3D Passive Walkers: Finding Periodic Gaits in the Presence of Discontinuities

This paper studies repetitive gaits found in a 3D passivewalking mechanism descending an inclined plane. By using directnumerical integration and implementing a semi-analytical scheme forstability analysis and root finding, we follow the corresponding limitcycles under parameter variations. The 3D walking model, which is fullydescribed in the paper, contains both force discontinuities andimpact-like instantaneous changes of state variables. As a result, thestandard use of the variational equations is suitably modified. Theproblem of finding initial conditions for the 3D walker is solved bystarting in an almost planar configuration, making it possible to useparameters and initial conditions found for planar walkers. The walkeris gradually transformed into a 3D walker having dynamics in all spatialdirections. We present such a parameter variation showing the stabilityand the amplitude of the hip sway motion. We also show the dependence ofgait cycle measurements, such as stride time, stride length, averagevelocity, and power consumption, on the plane inclination. The paperconcludes with a discussion of some ideas on how to extend the present3D walker using the tools derived in this paper.     

Cohesive properties of nickel-alumina interfaces determined via simulation of ductile bridging experiments

A combined experimental and computational study is carried out to characterize a nickel-alumina interface in terms of the two parameter (σ̂, Γ₀) computational cohesive zone (CCZ) model of Tvergaard and Hutchinson. Experiments were performed using a sandwich specimen consisting of a thin nickel foil bonded between two pre-cracked alumina plates. The specimen was loaded in tension with the nickel foil bridging the cracks in the ceramic. Numerical simulations of the experiments were used to extract the parameters for the CCZ model.Effects of various parameters of the CCZ model are investigated and it is found that the most dominant parameter is the interface strength, σ̂. Effects of the residual thermal stresses are also investigated and it is shown that these stresses can enhance the specimen fracture toughness by almost 16%. The parameters for the nickel-alumina interface are found to be σ̂ = 148 MPa and Γ₀ = 11 J m⁻². It is observed that for the foil thicknesses tested, the work of rupture does not vary linearly with the thickness as predicted by many theoretical models. We found that interfaces which are neither too strong nor too weak contribute most to the overall fracture toughness of such a composite. Although the macroscopic loading at the nickel-alumina interface is shear, the failure is primarily tensile due to the thinning that occurs in the metal as it is stretched.     

Sub-surface fracture of a thin metallic foil under impact loading

We examine here sub-surface fracture of a thin metallic foil sandwiched between two elastomeric layers under impact. In particular we generate a vertical stack consisting of alternate layers of soft elastomers and thin aluminum foils and place it on a rigid substrate; we then allow a rigid sphere to impact the stack from a small vertical height. We show that under impact the foil at the top of the stack undergoes buckling deformation; however the foil sandwiched between the two elastic layers undergoes both deformation and fracture. We show that because of friction at the contacting interfaces with the elastomer, the sandwiched foil is subjected to in-plane stretching which when exceeds a threshold limit causes fracture. Experiments show that this threshold condition is reached within a range of critical thicknesses of the top and bottom elastomeric layers, for a given height of impact of the rigid spherical indenter. We present a theoretical analysis to predict the critical thickness of the stack below which the foil is expected to undergo fracture and also the critical heights within this stack at which the foil would fracture.     

Improvement of cavitation mass transfer modeling based on local flow properties

This paper presents and studies the effect of two modifications to improve cavitation mass transfer source term modeling for transport equation based models by considering local flow properties. The first improvement is by creating an analogy between the phase change time scale and turbulence time scale, and have the model to automatically adjust mass transfer rate based on the flow. This will alleviate the manual calibration of model parameter that is often necessary in presently used models. The second modification introduces an influence of shear stress on the liquid rupture in flows relevant for hydromachinery. This relates to that the pressure threshold, which represents the criteria of when phase change occurs, is normally taken as the value relevant for a fluid at rest, but is in reality affected by the flow conditions.To demonstrate the effect of the model modifications, the three-dimensional, fully turbulent, cavitating flow around the Delft Twist11 foil is simulated. The suggested modifications are implemented in and evaluated using the Sauer mass transfer model, with simulations performed with an incompressible implicit LES flow model. The pressure distribution across different sections of the foil, lift force, and cavitation behavior, such as generation, separation, and collapse processes, are studied and compared with the experimental data. The comparison shows the capability of the presented model to improve the prediction of the complex physics of the cavitation around the Twist11 foil, compared with using only the original Sauer mass transfer model.     

