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.