Reducing the resistance of vehicles,ships,aircraft and other means of transport during movement can significantly improve the speed,save energy and reduce emissions.After billions of years of continuous evolution,orga...Reducing the resistance of vehicles,ships,aircraft and other means of transport during movement can significantly improve the speed,save energy and reduce emissions.After billions of years of continuous evolution,organisms in nature have gradually developed the ability to move at high speed to achieve better survival.These evolved organisms provide a perfect template for the human development of drag reduction materials.Revealing the unique physiological structural characteristics of organisms and their relationship with resistance during movement can provide a feasible approach tosolving the problem of reducing friction resistance.Whether flying in the sky,running on the ground,swimming in the water,or even living in the soil,many creatures in various environments have the ability to reduce resistance.Driven by these inspirations,researchers have done a lot of work to explore and imitate these biological epidermis structures to achieve drag reduction.In this paper,the biomimetic drag reduction materials is introduced in detail in the order of drag reduction mechanism,structural characteristics of biological epidermis(including marine animals,flying animals,soil animals and plants),biomimetic preparation methods,performance testing methods and application fields.Finally,the potential of various biomimetic drag reduction materials in engineering application and the problems to be overcome are summarized and prospected.This paper can help readers comprehensively understand the research progress of biomimetic drag reduction materials,and provide reference for further designing the next generation of drag reduction materials.展开更多
Bioinspired superhydrophobic surfaces have been used for drag reduction.However,the secondary structures and the air cushions on these surfaces could be destructed in a flow,losing the effect of drag reduction.Here,a ...Bioinspired superhydrophobic surfaces have been used for drag reduction.However,the secondary structures and the air cushions on these surfaces could be destructed in a flow,losing the effect of drag reduction.Here,a stainless-steel surface with mushroom-like cross-section(SMC)and diamond cavities(SMCD)having a drag reduction rate up to 19.37%is developed by 3D printing.The concealed re-entrant structures in SMCD prevent the infiltration of water into the chamber and form gas cushions,which converts the sliding friction at liquid-solid interface into rolling friction at liquid-gas interface,realizing the drag reduction.Meanwhile,98.3%of air can be maintained in the chamber in a flow with Reynolds number(Re)of 9×10^(5),ensuring the drag reduction in a high-velocity flow.Moreover,the continuous top stainless-steel surface and the supporting mesh network protect the critical re-entrant structures,ensuring the robustness of SMC.With the bioinspired design and one-step additive manufacturing process,SMC holds great potential for large-area production and applications requiring robust drag reduction.展开更多
Inspired by the aquatic-adapted pit structures of the Cybister beetles that enable high-speed swimming,this study employs warp-knitted technology to fabricate drag-reduction swimwear textiles.Eight distinct fabric mor...Inspired by the aquatic-adapted pit structures of the Cybister beetles that enable high-speed swimming,this study employs warp-knitted technology to fabricate drag-reduction swimwear textiles.Eight distinct fabric morphologies were produced,and a self-developed high-precision dynamic drag measurement device was used to systematically analyze the mechanisms underlying the drag-reduction performance of these biomimetic pit structures.The device incorporates a servomotor,ball screw linkage,and high-precision tension sensor,enabling real-time and accurate detection of fluid drag forces.It effectively overcomes the limitations of traditional indirect measurement methods,including dynamic response lag and insufficient accuracy.Experimental results demonstrate that the hydrophobic small-pit fabric(4^(#))achieves an 84% drag reduction at 400 mm/s,outperforming the control sample(warp-knitted fabric 7^(#)).This significant reduction is attributed to the Cassie state established on the hydrophobic surface,which substantially decreases viscous drag and the microvortices generated by the pit structures,which delay flow separation and effectively minimize pressure drag.Furthermore,small-pit fabrics demonstrate a drag reduction rate 26% to 50% higher than that of large-pit structures,highlighting the critical importance of matching the pit scale to the thickness of the near-wall viscous sublayer for optimal drag reduction.This study establishes a theoretical foundation for the biomimetic design of high-performance drag-reduction swimsuits.The developed drag-measuring device also provides a standardized experimental platform for hydrodynamic studies of flexible materials,supporting a shift from empirical design methodologies to theory-driven approaches in drag-reduction technology and exhibiting significant potential for future advancements.展开更多
