Patent Description:
When a rotor blade with a leading edge and a trailing edge is exposed to a fluid, such as air, flowing substantially from the leading edge to the trailing edge of the rotor blade, noise is generally generated at the edges of the rotor blade. The intensity of the noise and the frequency of the noise depends on many parameters, such as the properties of the fluid and the properties of the edge, namely the size of the edge and the shape of the edge, e.g. whether it is rounded or sharp.

The noise being generated at the edges is typically undesired. In the example of a wind turbine which is installed onshore, i.e. at the ground, noise which is generated by the rotating rotor blades can be perceived as a nuisance by people in the vicinity of the wind turbine. For this reason, there exist various legal provisions and restrictions regarding the noise level which is allowed to be produced by such a wind turbine at a certain distance from the wind turbine. As a consequence, wind turbines may either need to be placed further away from residential areas or they have to be operated in such a manner that the maximum allowed level of noise is not exceeded. This means that wind turbines may need to be curtailed in certain conditions because of the flow-induced edge noise.

As this issue is known since several years, various approaches for reducing flow-induced edge noise of a rotor blade have been proposed. These approaches include the shape and the design of the airfoil. In this context, particularly the shape and the design of the trailing edge of the rotor blade is of utmost importance. Aerodynamic add-ons may be added to or included in the rotor blade in order to minimize the flow-induced edge noise of the rotor blade. Well-known add-ons for noise reduction are serrations such as a serrated panel which is mounted to the pressure side or suction side of the rotor blade close to the trailing edge. However, the still existing and the still generated flow-induced edge noise could still be considerable.

<CIT> discloses to a system for suppressing and eliminating noise in wind turbines, formed by microphones disposed over the entire span of the blades and the towers. <CIT> discloses a wind turbine system. Where a noise controller implements a noise mitigation measure in response to the detected rotational position of the rotor blade. <CIT> discloses a controlling method for amplitude modulation of noise generated by wind turbines of wind farm.

<CIT> discloses another system for suppressing and eliminating noise in wind turbines.

Thus, there exists the desire to provide a concept how to further reduce flow-induced edge noise of a rotor blade.

This objective is solved by the present invention which is described in the independent claim. Advantageous embodiments and modifications are expressed in the dependent claims. According to the invention, there is provided a rotor blade with a leading edge and a trailing edge, wherein the rotor blade is designed and configured for being exposed to a fluid flowing substantially from the leading edge to the trailing edge of the rotor blade. The rotor blade comprises a portion which has the shape of an airfoil comprising a pressure side and a suction side, each confined by the trailing edge and the leading edge. The rotor blade further comprises.

In other words, the present invention describes a method how to reduce or even completely eliminate flow-induced edge noise from a rotor blade. This reduction or even elimination of the noise is achieved by the use of anti-noise. Since often times flow-induced edge noise is a broadband noise source, caused by a turbulent flow, the edge noise is random, i.e. stochastic. The acoustic pressure fluctuations are nondeterministic, meaning that they cannot be predicted in a temporal sense on the basis of the current or earlier acoustic signals, even when the statistical properties are known.

However, the unsteady surface pressure pattern, that generates the sound at the edge, can be considered to convect unchanged with the flow along the chord of the rotor blade. Note that in practice this unchanged convection happens until a certain degree. This means that minimum changes of the unsteady surface pressure pattern is possible. This phenomenon is referred to in the literature as the 'frozen turbulence' assumption. In the present invention, this fact is used to detect the unsteady surface pressures upstream of the edge so that a noise cancelling anti-noise signal can be constructed and emitted in anti-phase at the moment when the turbulent eddies, which are responsible for the unsteady surface pressure pattern and noise generation, pass the edge. Therefore, essential components of the inventive arrangement at the rotor blade are at least one sensor for detecting the flow characteristics of the fluid and at least one actuator for producing the anti-noise signal. Both the sensor and the actuator are arranged at the surface of the rotor blade. This means that they are somehow integrated or added to the rotor blade at its surface. In order to prevent disadvantageous aerodynamic effects one option is to submerge and insert the sensor and the actuator into the shell or surface part of the rotor blade such that they are in contact with the surrounding air but they do not stick out and produce additional turbulences on the surface of the rotor blade.

