Preface

Multibody System Dynamics -     

On the Gröbner basis triangularization of constraint equations in natural coordinates

Efficient dynamic simulation code is essential in many situations (including hardware-in-the-loop and model-predictive control applications), and highly beneficial in others (such as design optimization, sensitivity analysis, parameter identification, and controller tuning tasks). When the number of modeling coordinates n exceeds the degrees-of-freedom of the system f, as is often the case when closed kinematic chains are present, the governing dynamic equations consist of n second-order ordinary differential equations (ODEs) coupled with m=n?f algebraic constraint equations. This set of n+m index-3 differential-algebraic equations can be difficult to solve in an efficient yet accurate manner. Embedding (or generalized coordinate partitioning) can be used to obtain f ODEs (one for each independent acceleration), which are generally more amenable to numerical integration; however, the dependent positions are typically computed from the independent positions at each time step. Newton–Raphson iteration is often used for solving the position-level kinematics, but only provides solutions to within a specified tolerance, and can require several iterations to converge. In this work, Gröbner bases are used to obtain recursively solvable symbolic solutions for the dependent positions, which can then be evaluated to within machine precision using a fixed number of arithmetic operations. Natural coordinates are particularly attractive in this context, since the resulting constraint equations are maximally quadratic polynomials and are, therefore, easily triangularized. The proposed approach is suitable for use in an automated formulation procedure and, as demonstrated by three examples, is capable of generating highly efficient simulation code with minimal additional effort required at the formulation stage.     

Anisotropy of radiance in tissue phantoms and Dunning R3327 rat tumors: radiance measurements with flat cleaved fiber probes

OBJECTIVE: To compare the prevalence of conduct problems (CP) according to level of urbanization and to determine which factors account for the potential difference in prevalence rates. METHOD: Study 1 used a questionnaire survey of a nationally representative sample of 10,462 Norwegian adolescents. Study 2 used a questionnaire survey of a representative sample of 1,346 adolescents living in Oslo. Self-reported CP included most DSM-III-R criteria for conduct disorder. RESULTS: CP rates were similar in all levels of urbanization, except for the only semimetropolitan city in the country, the capital Oslo, which had CP rates twice those of the rest of the country. This increase rate could not be explained by a series of commonly advocated explanations: family structure and parental practices, social network, socioeconomic status, integration in community activities, religious involvement, and race. However, involvement in "soft" drugs and associating with antisocial peers could explain the statistically differential rates. Furthermore, in the Oslo study, adolescents' CP did not vary according to density of population or region within the city. CONCLUSIONS: The results support previous studies showing increased rates of CP in urban areas. However, urbanization must pass a certain threshold before it has this effect. Moreover, the lack of support for commonly advocated explanations for the difference between urban and nonurban areas suggests that investigations specifically addressing potential explanations for this difference should be conducted. The results indicate that the increased rates of substance use in highly urbanized areas may account for the difference in CP rates by prolonging and aggravating CP.     

Parameter identification in dynamic systems using the homotopy optimization approach

Identifying the parameters in a mathematical model governed by a system of ordinary differential equations is considered in this work. It is assumed that only partial state measurement is available from experiments, and that the parameters appear nonlinearly in the system equations. The problem of parameter identification is often posed as an optimization problem, and when deterministic methods are used for optimization, one often converges to a local minimum rather than the global minimum. To mitigate the problem of converging to local minima, a new approach is proposed for applying the homotopy technique to the problem of parameter identification. Several examples are used to demonstrate the effectiveness of the homotopy method for obtaining global minima, thereby successfully identifying the system parameters.     

A Deformation Field for Euler–Bernoulli Beams with Applications to Flexible Multibody Dynamics

The deformation field commonly used for Euler–Bernoulli beamsin structural dynamics is investigated to determine its suitability foruse in flexible multibody dynamics. It is found that the traditionaldeformation field fails to produce an elastic rotation matrix that iscomplete to second-order in the deformation variables. A completesecond-order deformation field is proposed along with the equationsneeded to incorporate the beam model into a graph-theoretic formulationfor flexible multibody dynamics [1]. This beam modeland formulation have been implemented in a symbolic computer programcalled DynaFlex that can use Taylor, Chebyshev, or Legendrepolynomials as the basis functions in a Rayleigh–Ritz discretizationof the beam's deformation variables. To demonstrate the effects of the proposed second-order deformationfield on the response of a flexible multibody system,two examples are presented.     