The cutaneous ridges on dolphin skin have long been believed to effectively reduce friction drag, thereby contributing to overall drag reduction. However, since these skin ridges are oriented perpendicular to the swim...The cutaneous ridges on dolphin skin have long been believed to effectively reduce friction drag, thereby contributing to overall drag reduction. However, since these skin ridges are oriented perpendicular to the swimming direction, they also generate additional pressure drag, raising questions about the impact of the shape-induced pressure forces on swimming. Inspired by the microvibrations observed on dolphin skin, we hypothesize that the microstructure on dolphin skin is not static but dynamically oscillates in the form of Longitudinal Micro-Ultrasonic Waves (LMUWs). To explore this, we carried out a series of Computational Fluid Dynamics (CFD) simulations based on Large Eddy Simulation (LES) model to investigate the impact of pressure drag on the total drag acting on an oscillating skin surface under realistic turbulent flow conditions. The results indicate that the dynamic skin oscillations induce a new dynamic Stokes boundary layer, which has the potential to convert pressure drag into a negative force, thereby reducing total drag under the influence of traveling LMUW excitations. Furthermore, a relative velocity ξ, defined as the difference between the wave speed c and the external flow speed U, is introduced to evaluate the drag-reduction effect dominated by pressure. The findings reveal that pressure drag remains negative when ξ > 0. As ξ increases, the thrust effect induced by negative pressure becomes increasingly significant, ultimately counteracting friction drag and eliminating total drag. This pressure-dominated drag reduction mechanism thus demonstrates a novel strategy for the drag reduction technology and the potential of unveiling the mysteries behind dolphin swimming.展开更多
The primary objective in aircraft transportation is to minimize turbulent drag, thereby conserving energy and reducing emissions. We propose a sector-shaped counter-flow dielectric barrier discharge plasma actuator, w...The primary objective in aircraft transportation is to minimize turbulent drag, thereby conserving energy and reducing emissions. We propose a sector-shaped counter-flow dielectric barrier discharge plasma actuator, which leverages jet synthesis for drag reduction. A drag control experiment was conducted in a low-speed wind tunnel with a controlled flow velocity of 9.6 m/s(Re = 1.445 × 10^(4)). This study investigated the effects of varying pulse frequencies and actuation voltages on the turbulent boundary layer. Using a hot-wire measurement system, we analyzed the pulsating and time-averaged velocity distributions within the boundary layer to evaluate the streamwise turbulent drag reduction. The results show that the local TDR decreases as the pulse frequency increases, reaching a maximum reduction of approximately 20.97% at a pulse frequency of 50 Hz. In addition, as the actuation voltage increases, the friction coefficient decreases, increasing the drag reduction rate. The maximum drag reduction of approximately 33.34% is achieved at an actuation voltage of 10 kV.展开更多
In the present paper, an ‘in-house＇ genetic algorithm was numerically and experimentally validated. The genetic algorithm was applied to an optimization problem for improving the aerodynamic performances of an aircr...In the present paper, an ‘in-house＇ genetic algorithm was numerically and experimentally validated. The genetic algorithm was applied to an optimization problem for improving the aerodynamic performances of an aircraft wing tip through upper surface morphing. The optimization was performed for 16 flight cases expressed in terms of various combinations of speeds, angles of attack and aileron deflections. The displacements resulted from the optimization were used during the wind tunnel tests of the wing tip demonstrator for the actuators control to change the upper surface shape of the wing. The results of the optimization of the flow behavior for the airfoil morphing upper-surface problem were validated with wind tunnel experimental transition results obtained with infra-red Thermography on the wing-tip demonstrator. The validation proved that the 2D numerical optimization using the ‘in-house＇ genetic algorithm was an appropriate tool in improving various aspects of a wing＇s aerodynamic performances.展开更多
A new idea of drag reduction and thermal protection for hypersonic vehicles is proposed based on the combination of a physical spike and lateral jets for shockreconstruction. The spike recasts the bow shock in front o...A new idea of drag reduction and thermal protection for hypersonic vehicles is proposed based on the combination of a physical spike and lateral jets for shockreconstruction. The spike recasts the bow shock in front of a blunt body into a conical shock, and the lateral jets work to protect the spike tip from overheating and to push the conical shock away from the blunt body when a pitching angle exists during flight. Experiments are conducted in a hypersonic wind tunnel at a nominal Mach number of 6. It is demonstrated that the shock/shock interaction on the blunt body is avoided due to injection and the peak pressure at the reattachment point is reduced by 70% under a 4° attack angle.展开更多
Improving vehicle fuel consumption,performance and aerodynamic efficiency by drag reduction especially in heavy vehicles is one of the indispensable issues of automotive industry.In this work,the effects of adding app...Improving vehicle fuel consumption,performance and aerodynamic efficiency by drag reduction especially in heavy vehicles is one of the indispensable issues of automotive industry.In this work,the effects of adding append devices like deflector and cab vane corner on heavy commercial vehicle drag reduction were investigated.For this purpose,the vehicle body structure was modeled with various supplementary parts at the first stage.Then,computational fluid dynamic(CFD) analysis was utilized for each case to enhance the optimal aerodynamic structure at different longitudinal speeds for heavy commercial vehicles.The results show that the most effective supplementary part is deflector,and by adding this part,the drag coefficient is decreased considerably at an optimum angle.By adding two cab vane corners at both frontal edges of cab,a significant drag reduction is noticed.Back vanes and base flaps are simple plates which can be added at the top and side end of container and at the bottom with specific angle respectively to direct the flow and prevent the turbulence.Through the analysis of airflow and pressure distribution,the results reveal that the cab vane reduces fuel consumption and drag coefficient by up to 20 % receptively using proper deflector angle.Finally,by adding all supplementary parts at their optimized positions,41% drag reduction is obtained compared to the simple model.展开更多
