Recent circulation control testing at West Virginia University, in a closed loop wind tunnel, has been conducted on models where the trailing edge radius was selected to be smaller than that used in literature, such as Loth and Boasson [1], 1.5 inches and Englar [2], 0.4375 inches. The reduced size was chosen in an attempt to minimize the drag experienced during periods of non-activation of the circulation control, and the smaller size was more compatible to the wind tunnel test section size. However, while the drag is lessened by a smaller trailing edge, the performance of circulation control also appears to be dependent upon a multitude of variables including, but not limited to, the trailing edge radius and jet velocity. Through a modeled experiment, the two attributes that influence the circulation control performance were concurrently manipulated by varying the radius of curvature and the velocity of the blown jet. The combination of these characteristics were experimentally explored to determine the location where the jet leaves the surface of the cylinder, also known as the separation point. The optimum separation point is defined as the farthest angular displacement from the plane of the blown jet exit slot, which corresponds to the greatest increase in the circulation around the cylinder, representing the trailing edge of a circulation control airfoil. From the known radius and jet velocity, an expression that relates the separation point and the mass flow rate velocity quantity are compared. Understanding the blowing coefficient and its impact on the separation point, results in a predictive relationship between these two attributes of circulation control. The results of this two-dimensional cylinder study found that an increase in trailing edge radius decreased the location of the separation point. In addition, an increase in the jet velocity resulted in an increase in the separation point location. The combination of these two quantities produced a relationship similar to each individually, illustrated by the mass flow rate velocity value, which is the blowing coefficient excluding free stream conditions, versus the angle of separation. Data is therein compared to the theory by Newman [3], which predicts a maximum separation point location at 245 degrees beyond the jet exit plane and an increase in the separation point as the radius of curvature increases. The results of this study found a separation point maximized at 231 degrees, and, contrary to Newman [3], a decrease in the separation point was found as the radius of the cylinders increased.

Ground effect is an aerodynamic phenomenon that occurs when moving bodies come in close proximity to the ground. A “cushion” of air is created underneath the moving body which provides additional lift by increasing the local pressure under the body surface. To experimentally test ground effect vehicles, a unique wind tunnel is currently being redesigned and constructed at West Virginia University. This wind tunnel incorporates a rotating belt as the ground plane and a centrifugal fan that generates the air flow through the test section in the same direction as the belt’s rotation. The combination of a rotating belt and airflow is used to mimic ground effect in that it is representative of a body moving through still air in close proximity to the ground. The test section and fan assembly sit on a platform that is connected to a movable base frame. The base and testing platform connect through a pivot point that is capable of being raised upward to a maximum angle of fifty degrees to account for gravitational vector alignment between modeled and real world conditions. When the platform is raised and the belt is spinning, the structure is less stable and has the potential to create errors in force readings due to these oscillations, as well as the potential to tip in extreme wind conditions. Thus, the evaluation of the original design and the subsequent redesign are addressed in this research effort. To stabilize the wind tunnel, additional structural elements have been added downstream of the test section. Two telescoping poles were added to the end of the platform that will connect onto outriggers attached to the base structure. These poles and outriggers will form an A-shape support system when the platform is raised to any degree between zero and fifty. The width of the outriggers was calculated and then modeled in conjunction with the existing base structure. The final design is presented in this paper.

Circulation control and a lifting surface influenced by ground effect have both individually been shown to augment the generation of lift. Recent research conducted at West Virginia University has explored the feasibility of amalgamating the two phenomena, in an attempt to enhance each other. While there are many variables that influence the two phenomena separately, such as the radius of curvature on the trailing edge, this computational effort considers the impact that the height to chord (h/c) ratio and jet blowing coefficient has on the coefficient of lift and drag. During this study, three h/c ratios are considered, 0.25, 0.50, and 0.75. Furthermore, four blowing coefficients for the jet are used, 0, 0.0106, 0.0675, and 0.1519. In general, as the h/c value is decreased, there is an increase in the lift coefficient, as well as the drag coefficient. As the blowing coefficient is decreased, there is a decrease in the lift and drag coefficients. The hallmark result of this effort investigates the L/D as the airfoil approaches the ground. For Cμ = 0.1519, as the airfoil approaches the ground, the L/D ratio increases, indicating that circulation control and ground effect enhance each other.

