Jun TAO,Gng SUN,Gng WU,Liqing GUO,Yongjin ZHONG,Mng WANG,Bo YOU

aDepartment of Aeronautics&Astronautics,Fudan University,Shanghai 200433,China

bDepartment of Mechanical&Aerospace Engineering,University of California,Irvine 92697,USA

cDepartment of Material Science,Fudan University,Shanghai 200433,China

dAECC Commercial Aircraft Engine Co.,Ltd.,Shanghai 200241,China

eAVIC Aerodynamics Research Institute,Shenyang 110034,China

KEYWORDS Coating techniques;Drag reduction;Laminar flow;Surface energy;Wind tunnel test

Abstract Laminar flow design is one of the most effective ways to reduce the drag of a commercial aircraft by expanding the laminar flow region on the surface of the aircraft.As material science develops,the emergence of new materials such as low surface energy materials has offered new choices for laminar flow design of commercial aircraft.Different types of low surface energy micro-nano coatings are prepared to verify the effects on the boundary layer transition position and the drag of the airfoil through wind tunnel tests.The infrared thermal imaging technology is adopted for measuring the boundary layer transition,while the momentum integral approach is employed to measure the drag coefficient through a wake rake.Infrared thermal imaging results indicate that the coatings are capable of moving backward the boundary layer transition position at both a low velocity of Mach number 0.15 and a high velocity of Mach number 0.785.Results of the momentum integral approach demonstrate that the drag coefficients are reduced obviously within the cruising angle of attack range from 1°and 5°by introducing the low surface energy micro-nano coating technology.

1.Introduction

Low-drag design has always been one of the most effective ways to improve the economy and environment protection of a commercial aircraft.Taking a large air freighter or airliner as an example,about 50%of the total drag is the friction drag generated from the interaction between the surface of the aircraft and the air in the cruise phase.1The flow around an aircraft includes two states which are laminar flow and turbulent flow.Under the same status of Reynolds number,the drag generated by laminar flow is far less than that generated by turbulent flow.Hence,expanding the region of laminar flow on an aircraft's surface can reduce the drag,thus contributing to enhancing the economy and decreasing the carbon emission of a commercial aircraft.

Scholars and researchers from all over the world have conducted a large number of investigations on laminar flow design for an aircraft over the past few decades.2-13Methods for laminar flow design can be classified into three types which are Natural Laminar Flow (NLF),Laminar Flow Control(LFC),and Hybrid Laminar Flow Control(HLFC).14However,with the rapid development of material science,the emergence of new materials such as low surface energy materials has offered new options for laminar flow design.15-17Generally,a low surface energy material is painted in the form of a coating on the surface of an object to reduce the drag by expanding the laminar flow region.Due to the low surface energy of the coating,the interaction between the coating and the fluid is weak,and the fluid medium has a tendency of slipping from the surface,which will lead to a nonzero slip velocity.Since the velocity on the surface of the object is not equal to zero,the velocity gradient as well as the shear stress is reduced on the wall boundary,which will result in a reduction of the friction coefficient.Precisely because of the reduction of the velocity gradient,the laminar flow status becomes more stable,and thus the transition position from laminar flow to turbulent flow is moved backward.

Therefore,the low surface energy coating moves backward the transition position from laminar flow to turbulent flow,and reduces the friction coefficient between the fluid and the surface of the object,which will lead to a drag reduction.

As for laminar flow design based on low surface energy coatings,many investigations have been conducted all over the world.Watanabe et al.conducted experiments on flow along a circular tube and a square tube,and found that a low surface energy coating led to a drag reduction of over 14%.18Bechert et al.did a lot of research work on the flow along a plate,and obtained a drag reduction of 7.3%by applying a low surface energy coating.19Yu and Wei conducted experiments on a low surface energy coating,and results indicated that the drag reduction was due to a combined effect of the low surface energy and the microstructure.20Luo et al.utilized sharkskin as a template,and successfully gained a low surface energy coating based on the epoxy resin.21Their experimental results indicated a drag reducing rate of over 12%.Wang et al.prepared a type of attachable coating by means of hot extrusion briquetting,which was proven to be effective on drag reduction.22Nonetheless,up to now,most studies just duplicated the microstructures of the surface of sharkskin,but few explored selections of materials and preparation of more re fined multilevel structures.Furthermore,almost all of these studies were conducted with water as the fluid medium,but few were implemented with air as the fluid medium.

