DRAG REDUCTION ARTICLE

Abstract: A conformable drag reduction article is provided having a patterned surface capable of reducing drag resistance by fluid flowing thereover.

Description: This invention relates to the reduction of drag caused by fluids (e.g., air, gas, water, etc.) flowing across surfaces and to articles which can be applied to surfaces to reduce such drag. It has long been desired to reduce drag caused by the passage of bodies through fluid media. Relatively small reductions in drag can significantly reduce the fuel needed to propel a body. For example, it has been estimated that a one percent reduction in drag across the leading edge of a wing of a Boeing 727 airliner could reduce fuel consumption by more than 20,000 gallons per airplane per year. See Automotive Engineering , February, 1982, pp. 73. The reduction of drag across surfaces of a body also is of benefit in other applications. By way of example, it is desirable to reduce the drag caused by water flowing past the hull of a boat, air flowing past a moving automobile or air flowing past the blades of a windmill fan, airfoil, fan, rotor, stator, inlet, etc. Many other examples are of course known as will be appreciated by those in the art. However, as yet there has not been provided a practical solution to the problem of reducing drag. Many techniques have been proposed which involve mechanically altering flow control surfaces. For example, the utilization of various devices to direct air into ducts that exit at the trailing edges of the flow control surface has been suggested. See, for example, United States Patents 2,742,247; 2,925,231; 3,117,751; 3,521,837; 4,114,836; 4,258,889; and 4,296,899. U.S. Patent 4,434,957 suggests the use of a corrugated control surface. The corrugations extend transversely to the direction of the fluid flow, temporarily retain vortices formed in the fluid flow on the flow control surface, and aid in regulating their passage across the surface. U.S. Patent 4,455,045 proposes the use of one or more 3-sided submerged channels in the flow control surface. Each channel includes two divergent walls which form a generally V-shaped ramp which is sloped downward so that the channel widens and deepens toward the downstream flow of the fluid. Such channels are V-shaped in a plane generally parallel to the flow control surface. They are intricate and are most effective when provided in a serial cascade wherein the last channel in the cascade ends at the trailing edge of the flow control surface. These techniques are expensive, time consuming to employ and do not address the problem of how to reduce drag across the surfaces of existing equipment (e.g., airplanes, automobiles, etc.) in a practical way. The use of smooth surface coatings on airplane skins has also been suggested. See Automotive Engineering , February, 1982, pp. 73-78. However, this article reported that liquid polymeric coatings and adhesively backed films, applied to the flow control surfaces in order to maintain a smooth, protected surface for drag reduction, performed poorly and were unsuitable for areas of high erosion such as wing and tail leading edges and nacelle inlets. In "Turbulent Drag Reduction for External Flows" AIAA 21st Aerospace Sciences Meeting, Jan 10-13, 1983, Reno, Nevada, USA there is disclosed a drag-reduction article comprising a series of parallel peaks separated from one another by a series of parallel valleys wherein said patterned surface reduces drag resistance when in contact with a fluid flowing thereon. US-A-4,414,316 discloses a method of applying a pattern to a heat and radiation curable composition in which the composition is applied to a pattern and cured but only in the context of a lenticular structure. The present invention as defined in independent claim 1 provides a practical technique for significantly reducing drag across flow control surfaces. It comprises a conformable sheet material which employs a patterned first surface to significantly reduce drag caused by fluid flowing over the surface of a body to which the sheet has been applied. As used herein, conformable means that the article assumes the shape of the surface to which it is applied without cracking or breaking and without the formation of any air bubbles or wrinkles. Additionally, it does not release from the surface even when it is under extreme stress, such as elongation and compression, once it has been applied. For example, the article of the invention will conform exactly to curved surfaces and riveted surfaces such as are encountered on the skin of an airplane. In a preferred embodiment of the invention, the article is capable of withstanding extended exposure to water, oil, hydraulic fluids and the like