IMPROVEMENTS IN OR RELATING TO REDUCTION OF DRAG

Abstract: Drag reducing riblets, which may be grooves (7) or formations of other shape, are produced in various ways. A coating (3) of a thermo-plastic material is first applied to the surface (4) and the coating is then deformed by the application of pressure to a mould (25; 25<1>, 25<11>) with heat. The mould may be a roller (11) or of generally planar form. In order to apply pressure to the mould an inflatable bag (32) (Figure 5) may be positioned between the mould (25<1>, 25<11>) and a rigid reaction member (34, 33), or the mould (25) may be covered with an impervious cover sheet (42) (Figure 6) to define a vacuum chamber which is then evacuated, a porous breather sheet (39) being provided beneath the cover sheet (42). A further method (Figure 7) employs chemical milling.

Description: IMPROVEMENTS IN OR RELATING TO REDUCTION OF DRAG This invention relates to the reduction of drag caused by relative flow between fluids and bodies. Such bodies may comprise, but are not confined to, marine craft, aircraft, motor vehicles, air compressor blades of jet engines, windmill blades and the interiors of fluid conducting pipes. Problems arise in reducing drag caused by relative flow between fluids and bodies. One proposed solution to the reduction of drag caused by relative flow between fluids and bodies comprises applying to the bodies, adhesive-backed material having a patterned surface defining a multiplicity of minute grooves aligned with the path of fluid flow relative to the bodies and operable so as to reduce drag forces of the turbulent boundary layer. See WO 84/03867 and the article entitled "A Smoother Mover", page 51 of the publication "Yachting World" dated June 1987. Such sheet material has been produced by extrusion. Such micro-grooves have been termed 'riblets'. Other shapes of drag reducing surface irregularities have been proposed, such as a distribution of hemi-spherical or spherical projections, regularly or randomly distributed over the body surface. The term 'riblets' used hereinafter should be understood to include not only micro-grooves but where appropriate any small scale surface irregularity in the nature of a plurality of projections and/or recesses in the surface and which are capable of reducing drag, at least for a range of Reynolds numbers. The present application is concerned with various improvements in the production of drag reducing riblets. According to one aspect of the present invention, riblets as hereinbefore defined are formed directly on the surface of a body by the steps of first applying a deformable coating to the surface and then deforming the coating so as to produce the riblets. This process overcomes a problem which can arise with adhesive- backed sheet material, that the material can sometimes begin to peel thereby increasing drag. The coating material may be thermoplastic. Heat and pressure may then be applied to the material so as to deform it. Alternatively, the coating material may be curable by exposure to radiation, preferably ultra-violet radiation. Heat and pressure may be applied by a heated roller or by a heated mould. An inflatable bag can be positioned between a rigid, stationary reaction member and the mould, and the bag inflated to apply a super-atmospheric pressure to the mould. Alternatively super-atmospheric pressure can be applied in a heated chamber. Instead of applying super-atmospheric pressure, a similar effect can conveniently be achieved by evacuating the region of contact between the mould and the surface. Preferably the mould is positioned on the surface, and an impervious cover is positioned over the mould to define a vacuum chamber, sealing means being provided to seal the margin of the cover to the surface around the mould, and a vacuum connection is provided to the chamber for connecting the chamber to a vacuum source to subject the mould to vacuum. The cover is preferably a sheet of flexible material and preferably a porous breather sheet is positioned between the flexible cover material and the mould. A heated blanket may then be positioned against the flexible cover material. According to a second aspect of the invention we provide a method of forming riblets on the surface of a body by chemical milling, the method comprising the steps of providing a mask with the desired riblet pattern but having a riblet spacing which is a multiple of the desired riblet spacing, using the mask to expose portions of a photosensitive resist layer applied to the surface, removing the exposed portions of the resist, applying an etching solution to the resist-covered surface to form a first plurality of riblets in the surface, providing a new photosensitive resist layer, and then applying the mask again to the resist but in a position displaced from the original position, the steps of exposing the resist layer, removing the exposed resist and applying the etching solution being repeated to produce a second plurality of riblets in the surface phase - displaced from the first plurality of riblets. According to a third aspect of the invention, a method of forming riblets on deformable flexible sheet material comprises embossing the material by bringing it into moving pressurised contact with a suitably patterned surface defined by a moving strip. The flexible sheet material is preferably