Wind tunnel experiments on a rotor model in yaw

Transcription

1 Nord-Jan ermeer Faculty of Civil Engineering and Geosciences Stevinweg 8 CN Delft The Netherlands Tel: + 78 Fax: n.vermeer@ct.tudelft.nl The has performed experiments in the open-jet wind tunnel with the aerodynamic rotor model in yaw, as a subcontractor of the ECN project An Engineering Model for Yawed Conditions (see reference). This project was sponsored by NOEM. Wind tunnel experiments have been done on both wake flow distribution close behind the rotor plane and rotor loads. The purpose of the measurements is to provide experimental data for the validation of an engineering model for yawed rotor conditions. The adaptation of the model consists of the inclusion of higher harmonics of the azimuthal distribution of the axial flow. Only the flow field measurements will be covered in this paper. Equipment As usual the aerodynamic rotor model has been used in the open-jet wind tunnel. The tunnel has a diameter of.m, with a maximum speed of m/s. The rotor model has a diameter of.m, with two blades with NACA aerofoil, the chord is.8m. There is a twist from % to 9% rotor radius, the outer % has no twist and is removable. A traversing system is used to set and change the position of the hot-wire probe. It is controlled by the measurement PC. With this traversing system only half the rotor plane can be covered. Because of this, it is necessary to move and re-adjust the traversing system. This is further discussed later. Instrumentation The tunnel velocity is measured at the exit of the tunnel by a Pitot tube, connected to an electronic pressure sensor. The flow field is been measured with hot-wires. The hot-wires are kept at constant temperature in a bridge circuit. The bridge voltage is a measure for the cooling-down of the wire and therefor a measure for the velocity. One important thing to understand is that the wire doe not sense the direction of the velocity, it only senses cooling-down. So, the wire also cools down by a flow parallel to the wire, but not as effective as with a perpendicular flow. This is taken into account by calibration over different inflow angles of the wire. Experiments To measure three components of the flow, at least three hot-wire orientations are needed. At first, this was attempted with a X-wire probe (a probe with two perpendicular wires), but during angle calibration, it appeared that the hind wire is strongly influenced by the front wire. So, it is decided to use only one wire, in three different orientations. The angle calibration is done using the effective velocity method: and. is the velocity perpendicular to the wire and parallel to the probe axis, is the velocity parallel to the wire and is the velocity perpendicular to both the wire and the probe axis (see figure). The coefficients k and k are the corresponding calibration constants.

2 So, with the hot-wire probe placed behind the rotor plane in three orientations, the three effective velocities can be specified by the following equation set: The subscripts vw, hw en aw denote respectively the vertical, horizontal and axial hot-wire orientation and the subscripts ax, lat en vert denote respectively the axial, lateral and vertical velocity component. In matrix form these equations become: During the data reduction the matrix with k-coefficients is being converted, to calculate the separate components from the measured velocities. With the coefficients k en k from the angle calibration (resp.. en., see sheets), this matrix gets the following content: The determinant of this matrix is -.79, so inverting is not a problem. The inverse matrix looks as follows: From the values of the inverse matrix, it can be seen that the vertical component can be reduced with least accuracy: the horizontal and vertical wire will measure the highest effective velocity, because they are most sensitive for the axial velocity component. In the matrix, these two have an almost equal factor, but with an opposite sign, giving a result with a low value. It is even possible to obtain a negative value for the square of the vertical component, which is of course physically impossible. Because in this project, the axial component was main interest, not much attention has been payed to this matter. Acquiring data for the axial component is most important and this can be done with highest accuracy. Conditions and configuration The settings of the rotor are chosen near the optimum power: tip-speed ratio =8, tip pitch angle tip =, operated at a rotational speed of.hz, this results in a Reynolds number around, and a tunnel velocity of =.m/s. The measuring plane is parallel with the rotor plane at.m downstream. Yaw angles are,, and, with yaw as a control for tunnel uniformity. Measuring positions are at %, %, 7% and 8% of the rotor radius and at every probe azimuth angle (not to confuse with blade azimuth angle).