The actuator comprises a membrane. The sensor also comprises a membrane.

In an embodiment of the invention, the rotor blade comprises a portion which has the shape of an airfoil comprising a pressure side and a suction side, and the pressure side and the suction side are each confined by the trailing edge and the leading edge of the rotor blade.

In other words, in a preferred embodiment of the invention, the rotor blade is a lift producing rotor blade which has at least partly the shape of an airfoil. An airfoil is characterized in that it comprises a pressure side and a suction side and is able to produce lift when it is exposed to a fluid flowing substantially from the leading edge to the trailing edge of the rotor blade. As it is well-known to the person skilled in the art, the outer surface of such an airfoil shaped rotor blade is characterized by one portion which is referred to as the pressure side and which is confined at one side by the trailing edge and on the other side by the leading edge and the remaining part of the surface is typically referred to as the suction side of the rotor blade.

The present invention particularly relates to a rotor blade of a wind turbine. However, the inventive concept is not limited to flow-induced edge noise of rotor blades of a wind turbine. It can also be applied to reduce flow-induced edge noise from, for example, aircraft wings, helicopter blades, fans, etc..

In a non claimed example, the actuator comprises a loudspeaker. Such a loudspeaker is a well-known and readily available, inexpensive device which can be integrated or added to a rotor blade at almost any size in a simple and inexpensive manner, without affecting the aerodynamic flow around the blade.

In a non claimed example, the sensor may exemplarily comprise a surface pressure transducer. Such a pressure transducer is also well-known and well-proven and may also be added or implemented and included to a rotor blade without large expenses or changes to the existing rotor blade.

Note that the flow-induced edge noise, which is at least partly cancelled out by the anti-noise signal being produced by the actuator, preferably relates to trailing edge noise of the rotor blade, i.e. to noise which is flow-induced edge noise and which is generated in the trailing edge of the rotor blade. However, in principle, the present invention may be applied to other flow-induced edge noise sources as well, such as, for instance, leading edge in-flow turbulence noise or tip noise of the rotor blade.

In another embodiment of the invention, the sensor is located upstream of the actuator with regard to the flow direction of the fluid.

As it has been described, the rotor blade is designed and configured for being exposed to the fluid flowing substantially from the leading edge to the trailing edge of the rotor blade; therefore a flow direction can be assigned and defined to the exposed and surrounded rotor blade. In order to efficiently monitor and determine the characteristics of the unsteady surface pressure pattern, these are first in a temporal sense detected by the sensor which is located upstream of the actuator and the resulting and produced anti-noise signal can slightly afterwards be emitted or induced by the actuator. In the case of a rotor blade of a wind turbine to which a chord is assigned, this preferred embodiment may also be described that the actuator is closer to the trailing edge regarding a chordwise distance than the sensor.

In a non claimed example, the sensor and the actuator are both located either on the pressure side or on the suction side of the rotor blade.

This includes the possibility that there are arranged two sensors, one on the pressure side and one on the suction side and there are arranged one or two actuators, one on the pressure side and one on the suction side.

In a non claimed example, the sensor is located at the suction side and the actuator is located at the pressure side, or the sensor is located at the pressure side and the actuator is located at the section side.

Note that the sensor and the actuator may be directly connected with each other via connections means, such as a tube or a channel.

This has the advantage of a very simple and robust arrangement. In this case, the distance, in particular the chord wise distance between the actuator and the sensor has to be carefully chosen based on expected unsteady surface pressure pattern and the velocity at which these surface pressure pattern travels towards the edge where the flow-induced edge noise is generated. If this is known or can be foreseen reasonably precisely, then this embodiment is a promising and advantageous way of implementing the inventive concept in a rotor blade.

Alternatively, the rotor blade may further comprise a control unit for constructing the anti-noise signal and the actuator is connected with the sensor via connection means and the control unit.

