Passive method of turbulent flow separation control for offshore wind energy

The research is devoted to understanding the impact of amplitude modulation of small scales by large-scale motion for a wide range of Reynolds numbers. Acquired knowledge has recently been used to develop a novel passive flow separation control method with a high Reynolds number (Rec>10 million) that can be used forr the longest offshore wind turbine blades.

Preventing the flow separation increases lift force generated by the blade and thus efficiency of the power produced by the turbine.

The method uses the stationary streamwise wavy wall. Ongoing research is devoted to application of the method on the curved surface, which will be a first step to commercialisation.

Innovative technology improves the aerodynamics of offshore wind turbines by using a wavy wall in streamwise direction to prevent turbulent flow separation on the suction side of the largest wind turbine blades. Experimental research conducted at Czestochowa University of Technology in Poland has shown that the wavy wall can be used as a device (like the vortex generators) to further increase the flow momentum near the surface upstream of the trailing edge. The increase of 15% in the skin friction coefficient locally behind the wavy wall has the potential to delay flow separation under on-design conditions. Further studies have shown that the method's efficiency for off-design conditions reduces the device efficiency by only 50%, which is crucial especially for floating wind turbines where wind inflow conditions are extremely unstable.

The new result shows that the skewed wavy wall, in particular the steeper of about 30% uphill side than the downhill side will increase the skin friction coefficient by up to 30%, which prevents flow separation under off-design conditions.

Two types of implementations of the wavy surface on the offshore wind turbine blade are planned to be assessed in the project. The first implementation involves forming the wavy surface directly on the blade surface during the manufacturing process. This approach ensures that the wavy surface is an inherent part of the blade’s structure enhancing its aerodynamic efficiency.
The second implementation focuses on retrofitting existing blades with the wavy surface overlay during maintenance and repair procedures. This method allows for the benefits of the wavy surface to be applied to blades already in service, potentially improving their performance and extending their operational lifespan. Both approaches aim to optimize the aerodynamic properties of the blades, thereby increasing the overall efficiency and effectiveness of offshore wind turbines.

There are a few economic values concerning cost and risk reduction, as well as saving the environmental impact of blade manufacturing:

- The device can increase the efficiency of energy production by a few percent or even more (fewer turbines needed for the same power)

- The device can improve the endurance of the blades by using blades that are a few percent shorter to produce the same power compared to current designs (requiring a lesser blade material to manufacture).

 Unstable wind flow, especially prevalent in floating wind turbines, can be harvested more efficiently using flow stabilisation technology, which further improves flow stability and the blade aerodynamics.

The main goal of the project is to measure the mean flow profile in the boundary layer in the halfway of the chord on the working 10MW+ wind turbine blade and design the wavy surface to improve the efficiency of the wind turbine. Therefore, it is extremely important to obtain flow characteristics that are needed to design a wavy surface. The inflow conditions on the blade are quite different along the height of wind turbine (about 40 deg.).

The final test that is slated is to modify the 10MW+ blade surface by placing a small wavy surface on the suction side. It is expected that it will stabilise the flow by shortening the separated flow time in the rotation cycle.