GB2468903A - Aerofoil tip vortex reducing structure - Google Patents

Abstract

For reducing wind turbine tip vortices, each blade tip comprises two or more vanes 3,6 or winglets separated by a gap 7 through which flow that would have formed a vortex core is passed from the upwind to the downwind blade sides. The trailing vane trailing edge is close to the main blade trailing edge. The pressure of fluid passing through the gap falls to the downwind side pressure so flow is ejected in a direction opposite that of blade motion to suppress rapidly revolving vortex core formation. A shroud 10 may cover two vane tips to prevent the formation of secondary vortices. The vane stagger at their roots is greater than that of the main foil and reduces along the length of the vane. The leading vane may be tilted towards the suction surface and the same length or shorter than the trailing vane which may tilt towards the pressure surface.

Description

IMPROVEMENTS TO WIND TURBINES BY REDUCING DRAG FROM TIP

VORTICES ALSO WITH APPLICATIONS TO PROPELLERS HELICOPTERS

AND AIRCRAFT.

Ronald Denzil Pearson of Curbar Edge, 2 Rowlands Close, Bathford, Bath, N.E Somerset. BA1 7TZ. Tel: 01225 858315: e-mail rdp(ronaIdpearson.pIus.com submits the following invention.

This invention relates to means for reducing the vortex cores leaving the blade tips of wind turbines for improving the power delivered whilst simultaneously reducing noise and the impact on wild flying creatures. The invention can also be applied to the wingtips of aircraft and the blades of propellers and helicopters to reduce drag.

The idea for the present invention arose from reading an article showing that large numbers of bats were being sucked into the tip vortices of wind turbines to be instantly killed by decompression. It then transpired that the solution forming the present invention would also reduce drag so improving power output. This is because the net drag of blade tips is transformed into propulsive thrust. Another benefit would be a reduction in noise. Tt is the turbulent motion of air inside vortices that forms the source of much noise created by wind turbines. The most severe vortices arise at the blade tips and so generate a large part of that noise. That pressures arc so low as to kill the bats sucked into them is proof of the severity of tip vortices. The tip vortex cores are also a major source of drag both for aircraft and wind turbines. The invention provides a means for greatly reducing the rotational motion inside the cores of tip vortices by changing the direction of this rotary motion to a substantially axial direction. In this way the shearing motion generating noise is greatly reduced and simultaneously the decompression effects so harmful to wildlife are eliminated. The energy wasted inside the cores of the tip vortices is redirected so that drag is reduced.

The result is an increase in power output of the turbine. For application to aircraft the reduction in drag will reduce fuel consumption.

The invention is easily retrofitted to existing apparatus by cutting off the existing tips and fixing replacements of the kind to be described. This will upgrade existing apparatus as welt as providing advantages for new products.

Attention now needs to be drawn to a possible source of confusion. Certain embodiments of the invention might appear to have been anticipated by the slots used to improve the lift of aircraft wings at low speeds. The similarity is entirely superficial since the purpose is entirely different. Geometrical details also differ.

As will be explained the production of vortices is fundamental to producing the lift force on aircraft wings and a similar connection applies to wind turbines. It may not be obvious therefore that vortex production can be minimised. It is certainly true that the large vortices associated with the production of lift on wings cannot be eliminated.

However, it is the rapidly revolving cores of those vortices to which this invention relates. A simple example that cannot be applied to wind turbines will show that in principle it is possible to usefully remove such vortex cores without affecting lift forccs. An aircraft is to be imagincd having a streamlined cxtcnsion bchind and fitted to each wingtip. The extension carries a set of de-swirling vanes of the kind forming the stators of axial flow compressors. The vortex cores would be transformed into a non-rotating fluid and a pressure rise would occur from inlet to outlet of the de-swirling vanes. This would produce a forward thrust by the redirection of flow without affecting lift Such a solution is not practical for application to wind turbines owing stresses caused by centrifugal forces but the example suggests that ft should be possible to eliminate such vortex cores in some other way.

This invention provides a practical alternative means.

The blades of turbines or the wings of aircraft have cross-sections of aerofoil shape and deflect flows in similar wayt Either will be defined as a main aerofoil. The term blade will refer to a turbine blade. A vane is to be defined as being of similar -to a wing or turbine blade being also of aerofbil cross section but of much smaller width as measured in the direction of flow than the main aerofoil to which ft is attached. The width of any aeroibil is known as the chord of that aerofoil.

