Numerical Study on Wave Induced Flow Field around a Vibrant Monopile Regarding Cross-Sectional Shape

Abstract

A three-dimensional numerical simulation of regular waves passing over a monopile with square and circular cross-sectional shape was carried out to investigate flow field and vortex induced vibration. The rectangular wave flume and monopile are modeled with a solver; available in the open-source CFD toolkit OpenFOAM®. This solver applies the Reynolds-Averaged Navier-Stokes (RANS) equations with the volume of fluid technic (VOF) for tracking free surface. The motion equation together with mesh deformation was applied to capture monopile displacement. To validate the numerical model, results were compared to experimental data, and an admissible agreement was seen. Computations were conducted for four cases with two different wave characteristics and different Keulegan-Carpenter (KC) numbers for square and circular cross-sectional shape. Vorticity field and Q criterion around the square and circular pile were depicted. It was seen that when KC increased, the difference in vortices around the square and the circular pile was more distinct. Investigations continued on transverse force coefficient and its oscillations. It was seen that by increasing KC, this coefficient and its frequency increased. When KC=20, the lift coefficient is larger for square pile compared to the circular pile. For both square and circular cross-sectional shape, the number of pile oscillation increased by increasing KC number. Also, the Strouhal number and vortex shedding frequency were larger for the circular pile compared to that of the square pile in vortex shedding regime. However, cross-flow vibration frequencies of the square and circular pile were close together.

Keywords

• openfoam®
• rans equations
• vortex induced vibration (viv)
• lock-in
• mesh deformation