Initial boundary layer state of typical model-scale jet nozzles and its impact on noise

Abstract

A set of 2-inch diameter nozzles is used to investigate the effect of varying exit boundary layer (BL) states on the radiated noise from high-subsonic jets. It is confirmed that nozzles involving turbulent boundary layers are the quietest while others, involving nominally-laminar BLs, are noisier. A turbulent BL is thicker and there is simply an effect of thickness on noise. A thicker BL results in a decrease in the sound pressure spectral amplitudes due to a less vigorous growth of instability waves in the jet’s shear layer. A nominally-laminar BL, besides being thinner, may also involve significantly higher turbulence intensities, much higher than that in a turbulent BL. Such a BL state, referred to as ‘highly disturbed laminar’, results in the largest noise amplitudes especially on the high-frequency side of the spectrum. This transitional state, often encountered with model scale nozzles, involves a ‘Blasius-like’ mean velocity profile but large velocity fluctuation intensities and intermittency. The higher initial turbulence adds to the increase in high-frequency noise. The results leave little doubt that an anomaly noted with subsonic jet noise databases in the literature is due to similar effects of differences in the initial boundary layer state.

Characteristics of jet noise: A synthesis

“…The use of the acceleration parameter K * for establishing the state of the boundary layer does not provide conclusive proof, though certain insights may be gleaned. Recently, Zaman 71 made boundary layer measurements from a conic and ASME nozzles; one of the variants of the ASME nozzles had a short straight section; another had an extension of a straight pipe of 3D to the end of the short section. The boundary layer characteristics were classified as turbulent, nominally laminar, and highly disturbed laminar.…”