a thermoplastic material such as PVC (Polyvinyl chloride). According to a fourth aspect of the present invention, flexible sheet material defining elongate riblets is manufactured by attaching a plurality of suitably shaped members of filamentary form in parallel array , to a backing whereby the filamentary members form the required elongate riblets. The filamentary members may be first wound onto a former defining a cylindrical surface, the backing attached to the members, and both the filamentary members and the backing then removed from the former. According to a fifth aspect of the invention, flexible sheet material defining riblets is manufactured by joining together, by weaving, a plurality of suitably shaped members of filamentary form disposed in substantially parallel array. The invention also comprises bodies and/or flexible material bearing riblets formed according to any of the above-defined methods. The invention further comprises apparatus for performing any of the above-defined methods. The various aspects of the invention will now be further described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a fragmentary end view, in section, of the body of a motor vehicle which bears a coating defining riblets; Figure 2 is a side view of a roller device; Figure 3 is a front view, to an enlarged scale, of the roller of the device; Figure 4 is a view similar to that shown by Figure 1 and illustrates a modification; Figure 5 shows a technique for subjecting tooling sheets to super-atmospheric pressure using inflatable bags; Figure 6 shows an arrangement for subjecting the tooling sheet to vacuum and the use of a heated blanket; Figure 7 shows the various stages of a chemical milling technique; Figure 8 illustrates how riblets may be produced by rotary embossing apparatus; and Figure 9 illustrates how riblets may be produced by use of a former. Figure 1 illustrates a part 1 of the metal body 2 of a motor vehicle. The body part 1 illustrated is a portion of one side of the vehicle. A coating 3 of thermoplastic paint has been applied to the outer surface 4 of the metallic body part 1 and subsequently deformed to produce a plurality of drag-reducing riblets 5 disposed in parallel array. The riblets 5 comprise a series of minute grooves with parallel peaks 6 and valleys 7. The riblets 5 extend lengthwise along the path of air flowing past the vehicle body, which path is normal to the plane of Figure 1. The coating 3 of thermoplastic paint is best applied to the body part 1 by a spray gun. A suitable paint is Type F190-0494 acrylic lacquer available from ICI Paints Division, Slough, Berkshire, or Type Z-4381, Aultragem single pack lacquer available from Ault and Wiborg Paints, Chadwell Heath, Dagenham. Figure 2 illustrates a roller device 10 whereby heat and pressure is applied to the thermoplastic coating 3 to form the riblets 5. The roller device 10 comprises a roller 11 formed by a metal cylinder 12 (Figure 3) with a wire 13 wound around the surface of the cylinder and secured thereto. The roller 11 is rotatable, about an axis 14, within a cowling 15. The cowling 15 is connected to the outlet nozzle 16 of a variable temperature hot air gun 17, as used for paint removal. The cowling 15 defines a small longitudinal gap 18 with the roller 11, which gap provides a restricted hot air exit. In operation, the hot air gun 17 is used to heat the roller 11 and the wire 13 thereon, and the device 10 then employed to move the roller along the coating 3 while applying pressure thereto. The heated roller 11 softens the coating 3, allowing the wire 13 of the roller to enter the coating and deform it so as to form the riblets 5. When the riblets are grooves they may be regularly or randomly distributed. They may, for example, be of triangular or semi-circular transverse cross-section. The temperature of the paint coating 3 during the formation of the riblets 5 is preferably 60° to 80° C. In a non-illustrated modification, the roller 11 is driven. It will be appreciated that the roller device 10 has been described herein for illustrative purposes only. Obviously, the device 10 would be replaced by larger, purpose-built equipment for full-scale production of riblets on vehicles produced in a factory. The principle of another method of producing riblets is illustrated schematically in Figure 4, wherein a heated mould 25 is pressed against a coating 3a of thermoplastic paint, as indicated by arrows 27. The working surface 26 of the mould is formed with a suitable pattern whereby the riblets 5a are produced. Some form of release agent or release layer may be required to assist removal of the mould 25 from coating 3a. A practical implementation of the method of Figure 4 for applying riblets to a thermoplastic paint-coated aerofoil member 29 is illustrated in Figure 5. The purpose of this process is to create riblets on the external surfaces of large structures such as aircraft wings and fuselages. It is intended to be the final production process in the manufacture of these finished surfaces, with no further painting or finishing being necessary. The surface of the body 29 to which the process is to be applied, is first thoroughly cleaned and degreased. It is then either sprayed with a suitable thermoplastic paint or covered with an appropriate pre-preg sheet. I II Special, custom designed, "tooling" sheets 