Analytical prediction of the unsteady lift on a rotor induced by downstream flow obstructions

Abstract

A two-dimensional, inviscid, incompressible procedure is presented for predicting the unsteady lift on turbomachinery blades caused by the upstream potential disturbance of downstream flow obstructions. The method is applied to a particular geometry which consists of a rotor, a downstream stator, and downstream struts which support the engine casing. Using the Douglas-Neumann singularity superposition computer program to model the downstream flow obstructions, classical equations of thin airfoil theory are then employed, to compute the unsteady lift on the upstream rotor blades. Very good agreement between the Douglas-Neumann program and experimental measurements was obtained for the downstream stator-strut flow field. The calculations for the unsteady lift due to the struts were in good agreement with the experiments in showing that the unsteady lift due to the struts decays exponentially with increased axial separation of the rotor and the struts. However, the calculations for the unsteady lift due to the stator were two orders of magnitude smaller than that measured in experiments. This is attributed to the strong viscous interaction between the rotor and stator blade rows.