The means provided by the invention for reducing tip vortices consists of at least one vane extending from the tip of a main aerofoil so arranged as to remove part of the vortex motion produced at that tip. With an embodiment of the invention incorporating only a single vane the latter is positioned as close as possible to the trailing edge of the main aeroibil. Other embodiments of the invention have two or more vanes arranged in line so that the trailing edge of a leading vane comes close to the leading edge of a trailing vane each vane being attached at its root to one main aeroibil. In a further embodiment a shroud is attached to the tips of all vanes extending from a single main acrofo The lift of aircraft wings depends upon the formation of a vortex pair each vortex being centred behind a wing tip. Each vortex has a diameter comparable with wingspan and so is very large and cannot be eliminated. However, close to the centre of each vortex rotational motion becomes very high and is a source of high drag. This region will be defined as a vortex core and it is to the reduction of losses in such cores that the invention is directed. The actual radius of the core is a matter of designer choice but an optimum radius can be chosen that when applied to the invention will cause drag on the entire structure of main aerofoil and vanes to be lower than that of a conventional main aerofoil.

Further definitions are required. The leading edge of an aerofoil is the edge that meets the flow of fluid whilst the trailing edge is the point at which flows rejoin. The chord of an aerofoil is a straight line extending from its leading edge to its trailing edge. Stagger is the angle made by the chord to the plane of rotation of the turbine blade. A shroud is defined as a cover formed as a sheet preferably of streamlined cross section and aligned substantially perpendicular to the turbine axis or flight direction. Said shroud is attached to the tips of the vanes and also aligned with the local direction of flow. The root of a vane is its point of attachment to a main aerofoil.

The pressure on the lower surface of the wing of an aircraft needs to exceed that on its upper surface in order to produce lift and this is directly connected with the formation of the large vortex pair previously described. At the wingtips this pressure difference causes air to flow upward from the underside to the upper more cambered side.

Conscqucntly flow on the lowcr sidc is divergent whilst on thc uppcr side the flow converges. When the two flows rejoin at the trailing edge of the wing a shear flow exists in which that from the lower surface has a an outward component of velocity whilst that leaving the upper surface has an inward component of velocity. The flows roll up together to form a set of small vortices leaving the trailing edge but at each wingtip a single vortex core of much larger radius arises. The invention is limited to minimising the parasitic effects of the core of such tip vortices.

Wind turbines produce similar vortices at their blade tips. In this case the upwind side of the blades are subjected to a higher pressure than the downwind and more cambered side. A lift force similar to that of an aircraft wing is produced but owing to the stagger of the blade chords the lift force is tilted so as to produce a tangential component of force. It is this tangential component that generates the power produced by the turbine but is partly offset by a drag force.

The way in which vanes minimise the parasitic effects of tip vortices will now be described. In the case of turbines these vanes extend substantially radially outward meaning in a direction away from the turbine hub and lie substantially in the same helical plane as the chord of the blade tip to which they are attached. Preferred embodiments of the invention have more than one vane. Each vane has a cross section of aerofoil shape and has a much smaller chord than the blade tip so that at least two of them can be arranged in line from the blade leading edge to its trailing edge. Close to the blade tip the trailing edge of a leading vane can overlap the leading edge of the next vane though this overlap is not essential. What is essential is the leaving of a gap between the two edges that is on the downwind side of the turbine that is also the most cambered side of both blade and vanes. The gap is arranged to permit air from the upwind side to flow over the cambered downwind surface of the said next vane. This occurs because the air-pressure on the upwind side of the turbine is greater than that on the downwind side. With the pressure differential now utilised to create a flow through the gap no tendency remains for air to move radially inwards and so the tip vortex core can be at least greatly reduced by redirecting flow from the rotational kind to motion opposite blade motion. Furthermore the convergence of flow on the cambered downswind side of the blade is reduced. This greatly reduces the vortex producing shear flow along the entire trailing edge of the blade. Lift is increased and noise reduced from the entire blade in addition to the main effect of harnessing the energy normally lost in the tip vortex core. It needs to be understood that the vanes are arranged with their chords set at a greater stagger than the blade tips to which they are attached. This is permissible since it is the flow from the upwind side that is being utilised and this increases the velocity component measured in the axial direction. It is this increased angle that permits most of the power increase by directing the force generated more to the tangential direction. Since this axial component of flow reduces with distance outward along a vane each vane is provided with a twist to accommodate the reduced axial velocity and the vane is tapered so that any overlap between the trailing edge of a leading vane and the leading edge of the following vane is eliminated at some position. It is desirable for the leading vane to be tilted from the radial to an angle in the direction of motion so that the gap for flow between its trailing edge and the leading edge of the following vane is increased. The same difference in tilt is then applied to any remaining vanes so that the vane closest to the blade trailing edge is tilted opposite blade motion. It is not essential for the leading vane to have its leading edge in line with the leading edge of the main aerofoil. Tndeed an optimum position may be for the leading vane to be positioned at a fraction of the main acrofoil chord toward the trailing edge. It is not essential to have all the roots of vanes at the same radius from the turbine hub, An undesirable tip vortex to be called a secondary vortex will form at the outer end of each vane but its core will have a radius diminished in direct proportion to the ratio of its chord to that of the blade tip. However, since the cross sectional area of such cores is proportional to the square of its radius the loss of energy from all vanes put together will be far less than that of the single vortex core arising from a conventional blade tip.

