Hydro Elastic Behavior of Vibrating Blades
Fluid-structure investigations in hydraulic machines using coupled simulations are extremely time-consuming; therefore we develop an alternative method. In this paper, a model is proposed to predict fluid-structure coupling by linearizing the hydrodynamic load acting on a rigid oscillating 2D hydrofoil surrounded by an incompressible turbulent flow. Forced and free pitching motions are considered with a mean incidence of 0° and maximum amplitude of 2°. Unsteady flow simulations, performed with ANSYS CFX, are presented and validated with experiments carried out in the EPFL High-Speed Cavitation Tunnel. The hydrodynamic moment is assumed to result from three actions: inertia, damping and stiffness. The forced motion is investigated for reduced frequencies ranging from 0.02 to 100. As expected by the potential flow analysis, the added moment of inertia is found constant, while the fluid damping and the fluid stiffness coefficients are found to depend solely on the reduced frequency after an appropriate scaling. Behavioral patterns are observed and two different cases are identified depending on the development of vortices in the hydrofoil wake. Using the coefficients identified in the forced motion case, the time history of the profile incidence is then predicted analytically in the free motion case. An excellent agreement is observed with results from coupled fluid-structure simulations. The model is validated and can then be extended to more complex cases such as blade assemblies.