A Numerical Study on Improving Airfoil Performance at Low Reynolds Numbers for Small-Scale Wind Turbines using Intentional Roughness

2012 ◽  
Author(s):  
Timothy Burdett ◽  
Jason Gregg ◽  
Kenneth Van Treuren
Keyword(s):  
Wind Turbines ◽  
Numerical Study ◽  
Reynolds Numbers ◽  
Small Scale ◽  
Low Reynolds Numbers ◽  
Low Reynolds

An Examination of the Effect of Reynolds Number on Airfoil Performance

2011 ◽  
Author(s):  
Tim Burdett ◽  
Jason Gregg ◽  
Kenneth Van Treuren
Keyword(s):  
Experimental Data ◽  
Reynolds Number ◽  
Wind Turbines ◽  
Large Scale ◽  
Reynolds Numbers ◽  
Energy Sources ◽  
Small Scale ◽  
Number Range ◽  
Low Reynolds Numbers ◽  
Low Reynolds

The standard of living throughout the world has increased dramatically over the last 30 years and is projected to continue to rise. This growth leads to an increased demand on conventional energy sources, such as fossil fuels. However, these are finite resources. Thus, there is an increasing demand for alternative energy sources, such as wind energy. Much of current wind turbine research focuses on large-scale (>1 MW), technologically-complex wind turbines installed in areas of high average wind speed (>20 mph). An alternative approach is to focus on small-scale (1–10kW), technologically-simple wind turbines built to produce power in low wind regions. While these turbines may not be as efficient as the large-scale systems, they require less industrial support and a less complicated electrical grid since the power can be generated at the consumer’s location. To pursue this approach, a design methodology for small-scale wind turbines must be developed and validated. This paper addresses one element of this methodology, airfoil performance prediction. In the traditional design process, an airfoil is selected and published lift and drag curves are used to optimize the blade twist and predict performance. These published curves are typically generated using either experimental testing or a numeric code, such as PROFIL (the Eppler Airfoil Design and Analysis Code) or XFOIL. However, the published curves often represent performance over a different range of Reynolds numbers than the actual design conditions. Wind turbines are typically designed from 2-D airfoil data, so having accurate airfoil data for the design conditions is critical. This is particularly crucial for small-scale, fixed-pitched wind turbines, which typically operate at low Reynolds numbers (<500,000) where airfoil performance can change significantly with Reynolds number. From a simple 2-D approach, the ideal operating condition for an airfoil to produce torque is the angle of attack at which lift is maximized and drag is minimized, so prediction of this angle will be compared using experimental and simulated data. Theoretical simulations in XFOIL of the E387 airfoil, designed for low Reynolds numbers, suggest that this optimum angle for design is Reynolds number dependent, predicting a difference of 2.25° over a Reynolds number range of 460,000 to 60,000. Published experimental data for the E387 airfoil demonstrate a difference of 2.0° over this same Reynolds number range. Data taken in the Baylor University Subsonic Wind Tunnel for the S823 airfoil shows a similar trend. This paper examines data for the E387 and S823 airfoils at low Reynolds numbers (75,000, 150,000, and 200,000 for the S823) and compares the experimental data with XFOIL predictions and published PROFIL predictions.

A numerical study on free-fall of a torus with initial inclination angle at low Reynolds numbers

2021 ◽  
Vol 107 ◽  
pp. 103389
Author(s):  
Tao Huang ◽  
Haibo Zhao ◽  
Sai Peng ◽  
Jiayu Li ◽  
Yang Yao ◽  
...  
Keyword(s):  
Inclination Angle ◽  
Numerical Study ◽  
Free Fall ◽  
Reynolds Numbers ◽  
Low Reynolds Numbers ◽  
Low Reynolds

scholarly journals The transient swimming of a waving sheet

2009 ◽  
Vol 466 (2113) ◽  
pp. 107-126 ◽  
Author(s):  
On Shun Pak ◽  
Eric Lauga
Keyword(s):  
Fluid Flow ◽  
Reynolds Numbers ◽  
Geometrical Model ◽  
Small Scale ◽  
Transient Effects ◽  
Initial Value ◽  
Low Reynolds Numbers ◽  
The Past ◽  
Low Reynolds Number Swimming ◽  
Low Reynolds

