Wake structure of tidal stream turbine arrays under increasing flow depth
Pablo Ouro Hydro-environmental Research Centre, School of Engineering, Cardiff University,CF24 3AA Cardiff UK
At every tidal site the local environmental conditions regarding water depth or bathymetry are different. These can notably change the flow dynamics, e.g. velocity profile distribution, impacting on the energy generation capabilities of tidal stream turbine arrays, and hence there is a need to individually investigate the layout that maximises the energy generation of the array. Relatively low submergences, i.e. turbines occupy a great proportion of the water level, can have an immediate effect in the tidal turbine flow dynamics, restricting the wake structure whilst diminishing wake velocity recovery rate. Consequently, the energy generation capabilities from secondary rows are greatly reduced. Due to this obvious implication, it is necessary to identify and quantify the changes in wake recovery mechanisms depending on the relative water depth in order to better plan the location of the turbines within the array to maximise energy generation. A Digital Offshore FArms Simulator (DOFAS)  is used to accurately predict the flow through a tidal turbine farm under four water depth conditions. DOFAS is based on the highly-accurate method of large-eddy simulation equipped with an actuator line model and immersed boundary method for there presentation of tidal turbines and bathymetry, respectively [1-3]. Stallard et al.  experimentally tested laboratory-scale tidal turbine arrays, operated in shallow flow conditions with a relative water depth of less than two equivalent turbine diameters. This setup can be considered of very shallow nature and is used to validate DOFAS, both in terms of performance prediction and wake velocities. Findings from previous wind turbine farm studies  with similar scope on the wake dynamics cannot be directly extrapolated to tidal turbines, due to the latter ones operating in flows constrained by two vertically bounded surfaces, i.e. water depth and bathymetry, and hence the wake’s vertical expansion can be notably constrained. In this work, the flow mechanisms involved in the wake behind three turbines in one row with the four different depths are explored, quantifying those main mechanisms involved in the recovery of kinetic energy which has been linked to the available power for secondary row of turbines. Our results show that turbulence has a great influence on kinetic energy recovery with the transport of mean kinetic energy due to Reynolds stress being determinant both in their horizontal and vertical contributions. Results of production of turbulent kinetic energy can also be linked to study the fatigue loads that turbines to be placed in the wake will undergo during their operating life, and how these vary depending on the water depth.
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