Scale effects in testing of a monopile support structure submerged in tidal currents.
Okorie, Oghenevwori Patricia
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Climate change and energy security are key issues motivating the development of Tidal Current Energy (TCE) technology. Small scale model testing of TCE devices is a major challenge facing the technology due to turbulent flow in tidal currents environment and scaling is a fundamental engineering procedure for prototype development and optimisation for cost effectiveness. This thesis presents the outcome from investigating Dimensional Scale Effects (DSE) and Turbulence Scale Effects (TSE) of a monopile support structure using Computational Fluid Dynamics (CFD). Scale models dimensioned according to the Froude scaling criterion was used for the DSE investigation whereas real time turbulent velocity profile sampled from the Firth of Forth, Scotland was used for the TSE investigation. The real time turbulent velocity profile is influenced by waves, channel sidewalls, seabed roughness and other natural, physical and biological processes occurring in the estuary. The following observations were made: 1) For the DSE, the drag coefficient of scale models within the subcritical flow regime varies as a function of a non-dimensional length and velocity scale. Equations for estimating the scale effects are presented. 2) The TSE investigation demonstrates a novel application of Acoustic Doppler Current Profiler (ADCP) data from a real site to generate significant upstream turbulence structures suitable for testing tidal current energy devices. The results show that, significant increase in turbulence in terms of maximum vorticity magnitude, by a factor more than 4, can be achieved in an empty channel and seabed drag coefficient is significantly reduced when compared with simulations done by specifying theoretical velocity profiles in a Large Eddy Simulation (LES). 3) Ambient turbulent flow around a monopile support structure causes a significant reduction in drag coefficient compared with simulations done by specifying the uniform and the 1/7th power law velocity profiles. The simulation results further demonstrate the possibility of representing a large scale prototype with a small scale numerical simulation domain that captures ambient vortex structures in real sites and that, the use of uniform flow and 1/7th power law velocity profiles in numerical simulations would lead to overestimation of hydrodynamic forces acting on an energy device and underestimation of available energy for extraction due to lower seabed drag coefficient. Further work is recommended to investigate effect of ambient turbulent structures on hydrodynamic loading and performance of a turbine undergoing sea trials using the methodology proposed in this study. The methodology has potential application to oil and gas subsea structures.