Computational fluid dynamics based dynamic modeling of parafoil system

The calculation of aerodynamic coefficients has been one of the key issues when modeling parafoil systems, that directly affects model precision. This study relates to investigate limitations of traditional calculation methods. As a result, we achieve aerodynamic parameters of a parafoil using computational fluid dynamics simulations. Also we employ the least square method as a tool for the rapid identification of deflection factors of aerodynamic coefficients. The estimated aerodynamic coefficients of the system were incorporated into the dynamic equations of the parafoil to implement a six degree of freedom model of a parafoil system according to the Kirchhoff equations. Numerical results generated by simulation and airdrop testing demonstrate that the established model can accurately describe the flight characteristics of the parafoil system.     

Computational fluid dynamics simulation of aerodynamic performance and flow separation by single element and slatted airfoils under rainfall conditions

This study investigated the effects of rainfall on flow separation and the aerodynamic performance of single element and slatted NACA 0012 airfoils by using a mathematical model developed with the commercial computational fluid dynamics solver ANSYS FLUENT 18.2. A two-way momentum coupled Eulerian–Lagrangian multiphase approach was used to simulate the formation of the water film layer on the airfoil's surface. According to the results, very low values of the lift-to-drag ratio at low angles of attack reflected severe degradation of the aerodynamic performance of the airfoil in the presence of water accumulated on its surface. The impact of rain droplets on the leading-edge slat surface led to less water accumulating on the main section of the airfoil. In particular, the maximum water film mass concentrated on the airfoil surface decreased from 15 g to 1 g compared with the single element airfoil. Hence, the thickness of the water film layer was not sufficiently large to significantly affect the aerodynamic coefficients of the slatted airfoil, especially the maximum lift coefficient, compared with the thicker water film layer on the single element airfoil. In addition, the use of slats clearly enhanced the aerodynamic coefficients and increased the stall angle from 13° to 22° in dry conditions, and from 16° to 24° in rainy conditions. Slats also significantly decreased the boundary layer thickness and delayed the separation at higher angles of attack.     

A modified discrete-vortex method algorithm with shedding criterion for aerodynamic coefficients prediction at high angle of attack

Low-order methods require less computing power than classical computational fluid dynamics and can be implemented on a laptop computer, which is needed for engineering tasks. Discrete vortex methods are such low order methods that can describe the unsteady separated flow around an airfoil. After a presentation of the leading edge suction parameter discrete vortex method, a modified algorithm is proposed, in order to reduce the computing cost, and compared with the previous one. Several reference unsteady airfoil motions are discussed in terms of gain in the computation time with comparisons between the previous scheme and the present one. The accuracy of the new method is demonstrated through aerodynamic coefficients. The application of the present discrete vortex method to a transient pitching motion of an airfoil is also presented, in order to understand the leading edge vortex formation, and its implication in terms of lift and drag coefficients. The method is not limited to unsteady or transient motions but can also simulate the flow around a constant angle of attack airfoil. In that case, an original method of fast summation of the vortices located far away from the airfoil, allows a linear dependence of the computation time versus the number of vortices shed, which is a great improvement over the quadratic dependence observed in the classical discrete vortex methods. The development of the aerodynamic coefficients with angle of attack, from values ranging between −10° and 90°, is obtained for a purely two-dimensional flow. In particular, the shape of the lift coefficient of the airfoil in the fully detached flow region is established. Comparisons with relevant experimental or computational fluid dynamics data are discussed in order to grasp the influence of upstream turbulence level and three-dimensional effects in the measured data in the fully detached flow region.     

RBF Neural Network Based Prediction on Blade Surface Pressure Fields in Compressors北大核心CSCD

The airflow characteristics of the internal flow path of an aero-engine compressor are complex, and the vortex flow field around the blade is characterized by high pressure, high speed, rotation, and unsteadiness. Therefore, there is an urgent need to calculate and predict the aerodynamic characteristics of the complex flow field around the compressor blade efficiently and accurately. The computational fluid dynamics (CFD) method was used to generate the aerodynamic load distribution on the blade surface under different operating conditions for the study of the complex flow fields around aero-engine blades. The radial based function (RBF) neural network was applied to establish the pressure surface aerodynamic load prediction model, and the neural network modeling method was combined with the flow field calculation. The neural network method can learn and train the CFD-based data set to properly compensate the errors from the CFD, which provides a reference for the effective prediction of the complex flow fields around aero-engine compressor blades. © 2023 Editorial Office of Applied Mathematics and Mechanics. All rights reserved.     