Evaporation of thin liquid droplets on heated surfaces

We carry out combined experimental and theoretical studies of liquid droplet evaporation on heated surfaces in a closed container filled with saturated vapor. The droplets are deposited on an electrically heated thin stainless steel foil. The evolution of droplet shapes is studied by optical methods simultaneously with high-resolution foil temperature measurements using thermochromic liquid crystals. A mathematical model is developed based on the assumptions that the droplet surface has uniform mean curvature and the contact line is pinned during evaporation. Both the dynamics of liquid–vapor interface and the temperature profiles at the foil are shown to be in good agreement with the experimental data.     

Stability boundaries for wrinkling in highly stretched elastic sheets

We determine stability boundaries for the wrinkling of highly unidirectionally stretched, finely thin, rectangular elastic sheets. For a given fine thickness and length, a stability boundary here is a curve in the parameter plane, aspect ratio vs. the macroscopic strain; the values on one side of the boundary are associated with stable unwrinkled (flat) states, while stable wrinkled configurations correspond to all values on the other. In our recent work we demonstrated the importance of finite elasticity in the membrane part of such a model in order to capture the correct phenomena. Here we present and compare results for four distinct models: (i) the popular Föppl–von Kármán plate model (FvK), (ii) a correction of the latter, used in our earlier work, in which the approximate 2D Föppl strain tensor is replaced by the exact Green strain tensor, (iii) and (iv): effective 2D finite-elasticity membrane models based on 3D incompressible neo-Hookean and Mooney–Rivlin materials, respectively. In particular, (iii) and (iv) are superior models for elastomers. The 2D nonlinear, hyperelastic models (ii)–(iv) all incorporate the same quadratic bending energy used in FvK. Our results illuminate serious shortcomings of the latter in this problem, while also pointing to inaccuracies of model (ii) – in spite of yielding the correct qualitative phenomena in our earlier work. In each of these, the shortcoming is a due to a deficiency of the membrane part of the model.     

Determination of material intrinsic length and strain gradient hardening in microbending process

Microbending experiments of pure aluminum show that the springback angles increase with the decrease of foil thickness, which indicates obvious size effects and attributes to plastic strain gradient hardening. Then a constitutive model, taking into accounts both plastic strain and plastic strain gradient hardening, is proposed to analyze the microbending process of thin foil. The model is based on the relationship between shear yield stress and dislocation density, in which the material intrinsic length is related to material properties and average grain numbers along the characteristic scale direction of part. It is adopted in analytical model to calculate the non-dimensional bending moment and predict the springback angle after microbending. It is confirmed that the predictions by the proposed hardening model agree well with the experimental data, while those predicted by the classical plasticity model cannot capture such size effects. The contribution of plastic strain gradient increases with the decrease of foil thickness and is independent on the bending angle.     

Change in thickness in curved fold model for axial crushing of tubes

A partly inside and partly outside curved fold model with variable straight length and stepped variation in the thickness of tube during folding has been developed in the present paper. The variation of circumferential strain during the formation of fold has been taken into consideration. All model parameters viz. size of fold, optimal value of folding parameter, maximum hinge angle and the final radius of curvature of fold have been evaluated analytically. An expression has been derived for determining the variation of crushing load during the formation of a fold. The total outside and total inside fold models can be easily derived from the present model. The results have been compared with experiments and reasonably good agreement has been observed. The incorporation of change in thickness of tube during folding has been found to reduce the folding parameter thus bringing it closer to the experiments. Some parametric studies by varying the length of straight portion of the fold have also been conducted. The results are of help in understanding the phenomenon of actual fold formation.     

Forces on oscillating foils for propulsion and maneuvering

Experiments were performed on an oscillating foil to assess its performance in producing large forces for propulsion and effective maneuvering. First, experiments on a harmonically heaving and pitching foil were performed to determine its propulsive efficiency under conditions of significant thrust production, as function of the principal parameters: the heave amplitude, Strouhal number, angle of attack, and phase angle between heave and pitch. Planform area thrust coefficients of 2.4 were recorded for 35° maximum angle of attack and efficiencies of up to 71.5% were recorded for 15° maximum angle of attack. A plateau of good efficiency, in the range of 50–60%, is noted. A phase angle of 90–100° between pitch and heave is found to produce the best thrust performance. Also, the introduction of higher harmonics in the heave motion, so as to ensure a sinusoidal variation in the angle of attack produced much higher thrust coefficient at high Strouhal numbers. Second, experiments on a harmonically oscillating foil with a superposed pitch bias, as well as experiments on impulsively moving foils in still water, were conducted to assess the capability of the foil to produce large lateral forces for maneuvering. Mean side force coefficients of up to 5.5, and instantaneous lift coefficients of up to 15 were recorded, demonstrating an outstanding capability for maneuvering force production.     

