During the re-entry of a hypersonic aircraft into the earth’s atmosphere,the surrounding air experiences dissociation,ionization,and other complex chemical phenomena due to extreme temperature by shock wave.To ensure...During the re-entry of a hypersonic aircraft into the earth’s atmosphere,the surrounding air experiences dissociation,ionization,and other complex chemical phenomena due to extreme temperature by shock wave.To ensure thermal safety,the thermochemical non-equilibrium effects resulting from real-gas behavior should be taken into account.In this paper,the characteristics of a double-cone hypersonic laminar flow,including distributions of wall pressure,heat flux,and species dissociation are numerically analyzed with incoming enthalpy of 9.65-21.77 MJ/kg.The thermochemical non-equilibrium flow at different enthalpy and wall temperatures is performed with two-temperature model and Park’s seven chemical reaction model.It is found that the doublecone flow features complex shock-shock interactions to form triple points.The flow topology is further brought out from the analysis of streamlines.At the lowest incoming enthalpy with isothermal wall conditions,two foci points appear.While others highlight only one focal point.As the increment of incoming enthalpy,the heat flux and dissociation of nitrogen and oxygen also increase.An increasing wall temperature leads to a larger separation bubble and a lower value of heat flux and pressure peak,while massive dissociation occurs without obvious ionization under considered cases.展开更多
Adaptive,morphing flaps are taking ever-increasing attention in civil aviation thanks to the expected benefits this technology can bring at the aircraft level in terms of high-lift performance improvement and related ...Adaptive,morphing flaps are taking ever-increasing attention in civil aviation thanks to the expected benefits this technology can bring at the aircraft level in terms of high-lift performance improvement and related fuel burnt reduction per flight.Relying upon morphing capabilities,it is possible to fix a unique setting for the flap and adapt the flap shape to match the aerodynamic requirements for take-off or landing.The proper morphed shapes can assure better high-lift performances than those achievable by referring to a conventional flap.Moreover,standing the unique flap setting for take-off and landing,a dramatic simplification of the flap deployment systems may be achieved.As a consequence of this simplification,the deployment system can be fully hosted in the wing,thus avoiding under-wing nacelles with significantly better aerodynamics and fuel consumption.The first step for a rational design of an adaptive flap consists in defining the target morphed shapes and the unique optimal flap setting in the take-off and landing phases.In this work,aerodynamic optimization analyses are carried out to determine the best flap setting and related morphed shapes in compliance with the take-off and landing requirements of a reference civil transport aircraft.Four different initial conditions are adopted to avoid the optimization falling into local optima,thus obtaining four groups of optimal candidate configurations.After comparing each candidate’s performance through 2D and 3D simulations,the optimal configuration has been selected.2D simulations show that the optimal configuration is characterized by a maximum lift increase of 31.92%in take-off and 9.04%in landing.According to 3D simulations,the rise in maximum lift equals 22.26%in take-off and 3.50%in landing.Numerical results are finally verified through wind tunnel tests,and the aerodynamic mechanism behind the obtained improvements is explained by carefully analyzing the flow field around the flap.展开更多
Beams,plates,and shells,as the fundamental mechanical structures,are widely used in microelectromechanical systems(MEMS)and nanoelectromechanical systems(NEMS)as sensors,actuators,energy harvesters,and among others.De...Beams,plates,and shells,as the fundamental mechanical structures,are widely used in microelectromechanical systems(MEMS)and nanoelectromechanical systems(NEMS)as sensors,actuators,energy harvesters,and among others.Deeply understand the electromechanical coupling of these dielectric structures is of crucial for designing,fabricating,and optimizing practice devices in these systems.Herein we demonstrate the electromechanical coupling in flexoelectric circular plate,in which higher-order strain gradients were considered to extend the classical electromechanical properties to isotropic materials,in which the non-uniform distribution of the electric potential along the radial direction was considered.Analytical solutions for the vibration modes of the flexoelectric circular plates showed that the dynamic modes were totally different from the piezoelectric circular plates owing to the inversion symmetry breaking by the strain gradient.The electromechanical coupling dynamic modes are sensitive to bending,twisting modes owing to the sensitivity of the flexoelectric effect to bending.This work provides a fundamental understanding of the electromechanical coupling in flexoelectric circular plate,which is helpful in designing novel flexoelectric circular plate-based devices,such as flexoelectric mirrors.展开更多
Corrections for wind tunnel experimental results are crucial when accounting for tun-nel wall interference.This study introduces a new method,the non-uniform wall pres-sure signature method(NUWPSM),which is designed t...Corrections for wind tunnel experimental results are crucial when accounting for tun-nel wall interference.This study introduces a new method,the non-uniform wall pres-sure signature method(NUWPSM),which is designed to address tunnel wall interfer-ence in airfoil.The improved wall pressure signature method(WPSM),an enhanced version of the WPSM,is developed to address the velocity disparities and systematic errors in pressure measurements between with and without model conditions.Fur-thermore,the NUWPSM considers the non-uniformity of the flow induced by the lim-ited far-field effect in wind tunnel experiments.Utilizing experimental data from three different scaled models of the WA210 airfoil,the efficacy of both the Improved WPSM and NUWPSM is verified.Results indicate that the Improved WPSM exhibits superior capabilities in simulating the distribution of axial induced velocity along the wall com-pared to the traditional WPSM.Additionally,both the Improved WPSM and NUWPSM demonstrate comparable abilities in correcting tunnel wall interference,achiev-ing precise corrections within an angle of attack range of-180°to+180°.Notably,the NUWPSM effectively captures the velocity non-uniformity induced by the limited far-field effect,thereby extending its applicability to a broader range of scenarios.Furthermore,the NUWPSM showcases enhanced robustness by eliminating human intervention in the singularity quantity and distribution.展开更多