In this study,low surface energy coatings with multilevel micro-nano structures are prepared for the purpose of investigating the effect on moving backward the transition position from laminar flow to turbulent flow as well as the drag reduction of an airfoil in a flow field of air.Due to the micro-nano scale of the coatings,it is extremely difficult to evaluate the aerodynamic performance of the airfoil by numerical methods,so wind tunnel experiments are conducted to assess the effect of the coatings on the aerodynamic performance of the airfoil.

This paper is organized as follows.In Section 2,the preparation methods and process of the multilevel micro-nano coatings are introduced.In Section 3,the equipment and measurement methods of the wind tunnel experiments are presented.In Section 4,experimental results are analyzed and discussed.Finally,in Section 5,some conclusions are drawn based on the results and analyses.

2.Preparation methods and process of micro-nano coatings

In this study,low surface energy coatings with a three-level micro-nano structure are prepared.To be specific,the first-level structure is a groove structure of which the scale is from 10 μm to 100 μm,the second-level structure is a microsphere structure of which the scale is about 2 μm,and the third-level structure is a nano-sized particle structure attached to the microsphere.

Firstly,the second-and third-level structures are prepared and combined.Then,the combined second-third-level structure is sprayed to the first-level structure,namely the groove structure,to form the final three-level micro-nano structure.

The preparation methods and process of the combined second-third-level structure are as follows.

(1)Add Methyl MethAcrylate(MMA),AzodiIsoButyroNitrile(AIBN),ethyl alcohol,and water in turn into a fl ask,and mechanically agitate them for dozens of minutes at normal temperature to remove oxygen by inletting nitrogen.

(2)Heat the above mixture for about 2 h,then add a compound of Methacryloyloxyethyl Trimethyl ammonium Chloride(MTC)and ethyl alcohol,and obtain positively charged microspheres of PolyMethyl MethAcrylate(PMMA)after several minutes of chemical reaction.

(3)Add a certain amount of silica sol into the aqueous dispersion of PMMA microspheres,and then obtain strawberry-like composite microspheres after a certain time of magnetic stirring.

Fig.1 shows the schematic of the combination.A positively charged PMMA microsphere is combined with negatively charged SiO2nanoparticles due to electrostatic attraction,and then a strawberry-like PMMA/SiO2microsphere is generated consequently.

Fig.1 Schematic combination of PMMA/SiO2microspheres.

Fig.2 shows a Scanning Electron Microscopic(SEM)photograph of PMMA microspheres without nanoparticles.As can be seen,all the microspheres are with a uniform size of about 2 μm and with high stability in structure.

Fig.3 displays SEM photographs of different types of PMMA/SiO2microspheres,where‘‘2+”and‘‘2-”after PMMA denote the carried charges of PMMA microspheres while the values after SiO2denote the sizes of nanoparticles.As shown in Fig.3(a),no SiO2nanoparticles are attached to PMMA microspheres.That is because PMMA microspheres are negatively charged without MTC added during the preparation.As a result,negatively charged PMMA microspheres and negatively charged SiO2nanoparticles mutually repel each other.As shown in Fig.3(b)-(f),when PMMA 2+microspheres are generated through copolymerizing MTC with MMA,strawberry-like microspheres are successfully obtained with different sizes of SiO2nanoparticles due to electrostatic attraction.

Fig.2 PMMA microspheres without nanoparticles.

After the second-third-level structure is prepared,what remains to do is to spray the second-third-level structure onto the first-level structure.To prepare the first-level structure,hydroxy acrylic resin is selected as the base resin.With isocyanate as the curing agent,a polyurethane coating can be obtained by mixing the hydroxy acrylic resin with a certain amount of inorganic fillers under the condition of high-speed shearing.

Thus,a complete three-level structure can be obtained by spraying strawberry-like microspheres to the polyurethane coating.A highly cost-effective drag reduction method is developed in the present study based on micro-nano coatings.The raw materials are abundant and cost less than RMB 100/m2.The synthesis procedure is easy to implement which can be finished within 12 h.

Fig.4(a)shows a microscope photograph of the threedimensional structure of the complete three-level coating,which is prepared in the shape of grooves attached with strawberry-like microspheres.By adopting different preparation techniques,various widths of the grooves from 10 μm to 100 μm can be realized.Fig.4(b)shows a Transmission Electron Microscopic(TEM)photograph of the cross section of the coating.PMMA microspheres and SiO2nanoparticles are displayed clearly,which indicates that the structures of strawberry-like microspheres have not been destroyed during the process of preparation.

