GB2477509A

GB2477509A - A vertical axis turbine foil structure with surface fluid transfer openings

Info

Publication number: GB2477509A
Application number: GB1001749A
Authority: GB
Inventors: Neil William Harvey
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2010-02-04
Filing date: 2010-02-04
Publication date: 2011-08-10
Also published as: GB201001749D0

Abstract

A vertical axis turbine foil structure 2 comprises a first opening 10 on a first foil surface 12 and a second opening 20 on a second foil surface 22. The openings are arranged to transfer fluid to or from the respective surfaces to suppres separation and stall to improve efficiency. There may be a passage 30 with a length to width ratio of 5 between the openings such that they are in fluid communication. The foil may be hollow and there may be a chamber between the openings. Fluid transfer may be passive i.e. driven by pressure differences between the surfaces or active by e.g. pump blowing or sucking. The openings may be holes, slots or a mixture thereof and located at between 15 and 55% chord e.g. approximately 40% chord. The turbine may be a wind or marine turbine e.g. VAWT and so the foils may be aerofoils or hydrofoils.

Description

A VERTICAL AXIS TURBINE AEROFOIL STRUCTURE

This invention relates to a vertical axis turbine aerofoil structure and particularly but not exclusively relates to an aerofoil structure for a vertical axis wind turbine.

A conventional vertical axis wind turbine (VAWT) is disclosed in US1835108 (Darrieus) and is shown in Figure 1. Other VAWTs are disclosed in, US1592417, U54087202, US4236866, US4264279, US4483657, US4561826, US4718821, US5405246 and GB2415750. A more recent, commercial VAWT is disclosed in reference 3 and is shown in isometric view in Figure 2. Both VAWTs of Figures 1 and 2 are "three-bladed", although that of Figure 2 has a more complex aerodynamic shape in part to reduce the noise from it perceived by the human ear (as it is intended for urban applications).

Comparison of a VAWT against other turbines The power output of a wind turbine rotor changes with its rotational speed.

As a result, the rotor performance is normally presented non-dimensionally as a power coefficient versus tip to wind speed ratio graph. The power coefficient Cp is defined by, c=P% / pU3A and the tip to wind speed ratio, tsr, is defined by, tsr where, P = Rotor power; p = Density of air; U = Incoming wind speed; A = Cross-sectional area of the wind turbine presented to the incoming wind; = Rotational speed of the aerofoil about the central (radial) axis; and RM = Maximum radius of aerofoil from the central axis.

An example of such a graph along with curves for other devices that extract energy from wind or water flows is shown in Figure 3. There is a well known theoretical limit to the power that such a device can extract from a fluid flow, and this is known as the Betz limit. The Betz limit corresponds to a value of Cp of about 59% and this is also shown in Figure 3.

The flatter curve in Figure 3 from the propeller type rotor (e.g. of a horizontal axis wind turbine) indicates that the rotor is able to maintain high efficiency over a large range of rotor rotational speed. By contrast, the "peaky" curve for the Darrieus rotor shows that it experiences a sharp drop in the power coefficient as it moves away from its optimum value of tsr. In addition, the Darrieus rotor typically has a low power coefficient at low values of tsr which indicates a weak self-starting ability. Typically they have to be spun up to higher speeds to begin working.

Fundamental equations for a VAWT The basic means of operation of such a VAWT is illustrated by reference to Figure 4 which shows a horizontal cross-section through one aerofoil section of such a machine, in a number of different angular positions. Figure 4 also shows the key elements of the "velocity triangle" (see reference 1) for the aerofoil section at the different angular positions. The lift and drag forces on the aerofoil are also shown. In this Figure the rotation is counter clockwise and the parameters shown can be defined as follows: = Rotational speed of the aerofoil about the central (radial) axis; R = Radius of aerofoil from the central axis; = Velocity of blade due to rotation; U' = Wind speed directly in front of the rotor (generally vertically down the page); U = Flow velocity relative to the aerofoil section (in its frame of reference); a = Angle of attack, or incidence, of the relative flow onto the aerofoil leading edge as measured from the direction of motion; and e = Angular position of the aerofoil, 00 being the most upstream point on its path.

The instantaneous power output from an aerofoil is given by, P' FTXRX@Xh where FT = FLsina-FDcosa and, h = Radial height of the turbine; FT = Tangential force on the aerofoil; FL = Lift force on the aerofoil; and FD = Drag force on the aerofoil.

The instantaneous power is integrated around one revolution of the turbine to give the net power output P. Clearly for the turbine to work the net tangential force on the aerofoil should be positive. To maximise the force, and thus the power, the following are then desirable: * Maximise the lift force FL. While the aerofoil behaves as a streamlined body this is achieved by increasing the angle of attack a. However, this only works up to a critical value of afor reasons explained later.

