Side loads and aeroelastic stability of rocket nozzle were studied by solving Navier-Stokes equation coupled with structural equation of motion.The computation was implemented at different total pressure inlet conditi...Side loads and aeroelastic stability of rocket nozzle were studied by solving Navier-Stokes equation coupled with structural equation of motion.The computation was implemented at different total pressure inlet conditions,and flow phenomena of free shock separation(FSS) and restricted shock separation(RSS) were captured.At certain total pressure inlet conditions,it was found that both kinds of separations existed in nozzle flow filed,while RSS exhibited combined space asymmetry and time unsteady characteristics.The corresponding asymmetric circumferential pressure distribution,strong pressure fluctuation in separation region and large range of displacement of shock wave all led to severe side loads.Besides,for flexible nozzles,the low pressure gradient in separation region might reduce structure stability at nozzle exit,resulting in large local deformation.It was also found that aeroelasticity exhibited buffeting characteristic due to the asymmetric separation,resulting in reduction of aeroelastic stability,even structure destruction.Moreover,aeroelasticity might amplify side loads and aggravate its growth rate.However,with increment of inlet pressure,nozzle aeroelastic stability was also increased when a full flow was nearly reached.展开更多
The lower-upper symmetric Gauss-Seidel (LU-SGS) implicit relaxation has been widely used because it has the merits of less dependency on grid topology, low numerical complexity and modest memory requirements. In ori...The lower-upper symmetric Gauss-Seidel (LU-SGS) implicit relaxation has been widely used because it has the merits of less dependency on grid topology, low numerical complexity and modest memory requirements. In original LU-SGS scheme, the implicit system matrix is constructed based on the splitting of convective flux Jacobian according to its spectral radius. Although this treatment has the merit of reducing computational complexity and helps to ensure the diagonally dominant property of the implicit system matrix, it can also cause serious distortions on the implicit system matrix because too many approximations are introduced by this splitting method if the contravariant velocity is small or close to sonic speed. To overcome this shortcoming, an improved LU-SGS scheme with a hybrid construction method for the implicit system matrix is developed in this paper. The hybrid way is that: on the cell faces having small contravariant velocity or transonic contravariant velocity, the accurate derivative of the convective flux term is used to construct more accurate implicit system matrix, while the original Jacobian splitting method is adopted on the other cell faces to reduce computational complexity and ensure the diagonally dominant property of the implicit system matrix. To investigate the convergence performance of the improved LU-SGS scheme, 2D and 3D turbulent flows around the NACA0012 airfoil, RAE2822 airfoil and LANN wing are simulated on hybrid unstructured meshes. The nu- merical results show that the improved LU-SGS scheme is significantly more efficient than the original LU-SGS scheme.展开更多
The improved line sampling (LS) technique, an effective numerical simulation method, is employed to analyze the probabilistic characteristics and reliability sensitivity of flutter with random structural parameter i...The improved line sampling (LS) technique, an effective numerical simulation method, is employed to analyze the probabilistic characteristics and reliability sensitivity of flutter with random structural parameter in transonic flow. The improved LS technique is a novel methodology for reliability and sensitivity analysis of high dimensionality and low probability problem with implicit limit state function, and it does not require any approximating surrogate of the implicit limit state equation. The improved LS is used to estimate the flutter reliability and the sensitivity of a two-dimensional wing, in which some structural properties, such as frequency, parameters of gravity center and mass ratio, are considered as random variables. Computational fluid dynamics (CFD) based unsteady aerodynamic reduced order model (ROM) method is used to construct the aerodynamic state equations. Coupling structural state equations with aerodynamic state equations, the safety margin of flutter is founded by using the critical velocity of flutter. The results show that the improved LS technique can effectively decrease the computational cost in the random uncertainty analysis of flutter. The reliability sensitivity, defined by the partial derivative of the failure probability with respect to the distribution parameter of random variable, can help to identify the important parameters and guide the structural optimization design.展开更多
Our research aim is to investigate the buffet alleviation effect of static or vibrating bulges attached on the forebody surface of the model.Experiments and numerical simulations on a model consisting of a sharp-edged...Our research aim is to investigate the buffet alleviation effect of static or vibrating bulges attached on the forebody surface of the model.Experiments and numerical simulations on a model consisting of a sharp-edged,70°-leading edge sweep delta wing and twin swept back vertical tails were conducted.Models with different bulges were tested and computed at 10 and 20 m/s of free stream velocity at angles of attack ranging from 20°–50°.Dynamic strain gauge and multichannel data acquisition and analysis system were employed for the measurement of unsteady root strain on the vertical tails.Experimental and computational results show that both static and vibrating bulges behave effectively as a novel tool to alleviate tail buffet,and the alleviation effect depends largely on the vibrating frequency.Besides,one-sided bulge can only alleviate the buffeting response for the tail of the same side,and it has no obvious alleviation effect for the opposite tail.Results of the spectral analysis reveal that there are generally three peaks of spectral density for aircrafts of this configuration,and bulges used in this paper could alleviate tail buffeting,but the total lift and drag of the whole model show no obvious deviation compared to the base model and the dominant frequency of the vibration of the tails has not shifted.展开更多
