FREQUENCY AND TIME DOMAIN FLUID-STRUCTURE COUPLING METHODS FOR TURBOMACHINERIES

Abstract

Two methods of fluid-structure coupling for turbomachinery are presented, the first one in the frequency domain and the second in both frequency and time domains, with the assumptions of linearized aerodynamics and cyclic symmetry. This paper is concerned with the coupled fluid-structure dynamic analysis of turbomachinery. The structure consists of a rotating bladed disk submitted to the unsteady aerodynamic forces exerted by the fluid, which are themselves generated by the structural motion. The structure and the fluid are assumed to have a perfect circumferential cyclic symmetry, so that the classical reduc- tion of the analysis to only one reference sector can be applied. The study of the structure comes down to that of the reference sector by applying the ap- propriate boundary conditions for each phase number. The displacements of the reference sector in the travelling wave coordinates are expressed as a lin- ear combination of the complex modes or Craig and Bampton's basis and the motion equations are projected on these bases to obtain a reduced system. In the coupling methods proposed here Tran et al., 2003, the unsteady aero- dynamic forces are assumed to depend linearly on the structural displacements and velocities and they are expressed in terms of those induced by the modes. The mode-induced aerodynamic forces are computed only once at the begin- ning of the simulation by using an aerodynamic code (solving the Euler equa- tions) with the assumption of harmonic motion of the modes, for an inter-blade phase angle and a number of reduced oscillation frequencies.