GB2187261A

GB2187261A - Controlling boundary layer

Info

Publication number: GB2187261A
Application number: GB08600175A
Authority: GB
Inventors: Srinivasan Raghunathan; Dr Dennis George Mabey
Original assignee: SECR DEFENCE; Queens University of Belfast; UK Secretary of State for Defence
Current assignee: SECR DEFENCE; Queens University of Belfast; UK Secretary of State for Defence
Priority date: 1986-01-06
Filing date: 1986-01-06
Publication date: 1987-09-03
Also published as: GB8600175D0

Abstract

A buffet breather system for aerofoils is used to alleviate buffet and energy losses caused by separation at a shock of subsonic and transonic airflow speeds. A network of breather orifices 12, 13 in the surface may be connected together, e.g. by chamber 14, so as to permit flow from one region to another. The orifices may be forward facing, normal or backward facing holes, or any suitable combination. The intensity of shock is also decreased. The improvement in efficiency can be achieved with very low levels of aerofoil porosity. <IMAGE>

Description

SPECIFICATION Buffet breathers for aerofoils The present invention is concerned with the alleviation of air stream buffeting and in particular with delaying the shock and the onset of flow separation from aerofoil surfaces at subsonic and transonic flow speeds.

The problem of airflow buffet is well known and documented. A suggested solution to alleviate the problem was proposed in the UK Patent Application 8419158. That application proposed a solution intended to alleviate the problem for bodies where on the body surface time mean pressures are similar but vary with time so as to be generally out of phase but for the presence of breather tubes. Such a breather system would be inappropriate for the present invention in which it is intended to delay the onset of flow separation from an aerofoil or other surface having air flowing over it such as diffusers. Under certain conditions the flow across an aerofoil surface will become detached and separate from the surface becoming non-laminar and resulting in undesirable losses. These losses can be particularly large in transonic flow conditions when a pronounced shock results.The consequent thickening of the boundary layer in the region after separation of the shock increasing the drag with a consequent decrease in efficiency.

According to the present invention there is provided a buffet breather for a body locatable in a fluid flow and where time mean pressures are different at adjacent points, comprising a plurality of breather orifices located on an outer surface of the body, the breather orifices located in the region of the flow separation point, the breather orifices connectable together by connecting means below the surface of the body, the connecting means permitting fluid to flow between the breather orifices.

The present invention advantageously provides a method of alleviating buffet resulting from fluid flow over an aerofoil surface by use of a breather orifices located in the region of a flow separation point, thereby enabling fluid to flow from the region of higher pressure to a region of lower pressure upstream of the flow separation point, and emerge via the upstream breather orifice. The use of this breather and pressure relief mechanism results in the flow separation and shock to being moved further downstream than would otherwise be the case.

According to a second aspect of the present invention there is provided a breather assembly wherein the means connecting the breather orifices is a plenum.

The presence of the plenum is advantageous because it assists the breather flow by provision of an area of lower pressure beneath the surface of the aerofoil at the downstream area of turbulence so that the suction effect can appreciably thin the separated shear layer.

According to a third aspect of the invention there is provided a buffet breather wherein the means connecting the breather orifices is a breather tube.

Advantageously a breather tube provides an improved flow passage between the two breather orifices, with a stronger bleed flow which should more effectively stabilise the shock and thereby delay separation.

According to a further aspect of the present invention a buffet breather having at least one orifice located upstream of a flow separation point and at least a second breather orifice located downstream of a flow separation point.

Advantageously the breather orifice located upstream of the flow sepration point points in a downstream direction so that fluid may be injected tangentially into the stream and thereby delay flow separation.

According to a further aspect of the present invention there is provided a buffet breather system having a plurality of interleaved pairs of breather orifices arranged in a substantially streamwise direction along the aerofoil surface, the pairs of orifices each connected by their own breather tube so enabling the flow separation point to be moved downstream under a range of flow conditions.

Advantageously the present invention could provide a plurality of breathers interleaved so that as flow conditions over the surface of the aerofoil changed different sets of breather tubes could be employed to delay the onset of flow separation under a variety of flow conditions.

According to a further aspect of the present invention there is provided a buffet breather in which the breather orifices are inclined in a chordwise plane at about 80" to the normal.