水底管道抛石加固过程的DEM-SPH耦合分析

水底管道的抛石加固过程是典型的颗粒-流体耦合问题.采用DEM-SPH耦合方法模拟颗粒-流体系统,其中离散元方法(DEM)用于模拟落石,光滑粒子流体动力学方法(SPH)用于模拟流体.通过三维Voronoi切割算法生成不规则形状的多面体,并基于闵可夫斯基原理构造扩展多面体形态的落石单元.通过SPH的边界排斥力模型计算颗粒与...     

A relationship between effective work of adhesion and peel force for thin hyperelastic films undergoing large deformation

A method to determine the effective work of adhesion for hyperelastic thin films undergoing large deformations is presented. Starting from energy balance equation a relationship between work of adhesion, the peel force, the peel angle, and the stretch is derived. Based on this relation a procedure to compute the energy of adhesion from peel tests is proposed. To this end the peel force as well as the engineering stress vs. engineering strain diagram for thin film is required. The derived relationship shows that the non-linearity of the stress-stain relation must be taken into account in computing the effective work of adhesion from the peel force. The processing of experimental data within the standard linear elasticity approach would lead to an overestimation of effective work of adhesion. The error would increase with a decrease of the peel angle.     

On the boundary conditions of the geometrically nonlinear Kirchhoff–Love shell theory

The present paper addresses the problem of establishing the boundary conditions of a geometrically nonlinear thin shell model, especially the kinematic ones. Our model is consistently derived from general 3D continuum mechanics statements. Generalized cross-sectional strains and stresses are based on the deformation gradient and the first Piola–Kirchhoff stress tensor. Since only the bending deformation is included in this model, no special technique needs to be adopted in order to avoid shear-locking. The theory is derived in such a way that any material model can be considered as a constitutive relation, once the zero transverse normal stress assumption is properly taken into account.     

Heaving and pitching oscillating foil propulsion in ground effect

A detailed series of experiments is performed to investigate the ‘ground effect’ experienced by propulsive flapping foils operating near a solid boundary. A high aspect ratio foil is towed at constant speed and oscillated in pitch and heave at varying distances from a rigid wall. It is shown that this distance has a significant impact on the lift and thrust forces generated by the foil, both in the time averaged mean forces and the phase averaged periodic forces. For some thrust producing kinematics, the instantaneous force profile may change significantly without altering the time averaged mean force; thus, mean force measurements alone are not sufficient to indicate the proximity, or the effect, of the solid boundary. Results are presented across a wide range of thrust generating kinematics, showing that the strength of the ground effect can be modulated, for any achievable level of thrust, through appropriate selection of kinematics. This finding in particular has significance for underwater vehicles propelled by oscillating foil thrusters, as it follows that the sensitivity of the thrusters to ground effect can be controlled independently of the desired thrust. While propulsive efficiency is increased slightly near the wall for some kinematics, in general this does not occur for kinematics where a strong ground cushion (repulsion) effect is observed. Finally, the results suggest that span-wise flow around the tip of the foil is important in determining whether the foil is repelled from or pulled into the wall.     

A MECHANICAL ANALYSIS OF THE PROBLEM: HOW MANY TIMES CAN A SHEET OF PAPER BE FOLDED 1)

通过将纸张折叠问题简化为矩形截面简支梁在中段集中力作用下的三点弯曲问题,通过最大弯曲挠度和最小折叠载荷讨论了A4打印纸和八开报纸在普通人力作用下的最大对折次数问题。结果表明对于普通A4打印纸,一般成年人只能对折6次,最多也只能对折7 次。对于八开报纸,一般成年人能轻松对折7次,但是基本不可能对折8次。试验测试结果与理论预测结果吻合良好。     

