CFD simulation of the flow over a 2-dimensional pipe and vortex induced vibration of the pipe with 1 degree of freedom

Abstract

The fluid that flows through a pipeline free span can generate vortex shedding around the pipe and triggers Vortex Induced Vibration (VIV). This VIV is a major source of fatigue damage due to dynamic stresses in the pipeline free span. Therefore, it is necessary to observe the vortex shedding around the pipe since it could help to understand the VIV better. The objective of this thesis is to simulate a steady uniform flow that flows over a 2-dimensional pipe, which in this case is represented by a circular cylinder, with different parameters such as Reynolds number and span height using a computational fluid dynamic software called as OpenFOAM. Flow with different Reynolds numbers are simulated as laminar flow (20 ≤ Re ≤ 1000) and turbulent flow (10,000 ≤ Re ≤ 10,000,000), and then it will be modeled to flow over a free fixed pipe. Afterwards, a fixed pipe with seabed proximity will be simulated in a laminar flow (Re = 400). Lastly, VIV simulations will be done for a free 2-dimensional pipe with 1 degree of freedom enabled in a laminar flow (Re = 500). In the VIV simulation, the mass and stiffness of the pipe will be modified to see how it will affect the VIV behaviors. For flow over a free fixed pipe in laminar flow, the value of drag coefficients stays almost always constant and the amplitude of the lift coefficients increases as the Reynolds number increases. The Strouhal – Reynolds number relationships of the laminar flow simulation have a good agreement with other experimental results. The Strouhal – Reynolds number relationships of the turbulent simulation have a good agreement with the Strouhal – Reynolds number relationships from other literature. However, the drag coefficients are much higher than the drag coefficients for cylinders that were given by Cengel and Cimbala (2010). Conclusively, the turbulent flow simulation still need improvements, one of it is by changing the turbulence model from RANS to LES that can handle sophisticated vertices. The span height of the pipeline free span suppresses the vortex shedding phenomenon as the gap ratio gets smaller. The drag coefficients decreases as the gap ratio gets smaller, while the lift coefficient has a unique relationship with the gap ratio, as described by Sumer and Fredsøe (1997). However, the simulation results in this thesis could not draw the same relationship due to lack of simulation data. In the VIV simulations, when the reduced velocity reaches 4, the pipe is in resonance. Conclusively, we shall keep the reduced velocity to be always less than 4 to avoid this resonance. This can be done by modifying the Eigen frequency, which is affected by the mass and stiffness of the pipe.