In this study,different types of three-level micro-nano coatings are prepared.Wind tunnel experiments are conducted to investigate the effects of different types of coatings on the boundary layer transition and the drag of the airfoil.

Fig.5 shows a schematic of the groove structure relative to the flow direction.The grooves are mounted on the surface of the airfoil model perpendicular to the flow direction.As shown in Fig.5,the width of the grooves is approximately equal to the depth of the grooves.

Fig.3 Different types of PMMA/SiO2microspheres.

Fig.4 Three-dimensional structure of complete three-level coating and cross-section of coating.

Fig.5 Schematic of groove structure relative to flow direction.

3.Equipment and measurement methods of wind tunnel test

Fig.6 shows photographs of the semi-return flow wind tunnel used for this study.The wind tunnel is called FL-1 wind tunnel located at the AVIC Aerodynamics Research Institute in Shenyang,China.The dimensions of the test section are 0.6 m by 0.6 m,the length of the test section is 1.575 m,and the maximum test Mach number can reach up to 4.0.

In this study,wind tunnel tests are conducted at both a low velocity of Mach number 0.15 and a high velocity of Mach number 0.785.As for the tests at a Mach number of 0.15,the NACA23012 low-speed airfoil23is chosen as the research object.As for the tests at a Mach number of 0.785,an airfoil named FDU0785 which is designed at a cruising Mach number of 0.785 is selected as the research object.Fig.7 shows schematics of the two airfoils.

The wind tunnel test models of the two airfoils are both machined with a span length of 600 mm and a chord length of 200 mm.Fig.8 shows the relative position of the airfoil model to the wind tunnel test section.The leading edge of the airfoil model is located 993 mm away from the inlet of the wind tunnel test section.To vary the angle of attack during the tests,a hinge is installed 80 mm away from the leading edge on the model.

Throwing himself down at the Princess s feet, he implored68 her to stay, and at least speak to him, and she at last consented, but only because he seemed to wish it so very much

Fig.6 Photographs of semi-return flow wind tunnel.

Fig.7 Schematics of ACA23012 and FDU0785 airfoils.

As for the wind tunnel tests,the airfoil model is equipped with three-level micro-nano coatings to investigate the effects of the coatings on the boundary layer transition and the drag.Thus,any contact measurement methods for the boundary layer transition and the drag may cause damage to the structures of the coatings,and impact the accuracy of measurement results consequently.In order to measure the status of the boundary layer transition,the temperature-dependent infrared thermal imaging technology is introduced instead of traditional contact measurement methods such as the oil- flow technology and the schlieren technology.In the meantime,for the purpose of measuring the drag of the FDU0785 airfoil at a cruising Mach number of 0.785,the momentum integral approach24is employed by installing a wake rake behind the airfoil model to measure static and total pressure distributions in the wake area.

As the convective heat transfer coefficients of the laminar flow boundary layer and the turbulent flow boundary layer are different,when there is a difference in temperature between the flow and the surface of the airfoil model,there will be a difference in temperature between the laminar flow region and the turbulent flow region on the surface of the airfoil model.

Fig.8 Relative position of airfoil model to wind tunnel test section.

Before conducting a test,the airfoil model is heated by a halogen lamp to make the temperature of the model higher than that of the flow.Because of the difference in temperature between the flow and the model,boundary layer transition can be observed during the test.An FLIR SC7750L thermal infrared imager is used for observing the difference in temperature between the laminar flow region and the turbulent flow region so that the laminar flow region and the turbulent flow region can be differentiated.

Fig.9 shows a schematic of the momentum integral approach.When air flows over the airfoil,a wake region is formed downstream.The higher the drag on the airfoil is,the smaller the kinetic energy of the flow in the wake region will be.Index I denotes a station far behind the model where the static pressure has recovered to p∞.Station II is the measurement plane where the pressure has not recovered to p∞in experiments.Invoking the assumptions of mass conservation and constant total pressure along streamlines,the quantities at station I are enabled to be related to the quantities at station II.Thus the drag coefficient can be expressed by the following equation:

where CDis the drag coefficient,p∞,pt∞,p2and pt2are static pressure of free stream,total pressure of free stream,static pressure at Station II and total pressure at Station II,respectively.y2is the y-coordinate at Station II,c is the chord length of the airfoil,and γ is the specific heat ratio of air.

