Passive method of turbulent flow separation control for wind energy

The knowledge of the impact of amplitude modulation of small scales by large-scale motion in a turbulent boundary layer across a wide range of Reynolds numbers has recently been applied to develop a new passive flow separation control method at a high Reynolds number (Rec>10 million) suitable for the longest offshore wind turbine blades.

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

The method utilizes a stationary streamwise wavy wall. Ongoing research aims to apply this method to curved surfaces, which will be a step towards commercialization. The technology aims to improve the aerodynamics of offshore wind turbines by using a wavy wall in the streamwise direction to prevent turbulent flow separation on the suction side of large wind turbine blades. Experimental research conducted at Czestochowa University of Technology in Poland indicates that the wavy wall can increase flow momentum near the surface upstream of the trailing edge. The local skin friction coefficient behind the wavy wall increased by 15%, potentially delaying flow separation under on-design conditions. Further studies suggest that the method's efficiency under off-design conditions reduces device efficiency by only 50%, which is particularly relevant for floating wind turbines where wind inflow conditions are highly unstable.

New findings indicate that a tilted wavy wall, especially one with an approximately 30% steeper uphill side compared to the downhill side, can increase the skin friction coefficient by up to 30%, aiding in preventing flow separation under off-design conditions.

Two types of implementations of the wavy surface on offshore wind turbine blades are planned for assessment. The first involves incorporating the wavy surface directly onto the blade during manufacturing, making it an integral part of the blade's structure and enhancing aerodynamic efficiency. The second implementation focuses on retrofitting existing blades with a wavy surface overlay during maintenance and repair, allowing for performance improvements and extending operational lifespan. Both approaches aim to optimize the aerodynamic properties of the blades, thereby increasing the overall efficiency and effectiveness of offshore wind turbines.

Unstable wind flow, particularly prevalent in floating wind turbines, can potentially be harvested more efficiently using flow stabilization technology, which further improves flow stability and blade aerodynamics.

The primary objective of the project is to measure the mean flow profile in the boundary layer at the midpoint of the chord on the working 10MW+ wind turbine blade and design the wavy surface to enhance wind turbine efficiency. Therefore, obtaining flow characteristics essential for designing a wavy surface is crucial. The inflow conditions on the blade differ significantly along the height of the wind turbine (about 40 degrees).

The final test involves modifying the 10MW+ blade surface by placing a small wavy surface on the suction side. It is expected that this modification will stabilize the flow by shortening the separated flow time in the rotation cycle and/or torsional deflection cycle.

There are several economic benefits related to cost and risk reduction, as well as reducing the environmental impact of blade manufacturing:

·         The device can increase energy production efficiency by a few percent or more, reducing the number of turbines needed for the same power output.

The device can enhance blade endurance by allowing shorter blades to produce the same power as current designs, requiring less material for manufacturing