Flow past a horizontal cylinder

Abstract

Underwater pipelines are used for the transportation of oil and gas across rivers and oceans.

They obstruct the flow of water which causes erosion beneath these pipelines. Once the gap

between the pipeline and the bed attains a critical value, the phenomenon of vortex shedding

starts. The pipeline under the influence of vortex induced vibrations (VIV) can undergo failure.

Simulations for flow past a horizontal cylinder is studied numerically in the present work.

The time-dependent, incompressible, two-dimensional and turbulent flow is solved using the

finite volume method (FVM) formulation of the Reynolds Averaged Navier-Stokes equation

and k − ω SST turbulence model. An open source CFD package (OpenFOAM) is used for

this purpose. Dissipation of the vortex shedding for a pipeline which has a gap more than the

critical gap is suggested using different shapes such as elliptical cylinder, airfoil fairing around

the cylinder, dimpled cylinder, split cylinder and cylinder with split shells. The flow is solved

for a gap ratio, G/D = 0.5 and inlet velocity, U = 0.2 m/s. The results for each of these

shapes are compared on the basis of flow parameters such as drag coefficient, lift coefficient

and the wall shear stress on the bed. In addition, a solver to calculate scour below the pipe

using flow fields obtained from OpenFOAM is developed. The flow parameters obtained are

used to calculate sediment transport, which in turn are used for the calculation of changes in

bed elevation. After obtaining these changes, a new mesh is generated and the entire process

is repeated until an equilibrium scour profile is obtained. The whole process is automated

by writing scripts in bash and python scripting languages. The numerical scour results are

underestimated as compared to the experimental results.