without noticeable deterioration in its physical properties or appearance. The article of the invention is convenient to use and provides a simple yet versatile technique for reducing drag across flow control surfaces. It can be applied to the flow control surfaces of existing equipment or it can be applied to flow control surfaces of new equipment at the time of manufacture. In either case the reduction in drag across the surface of such equipment is virtually identical. The reduction of drag achieved by the article of the invention varies somewhat depending upon the body to which it is applied and the conditions at which the body operates. However, reductions in drag of up to 10% or more can be achieved with the present invention. Figure 1 is a perspective view of one embodiment of the invention. Figures 2 and 3 are cross sectional views of two alternative embodiments of the invention. Figures 4-8 represent greatly enlarged cross sectional views of alternative patterned surfaces useful in the invention. Figure 9 represents a cross sectional view of the article of the invention bearing graphic indicia. The article of the invention is characterized by a number of tests. For example, the article has a tensile strength of greater than 200 kg/cm² when measured according to ASTM D 2370. The conformability of the article is demonstrated by subjecting the article to flexibility tests MIL-P-38477A and MIL-M-43719B. Specifically, the article will show no change after being subjected to the following tests (a) folding the article 20 times at room temperature (e.g., 20 o C); (b) dropping a 2.3 kg steel ball onto the article from a height of 3 meters at a temperature of -29 o C; and (c) bending the article around a 2.5 cm mandrel at temperatures of from -54 o C to 121 o C. Preferred embodiments also exhibit resistance to a number of solvents. Thus, when tested according to MIL-M-43719B the preferred embodiment shows no change after one hour in jet fuel. When tested according to MIL-P-38477A the preferred embodiment shows no change after either 2 hours at 107 o C in hot oil or 240 hours in salt spray. When tested according to BMS-10-30A, the preferred embodiment of the invention will show no change after 72 hours in TT-S-735 hydrocarbon fluid. Additionally, the preferred embodiment will show less than 0.15 cm edge lift after 72 hours in MIL-L-7808 lubricating oil and less than 0.3 cm edge lift after 24 hours in BMS-3-11. The article of the invention is typically a film (i.e., it has width to thickness ratio of at least 5:1). Generally it is from 50 to 650 micrometers thick. Preferably it is from 75 to 160 micrometers thick when used in high speed applications (e.g., aircraft) and from 150 to 500 micrometers thick when used in low speed applications (e.g., automobiles). Preferably the films have a weight of less than 300 g/m² and most preferably a weight of less than 150 g/m². Materials which may be employed to provide the patterned layer are heat or radiation curable materials which generally comprise a heat or radiation curable monomer or prepolymer and an initiator. The monomer or prepolymer generally contains at least two groups capable of reacting to form a polymer. A variety of such materials are known and include materials having functional ethylenically unsaturated groups (e.g., vinyl groups, acrylate and methacrylate groups, etc.), and polyurethanes (e.g., polyester and polyether urethanes). Plasticizers, extenders, antioxidants, ultraviolet light stabilizers and the like may be utilized in the article of the invention. Plasticizers (sometimes referred to as flexibilizers) are particularly helpful when the material used must be modified in order to meet the flexibility requirements of the invention. Materials useful for these purposes are known. Typically they each comprise up to 5 parts by weight of the article. While the exact material used to provide the article of the invention is not critical, it is noted that certain materials will be better suited to some applications than others. For example, thermoset flexible materials will be well suited in those uses where high temperatures will be encountered. Water-resistant (e.g., hydrolytically inert) materials will be better suited to use in water environments than water-sensitive materials. The article of the invention may be inherently adhesive, that is it need not have a separate layer of adhesive applied to it. Such articles may be either passively adhesive or actively adhesive. In the former case the adhesive may be activated by solvents, heat, pressure, or the like prior to application of the article to a substrate. In the latter case, such activation would not be necessary. In the event that a separate layer of adhesive is