Abstract:
A rotor blade of a wind turbine including at least one vortex generator is provided. The vortex generator is attached to the surface of the rotor blade and is located at least partially within the boundary layer of the airflow flowing across the rotor blade. The vortex generator is exposed to a stagnation pressure, which is caused by the fraction of the airflow passing over the vortex generator and of which the magnitude depends on the velocity of the fraction of the airflow passing over the vortex generator. The vortex generator is arranged and prepared to change its configuration depending on the magnitude of the stagnation pressure acting on the vortex generator. Furthermore, an aspect relates to a wind turbine for generating electricity with at least one such rotor blade.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to European application No. EP 16178023 having a filing date of Jul. 5, 2016, the entire contents of which are hereby incorporated by reference. 
       FIELD OF TECHNOLOGY 
       [0002]    The following relates to a rotor blade of a wind turbine comprising at least one vortex generator. Furthermore, embodiments of the invention relates to a wind turbine for generating electricity comprising at least one such rotor blade. 
       BACKGROUND 
       [0003]    Vortex generators are well known devices for manipulating the airflow flowing across the surface of a rotor blade of a wind turbine. The function of a vortex generator is to generate vortices downstream of the area where the vortex generator is mounted to the surface of the rotor blade. The generated vortices may re-energize the boundary layer close to the surface of the rotor blade. This re-energization of the boundary layer may delay or prevent stall. The delay or elimination of stall at the section of the rotor blade where the vortex generators are mounted generally increases the lift coefficient of the rotor blade at this section. The increase of the lift is generally desirable. An increase of the lift generally correlates with an increase of the load of the rotor blade. This increase of the load of the rotor blade may be undesirable. 
         [0004]    Therefore, there exists the desire to provide a concept how to selectively activate or deactivate, respectively, a vortex generator for a rotor blade of a wind turbine. 
       SUMMARY 
       [0005]    According to embodiments of the invention there is provided a rotor blade of a wind turbine comprising at least one vortex generator, wherein the vortex generator is attached to the surface of the rotor blade. The vortex generator is located at least partially within the boundary layer of the airflow flowing across the rotor blade. The vortex generator is exposed to a stagnation pressure, which is caused by the fraction of the airflow passing over the vortex generator and of which the magnitude depends on the velocity of the fraction of the airflow passing over the vortex generator. Furthermore, the vortex generator is arranged and prepared to change its configuration depending on the magnitude of the stagnation pressure acting on the vortex generator, such that, with increasing stagnation pressure in the boundary layer, the ability of the vortex generator to generate vortices decreases. 
         [0006]    The boundary layer is the layer of the airflow in the immediate vicinity of the surface of the rotor blade. The boundary layer is also referred to as “surface boundary layer”. 
         [0007]    In the boundary layer the effects of viscosity are significant. The thickness of the boundary layer is defined as the distance of the surface of the rotor blade at which the velocity of the airflow is 99% of the free stream velocity. For the surface of the rotor blade, the confining, i.e. limiting surface of the rotor blade is taken, which is typically referred to as the suction side and the pressure side of the rotor blade, respectively. 
         [0008]    The airflow flowing across the rotor blade is understood to be the typical airflow flowing from the leading edge section to the trailing edge section of the rotor blade. Depending on the particular set up and operation mode of the wind turbine, and depending on the concrete direction of the impinging airflow, the direction of the airflow may vary. However, in general, the airflow flowing across the rotor blade is substantially parallel to the chordwise direction of the rotor blade. 
         [0009]    The expression “the airflow passes over the vortex generator” can in other words be described as “the airflow impinges on the vortex generator”. 
         [0010]    The stagnation pressure, which is sometimes also referred to as the Pitot pressure, is defined as the pressure built up in a fluid which is brought to rest isentropically. For a low velocity flow which can be assumed to be incompressible, the stagnation pressure is equal to the sum of a static pressure and a dynamic pressure. The static pressure is to be understood as being the free stream hydrostatic pressure, which for example for an open flow application such as a wind turbine is equivalent with the ambient pressure. The term dynamic pressure refers to the kinetic energy of a fluid per unit mass, and is therefore dependent on the velocity of the airflow. If a fluid is brought to rest isentropically, all the kinetic energy per unit mass of the fluid is converted into pressure, and therefore the pressure at a stagnation point (a point where the fluid is at rest) equals the sum of the static and the dynamic pressure. The stagnation pressure can be measured using a so-called Pitot tube. 
