Motivated by recent experiments on Rayleigh-Benard convection, where spiral defect chaos was discovered, we have simulated the Boussinesq equations for the appropriate Rayleigh numbers not far from threshold for large aspect ratio systems using a Galerkin method. A detailed analysis of the results reproduces the experimental findings almost quantitatively. A critical comparison with recent model calculations is also presented. Our investigations show that the new spatiotemporal pattern must be considered generic for Rayleigh-Benard convection. PACS numbers: 47.10. +g, 47.20.Bp Rayleigh-Benard convection (RBC) in a fluid layer heated from below provides a canonical example for pattern-forming transitions in nonequilibrium Quid systems [1,2]. The convection sets in if the temperature difference AT across the fluid layer, characterized by the nondimensional Rayleigh number B, exceeds a certain

We calculate the threshold for electrohydrodynamic convection in homeotropically oriented nematic layers with negative dielectric anistropy that sets in after the bend-Fréedericksz transition to a quasi-planar alignment has taken place. Oblique rolls are found in a larger range than in the case of planar anchoring. The most interesting prediction is that in the weakly nonlinear analysis all roll solution are in fact unstable so that a direct transition to spatio-temporal chaos becomes possible. In the oblique-roll case this may be accompanied by a permanent local rotation of the director bend axis. Application of a planar magnetic field should stabilize the rolls

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The reduced description of the roll patterns and their stability near onset is

The effect of superimposed ac and dc electric fields on the formation of electroconvection and flexoelectric patterns in nematic liquid crystals was studied. For selected ac frequencies, an extended standard model of the electrohydrodynamic instabilities was used to characterize the onset of pattern formation in the two-dimensional parameter space of the magnitudes of the ac and dc electric field components. Numerical as well as approximate analytical calculations demonstrate that depending on the type of patterns and on the ac frequency, the combined action of ac and dc fields may either enhance or suppress the formation of patterns. The theoretical predictions are qualitatively confirmed by experiments in most cases. Some discrepancies, however, seem to indicate the need to extend the theoretical description.