employed, it is noted that the adhesive may be selected from a wide variety of materials such as heat activated adhesives, solvent (organic or inorganic) activated adhesives, or pressure sensitive adhesives. These adhesives preferably are compatible with the carrier to which they are applied and are resistant to water, oil, hydraulic fluids and the like. Furthermore, the separate adhesive layer preferably does not separate from the carrier during use. Pressure-sensitive adhesives represent a preferred adhesive for use as a separate layer. Examples of useful pressure-sensitive adhesives include polyacrylate adhesives, natural rubber adhesives and thermoplastic rubber adhesives. Polyacrylate adhesives comprise a polymer of an acrylate ester of acrylic acid with a non-tertiary alcohol. These adhesives may also contain a minor amount of a copolymerized acid or amide. Adhesives of this type and methods for their preparation are known. See U.S. Pat. No. Re. 24,906. The polyacrylate pressure-sensitive adhesives may be crosslinked if desired. Techniques for accomplishing this are known and described, for example, in U.S. Pat. No. 2,925,174. Useful natural rubber adhesives comprise natural rubber and, preferably, a tackifying resin. One such adhesive comprises natural pale crepe rubber (100 parts by weight), polyterpene resin (75 parts by weight), and antioxidant (1 part by weight). However, other useful natural rubber adhesives are also useful and will be apparent to those skilled in the art. Useful thermoplastic rubbery adhesives comprise a rubbery block copolymer and, preferably, at least one resin compatible with the block copolymer. The rubbery copolymers have the general configuration A-B-A wherein the A units represent a thermoplastic polymer block with a T g above 20 o C and the B units represent an elastomeric polymer block formed from a conjugated diene. The A units are relatively incompatible with the B units and have an average molecular weight of from about 5,000 to 125,000. Useful A units are styrene and useful B units are polybutadiene or polyisoprene. An example of block copolymer of this type is Shell Chemical Company Kraton® 1101, a block copolymer of styrene and butadiene having one butadiene central block of 70,000 molecular weight and two terminal blocks of styrene of 15,000 molecular weight each. Other block copolymers of this type may also be used. Resins compatible with the block copolymer are known and include, for example, hydrecarbon resins, coumarone-indene resins, dimerized rosins, metal resinates, hydrogenated rosins, poly-terpene resins and heat treated wood rosins. Still other resins are also useful and will be obvious to those skilled in the art. The thickness of the adhesive layer is not critical to the present invention. Typically, the adhesive layer is from 10 to 50 micrometers thick. Whether the article of the invention is inherently adhesive or uses a separate adhesive layer, it preferably adheres over the entire range of conditions normally encountered. Most preferably it exhibits a peel adhesion of at least 0.5 kg/cm width. The article of the invention may be readily applied to a variety of substrates. Preferably the article is positioned on the substrate such that the patterned surface will provide maximum drag reduction. Maximum drag reduction is achieved when the peaks and valleys are generally parallel to the fluid flow. After being positioned, the article may be stretched, if necessary, to conform to the substrate surface and to remove unwanted wrinkles and air bubbles. The film is then adhesively secured to the surface. Several embodiments of the invention are shown in the drawings. FIG. 1 illustrates an article of the invention comprising a sheet material 10 having a patterned first surface 12. Sheet material 10 may be inherently adhesive or, alternatively, it may have an adhesive second layer 14 applied to the surface of the sheet opposite patterned surface 12 as shown in FIG. 2. In yet another embodiment, shown in FIG. 3, the sheet material 10 comprises a carrier 16 and layer 18 bearing the patterned first surface 12. Adhesive second layer 14 may be provided on the surface of carrier 16 which is opposite the layer 18. The patterned surface 12 shown in FIGS. 1-3 and 9 comprises a series of parallel peaks 20 and valleys 22. The patterned surfaces 12 shown in FIGS. 1-3 have a symmetric, saw tooth cross section. FIGS. 4-8 illustrate a number of alternative patterned surfaces that may be used in the present invention. In all of these embodiments, the patterned surfaces comprise a series of essentially parallel valleys separated by a series of essentially parallel peaks. As can be seen from FIGS. 1-9, the cross section of the patterned layer