Wake characteristics of stationary catenary risers with different incoming flow directions

In this paper, we numerically investigate three-dimensional flow past a freely hanging stationary riser in catenary shape (referred to CR) with the angle of incoming flow direction with respect to the plane aligned with the CR ranging from α=0∘∼90∘ at a low Reynolds number of 100. The flow dynamics are governed by the incompressible Navier-Stokes equations and a high-order spectral/hp element method is employed for the solutions of the flow field. The numerical results present that the wake topology behind the CR is strongly influenced by the incoming flow direction. In particular, in the range of small angle of incoming flow direction (0∘≲α≲40∘), the wake is completely suppressed; in the medium range of 40∘≲α≲50∘, vortex shedding is developed firstly at the top side of the concave part, and then gradually spread over the span towards the convex part; wake instability is developed sufficiently along the whole span when the flow incidence is over 50∘. The wake dynamics is further explored by examining the temporal evolution of hydro-forces and field quantities along the span and their frequency characteristics. It is revealed that the mechanism of axial flow that is varied along the span of CR can explain hydro-dynamics of the wake behavior of the CR.