Generalized strain-based finite element for non-linear stability analysis of beams with thin-walled open cross-section

Based on the generalized strain theory a shear-deformable finite element is developed for nonlinear stability analysis of thin-walled open-section beams in multibody systems. In this formulation a finite number of deformations, characterized as generalized strains, is defined which are related to dual stress resultants in a co-rotational frame. The stiffness formulation is based on a second-order approximation of the local elastic displacement field. Timoshenko's beam theory and Vlasov's modified thin-walled beam theory are used to include the shear strain effects due to non-uniform bending and restrained warping torsion and their mutual coupling effects. Axial shortening associated with the Wagner Hypothesis is taken into account such that the nonlinear behaviour of the beam is predicted accurately, especially under large torsion. Coupling between bending and torsional deformation due to non-coincident centroid and shear centre is modelled using a second-order cross-section transformation matrix. Cubic Hermitian polynomials are used as shape functions for the lateral displacements and twist rotation to derive the elastic and geometric stiffness matrices. Geometric nonlinearities associated with axial elongation and bending curvatures are described by additional torsion, bending and warping-related quadratic deformation terms yielding a set of modified deformations. The inertia properties of the beam are described using both consistent and lumped mass formulations. The latter is used to model rotary and warping inertias of the beam cross-section. The accuracy and the computational efficiency of the new beam element is demonstrated in several static and dynamic examples.</p

Epitaxial Growth of an n-type Ferromagnetic Semiconductor CdCr2Se4 on GaAs(001) and GaP(001)

We report the epitaxial growth of CdCr2Se4, an n-type ferromagnetic semiconductor, on both GaAs and GaP(001) substrates, and describe the structural, magnetic and electronic properties. Magnetometry data confirm ferromagnetic order with a Curie temperature of 130 K, as in the bulk material. The magnetization exhibits hysteretic behavior with significant remanence, and an in-plane easy axis with a coercive field of ~125 Oe. Temperature dependent transport data show that the films are semiconducting in character and n-type as grown, with room temperature carrier concentrations of n ~ 1 x 10^18 cm-3.Comment: 12 pages, 3 figure

Wind and boundary layers in Rayleigh-Benard convection. Part 2: boundary layer character and scaling

The effect of the wind of Rayleigh-Benard convection on the boundary layers is studied by direct numerical simulation of an L/H=4 aspect-ratio domain with periodic side boundary conditions for Ra={10^5, 10^6, 10^7} and Pr=1. It is shown that the kinetic boundary layers on the top- and bottom plate have some features of both laminar and turbulent boundary layers. A continuous spectrum, as well as significant forcing due to Reynolds stresses indicates undoubtedly a turbulent character, whereas the classical integral boundary layer parameters -- the shape factor and friction factor (the latter is shown to be dominated by the pressure gradient) -- scale with Reynolds number more akin to laminar boundary layers. This apparent dual behavior is caused by the large influence of plumes impinging onto and detaching from the boundary layer. The plume-generated Reynolds stresses have a negligible effect on the friction factor at the Rayleigh numbers we consider, which indicates that they are passive with respect to momentum transfer in the wall-parallel direction. However, the effect of Reynolds stresses cannot be neglected for the thickness of the kinetic boundary layer. Using a conceptual wind model, we find that the friction factor C_f should scale proportional to the thermal boundary layer thickness as C_f ~ lambda_Theta, while the kinetic boundary layer thickness lambda_u scales inversely proportional to the thermal boundary layer thickness and wind Reynolds number lambda_u ~ lambda_Theta^{-1} Re^{-1}. The predicted trends for C_f and \lambda_u are in agreement with DNS results

MOVIEMOD: Een beslisingsondersteunend systeem voor de introductie van nieuwe films

Het produceren en distribueren van nieuwe films is risicovol. Distibuteurs en bioscoopexploitanten hebben daarom baat bij informatie over het succes van een film, voordat de film in de bioscoop draait. In dit artikel wordt een model gepresenteerd, genaamd MOVIEMOD, dat in staat is zulke voorspellingen te leveren door een voorvertoning van de film aan een steekproef van potentiële bezoekers. Daarnaast kan het model ook helpen bij het opstellen van het mediaplan, omdat het in staat is het effect van aanpassingen in het mediaplan te bepalen. Het model is twee keer toegepast in de praktijk: de eerste keer in de Verenigde Staten, en daarna in Nederland. Omdat in Nederland de distributeur en de belangrijkste bioscoopexploitant van de film deelnamen, waren we in staat om het gebruik van MOVIEMOD als decision support instrument te onderzoeken. De voorspelling van MOVIEMOD bij het gebruikte media- en distributieplan was redelijk nauwkeurig (een afwijking van 4% met het werkelijke bezoekersaantal) en deze voorspelling was beter dan die van een aantal benchmark modellen. De resultaten tonen aan dat MOVIEMOD een waardevol marketing instrument kan zijn voor de filmindustrie