed nozzle-exit boundary layers
Résumé
The effects of moderate Reynolds numbers on the flow and acoustic fields of initially highly disturbed isothermal round jets at Mach number M = 0.9 and diameter-based Reynolds numbers Re D between 2.5 × 104 and 2 × 105 are investigated using large-eddy simulation under carefully controlled conditions. To the best of our knowledge, this is the first comprehensive study of its kind. The jets originate at z = 0 from a pipe nozzle of radius r 0, in which a tripping procedure is applied to the boundary layers. At the nozzle exit, laminar-like mean velocity profiles of thickness δ ≃ 0.15r 0 and momentum thickness δθ ≃ 0.018r 0, yielding Reynolds numbers Reθ varying from 256 to 1856 depending on Re D , and peak turbulence intensities around 9% of the jet velocity, are thus obtained. As the Reynolds number increases, the mixing layers develop more slowly, with smaller integral length scales and lower levels of velocity fluctuations. The axial profiles of turbulence intensities become smoother, showing a clear overshoot around z = 2r 0 at Re D = 2.5 × 104, but a monotonical growth at Re D = 2 × 105. Velocity spectra downstream of the nozzle exit also broaden with Re D , as expected. Large-scale components usually observed in turbulent boundary layers and shear layers, characterized by Strouhal numbers Stθ ≃ 0.013 around z = r 0 and by azimuthal spacings λθ ≃ δ, remain dominant, although the contribution of fine-scale structures with λθ ⩽ δ/2 strengthens. Moreover, with rising Re D , the jet potential core lengthens slightly, but the flow properties do not change significantly farther downstream. Finally, lower sound pressure levels are generated, with a decrease of about 2 dB over the range of Re D considered.
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