         [0011]    A key aspect of embodiments of the present invention is that the varying magnitude of the stagnation pressure is used in order to activate or deactivate selectively the vortex generator of the turbine blade. As the static pressure is assumed to be substantially equal during the relevant operation conditions of the wind turbine, it is actually the variation of the dynamic pressure which causes the activation or deactivation of the vortex generator. 
         [0012]    The dynamic pressure may increase or decrease due to the increase or decrease of the thickness of the boundary layer. A thin boundary layer, for example, leads to a high dynamic pressure, while a thick boundary layer involves a small dynamic pressure. Therefore, in other words, the inventive concepts can be described as well by a selective activation or deactivation of the vortex generator depending on the thickness of the boundary layer. 
         [0013]    Advantageously, the activation or deactivation of the vortex generator occurs passively. Thus, an actively driven actuator is not necessary in order to activate the vortex generator. Instead, by the pure increase of the stagnation pressure, a change in the configuration of the vortex generator is caused. 
         [0014]    For this purpose, in a particular embodiment of the invention, the rotor blade comprises a pressure tube extending substantially upstream from the vortex generator for guiding a fraction of the airflow flowing across the rotor blade to an inflatable element. 
         [0015]    Depending on the thickness of the boundary layer, a high velocity airstream or a low velocity airstream flows through the pressure tube and impinges, i.e. hits or enters, the inflatable element. If the dynamic pressure, and consequently also the stagnation pressure, is small, the inflatable element is not inflated or only little inflated. In contrast, for high dynamic and stagnation pressures, a high velocity airflow is flowing through the pressure tube, leading to the inflatable element to be inflated to a significant extent. 
         [0016]    In an alternative embodiment, the varying stagnation pressure is only used as a trigger for triggering an actuator for activating the vortex generator. This actuator may e.g. be electrically or hydraulically driven. As an example, the actuator may inflate or deflate the inflatable element being associated with the vortex generator. 
         [0017]    Note that the ability of the vortex generator to generate vortices decreases with increasing stagnation pressure in the boundary layer. 
         [0018]    In other words, the rotor blade is designed such that a thin boundary layer leads to a high stagnation pressure and to a deactivation of the vortex generator, while a thick boundary layer leads to a small stagnation pressure, resulting in an activation of the vortex generator. 
         [0019]    It is known to place vortex generators almost at any spanwise position of the rotor blade. Therefore, the skilled person has to make a choice where to beneficially place the inventive vortex generator on the rotor blade. It is suggested to place and situate the vortex generator in the outboard half, in particular in the outboard third, of the rotor blade. 
         [0020]    This choice is preferred because here the impact on the lift coefficient, and hence, on the load of the rotor blade is particularly strong. With the notion “outboard”, the area adjacent to the tip of the rotor blade is meant. 
         [0021]    Examples of an inflatable element which is suitable to be used in the context of embodiments of this invention, a hose or a pressure chamber are to be mentioned. 
         [0022]    A hose has the advantage that it can be designed separately from the rest of the rotor blade and may also be exchanged easily after a certain time of operation. 
         [0023]    On the other hand, a pressure chamber, which has to be understood as a cavity running substantially in spanwise direction, is fully integrated in the profile of the rotor blade. No additional components and parts are used, which is advantageous. However, it is difficult to retrofit, for example, a rotor blade by a pressure chamber. 
         [0024]    In another embodiment of the invention, the vortex generator is at least partially embedded into the surface of the rotor blade. 
         [0025]    In this case, a vortex generator is seen as being active or activated when the vortex generator is sticking out from the surface. In other words, it is projecting away from the surface of the rotor blade. In contrast to this scenario, a vortex generator is seen as deactivated leading to a decrease in lift and load, if the vortex generator is entirely or at least partially embedded into the surface of the rotor blade. 
         [0026]    Note that the notion “into the surface of the rotor blade” signifies the presence of a recess or a groove or similar design options. As the remaining surface portions of the general surface of the rotor blade is unchanged, a vortex generator is called to be “embedded into the surface” if it somehow intersects the expected contour of the rotor blade. 