Bionic non-smooth surfaces （BNSS） can reduce drag. Much attention has been paid to the mechanism of shear stress reduction by riblets. The mechanism of pressure force reduction by bionic non-smooth surfaces on bodie...Bionic non-smooth surfaces （BNSS） can reduce drag. Much attention has been paid to the mechanism of shear stress reduction by riblets. The mechanism of pressure force reduction by bionic non-smooth surfaces on bodies of revolution has not been well investigated. In this work CFD simulation has revealed the mechanism of drag reduction by BNSS, which may work in three ways. First, BNSS on bodies of revolution may lower the surface velocity of the medium, which prevents the sudden speed up of air on the cross section. So the bottom pressure of the model would not be disturbed sharply, resulting in less energy loss and drag reduction. Second, the magnitude of vorticity induced by the bionic model becomes smaller because, due to the sculpturing, the growth of tiny air bubbles is avoided. Thus the large moment of inertia induced by large air bubble is reduced. The reduction of the vorticity could reduce the dissipation of the eddy. So the pressure force could also be reduced. Third, the thickness of the momentum layer on the model becomes less which, according to the relationship between the drag coefficient and the momentum thickness, reduces drag.展开更多
Partially hydrolyzed polyacrylamide（HPAM）as the main component of slickwater fracturing fluid is a shear-sensitive polymer,which suffers from mechanical degradation at turbulent flow rates.Five different concentrati...Partially hydrolyzed polyacrylamide（HPAM）as the main component of slickwater fracturing fluid is a shear-sensitive polymer,which suffers from mechanical degradation at turbulent flow rates.Five different concentrations of HPAM as well as mixtures of polyacrylamide/xanthan gum were prepared to investigate the possibility of improving shear stability of HPAM.Drag reduction（DR）measurements were performed in a closed flow loop.For HPAM solutions,the extent of DR increased from 30%to67%with increasing HPAM concentration from 100 to1000 wppm.All the HPAM solutions suffered from mechanical degradation and loss of DR efficiency over the shearing period.Results indicated that the resistance to shear degradation increased with increasing polymer concentration.DR efficiency of 600 wppm xanthan gum（XG）was 38%,indicating that XG was not as good a drag reducer as HPAM.But with only 6%DR decline,XG solution exhibited a better shear stability compared to HPAM solutions.Mixed HPAM/XG solutions initially exhibited greater DR（40%and 55%）compared to XG,but due to shear degradation,DR%dropped for HPAM/XG solutions.Compared to 200 wppm HPAM solution,addition of XG did not improve the drag reduction efficiency of HPAM/XG mixed solutions though XG slightly improved the resistance against mechanical degradation in HPAM/XG mixed polymer solutions.展开更多
Inspired by the fact that bogies and bottom equipment generally contribute a great deal of aerodynamic drag to high-speed trains,this paper puts forward a simple method of mounting some small deflectors before and/or ...Inspired by the fact that bogies and bottom equipment generally contribute a great deal of aerodynamic drag to high-speed trains,this paper puts forward a simple method of mounting some small deflectors before and/or after the bogie cabins to optimize the underbody flow and reduce the aerodynamic drag of high-speed trains.The flow fields of the high-speed train models with and without bottom deflectors are numerically studied by the IDDES method.The effectiveness and further mechanism of the bottom deflectors on aerodynamic drag reduction are analyzed.It is demonstrated that the bottom deflectors could guide the underbody flow to the ground and prevent it from hitting on the bogies and bottom equipment of the train,resulting in a significant aerodynamic drag reduction effect.Moreover,the effects of different mounting locations of bottom deflectors on drag reduction are discussed as well,and an optimal mounting configuration with a drag reduction effect of up to about 12%is finally obtained.Nevertheless,the mounted deflector is also proved capable of significantly reducing the interference range of the underbody flow and reducing the slipstream of the train,which possesses a higher guarantee for the safety of railway workers and passengers waiting on the platforms.This work provides a new idea for aerodynamic drag reduction of high-speed trains,and is of great significance in energy conservation and consumption reduction.展开更多
