Vortex-Induced Vibrations

Bluff_IA fluid flow past a bluff body, such as a circular cylinder, will result in the periodic shedding of vortices into the body’s wake for all but the lowest flow speeds. This process gives rise to oscillatory lift and drag forces which, if the body is compliant or elastically supported, can result in Vortex-Induced Vibrations (VIV). VIV can in turn lead to fatigue damage in vibrating structures, which makes it an important issue in the design of bridges, chimney stacks and marine riser pipes.

The work of the group is divided into two main areas: fundamental research into VIV of circular cylinders in low Reynolds number flows, and VIV of long riser pipes in flows with much higher Reynolds numbers.

The principal aim of the low Reynolds number VIV research is to use numerical models to further the understanding of the types of synchronisation that can exist between the vortex shedding form a cylinder submerged in a flow and the motion of the cylinder. A high order spectral element Computational Fluid Dynamics (CFD) code, Nektar, is employed to compute fluid flow past a circular cylinder that is undergoing vibrations transverse to the direction of the flow. Presently the mode of vortex shedding, which describes the manner in which the vortices are formed at the surface of the cylinder and is an indicator of the level of synchronisation that exists between the cylinder and the shedding, has been carefully analysed over a wide parameter space and has exposed some interesting trends. The lift component of the lift force acting in-phase with the cylinder’s velocity contains information relating to the direction of power transfer between the fluid and cylinder: whether the fluid is exciting or damping the motion of the cylinder; the present research has shown good agreement with experimental data in this respect, and has uncovered the hitherto unknown reason for which hysteretic responses are often observed in laboratory experiments.

Riser_IFatigue damage due to VIV in marine riser pipes represents a significant problem for the offshore oil industry. Current industry-standard VIV prediction strategies involve the use of very large safety factors, which is unsatisfactory from an engineering perspective. A means by which VIV response can be accurately and reliably predicted is therefore extremely desirable. The work of the group in this area focuses on the use of a two –dimensional CFD code, VIVIC, to predict such riser pipe vibrations. The code takes a strip theory approach, whereby fluid flow is computed on multiple two-dimensional planes that are positioned at intervals along the axis of a long vibrating pipe, modelled by a Finite Element assemblage. The model permits motions in both the in-line, x, and cross-flow, y, directions. By numerically simulating the experimental set-up of Trim et al (Journal of Fluids and Structures, 2005), the research has shown that VIVIC is capable of accurate simulation of in-line and cross-flow VIV, and has indicated that travelling waves are predominant along the length of the pipe, even when the flow profile is uniform. Super-harmonic responses have been also been observed, and they have been shown to be dominant with respect to riser curvature in a number of instances. Furthermore, fatigue analysis has revealed that the super-harmonic responses cab increase fatigue damage rate by an order of magnitude in the in-line direction and by two orders in the cross-flow direction.

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