scholarly journals Aerofoil behaviour at high angles of attack and at Reynolds numbers appropriate for small wind turbines

2014 ◽  
Vol 229 (11) ◽  
pp. 2007-2022 ◽  
Author(s):  
Longhuan Du ◽  
Arganthaël Berson ◽  
Robert G Dominy
Keyword(s):  
Wind Tunnel ◽  
Turbine Blades ◽  
Sudden Change ◽  
Reynolds Numbers ◽  
Surface Flow ◽  
Wind Tunnels ◽  
Small Scale ◽  
Flow Visualisation ◽  
Data Set ◽  
High Angles Of Attack

The aerodynamic characteristics of a NACA0018 aerofoil have been investigated experimentally for incidence angles ranging from [Formula: see text] to [Formula: see text] in closed-jet and open-jet wind tunnels with different blockage coefficients at Reynolds numbers from 60,000 to 140,000. The results provide a comprehensive data set for studying the performance of typical, small-scale Darrieus wind turbine blades which mainly operate at relatively low Reynolds number and experience extreme angles of attack, particularly during start-up. Measurements in both very high and very low blockage, open-jet wind tunnels capture a “second-stall” phenomenon at high angles of attack, but this behaviour is not observed in the closed-jet wind tunnel confirming the sensitivity of aerofoil performance at extreme incidence to wind tunnel configuration. Surface flow visualisation suggests that the “second-stall” occurs when the flow separation point near the leading edge of the aerofoil moves from the suction side to the pressure side which leads to a sudden change of wake structure. In the closed-jet wind tunnel, the tunnel walls constrain the wake and prevent the flow from switching from one regime to another. The measured data are also used to demonstrate that established wind tunnel blockage corrections break down under these extreme, post-stall angles of attack.

Turbulence reduction by screens

1988 ◽  
Vol 197 ◽  
pp. 139-155 ◽  
Author(s):  
Johan Groth ◽  
Arne V. Johansson
Keyword(s):  
Wind Tunnel ◽  
Reynolds Numbers ◽  
Wind Tunnels ◽  
Small Scale ◽  
Flow Conditions ◽  
Upstream Side ◽  
Turbulence Suppression ◽  
Controlled Flow ◽  
Reduction Effectiveness

Turbulence suppression by use of screens was studied in a small wind tunnel especially designed and built for the purpose. Wide ranges of mesh sizes and wire-diameter Reynolds numbers were covered in the present investigation, enabling the study of sub- and super-critical screens under the same, well-controlled, flow conditions. For the latter type small-scale fluctuations, produced by the screen itself, interact with the incoming turbulence. In the immediate vicinity of the screen the turbulence was found to be highly anisotropic and the intensities were higher than on the upstream side. Downstream of a short initial decay region, where the intensities decrease rapidly, the return to isotropy was found to be much slower than for the unmanipulated turbulence. The latter was generated by a square rod grid, and was shown to become practically isotropic beyond a distance of roughly 20 mesh widths from the grid. The role of the turbulence scales for the overall reduction effectiveness, and for the optimization of screen combinations for application in low-turbulence wind tunnels was studied.

Experimental investigations of flow over NACA airfoils 0021 and 4412 of wind turbine blades with and without Tubercles

2021 ◽  
pp. 0309524X2110071
Author(s):  
Usman Butt ◽  
Shafqat Hussain ◽  
Stephan Schacht ◽  
Uwe Ritschel
Keyword(s):  
Wind Tunnel ◽  
Drag Coefficient ◽  
Wind Turbine ◽  
Critical Region ◽  
Lift Coefficient ◽  
Turbine Blades ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Experimental Investigations ◽  
Wind Turbine Blades

Experimental investigations of wind turbine blades having NACA airfoils 0021 and 4412 with and without tubercles on the leading edge have been performed in a wind tunnel. It was found that the lift coefficient of the airfoil 0021 with tubercles was higher at Re = 1.2×105 and 1.69×105 in post critical region (at higher angle of attach) than airfoils without tubercles but this difference relatively diminished at higher Reynolds numbers and beyond indicating that there is no effect on the lift coefficients of airfoils with tubercles at higher Reynolds numbers whereas drag coefficient remains unchanged. It is noted that at Re = 1.69×105, the lift coefficient of airfoil without tubercles drops from 0.96 to 0.42 as the angle of attack increases from 15° to 20° which is about 56% and the corresponding values of lift coefficient for airfoil with tubercles are 0.86 and 0.7 at respective angles with18% drop.