can have a variety of wave forms. For example, the embodiments of FIGS. 1, 2 and 3 have a symmetric saw tooth cross section comprising peaks 20 and valleys 22. All of the peaks are identical as are all of the valleys. FIG. 4 shows a series of parallel, symmetric, peaks 24 equally separated by flat bottomed valleys 26. FIG. 5 shows a series of alternating, parallel peaks 28 and 30 separated by parallel valleys 22. Peaks 28 and 30 are parallel to one another but are of alternating heights. FIG. 6 shows a pattern of repeating, parallel, symmetric, rounded peaks 32. These peaks are separated by a series of parallel, symmetric valleys 34. FIG. 7 shows a series of parallel, symmetric, peaks 36 having concave side walls. These peaks are separated from one another by a series of parallel, round bottomed valleys 38. FiG. 8 shows a saw-tooth pattern of alternating, parallel, assymetric peaks 40 and assymetric valleys 42. While only continuous peaks and valleys are shown, a discontinuous pattern of peaks and valleys is also contemplated in this invention. Thus, for example, the peaks and valleys may terminate for a portion of the article. The valleys may either narrow or widen as the peak or valley progresses from one end of the article to the other. Still further, the height and/or width of a given peak or valley may change as the peak or valley progresses from one end of the article to the other. The dimensions of the peaks and valleys are not critical to the invention provided that whatever patterned surface is employed provides a reduction in drag. The optimum dimensions are somewhat dependent upon the speed at which the substrate to be covered passes through the fluid. However, it has been found that peaks of from 20 to 400 micrometers high are useful. Within this broad range it is preferred that the peaks be from 20 to 150 micrometers high for high speed uses (e.g., aircraft) and from 120 to 380 micrometers high for slow speed uses (e.g., automobiles). The peak to peak spacing between adjacent peaks likewise is not critical to the invention. It has been found that a spacing of from 20 to 400 micrometers is useful. Preferably the spacing is from 20 to 150 micrometers for high speed uses and from 120 to 380 micrometers for slow speed uses. The included angle between adjacent peaks can also vary within the scope of this invention. For example, while flat and round bottom valleys are useful, it is preferred that the valleys be generally V-shaped and have an included angle of from 15 o to 140 o (more preferably from 50 to 60 o ). The patterned surface 12 may be provided in-situ on the article of the invention (see FIGS. 1 and 2) or it may be applied thereto as layer 18 to carrier 16 (see FIG. 3). When layer 18 is employed to provide the patterned surface, the same or different materials may be used as are used for carrier 16. When selecting different materials for use as carrier 16 and layer 18 it is highly preferable that the two be compatible and either adhere together by themselves or be capable of being adhered together tenaciously. It is also preferred that the two materials have similar degrees of conformability so that stress at the interface between the two be minimized when the article is applied to a substrate. In constructions such as are shown in FIG. 3, the carrier is typically from 20 to 250 micrometers thick and the patterned layer is from 20 to 400 micrometers thick. The article of the invention may be prepared by several methods. A useful technique comprises applying a curable composition to a flexible carrier, contacting the radiation curable composition with a master bearing the negative structure of the desired pattern, and radiation curing the composition. The resulting structure may be removed from the master. While the foregoing description discusses coating techniques for applying a patterned layer to a carrier, it is also possible to employ laminating techniques and/or adhesives to bond the two together. Adhesive layers, when used, are applied to the surface of the article opposite the patterned surface. A variety of techniques, such as coating, spraying, or laminating, may be utilized to apply the adhesive layer. One useful method of applying the adhesive comprises contacting the surface of the carrier opposite the patterned surface to an adhesive carried on a release liner and passing the construction between rubber rolls. Either hot or cold lamination techniques can be employed. The article of the invention may be provided as an opaque, translucent or transparent item. Graphic designs 44 may be incorporated into the article, see FIG. 9, preferably between the carrier and the patterned layer. Alternatively, they may be placed on the patterned surface. When the