Fig.9 Schematic of momentum integral approach.

Note that this formulation only requires the static and total pressures in the wake region by measurement,as well as their values in the free stream.

In order to measure the static and total pressures in the wake region,a wake rake is designed for the wind tunnel and installed 320 mm behind the trailing edge ofthe FDU0785 airfoil model.Fig.10 shows a photograph of the wake rake along with the FDU0785 airfoil model in the wind tunnel.Eighty-nine measuring points for total pressure and four measuring points for static pressure are positioned on the wake rake.

4.Test results and analyses

In this study,wind tunnel tests are completed at different free stream Mach numbers.Firstly,for the purpose of validating the effect of the coatings on moving backward the boundary layer transition position,wind tunnel tests are conducted on the NACA23012 low-speed airfoil model at a free stream Mach number of 0.15.Then,wind tunnel tests are carried out on the FDU0785 airfoil model at a cruising Mach number of 0.785 in order to verify the effect of the coatings in the cruise phase.Lastly,the above test results are analyzed to draw some conclusions.

As for the experiments conducted in the present study,to make the start positions of coatings be before the transition position,the coatings are mounted in the scope from 16%of the chord length to the trailing edge of the airfoil model.

Fig.10 Photograph of wake rake along with FDU0785 airfoil model.

Table 1 Rearward displacements of transition positions with and without strawberry-like microspheres,Ma=0.15,α=4°.

For the purpose of investigating the effect of different groove widths on moving backward the transition position,wind tunnel tests are carried out on the NACA23012 airfoil model with different groove widths of the coatings.All the coatings for these tests are with nanoparticles of about 80 nm.Table 2 shows the rearward displacements of transition positions for different groove widths of the coatings.The tests are conducted at an α of 4°and a Mach number of 0.15,and the groove width varies from 0 μm to 80 μm,where 0 μm denotes that a coating is prepared without the groove structures.As can be seen,the maximum rearward displacement of the transition position occurs when the groove width is 20 μm.

Fig.11 shows the result by Probability Density Function(PDF)method in rearward displacement of the transition position on the surface of the NACA23012 airfoil model for the coating with nanoparticles of about 80 nm and a groove width of 20 μm.The top image shows the transition position of the airfoil without the coating,which is about 20%of the chord length.The bottom image shows the transition position of the airfoil with the coating,which is about 27%of the chord length.Thus,the transition position is moved backward obviously by about 7%of the chord length.

Therefore,at a Mach number of 0.15,the coating with nanoparticles of about 80 nm has the best effect on moving backward the transition position,and the groove width of 20 μm contributes to the best result in rearward displacement of the transition position.

As for the FDU0785 airfoil,wind tunnel tests are conducted at a Mach number of 0.785 to investigate the effects of the coatings on both the boundary layer transition and the drag.Table 3 shows the rearward displacements of transi-tion positions for the coatings with three different types of strawberry-like microspheres at a Mach number of 0.785 and an α of 4°.The groove widths of the three types of coatings are all 40 μm,and the sizes of nanoparticles on the three different types of strawberry-like microspheres are 20 nm,50 nm,and 80 nm,respectively.The rearward displacement of the transition position decreases as the size of nanoparticles becomes bigger.The coating with nanoparticles of about 20 nm leads to the best result in rearward displacement.The results demonstrate that the rearward displacement becomes smaller as the size of nanoparticles increases.

Table 2 Rearward displacements of transition positions with different groove widths,Ma=0.15,α=4°.

Fig.11 Best result in rearward displacement of transition position,Ma=0.15,α=4°.

Table 3 Rearward displacements of transition positions with different types of strawberry-like microspheres,Ma=0.785,α=4°.

Compared to the results of the NACA23012 airfoil,the rearward displacements of transition positions vary oppositely as the size of nanoparticles becomes bigger.That is because the thicknesses of boundary layers for the two airfoils are different.In the experiments,the free stream velocity for the FDU0785 airfoil is higher than that for the NACA23012 airfoil,so the thickness of the boundary layer of the FDU0785 airfoil is thinner than that of the NACA23012 airfoil.Therefore,for the FDU0785 airfoil,nanoparticles with smaller sizes have a better effect on moving backward the transition position than those with bigger sizes.In the same way,for the NACA23012 airfoil,nanoparticles with bigger sizes have a bet-ter effect on moving backward the transition position than those with smaller sizes.

