Blade Oscillations in One-Stage Axial Turbomachinery

Abstract

Two parallel straight cascades moving against one another with constant velocity are considered within plane incompressible potential flow. The position of the exciting active cascade is assumed upstream as well as downstream of the passive cascade. Free vortex sheets start only from the trailing edge of the passive cascade. Assuming infinite thin blades with small camber, small elongation and harmonic excitement, amplitudes of instationary lift and moment in relation to the corresponding stationary quantities are calculated as a function of pitch ratio of the two cascades, stagger angles of both cascades, and ratio of plate circulation to total circulation. These amplitudes reach up to 20 percent of the corresponding values in stationary flow but do not form a pronounced maximum in case of equal pitches. This effect increases if the vortex sheets from the neighboring passive profiles are neglected; it also increases with increasing pitch-chord ratio and decreasing number of harmonics. A comparison with the theoretical results of Kemps and Sears [15] shows up to 30 percent smaller lift amplitudes. Downstream position of the active cascade increases the amplitudes, especially in the case of increasing angle of attack.