article of the invention is inherently adhesive, the graphic designs 44 may be placed on either major surface. The article of the invention may be made breathable by, for example, perforating it. While the shape and type of the perforations are not of particular importance it is preferred that they be such that any adhesive used in the article does not flow together and seal the perforations. This can be achieved in many ways including, for example, by making the holes large enough to prevent such flow, and by deadening the adhesive around the walls of the perforations. Moreover, the perforations should not adversely affect the performance of the article, such as by causing it to separate from the substrate or by significantly adversely affecting its drag reduction capability. With these parameters in mind, it is preferred that the perforations have a maximum dimension of less than 200 micrometers and preferably one in the range of 10 to 100 micrometers. Laser drilling may be used to provide the perforations in the article. It has been found that the perforations provided by this technique are generally conical in shape and have a generally circular cross section. The size of the opening on the surface first struck by the laser is generally wider than that of the opposite surface. The size and shape of the opening is, of course, dependent upon the type of laser employed, the power of the laser, and the time of exposure to the laser beam. The present invention will be further described in the following examples wherein all "parts" are parts by weight unless otherwise indicated. These examples are provided by way of illustration of the invention and not by way of limitation. Example 1 A drag reduction film according to the invention was prepared by preparing a vinyl carrier from the following vinyl composition. COMPONENTS PARTS Polyvinyl chloride resin ("Diamond" 426 available from B. F. Goodrich) 100 Monomeric phthalate ("Santicizer" 711 available from Monsanto) 15 Monomeric phthalate ("Santicizer" 160 available from Monsanto) 4 Epoxy resin plasticizer ("Paraplex" G-62 available from Rohm and Haas) 4 Barrium/Cadmium/Zinc ("Ferro" 5649 available from Ferro Corp.) 0.5 UV light absorber ("Mark" 1413 available from Argus Chemical) 0.5 Phosphite stabilizer ("Ferro" 5227 available from Ferro Corp.) 0.5 Octadecanoic acid (available from Humko) 0.4 Inorganic silica ("Syloid" 244 available from Davidson Chemical) 1.0 Violet dye ("Violet" ZIRS available from American Cyanamid) 0.0073 The vinyl composition was coated onto a 75 micrometer thick polyester liner to a wet thickness of 100 micrometers and then passed through a forced air oven to dry it. The oven had five zones maintained respectively at 43 o C, 99 o C, 143 o C, 149 o C and 182 o C. A radiation curable composition having the following ingredients was roll coated onto the vinyl film to a thickness or 300 micrometers. COMPONENTS PARTS Urethane acrylate oligomer ("XP51"-85 available from Cargile) 67.9 Tetraethylene glycol diacrylate ("SR"-268 available from Sartomer Company) 19.2 Diethoxyacetophenone ("DEAP" available from Upjohn) 4.8 Fluorochemical surfactant ("FC"-431 available from Minnesota Mining and Manufacturing Company) 2.7 N-vinyl pyrrolidone (available from GAF) 4.9 UV light stabilizer ("Tinuvin" 770 available from Ciba Geigy) 0.5 The resulting construction was then laminated to an engraved surface using a laminator pressure of 3 kg/cm² and a laminator speed of 9 meters/minute. The radiation curable composition was exposed through the polyester and carrier to ultraviolet light in a Linde UV Processor from Union Carbide Corporation to cure it. Exposure conditions were: Processor length: 2.4 meters Processor speed: 9 meters/min. UV Exposure level: 2 passes at 1.45 J/cm² ea. Processor N₂ level: 220 ft³/hr. - ft. width The resulting construction was removed from the engraved surface revealing a patterned surface having a cross section similar to that of FIG. 2. The thickness of the patterned surface layer was 33 micrometers; the peak-to-peak distance between adjacent ridges was 33 micrometers; and the included angle was 53 o . The polyester liner was then removed and a pressure sensitive adhesive laminated to the side of the carrier opposite the patterned surface as follows:- A pressure sensitive adhesive was prepared as described in RE 24,906 using a blend of 90 parts by weight 2-methyl butyl acrylate and 10 parts by weight acrylic acid. The adhesive was applied to a silicone liner (SCW 106 from the General Electric Company) using a notched bar to a wet thickness of 125 micrometers at a coating speed of 51 meters/minute (m/min) and then dried in a forced air oven having four heated zones maintained respectively at 54 o C, 