Power Augmentation of a HAWT by Mie-type Tip Vanes, considering Wind Tunnel Flow Visualisation, Blade-Aspect Ratios and Reynolds Number

2003 ◽  
Vol 27 (3) ◽  
pp. 183-194 ◽  
Author(s):  
Yukimaru Shimizu ◽  
Edmond Ismaili ◽  
Yasunari Kamada ◽  
Takao Maeda
Keyword(s):  
Reynolds Number ◽  
Wind Tunnel ◽  
Aspect Ratio ◽  
Reynolds Numbers ◽  
Flow Visualisation ◽  
Velocity Measurements ◽  
Horizontal Axis Wind Turbine ◽  
Aspect Ratios ◽  
Detailed Surface ◽  
Blade Tips

Wind tunnel results are reported concerning the effects of blade aspect ratio and Reynolds number on the performance of a horizontal axis wind turbine (HAWT) with Mie-type1 tip attachments. The flow behaviour around the blade tips and the Mie-type tip vanes is presented. Detailed surface oil film visualization and velocity measurements around the blade tips, with and without Mie vanes, were obtained with the two-dimensional, Laser-Doppler Velocimetry method. Experiments were performed with rotors having blades with different aspect ratio and operating at different Reynolds numbers. The properties of the vortices generated by the Mie vanes and the blade tips were carefully studied. It was found that increased power augmentation by Mie vanes is achieved with blades having smaller aspect ratio and smaller Reynolds number.

scholarly journals On the prospects of improving the numerical analysis of symmetric airfoils at low Reynolds numbers and high angles of attack by a LES approach: the case of NACA0021 profile

2020 ◽  
Vol 197 ◽  
pp. 08015
Author(s):  
Simone Giaccherini ◽  
Filippo Mariotti ◽  
Lorenzo Pinelli ◽  
Michele Marconcini ◽  
Alessandro Bianchini
Keyword(s):  
Turbine Blades ◽  
Separated Flow ◽  
Reynolds Numbers ◽  
Aerodynamic Performance ◽  
Operating Conditions ◽  
High Fidelity ◽  
Complex Flow ◽  
Energy Applications ◽  
Low Reynolds ◽  
High Angles Of Attack

The working conditions of airfoils along modern wind turbine blades are putting new focus on the importance of properly characterizing the aerodynamic performance of different airfoil families also at high angles of attack (AoAs) beyond stall and at Reynolds numbers much lower (from few thousands to one million) than those commonly analyzed before. Several test cases are showing that even higher-order computational methods (like RANS/URANS CFD) are unable to properly capture the complex flow physics taking place past the blades, when deep stall occurs or when the AoA changes so rapidly to provoke the onset of dynamic stall. To fill this gap, the use of high-fidelity methods, like the Large Eddy Simulation (LES) is proposed, even though it implies a massive increase of the calculation cost. In order to analyze the prospects of using LES in comparison to RANS for low Reynolds, high AoAs, this work presents an in-depth study of the NACA 0021 aerodynamics at the Reynolds number of 80,000, by means of both traditional RANS approaches and high-fidelity (LES) simulations using the OpenFOAM suite. The selected airfoil has been showing in fact several issues in the correct characterization of its performance in similar conditions in many recent wind energy applications. The LES approach showed the ability to overcome the limitations of traditional RANS simulations, improving the accuracy of the results and reducing their dispersion thanks to the fact that the flow structures in the separated-flow regions are properly captured. Overall, this work underlines that accurate investigations of the aerodynamic performance of the NACA 0021 at low Reynolds require multiple sensitivity studies when RANS approaches are used, and suggests the use of LES simulations in order to increase the accuracy of estimations, especially when studying the stalledflow operating conditions of the airfoil.

Aerodynamic characteristics of delta wing–body combinations at high angles of attack

1994 ◽  
Vol 98 (975) ◽  
pp. 159-170 ◽  
Author(s):  
P. R. Viswanath ◽  
S. R. Patil
Keyword(s):  
Vortex Breakdown ◽  
Delta Wing ◽  
Aerodynamic Characteristics ◽  
Surface Flow ◽  
Flow Visualisation ◽  
Symmetric Flow ◽  
Gradual Process ◽  
Wing Sweep ◽  
The Mean ◽  
High Angles Of Attack

AbstractAn experimental study investigating the aerodynamic characteristics of generic delta wing-body combinations up to high angles of attack was carried out at a subsonic Mach number. Three delta wings having sharp leading edges and sweep angles of 50°, 60° and 70° were tested with two forebody configurations providing a variation of the nose fineness ratio. Measurements made included six-component forces and moments, limited static pressures on the wing lee-side and surface flow visualisation studies. The results showed symmetric flow features up to an incidence of about 25°, beyond which significant asymmetry was evident due to wing vortex breakdown, forebody vortex asymmetry or both. At higher incidence, varying degrees of forebody-wing vortex interaction effects were seen in the mean loads, which depended on the wing sweep and the nose fineness ratio. The vortex breakdown on these wings was found to be a gradual process, as implied by the wing pressures and the mean aerodynamic loads. Effects of forebody vortex asymmetry on the wing-body aerodynamics have also been assessed. Comparison of Datcom estimates with experimental data of longitudinal aerodynamic characteristics on all three wing-body combinations indicated good agreement in the symmetric flow regime.