         [0027]    A possibility to selectively embed the vortex generator is that in this case not only the ability to generate vortices is prevented or reduced but also the drag of the rotor blade. This is desirable because a reduced drag normally leads to an increase of the performance of the rotor blade and, hence, of the wind turbine. As mentioned above, it is preferred that the portion of the vortex generator which is embedded into the surface of the rotor blade increases with increasing stagnation pressure. This ultimately leads to a selective deactivation of the vortex generator with increasing stagnation pressure. 
         [0028]    In another embodiment of the invention, the vortex generator is able to bend depending on the value of the stagnation pressure acting on the vortex generator. 
         [0029]    In this embodiment, an inflatable element is not necessarily needed. Instead, by the mere presence and design of the vortex generator the configuration, in particular the shape of the vortex generator can be varied. This may be realized by an elastic portion at the vortex generator. 
         [0030]    Again, in a preferred configuration, at high velocities of the airflow, the vortex generator is bent down towards the surface of the rotor blade and is consequently at least partially deactivated. This has the consequence that the ability to generate vortices is reduced. 
         [0031]    If, on the other hand, the boundary layer thickness is large, thus, the stagnation pressure is small, the vortex generator moves away from the surface. Thus, a greater portion of the vortex generator is sticking out. In other words, the vortex generator projects away from the surface such that the ability of the vortex generator to generate vortices is increased. 
         [0032]    A similar, but slightly different, design is a vortex generator which is able to straighten relative to a direction of the airflow depending on the value of the stagnation pressure acting on the vortex generator. 
         [0033]    In this case, it is preferred that a pair of vortex generators exists at least at the surface of the rotor blade. Then, for example, at low velocity of the airflow (thick boundary layer), a large angle is present between the two vortex generators of the pair, while at high velocity of the airflow (thin boundary layer), the angle between the two vortex generators of the pair is small, even leading to a substantially parallel configuration of the two vortex generators of the pair. 
         [0034]    In practice, this can also be realized by an elastic portion of the vortex generator. 
         [0035]    Finally, embodiments of the present invention also relate to a wind turbine for generating electricity comprising at least one rotor blade according to one of the embodiments described above. 
     
    
     
       BRIEF DESCRIPTION 
         [0036]    Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
           [0037]      FIG. 1  shows a rotor blade of a wind turbine; 
           [0038]      FIG. 2  shows a cross sectional view of the rotor blade at a certain spanwise position; 
           [0039]      FIG. 3  shows a perspective view of a first embodiment of vortex generators; 
           [0040]      FIG. 4  shows a cut-away view of a first embodiment of a thick embodiment of a vortex generators; 
           [0041]      FIG. 5  shows a cut-away view of a first embodiment of a thin embodiment of a vortex generators; 
           [0042]      FIG. 6  shows a side view of a second embodiment of vortex generators having a deflated pressure chamber; 
           [0043]      FIG. 7  shows a side view of a second embodiment of vortex generators having an inflated pressure chamber; 
           [0044]      FIG. 8  shows a third embodiment of vortex generators with a slightly inflated pressure chamber; 
           [0045]      FIG. 9  shows a third embodiment of vortex generators with an inflated pressure chamber; 
           [0046]      FIG. 10  shows a fourth embodiment of vortex generators; 
           [0047]      FIG. 11  shows a fourth embodiment of vortex generators with a thick boundary layer; 
           [0048]      FIG. 12  shows a fourth embodiment of vortex generators with a thin boundary layer; 
           [0049]      FIG. 13  shows a fifth embodiment of vortex generators; and 
           [0050]      FIG. 14  shows a fifth embodiment of vortex generators wherein adjacent vortex generators comprise a significant angle relative to each other. 
       
    
    
       [0051]    Note that the following drawings are only schematically. Similar or identical reference signs are used throughout the drawings. 
