Interactions between tidal turbine wakes: experimental study of a group of three-bladed rotors

Abstract

It is well known that a wake will develop downstream of a tidal stream turbine owing to extraction of axial momentum across the rotor plane. To select a suitable layout for an array of horizontal axis tidal stream turbines, it is important to understand the extent and structure of the wakes of each turbine. Studies of wind turbines and isolated tidal stream turbines have shown that the velocity reduction in the wake of a single device is a function of the rotor operating state (specifically thrust), and that the rate of recovery of wake velocity is dependent on mixing between the wake and the surrounding flow. For an unbounded flow, the velocity of the surrounding flow is similar to that of the incident flow. However, the velocity of the surrounding flow will be increased by the presence of bounding surfaces formed by the bed and free surface, and by the wake of adjacent devices. This paper presents the results of an experimental study investigating the influence of such bounding surfaces on the structure of the wake of tidal stream turbines.

Towards a Low-Cost Modelling System for Optimising the Layout of Tidal Turbine Arrays

“…It can be seen that the wake is symmetrical about the centre of the turbine, exhibiting a bell-shaped profile, and that its width increases with distance downstream, growing from 2 RD width at 2 RD downstream to 6 RD width at 20 RD downstream. This is in agreement with the findings of [36] who studied the wake of a single scale model rotor in a recirculating flume; their wake exhibited a symmetrical, approximately bell-shaped, lateral profile, an increase in wake width with distance downstream and a comparable wake width of 2 RD at 4 RD downstream. If one assumes a similar vertical profile for the wake recorded by [36] to that recorded by [5],at the common measurement location of 4 RD downstream then one can compute a depth-averaged value for the centreline deficit of [36] of 0.18; the corresponding modelled value at 4 RD is 0.16 (from Figure 7).…”

“…This is in agreement with the findings of [36] who studied the wake of a single scale model rotor in a recirculating flume; their wake exhibited a symmetrical, approximately bell-shaped, lateral profile, an increase in wake width with distance downstream and a comparable wake width of 2 RD at 4 RD downstream. If one assumes a similar vertical profile for the wake recorded by [36] to that recorded by [5],at the common measurement location of 4 RD downstream then one can compute a depth-averaged value for the centreline deficit of [36] of 0.18; the corresponding modelled value at 4 RD is 0.16 (from Figure 7). These comparisons serve to demonstrate that the lateral extents of the modelled wake compare favourably with experimental measurements.…”

“…With regard to the low-cost modelling solution used here, the inclusion of a wave module would have significantly increased the computational time. However, [36] also studied the effects of waves on turbine wake characteristics and found that for the case of irregular waves opposing the direction of current flow there was no change in the vertical and lateral wake profiles and while there was a decrease in the velocity deficit immediately downstream of the turbine, beyond 10 RD downstream, the velocity deficits were similar to those generated in the absence of waves. Given that the author's model is primarily concerned with far-field wake characteristics, the findings of [36] suggest that the inclusion of a wave module is unnecessary.…”

“…Given that the authorʹs model is primarily concerned with far-field wake characteristics, the findings of [36] suggest that the inclusion of a wave module is unnecessary.…”

“…If one assumes a similar vertical profile for the wake recorded by [36] to that recorded by [5],at the common measurement location of 4 RD downstream then one can compute a depth-averaged value for the centreline deficit of [36] of 0.18; the corresponding modelled value at 4 RD is 0.16 (from Figure 7). These comparisons serve to demonstrate that the lateral extents of the modelled wake compare favourably with experimental measurements.…”