不可压饱和多孔弹性梁的大挠度非线性数学模型

在孔隙流体仅存在沿梁轴线方向扩散的假定下,建立了微观不可压饱和多孔弹性梁大挠度问题的非线性数学模型．利用Galerkin截断法,研究了固定端不可渗透、自由端可渗透的饱和多孔弹性悬臂梁在自由端突加集中载荷作用下的非线性弯曲,得到了梁骨架的挠度、弯矩以及孔隙流体压力等效力偶等的时间响应和沿轴线的分布．比较了大挠度非线性和小挠度线性理论的结果,揭示了两者间的差异．研究发现大挠度理论的结果小于相应的小挠度理论结果,并且,大挠度理论的结果趋于其稳态值的时间小于相应的小挠度理论结果趋于其稳态值的时间．     

径向基函数参数化翼型的气动力降阶模型优化

基于小扰动和弱非线性假设,提出了一种基于气动力降阶模型和径向基函数参数化的翼型优化方法.其主要方法是用径向基函数参数化翼型扰动;通过CFD辨识参数扰动对翼型气动力影响的降阶模型核函数;基于叠加法建立了参数变化对翼型气动力影响的降阶模型;最后基于该气动力降阶模型计算并优化翼型升阻特性.NACA0012翼型优化的结果表明基于气动力降阶模型的优化方法是可行的,可以极大地提高翼型优化速度.     

车身非光滑表面凹坑排布对气动性能的影响

受仿生学非光滑旋成体减阻启发,以SAE(美国机动车工程师协会)标准模型为研究对象,采用CFD(计算流体力学)数值模拟方法,在SAE模型顶部布置不同排布形式和不同排布密度的凹坑单元,研究其对车身气动性能的影响.通过比较各模型的尾流、气流速度、压力场、湍流动能等流场性能指标,分析非光滑表面减阻机理以及造成各模型流场性质差异的原因.计算结果显示:当凹坑型非光滑单元以矩形排布时模型具有最小的气动阻力,且气动阻力随着凹坑密度的增加而减小,减阻率最高达到4.1%.     

A novel dynamics model for railway ballastless track with medium-thick slabs

Motivated by the requirements for elaborated slab ballastless track dynamics analysis in practical engineering application, a novel dynamic model for the railway ballastless tracks with medium-thick slabs is proposed in this work based on the Reissner–Mindlin plate theory, and it is implemented into the coupled dynamics analysis of a vehicle and the ballastless track. First, an efficient and easily programmable computational algorithm is adopted to solve the transverse deflection of the Reissner–Mindlin plate, in which the displacements and shear strains are chosen as the independent variables and subsequently constructed by spline functions, resulting in no shear-locking effect. The involved partial differential equations are transformed into ordinary ones by using the energy variation principle. Further, a mathematical model for the ballastless track dynamics analysis is established, which can consider the effects of the shear deformation and moment of inertia involved in the medium-thick track slab. Experimental verification and comparative analysis with other models demonstrate the accuracy and efficiency of the proposed model. Finally, a spatially coupled dynamics model of a vehicle and the ballastless track is developed, and it is efficiently solved by using the hybrid explicit-implicit time integration method. Compared with the widely used modelling the track slab by elastic thin plate, the reliability and advantages of the proposed vehicle-slab track coupled dynamics model are demonstrated.     

Contribution to the propeller aerodynamic interference

Propeller wake can significantly change the flowfield at the downstream lifting surfaces and therefore influence its aerodynamic coefficients. The numerical model of the propeller presented here is using discrete vortices to form vortex sheet that is leaving each blade. Model is also applicable for combination of lifting surface and propeller using undeveloped propeller vortex sheet in determination of aerodynamic interference of the propeller on the downstream lifting surface, wing or tail for small angle of attack. This low computational cost numerical model is suitable for implementation in component build–up method used in preliminary estimation of aerodynamic coefficients for different propeller aircraft configurations.     

