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.
Highlights
In the last couple of decades, numerical simulation and Computational Fluid Dynamics (CFD) have become one of the pillars in fluid mechanical research, alongside experiments and analytical work
The structural domain is reduced by the use of modal decomposition, and solved for inside the commercial fluid solver ANSYS CFX
Based on the above it is concluded that the Crank-Nicolson scheme should be used, especially as it carries no additional computational cost
Summary
In the last couple of decades, numerical simulation and Computational Fluid Dynamics (CFD) have become one of the pillars in fluid mechanical research, alongside experiments and analytical work. The tools are in constant development and are in need of validation and testing. The turbine designs are usually confidential, which makes it difficult for academic institutions to do research on state-of-the-art geometries. The Francis-99 workshops aims to provide an open source geometry and experimental data for validation of numerical tools and methods [1]. The third Francis-99 workshop deals with Fluid-Structure Interaction (FSI). Two test cases are available to the public, one case on resonance in turbine runner channels, and one case on a more fundamental issue, hydrodynamic damping and eigenfrequencies of submerged hydrofoils. This paper will focus on the Francis-99 hydrofoil
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