* Minimise the drag force F0.

* Have a high angle of attack generally since this increases the component of the lift force that is resolved into the tangential direction and reduces the component of the drag force so resolved.

In actual operation, the wind speed U' local to the aerofoil rotor will be different, (typically less) than that far upstream, U. However, the reasons for the "peaky" power curve of the Darrieus rotor can be explained by reference to a simplified analysis, which ignores this slowing down of the wind speed as it passes through the turbine (and has some of its power extracted from it). We also assume the aerofoils are all at a constant radius from the central axis.

Some simple identities can then be derived: UWwR = \I(sin 9-1/tsr)2 + cos2 9 = relative wind speed ratio; and (sinG -(1/tsr) a 9-tan cosG The variation of the angle of attack with angular position and with varying tsr are plotted in Figures 5a and 5b. There a number of observations that can be made from these plots and these are as follows: * The angle of attack varies hugely, in fact over the range ± 180°. When the angle is greater than ± 900, this means that the aerofoil has a "tail wind" and the flow direction has a component going with the forward motion of the aerofoil. In this case the drag will effectively be pulling the aerofoil along rather than opposing its motion (as it would do in normal operation). At 0° angle of attack the flow is directly in line with the direction of motion, it is coming "straight at" the aerofoil. At ±90° angle of attack the aerofoil is "broadside on" to the flow, it is simply perpendicular to the direction of motion of the aerofoil.

* Typically, a commercial Darrieus type wind turbine (as shown in Figure 2) is operated such that the optimum value of tsr is over 4 in order to coincide with the peak power coefficient shown in Figure 3. It can be seen from Figure 5 that at such speeds the angle of attack reduces to less than ± 14° over its cycle.

Aerofoil shapes The aerofoil shapes used in VAWTs are typically chosen from a well known series such as the NACA series of aerofoils. For example, the Darrieus turbine disclosed in GB2415750 and reference 3 uses a NACAOO12 profile.

(See also references 4 and 5.) The important feature of such profiles is that they have low drag, which generally reduces the power output from an aerofoil, provided the flow over it remains "attached" and it operates as a streamlined body.

It is well known for such aerofoils that above a critical value of the angle of attack the flow will separate from one of the surfaces of the aerofoil.

Streamlined, attached flow round an aerofoil and the effects of separation are illustrated in Figures 6a and 6b respectively. For low drag profiles, such as those in the NACA series, the critical incidence is around ±12°. Once separation has occurred the lift is very much reduced while the drag significantly increases. Under these conditions there is no net positive (forward) force on the aerofoil and it typically ceases to provide power to the turbine. Of course the angle of attack varies with the angular position of the aerofoil. Thus at some values of tsr the aerofoil can experience positive and negative tangential net forces as it goes through one cycle of rotation.

Explanation of conventional VAWT performance Having conducted the analysis above, the shape of the power curve for the Darrieus turbine shown in Figure 3 can now be better understood as follows: * At the optimum value of tsr, slightly over 4, the aerofoil will always be within the critical angle of attack around the whole of its rotational cycle and does not suffer flow separation. That is, it experiences the maximum values of sub-critical angle of attack. From the previous definitions of P and FT the value of the power coefficient (integrated around one cycle of revolution) will then be at a maximum.

* At higher values of tsr, the angle of attack will reduce and the contribution of the lift force to the net tangential force will reduce while that due to the drag force relatively increases. Eventually, as the angle of attack tends to zero (at very high tsr), the aerofoil experiences only a drag force (tangentially) and the power output drops to zero. (It should be noted, however, that this is only a partial explanation of the shape of the curve at high values of tsr, because the curve additionally falls away due to the change in local wind speed as power is extracted from the wind flowing through the VAWT.) * As values of tsr fall below 4, the aerofoil will experience angles of attack greater than the critical value. For some parts of its revolution it will experience flow separation. During these periods the power output will significantly drop or even become negative. The integrated power output over a whole cycle may still be positive but is much reduced. Thus, the power curve in Figure 3 falls away sharply for values of tsr from 4 to 2.

* For values of tsr below 1 the aerofoil experiences angles of attack above 90°. In this case the flow onto the aerofoil is acting like a "tail wind", effectively coming from behind it. The behaviour is then complex and it tends to act like a "drag device". In these cases it may be possible for the wind to turn the turbine and start it. However, power levels are always low and frictional and other dissipative effects may prevent it from working. For this reason Darrieus wind turbines in commercial operation have systems that rotate them up to near the optimum value of tsr before letting them operate "under their own power".

