Inflow measurements from blade-mounted flow sensors: Flow analysis, application and aeroelastic response

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Abstract

The power and load performance of wind turbines are both crucial for the development and expansion of wind energy. The power and loads are highly dependent on the inflow conditions, which can be measured using different types of sensors mounted on nearby met masts, on the nacelle, at the spinner or at the blade. Each combination of sensor type and mounting position has advantages and shortcomings. To characterise the inflow that results in high and low fatigue loads, information about the temporal and spatial variations within the rotor area is required. This information can be obtained from a blade-mounted flow sensor, BMFS, e.g. a five-hole pitot tube, which has been used in several research experiments over the last 30 years. The BMFS measured flow velocity is, however, located inside the induction zone and thereby influenced by the aerodynamic properties, the control strategy and the operational status of the turbine. In this project, a method to estimate the free-inflow velocity from the BMFS measured flow velocity has been developed and implemented. The method is based on the aerodynamic engineering models that are used in well-established aeroelastic codes to describe the relation between the free-inflow and the velocity at the blades. Before these models can be applied, the measured local flow must be compensated for flow deflection and change of flow speed near the airfoil. Furthermore, the sensor velocity must be subtracted and the resulting absolute flow must be mapped into fixed ground coordinates. In these steps, uncertainty is introduced because the actual velocity and orientation of the BMFS are unknown due to the deflection and torsion of the blade. The introduced uncertainties have been investigated using HAWC2 simulations and simulations performed by Flex5 coupled with the LES flow solver, EllipSys3D. The uncertainties should, however, be considered in relation to the advantages of measuring the flow at the blade: a BMFS yaws with the turbine, measures the inflow at the rotor plane and sweeps different parts of the rotor. It is thereby exposed to exactly
the same inflow conditions as the turbine (including wake effects from upstream turbines) and able to provide valuable information about the instant inflow velocity as well as variations within the rotor plane, and that goes for all wind directions. From the BMFS measurements, estimates of the local aerodynamic forces, the angle-ofattack and relative flow speed, the rotor-plane velocity and the free-inflow velocity can be obtained. Applications of these measures have been investigated. It is concluded that a BMFS provides valuable information about the inflow, which can be used for the control of load alleviating concepts like individual pitch and trailing edge flaps, to investigate the complex relation between the inflow and the power and loads, to characterise the inflow conditions that yield high loads, and as input for aeroelastic simulations to improve the correlation between the measured and simulated loads.
Original languageEnglish
PublisherDTU Wind Energy
Number of pages190
DOIs
Publication statusPublished - 2018

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