Surface Flow Patterns as Visualised by Dust Deposits on the Blades of a Fan

1963 ◽  
Vol 67 (633) ◽  
pp. 589-594 ◽  
Author(s):  
E. T. Hignett ◽  
M. M. Gibson
Keyword(s):  
Boundary Layer ◽  
Wind Tunnel ◽  
Leading Edge ◽  
Surface Flow ◽  
Scale Model ◽  
Flow Visualisation ◽  
Pressure Gradients ◽  
Suction Surface ◽  
Layer Flow ◽  
Dust Deposits

Investigations by one of the authors in connection with the design of a fan for a blower type of wind tunnel showed that regular and repeatable dust patterns occurred on the blades of a one-quarter scale model fan of 18 inches diameter. Dust was deposited on the fan blades along the leading-edge and on the suction surface over an area thought to be the turbulent region of the boundary layer. The introduction of isolated protuberances on the dust free area of a blade gave rise to turbulence wedges in which dust was also deposited and this was interpreted as confirmation of the coincidence of the dust deposits with regions of turbulent boundary-layer flow. These deposits showed the existence of a considerable extent of laminar flow on the suction surface of each blade close to the root, a region where high lift coefficients would be expected with associated adverse pressure gradients. Two-dimensional wind tunnel experiments were made to confirm the interpretation of the observed dust patterns by comparison with the smoke filament and volatile liquid methods of flow visualisation and these are reported in Reference 2.

Flow separation on a spheroid at incidence

1979 ◽  
Vol 92 (4) ◽  
pp. 643-657 ◽  
Author(s):  
Taeyoung Han ◽  
V. C. Patel
Keyword(s):  
Reynolds Number ◽  
Wind Tunnel ◽  
Numerical Solutions ◽  
Three Dimensional ◽  
Inviscid Flow ◽  
Reynolds Numbers ◽  
Surface Flow ◽  
Laminar Separation ◽  
Low Reynolds Numbers ◽  
Turbulent Separation

Surface streamline patterns on a spheroid have been examined at several angles of attack. Most of the tests were performed at low Reynolds numbers in a hydraulic flume using coloured dye to make the surface flow visible. A limited number of experiments was also carried out in a wind tunnel, using wool tufts, to study the influence of Reynolds number and turbulent separation. The study has verified some of the important qualitative features of three-dimensional separation criteria proposed earlier by Maskell, Wang and others. The observed locations of laminar separation lines on a spheroid at various incidences have been compared with the numerical solutions of Wang and show qualitative agreement. The quantitative differences are attributed largely to the significant viscous-inviscid flow interaction which is present, especially at large incidences.

scholarly journals A technique to predict the aerodynamic effects of battle damage on an aircraft’s wing

2015 ◽  
Vol 119 (1218) ◽  
pp. 937-960 ◽  
Author(s):  
T.W. Pickhaver ◽  
P.M. Render
Keyword(s):  
Wind Tunnel ◽  
Lower Surface ◽  
Surface Flow ◽  
Internal Cavity ◽  
Flow Visualisation ◽  
Pitching Moment ◽  
Wind Tunnel Tests ◽  
Tunnel Model ◽  
Circular Holes ◽  
The Difference

Abstract A technique is developed that can be used to predict the effects of battle damage on the aerodynamic performance of an aircraft’s wing. The technique is based on results obtained from wind tunnel tests on a NASA LS(1)-0417MOD aerofoil with simulated gunfire damage. The wind tunnel model incorporated an internal cavity to represent typical aircraft construction and this was located between 24% and 75% of chord. The damage was simulated by circular holes with diameters between 20% and 40% of chord. To represent different attack directions, the inclination of the hole axis relative to the aerofoil chord was varied between ±60° pitch and 45° of roll. The aerofoil spanned the wind tunnel to create approximate two-dimensional conditions and balance measurements were carried out at a Reynolds number of 500,000 for incidences, increased in 2° increments, from –4° to 16°. Surface flow visualisation and pressure measurements were also carried out. For a given hole size, the increments in lift, drag and pitching moment coefficients produced trends when plotted against the difference between the upper and lower surface pressure coefficients on the undamaged aerofoil taken at the location of the damage. These trends are used as the basis of the predictive technique. The technique is used to predict the effects of a previously untested damage case, and these are compared with wind tunnel tests carried out on a half model finite aspect ratio wing. For all coefficients the trends in the predicted data are similar to experiment, although there are some discrepancies in absolute values. For the drag coefficient these discrepancies are partly accounted for by limitations in the technique, whilst discrepancies in the lift and pitching moment coefficients are attributed to limitations in the aerofoil test arrangements.