According to a further aspect of the present invention there is provided a buffet breather system having a downstream facing orifice located upstream of a flow separation point and an upstream facing breather orifice located downstream of the flow separation point.

According to a further aspect of the present invention there is provided a buffet breather system having detector means for detecting the point of separation of the flow, computation means for determining the optimum pair of breather orifices for a given flow separation, closure means for closing off the breather orifices not needed for stabilizing the fluid flow.

An advantage of the present invention is the provision of a plurality of sets of breather orifices, and detection means for determining the point of flow separation in order to enable the orifices not in use to be closed off and so maintain overall aerofoil efficiency.

In order that the present invention may be more easily and clearly understood it will now be described with respect to the accompanying drawings of which: Fig 1 shows a section of an aerofoil in a fluid flow, Fig 2 shows a section of an aerofoil in a fluid flow and having the breather orifices connected by a tube, Fig 3 shows a possible arrangement with a set of interleaved breather orifices adapted to cope with a range of flow separation conditions.

Fig 4 shows a pressure distribution for M = 1.35 in which the coefficient of pressure is plotted against chord x/c for a range of breather configurations.

Figs 5 & 6 represent similar pressure distributions to figure 4 but at different Mach numbers.

Figs 7-9 show examples of pressure profiles extending away from the surface in the y direction (y/c) with particular reference to the boundary layer.

Fig 10 shows a graph of drag vs Mach No.

for various types of breather.

Fig 11 shows a graphical plot of Mach Nos.

vs chord distance xs/c for various breather tube conditions.

Fig 12 shows a graph of pressure variations p/q, against chord measurement x/c, this being effectively a measure of pressure fluctuations over the length of the chord.

Figs 13 & 14 are tracings of spark Schlieren photographs taken in the wind tunnel showing the propagation of the shock for a pair of different Mach numbers under the different breather conditions described.

Figs 15 & 16 show plots of Vn F(n) against n, for two different points along the chord.

Figs 17 & 18 are two general illustrations of applications of the present invention, one applicable to flaps, the other to diffuser arrangements.

Fig 1 shows a section of an aerofoil 10 located in a flow stream. The flow separates at a point 11, the line indicating the approximate direction of the shock, and becomes turbulent thereafter. Located upstream of the flow separation point is a breather orifice 12 and a downstream breather orifice 13. These two orifices are connected to a plenum 14 which may include all or part of the aerofoil section. Fluid flow over the aerofoil surface is laminar and remains attached to the aerofoil until point 11 at which it separates from the aerofoil and becomes turbulent downstream of the shock. At a point downstream of flow separation the pressure is higher than that upstream of the flow separation. It is known that suction at the surface of the body can appreciably thin the separated boundary layer and so help move the shock further downstream in addition to reducing its effect.Thus a beneficial effect may be obtained by thinning of the separated shear layer.

It has also been found that tangential injection of fluid into an attached boundary layer tends to delay separation. If the two points 12 and 13 are connected by say a plenum 14 a clockwise bleed flow is induced by the pressure difference arising. This bleed layer will thin the boundary layer upstream of the separation points, and thin the layer of separation downstream of the separation point. The overall effect is, therefore, to move the mean separation position a little further downstream, and to reduce the degree of turbulence arising from separation.

Figure 2 shows a similar arrangement to that of Fig 1 in which an aerofoil section is located in a flow stream. In this instance the upstream breather orifice is arranged to point tangentially downstream, the downstream breather orifice is arranged to point upstream.

In order to induce a more positive fluid flow the two breather orifices are connected by a breather tube. The stronger flow resulting from the use of a breather tube results in a more effective stabilization of the shock.

Figure 3 shows an alternative arrangement in which there are a pair of interleaved breather orifices. Clearly two or more breather orifice pairs may be used but for clarity of illustration only two pairs are shown. By employing a range of breather orifices, a range of separation conditions could be adapted for.

It has been found by experiment that a system of breathers may just comprise a set of forward facing breathers or breather orifices pointing in an upstream direction. An alternative embodiment utilizing the upstream pointing breathers will now be described by way of further example.