Circulation control technology has proven aerodynamic benefits, however, along with an increase in the coefficient of lift, parasitic drag also increases. This paper explores two drag reduction techniques investigated in order to decrease the negative effects caused by the geometric requirements of a circulation control airfoil. The first method tests the viability of inducing a constant, low exit velocity jet, similar to a “leak” from the upper and lower plenums of the blowing jets, in an attempt to find a threshold where a specific rate of leak can alleviate the adverse effects of the circular trailing edge. A second drag mitigation technique included altering the shape of the trailing edge; attached to the trailing edge of a conventional circulation control surface is a sharp trailing edge, with an inner curved channel offset. By modifying the distance between the two geometries, an ideal separation distance is identified. The aerodynamic qualities of the two test models are therein compared to conventionally shaped airfoils. The results indicate that two techniques wherein have the ability to reduce the drag, proven by empirical experimentation.

The concept of harnessing wind power has been around for centuries, and is first recorded by the Persians in 900 AD. These early uses of wind power were for the processing of food, particularly grinding grains, and consisted of stationary blades around a horizontal axis, the precursor to today’s horizontal axis wind turbines (HAWT). Technology for these wind mills was essentially the same until the 1930’s when advances in aircraft propeller theories were applied to the blades of the turbine. During this development period, which has since remained basically unchanged, the design push was for increasingly larger propellers requiring heavy and costly transmissions, generators, and support towers to be installed. An alternative concept to the HAWT was developed by Georges Darrieus [1], which utilized a vertical shaft and is known as a vertical axis wind turbine (VAWT). The scientific development of the concept did not gain strong attention until the 1970’s due to the perceived low efficiency of this style. This perception was due in part to the portion of the blade’s rotary path that is adverse to the generation of power. This efficiency loss can be minimized by the mechanical movement of the blade, relative to the airflow during the upwind portion of the blades’ rotational path. Since, circulation control can alter the forces generated by an airfoil, it could be used to increase the efficiency of a VAWT by increasing Gerald M. Angle II, Franz A. Pertl, Mary Ann Clarke and James E. Smith International Journal of Engineering, (IJE) Volume (4): Issue (5) 431 the torque produced on the downwind portion of the path, while removing the need for a physical change in angle of attack. With the recent upturn in petroleum costs and global warming concerns, interest in renewable energy technologies have been reinvigorated, in particular the desire for advanced wind energy technologies, including the application of lift augmentation techniques. One of these techniques is to utilize circulation control to enhance the lifting capacity of the blades based on the location of the blade in the turbine’s rotation. Though this technology can be applied to any wind turbine, whether horizontal or vertical axis, this paper focuses on the application of circulation control for VAWT’s due primarily to reduced hardware complexities and to increase the performance of this design thus helping to level the playing field between the two styles. This performance enhancement coupled with the ability to locate the primary components near the ground allows for easier installation, troubleshooting, maintenance, and future improvement of the circulation control sub-system. By varying the circulation control performance with the blade position, the coefficient of performance, Cp, of the wind turbine can be altered. This variation in Cp resembles a change in the effective solidity factor, the non-dimensional characteristic that accounts for the number of turbine blades, chord length, and turbine radius. The solidity factor is typically used in the design of a wind turbine with its peak performance occurring at various tip speed ratios, at different solidity factors. Prior to the construction of physical models, numerical methods, namely a vortex model, was used to estimate the performance enhancement potential of the blade force augmentation via circulation control. These results were then used to construct and test a wind tunnel blade section model to obtain lift and drag values for a full range of rotational angles. These results were then supplied to the vortex model which indicated that through the addition of circulation control to the blades of a vertical axis wind turbine a wider coefficient of performance curve can be achieved, similar to a change in the solidity factor of the wind turbine.

