Spectral characteristics of the pulsation effect of a plane turbulent jet on a solid surface

Abstract

w 1. The present experiment was conducted with plane air jets issuing from a slot nozzle of width d=15 ram (length 350 ram) at velocities of 75-220 m/sec. The Reynolds numbers hence varied in the range 0.7- 1052.1- 105, and the Much number, in the range 0.2-0.64. The jet impinged on the flat surface of a massive turntable at distances of 360-640 mm from the nozzle exit. Modules with piezoelectric pressure fluctuation converters, similar to those described in [6], were mounted flush with the working surface of the slab. Spectral analysis of the signals from the converters was accomplished by an SI-1 spectrometer in three-octave frequency bands in the 50-10,000-Hz range. Transducers with a 1.3-ram-diameter detection surface had a practically constant response of about 4pV/Pa with respect to the frequency in this range. In order to check the vibration interference, the vibration response of the transducers was determined and the slab vibrations were measured during the tests. w It was clarified in an analysis of the measurement results that the governing parameters of the fluctuating effect of the jet on the surface perpendicular to the jet at a distance x from the nozzle exit are the mean characteristics (the density p, the axial velocity v, and the width 2b) of the equivalent free jet at a distance x. This agrees with the information that the zone of interaction in the case under consideration extends a distance on the order of the nozzle width along the normal to the surface [7, 8]. Therefore, the flow in this domain should be determined by the free jet characteristics at a distance on the order ofx-d or, for x >> d, at a distance x from the nozzle exit. It is seen from Fig. 1 that the values of the spectral density of the pressure fluctuations r 2v3b, reduced to dimensionless form, are functions of the reduced frequency vb/v and the relative removal y/b from the jet axis in the whole range of velocities and distances x investigated. Three cases with different values of y/b axe presented here: a) 0; b) 0.73; c) 2.7. The numbers correspond to the following values of v0inm/sec and x/d: 1) 75, 24; 2) 75, 33; 3) 75, 43; 4) 117, 24; 5) 117, 33; 6) 117, 43; 7) 218, 24; 8) 218, 33; 9) 218, 43.