In these graphs various breather orifice arrangements are used, these being represented by the initials FFH, for forward facing holes, BFH for backward facing holes, NT for the instances where there are no tubes interconnecting the breathers and Tubes for instances where the tubes are interconnected.

The results illustrating one embodiment of the present invention will be discussed in greater detail and described by way of example. The tests were conducted with a circular half arc aerofoil model. The region of porosity was located at about 80% chord, the porosity of the surface being about 1.6%. In this particular experiment the porosity was provided by a plurality of breather orifices in the aerofoil surface venting to either a plenum or connectable to a tube network. The breather orifices were in a network of orifices of Imm diameter spaced at 5mm intervals. Experiments were conducted at three shock Mach Numbers, 1.2, 1.30 and 1.37. Additionally different arrangements were used for the porous regions, these comprising forward facing holes (FFH), backward facing holes (BFH) and normal holes in the aerofoil surface.

Fig 4 shows a pressure distribution for M = 1.20. As can be seen from the plot of coefficient of pressure vs Chord (x/c), the datum shock is located at about 75% chord, upstream of the breathers.The inclusion of breather holes moves the shock further upstream, which although a negative effect,is mitigated by the reduced intensity of the shock.

The datum positon is shown with p = o-ie an aerofoil surface with zero porosity.

The two lines reflecting results in which breathers are used are to give a porosity of 1.6% BF/FF (Backward Facing and Forward Facing) indicate the two different sets of circumstances when the breathers are connected by tubes (T) and when they are not connected by tubes (NH). An advantageous effect noticed in the case of the connected tubes is that the trailing edge pressure divergence is reduced.

In this case the pressure shock is entirely upstream of porous region and so the mechanism operating to relieve the pressure is different to that postulated above. However, this factor is not the only one of interest or importance and later figures will indicate a drop in the amount of buffet experienced by the aerofoil.

Figures 7, 8 and 9 show examples of pressure profiles extending away from the aerofoil surface for the different breather configurations. These figues indicate increased aerodynamic losses in the boundary layer 0.02 < y/c < 0.1 for the aerofoils incorporating breathers whether or not they are connected by tubes, but show decreased losses beyond the boundary layer y/c > 0.1. This particular arrangement is shown for all three Mach numbers for which results are available and shows an improvement in aerodynamic performance for the arrangements incorporating some levels of buffet breather. The greatest improvement in performance is achieved using the FFH arrangments.

Figures 10 and 11 show the drag values obtained from investigation of wake traverses, illustrated in Figs 7-9. In these graphs the datum figure for drag C0 is derived from measurements on the surface with a porosity of p = o. Fig 10 shows the drag as a plot against Mach No. for a variety of breather configurations. Fig 11 shows the corresponding plot of drag vs chordwise distance for the same configurations. The scale CD/CDO < 1 represents a reduction in losses and thus an improvement in performance. From the annotations it can be seen that the BF/FF arrangement offers an improvement in performance over the datum, but a greater improvement is offered by the BF/FF arrangement with the breather tubes interconnected. A further improvement in performance is obtained by use of only FFH.This improvement with the FFH is particularly beneficial because in an aerofoil the range of conditions over which performance improve-ments are maintained is greater than would be the case with FFH and BFH. With a set of breathers comprising entirely of the FFH arrangement the position of the shock with respect to the breathers is less critical than in any other arrangement.

Figure 12 shows the variation of pressure against chord position, the measure p/q, being essentially a measure of pressure fluctuation. As can be seen the non-porous surface has a high peak near the trailing edge of the chord section. The symbols are used to indicate an approximate position of the peak pressures along the aerofoil chord section.

These lower pressure fluctuations achieved by use of breather orifices result in a further improvement in aerofoil performance.

Figures 13 and 14 are tracings of spark Schlieren photographs taken in the wind tunnel. These show changes in the mean shock patterns. The widths of the shocks provide an approximate indication of the shock amplitude.

The shock amplitudes are found to be appreciably decreased in the cases when breathers are used, the lines being much narrower than the non-breather case. A reduced amplitude of shock is desirable in that it means a lower loss of energy as a result of the shock and thereby increased efficiency.