To prevent vortices from all vanes combining to form a larger single vortex each vane can be made with a different length from any of the others.

These undesirable secondary vortices can be eliminated and the vanes reduced in length by the addition of a shroud to cover all vanes on any blade tip. The shroud is best formed with its upwind side bent in toward the hub in order to match the angle at which it meets the airflow. With such a shroud it is possible to achieve tip vortex suppression with only two vanes separated by a single gap. The addition of a shroud is only possible if permitted by stress considerations since at the high tip speeds in common use centrifugal stresses can be excessive.

In the case of application to aircraft wings a shroud can be added optionally since stressing considerations do not arise. For application to high speed aircraft that have swept wings the same angle of sweep can be maintained in the vanes though this is not essential.

The invention is also applicable to turbines used in seawater to harness the energy of ocean currents, to helicopter blades and ship or boat propellers.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:-Figure 1 shows a view from the windward side of a turbine blade in the direction of the axis of rotation showing a main aerofoil tip connecting with four vanes.

Figure 2 shows a cross section of a main aerofoil tip and vanes viewed in a direction toward the turbine hub and close to the root of the vanes.

Figure 3 shows a cross section of vanes at their outer ends.

Figure 4 shows an embodiment of the invention that has only two vanes capped by a shroud. A view from the windward side of a turbine blade is given at A with a section parallel to the shaft at B and a cross section through the two vanes is shown at C. Figure 5 shows an embodiment of the invention that has a multiplicity of vanes capped by a shroud. A view from the windward side of a turbine blade is given at A with a section parallel to the shaft at B and a cross section through the two vanes is shown at C. Referring to Figure 1 a turbine blade tip is shown looking in the axial direction from the upwind side and moving in the direction U. The blade has a leading edge at 1 and a trailing edge at 2 and carries four vanes 3 to 6 extending radially outward. The blade is shown so that the leading vane 3 has its leading edge coincident with that of the blade and has a shorter span than thc following vane 4. This is followcd by vane 5 and a final vane 6 is provided having the greatest span and has its trailing edge coincident with that of the blade at 2. The steadily increasing span of the vanes prevents the secondary vane tip vortices from combining to produce a single larger vortex though such means are not essential and all vanes could be made of equal length. A small overlap near the roots of the vanes is indicated between the trailing edges of a leading vane and the leading edge of a following vane. The gaps so provided re-direct the flow so that inward flow toward the axis is prevented and the formation of a large vortex core thereby eliminated.

Referring to Figure 2 a section close to the vane roots is shown looking in a radial direction toward the turbine hub. The wind direction is indicated by W and the blade motion is in direction U. The blade tip is shown with its chord line Ch inclined to the direction of motion by an angle of stagger S as required to produce a driving force. The trailing edges of leading vanes overlap the leading edges of following vanes to leave gaps or so called throats 7, 8 and 9. These throats are so designed as to utilise the pressure difference between upwind and downwind sides of the turbine to direct the flow rearwards so minimising the generation of a vortex core.

Referring to Figure 3 a cross section is taken at the radius of the tip of the leading vane 3. The throats between vanes are increased by having the vanes tilted slightly from the radial direction. Vane 3 tilts in the direction of blade motion whilst vane 6 tilts in the opposite direction with intermediate vanes tilted at intermediate angles.