This research effort provided a process and results for the prediction of the pressure gradient in the vicinity for a circulation controlled vertical axis wind turbine (CC-VAWT). The pressure gradient was determined using the Computational Fluid Dynamics (CFD) software, Fluent. The pressure gradient was then compared to the current commercial standard horizontal axis wind turbine (HAWT) the CC-VAWT and was shown to produce a much lower pressure drop within its immediate vicinity of the wind turbine. This difference shows evidence towards a potential solution for the current negative effects that the wind turbine industry is having on bat populations, namely barotrauma.

The effects of placing a slot through a two-dimensional Wortmann FX 63-137 airfoil in ground effect were examined using computational fluid dynamics. The geometric shape of the slot was varied in three different ways: the width of the slot (w/c = 0.02, 0.04, and 0.06), the angle of the slot with respect to the airfoil's chord line (d = 20, 30, and 40), and the position ofthe slot along the chord line, (x/c = 0.15,0.20, and 0.25). In addition, the airfoil was tested at five different angles of attack: ―3, 0, 5, 10, and 15 deg. The commercially available software Gambit 2.3.16 was used to create the computational grids. FLUENT 6.2.16 with the renormalized group k-e turbulence model was then used to simulate the flow. Pitch stability of the slotted airfoil was examined and results indicated that increasing the angle of attack of the slotted airfoil while in ground effect had a reduced increase in lift when compared with the lift generated by the baseline Wortmann FX 63-137 airfoil. Results also showed that the slot could be used to reduce center-of-pressure movement along the chord of the airfoil for the range of angles of attack investigated, thus improving the overall pitch stability of the airfoil. The slot geometry that produced a minimal center-of-pressure fluctuation was located at 20% of the chord length from the leading edge with a width of 2% and an angle of 20 deg between the slot and a line normal to the chord line.

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.

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.

Circulation control techniques have a long history of applications to fixed wing aircraft. General aviation has used circulation control to delay flow separation and increase the maximum lift coefficient achievable with a given airfoil. These techniques have been gradually expanded to other applications, such as ground vehicles, to reduce drag. Circulation control technology can, potentially, be applied also to each blade of the main rotor in a helicopter, in order to increase the lift capacity of the rotor. Applications of circulation control technologies to fixed wing aircraft have demonstrated the potential of a three-fold increase in the lift coefficient, as compared to a conventional airfoil. This finding would suggest that a rotorcraft equipped with circulation control of the main rotor blades could, conceivably, lift up a payload that is approximately three times heavier than the maximum lift capacity of the same helicopter without circulation control. Alternatively, circulation control could reduce the required rotor diameter by up to 48%, if the maximum lift capacity remains unaltered. A High Lift, Circulation Controlled Helicopter Blade will be undergoing initial testing in the subsonic wind tunnel facility at West Virginia University. Two-dimensional elliptic airfoil models with air blowing slots for circulation control will be used as specimens in these tests in order to determine the aerodynamic changes, especially in lift and drag forces, achievable with various blowing slot configurations. Based on the results of the wind tunnel testing, an improved, detailed design will be developed for the entire main rotor of a helicopter with circulation control.

Previous studies have shown that using blowing slots can reduce the effects of the rotor downwash on the main wing of a tilt-rotor aircraft, particularly the V-22 Osprey. The current study investigates the placement and air velocity of the leading edge blowing slot for optimization of the download reduction. The realizable turbulent kinetic energy - rate of dissipation (rke) numerical model available in Fluent 6.2.12 was used to model the flow involved under the rotors and the subsequent downwash around the main wing. It was found that the leading edge blowing slot is most beneficial when it is placed just upwind of the separation point without blowing slots. In the current investigation the optimal configuration is found between 0 percent and 1 percent of the chord length.