       DETAILED DESCRIPTION 
       [0052]      FIG. 1  shows a rotor blade  20  of a wind turbine. The rotor blade  20  comprises a root  21  and a tip  22 . The root  21  and the tip  22  are connected by a virtual line, which is referred to as the span  25 . The span  25  can be described as a virtual line, which is a straight line and which not necessarily exactly connects the root  21  and the tip  22 . This would be the case if the rotor blade was a straight rotor blade. If, however, as for example illustrated in the example of the rotor blade of  FIG. 1 , the rotor blade is a slightly swept rotor blade, the tip may be slightly separate from the span  25 . If the rotor blade is designed for a pitchable wind turbine, the span  25  can be associated and coincides with the pitch axis of the rotor blade. 
         [0053]    Another characteristic feature and parameter of rotor blades of a wind turbine are the chords of the rotor blade. The chords  26 , which are also referred to as the chord lines, can be defined and assigned for every spanwise position from the root to the tip of the rotor blade. The chord  26  is defined as the straight line being perpendicular to the span  25  and connecting the leading edge  23  of the rotor blade  20  with the trailing edge  24  of the rotor blade  20 . 
         [0054]    A particular chord length can be assigned to each chord  26 . The maximum chord  261  is understood to be that chord which has the maximum length. The portion of the rotor blade where the maximum chord  261  is present is referred to as the shoulder  262  of the rotor blade. The part of the rotor blade between the shoulder  262  and the tip  22  is also referred to as the airfoil portion of the rotor blade. On the other hand, the part of the rotor blade between the shoulder  262  and the root  21  is referred to a transition and root region of the rotor blade. 
         [0055]      FIG. 2  shows a cross sectional view at a certain spanwise position of the airfoil portion of the rotor blade. Again, the leading edge  23  and the trailing edge  24  can be seen. Additionally, the trailing edge section  241  and the leading edge section  231  are referenced in  FIG. 2 . The leading edge section  231  is defined as that section surrounding the leading edge  23  reaching from the leading edge  23  to a chordwise position of ten per cent of the chord length as measured from the leading edge  23 . Likewise, the trailing edge section  241  of the rotor blade is defined as that section of the rotor blade which extends between ninety per cent chordwise position as measured from the leading edge  23  until the very trailing edge  24 . 
         [0056]      FIG. 2  also illustrate the airflow  40  flowing from the leading edge section  231  to the trailing edge section  241  of the rotor blade. As can be seen, the airflow  40  is subdivided into a suction side airflow  41  and a pressure side airflow  42 . The separation of the airflow occurs at the stagnation point  29 . Typically, the stagnation point  29  is located at the pressure side  28  of the rotor blade, but may also be located at the suction side  27  of the rotor blade. The exact position of the stagnation point  29  depends on a variety of factors, mainly it depends on the angle of attack and the pitch movement of the rotor blade. 
         [0057]      FIG. 3-5  show a first embodiment of the present invention. In particular, a first embodiment of a vortex generator  30  is disclosed, which can be used and which is a part of a first embodiment of an inventive rotor blade. 
         [0058]      FIG. 3  shows a perspective view of four pairs of such vortex generators  30 . These vortex generators  30  are attached to a housing  35 , which, as a whole, can be attached and mounted onto the surface, e.g. the suction side surface, of the rotor blade. An important feature of the arrangement as illustrated in  FIG. 3  is the pressure tube  31 . The pressure tube  31  consists of a relatively small diameter tube which is arranged upstream of the vortex generator. The arrangement furthermore comprises an inflatable member, namely a hose  32 . This hose  32  is located within the housing  35 . The hose  32  is able to push the vortex generator  30  downwards towards the surface of the rotor blade which is exemplarily referenced by the suction side  27 . In order to facilitate or enable such a bending of the vortex generator, the vortex generator  30  comprises an elastic portion  34 . 
         [0059]      FIG. 4  illustrates the scenario of a thick boundary layer—confer to the shown velocity profile  43  in  FIG. 4 . In contrast to  FIG. 4 ,  FIG. 5  illustrates the scenario of a thin boundary layer—confer to the velocity profile  43  as illustrated in  FIG. 5 . As it can be seen, depending on the thickness of the boundary layer, the hose  32  is inflated or not which leads to an upwardly projecting vortex generator  30  or a vortex generator which is almost in contact with the suction side  27  of the rotor blade. 
