Oscillatory Behaviour (oscillatory + behaviour)

Distribution by Scientific Domains


Selected Abstracts


Parallel simulation of unsteady hovering rotor wakes

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 6 2006
C. B. Allen
Abstract Numerical simulation using low diffusion schemes, for example free-vortex or vorticity transport methods, and theoretical stability analyses have shown the wakes of rotors in hover to be unsteady. This has also been observed in experiments, although the instabilities are not always repeatable. Hovering rotor wake stability is considered here using a finite-volume compressible CFD code. An implicit unsteady, multiblock, multigrid, upwind solver, and structured multiblock grid generator are presented, and applied to lifting rotors in hover. To allow the use of very fine meshes and, hence, better representation of the flow physics, a parallel version of the code has been developed, and parallel performance using upto 1024 CPUs is presented. A four-bladed rotor is considered, and it is demonstrated that once the grid density is sufficient to capture enough turns of the tip vortices, hover exhibits oscillatory behaviour of the wake, even using a steady formulation. An unsteady simulation is then performed, and also shows an unsteady wake. Detailed analysis of the time-accurate wake history shows that three dominant unsteady modes are captured, for this four-bladed case, with frequencies of one, four, and eight times the rotational frequency. A comparison with theoretical stability analysis is also presented. Copyright © 2006 John Wiley & Sons, Ltd. [source]


A new algorithm for surface tension model in moving particle methods

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 3 2007
Shuai Zhang
Abstract A new algorithm for the surface tension model was developed for moving particle methods. The algorithm is based on the link-list search algorithm and the continuum surface tension (CST) model. The developed algorithm with the CST model was implemented to a kind of moving particle approach, the finite volume particle (FVP) method. The FVP method with the new algorithm was tested by oscillatory behaviour of a two-dimensional droplet. The oscillatory period agrees well with analytical one, and the transient shape of the droplet is also in good agreement with that obtained by other numerical methods. The droplet impact on a liquid surface was also studied using the new algorithm. The deposition and splashing phenomena were clearly reproduced. Simulated spread radius of the splashing phenomena was consistent with a power law. Copyright © 2007 John Wiley & Sons, Ltd. [source]


Parallel solution of lifting rotors in hover and forward flight

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 1 2007
C. B. Allen
Abstract An implicit unsteady, multiblock, multigrid, upwind solver including mesh deformation capability, and structured multiblock grid generator, are presented and applied to lifting rotors in both hover and forward flight. To allow the use of very fine meshes and, hence, better representation of the flow physics, a parallel version of the code has been developed. It is demonstrated that once the grid density is sufficient to capture enough turns of the tip vortices, hover exhibits oscillatory behaviour of the wake, even using a steady formulation. An unsteady simulation is then presented, and detailed analysis of the time-accurate wake history is performed and compared to theoretical predictions. Forward flight simulations are also presented and, again, grid density effects on the wake formation investigated. Parallel performance of the code using up to 1024 CPU's is also presented. Copyright © 2006 John Wiley & Sons, Ltd. [source]


Numerical analysis of deformed free surface under AC magnetic fields

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 12 2004
Haruhiko Kohno
Abstract A novel numerical scheme for the analysis of large deformation of electrically conducting liquid under alternating current magnetic fields is presented. The main features are characterized by two numerical tools; the level set method to calculate deformed free surface stably and the hybrid finite element method and boundary element method to discretize the electromagnetic field efficiently. Two-dimensional numerical simulation of conducting drop deformation is carried out to demonstrate the effectiveness of the present scheme, and the oscillatory behaviour, which depends on the magnitude of surface tension and Lorentz force, is investigated. Copyright © 2004 John Wiley & Sons, Ltd. [source]


Coherence length and time of excitons in ZnSe quantum wells

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3 2004
B. Dal Don
Abstract We investigate the in-plane transport of excitons in quantum wells by nano-photoluminescence. The experimental method is based on a confocal microscope with an enhanced resolution given by the introduction of a solid immersion lens. In combination with pulsed laser excitation and streak-camera detection, we have access to transport phenomena on a timescale faster than the time of scattering with acoustic phonons and a length scale of the light wavelength. We use ZnSe-based quantum wells as a model system since hot excitons with well defined excess energy can be formed assisted by the emission of optical phonons. This results e.g. in a periodic quenching of the excitonic transport length as function of excitation excess energy which, in comparison, is not found in GaAs quantum wells. Monte Carlo simulations of the nonlinear expansion of the luminescence spot observed as a function of time reveal the difference between the spatial profiles of the luminescence and the exciton density. The latter shows an oscillatory behaviour in time due to the dominant backscattering, when the first acoustic phonon is emitted. From this oscillation we can determine simultaneously the coherence time and length of the excitonic transport in ZnSe quantum wells. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Calmodulin activity and cAMP signalling modulate growth and apical secretion in pollen tubes

THE PLANT JOURNAL, Issue 6 2004
ClŠudia Rato
Summary Our present understanding implicates both calmodulin (CaM) and 3,,5,-cyclicAMP (cAMP) in the regulation of pollen tube growth. However, downstream molecules of these signalling pathways and the cellular processes they modulate remain largely unknown. In order to elucidate the role of CaM, we mapped its activity in growing pollen tubes. 2-chloro-(,-amino-Lys75)-[6-4-(N,N, -diethylaminophenyl)-1,3,5-triazin-4-yl]-calmodulin (TA-CaM) and fluorescein-calmodulin (FL-CaM), fluorescent analogues of CaM, were loaded into pollen tubes and CaM activity was mapped by fluorescence ratio imaging. It was found that CaM activity exhibits a tip-focused gradient, similar to the distribution of cytosolic-free calcium ([Ca2+]c). In long pollen tubes, apical CaM activity was also found to oscillate with a period similar to [Ca2+]c (40,80 sec). This oscillatory behaviour was not observed in small pollen tubes or in tubes that had stopped growing. Changes in CaM activity within the dome of the pollen tube apex resulting from the photolysis of caged photolysis of RS-20 (a peptide antagonist of CaM) induced re-orientation of the growth axis, suggesting that CaM is also involved in the guidance mechanism. CaM activity was strongly modulated by intracellular changes in cAMP (induced by activators and antagonists of adenylyl cyclase). These results indicate that the action of this protein is dependent not solely on [Ca2+]c but also on a cross-talk with other signalling pathways. A putative target of this cross-talk is the secretory machinery as observed in pollen tubes loaded with the FM (N -(3-triethylammoniumpropyl)-4-(4-dibutylamino)styryl)pyridinium dibromide 1-43 dye and exposed to different antagonists and activators of these molecules. Our data thus suggest that pollen tube growth and orientation depend on an intricate cross-talk between multiple signalling pathways in which CaM is a key element. [source]


The Real Exchange Rate Always Floats

AUSTRALIAN ECONOMIC PAPERS, Issue 4 2002
Thorvaldur Gylfason
This paper makes two main points. First, irrespective of nominal exchange rate arrangements, the real exchange rate always floats , if not through nominal exchange rate adjustment, then through price change. Further, because prices and wages tend to be sticky, the adjustment of real exchange rates towards long,run equilibrium takes time, as witnessed by long,lasting currency misalignments around the world. In second place, real exchange rates are rather likely to fluctuate on their way towards long,run equilibrium because of the dynamic interaction between real exchange rates and the current account or, put differently, because the structure of lags with which exchange rates impact the volume of exports and imports may give rise to oscillatory behaviour. [source]