Dynamic stall under large Reynolds numbers and large reduced frequencies has a significant effect on the performance of the wind turbine blades,helicopter rotors,etc.So the dynamic stall physics of the NACA0012 airfoi...Dynamic stall under large Reynolds numbers and large reduced frequencies has a significant effect on the performance of the wind turbine blades,helicopter rotors,etc.So the dynamic stall physics of the NACA0012 airfoil under a large Reynolds number of Re=1.5×10^(6) was studied using experimental and numerical methods.The reduced frequency range was k=0.035-0.1.The unsteady flow field in dynamic stall was studied in detail by using the transient pressure measurement and the numerical simulation based on the unsteady Reynolds-averaged Navier-Stokes(URANS)equation.And the time-frequency characteristics of the dynamic stall were studied using the wavelet analysis.The study showed that the aerodynamic performance during the dynamic stall was dominated by the shear layer vortex(SLV)and the leading edge vortex(LEV),and the phase difference between the SLV and the LEV was the key factor in the exist-ence of the bimodal characteristics of the aerodynamic force/moment.There was a significant linear correlation between the negative peak of the vortex-induced Cp and the Cn in the reduced frequency range studied in this paper.During the convection of the near-wall LEV to the trailing edge,the high-frequency features firstly decay,and the multi-scale structures of the LEV become more significant as the reduced frequency gradually increases.展开更多
Single-atom(SA)catalysts represent the ultimate limit of atom use efficiency for catalysis.Promising experimental progress in synthesizing SA catalysts aside,the atomic-scale transformation mechanism from metal nanopa...Single-atom(SA)catalysts represent the ultimate limit of atom use efficiency for catalysis.Promising experimental progress in synthesizing SA catalysts aside,the atomic-scale transformation mechanism from metal nanoparticles(NPs)to metal SAs and the stabilization mechanism of SA catalysts at high temperature remain elusive.Through systematic molecular dynamics simulations,for the first time,we reveal the atomic-scale mechanisms associated with the transformation of a metal NP into an array of stable SAs on a defective carbon surface at a high temperature,using Au as a model material.Simulations reveal the pivotal role of defects in the carbon surface in trapping and stabilizing the Au-SAs at high temperatures,which well explain previous experimental observations.Furthermore,reactive simulations demonstrate that the thermally stable Au-SAs exhibit much better catalyst activity than Au-NPs for the methane oxidation at high temperatures,in which the substantially reduced energy barriers for oxidation reaction steps are the key.Findings in this study offer mechanistic and quantitative guidance for material selection and optimal synthesis conditions to stabilize metal SA catalysts at high temperatures.展开更多
The transition characteristics of dynamic airfoil have significant effects on the aerodynamic performance of wind turbines,helicopter rotor blades,jet engine compressor blades,etc.The time domain and time-frequency do...The transition characteristics of dynamic airfoil have significant effects on the aerodynamic performance of wind turbines,helicopter rotor blades,jet engine compressor blades,etc.The time domain and time-frequency domain characteristics of transition on a NACA0012 airfoil during its pitching oscillation were experimentally studied using wall pressure measurement technology with high time accuracy in this paper.The vari-able slip window technology was used to detect the transition position,and the proper orthogonal decomposition(POD)method and wavelet analysis were combined to perform the time-frequency analysis.In the gradual forward movement of the transi-tion,the low-frequency instability is gradually enhanced by the main flow and the inverse pressure gradient,and significantly submerges the high-frequency fluctuated feature.The higher order moments of the wall pressure during dynamic airfoil transi-tion deviate significantly from the Gaussian characteristics,which is caused by the low-frequency instability and high-frequency burst.The POD method is able to distinguish low-frequency instability from the high-frequency feature.The reduced frequency had significant effects on the transition.With the increase of the reduced frequency,the hysteresis effect of the transition became more and more significant,and the fre-quency component of transition was more concentrated and the energy was stronger.展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.12090030 and 12002261)National Postdoctoral Program for Innovative Talents,China(Grant No.BX20200267)the Fundamental Research Funds for the Central Universities,China(Grant No.xzy012024019)。
文摘During the re-entry of a hypersonic aircraft into the earth’s atmosphere,the surrounding air experiences dissociation,ionization,and other complex chemical phenomena due to extreme temperature by shock wave.To ensure thermal safety,the thermochemical non-equilibrium effects resulting from real-gas behavior should be taken into account.In this paper,the characteristics of a double-cone hypersonic laminar flow,including distributions of wall pressure,heat flux,and species dissociation are numerically analyzed with incoming enthalpy of 9.65-21.77 MJ/kg.The thermochemical non-equilibrium flow at different enthalpy and wall temperatures is performed with two-temperature model and Park’s seven chemical reaction model.It is found that the doublecone flow features complex shock-shock interactions to form triple points.The flow topology is further brought out from the analysis of streamlines.At the lowest incoming enthalpy with isothermal wall conditions,two foci points appear.While others highlight only one focal point.As the increment of incoming enthalpy,the heat flux and dissociation of nitrogen and oxygen also increase.An increasing wall temperature leads to a larger separation bubble and a lower value of heat flux and pressure peak,while massive dissociation occurs without obvious ionization under considered cases.