Transonic single-degree-of-freedom(SDOF) flutter and transonic buffet are the typical and complex aeroelastic phenomena in the transonic flow. In this study, transonic aeroelastic issues of an elastic airfoil are inve...Transonic single-degree-of-freedom(SDOF) flutter and transonic buffet are the typical and complex aeroelastic phenomena in the transonic flow. In this study, transonic aeroelastic issues of an elastic airfoil are investigated using Unsteady Reynolds-Averaged Navier-Stokes(URANS) equations. The airfoil is free to vibrate in SDOF of pitching. It is found that, the coupling system may be unstable and SDOF self-excited pitching oscillations occur in pre-buffet flow condition, where the free-stream angle of attack(AOA) is lower than the buffet onset of a stationary airfoil. In the theory of classical aeroelasticity, this unstable phenomenon is defined as flutter. However, this transonic SDOF flutter is closely related to transonic buffet(unstable aerodynamic models) due to the following reasons. Firstly, the SDOF flutter occurs only when the free-stream AOA of the spring suspended airfoil is slightly lower than that of buffet onset, and the ratio of the structural characteristic frequency to the buffet frequency is within a limited range. Secondly, the response characteristics show a high correlation between the SDOF flutter and buffet. A similar "lock-in" phenomenon exists, when the coupling frequency follows the structural characteristic frequency. Finally, there is no sudden change of the response characteristics in the vicinity of buffet onset, that is, the curve of response amplitude with the free-stream AOA is nearly smooth. Therefore, transonic SDOF flutter is often interwoven with transonic buffet and shows some complex characteristics of response, which is different from the traditional flutter.展开更多
For the stochastic structure with stochastic excitation, an advanced stratified line sampling (SLS) method is presented to obtain the cumulative distribution function (CDF) of the structural response and its sensitivi...For the stochastic structure with stochastic excitation, an advanced stratified line sampling (SLS) method is presented to obtain the cumulative distribution function (CDF) of the structural response and its sensitivity. The advanced stratified line sampling method introduces a set of middle failure subsets firstly. And for each subset, the conventional line sampling can be used to obtain the corresponding value of the response's CDF. At the same time, the sensitivity estimations of each failure subset can also be computed by modifying the important direction and corresponding reliability coefficients. The properties of CDF sensitivity are proved while the performance function is linear with normal random variables. After two simple examples are used to demonstrate the properties of CDF sensitivity and the feasibility of the presented method, the method employed to analyze the CDF and corresponding sensitivity of root bending moment (RBM) responses for the stochastic BAH is wing with gust excitation to a square-edged gust and to a Dryden gust. The results show that the parameters of the second and the fifth order modals exert more influence on the CDF of response than the other ones, and the presented SLS method can more significantly reduce the computational cost compared with Monte Carlo simulation (MCS).展开更多
A method of unstructured dynamic overset grids is developed for the numerical simulation of helicopter unsteady rotorairframe aerodynamic interaction.For the effective treatment of the relative motion between the roto...A method of unstructured dynamic overset grids is developed for the numerical simulation of helicopter unsteady rotorairframe aerodynamic interaction.For the effective treatment of the relative motion between the rotor and the airframe,the domain of flowfield is divided into two overset subzones,namely,a rotational subzone containing the blades and a stationary subzone containing the airframe.The overset part of two subzones is used to convect the flow variables of the two zones.The Taylor series expansion is used to obtain a second-order spatial accuracy,and dual-time stepping is adopted to improve the solution accuracy.Mesh deformation from the blade motion in forward flight is treated by using a spring analogy.Validation is made by numerically simulating the flows around a wind tunnel configuration and comparing the predicted time-averaged and instantaneous inflow and airframe surface pressure distributions with the experimental data.It shows that the present method is efficient and robust for the prediction of complicated unsteady rotor-airframe aerodynamic interaction phenomena.展开更多