Dynamics of Flexible Multibody Systems Using Virtual Work and Linear Graph Theory

By combining linear graph theory with the principle of virtualwork, a dynamic formulation is obtained that extends graph-theoreticmodelling methods to the analysis of flexible multibody systems. Thesystem is represented by a linear graph, in which nodes representreference frames on rigid and flexible bodies, and edges representcomponents that connect these frames. By selecting a spanning tree forthe graph, the analyst can choose the set of coordinates appearing inthe final system of equations. This set can include absolute, joint, orelastic coordinates, or some combination thereof. If desired, allnon-working constraint forces and torques can be automaticallyeliminated from the dynamic equations by exploiting the properties ofvirtual work. The formulation has been implemented in a computerprogram, DynaFlex, that generates the equations of motion in symbolicform. Three examples are presented to demonstrate the application of theformulation, and to validate the symbolic computer implementation.     

General schema theory for genetic programming with subtree-swapping crossover: part I

This is the first part of a two-part paper which introduces a general schema theory for genetic programming (GP) with subtree-swapping crossover. The theory is based on a Cartesian node reference system which makes it possible to describe programs as functions over the space N(2) and allows one to model the process of selection of the crossover points of subtree-swapping crossovers as a probability distribution over N(4). In Part I, we present these notions and models and show how they can be used to calculate useful quantities. In Part II we will show how this machinery, when integrated with other definitions, such as that of variable-arity hyperschema, can be used to construct a general and exact schema theory for the most commonly used types of GP.     

Control of the Market for a Natural Fibre

Woolgrower organisations in Australia, New Zealand, South Africa and Uruguay directly control wool fibre quality and specification. The quality of end products is also controlled through the IWS Woolmark programme. Moreover, woolgrower organisations make important contributions to the control of design and wool textile processing, in order to provide consumers with attractive, value-for-money wool products.

For the last twenty years, wool prices have been controlled through reserve price schemes, but these have now been abandoned due to changes in markets for wool arising from political and economic events far beyond the influence, let alone control, of woolgrowers. This exemplifies the difficulty, if not impossibility, of control of prices, as well as of production volumes, which are inextricably-linked.

With regard to wool end products, control (i.e., direction and regulation) of price and supply would infringe individual freedom of choice and in textiles is impossible, not least because purchasing decisions are frequently emotionally, rather than rationally, based. However, demand (i.e., price times volume) for wool end products can be influenced by promotion and woolgrowers have achieved high demand for these by influencing consumers to exhibit a strong preference for wool products in the premium sectors of the textile market. This is a key factor in allowing wool to command a price of 3 times that of competitive fibres.     

Accumulation of unintegrated circular viral DNA in monocytes and growth-arrested T cells following infection with HIV-1

Cytocidal retrovirus infection is characterized by rapid accumulation of unintegrated viral DNA forms. These are thought to be generated by multiple rounds of reinfection and have been suggested to play a central role in cytopathogenesis. Here we have reviewed the work done in this area with HIV-1, mostly using acutely and chronically infected T cell and monocytic cell lines and in some cases T cells blocked at S phase of the cell cycle by aphidicolin treatment. To these studies, we have compared our findings with HIV-1 infected primary peripheral blood monocyte-derived macrophages and untreated and growth-arrested MT-2 cells, two biologically disparate cell populations. Using 1- and 2-long terminal repeat (LTR) circular forms as indicators of unintegrated viral DNA, we found similar rapid accumulation in both untreated and growth-arrested MT-2 cells. In contrast, we found much lower levels in monocyte/macrophages. Our findings suggest that accumulation of unintegrated viral DNA does not require virus production and reinfection in growth-arrested T cells. The significantly lower levels found in monocyte/macrophages may reflect superinfection resistance, allowing the maintenance of a persistent infection.