Figures 1 5 and 16 show plots of Vw(n) vs n, these plots providing an indication of the loss of energy due to buffet. A decreasing excitation energy is shown by a lower value of VnF(n). Figure 15 is the results of the measurements taken at a point well upstream of the shock and it is clear that the breather facilities result in a substantial drop in buffet energy. A similar plot is made in Fig 16 except that this set of measurements was taken much further downstream at a point beyond the shock. Here too the level of buffet excitation is generally found to be appreciably lower with breathers in use than with not.

The breather orifices in the present experiments have been inclined at 60 to the normal, in either a forward facing direction or a backward facing direction. In some instances they are just normal to the surface. A range of angles will function in this invention, the particular angle can be optimised to cater for the flow conditions to be experienced by the chosen aerofoil.

As indicated above in the present experiments the breather holes are of approximately 1 mum in diameter, although they need not be circular. In a full size large aircraft wing these breathers could be as large as 5mm diameter and spaced approximately 1cm apart.

This invention is envisaged as having a very wide range of possible uses and applications.

As described above the use of buffet breather orifices in the region of andbehind the shock wave separation region results in improved aerodynamic performance as a result of lower energy loss through buffeting and other associated drag penalties. These applications are appropriate to subsonic and transonic cases.

An example of a place where a reduction in buffet would be beneficial is in trailing edge flaps; the benefits would manifest themselves in the form of lighter flaps and structural supports therefor, particularly if the pressure fluctuations were reduced. This could in turn lead to lighter wing structures with the consequent benefits to aircraft performance. A reduction in buffet levels experienced by flaps would be executed as illustrated in Fig 17.

Additionally devices according to this invention could be used in diffusers, those diffusers being located for example in gas turbine engines.

There are of course numerous other applications where it is desirable to delay the onset of flow separation, and to reduce the energy losses resulting from flow separation. The present invention is particularly useful at transonic airspeeds where the degree of unsteadiness may be high and the pressure differences large.

Clearly the tubes connecting the buffet breather orifices need not be rigid, straight or of constant section although it is desirable that they are not unduly twisted or contorted and that the size does not alter so radically that the internal configuration of the tube itself might cause a flow constriction and thereby decrease the effectiveness of the breather system.

Claims

1. A buffet breather for a body locatable in a fluid flow at a position where time mean pressures are different at adjacent points comprising a plurality of breather orifices located on an outer surface of the body, the breather orifices located in the region of the fluid flow separation point, the breathers connectable by connecting means below the surface of the body, the connecting means permitting fluid to flow between the breather orifices.

2. A buffet breather according to claim 1 wherein the means connecting the breather orifices is a plenum.

3. A buffet breather according to claim 1 wherein the means connecting the breather orifices is a breather tube.

4. A buffet breather according to claim 1 having at least one breather orifice located upstream of a flow separation point and a second breather orifice located downstream of the flow separation point.

5. A buffet breather according to claim 4 and having a downstream facing breather orifice located upstream of the flow separation point and an upstream facing breather located downstream of the flow separation point.

6. A buffet breather according to any preceding claim in which the breather orifices are inclined at about 60 to the normal of the surface.

7. A buffet breather system according to any preceding claim having a plurality of downstream facing breather orifices aligned in a substantially streamwise direction, each of the breather orifices connected to one of a corresponding upstream facing breather orifices located downstream of the downstream facing orifice and flow separation point.

8. A buffet breather system according to claim 1 having a plurality of upstream facing breather orifices aligned in a substantially streamwise direction, the breather orifices being connected in pairs by connecting tubes.

9. A buffet breather according to claim 8 in which the breathers are inclined in a chordwise plane at about 60 to the normal to the surface.

10. A buffet breather system according to claim 1 wherein the breather orifices are inclined in a chordwise plane at about 60 to the normal in an upstream facing direction.

11. A buffet breather system according to any preceding claim and wherein each set of orifices is provided with independently operable closure means, the valve means operable to seal the breather system.

12. A buffet breather according to any preceding claim having detector means for detecting the point of separation of the flow, computation means for determining the optimum pairs of breathers for buffet alleviation under particular flow conditions, closure means for closing off the breather orifices not needed for stabilizing the fluid flow.

13. A buffet breather system substantially as described in the accompanying specification with reference to the drawings.