It is further shown in Figure 3 that the Darrieus type wind turbine has a maximum power coefficient that is well below the Betz limit and, as already mentioned, the power vs. tsr curve is very "peaky". A horizontal axis wind turbine (HAWT; "propeller" type) has a much flatter characteristic except at very low tsr. Conversely, the VAWT effectively responds instantaneously to changes in wind direction, which is always normal to the axis of rotation of the VAWT. In contrast, an HAWT does not respond very quickly. Larger HAWT machines have to be turned into the wind, having to operate at reduced power until they are facing the wind (with the blades possibly partially stalled). Smaller machines can be self correcting, with some sort of tail fin device, but the motion may still be erratic, again with loss of power. VAWTs may thus be much better suited to the urban environment where there are considerable wind gusts giving large variations in the local wind flow direction. It has also been shown that VAWTs, such as in reference 3, are very quiet and this is ideal (in fact commercially vital) for the urban environment. The low noise is due to the aerofoils not experiencing stall (flow separation off the aerofoils), which is what happens to an HAWT when the wind direction suddenly changes.

However, a drawback for the VAWT is the response of the aerofoils not to sudden changes in wind direction, but in speed. This may be a problem in the urban environment where the wind gusts that suddenly change wind direction may exhibit corresponding large changes in wind speed.

Looking again at Figure 3, one would ideally want to operate the machine at a value of tsr which achieved something near the maximum value of power coefficient. However, a sudden increase in the wind speed would drop the tsr causing the aerofoils to (at least partially) stall. The result would be a significant drop in power and increase in noise. In the worst case the machine might lose power entirely and then have to be spun up to the required operating speed again. This operation would be highly erratic and the net power coefficient (averaged over time) would be significantly less than if the machine operated in steady flow conditions (at a comparable average tsr).

An apparently safer operating point would be to run at a value of tsr well above the maximum power coefficient point. However, the power coefficient will be well below the maximum and any sudden droi in wind speed, due to wind gusts, will result in tsr increasing and the power falling further.

One solution is to make use of an aerofoil shape which produces a less "peaky" power coefficient vs. tsr curve and thus is less sensitive to wind gusts.

However, this typically lowers the maximum achievable power coefficient.

To resolve this problem, the commercial VAWTs disclosed in GB2415750 and reference 3 have a sophisticated (and expensive) feedback and control system which monitors the incoming wind speed and adjusts the power extraction for near optimum performance (see reference 4). As a result they are able to use a NACA-0012 aerofoil with a very peaky characteristic instead of an aerofoil with a smoother characteristic (for example NACA-0018), which would be more likely to be used in previous VAWTs for the reasons noted above. The Cp vs. tsr characteristics for the NACA-001 2 and NACA-001 8 are compared in Figure 7.

As already mentioned, the poor performance of the VAWT at low values of tsr (i.e. typically below 2, but for the aerofoils shown in Figure 7 values of tsr below 3.5), means the machine has to be spun up using a separate power source until it is able to run under its own power. Means are therefore needed to improve the C vs. tsr characteristic of a VAWT for values of tsr below 3.5.

Such means would: * Reduce the power needed for initial run up (by reducing the value of tsr it has to be run up to); * Make the power coefficient curve less peaky without compromising the maximum value achievable; * Reduce the complexity, and thus cost, of the control system; and * Allow the machine to run at higher absolute maximum wind speeds, and thus achieve higher power output over the lifecycle of the machine.

This last point is explained as follows. It must be remembered that the parameter tsr is non-dimensional and is defined as the speed of the aerofoils normalised by the wind speed. Now, the VAWT will have a maximum safe rotational speed, which is limited by the maximum stresses the aerofoils in the VAWT can sustain. This will then prescribe a maximum wind speed the VAWT can safely operate in, which may be calculated as follows: Maximum wind speed = Maximum safe blade speed I minimum operating tsrfor the VAWT.

Thus, trying to run at higher wind speeds requires either the blades to run beyond their maximum allowable speeds (and thus stresses) or the tsr must be so low that the power drops to zero, in which case there is no point running the machine. However, if the VAWT can run usefully at low values of tsr then the VAWT can continue running in higher wind speeds. Again, the usefulness of this may not be readily apparent because we have been using non-dimensional parameters. For instance, say that in Figure 7 the same peak value of Cp could be gained at a tsr of 2 instead of 4, and that the reduction in tsrwas just due to increased wind speed (the blade speed being unchanged). This would mean the wind speed U had gone up by a factor of 2. However, since Cp is a function of the U3 then the absolute power would have gone up by a factor of 8.

Obviously this would only apply at high wind speeds, but clearly enabling a VAWT to continue to run at higher wind speeds than previously possible (because aerodynamic stall had been avoided), makes useful increases in total power achievable.