3 Data acquisition For each probe position and wire orientation, samples are taken every rotor azimuth angle over revolutions. Also, the temperature, air pressure and pressure from the Pitot tube are taken. During the experiments, the tunnel velocity is kept as constant as possible, but due to warming up of the tunnel drive, the velocity increases a little. This is compensated for by measuring the tunnel velocity during the measurements and processing this in the data reduction. Data reduction Because the measured wake velocity changes both in magnitude and in direction during revolution and because the effective velocity is determined by the squares of the components, it would be incorrect to average all samples first and then calculate the components. First the components have to be determined, only then the average can be taken. For each measuring position of the hot-wire probe, 8 samples are taken ( revolutions of 8 samples), for all three wire orientations. First, a azimuthal average has been taken: all values for each azimuth angle of the rotor are averaged, leaving 8 samples. Next, for each azimuth angle, the three effective velocities (the horizontal, vertical and axial orientation of the wire) are reduced to three components (the axial, lateral and vertical velocity component) according to the matrix from the angle calibration: The root of the obtained squared velocity components can be given a sign, because there is knowledge of the direction of the components. Finally, for each component, the 8 values are averaged to one: for each hot-wire position, there are now three velocity components. To take account of the possible changes in tunnel speed, all components are divided by the tunnel speed during its measurement and multiplied by.m/s. The edges of the two halves ( en 8 ) are measured twice, to have a control over the movement and adjustment of the traversing system. Therefor, the plots are drawn with an azimuth angle from to, with the part from to equal to to 9. Results The final results for all yaw angles are given in the accompanying sheets. The control measurement at yaw, shows that there is a slight variation in the uniformity of the tunnel. However, these measurements demonstrate the validity of the data reduction process, because the edges of the two halves show fairly continuous curves at 8 and. Although there have been numerous difficulties during the experiments, the velocity measurements have been finalised fairly successful, and the following conclusions can be drawn: S measuring with a hot-wire in three orientations and reducing velocity components goes well for the axial and lateral component, because there is enough knowledge of the direction. S with the equipment and set-up used, the vertical component can not be obtained with enough accuracy. S the limitation of the traverse system to cover only half the rotor plane, has not negatively influenced the results: the switches from one side to the other ( en 8 probe azimuth angle) are hardly noticeable in the axial and lateral component. S the changes in velocity over a rotor revolution, as experienced by a hot-wire at a fixed position, leave enough matter for further investigations. Reference. Schepers, J.G., An engineering model for yawed conditions, developed on basis of wind tunnel measurements, th IEA Symposium on Rotor Aerodynamics, Lyngby, 998.

4 Wind tunnel experiments on a rotor model in yaw Nord-Jan ermeer th IEA symposium and December 998

5 Introduction TUDelft/IvW subcontractor for ECN project: An Engineering model for Yawed Conditions Project is sponsored by NOEM TUDelft/IvW part: wind tunnel measurements of wake flow distribution and rotor loads ECN will give overview of project and interpretation of results (see other presentation)

6 Equipment Open-jet wind tunnel, aerodynamic model and traversing system

7 Equipment (cnt d) Definitions and conventions Ω c=.8m φ y Hot-wire probe vert B ax lat R=. m t u n n e l A φ r,b TOPIEW FRONTIEW

8 Instrumentation Wake flow distribution by hot-wire measurements: hot-wires work by means of cooling down, so also parallel velocity is measured angle calibration of hot-wire by effective velocity approach: eff eff = = + k + k Effective velocity (m/s) k =. Effective velocity (m/s) k = Hot-wire yaw angle ( ) Hot-wire pitch angle ( )

9 Instrumentation (cnt d) To measure three components, at least three wires are required X-wire is useless, because front wire influences hind wire One wire is used in orientations vw hw aw = = = k ax ax + k + k ax + lat lat lat + k + k + k vert vert vert

10 Experiments Configuration and conditions: tip pitch angle tip-speed ratio 8 tunnel speed.m/s yaw angles,, and Data acquisition: measuring plane.m behind rotor plane measuring positions,, 7 and 8% radius every probe azimuth angle three wire orientations: vertical, horizontal and axial for each probe position and wire orientation: samples at every rotor azimuth for revolutions

11 Three orientations of wire: vertical, horizontal and axial transformed to three components Experiments (cnt d) ax lat vert = vw hw aw Effective velocity (m/s) Probe orientation: vertical horizontal axial Rotor azimuth angle ( ) elocity (m/s) Rotor azimuth angle ( ) Component: axial lateral vertical

12 Data reduction Data reduction : for each probe position: three wire orientations, with 8 samples ( rev. every ) azimuthal averaging to 8 samples ( rev. ) for each azimuth sample: transfer of effective velocities to velocity components for each velocity component averaging over revolution.

13 Results Axial velocity (m/s) Yaw angle: Radial position: % % 7% 8% Axial velocity (m/s) Yaw angle: Radial position: % % 7% 8% Hot-wire azimuth angle ( ) Hot-wire azimuth angle ( ) Axial velocity (m/s) Yaw angle: Radial position: % % 7% 8% Axial velocity (m/s) Yaw angle: Radial position: % % 7% 8% Hot-wire azimuth angle ( ) Hot-wire azimuth angle ( )

14 Results (ctn d) Lateral velocity (m/s) Yaw angle: Radial position: % % 7% 8% Lateral velocity (m/s) Radial position: % % 7% 8% Yaw angle: Hot-wire azimuth angle ( ) Hot-wire azimuth angle ( ) Lateral velocity (m/s) Yaw angle: Radial position: % % 7% 8% Lateral velocity (m/s) Yaw angle: Radial position: % % 7% 8% Hot-wire azimuth angle ( ) Hot-wire azimuth angle ( )

15 Yaw angle, probe at 8% radius, 9 azimuth Examples Blade passage ortex sheet

16 Examples (cnt d) Yaw angle, probe at 8% radius, at, 9, 8 and 7

17 Conclusions Conclusions: measuring with a hot-wire in three orientations turns out alright for axial and lateral component with current set-up, the vertical component can not be obtained with enough accuracy limitation of the traversing system to cover half the rotor plane, has no negative influence on the results velocity changes over a rotor revolution at a fixed position are not yet fully understood