Inlet flow field of transonic rotors and the generation of buzz-saw noise

Inlet flow field upstream of supersonic cascades has been studied using the rotational method of characteristics. The study includes a technique to calculate the curved shocks in the vicinity of the blade leading edge, taking into consideration the gradient of the flow properties upstream of the shock. For a cascade of identical blades the pressure pattern consists of repetitive wave form of wavelength equal to blade spacing. At upstream locations far from the rotor, the shock strength is inversely proportional to the axial upstream distance. Larger rate of decay of shock wave strength is found close to the rotor. The dependence of flow incidence on inlet Mach number is investigated, and for the range of flow conditions considered here, the flow incidence for a fixed cascade geometry, was found to decrease with inlet Mach number. Noise produced by rotating the pressure pattern upstream of the rotor relative to a fixed observer were calculated at various axial locations and dependence of the noise spectra on the pressure wave form is obtained. Flow fields upstream of cascades of non-identical blades has also been caculated. Perturbations in flow properties due to the presence of manufacturing errors in the blades were found to be additive. Influence functions for stagger angle errors in the blades have been deduced and the pressure pattern due to variety of error configurations have been obtained.Rotation of these pressure patterns results in noise spectra at muliples of the shaft rotation frequency. The relative intensity of the various harmonics is dependent on the error configurations and the axial location upstream of the rotor. Slight deviations in blades stagger angle, which are typical of actual transonic rotor resulted in sub-harrmonies noise levels, which are comparable to that at blade passing frequency.