Small-scale locomotion plays an important role in biology. Different modelling approaches have been proposed in the past. The simplest model is an infinite inextensible two-dimensional waving sheet, originally introduced by Taylor, which serves as an idealized geometrical model for both spermatozoa locomotion and ciliary transport in Stokes flow. Here, we complement classic steady-state calculations by deriving the transient low-Reynolds number swimming speed of such a waving sheet when starting from rest (small-amplitude initial-value problem). We also determine the transient fluid flow in the ‘pumping’ setup where the sheet is not free to move but instead generates a net fluid flow around it. The time scales for these two problems, which in general govern transient effects in transport and locomotion at low Reynolds numbers, are also derived using physical arguments.

Static and Dynamic Analysis of a NACA 0021 Airfoil Section at Low Reynolds Numbers: Drag and Moment Coefficients

2019 ◽  
Author(s):  
David Holst ◽  
Francesco Balduzzi ◽  
Alessandro Bianchini ◽  
Christian Navid Nayeri ◽  
Christian Oliver Paschereit ◽  
...  
Keyword(s):  
Experimental Data ◽  
Wind Tunnel ◽  
Wind Turbines ◽  
Reynolds Numbers ◽  
Correction Method ◽  
Hysteresis Cycle ◽  
Low Reynolds Numbers ◽  
Static And Dynamic Analysis ◽  
Wide Range ◽  
Low Reynolds

Abstract Wind industry needs high quality airfoil data for a range of the angle of attack (AoA) much wider than that often provided by the technical literature, which often lacks data i.e. in deep- and post-stall region. Especially in case of vertical axis wind turbines (VAWTs), the blades operate at very large AoAs, which exceed the range of typical aviation application. In a previous study, some of the authors analyzed the trend of the lift coefficient of a NACA 0021 airfoil, using the suggestions provided by detailed CFD analyses to correct experimental data at low Reynolds numbers collected in an open-jet tunnel. In the present study, the correction method is extended in order to analyze even the drag and moment coefficients over a wide range of AoAs for two different Reynolds numbers (Re = 140k and Re = 180k) of particular interest for small wind turbines. The utility of these data is again specifically high in case of VAWTs, in which both the drag and the moment coefficient largely contribute to the torque. The investigation involves tunnel data regarding both static polars and dynamic sinusoidal pitching movements at multiple reduced frequencies. Concerning the numerical simulations, two different computational domains were considered, i.e. the full wind tunnel and the open field. Once experimental data have been purged by the influence of the wind tunnel by means of the proposed correction method, they were compared to existing data for similar Reynolds both for the NACA0021 and for similar airfoils. By doing so, some differences in the static stall angle and the extent of the hysteresis cycle are discussed. Overall, the present paper provides the scientific community with detailed analysis of low-Reynolds NACA 0021 data in multiple variations, which may enable, inter alia, a more effective VAWT design in the near future.

Numerical study of instability mechanisms in a circular jet at low Reynolds numbers

2012 ◽  
Vol 64 ◽  
pp. 1-18 ◽  
Author(s):  
Trushar B. Gohil ◽  
Arun K. Saha ◽  
K. Muralidhar
Keyword(s):  
Numerical Study ◽  
Reynolds Numbers ◽  
Low Reynolds Numbers ◽  
Low Reynolds

Small-scale chaos at low Reynolds numbers

1991 ◽  
Vol 24 (24) ◽  
pp. 5763-5773 ◽  
Author(s):  
G A Kuz'min ◽  
A Z Patashinskii
Keyword(s):  
Reynolds Numbers ◽  
Small Scale ◽  
Low Reynolds Numbers ◽  
Low Reynolds
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