Hydrodynamics and cell motion during the rounding of two dimensional hydra cell aggregates

Using a two dimensional experimental geometry, we study hydrodynamics and cell motion during the rounding-up of three kinds of hydra aggregates (ectodermal, endodermal and mixed). The relaxation of initially elongated aggregates toward a circular shape is exponential and the relaxation time is proportional to the aggregate size as expected from hydrodynamics laws. As for viscous liquids, aggregate rounding is driven by tissue surface tensions σ and resisted by tissue viscosity η. The ratio η/σ is the same for the three kinds of aggregates. With a reasonable value σ = 1 mN/m, we obtain an estimate larger than 10⁴ Pa s for the tissue viscosity. Cell motion during rounding is strongly cooperative and cell displacements exhibit some specific patterns in each aggregate. These results point out the role of adhesive bonds in the observed kinetics. Received 21 December 2001     

Evolutionary optimization techniques as versatile solvers for hard‐to‐converge problems in computational fluid dynamics

Evolutionary algorithms mimic the process of natural evolution governed by the ‘survival of the fittest’ principle. In this work, a genetic algorithm (GA) is successfully used to solve problems in potential flow in a gradual contraction, viscous flow over a backward facing step, and non‐Newtonian flow using the power law model. Specifically, the GA heuristically searches the domain for potential solutions, precluding many convergence difficulties associated with the stiffness of a problem. The GA was able to solve problems that the gradient‐based method could not mainly because of its relative indifference to regions of high gradients when performing the search and that systems of discretized equations are never actually solved. The GA exhibited excellent scalability properties for solving problems with a large number of nodes. These results show evolutionary techniques to be of great utility in solving stiff problems in fluid flow. Copyright © 2006 John Wiley & Sons, Ltd.     

The topology of stirred fluids

There are simple idealized mathematical models representing the stirring of fluids. The models we consider involve two fluids entering a chamber, with the overflow leaving it. The stirring creates a Cantor-like, but connected, boundary between the fluids that is best described point-set topologically. We prove that in many cases the boundary between the fluids is an indecomposable continuum.     

磁流变智能液场致微结构变化的Monte Carlo模拟

从磁流变液中存在的各种相互作用势出发，对Ｎ＝１００个球形羰基铁颗粒均匀分布于一正方形二维体系之中的物理模型，利用ＭｏｎｔｅＣａｒｌｏ方法和Ｍｅｔｒｏｐｌｉｓ算法着重计算了磁流变智能液的场致微结构变化。计算结果表明，磁流变液（ＭＲＦ）的流变效应在外磁场中的变化是由于其ＭＲＦ中磁性颗粒的成链团簇有序化所致。这与磁流变液在外磁场中熵的下降和对ＭＲＦ的光学显微镜观察结果是一致的。     

On the interaction of mesoscopic quantum systems with gravity

We review the different aspects of the interaction of mesoscopic quantum systems with gravitational fields. We first discuss briefly the foundations of general relativity and quantum mechanics. Then, we consider the non‐relativistic expansions of the Klein‐Gordon and Dirac equations in the post‐Newtonian approximation. After a short overview of classical gravitational waves, we discuss two proposed interaction mechanisms: (i) the use of quantum fluids as generator and/or detector of gravitational waves in the laboratory, and (ii) the inclusion of gravitomagnetic fields in the study of the properties of rotating superconductors. The foundations of the proposed experiments are explained and evaluated.     

Hydrodynamics of compressible liquids: Influence of the piston effect on convection and internal gravity waves

The onset of stability in strongly compressible liquid with fixed volume is found by linear stability analysis. The results coincide with those at fixed pressure proving thereby that the piston effect does not influence the onset of convection. However, the dynamic phenomenon of internal gravity waves essentially depends on the piston effect.     

Statistics of Stretching Fields in Experimental Fluid Flows Exhibiting Chaotic Advection

Stretching fields and their statistical properties are studied experimentally for four distinct two-dimensional time-periodic confined fluid flows exhibiting chaotic advection: a random vortex array for two different Reynolds numbers, a set of parallel shear layers, and a vortex lattice. The flows are driven electromagnetically, and they are studied by means of precise particle velocimetry. We find that for a given flow, the probability distributions of log S (where S is the local stretching in N cycles) can be nearly superimposed for different N when log S is rescaled using the geometrical mean of the stretching distribution. The rescaled stretching fields for a given flow at various N are highly correlated spatially when N is large. Finally, the scaled distributions for different flows are similar, though there are some differences connected to the degree of spatial symmetry and time-reversibility of the flows.     

Internal wave computations using the ghost fluid method on unstructured grids

Two‐layer incompressible flows are analysed using the ghost fluid method on unstructured grids. Discontinuities in dynamic pressure along interfaces are captured in one cell without oscillations. Because of data reconstructions based on gradients, the ghost fluid method can be adopted without additional storages for the ghost nodes at the expense of modification in gradient calculations due to the discontinuity. The code is validated through comparisons with experimental and other numerical results. Good agreements are achieved for internal waves generated by a body moving at transcritical speeds including a case where upstream solitary internal waves propagate. The developed code is applied to analyse internal waves generated by a NACA0012 section moving near interfaces. Variations of the lift acting on the body and configurations of the interfaces are compared for various distances between the wing and the interface. The effects of the interface are compared with the effects of a solid wall. Copyright © 2004 John Wiley & Sons, Ltd.     

Multilayer film casting of modified Giesekus fluids Part 2. Linear stability analysis

A linear stability analysis of the multilayer film casting of polymeric fluids has been conducted. A modified Giesekus model was used to characterize the rheological behaviors of the fluids. The critical draw ratio at the onset of draw resonance was found to depend on the elongational and shear viscosities of the fluids. Extensional-thickening has a stabilizing effect, whereas shear-thinning and extensional-thinning have destabilizing effects. The critical draw ratios for bilayer films of various thickness fractions are bounded by those for single layer films of the two fluids. When the two fluids have a comparable elongational viscosity, the critical draw ratio at a given Deborah number varies linearly with thickness fraction. When one fluid has a much larger elongational viscosity, it dominates the flow and the critical draw ratios at most thickness fractions remain close to its critical draw ratio as a single layer film. When the dominating fluid exhibits extensional-thickening, a film with a certain thickness fraction has more than one critical draw ratio at a given Deborah number and may not exhibit draw resonance within some range of the Deborah number.