65 o C, 77 o C and 87 o C. The adhesive was then laminated at room temperature to the second surface of the patterned carrier. The film was applied to a flow control surface on an airplane. The film conformed exactly to the surface and provided a 6.5% reduction in drag across the surface. Example 2 A drag reduction film was prepared. The radiation curable composition of Example 1 was applied to a polyester film and cured as described in Example 1 to provide a grooved liner. The grooved surface of the liner was treated with a release coating and a polyurethane composition having the following ingredients was applied to the patterned surface of the liner. COMPONENT PARTS Polyester polyol (available from Mobay Chemical as "Desmophen" 670) 65.9 0.1 Second cellulose butyrate (available from Eastman Chemical as "CAB" 381-0.1) 2.4 Methyl isobutyl ketone 10.9 Polyglycol ether ester (Cellosolve acetate available from Dow Chemical Co.) 16.8 UV Stabilizer ("Irganox" 1010 available from Ciba Geigy) 1 UV Stabilizer ("Tinuvin" 328 available from Ciba Geigy) 1 UV Stabilizer ("Tinuvin" 770 available from Ciba Geigy) 2 Aliphatic isocyanate (Aliphatic isocyanate available from Mobay Chemical as "Desmodur" N-100) 36.3 Dibutyl Tin Dilaurate (1% by weight in Xylene) 0.4 The polyurethane composition was cured to a tack free film by sequentially heating it to 65 o C for one minute, 107 o C for one minute, 121 o C for one minute, and 163 o C for one minute. The polyurethane was then fully cured by exposing it to room temperature for 7 days. The film was then stripped from the patterned liner and a pressure sensitive adhesive applied as described in Example 1. The resulting drag reduction film could be applied to and conform to compound surfaces to reduce drag thereacross.

Claims: A drag reduction article comprising a sheet (10) having on one side a patterned surface (12) comprising a series of parallel peaks (20; 24; 28; 30; 32; 36; 40) separated from one another by a series of parallel valleys (22; 26; 34; 38) wherein said patterned surface reduces the drag resistance when in contact with a fluid flowing thereover, characterised in that the sheet (10) has a thickness of 40 to 650 µm, the patterned surface (12) consists of a layer of heat or radiation cured polymer and the sheet has a tensile strength greater than 200 kg/cm² and the conformability complies with the requirements that the sheet shows no change after being subjected to the following tests (a) folding the article 20 times at room temperature (e.g., 20 o C); (b) dropping a 2.3 kg steel ball onto the article from a height of 3 meters at a temperature of -29 o C; and (c) bending the article around a 2.5 cm mandrel at temperatures of from -54 o C to 121 o C. An article according to claim 1 wherein said polymeric sheet demonstrates no change after being placed in TT-3-735 hydrocarbon fluid for 72 hours according to test method BMS-10-30A. An article according to either of claims 1 and 2 wherein parallel peaks are 20-400µm high. An article according to any one of claims 1 to 3 characterised in that said article is a composite sheet comprising a conformable polymeric carrier sheet (16) having thereon a layer (18) of cured polymer whose outer surface is said patterned surface (12). An article according to any one of claims 1 to 4 wherein said cured polymeric sheet (10) comprises a thermoset polymer. An article according to claim 5 wherein said thermoset polymer comprises the reaction product of an isocyanate and a polyol. An article according to claim 6 wherein said polyol is selected from the group consisting of polyester polyols and polyether polyols. An article according to any preceding claim wherein said polymeric sheet (10) has an adhesive second surface. An article according to any preceding claim characterised in that it is perforated for breathing. An article according to any one of claims 1 to 9 wherein said thickness is from 75 µm to 160 µm. An article according to any one of claims 1 to 9 wherein said thickness is from 150 µm to 500 µm. An aircraft having a patterned drag-reduction surface (12) on one side of a sheet (10) attached to a flow control surface of the aircraft, said patterned surface comprising a series of parallel peaks (20; 24; 28; 30; 32; 36; 40) separated from one another by a series of parallel valleys (22; 26; 34; 38) wherein said patterned surface reduces the drag resistance experienced by the aircraft, characterised in that said sheet (10) is as defined in any of claims 1 to 11.

Inventors: MARENTIC, FRANCIS J. MINNESOTA MINING, MORRIS, TERRY L. MINNESOTA MINING
Applicants: MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1986
Main classification: B64C21
IPC codes: B05D5, F15D1, B62D35, B63B1, B64C21, B29C59