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ECN publication
The April 95 procedure to measure the pressure coefficient Cp on a wind turbine in the field: with the December 1998 proposal for a Cp-derivation
Published by: Publication date:
ECN 1-12-1998
ECN report number: Document type:
ECN-RX--98-066 Article (scientific)
Number of pages:

Published in: Paper presented at the Annual IEA Symposium on the aerodynamics of wind turbines, December 3-4, 1998, Lyngby, Denmark (), , , Vol., p.-.

To study the three-dimensional flow around the rotor blades of windturbines, field experiments were carried out at DUT (Delft University of Technology in Delft, Netherlands), at ECN (Netherlands Energy Research Foundation in Petten, Netherlands) and at NREL (National Renewable Energy Laboratories, USA). In all cases a test turbine was equipped with pressure measurement equipment inside one of the blades. The purpose of the experiments was to enable the description of the differences between the pressure distributions on the blades while they rotate in the field (3d-flow) and the pressure distributions on the same blade in the wind tunnel (2d-flow). In 1992 the first measurements on the 10m diameter test turbine of DUT were carried out. These measurements are described elsewhere and showed a sinusoidal variation of 1P in all pressure signals. A damper was inserted in the tubing at the reference side of the system in an attempt to suppress the variation. However after this change, the variation was still present. Therefore, after about two years of preliminary measurements, it was realised that the measuring system required a more thorough analysis to improve and understand the results. This report is written in that context. The primary goal is to accurately predict both the loads on and the power of a wind turbine rotor. Present prediction methods often are inaccurate and therefore better understanding in the rotor aerodynamics is required. Power and loads origin from pressure distributions over the rotor blades, for this reason it was decided to measure these distributions. The measured local pressures on the blades mostly are made dimensionless with the dynamic pressure, which will be defined later. For the dimensionless pressure or pressure coefficient the symbol cp is used. The local values of cp can be seen as the basic information on the aerodynamic behaviour of wind turbines. Many other relevant parameters, e.g. loads and power, can be derived from the cp-values. For this reason this report will focus on the derivation of cp from measured values. 8 refs.

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