Experimental testing was performed on a circulation controlled airfoil with upper and lower trailing edge blowing slots, controlled by span wise pneumatic valves. The augmented blade was designed for application to a circulation controlled vertical axis wind turbine. The design is based upon a conventional NACA0018 shape, replacing the sharp trailing edge with a rounded Coanda surface and blowing slots. A scale model with a chord of 8 inches and span of 16.5 inches was created using an ABS plastic rapid prototyping machine. In the past, circulation control wind tunnel models have been constructed with a separate blowing slot and trailing edge using conventional machining methods. The slot must be tediously aligned along the span for a consistent height which ultimately affects the uniformity and performance of the circulation control jet in combination with the flow rate. The rapid prototyping machine eased fabrication as a modular trailing edge section was printed which includes the Coanda surface, blowing slot, and diffuser all in one piece. Pressure taps were integrated by the prototyping machine into both the printed skin and trailing edge module. This method left additional space inside the model for circulation control valving components and eliminated the need for machining pressure ports. This paper will outline the model building procedures, wind tunnel test rig, and experimental results. Aerodynamic forces were determined by both load cells and surface pressure measurements; the agreement between the two methods will be analyzed and addressed. Test conditions include various angles of attack (±20°) at Cμ = 0, 0.02, 0.06, and 0.10; the test Reynolds number was kept constant at 300K. The results indicate that the blade performed at ΔCl/Cμ near 30 for Cμ = 0.02.

A possible method for modeling a Circulation Controlled - Vertical Axis Wind Turbine (CC-VAWT) is a vortex model, based upon the circulation of a turbine blade. A vortex model works by continuously calculating the circulation strength and location of both free and blade vortices which are shed during rotation. The vortices’ circulation strength and location can then be used to compute a velocity at any point in or around the area of the wind turbine. This model can incorporate blade wake interactions, unsteady flow conditions, and finite aspect ratios. Blade vortex interactions can also be studied by this model to assist designers in the avoidance of adverse turbulent operational regions. Conventional vertical axis wind turbine power production is rated to produce power in an operating wind speed envelope. These turbines, unless designed specifically for low speed operation require rotational start-up assistance. The VAWT blade can be augmented to include circulation control capabilities. Circulation control can prolong the trailing edge separation and can be implemented by using blowing slots located adjacent to a rounded trailing edge surface; the rounded surface of the enhanced blade replaces the sharp trailing edge of a conventional airfoil. Blowing slots of the CC-VAWT blade are located on the top and bottom trailing edges and are site-controlled in multiple sections along the span of the blade. Improvements in the amount of power developed at lower speeds and the elimination or reduction of start-up assistance could be possible with a CC-VAWT. In order to design for a wider speed operating range that takes advantage of circulation control, an analytical model of a CC-VAWT would be helpful. The primary function of the model is to calculate the aerodynamic forces experienced by the CC-VAWT blade during various modes of operation, ultimately leading to performance predictions based on power generation. The model will also serve as a flow visualization tool to gain a better understanding of the effects of circulation control on the development and interactions of vortices within the wake region of the CC-VAWT. This paper will describe the development of a vortex analytical model of a CC-VAWT.

A vertical axis wind turbine (VAWT) prototype is being developed at West Virginia University that utilizes circulation control to enhance its performance. An airfoil was chosen for this turbine based on its performance potential, and ability to incorporate circulation control. The selection process for the airfoil involved the consideration of camber, blade thickness, and trailing edge radius and the corresponding impact on the lift and drag coefficients. The airfoil showing the highest lift/drag ratio augmentation, compared to the corresponding unmodified airfoil was determined to be the most likely shape for use on the circulation control augmented vertical axis wind turbine. The airfoils selected for this initial investigation were the NACA0018, NACA2418, 18% thick elliptical, NACA0021, and the SNLA2150. The airfoils were compared using the computational fluid dynamics program FLUENT v.6.3.26 with a blowing coefficient of 1% [1]. The size of the trailing edge radius and the slot heights were varied based on past experimental data [2]. The three trailing edge radii and two blowing slot heights were investigated. The thickness of the airfoil impacts the circulation control performance [3], thus it was studied by scaling the NACA0018 to a 21% thickness and compared to an SNLA2150 airfoil. The airfoils’ lift and drag coefficients were compared to determine the most improved lift-drag ratio (L/D). When comparing the increases of the L/D due to circulation control, the NACA0018 and 2418 airfoils were found to outperform the elliptical airfoil; the NACA0018 performed slightly better than the 2418 when comparing the same ratio L/D. The results showed that the 21% thick airfoils produced a decreased L/D profile compared to the NACA0018 airfoils. Therefore, the NACA0018 was found to be the optimal airfoil based from this initial investigation due to an increased L/D compared to the other airfoils tested.