         [0060]    Note that in the first embodiment of the invention, the housing  35  is designed as a relatively stiff and rigid element. This means that its shape is substantially independent on the state of the hose  32 . Whether the hose  32  is inflated (as in  FIG. 5 ) or not (as in  FIG. 4 )—the housing has the same cross-sectional profile. As a consequence, the airflow, which is passing over the housing  35  is not influenced by the fact whether the hose  32  is inflated or deflated. 
         [0061]      FIGS. 6 and 7  shows a second embodiment of the invention. Here, inflatable element is exemplarily designed as a pressure chamber  33 . The pressure chamber may be in a deflated state (confer  FIG. 6 )—which is the case for a thick boundary layer, i.e. for a low stagnation pressure—or it may be in an inflated state (confer  FIG. 7 )—which is the case for a thin boundary layer, i.e. for a high stagnation pressure. 
         [0062]    The pressure chamber  33  is accommodated and surrounded by a housing  35 . In this embodiment, the housing is made of a flexible material. As a consequence, and contrary to the first embodiment as illustrated in  FIGS. 3 to 5 , the housing does change its shape depending of the state of the inflatable element. 
         [0063]    Descriptively speaking, the housing  35  represents a “bump” for the airflow passing over it. Note that the airflow, which is passing over the housing  33 , is influenced by the fact whether the pressure chamber  33  is inflated or deflated. 
         [0064]      FIGS. 8 and 9  disclose a third embodiment of a vortex generator. This time, the vortex generator  30  is partially embedded into the surface, e.g. into the suction side  27  of the rotor blade. In other words, the rotor blade is provided with a recess or groove at its suctions side  27 . In this groove, a device or arrangement comprising a pressure chamber  33  can b seen. This pressure chamber is connected with a pressure tube  31 . Depending on the stagnation pressure which is guided through the pressure tube  31 , the pressure chamber  33  is either inflated (confer  FIG. 9 ) or it is not or only slightly inflated (confer  FIG. 8 ). As a consequence, the vortex generator  30  is either submerged and embedded into the surface of the rotor blade (confer  FIG. 9 ) or it projects away and sticks out of the surface (confer  FIG. 8 ). 
         [0065]    This third embodiment has the advantage that additional drag from the attachment portion as shown in the first embodiment as illustrated in  FIGS. 3-5 , is avoided. Thus, additional drag from the attachment portion, but also from the vortex generator as such, is reduced. 
         [0066]      FIGS. 10, 11 and 12  disclose a fourth embodiment of an inventive vortex generator. This time no inflatable element, such as a pressure chamber or a hose, is used. Instead, it is the direct and sole design and configuration of the vortex generator  30  which leads to a changing configuration of the vortex generator in dependence of the velocity profile  43 . See  FIG. 11  for these scenarios of a thick boundary layer. As a consequence, the stagnation pressure at the position of the vortex generator  30  is small, thus the vortex generator which comprises an elastic portion  34  and which is bent upwards, i.e. away from the surface of the rotor blade, is projecting away and is able to generate vortices to a considerable extent. In contrast to that,  FIG. 12  shows these scenarios of a thin boundary layer which can be seen by the velocity profile  43  leading to the bending down of the vortex generator  30  towards the suction side surface of the rotor blade. In the configuration as illustrated in  FIG. 12  the ability of generating vortices by the vortex generator is heavily reduced. 
         [0067]    Finally, the  FIGS. 13 and 14  disclose a fifth embodiment of the invention. Similar to the fourth embodiment, no inflatable element or the like is present. Instead, the vortex generators themselves again comprise an elastic portion  34 . This elastic portion  34  is designed such that for a thin boundary layer, as illustrated in  FIG. 13 , the vortex generators  30  are almost parallel to each other. They can also be described as being straightened by the high stagnation pressure of the airflow impinging on the vortex generator. In contrast to  FIG. 13 ,  FIG. 14  shows this scenario of a thick boundary layer, wherein the relatively small stagnation pressure is not able to overcome the pre-bent of the elastic portion  34  of the vortex generators  30 . Thus, the adjacent vortex generators comprise a significant angle relative to each other. In this case, the ability to generate vortices is increased, compared to the straightened scenario as illustrated in  FIG. 13 . 
         [0068]    Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. 
         [0069]    For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.