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

TL;DR

This study investigates how elevated free stream turbulence affects the near-wake behavior of tidal turbines, revealing increased torque variability, faster energy recovery, and disrupted wake rotation, with implications for turbine design in turbulent environments.

Contribution

It provides detailed experimental insights into the effects of elevated free stream turbulence on tidal turbine near-wake dynamics, including turbulence intensity and length scale impacts.

Findings

Elevated turbulence increases rotor torque variability by 4.5 times.

Energy recovery at X/D=4 is twice that in quasi-laminar flow.

Wake swirl decreases significantly with elevated 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.