Design Considerations, Performance Enhancing Techniques, and Wind Tunnel Testing for Small-Scale, Low Reynolds Number Wind Turbines

2011 ◽  
Author(s):  
Jason R. Gregg ◽  
Timothy A. Burdett ◽  
Kenneth W. Van Treuren ◽  
Stephen T. McClain
Keyword(s):  
Reynolds Number ◽  
Wind Tunnel ◽  
Wind Turbine ◽  
Wind Turbines ◽  
Power Output ◽  
Low Reynolds Number ◽  
Small Scale ◽  
Wind Tunnel Testing ◽  
Low Reynolds ◽  
Lift And Drag

Wind turbines have become a significant part of the world’s energy equation and are expected to become even more important in the years to come. A much-neglected area within wind turbine research is small-scale, fixed-pitch wind turbines with typical power outputs in the 1–10 kW range. This size wind system would be ideal for residential and small commercial applications. The adoption of these systems could reduce dependence on the aging U.S. power grid. It is possible to optimize a small-scale system to operate more efficiently at lower wind speeds, which will make wind generation possible in areas where current wind technology is not feasible. This investigation examines the use of the S818 airfoil, a typical blade root airfoil designed by the National Renewable Energy Laboratory (NREL), as a basis for the design of low Reynolds number (less than 200,000) systems. The literature shows that many of the airfoils proposed for wind turbine applications, including the S818, only have lift and drag data generated by numerical simulations. In previous research at Baylor, 2-D simulations published by NREL have been shown to predict an optimal design angle of attack (which is the angle at which L/D is maximized) up to 2.25° different from actual wind tunnel data. In this study, the lift and drag generated by the S818 airfoil has been measured experimentally at a Reynolds number of approximately 150,000 and compared with NREL simulation data, showing a discrepancy of 1.0°. Using the S818 airfoil, a set of wind turbine blades has been designed to collect wind turbine power data in wind tunnel testing. Design parameters investigated include the effect of design tip speed ratios (TSR) (1, 3, and 7) and the influence of the number of blades (2, 3 and 4) on power generated. At the low Reynolds numbers tested (ranging from 14,000–43,200 along the blade for a design TSR of 3 and a wind speed of 10 mph), the effect of roughness was explored as a performance enhancing technique and was seen to increase power output by delaying separation. Under these low Reynolds number conditions, separation typically occurs on smooth blades. However, the roughness acted as a passive flow control, keeping the flow attached and increasing power output. Preliminary data suggest that as much as a 50% improvement can be realized with the addition of roughness elements for a TSR of 3. Additionally, the increase in power output due to roughness is comparable with the increase in power due to adding another smooth blade.

A bioinspired Separated Flow wing provides turbulence resilience and aerodynamic efficiency for miniature drones

2020 ◽  
Vol 5 (38) ◽  
pp. eaay8533 ◽  
Author(s):  
Matteo Di Luca ◽  
Stefano Mintchev ◽  
Yunxing Su ◽  
Eric Shaw ◽  
Kenneth Breuer
Keyword(s):  
Reynolds Number ◽  
Wind Tunnel ◽  
Low Reynolds Number ◽  
Substantial Reduction ◽  
Separated Flow ◽  
Small Scale ◽  
Freestream Turbulence ◽  
Flight Duration ◽  
Aerodynamic Efficiency ◽  
Low Reynolds

Small-scale drones have enough sensing and computing power to find use across a growing number of applications. However, flying in the low–Reynolds number regime remains challenging. High sensitivity to atmospheric turbulence compromises vehicle stability and control, and low aerodynamic efficiency limits flight duration. Conventional wing designs have thus far failed to address these two deficiencies simultaneously. Here, we draw inspiration from nature’s small flyers to design a wing with lift generation robust to gusts and freestream turbulence without sacrificing aerodynamic efficiency. This performance is achieved by forcing flow separation at the airfoil leading edge. Water and wind tunnel measurements are used to demonstrate the working principle and aerodynamic performance of the wing, showing a substantial reduction in the sensitivity of lift force production to freestream turbulence, as compared with the performance of an Eppler E423 low–Reynolds number wing. The minimum cruise power of a custom-built 104-gram fixed-wing drone equipped with the Separated Flow wing was measured in the wind tunnel indicating an upper limit for the flight time of 170 minutes, which is about four times higher than comparable existing fixed-wing drones. In addition, we present scaling guidelines and outline future design and manufacturing challenges.

