Abstract
This paper presents the experimental results on the acoustic power of flow-generated noise and the head losses of silencers and methods of estimating them. The experimental apparatus is schematically shown in Fig. 1. The PWL (the over all power level, re. 10^<-12> watt) was calculated by Eq. (1). The geometries of the test silencers are given in Table 1. Figure 2 shows the relation of SPL to the frequency when flow velocity was changed. The sound of high frequency is in propotion to 6-7th power of the mean flow velocity. Figure 4 illustrates the dependence of the PWL of flow-generated noise on the geometric parameters of silencers. It can be expressed from these experimental results that the PWL depends primarily on mean Mach number. The PWL increases with increase of the expansion ratio of cross sectional open area and of the diameter of the tail pipe, but it decreases with increase of the length of the chamber. It is shown in Figs. 8 to 13 that the variation of the PWL depends on the pressure fluctuation producted by the separation of flow at the inlet of silencers. Table 3 gives the PWL of noise generated by flow through the silencers. It is shown clearly that the PWL depends on the geometrical parameters of the silencers and the mean flow velocity. Figures 14 and 15 show the band PWL in relation to overall PWL as the relative level. The relationship between Strouhal number and the frequency is in accordance with Eq. (2). It may be seen that the power spectra of flow-generated noise show the maximum level around Strouhal number of about 0. 2, and above this frequency, it decreases gradually. Table 5 gives the experimental results of the head losses in the silencers. It should be noticed that the head loss ⊿P depends primarily on the mean velocity of air flow. The ⊿P increases with increase of the length of the silencers, and it decrease with increase of expansion ratio of the cross sectional open area.
