Calculation of the forces acting upon a rigid structure from an aircraft impact

A numerical method is presented which permits the calculation of the loads produced by an aircraft impacting upon a rigid structure at an arbitrary angle to the surface normal. This method is utilized to compute the variation of the horizontal and vertical components of the corresponding reaction with aircraft impact angle for a Boeing 707-320 Airliner and Phantom F4 Military Jet. Furthermore, three flight trajectory distributions have been studied which are relevant to the aircrash problem. Firstly, an idealised rectangular distribution; secondly, a sin 2 distribution corresponding to a undirectional horizontal beam onto a hemispherical dome or an isotropic trajectory distribution onto an upright rectangular (or cylindrical) structure. Finally, a ‘well’ distribution is considered which approximates to the trajectory distribution observed from aircrash data. For each angular distribution, the vertical and horizontal reaction histories have been calculated to 50 and 90% confidence levels for both types of aircraft. Finally, it is concluded that the application of the normal load case in a design code is conservative in nature, corresponding to a 90% confidence level for the flight trajectory distributions likely to occur.     

System for measuring aerodynamic forces on models

The structure and basic characteristics are considered for a system for measuring aerodynamic forces on a flying vehicle. The structure and the algorithm are discussed, which are intended to increase the accuracy. Translated from Izmeritel'naya Tekhnika, No. 5, pp. 43–46, May, 1998.     

既有桥梁对相邻车-桥系统气动力的影响分析

为了研究既有桥梁对相邻车～桥系统气动力的影响,通过节段模型风洞试验,测试了考虑、不考虑既有桥梁干扰工况下相邻车-桥系统的气动力系数,数值模拟了作用在车-桥系统上的抖振力时程,并计算了抖振力极值.结果表明:既有桥梁对相邻车-桥系统气动力的影响显著,导致部分工况下列车、桥梁的气动力显著增大,结构设计中应充分考虑邻近桥梁的气...     

Vertical forces acting on horizontal disks in a fluidized bed

Experimental studies of vertical forces were made with a turbulimeter. A dimensionless expression was obtained for the determination of the average vertical forcing acting on horizontal disks submerged in a fluidized bed.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 27, No. 6, pp. 978–981, December, 1974.     

Lifting forces acting on magnets placed above a superconducting plane

Based on the London model, we have calculated the lifting forces acting on permanent (cylinder, ring, sphere, cone, etc.) magnets placed above an infinite superconducting plane. The theory has also been applied to obtain theoretical estimates of the forces acting on the semispherical and conical-shaped tips of a magnetic force microscope (MFM) over a high-T _c superconductor.On leave from Department of Physics, Dalhousie University, Halifax, Nova Scotia, Canada, B3H 3J5.     

Controlling aerodynamic forces with the aid of gas-permeable porous materials

We have experimentally studied the effect of asymmetric heating of gas-permeable porous inserts situated in front of a cylinder on the aerodynamics of this system in supersonic airflow with a Mach number of M_∞ = 4.85. The inserts were heated by electric glow discharge. It is established that side forces perpendicular to the airflow axis arise and their quantitative characteristics are determined.     

Cracks located on a single line in an orthotropic elastic medium in which body forces are acting

Plane strain problem of determining the distribution of stress in the neighborhood of cracks located on a single line is examined. The medium containing cracks is infinite, orthotropic and acted upon by an asymmetrical distribution of body forces. The crack surfaces are free from tractions and the crackline coincides with an axis of orthotropy. Fourier transform methods are employed to reduce the problem to that of solving a singular integral equation of Cauchy type.The solution is completed in the case when the infinite orthotropic medium contains a single crack. Particular distributions of concentrated loads and moments are considered in detail. The stress intensity factor and crack shape are determined and shown graphically for oak wood. The results in special and limiting cases are compared with those available in the literature.     

Method of measuring transient aerodynamic forces and moments on a vibrating cascade

A method is described for experimentally determining the aerodynamic forces and moments on a vane cascade that models the blading ring of a turbine. The vanes have two degrees of freedom and undergo complex vibration having different translational and angular components. Aerodynamic induction factors are used as the aerodynamic characteristics.Translated from Problemy Prochnosti, No. 2, pp. 100–109, February, 1996.     