大气制动期间探测器的气动特性和轨道计算

以NASA火星全球勘测（MGS）号探测器作为大气制动计算模型,应用DSMC方法模拟了探测器在大气制动期间的稀薄气体动力学特性,分析了不同来流密度情况下探测器的流场特性、气动特性的变化情况以及气体动力学系数的变化关系．并提出了气动力-气动热-轨道一体化计算模型,利用DSMC模拟技术以及经典动力学理论对大气制动轨道变化进行了计算和分析．研究结果表明:行星大气密度、探测器俯仰角、偏航角的变化对探测器的气动特性有重要影响,计算结果与文献中的结果表现出很好的一致性;气动力-气动热-轨道一体化计算可以模拟大气制动过程,模拟所得到的大气制动轨迹反映出较好的制动效果．     

A partitioned strongly coupled fluid-structure interaction method to model heart valve dynamics

A new method for the calculation of fluid-structure interaction (FSI) of highly flexible bodies is presented. This innovative algorithm demonstrates the strong coupling of a commercial computational fluid dynamics code with an in-house coded structural solver. The strong response of the pressure distribution to the displacement can be approximated by a reduced order model for the fluid solver. The Jacobian of this reduced order model is then used in the structural solver to obtain a stable and full implicit iteration scheme. The method is demonstrated on a 2D model of a flexible aortic valve during the cardiac cycle. Furthermore, the model is able to calculate shear stresses on the leaflet.     

Computation of multispecies diffusion rates

The Fick diffusion law is generally used in combustion theory. A shortcoming of this model is that it violates the momentum conservation law. The hydrodynamic theory of multispecies diffusion is free of this disadvantage. A highly effective method for determining an approximate solution to the multispecies fluid dynamics equations is proposed. The method is comparable to the Fick diffusion model in terms of computational costs.     

基于Schwarz-Christoffel变换的非圆截面血管流场分布研究

应用Schwarz-Christoffel(S-C)变换方法,实现从复平面单位圆到多边形区域的共形映射,结合圆形管道下完全发展脉动流的Womersley算法理论,建立了基于S-C映射的非圆入口截面下的Womersley速度边界模型.在边界模型建立的基础上,应用计算流体力学方法,对基于生理真实的人体肺动脉二级分支血管在一个心动周期内的血流流动情况进行了数值模拟,并与通过外接圆管法设定入口速度边界条件得到的流场模拟结果进行了对比分析.分析结果表明,两者的模拟结果高度一致,但考虑到模拟效率和数值模拟结果的确定性,基于S-C映射的Womersley速度边界模型优于外接圆管方法,对于血管血流动力学的模拟研究更具有现实意义.     

Feedback flow control employing local dynamical modelling with wavelets

In this paper, we utilize wavelet transform to obtain dynamical models describing the behaviour of fluid flow in a local spatial region of interest. First, snapshots of the flow are obtained from experiments or from computational fluid dynamics (CFD) simulations of the governing equations. A wavelet family and decomposition level is selected by assessing the reconstruction success under the resulting inverse transform. The flow is then expanded onto a set of basis vectors that are constructed from the wavelet function. The wavelet coefficients associated with the basis vectors capture the time variation of the flow within the spatial region covered by the support of the basis vectors. A dynamical model is established for these coefficients by using subspace identification methods. The approach developed is applied to a sample flow configuration on a square domain where the input affects the system through the boundary conditions. It is observed that there is good agreement between CFD simulation results and the predictions of the dynamical model. A controller is designed based on the dynamical model and is seen to be successful in regulating the velocity of a given point within the region of interest.     