For the purpose of investigating the effect of different groove widths on moving backward the transition position,tests are carried out on the FDU0785 airfoil model with different groove widths of the coatings.All the coatings for these tests are with nanoparticles of about 20 nm.Table 4 shows the rearward displacements of transition positions for different groove widths of the coatings at a Mach number of 0.785 and an α of 4°.The groove widths are 20 μm,40 μm,and 80 μm,respectively.As can be seen,the maximum rearward displacement of the transition position occurs when the groove width is 20 μm,which is similar to the situation of the NACA23012 airfoil model at a Mach number of 0.15.

Fig.12 shows the result in rearward displacement of the transition position on the surface of the FDU0785 airfoil model for the coating with nanoparticles of about 20 nm and a groove width of 20 μm at a Mach number of 0.785 and an α of 4°.The top image shows the transition position of the airfoil without the coating,which is about 26.2%of the chord length.The bottom image shows the transition position of the airfoil with the coating,which is about 31.6%of the chord length.Thus,the transition position is moved backward obviously by about 5.4%of the chord length.

In order to investigate the effect of the coatings on the drag of the airfoil,a wake rake is installed behind the airfoil model to measure the drag through the momentum integral approach.Fig.13 shows a comparison of the drag coefficient curves between the airfoil without the coating and that with the coating,of which the size of nanoparticles is about 20 nm and the groove width is about 20 μm.As can be seen from the figure,the drag coefficients of the airfoil with the coating are lower than those without the coating in the angle of attack range from 1°to 5°.In general,for a commercial aircraft,the angle of attack range of cruising is from 1°to 5°,so the coating can obviously reduce the drag in the cruise phase for the airfoil.

Synthesizing all the above results,the low surface energy micro-nano coatings are capable of moving backward the transition position of the airfoil at both a low velocity of Mach number 0.15 and a high velocity of Mach number 0.785.At a low velocity of Mach number 0.15,the coating with nanoparticles of about 80 nm and a groove width of 20 μm leads to the best result in rearward displacement of the transition position.At a high velocity of Mach number 0.785,the coating with nanoparticles of about 20 nm and a groove width of 20 μm obtains the best result in rearward displacement of the transition position.Within the cruising angle of attack range from 1°to 5°,the drag coefficients of the airfoil are reduced obviously by introducing the low surface energy micro-nano coating technology.

Table 4 Rearward displacements of transition positions with different groove widths,Ma=0.785,α=4°.

Fig.12 Best result in rearward displacement of transition position,Ma=0.785,α=4°.

Fig. 13 Comparison between drag coefficient curves,Ma=0.785.

5.Conclusions

In this study,for the purpose of reducing drag,a laminar flow design technology based on low surface energy micro-nano coatings with multilevel structures is presented.

Different types of coatings with three-level structures are prepared to investigate the effect on moving backward the transition position as well as the drag of an airfoil through wind tunnel tests.The NACA23012 low-speed airfoil is selected as the research object under the low velocity condition of Mach number 0.15,while the FDU0785 airfoil is chosen as the research object under the high velocity condition of Mach number 0.785.

The results of wind tunnel tests indicate that low surface energy micro-nano coatings are capable of moving backward the transition positions for both the NACA23012 airfoil and the FDU0785 airfoil.As for the NACA23012 airfoil,the rearward displacement of the transition position increases as the size of nanoparticles becomes bigger,and the coating with nanoparticles of about 80 nm and a groove width of 20 μm moves the transition position backward most.As for the FDU0785 airfoil,the rearward displacement of the transition position decreases as the size of nanoparticles increases,and the coating with nanoparticles of about 20 nm and a groove width of 20 μm leads to the maximum rearward displacement of the transition position.

Moreover,the drag coefficients of the FDU0785 airfoil are decreased obviously within the cruising angle of attack range from 1°to 5°by employing the technology of low surface energy coating.

Acknowledgements

The authors are grateful for the support by the United Innovation Program of Shanghai Commercial Aircraft Engine,which was founded by Shanghai Municipal Commission of Economy and Informatization,Shanghai Municipal Education Commission,and AECC Commercial Aircraft Engine Co.,Ltd.(No.AR909),the Aeronautical Science Foundation of China(No.2015ZBP9002),and the China Scholarship Council.