Referring to Figure 4 views of a twin vane with shroud embodiment of the invention arc shown. At 4A a view from thc upwind side of a turbinc is shown. A leading vane 3 is followed by a trailing vane 6 separated by gap 7. A shroud 10 is attached to both vanes so that the gap is covered. At 4B a cross section through the vanes shows their aerofoil section and the nozzle shaped gap 7 formed between them. The throat area is arranged to permit the amount of air to pass through it that would have passed over the tip to form a vortex core. In this way the static pressure of the air is caused to fall to the value existing on the downwind cambered side of the blade and flow over vane 6 to produce useful thrust instead of drag. No tendency remains for flow 7 to pass over the outer surface of the shroud to cause a vortex core. The remaining and major component of flow does pass over the outside of shroud 10 since when applied to an aircraft wing this will form half of the vortex pair responsible for lift. However, this vortex now has an inner radius of substantial size so that it can no longer form a vortex core of excessive rotational speed. At 4C a view of the shrouded vanes is shown as a section parallel to the axis of rotation and looking onto the leading edge. This section passes through the gap 7. The leading edge of the shroud 11 projects forward and is curved toward the axis to meet the air at its angle of flow. With this geometry the shroud acts like a pitot tube to trap fluid at a higher pressure than that flowing round outside. In this way the pressure on the upwind side of the blade is prevented from falling too much.

The entrance to the slot as shown in this view is also rounded as shown at 12 in order to prevent the separation of flow. Separation would arise at a sharp corner due to the radial component in the approaching flow. The leading vane 3 is shown tilted in the direction of blade motion or upwards in the case of a wing whilst the trailing vane 6 is tilted in the opposite direction. This enables the gap 7 to be wider at the shroud than at the root to match the increased rate of flow near the shroud. This tilting is desirable for aerodynamic reasons but not essential. Tilting is undesirable for turbine blades owing to the bending stresses caused by centrifugal forces and so may need to be omitted in practical apparatus.

Referring to Figure 5 a further embodiment of the invention in which a shroud is attached to a set of vanes is shown. A multiplicity of vanes is shown at A and B. A view looking in the axial direction from the windward side is shown at A whilst B is a cross section of the vanes viewed in a radial direction. At C a section through a gap between vanes is shown viewed perpendicular to the axial direction and looking onto the leading edge. The same description as for Figure 4 is applicable except for the increase in number of vanes. In this case of multiple vanes a smaller area of blade is involved for vortex minimisation.

Claims (6)

1. CLAIMS1 A structure in which a main aerofoil that forms the blade of a wind turbine helicopter propeller or wing of an aircraft has a vane attached or made integral with the main aerofoil said vane having much shorter chord than the main aerofoil with that vane also being of aerofoil section and preferably though not necessarily tapered to an even smaller chord at its tip the vane also arranged with its trailing edge close to that of said main aerofoil so as to reduce flow over the tip of the main aerofoil by diversion to flow over the vane so reducing the rotational speed of the vortex core that would otherwise form.
2. 2 A structure as claimed in claim 1 in which the stagger of the vane at its root is greater than the stagger of the main aerofoil but with this stagger reducing toward the vane tip the stagger at any section along the vane being arranged to suit the local direction of flow.
3. 3 A structure as claimed in Claim 1 or 2 in which a main aerofoil has two vanes of the kind described in which one vane is a leading vane defined as having its leading edge close but not necessarily coincident with the projected line of the leading edge of the main aerofoil and with the other vane as described in Claim 1 but now defined as a trailing vane having its trailing edge close to that of the main aerofoil the two vanes so arrangcd as to leave a gap bctwccn thc trailing edge of thc lcading vanc and the leading edge of the trailing vane the gap being on the more cambered and lower pressure side of the structure so that the flow that would have travelled over the tip of the main aerofoil to form a rapidly rotating vortex core is instead diverted over the trailing vane to at least reduce drag by producing some forward thrust.
4. 4 A structure as claimed in Claim 3 in which the tips of the vanes and the gap between them are covered by a so called shroud which is a sheet aligned in the local direction of flow and can be of streamlined form whose purpose is to enclose the flow that passes between the vanes and so prevent secondary vortices from being shed from the tips of the vanes.
5. A structure as claimed in Claim 3 or 4 in which the leading vane is tilted in the direction of the more highly cambered and lower pressure surface which is also the downwind direction in the case of a wind turbine whilst the trailing vane is tilted in the opposite direction so that the gap between the vanes is greater at the vane tip than at the vane root.
6. 6 A structure as claimed in Claims 3 to 5 in which a multiplicity of vanes replaces the two vanes described in Claim 3 and extend from the main aerofoil and in which a gap is arranged between the trailing edge of any leading vane and the leading edge of any trailing vane on the more cambered and lower pressure side of the structure whereby much of the flow that would otherwise form a rapidly rotating vortex core is diverted to form at least a more slowly rotating vortex core.