25 and 25 containing the negative form of the desired riblet profile are then carefully positioned on top of and/or underneath the body. Great care is taken to ensure the correct orientation of the riblet grooves. The tooling sheet is so constructed that the alignment of the riblets is appropriate to the aerodynamic or hydrodynamic characteristics of the body, and incorporates datum locations to mate with appropriate features in the body itself. Usually the riblets on a given tooling sheet will not all be parallel because of the 3D characteristics of the flow and the surfaces. The tooling sheet 25 , 25 can be produced in an appropriate grade of silicon rubber for the application, and incorporates a heating element 30 and thermocouples 31 in its construction. The heating elements are close to the working surface of the tool which is in contact with the body 29. A respective inflatable pressure bag 32 -is put in position covering the tooling sheet(s). In general there will be a bag 32 on either side of the body 29 even when no tool is present on one side; this is to ensure that the forces applied to the body are balanced. This complete assembly is then positioned between reaction frames 33, 34. As illustrated, there is an upper frame 33 and a lower frame 34. These frames will be connected together by structural members so that the forces on each frame react against one another. Pressure connections 35 and air supply lines 36 are shown in the diagram. The complete assembly is pressurised and heater supplies 37 are energised. The body 29 then undergoes a carefully controlled thermal cycle at a predetermined temperature and time. The temperature and the time depend on the surface coating materials which have been applied to the body. The thermocouples 31 incorporated in the tools 25 , 25 are used both to monitor and control the temperature during this riblet forming process. A typical height and width of riblet grooves applied to commercial aircraft (eg Boeing 747 or 767) is about 50 um. A vacuum forming technique for producing riblets will now be described with reference to Figure 6. The surface 1 of the body 1 to be treated is painted as previously described. After the paint is fully cured a sheet tool 25 containing the negative form of the desired riblet profile is then carefully applied to the surface 1 , and is held in place temporarily, eg with adhesive tape. This tool 25 is so constructed that the alignment of the riblets is appropriate to the aerodynamic or hydrodynamic characteristics of the body. In many cases this means that the riblets on a given tool will not all be parallel. A release sheet 38 of halar is preferably used to cover the area of the tooling sheet 25 and ensure that a porous breather sheet 39 does not stick to sheet 25. Both the release 38 and breather 39 sheets are very slightly larger than the tooling sheet 25 itself. A strip of adhesive tape 40 is applied around the perimeter of these sheets. This is to protect the thermoplastic coating from damage caused by the removal of sealing putty 41, which is applied on top of the tape 40. The complete assembly is covered with a sheet of capran 42 which is sealed against the putty 41. Vacuum fittings 43 are incorporated in one or more positions, depending on the size of the assembly, and the whole assembly is subjected to a vacuum. After a satisfactory vacuum has been achieved, the assembly is covered with a heater blanket 43 and subjected to a carefully controlled thermal cycle at a controlled temperature for fixed time. The temperature and the time depend on the thermoplastic material being used. Thermocouples 44 are attached to the outer surface of the capran sheet 42 both to monitor and control the temperature. The construction of the heater blanket 43 is also shown in Figure 6. A woven copper heating element 45 is protected on both sides by glass cloth 46. Insulation 47 on one side helps to retain the heat during the process. The outer casing 48 of the blanket is made from silicon rubber in order to withstand the heat and keep out moisture. Electrical connections 49, 50 respectively from the blanket 43 and the thermocouples 44 are both connected to the power supply/ temperature controller for the blanket. Instead of using the heated blanket 43 it will be possible for some articles, such as external surface panels for vehicles, to be placed in an oven or autoclave to provide the necessary thermal cycle. In that case the tooling sheet 25, halar release sheet 38, breather sheet 39 and capran sheet 42 are assembled as shown in Figure 6, and that assembly after evacuation is inserted into the oven or autoclave where the assembly is subjected to a carefully controlled thermal cycle. The temperatures and times depend on the materials used. The thermocouples 44 are connected into the oven/autoclave control system, for monitoring and controlling the temperature during the heat curing process. A process for chemical milling of riblets, in the form of recesses, will now be described with reference to Figure 7. This process can be used for chemical milling of semi-circular section riblet grooves directly into metal surfaces. These surfaces may be part of the body which is in the fluid flow itself, or a thin laminate sheet which will subsequently be attached to the body during part of the manufacturing process. The process of