A possible method for analytically modeling a CC-VAWT (Circulation Controlled Vertical Axis Wind Turbine) is the momentum model, based upon the conservation of momentum principal. This model can consist of a single or multiple stream tubes and/or upwind and downwind partitions. A large number of stream tubes and the addition of the partition can increase the accuracy of the model predictions. The CC-VAWT blade has blowing slots located on the top and bottom trailing edges and have the capability to be site controlled in multiple sections along the span of the blade. The turbine blade, augmented to include circulation control capabilities, replaces the sharp trailing edge of a standard airfoil with a rounded surface located adjacent to the blowing slots. Circulation control (CC), along with a rounded trailing edge, induces the Coanda effect, entraining the flow field near the blowing slots thus preventing or delaying separation. Ultimately, circulation control adds momentum due to the mass flow of air coming out of the blowing slots, but is negligible compared to the momentum of the free stream air passing through the area of the turbine. In order to design for a broader range of operating speeds that will take advantage of circulation control, an analytical model of a CC-VAWT is helpful. The analytical modeling of a CC-VAWT could provide insight into the range of operational speeds in which circulation control is beneficial. The ultimate goal is to increase the range of operating speeds where the turbine produces power. Improvements to low-speed power production and the elimination or reduction of startup assistance could be possible with these modifications. Vertical axis wind turbines are typically rated at a particular ratio of rotational to wind speed operating range. In reality, however, wind speeds are variant and stray from the operating range causing the power production of a wind turbine to suffer. These turbines, unless designed specifically for low speed operation, may require rotational startup assistance. The added lift due to circulation control at low wind speeds, under certain design conditions, will allow the CC-VAWT to produce more power than a conventional VAWT of the same size. Circulation control methods, such as using blowing slots on the trailing edge are modeled as they are applied to a VAWT blade. A preliminary CC-VAWT was modeled using a standard NACA 0018 airfoil, modified to include blowing slots and a rounded trailing edge. This paper describes an analytical momentum model that can be used to predict the preliminary performance of a CC-VAWT.

Wind turbines are a source of renewable energy with an endless supply. The most efficient types of wind turbines operate by utilizing the lift force of its blades to create a rotational force. The power capabilities of a wind turbine are tied to the blades’ ability to convert the aerodynamic forces into rotational energy. Vertical axis wind turbines (VAWT), unlike the more common horizontal axis (HAWT) type, do not need to be directed into the wind and can place the transmission and electrical power generation components at the bottom of the turbine shaft, near the ground. Currently VAWTs cannot feather or pitch the blades, in the same fashion as a HAWT, for a lift change to control power generation and/or rotational speed at different or changing wind speeds. A method of increasing the lift of a blade without physically moving the blade is to use circulation control (CC), via a blowing slot over a rounded trailing edge. The CC air flow entrains the air around the blade to create more lift. Adding an actuated valve for the blowing slot allows a CC-VAWT to control the amount of lift generated, as well as the location of the augmentation relative to the wind direction, resulting in augmented power generation. In order to study the performance capabilities of a CC-VAWT, a NACA0018 blade was modified to incorporate circulation control. This modified shape was analyzed using computational fluid dynamics at two Reynolds numbers and a wide range of angles of attack. The lift to drag ratio of the CC-VAWT blade shows benefits at low Reynolds numbers over a NACA0018 blade for post stall angles of attack, but there is a decrease in the lift to drag before stall due to a significant increase in drag of the circulation control models. Further CFD refinement and experimental investigations are recommended to validate the predicted effects circulation control will have on the performance of a VAWT.