Numerical investigation of synthetic jet based flow control for vertical axis wind turbines

Abstract

This numerical study focuses on the implementation of active flow control using synthetic jets on vertical-axis wind turbine (VAWT) blades. This study demonstrates that synthetic-jet based flow control improves the efficiency of the turbine and reduces the risk of structural fatigue.

The idea of using synthetic jets is to control the variation in flow-induced loading during each rotation cycle. At first the dominant location of the flow separation is determined for both airfoils. The jets are then placed at this location. Jet parameters of blowing ratio and reduced frequency are specified within a range (i.e., O(0.5-1.5) and O(1-10), respectively) and their effects on jet performance are studied. The jets are activated only in a selected portion of the rotation cycle. This is referred to as the partial cycle control in contrast to the full cycle (the latter is found to be detrimental). For given jet parameters, simulations results are used to determine whether the jets improve axial force, flow separation and blade-vortex interaction. At blowing ratio of 1.5 and reduced frequency of 5, we observe above 12% increase in the average axial force over the rotation cycle for both airfoils.

In VAWTs, the blades experience a significant variation in the angle of attack over each rotation cycle and associated with it are sudden changes in the flow-induced loading on the blades. For example, a sudden variation in blade loading is experienced due to the detachment of the leading edge vortex at high angles of attack. This is in-turn reduces the axial force and hence the overall power output of the turbine. Additionally, such force variations lead to structural fatigue and possibly failure. Current simulations consider a cross-section of a three-blade VAWT model (with straight blades). VAWT models with two different airfoils, NACA 0018 and DU 06-W-200, are considered at tip-speed-ratios of 2 and 3. In these simulations, unsteady, Reynolds-averaged Navier-Stokes equations along with the Spalart-Allmaras turbulence model are employed, where stabilized finite element method is utilized along with an implicit time-integration scheme.