Figure 7 employs the normal techniques of chemical milling but with the addition of a novel indexing arrangement to achieve the higher resolution required to give very closely spaced riblet grooves. In Figure 7 the four stages of the chemical milling process are shown. Stage 1. The substrate material 1 is coated with an appropriate etch resistant polymer layer 51. A photographically produced mask 52 is then used to expose this photosensitive resist layer to the desired riblet pattern. This pattern has grooves which are spaced apart by 2 or 3 times the desired spacing for the finished surface. T e exposed portion of the resist is illustrated as 53 in the Stage 1 diagram. The accurate positioning of the mask is critical to the proper formation of the riblet grooves and a suitable jig or fixture is required to control the relative positions of the two. Stage 2. The areas of the etch resistant coating which have been exposed to ultra-violet light by the mask are fixed by the cross-linking of polymer and the unexposed areas washed away. The sheet is then immersed in acid or sprayed with an appropriate etchant which etches away the metal where it is not protected by the etch resistant coating. Controlling the time and strength of the etchant determines the depth and width of the grooves 54. Stage 3. After removing the original resist layer and applying a fresh resist layer 51, the whole of Stage 1 of the process is then repeated, but with the mask 52 displaced by one groove spacing. As noted above a suitable jig with micrometer adjustment or ratchet mechanism is required to ensure that the displacement of the mask can be controlled with sufficient accuracy. In this way the grooves 54 produced during the first etching are protecting during the second and subsequent etching processes. Stage 4. Stage 2 is then repeated to remove the etch resistant layer where the grooves are to be formed, followed by etching to produce the further grooves 55 themselves. The number of times which the process is repeated depends on the size and spacing of the riblet grooves to be produced for the specific application. An alternative method of forming riblets directly on a body comprises a coating which is sensitive to and curable by ultra¬ violet light radiation. The riblets are first produced in an uncured coating by any of the techniques described above, and the deformed coating subsequently cured by exposing it to the radiation, which may conveniently be shone through a suitably transparent mould or roller. A suitable material which is sensitive to and curable by ultra¬ violet radiation is UV varnish, available from the aforesaid ICI Paints Division. The body parts 1 need not in general be metallic. They may, for example, by GRP (glass-reinforced plastics) material. Figures 8 and 9 illustrate methods of forming riblets on deformable and conformable flexible sheet material. Such material per se has application, for example, as yacht sails and coverings for loads carried by road and other vehicles. The flexible sheet material may also be applied to a body, using adhesive. The adhesive may be in the form of a backing on the flexible sheet material. With reference to Figure 8, rotary embossing apparatus 60 comprises a pair of rolls 61, 62 through the nip of which pass, in moving contact with each other, a strip 63 of paper and a strip 64 of PVC (Polyvinyl chloride). The ends of the paper strip 63 are mounted on rollers 65, 66 and the ends of the PVC strip 64 are mounted on rollers 67, 68. Rollers 65 and 67 are supply rollers. Rollers 66 and 68 are take-up rollers. Paper strip 63 is a master or casting strip, embossed with a suitable pattern, eg a plurality of grooves comprising a mirror image of the desired riblets. The rolls 61, 62 are heated and the apparatus 60 is set up and operated so that as the strips 63, 64 pass through the nip of the rolls 61, 62, controlled heat and pressure are applied to the PVC strip whereby it is deformed so as to define the desired riblets. The PVC strip may be replaced by composite strip material wherein only the surface contacted by the patterned paper strip 63 is of thermoplastic material, such as PVC. All or only some of the rolls 61, 62 and rollers 65, 66, 67, 68 may be driven. With reference to Figure 9, another method of manufacturing sheet material defining riblets employs a mandrel 75 around the cylindrical surface of which is wound filamentary members 76 of triangular or cusped semi-circular cross-section. The filamentary members 76, which are disposed in parallel array, are subsequently covered by a backing 77, the members and the backing being attached to each other by adhesive. The backing 77 is dimensioned so that its ends meet to form a tube enclosing the members. The tube and members beneath are then cut along a line 78 parallel to the axis 79 of the mandrel 75, and the material peeled off the mandrel. The contiguous filamentary members 76 form the required riblets 80. Flexible sheet material in the form of a mat (which may carry an adhesive backing) may also be produced by weaving together filamentary members of suitable cross-section disposed in parallel array. The resulting material is similar to that produced using the filament to backing technique described above with reference to Figure 9, but with cross-stitching holding the filaments in place instead of adhesive.