Experimental and numerical studies are carried out to validate the potential of opposing Plasma Synthetic Jet(PSJ)for drag reduction for a hemisphere.Firstly,flow field changes of opposing PSJ are analyzed by comparin...Experimental and numerical studies are carried out to validate the potential of opposing Plasma Synthetic Jet(PSJ)for drag reduction for a hemisphere.Firstly,flow field changes of opposing PSJ are analyzed by comparing the experimental schlieren images and simulation results in a supersonic free stream of Mach number 3.As PSJ is a kind of unsteady pulsed jet,the shock standoff distance increases initially and then decreases under the control of PSJ,which corresponds to the change of the strength of PSJ.Accordingly,the amount of drag reduction of the hemisphere increases initially and then decreases.It is found that there is a short period of“drag rise”during the formation of PSJ before the drag reduction,which is induced by the generation of normal shock waves and the area difference of the cavity wall of PSJ Actuator(PSJA).Secondly,the effects of five parameters,including exit diameter,discharge energy of PSJA,Mach number,static pressure of incoming flow and angle of attack,on drag reduction of opposing PSJ were studied in detail by using numerical method.It is found that the Maximum Pressure Ratio(MPR)has a significant impact on the average drag reduction for a configuration-determined PSJA.For the configuration selected in this study,the flow field of opposing PSJ shows typical Short Penetration Mode(SPM)in a control cycle of PSJ when the MPR is less than 0.89.However,the flow field shows typical Long Penetration Mode(LPM)at some time when the MPR is bigger than 0.89.Relatively better drag reduction is achieved in this case.展开更多
Ship hull form of the underwater area strongly influences the resistance of the ship. The major factor in ship resistance is skin friction resistance. Bulbous bows, polymer paint, water repellent paint （highly water-...Ship hull form of the underwater area strongly influences the resistance of the ship. The major factor in ship resistance is skin friction resistance. Bulbous bows, polymer paint, water repellent paint （highly water-repellent wall）, air injection, and specific roughness have been used by researchers as an attempt to obtain the resistance reduction and operation efficiency of ships. Micro-bubble injection is a promising technique for lowering frictional resistance. The injected air bubbles are supposed to somehow modify the energy inside the turbulent boundary layer and thereby lower the skin friction. The purpose of this study was to identify the effect of injected micro bubbles on a navy fast patrol boat （FPB） 57 m type model with the following main dimensions： L=2 450 ram, B=400 mm, and T=190 mm. The influence of the location of micro bubble injection and bubble velocity was also investigated. The ship model was pulled by an electric motor whose speed could be varied and adjusted. The ship model resistance was precisely measured by a load cell transducer. Comparison of ship resistance with and without micro-bubble injection was shown on a graph as a function of the drag coefficient and Froude number. It was shown that micro bubble injection behind the mid-ship is the best location to achieve the most effective drag reduction, and the drag reduction caused by the micro-bubbles can reach 6%-9%.展开更多
In the present paper, a physical model is proposed for reducing the problem of the drag reduction of an attached bow shock around the nose of a high-speed vehicle with on-board discharge, to the problem of a balance b...In the present paper, a physical model is proposed for reducing the problem of the drag reduction of an attached bow shock around the nose of a high-speed vehicle with on-board discharge, to the problem of a balance between the magnetic pressure and gas pressure of plane shock of a partially ionized gas consisting of the environmental gas around the nose of the vehicle and the on-board discharge-produced plasma. The relation between the shock strength and the discharge-induced magnetic pressure is studied by means of a set of one-fluid, hydromagnetic equations reformed for the present purpose, where the discharge-induced magnetic field consists of the electron current （produced by the discharge）-induced magnetic field and the partially ionized gas flow-induced one. A formula for the relation between the above parameters is derived. It shows that the discharge-induced magnetic pressure can minimize the shock strength, successfully explaining the two recent experimental observations on attached bow shock mitigation and elimination in a supersonic flow during on-board discharge [Phys. Plasmas 9 （2002） 721 and Phys. Plasmas 7 （2000） 1345]. In addition, the formula implies that the shock elimination leaves room for a layer of higher-density plasma rampart moving around the nose of the vehicle, being favourable to the plasma radar cloaking of the vehicle. The reason for it is expounded.展开更多
The body surface of some organisms has non-smooth structure, which is related to drag reduction in moving fluid. To imitate these structures, models with a non-smooth surface were made. In order to find a relationship...The body surface of some organisms has non-smooth structure, which is related to drag reduction in moving fluid. To imitate these structures, models with a non-smooth surface were made. In order to find a relationship between drag reduction and the non-smooth surface, an orthogonal design test was employed in a low speed wind tunnel. Six factors likely to influence drag reduction were considered, and each factor tested at three levels. The six factors were the configuration, diameter/bottom width, height/depth, distribution, the arrangement of the rough structures on the experimental model and the wind speed. It was shown that the non-smooth surface causes drag reduction and the distribution of non-smooth structures on the model, and wind speed, are the predominant factors affecting drag reduction. Using analysis of variance, the optimal combination and levels were obtained, which were a wind speed of 44 m/s, distribution of the non-smooth structure on the tail of the experimental model, the configuration of riblets, diameter/bottom width of i mm, height/depth of 0.5 mm, arranged in a rhombic formation. At the optimal combination mentioned above, the 99% confidence interval for drag reduction was 11.13% to 22.30%.展开更多
基金the National Natural Science Foundation of China(No.52305236)supported by National Natural Science Foundation of China.
