The effect of friction contacts on the dynamics of a rotating vane segments array : simulation and comparison with experimental result

One important concern in the design of a counter rotating turbine (CRT) is the structural vibration of the turbine components and its passive control by means of friction contacts in the joints whose design is a critical issue. A test campaign was performed on a simplified counter-rotating turbine stage. A test article which reproduces the dynamic behavior of a real CRT stage was purposely designed and constructed. The test article is composed of 24 segments, each one with 3 blades. As in the real case the segments are constrained at the outer radius and mounted with interlocking with friction contacts at the inner radius. The segments are connected to an outer ring by means of a double hook attachment with contact surfaces which can be characterized by sliding or sticking depending on the constraint which can be tight or loose. The tests were carried out with two aims: - evaluate the dynamic behavior and the vibration damping capability due to friction of the dummy CRT stage - produce experimental data in order to validate the numerical model of the non linear forced response of the test article. The calculation of the non linear forced response is performed in Matlab by reducing the Finite Element mass and stiffness matrices by means of a suitable reduction technique that is able to account for the cyclic symmetry of the rotor. Contact elements are located where the vane segment is expected to slip and different conditions for the hook attachment (tight or loose) and for the interlocking (slip or stick) are investigated in order to simulate the response of the test rig under different configurations. Harmonic Balance Method is used to reduce the calculation time requested by the iterative procedure. Sensitivity analysis of the parameters involved in the calculation is performed to verify the consistency of the mathematical approach and their effect on the dynamics of the CR turbine.