Computational flowfield analyses of hypersonic problems with reacting boundary layer

A mathematical model able to deal with high temperature gas effects in hypersonic flow fields is presented. In order to assess this model, four typical hypersonic applications are considered. The numerical results are presented and compared with experimental data. The effects of the catalyticity of the materials on the heat fluxes are also highlighted.     

Investigation of the Influence of Pseudoinverse Matrix Calculations on Multibody Dynamics Simulations by Means of the Udwadia-Kalaba Formulation

The formulation of the dynamic equations of motion proposed by Udwadia-Kalaba is discussed from the point of view of numerical efficiency. Since this formulation requires the computation of a pseudoinverse matrix, it has investigated the influence of the method of pseudoinverse computation on the dynamic simulation of an overconstrained linkage. Finally, it has been proposed that a new dynamic equation which explicitly computes accelerations also in the case of mathematical models with rank deficient Jacobian and mass matrices.     

Simplified Impedance Model for Adhesively Bonded Piezo-Impedance Transducers

The electromechanical impedance technique employs surface-bonded lead zirconate titanate piezoelectric ceramic patches as impedance transducers for structural health monitoring and nondestructive evaluation. The patches are bonded to the monitored structures using finitely thick adhesive bond layer, which introduces shear lag effect, thus invariably influencing the electromechanical admittance signatures. This paper presents a new simplified impedance model to incorporate shear lag effect into electromechanical admittance formulations, both one-dimensional and two-dimensional. This provides a closed-form analytical solution of the inverse problem, i.e. to derive the true structural impedance from the measured conductance and susceptance signatures, thus an improvement over the existing models. The influence of various parameters (associated with the bond layer) on admittance signatures is investigated using the proposed model and the results compared with existing models. The results show that the new model, which is far simpler than the existing models, models the shear lag phenomenon reasonably well besides providing direct solution of a complex inverse problem.     

Ballistic Walking Design via Impulsive Control

Mathematical models of a biped and quadruped walking are considered. The planar five-link biped consists of a one-link trunk and two identical two-link legs. In the single support motion (swing phase), the biped has five degrees of freedom and is described by a system of nonlinear ordinary differential equations of the 10th order. These equations are written in a visible matrix form. A seven-link planar biped with massless feet is also considered. In this paper, the swing motion of the biped is assumed a ballistic (passive) one. There are no active torques in the interlink joints during the single support motion—only the gravity force and ground reaction forces are applied to the biped. The problem of design of ballistic swing motion is reduced to the boundary-value problem for the system of nonlinear differential equations with given initial and final configurations and the duration of the half-step. It is assumed that there is no friction in the interlink joints (note that the friction in the human joints is very small). Therefore, in the ballistic swing motion the complete energy of the system (kinetic energy plus potential one) is conserved and the system has the energy integral. Due to this fact some properties of symmetry of ballistic motions are proved. Linearized model can be reduced to the canonical Jordan form. Then the linear boundary-value problem can be solved analytically. Using numerical investigations of the linear model we have animated the biped walking, which occurs “similar” to the human gait: the transferring leg moves over the support, the legs bend with knees forward, and the trunk makes one vibration during one half-step. All these features have not been prescribed beforehand in the statement of the problem. Iteration process is used to solve the nonlinear boundary-value problem. For some cases, several solutions of this nonlinear problem are found numerically. The symmetry properties of the ballistic motions help to find numerically the solutions of this complex nonlinear problem. The ballistic motion is also designed numerically for the three-dimensional biped model with 6, 8, 9, and finally with 11 degrees of freedom. The double-support phase is assumed an instantaneous one. During this phase there is a collision of the transferring (swing) leg and the support. Active impulsive torques are applied in the interlink joints at this instant. These impulsive torques and ground reaction forces are described by delta functions of Dirac. Thus, with this impulsive control, most of the efforts are applied in the double-support phase. Formulas for the energy cost of impulsive control actions have been found. Some problems of optimal distribution of the impulsive actions between the joints are discussed. A planar seven-link model of biped with arms is considered. This model consists of a one-link trunk, two identical one-link arms, and two identical two-link legs with point feet. Ballistic gait of this biped model is studied. The goal of this study is to find the optimal amplitude of the arms swinging, minimizing the energy consumption. We show numerically the existence of the optimal amplitude of the arms swinging. Ballistic walking of a quadruped is investigated too. Three types of quadruped gaits, bound, amble, and trot, are considered. For each kind of the gait, ballistic locomotion is designed and energy consumption is evaluated.     