超声波测速仪的计算流体力学数值模拟研究

超声波测速仪是一种利用超声波发射接收装置,通过发射接收时间来计算来流速度的一种仪器装置.目前国内许多研究都关注在如何消除测速仪的测量误差上,对于模型结构对测量风场的影响研究较少.为了考察测速仪的测量精度,本研究以模型结构对测量区域风速的影响作为关注的重点,采用计算流体力学方法模拟了从低速到高速的不同来流风速下的绕流流场,计算出位于测速仪中心区域的速度分布和不同截面上的平均速度,以判断测速仪模型结构对中心测量区域风速的影响程度.该研究结果表明在测速探头平面内测量到的速度值,无论在低速和高速时均最为准确.采用计算流体力学数值模拟方法可为今后的测速仪模型设计和改造提供准确的依据.     

Computational fluid dynamics and experimental study of the hydrodynamics of a gas–solid tapered fluidized bed

In the present work, experimental and numerical studies for the hydrodynamics in a gas–solid tapered fluidized bed have been carried out. The experimental results obtained by carrying out experiments in a tapered fluidized bed for glass bead (spherical) of 2.0 mm and dolomite (non-spherical particles) of 2.215 mm in diameter, were compared with the computational fluid dynamics (CFD) simulation results, using a commercial CFD software package, Fluent. The gas–solid flow was simulated using the Eulerian–Eulerian model and applying the kinetic theory of granular flow for solid particles. The Gidaspow drag model was used to calculate the gas–solid momentum exchange coefficients. Pressure drops predicted by the CFD simulations agreed reasonably well with experimental measurements for both types (spherical and non-spherical) of particles. Good agreement was also obtained between experimental and CFD predicted bed expansion ratios for both types of particles. Present study provides a useful basis for further works on the CFD of tapered fluidized bed.     

微曲输流管道振动固有频率分析与仿真北大核心CSCD

首次建立了基于Timoshenko梁理论的微曲输流管道横向振动的动力学模型,并分析了流体流动影响下微曲管道横向自由振动的固有特征.采用广义Hamilton原理,导出了考虑流体影响的微曲管道横向振动的控制方程,通过Galerkin截断对控制方程离散化,再由广义本征值问题得到管道横向振动的固有频率,并研究了液体流速和弯曲幅度对管道横向固有振动特征的影响.发展了基于等效刚度和等效阻尼方法的考虑流体影响的微曲管道振动分析的有限元仿真计算方法,并通过有限元软件实现数值仿真,验证了Galerkin截断的分析结果以及所建立的Timoshenko微曲管道动力学模型的有效性.研究表明,流体的流速以及管道的弯曲幅度对管道横向振动固有频率均有显著影响.     

粘弹性板混沌振动的输出变量反馈线性化控制

研究了粘弹性板混沌振动的控制问题·　应用非线性系统精确线性化控制理论导出了一类非仿射控制系统的非线性反馈控制律·　建立了描述材料非线性的粘弹性板运动的数学模型并利用Ｃａｌｅｒｋｉｎ 方法进行简化·　采用相空间曲线和频率谱密度函数说明了在特定参数条件下系统将出现混沌振动,并以位移为输出变量将混沌振动控制为给定的周期运动·     

Nonstationary breakaway flow over a permeable parachute

The properties of a uniformly permeable surface that models the canopy of a real parachute are presented. A modernization of the discrete vortex method is proposed that makes it possible to apply it to calculate the flow over uniformly permeable surfaces. The results of computations of the aerodynamic characteristics of permeable parachutes are presented and compared with experimental data.Translated fromDinamichekie Sistemy, No. 6, pp.3–10, 1987.     

带控制面机翼结构基于弧长数值连续法的颤振特征研究

以三自由度二元机翼为研究对象,将浮沉位移和俯仰位移方向的非线性刚度简化为立方非线性,对于存在间隙的控制面采用双线性刚度代替.考虑准定常气流,建立气动弹性运动方程,通过数值模拟构造峰值-峰值图,反映其在不同气流速度下的振动特征.通过弧长数值连续法构造系统的分岔图,结合Floquet算子研究其稳定性及其分岔类型,所得分岔图和数值模拟的结果相吻合.由分岔图可得系统由于控制面双线性的存在,导致机翼结构振动形态多变,存在多个分岔点和多个不稳定区间,不仅存在极限环振动和非光滑准周期振动,而且在某些不稳定区间出现混沌现象.