文摘Reducing the resistance of vehicles,ships,aircraft and other means of transport during movement can significantly improve the speed,save energy and reduce emissions.After billions of years of continuous evolution,organisms in nature have gradually developed the ability to move at high speed to achieve better survival.These evolved organisms provide a perfect template for the human development of drag reduction materials.Revealing the unique physiological structural characteristics of organisms and their relationship with resistance during movement can provide a feasible approach tosolving the problem of reducing friction resistance.Whether flying in the sky,running on the ground,swimming in the water,or even living in the soil,many creatures in various environments have the ability to reduce resistance.Driven by these inspirations,researchers have done a lot of work to explore and imitate these biological epidermis structures to achieve drag reduction.In this paper,the biomimetic drag reduction materials is introduced in detail in the order of drag reduction mechanism,structural characteristics of biological epidermis(including marine animals,flying animals,soil animals and plants),biomimetic preparation methods,performance testing methods and application fields.Finally,the potential of various biomimetic drag reduction materials in engineering application and the problems to be overcome are summarized and prospected.This paper can help readers comprehensively understand the research progress of biomimetic drag reduction materials,and provide reference for further designing the next generation of drag reduction materials.
基金supported by National Natural Science Foundation of China(52373119,52475310)the National Key R&D Program of China(2022YFB4701000).
文摘Bioinspired superhydrophobic surfaces have been used for drag reduction.However,the secondary structures and the air cushions on these surfaces could be destructed in a flow,losing the effect of drag reduction.Here,a stainless-steel surface with mushroom-like cross-section(SMC)and diamond cavities(SMCD)having a drag reduction rate up to 19.37%is developed by 3D printing.The concealed re-entrant structures in SMCD prevent the infiltration of water into the chamber and form gas cushions,which converts the sliding friction at liquid-solid interface into rolling friction at liquid-gas interface,realizing the drag reduction.Meanwhile,98.3%of air can be maintained in the chamber in a flow with Reynolds number(Re)of 9×10^(5),ensuring the drag reduction in a high-velocity flow.Moreover,the continuous top stainless-steel surface and the supporting mesh network protect the critical re-entrant structures,ensuring the robustness of SMC.With the bioinspired design and one-step additive manufacturing process,SMC holds great potential for large-area production and applications requiring robust drag reduction.
基金the financial support from the Fundamental Research Funds for the Central Universities(JUSRP122003)the fellowship of China Postdoctoral Science Foundation(2022TQ0123).
文摘Inspired by the aquatic-adapted pit structures of the Cybister beetles that enable high-speed swimming,this study employs warp-knitted technology to fabricate drag-reduction swimwear textiles.Eight distinct fabric morphologies were produced,and a self-developed high-precision dynamic drag measurement device was used to systematically analyze the mechanisms underlying the drag-reduction performance of these biomimetic pit structures.The device incorporates a servomotor,ball screw linkage,and high-precision tension sensor,enabling real-time and accurate detection of fluid drag forces.It effectively overcomes the limitations of traditional indirect measurement methods,including dynamic response lag and insufficient accuracy.Experimental results demonstrate that the hydrophobic small-pit fabric(4^(#))achieves an 84% drag reduction at 400 mm/s,outperforming the control sample(warp-knitted fabric 7^(#)).This significant reduction is attributed to the Cassie state established on the hydrophobic surface,which substantially decreases viscous drag and the microvortices generated by the pit structures,which delay flow separation and effectively minimize pressure drag.Furthermore,small-pit fabrics demonstrate a drag reduction rate 26% to 50% higher than that of large-pit structures,highlighting the critical importance of matching the pit scale to the thickness of the near-wall viscous sublayer for optimal drag reduction.This study establishes a theoretical foundation for the biomimetic design of high-performance drag-reduction swimsuits.The developed drag-measuring device also provides a standardized experimental platform for hydrodynamic studies of flexible materials,supporting a shift from empirical design methodologies to theory-driven approaches in drag-reduction technology and exhibiting significant potential for future advancements.
基金supported by the Japan Society for the Promotion of Science(JSPS KAKENHI No.23H01373).