Aerodynamic Performance Analysis of Three Bladed Savonius Wind Turbine With Different Overlap Ratios and at Various Reynolds Number

2010 ◽  
Author(s):  
Mosfequr Rahman ◽  
Khandakar N. Morshed ◽  
Ahsan Mian
Keyword(s):  
Reynolds Number ◽  
Wind Tunnel ◽  
Wind Turbine ◽  
Wind Turbines ◽  
Alternative Energy ◽  
Vertical Axis ◽  
Aerodynamic Performance ◽  
Small Scale ◽  
Scale Models ◽  
Savonius Wind Turbine

Considerable improvements in the aerodynamic performance of a vertical axis wind turbine (VAWT) can be achieved by integrating computational fluid dynamics (CFD) simulation and wind tunnel investigation in their design improvement. With the growing demand for energy worldwide, conventional sources are becoming more scarce and expensive. Wind is among the most popular and fastest growing sources of alternative energy in the world. It is an inexhaustible, indigenous resource, pollution-free, and available almost any time of the day, especially in coastal regions. Industry experts predict that, with proper development, wind energy could provide 20% of the nation’s energy needs. Vertical axis wind turbines (VAWTs) may be as efficient and practical as, and simpler, and significantly cheaper to build and maintain than, horizontal axis wind turbines (HAWTs). They have other inherent advantages; for example, they always face the wind. VAWTs include both a drag-type configuration, such as the Savonius rotor, and a lift-type configuration, such as the Darrieus rotor. The Savonius wind turbine is the simplest. Its operation depends on the difference in drag force when the wind strikes either the convex or concave part of its semi-cylindrical blades. It is good at self-starting and works independently of wind direction. However, its efficiency is relatively lower than that of the lift-type VAWTs. Due to its simple design and low construction cost, Savonius rotors are primarily used for water pumping and to generate wind power on a small scale and its large starting torque makes it suitable for starting other types of wind turbines that have inferior starting characteristics. Recently, some generators with high torque at low rotational speed, suitable for small-scale wind turbines, have been developed, suggesting that Savonius rotors may yet be used to generate electric power. The main goal of this research work is to improve the aerodynamic performance of the three bladed vertical axis Savonius wind turbine. Based on this goal, the objective of this project is to study the performance characteristics of the Savonius wind turbine scale models both experimentally and numerically. The turbine scale models will have different designs with different overlap ratios (ratio of gap between two adjacent blades and the rotor diameter) and without overlap within three blades. The experimental measurements and testing will be conducted in front of a low speed subsonic wind tunnel at different Reynolds number and the computational fluid dynamic (CFD) flow simulation around those design models will be performed by commercial CFD software FLUENT and GAMBIT.

Small-Scale Wind Turbine Testing in Wind Tunnels Under Low Reynolds Number Conditions

2015 ◽  
Vol 137 (5) ◽  
Author(s):  
Kenneth W. Van Treuren
Keyword(s):  
Wind Turbine ◽  
Wind Turbines ◽  
Reynolds Numbers ◽  
Wind Tunnels ◽  
Small Scale ◽  
Predicting Performance ◽  
Accurate Design ◽  
Low Reynolds ◽  
Lift And Drag ◽  
Turbine Airfoils

Much of the aerodynamic design of wind turbines is accomplished using computational tools such as XFOIL. These codes are not robust enough for predicting performance under the low Reynolds numbers found with small-scale wind turbines. Wind tunnels can experimentally test wind turbine airfoils to determine lift and drag data over typical operating Reynolds numbers. They can also test complete small wind turbine systems to determine overall performance. For small-scale wind turbines, quality experimental airfoil data at the appropriate Reynolds numbers are necessary for accurate design and prediction of power production.

Scaling Small-Scale Wind Turbines for Wind Tunnel Testing

2012 ◽  
Author(s):  
Timothy A. Burdett ◽  
Kenneth W. Van Treuren
Keyword(s):  
Reynolds Number ◽  
Wind Tunnel ◽  
Wind Turbines ◽  
Reynolds Numbers ◽  
Speed Ratio ◽  
Small Scale ◽  
Wind Tunnel Testing ◽  
Tip Speed Ratio ◽  
Multiple Challenges ◽  
Tunnel Testing

Wind tunnel testing of wind turbines can provide valuable insights into wind turbine performance and provides a simple process to test and improve existing designs. However, the scale of most wind turbines is significantly larger than most existing wind tunnels, thus, the scaling required for testing in a typical wind tunnel presents multiple challenges. When wind turbines are scaled, often only geometric similarity and tip speed ratio matching are employed. Scaling in this manner can result in impractical rotational velocities. For wind tunnel tests that involve Reynolds numbers less than approximately 500,000, Reynolds number matching is necessary. When including Reynolds number matching in the scaling process, keeping rotational velocities realistic becomes even more challenging and preventing impractical freestream velocities becomes difficult. Turbine models of 0.5, 0.4, and 0.3 m diameter, resulting in wind tunnel blockages up to 52.8%, were tested in order to demonstrate scaling using Reynolds number matching and to validate blockage corrections found in the literature. Reynolds numbers over the blades ranged from 20,000 to 150,000 and the tip speed ratio ranged from 3 to 4 at the maximum power point for each wind speed tested.

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