Reynolds number effects All the Cp curves shown (in Figures 3 and 7) have been for aerofoils at relatively high values of Reynolds numbers. This parameter is very well known in aerodynamics and is defined as follows:

R _PURC /1

where, Re = Reynolds number based on true chord; = Fluid density; = Flow velocity relative to the aerofoil section (in its frame of reference); = Fluid viscosity; and c = Characteristic length, typically the aerofoil chord.

The phrase "relatively high" typically means over about 500,000.

However, for smaller machines operating in the urban environment values of 100,000 or lower may easily be experienced. Typically, as the Reynolds number decreases, the aerofoil may become more prone to stall, reducing the angle of attack at which it occurs. The effect on curves such as those in Figures 3 and 7 is to generally lower the levels of Cp achieved and/or push up the minimum value of tsrfor useful operation.

Pressure distributions In addition to the above, it is useful to illustrate the typical surface static pressure distributions around the aerofoils in a VAWT at key points of operation.

Figure 8 shows the variation of pressure distribution with angle of attack a of a wing section with 12% thickness / chord (the same as a NACA-0012 profile) and 2% camber. This is not the exactly the same as the profile used in a VAWT since this typically has zero camber, due to it experiencing positive and negative angles of attack equally during its revolution. However, Figure 8 still usefully illustrates the pressure distributions, even if they are not exact. The pressure axis shown in Figure 8 is expressed non-dimensionally as a static pressure coefficient, c, which is defined by, p-pin cp= p0 -pin where, = Relative stagnation pressure around the aerofoil; p = Local surface static pressure; and Pm = Static pressure of the relative inlet wind speed.

Figure 8 shows that the inlet flow stagnates on the "lower" surface, which then has the higher static pressures. Most of the lift comes then from the "upper" surface on the other side of the aerofoil. For an aerofoil in a VAWT experiencing angles of attack oscillating between positive and negative, its two surfaces will correspondingly oscillate between being "upper" and "lower" in these definitions.

It can further be seen from Figure 8 that as the angle of attack, a, increases the "suction" increases over the upper surface, particularly towards the leading edge, and the lift (which is a function of the area between the two curves for the upper and lower surfaces) increases with it. At a sufficiently high (critical) angle of attack, the adverse pressure gradient following the peak suction approaches the value for which boundary layer separation develops.

With a further increase in the angle of attack, flow separation rapidly spreads over the upper surface. As a consequence, the peak suction falls and the pressure becomes almost constant over the region of separated flow, whilst the trailing edge pressure and the lift fall. The aerofoil is then said to be stalled. For the wing section in Figure 8, the critical value of angle attack is just over 12°, similar to the NACA aerofoil sections typically used in VAWT5 (as already noted).

Figure 9 shows a similar plot to Figure 8. However, Figure 9 is for an aerofoil near its critical angle of attack (nominally of 10.7°) and shows two different Reynolds numbers: 0.4 x i05 (curve (a)) and 4.2 x i05 (curve(b)). At the lower Reynolds number the boundary layer undergoes laminar separation almost immediately and achieves low lift and high drag. At the higher Reynolds number the separation point ("S") occurs later on the "upper" surface, where the boundary layer undergoes turbulent separation (having undergone transition at point "T"). In fact, at this Reynolds number the aerofoil can operate at a higher angle of attack before becoming truly stalled.

References The following references are referred to in this specification: 1) Duncan W J, Thom AS, Young A D. "Mechanics of Fluids". Edward Arnold (Printers) Ltd., 1970.

2) Schlichting H. "Boundary Layer Theory". McGraw-Hill Book Company, 1979.

3) quietrevolution. "Vertical axis wind turbine specification".

http:!twwwquetrevolution.co.uk/about smaH wind 4) McIntosh S C, Babinsky H, Bertenyi T, (2007), "Optimizing the Energy Output of Vertical Axis Wind Turbines for Fluctuating Wind Conditions", Paper AIAA 2007-1368.

5) Sheldahl R E, Klimas P C, (1980), "Aerodynamic Characteristics of Seven Symmetrical Aerofoil Sections Through 180-Degree Angle of Attack for Use in Aerodynamic Analysis of Vertical Axis Wind Turbines". Sandia National Laboratories Energy Report. SAND8O-21 14.

6) FloWind Corporation, (1996), "Final Report: High-Energy Rotor Development, Test and Evaluation", SAND96-2205.

According to a first aspect of the present invention there is provided a vertical axis turbine aerofoil structure comprising a first opening on a first surface of the aerofoil structure and a second opening on a second surface of the aerofoil structure, the first and second openings being arranged to selectively transfer fluid to or from the first and/or second surfaces of the aerofoil structure. For example, by expelling, blowing, sucking or removing fluid from the surfaces of the aerofoil structure, flow stall may be delayed and the performance of the vertical axis turbine improved.