This has the advantage that a more optimized and tailored anti-noise signal may be produced and that the elimination of the flow induced noise may possibly be achieved in a larger extent. Such a control unit is advantageously located inside the rotor blade, which has the advantage that it does not disturb the air flow which is flowing across the rotor blade and that it is protected from the ambient and the influences of the ambient. Note that the actuator may be arranged and prepared to either induce surface pressure cancellation or to emit a sound signal.

It is also conceivable that the air flow is somehow manipulated, for example by passive or active aerodynamic devices, in order to obtain a certain (fixed) thickness of the trailing edge boundary layer, so that a tuned (passive or active) anti-noise system can be applied. Having a fixed boundary layer thickness makes it easier to design an effective, tuned anti-noise system. Likewise, it is also conceivable that the air flow is somehow manipulated such that a particular (fixed) frequency content of the pressure fluctuations is obtained, so that a tuned (passive or active) anti-noise system can be applied. In other words, the thickness of the boundary layer and/or the frequency of the surface pressure fluctuations could be tuned in order to match a given anti-noise concept. This general principle, of manipulating the air flow to obtain a certain thickness of the boundary layer or a specific frequency content of the pressure fluctuations, can also be applied to other noise reduction concept, such as e.g. trailing edge serrations.

The invention is also related to a wind turbine with at least one rotor blade as described in various embodiments above.

Embodiments of the invention are now described, by way of example only, by the accompanying drawings, of which:.

The illustration and the drawings is in schematic form. It is noted that in different figures, similar or identical elements may be provided with the same reference sign.

<FIG> shows a rotor blade <NUM>. The rotor blade <NUM> comprises a root <NUM> and a tip <NUM>. The rotor blade <NUM> furthermore comprises a leading edge <NUM> and a trailing edge <NUM>. <FIG> illustrates a top view or planar view onto the rotor blade <NUM>, namely onto the pressure side of the rotor blade. The rotor blade <NUM> is characterized by a span <NUM>, which is a straight line projecting away from the root <NUM>. In the case of a rotor blade of a wind turbine, wherein the rotor blade is pitchable about a pitch axis, the pitch axis coincides with the span <NUM> as defined in the context of this patent application.

Furthermore, a plurality of chords <NUM> may be assigned to the rotor blade <NUM>. Each chord <NUM> is oriented perpendicular to the span <NUM>. Therefore, for each spanwise position starting at the root <NUM> and reaching to the tip <NUM>, a chord <NUM> may be defined. The chord <NUM> which is maximum, in other words which has a maximum length or extension, is referred to as the maximum chord <NUM>. The area where the maximum chord <NUM> is located is referred to as the shoulder <NUM> of the rotor blade <NUM>.

<FIG> shows a schematic view of an airfoil of the rotor blade <NUM>. This airfoil or profile is taken perpendicular to the span <NUM> of the rotor blade <NUM>. In other words this profile is a cross-sectional view at a specific radial position or spanwise position of the rotor blade. The leading edge <NUM> can be seen and described as a relatively round edge, whereby the trailing edge <NUM> is relatively sharply designed. The straight line connecting the leading edge <NUM> with the trailing edge <NUM> is referred to as the chord <NUM>.

Note that the whole area from the leading edge <NUM> up to ten per cent of the chord length of the chord <NUM> as measured from the leading edge <NUM> is referred to as the leading edge section <NUM>. Likewise, the area which is within ten per cent chord wise length away from the trailing edge <NUM> is referred to as the trailing edge section <NUM>.

Note that in this schematic view the maximum thickness of the airfoil which is defined as the distance between the pressure side <NUM> of the suction side <NUM> is relatively large. This thickness often times considerably decreases towards the tip <NUM> of the rotor blade, at least in modern rotor blades of wind turbines.

<FIG> show three exemplary embodiments of the invention.

<FIG> shows a trailing edge section <NUM> comprising two sensors <NUM>, one sensor <NUM> being positioned at the suction side <NUM> of the rotor blade, and one sensor <NUM> being positioned at the pressure side <NUM> of the rotor blade. These sensors <NUM> are located upstream with regard to the actuator <NUM> which is arranged and positioned at the suction side <NUM> of the rotor blade <NUM>.

