icoev 2017

A large amount of work has been successfully carried out to date by the author and others on the modelling of symmetrical momentum exchange space tethers, and this has allowed some important insights into on-orbit propulsion performance. The incorporation of a prime mover such as a solar-electric drive motor into a momentum exchange tether introduces the potential for greatly enhanced performance over passive tethers. But so far the work done has concentrated solely on tethers which are symmetrical in the distribution of mass about the assumed geometrical centre, the point at which the motor drive location has been defined. This has operational relevance in that symmetry issues can be powerfully exploited for two-way interplanetary payload exchange, and so a tether that carries two payloads with identical mass properties, or one that has divested itself of both payloads simultaneously, can be readily applied to an ongoing continuous mission programme. However, there are occasions when a motorised momentum exchange tether may be forced to operate in an asymmetrical mode, where one of the two payloads is detached but the other remains attached. From a mission architecture and logistics point of view this scenario has already been accommodated, and a solution proposed, but from the perspective of the three dimensional orbital dynamics it has remained an unsolved problem until very recently. The work to be presented in this lecture will summarise the modelling of a symmetrical motorised tether and will also summarise the logistics of a two-way interplanetary freight mission on which this depends. It will then go on to examine those scenarios when asymmetry can occur and will show how very recent work has led to new models of the three dimensional orbital dynamics of tethers operating under these conditions. It will be shown that such tethers can indeed continue to be operated and that steps for mission recovery can be defined and proposed as a consequence of this further modelling.

 Rate-independent hysteresis in energy dissipation within many materials and structures has been empirically observed by many researchers over more than a century. In such materials and structures the use of viscous damping in vibrations is therefore incorrect in principle, although it is analytically convenient.
In hysteresis, the load-displacement or stress-strain curve is irreversible and changes slope whenever the direction of loading is changed, leading to loops with sharp corners. Small reversals of loading within larger loading paths lead to minor loops in the load-displacement curve. Under complex loading, the load-displacement curve can have many loops within loops.
One can attempt to describe the load-displacement curve in terms of evolution equations. In structural mechanics, the famous Bouc-Wen model has been used for decades to describe hysteresis. This model is a differential equation with one internal state, captures single loops well, but cannot capture minor loops. One can show that, in order to capture minor loops within such larger loops, the hysteresis model should have at least two internal states.
In this talk, I will describe our mechanistic approach toward purposefully constructing a simple two-state model that captures such hysteresis loops. Our approach differs from other conceivable empirical approaches where ad hoc relations are guessed and shown to produce hysteresis loops. Here, we start with a physically based micromechanical model with many springs and sliding elements, i.e., a high dimensional frictional system resembling smaller systems studied by Iwan half a century ago.
We first solve the resulting equations incrementally via the linear complementarity problem. Then, using new approximation methods suitable for such a system, we are able to reduce the order of the system to the theoretical minimum requirement of two internal states. This reduction involves identification of shape functions, use of a work inequality, a simple fortuitous approximation of a complicated dissipation function, solution of a slightly offbeat minimization problem, reduction of the resulting equations to easily tractable form, elimination of superfluous fitted parameters through coordinate changes, and finally, separation of the roles of the remaining fitted parameters through a combination of further analytical simplification and approximation.
In summary, I will start with initial motivation, present the high dimensional model, and trace the route of reduction, approximation and simplification leading to a final novel and simple two-state hysteresis model.

Keywords: Stochastic model updating, parameter selection, covariance and interval methods, Bayesian inference, AIRMOD database

Deterministic model updating is now regularly applied to large scale industrial structures. However, it is well known that different analysts produce different finite element (FE) models, make different physics-based assumptions, and parameterise their models differently. Also, tests carried out on the same structure, by different operatives, at different times, under different ambient conditions produce different results. This provides the motivation for the presented work.
Interval, perturbation (covariance) and Bayesian methods are described and new results are presented on the selection of updating parameters. The practical application of these techniques is demonstrated using the DLR AIRMOD structure, which was disassembled, reassembled and tested 130 times to produce a database of modal-test variability. The Bayesian updating procedure gives a measure of the most probable model whereas the covariance updating method is predicated on the assumption of Gaussian distributions. Interval model updating is generally the more conservative of the 3 methods. Updating equations produced by the sensitivity method are generally ill-conditioned requiring regularisation, which is applied automatically in Bayesian model updating. Covariance model updating can be carried out on a standard desk-top PC in a few minutes whereas Bayesian updating requires parallelised code on multiple processors and errors in surrogate models can affect the updated distributions. 

Inspired by nature, researchers are trying to create systems and structures that present adaptive behavior according to its environment. The use of nature as an inspiration has the objective to extract design principles from biological systems. Among many options of smart sensors and actuators, shape memory alloys (SMAs), piezoelectric and magnetostrictive materials are some interesting options. Due to their remarkable properties, smart materials have been used in many areas of human knowledge. This seminar presents some smart material system applications and their thermomechanical behaviors. It is discussed constitutive models that explore phenomenological features of the thermomechanical response of smart materials. This constitutive model is employed in order to describe some interesting behaviors and potential applications of smart devices involving dynamical behavior. Adaptive structures and origami systems are discussed. In this regard, it should be highlighted their rich response including chaos and attractor multistability. Chaos control is also employed showing situations where tiny perturbations can avoid undesirable behaviors.

We present a hybrid model of church bell. The dynamical system of yoke-bell-clapper is nonlinear and discontinuous. We use the Lagrange equations of the second type to derive formulas that describe the system's motion. The energy between the bell and the clapper is transmitted via impacts, here modeled using a coefficient of energy restitution. The values of the system's parameters have been determined basing on the measurements of the biggest bell ‘‘The Heart of Lodz’’ in the Cathedral Basilica of St Stanislaus Kostka, Lodz, Poland. Using the same bell we also validate the model by comparing the results of numerical simulations with experimental data. The presented results show that the described model is a reliable predictive tool which can be used both to simulate the behavior of the existing yoke-bell-clapper systems as well as to design the yokes and predict the motion of new bells.

Active control has witnessed a great change in the design of aircraft structures, from the passive design of increasing structure stiffness to the active design in view of control configured aircraft. The idea of active design does not avoid the aeroelastic problem, but adjust the structural aeroelasticity via active control so as to reduce the structure weight and optimize the aircraft performance. The aeronautical community has made great efforts to study the corresponding aeroelastic problems and gain an insight into the coupling among aircraft structure, aerodynamics and active control. Most studies, however, have been based on the simplified models. As such, it is difficult to apply the research achievements to aeronautical industry.

This lecture presents the recent advances in the dynamic problems of aircraft aeroelasticity including the aerodynamic nonlinearity from supersonic flow, the backlash nonlinearity from multiple control surfaces, the instability induced by time delays in control loop, the active flutter suppression when the above nonlinearities and time delays appear, as well as the validation of wind tunnel tests. The lecture focuses on the new approaches proposed at the author’s lab, and the corresponding numerical simulations and wind tunnel tests over the past decade. Finally, the lecture addresses a number of open problems related to the aeroelastic analysis and control.