1,721,216 research outputs found
On a class of bounded quantum Bose fields
No full textLocal quantum fields in 1+1 dimensions can have bounded field operators. The class of such fields which in addition obey Huygens' principle (time-like commutativity) and conformal covariance, is completely determined. The result confirms and qualifies a conjecture by K. Baumann
Simulation of stellar instabilities with vastly different time-scales using domain decomposition
No full textStrange mode instabilities in the envelopes of massive stars lead to shock waves, which can oscillate on a much shorter time-scale than that associated with the primary instability. The phenomenon is studied by direct numerical simulation using a, with respect to time, implicit Lagrangian scheme, which allows for a variation of the dependent variables by several orders of magnitude. The time-step for the simulation of the system is reduced appreciably by the shock oscillations and prevents its long-term study. A procedure based on domain decomposition is proposed to surmount the difficulty of vastly different time-scales in various regions of the stellar envelope and thus to enable the desired long-term simulations. Criteria for domain decomposition are derived and the proper treatment of the resulting inner boundaries is discussed. Tests of the approach are presented and its viability is demonstrated by application to a model for the star P Cygni. In this investigation primarily the feasibility of domain decomposition for the problem considered is studied. We intend to use the results as the basis of an extension to two-dimensional simulations
A grid reconstruction procedure for the simulation of instabilities in luminous blue variables
No full textThe simulation of stellar instabilities requires a sufficient resolution of shocks, ionization zones and instability regions. Resolution problems in the latter may occur in simulations of the evolution of strange mode instabilities, if a Lagrangian description is used and the instability leads to a mass transfer in the stellar envelope, which is found for selected models of luminous blue variables. This necessitates a reconstruction of the Lagrangian grid. A grid reconstruction procedure suitable for the treatment of resolution problems occurring in simulations of stellar instabilities is presented and verified. This includes a systematic study of the influence of grid resolution and artificial viscosity on the result of numerical calculations
Instabilities of captured shocks in the envelopes of massive stars
No full textThe evolution of strange-mode instabilities into the non-linear regime has been followed by a numerical simulation for an envelope model of a massive star with a solar chemical composition, M=50 M-circle dot, T-eff=10(4) K and L=1.17x10(6) L-circle dot. Contrary to previously studied models, for these parameters, shocks are captured in the H-ionization zone and perform rapid oscillations within it. A linear stability analysis is performed to verify that this behaviour is physical. The origin of an instability discovered in this way is identified by the construction of an analytical model. As a result, stratification turns out to be essential for instability. The difference from common stratification instabilities-for example, convective instabilities-is discussed
SEMI-ACTIVE SUSPENSION SYSTEMS FOR HEAVY-DUTY VEHICLES: MULTIBODY MODEL DEVELOPMENT, IDENTIFICATION AND CONTROL ALGORITHM EVALUATION
No full textThe design of suspension systems for heavy-duty vehicles covers a specific field of automotive industry. During the past few years there has been an increasing demand in power capabilities, loads and driving speeds of heavy duty vehicles. Therefore, off-highway vehicle manufacturers have shown their interest in employing new technologies. This work focuses on the investigation of hydro-pneumatic suspension systems for heavy duty vehicles, in particular on the benefits of a semi-active solution compared to a passive one. The main targets of this activity is the study of the dynamical behaviour of agricultural tractors and the design of a cost-effective controllable suspension, capable to adapt the tractor dynamical behaviour, under different road and load conditions. The work started with the development of a multibody model of the suspension test bench to be used for control solution comparisons. The multibody model was experimentally validated by characterizing the cylinder friction, tire parameters and Frequency Response (F.R.) of the suspension bench test equipped with a passive solution. As a last step the evaluation of different control algorithms for hydraulic semi-active suspension was carried out via Adams/Matlab co-simulation technique
Vehicle simulation for the development of an active suspension system for an agricultural tractor
No full textThe design of suspension systems for heavy-duty vehicles covers a specific field of automotive industry. The proposed work focuses on the design development of a front controllable suspension for an agricultural tractor capable to satisfy the system requirements under different operating conditions. The design of the control algorithms is based on the developed multibody model of the actual tractor, including the pitch motion of the sprung mass, the anti-dive effects during braking and forward-reverse maneuvers and the non-linear dynamics of the actuation system. For an advanced analysis, a novel thermo-hydraulic model of the hydraulic system has been implemented. Several semi-active damping controls are analyzed for the specific case study. Therefore, the most promising damping strategy is integrated with other control functions, namely a self-leveling control, an original control algorithm for the reduction of the pitch motion, an anti-impact system for the hydraulic actuator and an on-line adaptation scheme, which preserves an optimal damping ratio of the suspension, even against large variations in operating conditions
The simulation of non-linear stellar pulsations
No full textDetails of a method for the numerical simulation of stellar pulsations and instabilities in the non-linear regime are presented. Important properties of the method are intrinsic mass and energy conservation. These are indispensable if the connection between stellar pulsations and mass loss is to be investigated. A prescription for the construction of initial models is given, which allows for the study of the linear regime of exponential growth of instabilities and a comparison with independent linear stability analyses. The dependence on numerical parameters of the simulations is investigated in detail. As a result, the acoustic luminosity generated by strange mode instabilities in a massive star is found to depend sensitively on the numerical viscosity necessary to handle shock waves. The values derived have to be regarded as lower limits and reach the order of magnitude of the kinetic luminosities observed in the winds of corresponding objects. This result is taken as further indication for a connection between strange mode instabilities and stellar mass loss
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Modelling, control and design of heavy duty suspension systems
No full textThe design of suspension systems for off-highway vehicles covers a specific field of automotive industry. During the last years, due to the working conditions in agricultural environments, there have been an increasing demand in power capabilities, loads and driving speeds of heavy duty vehicles. In this way the industrial design and consequently the research activity present a big potential of improvement. This paper describes the development, modeling and testing of suspension systems and control algorithms for heavy vehicles. The main targets of this activity, currently under development, consist in the study of the dynamical behaviour of agricultural tractors and the design of a cost-effective industrial solution. The work is structured in different steps: the definition of the project requirements (also by consultations with manufacturers), the modeling of agricultural tractors by symbolic and numerical multibody codes, the investigation of commercial solutions in terms of comfort, ride and energetic requirements and finally the development of control algorithms for hydro-pneumatic semi- active suspension systems. The main results are validated through experimental activities done by a suspension bench test purposely designed. The experimental sessions consist in the identification of the frequency response of the suspension system and the computation of performance indexes
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