Note that the invention is not limited to the case that the sensors are arranged in the trailing edge section of the rotor blade, i.e. in the area which is within ten per cent chord wise length away from the trailing edge of the rotor blade towards the leading edge of the rotor blade. Moreover, in an alternative embodiment of the invention, the sensor may also be located further upstream, e.g. twenty per cent, or thirty per cent, or even forty per cent chord wise length away from the trailing edge of the rotor blade towards the leading edge.

Both sensors <NUM> are connected with the actuator <NUM> by means of connection means <NUM>. Between the two sensors <NUM> and the actuator <NUM> is arranged and located a control unit <NUM>. The control unit is configured to produce the anti-noise signal based on the input which is received by the sensors <NUM>. The anti-noise signal which is emitted by the actuator <NUM> is symbolized by the arrows <NUM>.

The anti-noise <NUM> is deliberately chosen such that it destructively interferes with the noise <NUM> which is generated and emitted at the trailing edge <NUM> of the rotor blade <NUM>. Note that in the illustration of <FIG>, two main directions of the noise are visualized by the two arrows <NUM>, one projecting away from the trailing edge <NUM> into the direction of the suction side, and one projecting away from the trailing edge <NUM> into the direction of the pressure side. In the exemplary embodiment of <FIG>, the anti-noise <NUM> mainly cancels out or minimizes the noise <NUM> which is emitted and generated at the trailing edge <NUM> into the direction of the suction side <NUM>. If desired a pressure-side actuator could be added to also eliminate the noise emitted to the pressure side. Also note the turbulent boundary layer which is symbolized by suction side eddies <NUM> and pressure side eddies <NUM>. The overall flow direction of the fluid is symbolized by reference sign <NUM>.

It should be noted that, instead of anti-noise, the actuator (for example a membrane) may also produce anti-pressure, thus canceling the fluctuating surface pressures which are the source of the trailing edge noise. By (partly) eliminating the fluctuating surface pressures, noise radiation at the trailing edge is suppressed or completely prevented.

In comparison with the embodiment as illustrated in <FIG>, <FIG> shows another embodiment of the invention wherein the control unit <NUM> is omitted. In contrast, the sensor <NUM> which is arranged at the suction side <NUM> is directly connected via connection means <NUM> with the actuator <NUM>. These connection means may be designed as flexible tubes or channels. The effect, namely the generated anti-pressure which is arranged and prepared to at least partly cancelling out the fluctuating surface pressures on the airfoil surface <NUM> is in principle comparable to the embodiment as illustrated in <FIG>. However, the input for the actuator <NUM> only comes from one sensor at the suction side in the embodiment of <FIG> compared to the embodiment of <FIG> which implies that the arrangement is more simple, i.e. more robust and more inexpensive, but that it may eliminate slightly less of the noise <NUM> being generated at the trailing edge <NUM>.

Claim 1:
Rotor blade (<NUM>) with a leading edge (<NUM>) and a trailing edge (<NUM>), designed and configured for being exposed to a fluid flowing substantially from the leading edge (<NUM>) to the trailing edge (<NUM>) of the rotor blade (<NUM>), which rotor blade (<NUM>) comprises a portion which has the shape of an airfoil comprising a pressure side (<NUM>) and a suction side (<NUM>), each confined by the trailing edge and the leading edge (<NUM>), characterized in that the rotor blade (<NUM>) comprises
- two membranes (<NUM>, <NUM>) arranged at the surface of the rotor blade (<NUM>), one on the pressure side and one on the suction side of the airfoil, and connected by a flexible tube (<NUM>),
- each membrane (<NUM>, <NUM>) acting as a sensor (<NUM>) for detecting flow characteristics of the fluid and as an actuator (<NUM>) for producing an anti-noise signal (<NUM>),
- the membranes (<NUM>, <NUM>) arranged and prepared such that flow-induced edge noise (<NUM>) of the rotor blade (<NUM>), which is generated by the fluid at the trailing edge (<NUM>) of the rotor blade (<NUM>), is at least partly cancelled out by the anti-noise signal (<NUM>) being produced by the membranes (<NUM>, <NUM>).