文摘The cutaneous ridges on dolphin skin have long been believed to effectively reduce friction drag, thereby contributing to overall drag reduction. However, since these skin ridges are oriented perpendicular to the swimming direction, they also generate additional pressure drag, raising questions about the impact of the shape-induced pressure forces on swimming. Inspired by the microvibrations observed on dolphin skin, we hypothesize that the microstructure on dolphin skin is not static but dynamically oscillates in the form of Longitudinal Micro-Ultrasonic Waves (LMUWs). To explore this, we carried out a series of Computational Fluid Dynamics (CFD) simulations based on Large Eddy Simulation (LES) model to investigate the impact of pressure drag on the total drag acting on an oscillating skin surface under realistic turbulent flow conditions. The results indicate that the dynamic skin oscillations induce a new dynamic Stokes boundary layer, which has the potential to convert pressure drag into a negative force, thereby reducing total drag under the influence of traveling LMUW excitations. Furthermore, a relative velocity ξ, defined as the difference between the wave speed c and the external flow speed U, is introduced to evaluate the drag-reduction effect dominated by pressure. The findings reveal that pressure drag remains negative when ξ > 0. As ξ increases, the thrust effect induced by negative pressure becomes increasingly significant, ultimately counteracting friction drag and eliminating total drag. This pressure-dominated drag reduction mechanism thus demonstrates a novel strategy for the drag reduction technology and the potential of unveiling the mysteries behind dolphin swimming.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 61971345 and 52107174)。
文摘The primary objective in aircraft transportation is to minimize turbulent drag, thereby conserving energy and reducing emissions. We propose a sector-shaped counter-flow dielectric barrier discharge plasma actuator, which leverages jet synthesis for drag reduction. A drag control experiment was conducted in a low-speed wind tunnel with a controlled flow velocity of 9.6 m/s(Re = 1.445 × 10^(4)). This study investigated the effects of varying pulse frequencies and actuation voltages on the turbulent boundary layer. Using a hot-wire measurement system, we analyzed the pulsating and time-averaged velocity distributions within the boundary layer to evaluate the streamwise turbulent drag reduction. The results show that the local TDR decreases as the pulse frequency increases, reaching a maximum reduction of approximately 20.97% at a pulse frequency of 50 Hz. In addition, as the actuation voltage increases, the friction coefficient decreases, increasing the drag reduction rate. The maximum drag reduction of approximately 33.34% is achieved at an actuation voltage of 10 kV.
基金Bombardier Aerospace,Thales Canada,The Consortium in Research and Aerospace in Canada(CRIAQ)the Natural Sciences and Engineering Research Council of Canada(NSERC)for their financial support
文摘In the present paper, an ‘in-house＇ genetic algorithm was numerically and experimentally validated. The genetic algorithm was applied to an optimization problem for improving the aerodynamic performances of an aircraft wing tip through upper surface morphing. The optimization was performed for 16 flight cases expressed in terms of various combinations of speeds, angles of attack and aileron deflections. The displacements resulted from the optimization were used during the wind tunnel tests of the wing tip demonstrator for the actuators control to change the upper surface shape of the wing. The results of the optimization of the flow behavior for the airfoil morphing upper-surface problem were validated with wind tunnel experimental transition results obtained with infra-red Thermography on the wing-tip demonstrator. The validation proved that the 2D numerical optimization using the ‘in-house＇ genetic algorithm was an appropriate tool in improving various aspects of a wing＇s aerodynamic performances.
基金the National Natural Science Foundation of China (10632090)the National Aerodynamic Pre-Research Foundation (513130401)
文摘A new idea of drag reduction and thermal protection for hypersonic vehicles is proposed based on the combination of a physical spike and lateral jets for shockreconstruction. The spike recasts the bow shock in front of a blunt body into a conical shock, and the lateral jets work to protect the spike tip from overheating and to push the conical shock away from the blunt body when a pitching angle exists during flight. Experiments are conducted in a hypersonic wind tunnel at a nominal Mach number of 6. It is demonstrated that the shock/shock interaction on the blunt body is avoided due to injection and the peak pressure at the reattachment point is reduced by 70% under a 4° attack angle.
文摘Improving vehicle fuel consumption,performance and aerodynamic efficiency by drag reduction especially in heavy vehicles is one of the indispensable issues of automotive industry.In this work,the effects of adding append devices like deflector and cab vane corner on heavy commercial vehicle drag reduction were investigated.For this purpose,the vehicle body structure was modeled with various supplementary parts at the first stage.Then,computational fluid dynamic(CFD) analysis was utilized for each case to enhance the optimal aerodynamic structure at different longitudinal speeds for heavy commercial vehicles.The results show that the most effective supplementary part is deflector,and by adding this part,the drag coefficient is decreased considerably at an optimum angle.By adding two cab vane corners at both frontal edges of cab,a significant drag reduction is noticed.Back vanes and base flaps are simple plates which can be added at the top and side end of container and at the bottom with specific angle respectively to direct the flow and prevent the turbulence.Through the analysis of airflow and pressure distribution,the results reveal that the cab vane reduces fuel consumption and drag coefficient by up to 20 % receptively using proper deflector angle.Finally,by adding all supplementary parts at their optimized positions,41% drag reduction is obtained compared to the simple model.
