The nonlinear harmonic method: from single stage to multi-row effects

Abstract

The nonlinear harmonic method (NLH) and its extension to the modelling of the clocking effects can simulate the unsteady flows in multistage machines at much lower CPU cost. The bladerow interactions that can be reproduced are those between adjacent rotors and stators and also between successive rotors or successive stators, including the contra-rotating open rotors (CROR). However, the simulation of the unsteady effects of the engine pylon on the flow in the rear rotor requires the modelling of the time-varying disturbance from a stator of periodicity 1 (the pylon) into a rotor that is not adjacent, requiring an extension of the current NLH method. In addition, it has been observed that in some stator1/rotor/stator2 configurations, the amplitude of the perturbation associated with a blade passing frequency (BPF) in a row is not only created by the adjacent row but is also modified in space by the interaction of the periodic disturbances coming from the other two rows. For instance, for a struts/rotor/stator machine, the spatial frequencies of the periodic disturbance in the stator could be provoked not only by the interaction with the adjacent rotor but also by an additional interaction with the periodic flow coming from the struts. The present paper introduces a method that enables the identification of these spatial frequencies for a general row1/row2/row3 configuration. Besides, any row can be a stator or a rotor and the time-varying disturbances are not only produced by the interaction with the adjacent row but also with the other rows. A more general NLH method is therefore obtained. The proposed method was validated against a three-cylinder test case, a CROR test case, a struts/rotor/stator case and an impeller/bladed diffuser/volute case.