Shock wave driven microparticles for pharmaceutical applications

Ablation created by a Q-switched Nd:Yttrium Aluminum Garnet (Nd:YAG) laser beam focusing on a thin aluminum foil surface spontaneously generates a shock wave that propagates through the foil and deforms it at a high speed. This high-speed foil deformation can project dry micro- particles deposited on the anterior surface of the foil at high speeds such that the particles have sufficient momentum to penetrate soft targets. We used this method of particle acceleration to develop a drug delivery device to deliver DNA/drug coated microparticles into soft human-body targets for pharmaceutical applications. The device physics has been studied by observing the process of particle acceleration using a high-speed video camera in a shadowgraph system. Though the initial rate of foil deformation is over 5 km/s, the observed particle velocities are in the range of 900–400 m/s over a distance of 1.5–10 mm from the launch pad. The device has been tested by delivering microparticles into liver tissues of experimental rats and artificial soft human-body targets, modeled using gelatin. The penetration depths observed in the experimental targets are quite encouraging to develop a future clinical therapeutic device for treatments such as gene therapy, treatment of cancer and tumor cells, epidermal and mucosal immunizations etc.      

A heat transfer measurement of jet impingement with high injection temperature

Local heat transfer from an impinging high temperature jet is studied using a method based on the heat thin foil technique and on the infrared thermography. Heat thin foil technique is used to impose several heat fluxes. For each flux, the temperature distribution is recorded using infrared imaging. Local heat transfer coefficients and adiabatic wall temperatures are determined by means of a linear regression method. This procedure is validated for a single round jet impinging on a flat plate for a range of injection temperatures. To cite this article: M. Fénot et al., C. R. Mecanique 333 (2005).     

高超声速风洞短时气动力智能辨识算法研究

风洞测力试验是高超声速飞行器研发的重要环节,随着这项技术的发展,试验模型的大尺度化成为高超声速风洞试验的趋势.在几百毫秒的有效测试时间内,大尺度测力系统刚度减弱等问题会严重导致气动力辨识精度变差,试验模型大尺度化对短时脉冲燃烧风洞精确气动力辨识带来了挑战.对此本文提出了一种新的基于传统信号处理结合深度学习的智能气动力辨...     

Adhesive contact of an elastic semi-infinite solid with a rigid rough surface: Strength of adhesion and contact instabilities

The effect of adhesion on the contact behavior of elastic rough surfaces is examined within the framework of the multi-asperity contact model of Greenwood and Williamson (1966), known as the GW model. Adhesive surface interaction is modeled by nonlinear springs with a force–displacement relation governed by the Lennard–Jones (LJ) potential. Constitutive models are presented for contact systems characterized by low and high Tabor parameters, exhibiting continuous (stable) and discontinuous (unstable) surface approach, respectively. Constitutive contact relations are obtained by integrating the force–distance relation derived from the LJ potential with a finite element analysis of single-asperity adhesive contact. These constitutive relations are then incorporated into the GW model, and the interfacial force and contact area of rough surfaces are numerically determined. The development of attractive and repulsive forces at the contact interface and the occurrence of instantaneous surface contact (jump-in instability) yield a three-stage evolution of the contact area. It is shown that the adhesion parameter introduced by Fuller and Tabor (1975) governs the strength of adhesion of contact systems with a high Tabor parameter, whereas the strength of adhesion of contact systems with a low Tabor parameter is characterized by a new adhesion parameter, defined as the ratio of the surface roughness to the equilibrium interatomic distance. Applicable ranges of aforementioned adhesion parameters are interpreted in terms of the effective surface separation, obtained as the sum of the effective distance range of the adhesion force and the elastic deformation induced by adhesion. Adhesive strength of rough surfaces in the entire range of the Tabor parameter is discussed in terms of a generalized adhesion parameter, defined as the ratio of the surface roughness to the effective surface separation.     

Modelling of inflow-conditions for vortex particle methods to simulate atmospheric turbulence and its induced aerodynamic admittance on line-like bluff bodies

ABSTRACT

This paper presents a novel method for the simulation of aerodynamic admittance of turbulent wind on bluff line-like structures using a pseudo 3D model of Vortex Particle Method (VPM). The method is a computationally efficient extension of the 2D VPM, where a coupled set of simulation slices accounts for the 3D nature of the oncoming wind flow. Pre-computed vortex particles are seeded in each of the parallel 2D simulation slices in order to model the turbulent velocity perturbations. Here, the modelling of the inflow seeding particles is enhanced, reducing the computational cost and allowing extendibility into quasi 3D domain. This a priori computation of the seeding vortex particles is based on modelling the atmospheric turbulence characteristics. The method is applied to simulate turbulent flow around an infinitesimally thin flat-plate, to asses its validity at the viscous-rotational boundary layer, which is important for accurate fluid-structure Interaction simulations. Furthermore, sensitivity analysis to different attributes is assessed