基金National Natural Science Foundation of China （Grant No.50635030） the International Cooperation key Project of Ministry of Science and Technology of China （Grant No. 2005DFA00850）+2 种基金 The key project about ministry of education of science and technology （Grant No. 105059） the international cooperative of Jilin Province （Grant No.20040703-1） Specialized Research fund for the Doctoral Program of higher Education （Grant No. 20050183064）.
文摘Bionic non-smooth surfaces （BNSS） can reduce drag. Much attention has been paid to the mechanism of shear stress reduction by riblets. The mechanism of pressure force reduction by bionic non-smooth surfaces on bodies of revolution has not been well investigated. In this work CFD simulation has revealed the mechanism of drag reduction by BNSS, which may work in three ways. First, BNSS on bodies of revolution may lower the surface velocity of the medium, which prevents the sudden speed up of air on the cross section. So the bottom pressure of the model would not be disturbed sharply, resulting in less energy loss and drag reduction. Second, the magnitude of vorticity induced by the bionic model becomes smaller because, due to the sculpturing, the growth of tiny air bubbles is avoided. Thus the large moment of inertia induced by large air bubble is reduced. The reduction of the vorticity could reduce the dissipation of the eddy. So the pressure force could also be reduced. Third, the thickness of the momentum layer on the model becomes less which, according to the relationship between the drag coefficient and the momentum thickness, reduces drag.
基金the Research Partnership to Secure Energy for America (RPSEA)Oklahoma State University Chemical Engineering Department for partial support of this project
文摘Partially hydrolyzed polyacrylamide（HPAM）as the main component of slickwater fracturing fluid is a shear-sensitive polymer,which suffers from mechanical degradation at turbulent flow rates.Five different concentrations of HPAM as well as mixtures of polyacrylamide/xanthan gum were prepared to investigate the possibility of improving shear stability of HPAM.Drag reduction（DR）measurements were performed in a closed flow loop.For HPAM solutions,the extent of DR increased from 30%to67%with increasing HPAM concentration from 100 to1000 wppm.All the HPAM solutions suffered from mechanical degradation and loss of DR efficiency over the shearing period.Results indicated that the resistance to shear degradation increased with increasing polymer concentration.DR efficiency of 600 wppm xanthan gum（XG）was 38%,indicating that XG was not as good a drag reducer as HPAM.But with only 6%DR decline,XG solution exhibited a better shear stability compared to HPAM solutions.Mixed HPAM/XG solutions initially exhibited greater DR（40%and 55%）compared to XG,but due to shear degradation,DR%dropped for HPAM/XG solutions.Compared to 200 wppm HPAM solution,addition of XG did not improve the drag reduction efficiency of HPAM/XG mixed solutions though XG slightly improved the resistance against mechanical degradation in HPAM/XG mixed polymer solutions.
基金supported by the National Key Research&Development Projects(Grant No.2017YFB0202801)the Strategic Priority Research Program of the Chinese Academy of Sciences(class B)(Grant No.XDB22020000)Research project of Chinese Academy of Sciences(Grant No.XXH13506-204).
文摘Inspired by the fact that bogies and bottom equipment generally contribute a great deal of aerodynamic drag to high-speed trains,this paper puts forward a simple method of mounting some small deflectors before and/or after the bogie cabins to optimize the underbody flow and reduce the aerodynamic drag of high-speed trains.The flow fields of the high-speed train models with and without bottom deflectors are numerically studied by the IDDES method.The effectiveness and further mechanism of the bottom deflectors on aerodynamic drag reduction are analyzed.It is demonstrated that the bottom deflectors could guide the underbody flow to the ground and prevent it from hitting on the bogies and bottom equipment of the train,resulting in a significant aerodynamic drag reduction effect.Moreover,the effects of different mounting locations of bottom deflectors on drag reduction are discussed as well,and an optimal mounting configuration with a drag reduction effect of up to about 12%is finally obtained.Nevertheless,the mounted deflector is also proved capable of significantly reducing the interference range of the underbody flow and reducing the slipstream of the train,which possesses a higher guarantee for the safety of railway workers and passengers waiting on the platforms.This work provides a new idea for aerodynamic drag reduction of high-speed trains,and is of great significance in energy conservation and consumption reduction.
