Performance and near-wake characterization of a tidal current turbine in elevated levels of free stream turbulence

Abstract

Tidal turbines are deployed in sites which have elevated levels of free stream turbulence (FST).Accounting for elevated FST on their operation become vital from a design standpoint. Detailed experimental measurements of the dynamic near-wake of a tidal turbine model in elevated FST environments is presented; an active grid turbulence generator developed by our group was used to seed in the elevated FST and evaluate the influence of turbulence intensity (Ti) and inflow integral length scale (L) on the near-wake of the turbine. Three inflow conditions are tested: a quasi-laminar flow with Ti ~ 2.2% and two elevated Ti (~12-14%) cases, one with L ~ 0.4D (D is the turbine diameter) and the other where L~ D. Elevated Ti cases was found to increase the standard deviation of rotor torque by 4.5 times the value in quasi-laminar flow. Energy recovery was also found to be accelerated; at X/D=4, the percentage of inflow energy recovered was 37% and was twice the corresponding value in quasi-laminar flow. Elevated FST was observed to disrupt the rotational character of the wake; the drop in swirl number ranged between 12% at X/D=0.5 to 71% at X/D=4. Elevated Ti also resulted in L that were considerably larger (> 2 times) than the quasi-laminar flow case. An increase in inflow integral length scale (from 0.4D to D) was observed to result in enhanced wake Ti, wake structures and anisotropy; however, no noticeable influence was found on the rate of wake recovery.

On Blockage Effects for a Tidal Turbine in Free Surface Proximity

“…Flow velocity can be varied from 0.03 m/s to 0.94 m/s and can be measured within an accuracy of ±2%. The flow quality is such that the turbulence intensity is maintained to a value of~2% [25].…”

The effects of free-stream turbulence on the performance of a model wind turbine

“…Previous studies 11 identify the free-stream turbulence intensity as the main parameter driving drag variations. For model wind turbines at Reynolds numbers in the order of 10 5 and turbulence intensities under 20 %, conflicting results are reported in literature: some studies observe consistent reductions in the thrust generated by model turbines with increasing turbulence intensity 5,7 , while other studies report an increase in thrust for more turbulent flows 6 or comparable values of velocity deficit in the near wake a) Electronic mail: s.gambuzza@soton.ac.uk (thus, comparable thrust) for different turbulence intensities 12 . It must be noted that none of these studies (and neither does this one) employ a common model wind turbine design, and thus different results can, in principle, be expected; however, such large discrepancies in the general trends cannot be explained by different designs alone, and some other mechanisms previously overlooked could affect these trends.…”

“…To study how wind turbine performance figures (namely, the thrust generated or the power harvested by the turbine) are affected by different turbulent flows, a wide number of experimental studies have been carried out. These range from experiments with porous discs 3,4 , to model-scale turbines in wind tunnels or water channels [5][6][7] , to observations on real-scale turbines generating usable electrical power [8][9][10] ; the last are obviously scarcer due to inherent difficulties in undertaking rigorous measurements in natural environments.…”

Wake characterization of an undulating membrane tidal energy converter

“…While horizontal and vertical axis tidal turbines' wakes have been already studied [4,15,17,25] and their interaction effects well-known [16,2], no measurement has been done on an undulating tidal energy converter. Indeed, the tidal energy converter studied here is based on the fluid-structure interactions that occur between a flexible membrane and an axial flow, resulting in an undulating motion that can be used to harvest energy.…”