基金co-supported by the National Natural Science Foundation of China (Nos. 12172275, 12090030)the “111” Program, China (No. B18040)
文摘Adaptive,morphing flaps are taking ever-increasing attention in civil aviation thanks to the expected benefits this technology can bring at the aircraft level in terms of high-lift performance improvement and related fuel burnt reduction per flight.Relying upon morphing capabilities,it is possible to fix a unique setting for the flap and adapt the flap shape to match the aerodynamic requirements for take-off or landing.The proper morphed shapes can assure better high-lift performances than those achievable by referring to a conventional flap.Moreover,standing the unique flap setting for take-off and landing,a dramatic simplification of the flap deployment systems may be achieved.As a consequence of this simplification,the deployment system can be fully hosted in the wing,thus avoiding under-wing nacelles with significantly better aerodynamics and fuel consumption.The first step for a rational design of an adaptive flap consists in defining the target morphed shapes and the unique optimal flap setting in the take-off and landing phases.In this work,aerodynamic optimization analyses are carried out to determine the best flap setting and related morphed shapes in compliance with the take-off and landing requirements of a reference civil transport aircraft.Four different initial conditions are adopted to avoid the optimization falling into local optima,thus obtaining four groups of optimal candidate configurations.After comparing each candidate’s performance through 2D and 3D simulations,the optimal configuration has been selected.2D simulations show that the optimal configuration is characterized by a maximum lift increase of 31.92%in take-off and 9.04%in landing.According to 3D simulations,the rise in maximum lift equals 22.26%in take-off and 3.50%in landing.Numerical results are finally verified through wind tunnel tests,and the aerodynamic mechanism behind the obtained improvements is explained by carefully analyzing the flow field around the flap.
基金supported by the National Natural Science Foundation of China(Grant Nos.12122209,12072251,and 12102153)the Project B18040.
文摘Beams,plates,and shells,as the fundamental mechanical structures,are widely used in microelectromechanical systems(MEMS)and nanoelectromechanical systems(NEMS)as sensors,actuators,energy harvesters,and among others.Deeply understand the electromechanical coupling of these dielectric structures is of crucial for designing,fabricating,and optimizing practice devices in these systems.Herein we demonstrate the electromechanical coupling in flexoelectric circular plate,in which higher-order strain gradients were considered to extend the classical electromechanical properties to isotropic materials,in which the non-uniform distribution of the electric potential along the radial direction was considered.Analytical solutions for the vibration modes of the flexoelectric circular plates showed that the dynamic modes were totally different from the piezoelectric circular plates owing to the inversion symmetry breaking by the strain gradient.The electromechanical coupling dynamic modes are sensitive to bending,twisting modes owing to the sensitivity of the flexoelectric effect to bending.This work provides a fundamental understanding of the electromechanical coupling in flexoelectric circular plate,which is helpful in designing novel flexoelectric circular plate-based devices,such as flexoelectric mirrors.
基金This work was sponsored by the National Natural Science Foundation of China(No.12302303)the China Postdoctoral Science Foundation(No.2023M732777)+1 种基金the foundation of National Key Laboratory of Aircraft Configuration Design(No.ZYTS-202403)the foundation of National Key Laboratory of Science and Technology on Aerodynamic Design and Research(No.2023-JCJQ-LB-070).
