GB2469314A

GB2469314A - Leading edge flow control device for sails

Info

Publication number: GB2469314A
Application number: GB0906148A
Authority: GB
Inventors: Nathan Michael Williams
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-04-09
Filing date: 2009-04-09
Publication date: 2010-10-13
Anticipated expiration: 2029-04-09
Also published as: GB2469314B; GB0906148D0

Abstract

The active or passive leading edge flow control device for sails is a device designed to increase the force developed over a sail. The flow control device 2 is a flap (Fig 5 and 6), additional piece of sail or integrated section of sail (Fig 7) with specific material properties and tension attached to, or as part of the leading edge/luff of the sail 1. The control device oscillates, stimulating and organising instabilities in the shear layer, and increasing the level or attached flow over the sail. The device may be oscillated actively or passively.

Description

UK Patent Specification

TITLE

Active or Passive Leading Edge Flow Control Device for Sails

BACKGROUND

This invention relates to yacht sails. The speed of a yacht is restricted by the force produced by it's sail.

This invention aims to increase the maximum force generated by sails, and allow the sail to operated at a larger angle of attack without staUing. In certain sailing conditions, flow can separate from the sail.

Depending on the conditions, the flow may reattach, or separate completely. This invention is designed to stimulate the flow of air near the leading edge of the sail, organising the vorticity in the shear layer, encouraging reattachment over a wider range of apparent wind angles (and stronger reattachment of already attached flow) and increasing the driving force developed by the sail.

STATEMENT OF INVENTION

The invention comprises of a eading edge flap that is attached near or onto the leading edge of the sail.

The tension and weight (or area density) of the flap are selected to ensure that is oscillates at a frequency and amplitude that excites the shear layer increasing reattachment of the flow over the sail.

Passive excitation: If tuned to the correct frequency, natural instability's in the flow can excite the flap and encourage flow reattachment.

Active excitation: The flap can be oscillated actively by some outside source if a higher level of leading edge stimulation is required and/or to ensure excitation at a specific frequency.

ADVANTAGES

The invention can be excited passivey or actively.

This flap can have it's tension adjusted so that it has an adjustable natural frequency of vibration, and can be tuned to oscillate at a frequency that causes the optimal level of reattachmertt. The flap can be tuned' by altering the stiffness for optimal performance.

The flap may be set up so that no adjustment of tension is needed.

The invention can cause reattachment of separated flow, but also may have beneficial effects on un-separated flow.

The invention can either be integrated into the sail itself, or operated as a small flap at the eading edge, documented in this document. For the case of integrating the flap into the sail, it may simply be altering the material properties of the sail and the tension of a strip along the leading edge, allowing these oscillations to take place.

The flap can be made from a different material to the sail.

The concept can be used on non conventional sail shapes or rigs, such as the crab claw rig.

The flap need not be along the entire leading edge of the saiL The flap may be able to have ft's angle of attack to the oncoming flow adjusted independently to that of the sail that it is attached to or integrated into.

Introduction to drawings

An example of the invention will now be described by the following drawings: The following is a description of each number used to identify different drawing elements, and is applicable to all drawings: 1 sail, spinnaker or gennaker 2 leading edge flap, or in cases where the flap is not identifiable, the location of the leading edge flap in relation to the sail (1) 3 the spinnaker pole 4 mainsail mast 6 boom 7 join between the flap and sail (normal or loose) 8 join between the sail or flap and the supporting pole (3) 9 extra line added to adjust tension separation point 11 slow moving flow 12 fast moving flow 13 apparent wind 14 reattachment point Figure 1 Shows the position of the leading edge flap (2), from a top view of the sailboat, with other features standard to many sail boats identifiable.

Figure 2 Shows the position of the leading edge flap (2) in a three dimensional view.

Figure 3 Shows wind flow over a cross section of the sail, from the top view without the flow control device. The shear layer is the thin region of air between the slow moving flow and the fast moving flow.

Figure 4 Shows wind flow over a cross section of the sail, from the top view with the flow control device.

The shear layer is the thin region of air between the slow moving flow and the fast moving flow. The leading edge flap position is also shown(2).

Figure 5 Shows a simple method that the flap could be attached to the sail, a simple join (7), Figure 6 Shows an additional method that the flap could be attached to the sail, a loose join (7).

Figure 7 Shows an additional method where the flap is integrated into the sail.

Figure 8 Shows the sail (1) and the flap (2) attached to the supporting spinnaker pole (3).

Figure 9 Shows the sail (1) and the flap (2) attached to the supporting spinnaker pole (3), with the addition of an extra line added to adjust the flap tension (9) for tuning the natural frequency of the flap or for active oscillation of the flap.

Detailed description

A small (in chord) flap is placed upstream of the sail, in order that the oncoming wind flows over the flap before flowing over the sail, in a position shown in Figure 1, Figure 2 and Figure 4. Natural fluctuations in wind strength and direction and other instabilities, cause the flap to oscillate. There may or may not be provision for the angle of attack of the flap to be adjusted or set independent to that of the sail to further increase oscillations. In addition to passive oscillations, active oscillation can be used to increase the amplitude of flap oscillation or encourage oscillation at certain frequencies. For some cases, the flow will separate from the sails surface and a shear layer is formed between the fast moving and slow moving fluids, shown in Figure 3 for a case with no flow control flap. If the shear layer does not reattach to the surface of the sail, as in Figure 3, lift is reduced. For the case in Figure 4, with a flow control flap, as the flow passes over the flap, the flap oscillations increase the organisation of vorticity in the shear layer. This increased organisation of vorticity causes the separated shear layer to reattach to the sails surface, and higher levels of lift can be achieved, shown by the reattached flow in Figure 4. Oscillations of the flap may also prevent separation from occurring, maintain lift. Figure 5, Figure 6 and Figure 7 show various ways that the flap can be attached to the sail. Figure 5 shows the flap joined simply upstream of the sail. There may be an additional line at the junction of the flap and the sail. Figure 6 shows the flap joined partially to the sail, so that the tension of the flap can be adjusted easily independently of the sail. Figure 7 shows the flap integrated into the sail itself, and in effect may simply be a change in material properties or tension at the leading edge of the sail, with the specific intension of allowing oscillations to excite the oncoming flow.

Figure 8 shows a potential way by which the sail and flap are attached to the supporting pole. An addition to this setup is to have an extra line attached to the foot of the flap, as shown in Figure 9 or running up the centre of the flap, attached at specific points. This extra line would alow further independent adjustment of the flaps tension, with the purpose of changing the natural frequency of oscillation, and potentially matching it to the frequency that most effectively excites the shear layer instabilities. The extra line could also be a means of actively oscillating the leading edge, buy applying a periodic tension adjustment. Further active oscillations could be induced by integrating piezoelectric actuators into the flap.