scholarly journals Validation of CFD Predictions of Urban Wind - Developing a Methodology

2021 ◽  
Author(s):  
◽  
Riley Willis
Keyword(s):  
Wind Tunnel ◽  
City Council ◽  
Wind Tunnels ◽  
Small Scale ◽  
Cfd Model ◽  
Validation Process ◽  
Cfd Simulations ◽  
Entire Area ◽  
Cfd Models ◽  
Wind Tunnel Measurements

<p>“Good mental health in a fluid or CFD modeller is always indicated by the presence of a suspicious nature, cynicism and a ‘show me’ attitude. These are not necessarily the best traits for a life mate or a best friend, but they are essential if the integrity of the modelling process is to be maintained.” (Meroney, 2004)  Over the past 50 years, Computational Fluid Dynamics (CFD) computer simulation programs have offered a new method of calculating the wind comfort and safety data for use in pedestrian wind studies. CFD models claim to have some important advantages over wind tunnels; which remain the most common method of wind calculation. While wind tunnels provide measurements of selected points, CFD simulations provide whole-flow field data for the entire area under investigation (Blocken, 2014; Blocken, Stathopoulos, & van Beeck, 2016). Similarly, wind tunnel measurements must consider the similarity requirements involved with testing a model at small scale, while CFD simulations can avoid this as they are conducted at full scale (Ramponi & Blocken, 2012a).  However, CFD simulations can also often be misleading; and they should only be trusted once they can be proven to be accurate. To appease the requirements for this cynical view- referenced in the above quote- proper verification and validation of a model is imperative.  This thesis investigated and tested the current best practice guidelines around CFD model validation, using existing wind tunnel measurements of generic urban arrays. The goal of the research was to determine whether the existing data and guidance around the validation process was sufficient for a consultant user to trust that a CFD model they created was sufficiently accurate to base design decisions from.  The CFD code Autodesk CFD was used to simulate two configurations first tested as wind tunnel models by the Architectural Institute of Japan, and Opus labs in Wellington. The Wellington City Council wind speed criteria were used to determine whether the CFD simulations met the required accuracy criteria for council consent.  Results from the study found that the CFD models could not meet the accuracy criteria. It concluded that while the validation process provided sufficient guidance, there is a lack of available data which is relevant to CFD validation for urban flows.  It was recommended that at least one improved dataset was required, to build a system by which a consultant can identify what the requirements of a CFD model are to provide accurate CFD analysis of the site under investigation. To accommodate the range of sites likely to be present in urban wind studies, it was recommended that the new dataset provided data for a variety of wind flows likely to be found in cities.</p>

scholarly journals Demonstration and uncertainty analysis of synchronised scanning lidar measurements of 2-D velocity fields in a boundary-layer wind tunnel

2017 ◽  
Vol 2 (1) ◽  
pp. 329-341 ◽  
Author(s):  
Marijn Floris van Dooren ◽  
Filippo Campagnolo ◽  
Mikael Sjöholm ◽  
Nikolas Angelou ◽  
Torben Mikkelsen ◽  
...  
Keyword(s):  
Wind Tunnel ◽  
Uncertainty Analysis ◽  
Wind Turbine ◽  
Short Range ◽  
Goodness Of Fit ◽  
Wind Tunnels ◽  
Small Scale ◽  
Measurement Technology ◽  
Lidar Measurement ◽  
Hot Wire

Abstract. This paper combines the research methodologies of scaled wind turbine model experiments in wind tunnels with short-range WindScanner lidar measurement technology. The wind tunnel at the Politecnico di Milano was equipped with three wind turbine models and two short-range WindScanner lidars to demonstrate the benefits of synchronised scanning lidars in such experimental surroundings for the first time. The dual-lidar system can provide fully synchronised trajectory scans with sampling timescales ranging from seconds to minutes. First, staring mode measurements were compared to hot-wire probe measurements commonly used in wind tunnels. This yielded goodness of fit coefficients of 0.969 and 0.902 for the 1 Hz averaged u and v components of the wind speed, respectively, validating the 2-D measurement capability of the lidar scanners. Subsequently, the measurement of wake profiles on a line as well as wake area scans were executed to illustrate the applicability of lidar scanning to the measurement of small-scale wind flow effects. An extensive uncertainty analysis was executed to assess the accuracy of the method. The downsides of lidar with respect to the hot-wire probes are the larger measurement probe volume, which compromises the ability to measure turbulence, and the possible loss of a small part of the measurements due to hard target beam reflection. In contrast, the benefits are the high flexibility in conducting both point measurements and area scanning and the fact that remote sensing techniques do not disturb the flow during measuring. The research campaign revealed a high potential for using short-range synchronised scanning lidars to measure the flow around wind turbines in a wind tunnel and increased the knowledge about the corresponding uncertainties.

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