文摘Corrections for wind tunnel experimental results are crucial when accounting for tun-nel wall interference.This study introduces a new method,the non-uniform wall pres-sure signature method(NUWPSM),which is designed to address tunnel wall interfer-ence in airfoil.The improved wall pressure signature method(WPSM),an enhanced version of the WPSM,is developed to address the velocity disparities and systematic errors in pressure measurements between with and without model conditions.Fur-thermore,the NUWPSM considers the non-uniformity of the flow induced by the lim-ited far-field effect in wind tunnel experiments.Utilizing experimental data from three different scaled models of the WA210 airfoil,the efficacy of both the Improved WPSM and NUWPSM is verified.Results indicate that the Improved WPSM exhibits superior capabilities in simulating the distribution of axial induced velocity along the wall com-pared to the traditional WPSM.Additionally,both the Improved WPSM and NUWPSM demonstrate comparable abilities in correcting tunnel wall interference,achiev-ing precise corrections within an angle of attack range of-180°to+180°.Notably,the NUWPSM effectively captures the velocity non-uniformity induced by the limited far-field effect,thereby extending its applicability to a broader range of scenarios.Furthermore,the NUWPSM showcases enhanced robustness by eliminating human intervention in the singularity quantity and distribution.
基金the Key Laboratory of Flow Visualization and Measurement Techniques,AVIC Aerodynamics Research Institute(XFX20220502)the foundation of National Key Laboratory of Science and Technology on Aerodynamic Design and Research(No.61422010401).
文摘Dynamic stall under large Reynolds numbers and large reduced frequencies has a significant effect on the performance of the wind turbine blades,helicopter rotors,etc.So the dynamic stall physics of the NACA0012 airfoil under a large Reynolds number of Re=1.5×10^(6) was studied using experimental and numerical methods.The reduced frequency range was k=0.035-0.1.The unsteady flow field in dynamic stall was studied in detail by using the transient pressure measurement and the numerical simulation based on the unsteady Reynolds-averaged Navier-Stokes(URANS)equation.And the time-frequency characteristics of the dynamic stall were studied using the wavelet analysis.The study showed that the aerodynamic performance during the dynamic stall was dominated by the shear layer vortex(SLV)and the leading edge vortex(LEV),and the phase difference between the SLV and the LEV was the key factor in the exist-ence of the bimodal characteristics of the aerodynamic force/moment.There was a significant linear correlation between the negative peak of the vortex-induced Cp and the Cn in the reduced frequency range studied in this paper.During the convection of the near-wall LEV to the trailing edge,the high-frequency features firstly decay,and the multi-scale structures of the LEV become more significant as the reduced frequency gradually increases.
文摘Single-atom(SA)catalysts represent the ultimate limit of atom use efficiency for catalysis.Promising experimental progress in synthesizing SA catalysts aside,the atomic-scale transformation mechanism from metal nanoparticles(NPs)to metal SAs and the stabilization mechanism of SA catalysts at high temperature remain elusive.Through systematic molecular dynamics simulations,for the first time,we reveal the atomic-scale mechanisms associated with the transformation of a metal NP into an array of stable SAs on a defective carbon surface at a high temperature,using Au as a model material.Simulations reveal the pivotal role of defects in the carbon surface in trapping and stabilizing the Au-SAs at high temperatures,which well explain previous experimental observations.Furthermore,reactive simulations demonstrate that the thermally stable Au-SAs exhibit much better catalyst activity than Au-NPs for the methane oxidation at high temperatures,in which the substantially reduced energy barriers for oxidation reaction steps are the key.Findings in this study offer mechanistic and quantitative guidance for material selection and optimal synthesis conditions to stabilize metal SA catalysts at high temperatures.
基金the Key Laboratory of Flow Visualization and Measurement Techniques,AVIC Aerodynamics Research Institute(XFX20220502)。
文摘The transition characteristics of dynamic airfoil have significant effects on the aerodynamic performance of wind turbines,helicopter rotor blades,jet engine compressor blades,etc.The time domain and time-frequency domain characteristics of transition on a NACA0012 airfoil during its pitching oscillation were experimentally studied using wall pressure measurement technology with high time accuracy in this paper.The vari-able slip window technology was used to detect the transition position,and the proper orthogonal decomposition(POD)method and wavelet analysis were combined to perform the time-frequency analysis.In the gradual forward movement of the transi-tion,the low-frequency instability is gradually enhanced by the main flow and the inverse pressure gradient,and significantly submerges the high-frequency fluctuated feature.The higher order moments of the wall pressure during dynamic airfoil transi-tion deviate significantly from the Gaussian characteristics,which is caused by the low-frequency instability and high-frequency burst.The POD method is able to distinguish low-frequency instability from the high-frequency feature.The reduced frequency had significant effects on the transition.With the increase of the reduced frequency,the hysteresis effect of the transition became more and more significant,and the fre-quency component of transition was more concentrated and the energy was stronger.