Dynamic Model of an Astronaut Equipped with a Manned Maneuvering Unit in Virtual Reality

A virtual reality simulation system for a manned maneuvering unit (MMU) and a training system for an astronaut space walk is introduced. The system can simulate an astronaut’s outer space walk by means of manipulating the MMU, and it also can be used as a training environment by taking advantage of virtual reality properties of imagination, interaction, and immersion. As in microgravity space, an astronaut’s moving limbs cause changes in the MMU posture; these changes result in great offsets to correct direction and have a great effect on the MMU’s manipulation performance. A multibody astronaut model is developed by using relative coordinates according to the motion characteristics of an astronaut equipped with MMU. Motion information on the user’s limbs are captured by means of a motion capture subsystem and are mapped to an astronaut in virtual environment to simulate the astronaut’s limb motions while the astronaut is navigating in outer space. It is an open-chain system for an astronaut multibody model. As an astronaut can control his limbs, the whole system dynamics is a hybrid forward and inverse dynamic problem. Recursive dynamic formulations of a previous method are introduced to solve the forward dynamic problem. For the inverse dynamic part, because the system can get motion characteristics of parts of the relative coordinates, unknown generalized driving forces are applied on these known relative coordinates. All the unknown generalized driving forces and unknown relative coordinates can be combined in a set of hybrid dynamic formulations based on a forward dynamic formulation. The motion characteristics of an astronaut multibody model can be obtained by solving the hybrid formulations. Finally, some simulation results for astronaut forward-flying mode with moving limbs are presented; these results show that the movement of an astronaut’s limbs has great effects on astronaut navigation and MMU manipulation performance. The multibody astronaut model has the capability of simulating real space walk with an MMU equipped astronaut.     

Robust Control of PVTOL Aircraft with a Nonlinear Optimal Control Solution

In this paper, a new nonlinear control strategy is proposed for hovering control of planar vertical takeoff and landing (PVTOL) aircraft. The control of this typical nonlinear, nonminimum-phase system is treated as a robust control problem. The robust control design is then converted to an equivalent optimal control design so that both stabilization and performance can be guaranteed. The primary contribution of this work is to provide a closed-form solution to the resultant nonlinear optimal control problem by employing a recently emerging nonlinear control approach, called the θ?D technique. The θ?D technique is based on optimal control theory and derived from an approximate solution to the Hamilton-Jacobi-Bellman equation via a perturbation process. This formulation will not only enhance the system performance but also simplify the design process in that robustness and optimality are integrated in one unified optimal control framework. Furthermore, the closed-form control law is very suitable for onboard implementation. The effectiveness of the proposed method is demonstrated by simulation results and comparison studies with a similar linearization based optimal control formulation.     

Strong bonding of infrared brazed α2-Ti3Al and Ti–6Al–4V using Ti–Cu–Ni fillers

Infrared dissimilar brazing of α₂-Ti₃Al and Ti–6Al–4V using Ti–15Cu–25Ni and Ti–15Cu–15Ni filler metals has been performed in this study. The brazed joint consists primarily of Ti-rich and Ti₂Ni phases, and there is no interfacial phase among the braze alloy, α₂-Ti₃Al and Ti–6Al–4V substrates. The existence of the Ti₂Ni intermetallic compound is detrimental to the bonding strength of the joint. The amount of Ti₂Ni decreases with increasing brazing temperature and/or time due to the depletion of Ni content from the braze alloy into the Ti–6Al–4V substrate during brazing. The shear strength of the brazed joint free of the blocky Ti₂Ni phase is comparable with that of the α₂-Ti₃Al substrate, and strong bonding can thus be obtained.     

Effectiveness and resolution of tests for evaluating the performance of cutting fluids in machining aerospace alloys

The paper discusses effectiveness and resolution of five cutting tests (turning, milling, drilling, tapping, VIPER grinding) and their quality output measures used in a multi-task procedure for evaluating the performance of cutting fluids when machining aerospace materials. The evaluation takes into account the following process output measures: tool wear, cutting forces, torque, spindle power, geometrical accuracy, texture and integrity of workpiece surface. Using statistics, through calculation of the Hellinger distance, the resolution given by experimental data was evaluated and a comparison of robustness in ranking the performance of cutting fluids based on different output measures and cutting tests is presented.     

前缘凸脊倾斜气膜冷却效果

为了研究前缘凸脊结构对气膜冷却的影响规律,设计3种不同堵塞比的凸脊模型,研究气膜冷却效率随吹风比、堵塞比的变化规律,并运用数值模拟方法分析再附区域的流场分布。研究结果表明,与典型的圆柱形气膜孔相比,前缘凸脊的存在可以使气膜冷却效率提高130%以上;前缘凸脊在大吹风比下的作用更加明显;堵塞比B为0.21时气膜冷却效率最高;数值模拟结果与试验结果吻合得较好;在特定范围内,吹风比的增大或凸脊堵塞比的增大有利于提高气膜再附区域的展向覆盖,进而提高气膜冷却效率。     

全面质量管理对航天工业的重要影响

21世纪将作为质量世纪载入史册。以高质量、低成本和快速反应为焦点的激烈的航天市场的国际竞争将更加尖锐 ,介绍一种可以在航天工业领域内进行设计、执行和评估航天产品持续质量改进的管理方法 ,重点讨论美国马尔科姆·布莱尔金国家质量奖—— MBNQA ( MalcolmBaldrige National Quality Award)对航天工业的重要影响。