Claims: C A I M S 1. A method of forming riblets (5), as hereinbefore defined, on the surface (4) of a body (1), characterised by the steps of first applying a deformable coating (3) to the surface and then deforming the coating so as to produce the riblets. 2. The method claimed in Claim 1, wherein the coating material is thermoplastic and heat and pressure are subsequently applied to the coating so as to deform it. 3. The method claimed in Claim 2, wherein heat and pressure are applied by a heated roller (11). 4. The method claimed in Claim 2, wherein heat and pressure are applied by a heated mould (25 ). 5. The method claimed in Claim 4 wherein an inflatable bag (32) is positioned between a rigid, stationary reaction member (33, 34) and the mould, and the bag is inflated to apply a super-atmospheric pressure to the mould. 6. The method claimed in Claim 2, wherein heat and pressure are applied in a chamber. 7. The method claimed in any one of Claims 2, 4 and 6, wherein the mould is subjected to vacuum. 8. The method claimed in Claim 7 wherein the mould is positioned on the surface, and an impervious cover (42) is positioned over the mould (25) to define a vacuum chamber, sealing means (40, 41) being provided to seal the margin of the cover to the surface (1 ) around the mould (25) , and a vacuum connection (43 ) being provided to the chamber for connecting the chamber to a vacuum source to subject the mould to vacuum. 9. The method claimed in Claim 8 wherein the cover is a sheet of flexible material (42). 10. The method claimed in Claim 9 wherein a porous breather sheet (39) is positioned between the flexible cover material and the mould. 11. The method claimed in Claim 10 wherein a release sheet (38) is sandwiched between the breather sheet (39) and the mould. 12. The method claimed in any one of Claims 8 to 11 in which a heated blanket (43) is positioned against the flexible cover material (42). 13. The method claimed in Claim 1, wherein the coating material is sensitive to and is curable by exposure to ultra-violet radiation. 14. A method of forming riblets (54, 55), as hereinbefore defined, on the surface of a body (1) characterised by the use of chemical milling, the method comprising the steps of providing a mask (52) with the desired riblet pattern but having a riblet spacing which is a multiple of the desired riblet spacing, using the mask to expose portions of a photosensitive resist layer (51) applied to the surface, removing the exposed portions of the resist, applying an etching solution to the resist-covered surface to form a first plurality of riblets (54) in the surface, providing a new photosensitive resist layer, and then applying the mask again to the resist but in a position displaced from the original position, the steps of exposing the resist layer, removing the exposed resist and applying the etching solution being repeated to produce a second plurality of riblets (55) in the surface phase - displaced from the first plurality of riblets (54). 15. A body formed with riblets produced by the method as claimed in any one of Claims 1 to 14. 16. A method of forming riblets on deformable sheet material , (64) characterised by embossing the material by bringing it into moving pressurised contact with a suitably patterned surface defined by a moving strip (63) . 17. The method claimed in Claim 16, wherein at least the surface of the deformable sheet material (64) contacted by the patterned surface is thermoplastic. 18. The method claimed in Claim 16 or 17, wherein the deformable material is Polyvinyl chloride. 19. A method of manufacturing flexible sheet material defining riblets characterised by attaching a plurality of suitably shaped members (76) of filamentary form disposed in parallel array to a backing, whereby the filamentary members form the required riblets. 20. The method claimed in Claim 19, wherein the filamentary members are first wound on to a former (75) defining a cylindrical surface, and both the filamentary members and the backing then removed from the former. 21. A method of manufacturing flexible sheet material characterised by joining together a plurality of suitably shaped members of filamentary form disposed in substantially parallel array, by weaving. 22. Flexible sheet material manufactured by the method claimed in any one of Claims 16 to 21. 23. Flexible sheet material as claimed in Claim 22 provided with an adhesive backing.

Inventors: CHOI KWING-SO, CHOI, KWING-SO
Applicants: BMT FLUID MECHANICS LIMITED
Application year: 1989
Main classification: B05D3
IPC codes: B05D1, B05D3, B05D5, B29C59, B64C21, F15D1