边界条件对有限长正方形棱柱气动力的影响

通过风洞实验研究了均匀流与湍流边界层中一端固定于壁面、另一端为自由端的有限长正方形棱柱气动力特性。实验模型宽度d=200 mm,高宽比H/d=5。基于自由流风速U_∞与d的雷诺数Re_d=0.68×10~5~5.47×10~5。研究发现,均匀流与湍流边界内有限长正方形柱气动力均不存在明显的雷诺数效应。棱柱局部时均阻力系数C_D、脉动阻力系数C'_D和脉动升力系数C'_L沿展向存在明显差异。湍流边界层会增大C'_D与C'_L。且均匀流中脉动风压与气动力的双稳态现象在湍流边界层内基本消失。经POD分析,确定了对称与反对称等两种典型风压分布状态对脉动风压的贡献率。均匀流中,柱体两端附近风压脉动由对称或准对称风压分布所控制;而柱体中部则由反对称风压分布控制。湍流边界层中所有高度风压脉动均由反对称风压分布控制。两种边界条件下,仅反对称形态的POD模态具有明显周期性并对柱体气动力周期性起支配作用。尽管对称风压POD模态在某些位置占支配地位,但其无明显周期性。     

Simplified description of optical forces acting on a nanoparticle in the Gaussian standing wave

We study the axial force acting on dielectric spherical particles smaller than the trapping wavelength that are placed in the Gaussian standing wave. We derive analytical formulas for immersed particles with relative refractive indices close to unity and compare them with the numerical results obtained by generalized Lorenz-Mie theory (GLMT). We show that the axial optical force depends periodically on the particle size and that the equilibrium position of the particle alternates between the standing-wave antinodes and nodes. For certain particle sizes, gradient forces from the neighboring antinodes cancel each other and disable particle confinement. Using the GLMT we compare maximum axial trapping forces provided by the Gaussian standing-wave trap (SWT) and single-beam trap (SBT) as a function of particle size, refractive index, and beam waist size. We show that the SWT produces axial forces at least ten times stronger and permits particle confinement in a wider range of refractive indices and beam waists compared with those of the SBT.     

Griffith cracks in an elastic medium in which body forces are acting

The plane strain problem of determining the distribution of stress in an infinite isotropic elastic medium containing Griffith cracks located on a single line is examined. The crack surfaces are assumed to be free from tractions, and the stress distribution in the medium is due to the action of body forces. Fourier transform methods are employed to reduce the problem to that of solving a singular integral equation of Cauchy type. The solution is completed in the case in which the medium contains a single crack. Particular distributions of concentrated loads are considered in detail, and the results are compared with those available in the literature.     

A strategy to identify exciting forces acting on structures

The strategy for identifying the external forces acting on structures sketched by Turco (Comput. Method Appl. Mech. Eng. 1998; 165 :291–306) is extended to the dynamic case. Starting from the measurement of the stress or strain history in a suitable number of points of the structure, the exciting forces are reconstructed. This inverse problem is tackled in the frequency domain by using the standard tools of computational mechanics along with a classical approach to treat ill‐conditioned problems. Some numerical results relating to pin‐jointed trusses are discussed in order to validate the proposed strategy. Copyright © 2005 John Wiley & Sons, Ltd.     

基于数论网格法与Morris法的翼型气动特性敏感性分析

针对在风力机翼型气动特性敏感性分析中计算效率较低的问题,提出了一种基于数论网格（number theoretic net,NT-net）法与Morris法的翼型气动特性敏感性分析方法。首先,构建了拟合精度较高的翼型参数化模型;其次,阐述了NT-net法的计算原理,采用NT-net法对翼型参数化模型的多项式系数进行抽样;然后,以风力机翼型S832为研究对象,采用Morris法进行翼型气动特性的全局敏感性分析;最后,进一步分析在特定工况下风力机翼型参数化模型的多项式系数对翼型外形及气动特性的影响。结果表明：影响翼型气动特性的主要因素依次为翼型的最大相对厚度和最大相对弯度、前缘半径和后缘厚度;当来流攻角较小时,最大相对厚度和最大相对弯度取适当的较小值、前缘半径取适当的较大值可有效增强该工况下风力机翼型的气动特性,同时也验证了NT-net法的计算效率更高。研究结果为风力机翼型气动设计提供了理论参考。     

Low-frequency expansions for a penetrable ellipsoidal scatterer in an elastic medium

In this paper the scattering of a plane longitudinal or transverse wave by a penetrable ellipsoid in an isotropic and homogeneous elastic medium in the low-frequency region is examined. Using low-frequency expansions the scattering problem is reduced to a sequence of potential problems. Explicit closed-form solutions for the zeroth and first-order approximations are obtained. The solution of the problem was made possible by using an analytical technique based on Papkovich-Grodski-Neuber potentials. The normalized scattering amplitudes and the scattering cross-section are evaluated up to k ³-order terms, respectively.The author would like to thank the Greek Ministry of Research and Technology for partially supporting the present work.