Numerical simulations are performed to study the aeroelastic responses of an elastically suspended airfoil in transonic buffet flow, by coupling the unsteady Reynolds-averaged Navier- Stokes (RANS) equations and str...Numerical simulations are performed to study the aeroelastic responses of an elastically suspended airfoil in transonic buffet flow, by coupling the unsteady Reynolds-averaged Navier- Stokes (RANS) equations and structural motion equation. The current work focuses on the char- acteristic analysis of the lock-in phenomenon. Great attentions are paid to studying the frequency range of lock-in and the effects of the three parameters, namely the structural natural frequency, mass ratio and structural damping, on lock-in characteristic of the elastic system in detail. It is found that when the structural natural frequency is close to the buffet frequency, the coupling fre- quency of the elastic system is no longer equal to the buffet frequency, but keeps the same value as the structural natural frequency. The frequency lock-in occurs and stays present until the structural nature frequency is near the double buffet frequency. It means that the lock-in presents within a broad range, of which the lower threshold is near the buffet frequency, while the upper threshold is near the double buffet frequency. Moreover, the frequency range of lock-in is affected by mass ratio and structural damping. The lower the mass ratio and structural damping are, the wider the range of lock-in will be. The upper threshold of lock-in grows with the mass ratio and structural damping decreasing, but the lower threshold always keeps the same.展开更多
It is scientifically important science value and engineering promising to develop the buoyancy-lift integrated hybrid airship for high attitude platform.Through the numerical method,a new tandem wings hybrid airship w...It is scientifically important science value and engineering promising to develop the buoyancy-lift integrated hybrid airship for high attitude platform.Through the numerical method,a new tandem wings hybrid airship with both higher utility value and economy efficiency was obtained and its total performance and technical parameters were analyzed in detail.In order to further improve the lift-drag characteristics,we implemented the optimization design for aerodynamic configuration of tandem wings hybrid airship via the response surface method.The results indicate that the tandem wings hybrid airship has considerable volume efficiency and higher aerodynamic characteristics.After optimization,the lift-drag ratio of this hybrid airship was increased by 6.08%.In a given gross lift condition,tandem wings hybrid airship may provide more payload and specific productivity.Furthermore,the size of tandem airship is smaller so the demand for skin flexible materials can be reduced.Results of this study could serve as a new approach to designing buoyancy-lifting integrated hybrid airship.展开更多
基金supported by the National Natural Science Foundation of China (Grant No. 11072199)
文摘Side loads and aeroelastic stability of rocket nozzle were studied by solving Navier-Stokes equation coupled with structural equation of motion.The computation was implemented at different total pressure inlet conditions,and flow phenomena of free shock separation(FSS) and restricted shock separation(RSS) were captured.At certain total pressure inlet conditions,it was found that both kinds of separations existed in nozzle flow filed,while RSS exhibited combined space asymmetry and time unsteady characteristics.The corresponding asymmetric circumferential pressure distribution,strong pressure fluctuation in separation region and large range of displacement of shock wave all led to severe side loads.Besides,for flexible nozzles,the low pressure gradient in separation region might reduce structure stability at nozzle exit,resulting in large local deformation.It was also found that aeroelasticity exhibited buffeting characteristic due to the asymmetric separation,resulting in reduction of aeroelastic stability,even structure destruction.Moreover,aeroelasticity might amplify side loads and aggravate its growth rate.However,with increment of inlet pressure,nozzle aeroelastic stability was also increased when a full flow was nearly reached.
基金Foundation item: National Natural Science Foundation of China (10802067)
文摘The lower-upper symmetric Gauss-Seidel (LU-SGS) implicit relaxation has been widely used because it has the merits of less dependency on grid topology, low numerical complexity and modest memory requirements. In original LU-SGS scheme, the implicit system matrix is constructed based on the splitting of convective flux Jacobian according to its spectral radius. Although this treatment has the merit of reducing computational complexity and helps to ensure the diagonally dominant property of the implicit system matrix, it can also cause serious distortions on the implicit system matrix because too many approximations are introduced by this splitting method if the contravariant velocity is small or close to sonic speed. To overcome this shortcoming, an improved LU-SGS scheme with a hybrid construction method for the implicit system matrix is developed in this paper. The hybrid way is that: on the cell faces having small contravariant velocity or transonic contravariant velocity, the accurate derivative of the convective flux term is used to construct more accurate implicit system matrix, while the original Jacobian splitting method is adopted on the other cell faces to reduce computational complexity and ensure the diagonally dominant property of the implicit system matrix. To investigate the convergence performance of the improved LU-SGS scheme, 2D and 3D turbulent flows around the NACA0012 airfoil, RAE2822 airfoil and LANN wing are simulated on hybrid unstructured meshes. The nu- merical results show that the improved LU-SGS scheme is significantly more efficient than the original LU-SGS scheme.
