Numerical investigation of boundary layer effects on vortex shedding frequency and forces acting upon marine pipeline

Abstract

In this study the force components and vortex shedding frequency of a pipe exposed to a steady current were numerically investigated in terms of the drag coefficient, lift coefficient and Strouhal number. The effects of the bed proximity and boundary layer thickness on these parameters were studied extensively. The Reynolds-averaged-Navier-Stokes equations with a k-e turbulence closure model were numerically solved to approximate both the flow pattern and pressure distribution around the pipe. The instantaneous drag and lift coefficients were calculated based on the pressure distribution around the pipe, while the Strouhal number was estimated by the spectral analysis of the predicted instantaneous lift force. Evaluation of the numerical model revealed that the model well predicted the velocity profile around the pipe, force coefficients and Strouhal number. The results showed that the model slightly over-predicts the mean force coefficients and Strouhal number. It is concluded that the mean force coefficients and the root-mean-square (RMS) lift coefficient are strongly affected by the gap to diameter ratio while the Strouhal number is slightly affected by the gap ratio. The results also indicated that the mean lift force acting on the pipe is upward in all boundary layers and is rapidly decreased by increasing the gap ratio, whereas the mean drag force is slightly increased as the gap ratio is increased up to a certain value. The RMS lift force also increased as the gap ratio increased up to a certain value and remained approximately constant for further increase of the gap ratio. The mean force coefficients and Strouhal number calculated in terms of the flow velocity at the pipe axis are marginally influenced by the boundary layer thickness, while the parameters calculated in terms of the free-stream velocity are strongly affected by the boundary layer thickness.