W.M.J. Batten

Influence of turbulence on the wake of a marine current turbine simulator

Marine current turbine commercial prototypes have now been deployed and arrays of multiple turbines under design. The tidal flows in which they operate are highly turbulent, but the characteristics of the inflow turbulence have not being considered in present design methods. This work considers the effects of inflow turbulence on the wake behind an actuator disc representation of a marine current turbine. Different turbulence intensities and integral length scales were generated in a large eddy simulation using a gridInlet, which produces turbulence from a grid pattern on the inlet boundary. The results highlight the significance of turbulence on the wake profile, with a different flow regime occurring for the zero turbulence case. Increasing the turbulence intensity reduced the velocity deficit and shifted the maximum deficit closer to the turbine. Increasing the integral length scale increased the velocity deficit close to the turbine due to an increased production of turbulent energy. However, the wake recovery was increased due to the higher rate of turbulent mixing causing the wake to expand. The implication of this work is that marine current turbine arrays could be further optimized, increasing the energy yield of the array when the site-specific turbulence characteristics are considered.

Inlet grid-generated turbulence for large-eddy simulations

A new technique of generating turbulence in large-eddy simulations (LES) has been investigated and results compared with previous studies for validation. The proposed gridInlet technique uses a grid pattern on the inlet boundary patch to produce grid-generated turbulence as used in wind tunnel experiments. This allows the turbulence integral length scale to be controlled by changing the grid size, while the turbulence intensity is controlled by changing the inlet distance. The objective of this paper is to investigate domain and mesh requirements to implement the gridInlet technique. This technique is most suited to studies on the influence of high-intensity isotropic turbulence on objects, particularly if comparisons are to be made to experimental data obtained with grid-generated turbulence.

Simultaneous Wake- and Vortex-Induced Vibrations of a Cylinder With Two Degrees of Freedom in Each Direction

The flow-induced vibration of one cylinder in the wake of another is the subject of continuing interest in connection with interactions between vertical tension risers in deep water. When one riser is downstream of another it is likely to be subject to wake-induced and vortex-induced excitations at different frequencies simultaneously. Both are complex mechanisms, and it is reasonable to assume that they interact. To begin to understand this complicated process it is desirable that any modelling should incorporate some features of a multi-degree-of-freedom structural response. With this aim, this paper describes experiments in which one cylinder was free to undergo simultaneous wake- and vortex-induced vibrations downstream of a similar but stationary cylinder in a steady flow. The downstream cylinder was mounted on an elastic system that had two natural frequencies in both the in-line and cross-flow directions. Mass ratios were almost the same in all four modes. Measurements are presented of simultaneous wake- and vortex-induced vibrations for cylinder separations of 5 and 10 diameters in the in-line direction, and up to 4 diameters transversely. At a reduced velocity of 83 (based on the cylinder’s lower submerged natural frequency) and a separation of 5 diameters, excursions of wake-induced vibrations peaked at almost 5 diameters, when the downstream cylinder was near the edge of the upstream cylinder’s wake

Comparing Energy Yields From Fixed and Yawing Horizontal Axis Marine Current Turbines in the English Channel

At many locations with high tidal stream velocities – and potential for tidal stream energy generation – the flow is approximately rectilinear, that is to say the flow direction is always 0 degrees or 180 degrees with respect to a particular orientation. At some sites, however, there is an appreciable change in flow direction (‘swing’) away from 180 degrees between the two maxima of flow speed. In order to assess the performance of horizontal axis marine current turbines in non rectilinear currents, measurements of a model rotor have been made in a towing tank. Curve fits have been calculated as a function of the cosine of the yaw angle squared and the thrust as cosine of the yaw angle. The curve fits have been used in a case study to investigate the impact of fixed-orientation or yawing rotor designs on average annual energy output, at three locations in the English Channel. All three sites are of the type where flow is accelerated around a headland or cape, but their tidal streams vary in deviation from rectilinearity. For two of the sites - Portland Bill (Dorset, UK) and Race of Alderney (Alderney, Channel Islands/Normandy, France) - available data consisted of tidal stream diamonds printed on Admiralty navigational charts. These rely on local tidal elevations for interpolation of tidal streams. At the other site – St. Catherine’s Point, Isle of Wight, Hampshire – current meter measurements of duration one month were available from the British Oceanographic Data Centre (BODC), allowing a direct tidal analysis. …