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WP2 results: Active Flow Control (AFC) appendages

Last updated: 5 November 2023

Active Flow Control in tidal composite blades

WP2 reviewed existing active and passive flow control devices for turbine blades to improve blade efficiency. The wall-flow control concept originated from aeronautics, where the lift force created over an airfoil with a high angle of attack is desirable. As we use airfoils (and hydrofoils) in other applications such as turbines, boundary layer flow control devices are becoming important for devices other than airplane wings.

Many flow control devices have been developed for wind turbine blades, ship propellers and ship rudders over many years. Therefore, it is likely to benefit the type of hydrokinetic turbine such as the Magallanes’ turbine. As the flow conditions compared to the Magellan’s turbine for wind turbines (similar blade geometry but different scale and fluid) and for ship propellers and rudders (significantly higher revolution speed and nonrotating respectively) are different, NEMMO partner only outlined possible efficiency avenues, and any performance enhancing device probably will need to be redesigned to improve the Magellan’s device.

Flow control methods use two different active and passive separation control devices. The active control method requires a control device and additional energy to activate the flow control. Although it requires a more complicated system, the benefit of using this method is that it can be deployed when needed and will not create additional drag when the flow condition changes. On the other hand, passive control devices are simple to implement but cannot adjust themselves to the variation in flow conditions. The theoretical aspect of wall-bounded flow separation control is covered by [Gad-el-hak 2000] and the application of both methods in turbomachines is discussed in [Tiainen 2018].

Deliverable 2.1 describes possible avenues for increasing the efficiency of the Magallanes turbine blades. Most originate from wind turbines because wind turbines are a more mature technology than the case for a turbine of the type of Magallanes. However, there is no reason to believe that the same technology cannot be used for a different fluid. The flow control measures discussed are deemed the most promising candidates, and two of those were chosen for further investigation and design in D2.3.

(Deliverable 2.1 and 2.3 are not available for download)

Testing of AFC appendages on tidal turbine and validation of High Fidelity LES Models

The work included a large test programme at SSPA, including 3 sets of blades – carbon fibre blades from Canoe and bronze blades from SSPA for blade geometry 1 and 2. It also included testing of constant cross-sectional profiles at 0.5R and 0.8R on the original blades. Two types of flow control were tested but reported in D2.3. Furthermore, Laser Doppler Velocimetry (LDV) was used to measure the flow field at 3 planes upstream and downstream of the blades. Finally, surface roughness modelling is described. CFD and LES modelling are mainly described in WP1.

The ALM-LES model was used to simulate the full-scale Magallanes turbine accounting for the effect of surface texturing. Lift and drag coefficients were used to model the effects of blades, and the coefficients were extracted from the Xfoil-Qblade software. It was concluded that with the inclusion of surface texturing, the coefficient of lift (Cl) value could be expected to decrease by a maximum of 1.5% while the drag coefficient (Cd) would increase by 3% at most. To understand this effect on the performance of the turbines, the full-scale Magallanes turbine was modelled using the ALM-LES method with the modified coefficients (of lift and drag.) at the standard tip speed ratio (TSR=6.01) and inlet flow velocity (u∞=2.5 m/s). The comparison of the performance with and without texturing is shown in the table below.

It can be concluded that the effect of the proposed surface texturing of blade on the performance characteristics of the Magallanes turbine can be neglected.

Learn more about the setup of the novel test-rigs and the best AFC strategies for tidal turbine blades

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Best performing AFC appendages for tidal blades composite. Performance & operation evaluation and characterisation & features evaluation

This specific work focused on LES simulations/cavitation tunnel evaluation of Active Flow Control (AFC), including tidal composite blades. Two types of flow control, Vortex generators (VG) and blowing (injection) of water in the boundary layer, were tested in a cavitation tunnel. The aim is to delay the insert of a stall and thus increase the lift at higher angles of attack.

The results show that the Vortex generators delay the stall and thus increase the lift at angles of attack above 10°. However, this comes at the expense of increased drat at positive angles of attack. At negative angles of attack, there is no effect on the VGs. The measurements at 9 m/s without VGs were only made up to 10° angle of attack.

Regarding flow control based on blowing, the measurements at 7 m/s could not be performed up to stall due to restrictions in maximum force. The measurements at 5 m/s show some tendency to stall above 30° angle of attack. The slot and chamber seem to have a small negative effect on the lift between approximately -5° to 30° angle of attack. At angles of attack above 30° we can see that the stall is delayed at the two higher volume flows (2 and 3 dm3/s) compared to the lower ones (0.3 and 1 dm3/s). The water flow velocity around the wing generates a small volume flow through the water injection system at zero RMP on the pump.

However, implementing any of these AFC techniques is considered industrially inviable at the current state of the art in blade manufacturing, so they will not be implemented in the full-scale blades manufactured in WP5.

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