Numerical studies of viscous flow around bluff bodies
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- Institutt for marin teknikk 
A bluff body, i.e. an object with blunt rather than streamlined shape, exposed to external flow gives rise to a multitude of flow patterns depending on the timeand length-scales, the free-stream velocity, and the viscosity of the fluid. In this work, three-dimensional numerical solutions of the Navier–Stokes equations are used to study the physics of two interesting cases of flow past bluff bodies. The first is the turbulent wake behind a curved circular cylinder. This is an important flow configuration for the offshore industry because it represents a model to study the flow past catenary risers and curved pipelines. Carefully validated direct numerical simulations are the basis for the identification of upper and lower wake regions with distinctly different flow dynamics. Results concerning turbulence in the wake, vortex dynamics, secondary flows, and instabilities in the shear layers, are presented and discussed. The second problem addressed is that of oscillatory flow around cylinders, with focus on the primary vortical instability for circular and elliptic cross-sections. New findings for both cylindrical configurations are reported in the light of vortical structures and time-series analysis of velocity and vorticity samples. Furthermore, predictions from a previous stability theory for elliptic cylinders are confirmed for the first time.