| Passive Turbulence Control (PTC) in the form of selectively distributed surface roughness is used to alter Flow Induced Motion (FIM) of a circular cylinder. Recent developments in the Marine Renewable Energy Laboratory (MRELab) of the University of Michigan on PTC of spring-mounted rigid circular-cylinders have improved dramatically the hydrodynamics of hydrokinetic energy harnessing. In the present study, two-dimensional RANS with the Spalart-Allmaras turbulence model is used to simulate flow past a circular cylinder undergoing FIM transversely to the flow direction. Flow simulation is performed using a special flow solver built upon the OpenFOAM, open-source CFD library. The governing equations are solved numerically with a finite volume discretization method. Gaussian integration with a linear interpolation scheme is selected for spatial terms and a backward Euler scheme is used for time integration. The momentum equations are solved in a segregated manner, and the pressure and momentum variables are coupled using a PISO algorithm. Roughness parameters model accurately the PTC applied in the corresponding model tests in the MRELab. Sand paper with thickness of P attached to the cylinder wall is modeled as part of the body geometry. The average height of sand grit (k) is taken into account in the wall function as boundary conditions. Series of cases with different system parameters (spring constant, damping) are simulated and the results are compared with experimental results derived in the Low Turbulence Free Surface Water Channel of the MRELab. Amplitude ratio (A/D) curves show clearly 3 different branches: the VIV initial branch, the VIV upper branch, and a galloping branch, similar to those derived experimentally. Frequency ratio, vortex pattern, transitional behavior and lift are also investigated and provide good agreement with experimental measurements. The system parameters are: Mass ratio 1.88, damping ratio zetaharn 0--0.12, and spring constant 1,200--2,000 N/m. Reynolds number ranges from 30,000 to 130,000. The effects of system damping, stiffness and placement angle are further studied. FIM enhancement and suppression in various zones are investigated under the guidance of PTC-to-FIM map derived in the MRELab. The phase angle by which the lift force leads the displacement is found to have a major effect on determining the FIM behavior in different zones. 2D-RANS simulation of circular-cylinder flows for Re>10,000 is challenging. Nevertheless, with proper modeling of PTC, 2D-RANS simulations exhibit several of the salient features of experiments and can provide complementary flow information, which would be difficult to measure experimentally at such high speeds and turbulence levels. |