基金co-supported by the National Natural Science Foundation of China(Nos.52075538,12002377,11872374)the Natural Science Foundation of Hunan Province,China(Nos.2020JJ5670,2020JJ2031)+1 种基金the research program of National University of Defense Technology(No.ZK18-03-11)China Postdoctoral Science Foundation(No.2019M652754)。
文摘Experimental and numerical studies are carried out to validate the potential of opposing Plasma Synthetic Jet(PSJ)for drag reduction for a hemisphere.Firstly,flow field changes of opposing PSJ are analyzed by comparing the experimental schlieren images and simulation results in a supersonic free stream of Mach number 3.As PSJ is a kind of unsteady pulsed jet,the shock standoff distance increases initially and then decreases under the control of PSJ,which corresponds to the change of the strength of PSJ.Accordingly,the amount of drag reduction of the hemisphere increases initially and then decreases.It is found that there is a short period of“drag rise”during the formation of PSJ before the drag reduction,which is induced by the generation of normal shock waves and the area difference of the cavity wall of PSJ Actuator(PSJA).Secondly,the effects of five parameters,including exit diameter,discharge energy of PSJA,Mach number,static pressure of incoming flow and angle of attack,on drag reduction of opposing PSJ were studied in detail by using numerical method.It is found that the Maximum Pressure Ratio(MPR)has a significant impact on the average drag reduction for a configuration-determined PSJA.For the configuration selected in this study,the flow field of opposing PSJ shows typical Short Penetration Mode(SPM)in a control cycle of PSJ when the MPR is less than 0.89.However,the flow field shows typical Long Penetration Mode(LPM)at some time when the MPR is bigger than 0.89.Relatively better drag reduction is achieved in this case.
基金Supported by the Directorate for Research and Community Service,University of Indonesia(RUUI Research Laboratory 2010),Jakarta,Indonesia
文摘Ship hull form of the underwater area strongly influences the resistance of the ship. The major factor in ship resistance is skin friction resistance. Bulbous bows, polymer paint, water repellent paint （highly water-repellent wall）, air injection, and specific roughness have been used by researchers as an attempt to obtain the resistance reduction and operation efficiency of ships. Micro-bubble injection is a promising technique for lowering frictional resistance. The injected air bubbles are supposed to somehow modify the energy inside the turbulent boundary layer and thereby lower the skin friction. The purpose of this study was to identify the effect of injected micro bubbles on a navy fast patrol boat （FPB） 57 m type model with the following main dimensions： L=2 450 ram, B=400 mm, and T=190 mm. The influence of the location of micro bubble injection and bubble velocity was also investigated. The ship model was pulled by an electric motor whose speed could be varied and adjusted. The ship model resistance was precisely measured by a load cell transducer. Comparison of ship resistance with and without micro-bubble injection was shown on a graph as a function of the drag coefficient and Froude number. It was shown that micro bubble injection behind the mid-ship is the best location to achieve the most effective drag reduction, and the drag reduction caused by the micro-bubbles can reach 6%-9%.
基金Project supported by the National Natural Science Foundation of China （Grant Nos 40390150 and 10005001）.
文摘In the present paper, a physical model is proposed for reducing the problem of the drag reduction of an attached bow shock around the nose of a high-speed vehicle with on-board discharge, to the problem of a balance between the magnetic pressure and gas pressure of plane shock of a partially ionized gas consisting of the environmental gas around the nose of the vehicle and the on-board discharge-produced plasma. The relation between the shock strength and the discharge-induced magnetic pressure is studied by means of a set of one-fluid, hydromagnetic equations reformed for the present purpose, where the discharge-induced magnetic field consists of the electron current （produced by the discharge）-induced magnetic field and the partially ionized gas flow-induced one. A formula for the relation between the above parameters is derived. It shows that the discharge-induced magnetic pressure can minimize the shock strength, successfully explaining the two recent experimental observations on attached bow shock mitigation and elimination in a supersonic flow during on-board discharge [Phys. Plasmas 9 （2002） 721 and Phys. Plasmas 7 （2000） 1345]. In addition, the formula implies that the shock elimination leaves room for a layer of higher-density plasma rampart moving around the nose of the vehicle, being favourable to the plasma radar cloaking of the vehicle. The reason for it is expounded.
基金support provided by the National Key Grant Program of Basic(Grant No.2002CCA01200)the National High Technol-ogy Research and Development Program of China(863 Program)(Grant No.2003AA305080)+1 种基金the Key Project of Chinese Ministry of Education(No,02089)the Natural Science Foundation of Jilin Province(No.20040703-1).
文摘The body surface of some organisms has non-smooth structure, which is related to drag reduction in moving fluid. To imitate these structures, models with a non-smooth surface were made. In order to find a relationship between drag reduction and the non-smooth surface, an orthogonal design test was employed in a low speed wind tunnel. Six factors likely to influence drag reduction were considered, and each factor tested at three levels. The six factors were the configuration, diameter/bottom width, height/depth, distribution, the arrangement of the rough structures on the experimental model and the wind speed. It was shown that the non-smooth surface causes drag reduction and the distribution of non-smooth structures on the model, and wind speed, are the predominant factors affecting drag reduction. Using analysis of variance, the optimal combination and levels were obtained, which were a wind speed of 44 m/s, distribution of the non-smooth structure on the tail of the experimental model, the configuration of riblets, diameter/bottom width of i mm, height/depth of 0.5 mm, arranged in a rhombic formation. At the optimal combination mentioned above, the 99% confidence interval for drag reduction was 11.13% to 22.30%.