基金Foundation items: National Natural Science Foundation of China (NSFC 10572117, 10802063, 50875213) National High-tech Research and Development Program (2007AA04Z401)+2 种基金 Aeronautical Science Foundation of China (2007ZA53012) New Century Program For Excellent Talents of Ministry of Education of China (NCET-05-0868) Ph.D. Program Foundation of Northwestern Polytechnical University (CX200801).
文摘The improved line sampling (LS) technique, an effective numerical simulation method, is employed to analyze the probabilistic characteristics and reliability sensitivity of flutter with random structural parameter in transonic flow. The improved LS technique is a novel methodology for reliability and sensitivity analysis of high dimensionality and low probability problem with implicit limit state function, and it does not require any approximating surrogate of the implicit limit state equation. The improved LS is used to estimate the flutter reliability and the sensitivity of a two-dimensional wing, in which some structural properties, such as frequency, parameters of gravity center and mass ratio, are considered as random variables. Computational fluid dynamics (CFD) based unsteady aerodynamic reduced order model (ROM) method is used to construct the aerodynamic state equations. Coupling structural state equations with aerodynamic state equations, the safety margin of flutter is founded by using the critical velocity of flutter. The results show that the improved LS technique can effectively decrease the computational cost in the random uncertainty analysis of flutter. The reliability sensitivity, defined by the partial derivative of the failure probability with respect to the distribution parameter of random variable, can help to identify the important parameters and guide the structural optimization design.
基金supported by the National Natural Science Foundation of China(Grant No.11072199)
文摘Our research aim is to investigate the buffet alleviation effect of static or vibrating bulges attached on the forebody surface of the model.Experiments and numerical simulations on a model consisting of a sharp-edged,70°-leading edge sweep delta wing and twin swept back vertical tails were conducted.Models with different bulges were tested and computed at 10 and 20 m/s of free stream velocity at angles of attack ranging from 20°–50°.Dynamic strain gauge and multichannel data acquisition and analysis system were employed for the measurement of unsteady root strain on the vertical tails.Experimental and computational results show that both static and vibrating bulges behave effectively as a novel tool to alleviate tail buffet,and the alleviation effect depends largely on the vibrating frequency.Besides,one-sided bulge can only alleviate the buffeting response for the tail of the same side,and it has no obvious alleviation effect for the opposite tail.Results of the spectral analysis reveal that there are generally three peaks of spectral density for aircrafts of this configuration,and bulges used in this paper could alleviate tail buffeting,but the total lift and drag of the whole model show no obvious deviation compared to the base model and the dominant frequency of the vibration of the tails has not shifted.
基金supported by the New Century Program for Excellent Talents of Ministry of Education of China(Grant No.NCET-13-0478)National Natural Science Foundation of China(Grant No.11172237)
文摘Transonic single-degree-of-freedom(SDOF) flutter and transonic buffet are the typical and complex aeroelastic phenomena in the transonic flow. In this study, transonic aeroelastic issues of an elastic airfoil are investigated using Unsteady Reynolds-Averaged Navier-Stokes(URANS) equations. The airfoil is free to vibrate in SDOF of pitching. It is found that, the coupling system may be unstable and SDOF self-excited pitching oscillations occur in pre-buffet flow condition, where the free-stream angle of attack(AOA) is lower than the buffet onset of a stationary airfoil. In the theory of classical aeroelasticity, this unstable phenomenon is defined as flutter. However, this transonic SDOF flutter is closely related to transonic buffet(unstable aerodynamic models) due to the following reasons. Firstly, the SDOF flutter occurs only when the free-stream AOA of the spring suspended airfoil is slightly lower than that of buffet onset, and the ratio of the structural characteristic frequency to the buffet frequency is within a limited range. Secondly, the response characteristics show a high correlation between the SDOF flutter and buffet. A similar "lock-in" phenomenon exists, when the coupling frequency follows the structural characteristic frequency. Finally, there is no sudden change of the response characteristics in the vicinity of buffet onset, that is, the curve of response amplitude with the free-stream AOA is nearly smooth. Therefore, transonic SDOF flutter is often interwoven with transonic buffet and shows some complex characteristics of response, which is different from the traditional flutter.