The first and second openings may concurrently expel or remove fluid from both the first and second surfaces of the aerofoil structure. Alternatively, the first and second openings may alternate between expelling or removing fluid from the first and second surfaces of the aerofoil structure. For example, the first opening may expel or remove fluid from the first surface when the first surface is the suction surface. The second opening may or may not expel or remove fluid from the second surface when the first surface is the suction surface. Likewise, the second opening may expel or remove fluid from the second surface when the second surface is the suction surface. The first opening may or may not expel or remove fluid from the first surface when the second surface is the suction surface. The first and/or second openings may remove or expel fluid by virtue of a fluid suction or pressure means, for example a pump.

The first and second surfaces may be opposite faces of the aerofoil structure. The first and second openings may be arranged to be in fluid communication with one another. The vertical axis turbine aerofoil structure may further comprise a passage between the first and second openings. The first opening may expel fluid whilst the second opening removes fluid and vice versa. For example, when the first surface is the suction surface, fluid may be removed from the second surface through the second opening and passed through the passage to be expelled to the first surface through the first opening.

Similarly, when the second surface is the suction surface, fluid may be removed from the first surface through the first opening and passed through the passage to be expelled to the second surface through the second opening. Accordingly, the use of a pump to expel or remove the fluid may be obviated.

The vertical axis turbine aerofoil structure may further comprise a chamber provided between the first and second openings.

A portion of the passage leading to the first or second opening may comprise a length to width ratio greater than or equal to approximately 5.

One or more of the first and second openings may be arranged to be downstream of a boundary layer transition point. The first or second opening may be located at approximately 40% chord from the leading edge. The first or second opening may be located at approximately 55% chord from the leading edge. The first or second opening may be located at approximately less than 15% chord from the leading edge.

The first and/or second openings may be disposed such that a flow leaving said first and/or second openings may leave at an angle of less than substantially 200 with respect to the local surface of the aerofoil structure. The first and/or second openings may be disposed such that a flow leaving said first and/or second openings may leave substantially tangentially with respect to the local surface of the aerofoil structure.

The first and/or second openings comprise one or more spanwise slots provided along at least a portion of the span of the aerofoil structure.

A vertical axis wind turbine may comprise the aerofoil structure described above. A vertical axis marine turbine, for example a water flow turbine, may comprise the aerofoil structure described above. Accordingly, the fluid may be air or water.

According to a second aspect of the present invention there is provided a method of controlling the flow on a vertical axis turbine aerofoil structure, the method comprising: providing the vertical axis turbine aerofoil structure with a first opening on a first surface of the aerofoil structure and a second opening on a second surface of the aerofoil structure; and transferring fluid from one or more of the first and second surfaces of the aerofoil structure through the first and second openings.

The method may further comprise permitting flow between the first and second openings. The method may further comprise permitting the flow direction to oscillate between the first and second openings as the turbine rotates about its axis.

The method may further comprise delaying flow stall on one of the first and second surfaces of the aerofoil structure.

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:-Figure 1 is taken from US1835108 and shows a Darrieus turbine in isometric view; Figure 2 shows an isometric view of the VAWT of GB241 5750; Figure 3 is a graph of power coefficient vs. tip to wind speed ratio for different wind and water turbines; Figure 4 is a horizontal (plan) view through an aerofoil of a VAWT; Figure 5a shows the variation of aerofoil angle of attack with angular position about the axis of a VAWT, for various values of tip speed ratio; Figure 5b shows the variation of aerofoil angle of attack with angular position about the axis of a VAWT, for higher values of tip speed ratio; Figure 6a shows the flow pattern for an aerofoil at incidence with flow still attached (flow from right to left); Figure 6b shows the flow pattern for an aerofoil with separation due to excessive incidence (flow from right to left); Figure 7 is a graph of power coefficient vs. tip to wind speed ratio for VAWT's with different aerofoils; Figure 8 is taken from reference 1 and shows typical pressure distributions on an aerofoil of 12% thickness and 2% camber at different angles of attack; Figure 9 is taken from reference 2 and shows pressure distributions on an aerofoil in separated flow, at two different Reynolds numbers: 0.4 x iü (curve (a)) and 4.2 x iü (curve(b)); Figure 10 shows an aerofoil (in cross-section) according to a first example of the present invention with cross-bleed holes or slots applied at approximately 40% of the chord from the leading edge; Figure 11 shows an aerofoil (in cross-section) according to a second example of the present invention with cross-bleed holes or slots applied near the leading edge; Figure 12 shows a side view onto an example aerofoil of a VAWT according to the second example of the present invention; Figure 13 shows a side view onto an example aerofoil of a VAWT according to the first example of the present invention; Figure 14 is a graph of calculated power coefficient vs. tip to wind speed ratio; Figure 15 is a third example of the present invention applied to a hollow aerofoil; Figure 16 is taken from Reference 6 and shows an example of a laminar flow aerofoil; and Figure 17 shows an aerofoil (in cross-section) according to a fourth example of the present invention.

