Abstract:
A new formulation for predicting sound radiation from a centrifugal blower in a free field is derived. The fundamental difference between the present formula and the previously derived ones is that the former considers a new monopole, while the latter assumes the dipole as the primary source. For an axial flow fan or a rotor blade under steady flow condition, the usual monopole source proportional to ((rho)[inf o](cursive beta)v[inf n]/(cursive beta)t) is negligible [Siddon, J. Acoust. Soc. Am. 53, 616--633 (1973)]. However, for a centrifugal blower the monopole source induced by Coliolis acceleration ((omega)xv[inf t]) cannot be neglected because of a large inclination angle of the impeller. With the inclusion of this monopole source term, the formula yields a dependence of the radiated sound power on the fourth power of the azimuthal speed, which agrees with the experimental data [Maling, J. Acoust. Soc. Am. 35, 1556--1564 (1963); Neise, J. Sound Vib. 43, 61--75 (1975)]. The resulting formula can be used to derive the radiated sound power spectrum using the turbulence correlation curve. The spectra thus obtained are compared with the measured ones from a centrifugal blower mounted in a free field. A favorable agreement is obtained in all test cases.