Two-way coupled simulation of the Francis-99 hydrofoil using model order reduction

Abstract

The Francis-99 hydrofoil is simulated using a quasi two-way Fluid-Structure Interaction procedure. The structural domain is reduced by the use of modal decomposition, and solved for inside the commercial fluid solver ANSYS CFX. Both the first order Backward Euler and second order Crank-Nicolson time discretization scheme is used in the structural equations, with significantly different results. Several coupled fluid-structure phenomena is observed that would be unobtainable in a normal one-way approach. The most interesting is an "added stiffness" effect, where the eigenfrequency of the foil increases when the flow velocity is increased. This trend corresponds well with available experimental results. The same phenomenon is observed in the hydrodynamic damping on the foil. Self-induced vibration due to vortex shedding is also simulated with good results. The implemented two-way approach allows the different forcing terms to be tracked individually, due to the discretization of the second order structural system. This provides insight into the underlying physics behind the different FSI phenomena seen, and helps us explain why the damping and eigenfrequency characteristics change as the flow velocity passes the lock-in region.