With reference to Figure 10, a vertical axis turbine aerofoil structure 2 according to a first example of the present invention, comprises a first opening on a first surface 12 of the aerofoil structure and a second opening 20 on a second surface 22 of the aerofoil structure. The first and second openings 10, are arranged to selectively expel or remove fluid to or from the first and second surfaces 12, 22 of the aerofoil structure 2.

The aerofoil structure 2 may comprise a NACA-0012 aerofoil.

Accordingly, the aerofoil structure 2 comprises a leading edge 4 and a trailing edge 6. Depending on the wind direction and the position of the aerofoil structure 2 as it rotates about the vertical axis of the turbine, the first surface 12 may be the suction surface or pressure surface and the second surface 22 may be the pressure surface or suction surface respectively. In other words, the first surface alternates between the suction surface and the pressure surface and the second surface alternates between the pressure surface and the suction surface respectively. In the example shown in Figure 10, the instantaneous direction of the flow 8 dictates that the second surface 22 is the pressure surface and the first surface 12 is the suction surface.

The first and second openings 10, 20 may be arranged to be in fluid communication with one another via a passage 30 provided between the first and second openings 10, 20. The first opening 10 may expel fluid whilst the second opening 20 removes fluid and vice versa. For example, when the first surface 12 is the suction surface, fluid may be removed from the second surface 22 through the second opening 20 and passed through the passage 30 to be expelled to the first surface 12 through the first opening 10. Similarly, when the second surface 22 is the suction surface, fluid may be removed from the first surface 12 through the first opening 10 and passed through the passage 30 to be expelled to the second surface 22 through the second opening 20.

Accordingly, the use of a pump to expel or remove the fluid may be obviated.

Thus, in one example, the present invention uses passive aspiration to bleed fluid flow from the higher pressure to the lower pressure surface of the aerofoil ofaVAWT.

The passage 30 may be symmetrical about the centreline of the aerofoil 2. The first and second openings 10, 20 may also be symmetrical about the centreline of the aerofoil 2. The first and second openings 10, 20 may comprise holes or slots in the corresponding aerofoil surface.

By transferring fluid to or from the surfaces 12, 22 of the aerofoil structure, flow stall may be delayed and the performance of the vertical axis turbine improved. The boundary layer flow is reenergised by the addition of fluid or removal of low energy fluid (in some cases promoting transition from a laminar to a turbulent boundary layer), thereby delaying boundary layer separation and minimising the drag. In other words, the bleed flow re-energises the boundary layer on the surface it is ejected from; thus increasing the suction that can be sustained on that surface before boundary layer separation occurs; and thus increasing the critical angle of attack the aerofoil can sustain.

For the NACA series of aerofoils the point of maximum thickness is typically in the front half of the aerofoil. For such aerofoils, the first and second openings may be at approximately 40% of the aerofoil's chord from the leading edge 4. This may be suitable for aerofoils operating at a relatively high Reynolds number (e.g. 4.2 x i05 as for the aerofoil shown in Figure 9), because the bleed exit is downstream of the boundary layer transition point on the aerofoil. This helps keep the drag coefficient down while still increasing the critical angle of attack of the aerofoil. In particular the drag at low angles of attack may not be significantly increased, otherwise the power output of the VAWT will be compromised. The presence of slots or holes may have the effect of "tripping" the boundary layer from laminar to turbulent early (even when there was no bleed flow) and thus increasing the drag. Placing the bleed exit downstream of the natural transition point avoids this.

Ideally, the first and/or second openings 10, 20 may be disposed such that a flow leaving said first and/or second openings may leave substantially tangentially with respect to the local surface of the aerofoil structure. This maximises its beneficial effect (of re-energising the boundary layer). However, the first and/or second openings 10, 20 may be disposed such that a flow leaving said first and/or second openings may leave at an angle of less than substantially 20° with respect to the local surface of the aerofoil structure.

Important features and advantages of the present invention are: * At 00 incidence, there is no flow in the bleed holes or slots; * As the incidence increases, the pressure difference between the aerofoil surfaces increases and thus the bleed flow will increase; and * The flow in the passage 30 reverses direction around the rotational cycle of the aerofoil in the VAWT.

In alternative arrangements (not shown), the first and/or second openings may remove or expel fluid by virtue of a fluid suction or pressure means, for example a pump. The first and second openings may concurrently expel or remove fluid from both the first and second surfaces of the aerofoil structure.

Alternatively, the first and second openings may alternate between expelling or removing fluid from the first and second surfaces of the aerofoil structure. For example, when the first surface is the suction surface, the first opening may expel or remove fluid from the first surface, whilst the second opening may or may not expel or remove fluid from the second surface. Likewise, when the second surface is the suction surface, the second opening may expel or remove fluid from the second surface, whilst the first opening may or may not expel or remove fluid from the first surface.

