Showing papers by "Christopher K. W. Tam published in 1974"

Discrete tones of isolated airfoils

Abstract: Recent experimental measurements reveal that discrete tones are emitted by isolated airfoils under certain operating conditions. Arguments are presented that these tones are unrelated to vortex shedding processes as has been suggested. It is proposed that the tones are generated by a self‐excited feedback loop of aerodynamic origin. The loop consists of large‐scale unstable disturbances in the boundary layer and wake flow and the feedback acoustic waves. Quantitative deductions based on the feedback loop model compare favorably with experimental measurements. It is also found that the proposed model is consistent with the observed characteristic features associated with the discrete tone phenomenon.

Intensity, spectrum, and directivity of turbulent boundary layer noise

Abstract: The problem of noise radiated from a turbulent boundary layer is studied. It is found that the farfield noise intensity, spectrum, and directivity can be determined completely if the nearfield pressure cross‐correlation function is known. The role of supersonic and subsonic pressure components associated with the turbulent boundary layer in relation to noise radiation is examined. By using an empirical model of the wall pressure cross‐correlation function it is found that the directivity pattern of turbulent boundary‐layer noise differs considerably from that of a free dipole. One principal reason for this difference is the fact that the noise sources of a turbulent boundary layer are constantly in motion. The effect of mean flow outside the boundary layer on the radiated noise intensity and spectrum is also investigated. Numerical results indicate that this effect is very important for low‐frequency noise components and for high subsonic flow Mach number.