This paper presents our work of an experimental examination of the flow over a NACA 0012 Airfoil at low Reynolds numbers and large angle of attack using particle imaging velocimetry. The Reynolds numbers examined were 5,000, 30,000, and 60,000, while the angles of attack ranged from 8 to 12 degrees in 2 degree increments. This work was motivated by reports that lift and drag measurements for airfoils operating at Reynolds numbers less than roughly 40,000 could not be made due to flow unsteadiness. This is puzzling in that the flow should be laminar at these Reynolds numbers, which are an order of magnitude lower than the flat-plate transition Reynolds number of 500,000. To this end we examined a sequence of flow field measurements of the instantaneous velocity field. We observed mean streamline patterns that were very representative of those we would find for a strictly steady flow, however a random pattern of significant fluctuations in the velocity and vorticity were observed. The intensity of these fluctuations increased with Reynolds number and angle of attack.

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Airfoils at low speeds

Characteristics Of The Flow Over A NACA 0012Airfoil At Low Reynolds Numbers

Small wind turbines that operate in low-wind environments are prone to suffer performance degradation as they often fail to accelerate to a steady, power-producing
condition. The behaviour during this process is called “starting behaviour” and it is the subject of this present work.

This thesis evaluates potential benefits that can be obtained from the improvement of starting behaviour, investigates, in particular, small wind turbine starting
behaviour (both horizontal- and vertical-axis), and presents aerofoil performance characteristics (both steady and unsteady) needed for the analysis.

All of the investigations were conducted using a new set of aerodynamic performance data of six aerofoils (NACA0012, SG6043, SD7062, DU06-W-200, S1223, and S1223B). All of the data were obtained at flow conditions that small wind turbine blades have to operate with during the startup - low Reynolds number (from 65000 to 150000), high angle of attack (through 360◦), and high reduced frequency (from
0.05 to 0.20). In order to obtain accurate aerodynamic data at high incidences, a series of CFD simulations were undertaken to illustrate effects of wall proximity and
to determine test section sizes that offer minimum proximity effects.

A study was carried out on the entire horizontal-axis wind turbine generation system to understand its starting characteristics and to estimate potential benefits
of improved starting. Comparisons of three different blade configurations reveal that the use of mixed-aerofoil blades leads to a significant increase in starting capability.
The improved starting capability effectively reduces the time that the turbine takes to reach its power-extraction period and, hence, an increase in overall energy yield.
The increase can be as high as 40%.

Investigations into H-Darriues turbine self-starting capability were made through the analogy between the aerofoil in Darrieus motion and flapping-wing flow mechanisms. The investigations reveal that the unsteadiness associated with the rotor is key to predicting its starting behaviour and the accurate prediction can be made
when this transient aerofoil behaviour is correctly modelled. The investigations based upon the analogy also indicate that the unsteadiness can be exploited to promote
the turbine ability to self-start. Aerodynamically, this exploitation is related to the rotor geometry itself.

background

Small Wind Turbine Starting Behaviour

A numerical study of the effects of airfoil shape on low Reynolds number aerodynamics is presented. The large-eddy simulations are performed with 6-order compact finite difference scheme and 10-order low pass filter, and 2-order backward implicit time integration with inner iterations. Systematic numerical excesses show the feasibility of the current simulations to predict flow fields around fixed-wing configurations involving a laminar separation and laminar-to-turbulence transition at low Reynolds number. At the Reynolds number of 2.3×10, two types of thin and asymmetric airfoils as a target airfoil shape of micro-size air vehicle are considered. The results show that the airfoil cross section affects the formation of a laminar separation bubble and the transition to turbulence in the three-dimensional flow around the wings at low angle of attack and hence significant influence on the aerodynamic performance.

methods

A numerical study of the effects of aerofoil shape on low reynolds number aerodynamics

The need for better quality aerofoil data, extending to incidences well beyond stall, for the modelling of vertical-axis wind turbine (VAWT) start-up is established through blade elementmomentum modelling. The model is used to show that differences between existing post-stall data are large enough to impact on turbine performance. The differences between the existing aerofoil data are found to result primarily from inconsistencies introduced by wind tunnel blockage and the potential flow-derived formulae used to correct for it. A blockage tolerant wind tunnel test section has been constructed and calibrated for aerofoil testing. The tunnel has two semi-permeable walls made up of an array of transverse aerofoil-shaped slats. It produces free-air equivalent data without the need for blockage corrections. The ratio of slat to open area in the permeable walls that best minimises blockage has been obtained through testing of five different-sized NACA 0015 aerofoils. In free-air, results for the five aerofoils would be identical. The open area ratio that produces the most consistent results for the five aerofoils, based on a standard deviation analysis of the results, is therefore judged to be the best. The aerofoils are tested in a solidwalled wind tunnel, and the data processed using a selection of blockage corrections. The corrections are also judged using a standard deviation analysis. The tolerant tunnel, configured with the best open area ratio, outperforms the best corrections. Comparisons are made between results from the tolerant and solid-walled tunnels for the smallest (least blocked) aerofoil, with the latter corrected for blockage. Results are equal to within experimental error. Three additional aerofoils (a symmetrical NACA 0018 and two cambered versions of it) are tested in the tunnel. Results are used in a numerical study of virtual camber effects on VAWT blades. The effect is found to be significant in turbines with large blade chord to

background or methods

Blockage tolerant wind tunnel testing of aerofoils at angles of incidence from 0 to 360 degrees, with respect to the self-start of vertical-axis wind turbines

Airfoils at low speeds

Citations

Characteristics Of The Flow Over A NACA 0012Airfoil At Low Reynolds Numbers

Small Wind Turbine Starting Behaviour

A numerical study of the effects of aerofoil shape on low reynolds number aerodynamics

Blockage tolerant wind tunnel testing of aerofoils at angles of incidence from 0 to 360 degrees, with respect to the self-start of vertical-axis wind turbines

Numerical Analysis of the Flow Around the SD 7003 Airfoil

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