With reference to Figure 11, a vertical axis turbine aerofoil structure 102 according to a second example of the present invention, comprises a first opening 110 on a first surface 112 of the aerofoil structure and a second opening 120 on a second surface 122 of the aerofoil structure. The aerofoil structure 102 may be a NACA-0012 aerofoil and may comprise a leading edge 104 and a trailing edge 106. As before, first and second openings 110, 120 are arranged to selectively expel or remove fluid to or from the first and second surfaces 112, 122 of the aerofoil structure 102. However, in contrast with the first example, the first and second openings 110, 120 may be placed near the leading edge 104. For example, the first and second openings 110, 120 may be placed within 15% of the chord from the leading edge or they may be within 10% of the chord from the leading edge. This may be particularly beneficial in the case of aerofoils operating at lower Reynolds numbers, because the bleed flow is now being used to prevent laminar boundary layer separation, such as that shown for the aerofoil in Figure 9 with a Reynolds number of 0.4 x 1 ü.

The first and second openings 110, 120 may be formed by a leading edge skin 140, which follows the aerofoil 102 over a portion of the leading edge 104. A passage 130 may therefore be formed between the leading edge skin and the leading edge 104 of the aerofoil 102. The passage 130 provides a flow passage between the first and second openings 110, 120.

The second example of the present invention operates in substantially the same way as the first example. For example, the when the first surface 112 is the suction surface (as determined by the flow direction 108 shown in Figure 11), fluid may be removed from the second surface 122 through the second opening 120 and passed through the passage 130 to be expelled to the first surface 112 through the first opening 110. The flow in the passage 130 reverses when the second surface 122 is the suction surface.

With reference to Figures 12 and 13 the openings may be at approximately the same chordwise position across the span of the aerofoils 2, 102 because the Reynolds number would not be expected to change substantially across the span. Figure 12 shows a curved aerofoil with holes near the leading edge, as per the second example of the present invention, and Figure 13 shows slot exits at about 40% chord, as per the first example of the present invention. However, the openings of either the first or second examples of the present invention may comprise holes, slots or a mixture thereof.

With reference to Figure 14, the effect of the present invention on a VAWT has been estimated by making use of the simple flow model for a VAWT with a NACA-0012 aerofoil but with the critical angle of attack increased by 10° (the slope of the lift vs. angle of attack relationship for the aerofoil is assumed to be unchanged). Figure 14 shows the power coefficient, c, vs. tip to wind speed ratio, tsr, for a NACA-001 2 aerofoil (data taken from Figure 7) and the calculated curve from the simple model. In addition, the simple model curve with the aerofoil with increased critical angle of attack is included. As can be seen, the VAWT is now expected to operate effectively at lower values of tsr than before and the following benefits result: * Reduce the power needed for initial run up (by reducing the value of tsr it has to be run up to); * Make the power coefficient curve less peaky without compromising the maximum value achievable; * Reduce the complexity, and thus cost, of the control system; and * Allow the machine to run at higher absolute maximum wind speeds, and thus achieve higher power output over the lifecycle of the machine.

With reference to Figure 15 a vertical axis turbine aerofoil structure 202 according to a third example of the present invention, further comprises a chamber 250 provided between the first and second openings 210, 220. The aerofoil structure 202 may be a NACA-0012 aerofoil and may comprise a leading edge 204 and a trailing edge 206. The aerofoil 202 may be provided with additional chambers such that the aerofoil is substantially hollow. As before, first and second openings 210, 220 are arranged to selectively expel or remove fluid to or from the first and second surfaces 212, 222 of the aerofoil structure 202. The bleed flow from the (transiently) higher pressure surface feeds into the chamber 250, which then blows the fluid onto the adjacent suction surface.

The first and second openings 210, 220 are shown in the same position as for the first example (i.e. at 40% of chord), but they may alternatively be provided within 10% or 15% of the chord length from the leading edge as for the second example. The length to diameter (or height) ratio of the hole (or slot) may be at least 5 in order that it correctly guides the bleed flow onto the suction surface at the required angle. In other words, a portion of the passage from the chamber 250 leading to the first or second opening 210, 220 may comprise a length to width ratio greater than or equal to approximately 5. This ensures that the bleed flow is adequately bounded to achieve the intended exit flow direction and thus have the maximum beneficial effect after entering the mainstream flow.