基金the National Nature Science Foundation of China (Grant No. 51175425)the Aviation Science Foundation (Grant No. 2011ZA53015)+1 种基金the Aerospace Science and Technology Innovative Foundation (Grant No. 2011200093)the Nature Science Basic Research Fund of Shaanxi Province (Grant No. 2012JQ1015)
文摘For the stochastic structure with stochastic excitation, an advanced stratified line sampling (SLS) method is presented to obtain the cumulative distribution function (CDF) of the structural response and its sensitivity. The advanced stratified line sampling method introduces a set of middle failure subsets firstly. And for each subset, the conventional line sampling can be used to obtain the corresponding value of the response's CDF. At the same time, the sensitivity estimations of each failure subset can also be computed by modifying the important direction and corresponding reliability coefficients. The properties of CDF sensitivity are proved while the performance function is linear with normal random variables. After two simple examples are used to demonstrate the properties of CDF sensitivity and the feasibility of the presented method, the method employed to analyze the CDF and corresponding sensitivity of root bending moment (RBM) responses for the stochastic BAH is wing with gust excitation to a square-edged gust and to a Dryden gust. The results show that the parameters of the second and the fifth order modals exert more influence on the CDF of response than the other ones, and the presented SLS method can more significantly reduce the computational cost compared with Monte Carlo simulation (MCS).
基金supported by the Basic Research Program of Northwestern Polytechnical University (Grant No. JC201219)the Postdoctoral Science Foundation of China (Grant No. 20100481368)
文摘A method of unstructured dynamic overset grids is developed for the numerical simulation of helicopter unsteady rotorairframe aerodynamic interaction.For the effective treatment of the relative motion between the rotor and the airframe,the domain of flowfield is divided into two overset subzones,namely,a rotational subzone containing the blades and a stationary subzone containing the airframe.The overset part of two subzones is used to convect the flow variables of the two zones.The Taylor series expansion is used to obtain a second-order spatial accuracy,and dual-time stepping is adopted to improve the solution accuracy.Mesh deformation from the blade motion in forward flight is treated by using a spring analogy.Validation is made by numerically simulating the flows around a wind tunnel configuration and comparing the predicted time-averaged and instantaneous inflow and airframe surface pressure distributions with the experimental data.It shows that the present method is efficient and robust for the prediction of complicated unsteady rotor-airframe aerodynamic interaction phenomena.
基金supported by the project of the National Natural Science Foundation of China(No.11272262)
文摘Numerical simulations are performed to study the aeroelastic responses of an elastically suspended airfoil in transonic buffet flow, by coupling the unsteady Reynolds-averaged Navier- Stokes (RANS) equations and structural motion equation. The current work focuses on the char- acteristic analysis of the lock-in phenomenon. Great attentions are paid to studying the frequency range of lock-in and the effects of the three parameters, namely the structural natural frequency, mass ratio and structural damping, on lock-in characteristic of the elastic system in detail. It is found that when the structural natural frequency is close to the buffet frequency, the coupling fre- quency of the elastic system is no longer equal to the buffet frequency, but keeps the same value as the structural natural frequency. The frequency lock-in occurs and stays present until the structural nature frequency is near the double buffet frequency. It means that the lock-in presents within a broad range, of which the lower threshold is near the buffet frequency, while the upper threshold is near the double buffet frequency. Moreover, the frequency range of lock-in is affected by mass ratio and structural damping. The lower the mass ratio and structural damping are, the wider the range of lock-in will be. The upper threshold of lock-in grows with the mass ratio and structural damping decreasing, but the lower threshold always keeps the same.
基金supported by the National High-Tech Research and Development Program of China (Grant No. 863-2007AA11Z243)Foundation for Basic Research of Northwestern Polytechnic University (Grant No.JC-201103)
文摘It is scientifically important science value and engineering promising to develop the buoyancy-lift integrated hybrid airship for high attitude platform.Through the numerical method,a new tandem wings hybrid airship with both higher utility value and economy efficiency was obtained and its total performance and technical parameters were analyzed in detail.In order to further improve the lift-drag characteristics,we implemented the optimization design for aerodynamic configuration of tandem wings hybrid airship via the response surface method.The results indicate that the tandem wings hybrid airship has considerable volume efficiency and higher aerodynamic characteristics.After optimization,the lift-drag ratio of this hybrid airship was increased by 6.08%.In a given gross lift condition,tandem wings hybrid airship may provide more payload and specific productivity.Furthermore,the size of tandem airship is smaller so the demand for skin flexible materials can be reduced.Results of this study could serve as a new approach to designing buoyancy-lifting integrated hybrid airship.