Browsing by Author "Hales, R. L."
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Item Open Access Aerodynamic measurements on a small HAWT rotor in axial and yawed flow(Cranfield University, 1990-07) Bellia, J. M.; Hales, R. L.; Moss, J. B.Current wind turbine performance codes are not yet able to predict the rotor aerodynamic behaviour with sufficient certainty. This has led to both the over-design of blades and to operational restrictions in certain wind conditions. Essentially the problem is one of aerodynamic stall. Steady 3-dimensional stall can occur near the blade root in high wind conditions and may produce more power than predicted. Dynamic stall can also be expected due to the effects of yawed operation, turbulence, tower shadow and the earth's boundary layer. The main aim of this work is to provide a coherent set of measured aerodynamic data accounting for both axial/non-axial flow and stall in high winds. These measurements are designed to highlight the effects of both steady and dynamic stall on the rotor aerodynamic performance. In addition, the data will enable current performance prediction codes to be developed and validated. A completely new turbine has been designed and built at Cranfield to make aerodynamic measurements using pressure transducers. The design has been dominated by the requirements of accommodating the transducer signal processing equipment and allowing variation of many of the rotor parameters. Three commercial glass fibre blades were installed and performance curves measured on a conventional field site at a height of 11.5m for three rotor speed settings. These measurements show the turbine to give adequate power performance. A mobile trailer has been used to tow the turbine at a height of 4m along the Cranfield runways. Mobile testing facilitates an accelerated test schedule and allows aerodynamic data to be acquired under controlled wind conditions. A fully instrumented blade, fitted with forty transducers, has been tested under these circumstances and produced a large database of pressure measurements covering operation in winds up to 25 iq/s and yaw angles between -4511 and +55°. Analysis of the data has shown it to be of good quality and allowed some of the effects of yaw and stall to be identified. The use of the data base for performance prediction code validation has also been established.Item Open Access Aerodynamic surface pressure measurement in atmosphere and wind tunnel on a vertical axis wind turbine blade using pressure transducers(1989-09) Oram, C. E.; Garside, A. J.; Hales, R. L.; Moss, J. B.Blade aerodynamic loads during operation of a wind turbine are poorly understood. Better measurements of these forces are needed for the design of more cost effective turbines. Existing estimates have been inferred from strain gauges in the blade structure, but such measurements are confused by the complexity of the blade's structural response. This present work has pressure tapped the blade surface and recorded at high speed, giving a highly detailed picture of dynamic events and blade loads. A modular blade was constructed for the 6m diameter Rutherford Appleton Vertical Axis Wind Turbine and instrumented with 32 semiconductor pressure transducers. A flow probe using a further three transducers projected ahead of the blade during some of the tests. The instrumented module was also operated in the oscillating mounting of the University of Glasgow's 1.6m by 2.4m wind tunnel. 1760 revolutions of data were collected in atmosphere with blade speed ratios down to about 2.0 and with a variety of blade leanout angles. Some 118 runs of a variety of waveform types and Reynolds numbers from 0.3 to 1.1 million, were collected from the blade in tunnel. Analysis of transducer calibrations concluded that the novel temperature correction technique used allowed pressure readings to 40Pa or better with 95% confidence. Confidence in measured pressures has allowed use of leading edge stagnation point pressure and position as a measure of wind in the rotor flowfield and as a surrogate measure of blade angle of attack. Blade surface pressures and integrated forces show the progression of dynamic stall with increase in wind speed, the dominance of the upstream blade pass in usefulness at low and moderate windspeeds and the rough equality at high windspeeds. Comparisons of wind tunnel and on-rotor data show much lower force and moment coefficients in atmosphere. Rotor 'goodness1 ratios of average blade tangential force to maximum blade and rotor force coefficients peak at a blade speed ratio of about 2.8. Goodness ratios show a mixed pattern with blade leanout. Analysis of pressure coefficient traces through stall indicate a mixed stall type starting at mid chord, moving to leading edge and thence to trailing edge.Item Open Access Computational Study of Stalled Wind Turbine Rotor Performance(Cranfield University, 1991-01) Rawlinson-Smith, R. I.; Hales, R. L.; Moss, J. B.Simplification of the aerodynamic control of large horizontal axis wind turbines (HAWTs) has been identified as an important step towards improved reliability and reduced cost. At present the majority of large HMrrs use active control to regulate power and loads. A simpler strategy is to use the inherent stalling of the rotor blades in high winds to limit power and loads. Unfortunately the performance of stall regulated HAWTs 1S poorly understood; current performance models often fail to correctly predict peak power levels. The benefits of passive control of power and loads cannot be utilised because of this uncertainty. This study examines the possible reasons for the poor performance of current prediction techniques 1n high winds with the objective of fonmulating a new model. The available experimental evidence suggests that rotor stall is caused by turbulent separation at the rear of the blade aerofoil, growing in extent from the root in increasing wind. This 'picture' of the stalling HAW! rotor forms the basis of the approach. The new model consists of a prescribed vortex wake, first order panel method (extended to represent the viscous region of trailing edge separation) and three dimensional integral boundary layer directly coupled in an iterative scheme. A sensitivity study of rotor indicates that the most important performance to wake geometry factor is the rate at which the wake is convected downstream. However, it is found that stalled power levels are insensitive to wake geometry; the study concludes that the problem of poor prediction of high wind performance lies on the rotor blades. Before using the complete code to calculate the performance of a rotor it 1S first tuned for the aerofoils used on the blade. Aerofoil perfonmance characteristics measured in a wind tunnel are synthesised by the model. Ideally these characteristics should include measured pressure profiles below and above stall. Validation of the complete code against detailed measurements taken under controlled conditions on a three metre diameter machine indicates significant differences in the perfonmance of aerofoil sections on a wind turbine blade when compared to the same section when tested in a wind tunnel. Derived lift coefficients show a reduced lift curve slope and more gentle delayed stall. Similar results are found when the code is applied to two Danish stall regulated machines. These two machines although having very similar geometries and using the same family of aerofoils do however show differences in derived post stall drag. This is thought to be due to the different thickness distributions of the two rotors. The validation and applications of the new model show that it can accurately predict the peak power level of stall regulated machines.