With reference to Figure 16, an aerofoil shape as disclosed in reference 6 and the intended flow regime over the aerofoil is shown. The aerofoil disclosed therein was designed to achieve laminar flow (at 0° angle of attack) over as much of the aerofoil surface as possible in order to minimise drag. For example, a laminar boundary layer 360 separates from a surface of the aerofoil, undergoes transition at 364 and reattaches to the surface. A separation bubble 362 is thus formed and the turbulent boundary layer 366 remains attached to the surface.

With reference to Figure 17, a vertical axis turbine aerofoil structure 302 according to a fourth example of the present invention may further comprise the aerofoil shape disclosed in reference 6. The aerofoil structure 302 may comprise a leading edge 304 and a trailing edge 306 with first and second surfaces 312 and 322 provided therebetween. First and second openings 310, 320 may be provided just at the start of the laminar separation bubble 362. The laminar separation bubble may be just after the point of maximum thickness, where flow on the surface (at 0° angle of attack) has just started to diffuse. In fact the presence of the hole or slot exits, will cause boundary layer transition instead of this happening via the separation bubble. This will not compromise the drag, and will confer similar benefits, in terms of being able to run the VAWT at lower tip to wind speed ratios, as before.

The first or second openings 310, 320 may be located at approximately 55% chord from the leading edge. A passage 330 may be provided between the first and second openings 310, 320. As for the third example of the present invention, a chamber may be optionally provided between the first and second openings 310, 320. This style of aerofoil seems to have been developed for larger VAWT5, the size of whose aerofoils means that they will optimally run at relatively high values of Reynolds number, over 5 x i05. The positioning of the hole exits relatively late on the aerofoil surfaces is consistent with this.

Claims

Claims 1. A vertical axis turbine aerofoil structure comprising a first opening on a first surface of the aerofoil structure and a second opening on a second surface of the aerofoil structure, the first and second openings being arranged to selectively transfer fluid to or from the first and second surfaces of the aerofoil structure.
2. A vertical axis turbine aerofoil structure as claimed in claim 1 further comprising a passage between the first and second openings such that the first and second openings are in fluid communication with one another.
3. A vertical axis turbine aerofoil structure as claimed in claim 2 further comprising a chamber provided between the first and second openings.
4. A vertical axis turbine aerofoil structure as claimed in claim 2 or 3, wherein a portion of the passage leading to the first or second opening comprises a length to width ratio greater than or equal to approximately 5.
5. A vertical axis turbine aerofoil structure as claimed in any preceding claim, wherein one or more of the first and second openings are arranged to be downstream of a boundary layer transition point of the aerofoil structure in flow at zero angle of attack.
6. A vertical axis turbine aerofoil structure as claimed in any preceding claim, wherein the first or second opening is located at approximately 40% chord from the leading edge.
7. A vertical axis turbine aerofoil structure as claimed in any one of claims 1 to 5, wherein the first or second opening is located at approximately 55% chord from the leading edge.
8. A vertical axis turbine aerofoil structure as claimed in any one of claims 1 to 5, wherein the first or second opening is located at approximately less than 15% chord from the leading edge.
9. A vertical axis turbine aerofoil structure as claimed in any preceding claim, wherein the first and/or second openings are disposed such that a flow leaving said first and/or second openings leaves at an angle of less than substantially with respect to the local surface of the aerofoil structure.
10. A vertical axis turbine aerofoil structure as claimed in claim 9, wherein the first and/or second openings are disposed such that a flow leaving said first and/or second openings leaves substantially tangentially with respect to the local surface of the aerofoil structure.
11. A vertical axis turbine aerofoil structure as claimed in any preceding claim, wherein the first and/or second openings comprise one or more spanwise slots provided along at least a portion of the span of the aerofoil structure.
12. A vertical axis wind turbine or a vertical axis marine turbine comprising the aerofoil structure as claimed in any one of claims 1 to 11.
13. A method of controlling the flow on a vertical axis turbine aerofoil structure, the method comprising: providing the vertical axis turbine aerofoil structure with a first opening on a first surface of the aerofoil structure and a second opening on a second surface of the aerofoil structure; and transferring fluid from one or more of the first and second surfaces of the aerofoil structure through the first and second openings.
14. A method of controlling the flow on a vertical axis turbine aerofoil structure as claimed in claim 13, wherein the method further comprises: permitting flow between the first and second openings.
15. A method of controlling the flow on a vertical axis turbine aerofoil structure as claimed in claim 14, wherein the method further comprises: permitting the flow direction to oscillate between the first and second openings as the turbine rotates about its axis.
16. A method of controlling the flow on a vertical axis turbine aerofoil structure as claimed in any one of claims 13 to 15, wherein the method further comprises: delaying flow stall on one of the first and second surfaces of the aerofoil structure.
17. A vertical axis turbine aerofoil structure, substantially as described herein, with reference to and as shown in the accompanying drawings.
18. A method of delaying flow stall on a vertical axis turbine aerofoil structure substantially as described herein.