No abstract

Simulation results for the motion of flexible fibers modeled as rigid spheres connected by ball and socket joints are presented. Simulations of isolated stiff fibers reproduce such features of Jeffery orbits as orbit stability, the dependence of the dimensionless orbit period on only the fiber aspect ratio ͑independent of shear rate and orientation͒, and trajectories identical to those of prolate spheroids of the same equivalent aspect ratio. Simulations of stiff fibers ''pole-vaulting'' near a bounding surface qualitatively reproduce experimental observations. Fiber trajectories are very sensitive to the short-range interactions between a fiber and a bounding surface. In contrast to rigid fibers, flexible fiber orientations drift in unbounded simple shear and parabolic shear flows. The drift direction and rate depend on fiber stiffness, initial orientation, as well as the ambient flow field. A wide variety of configurational dynamics are observed, which also depend on the fiber stiffness, initial orientation, and the ambient flow field. These results agree with previous experimental observations of flexible fibers in shear flows.

No abstract

No abstract

Polymers consisting of rigid segments connected by flexible joints ͑needle chains͒ constitute an important class of biopolymers. Using kinetic theory as a starting point, we first derive the generalized coordinate-space diffusion ͑Fokker-Planck͒ equation for the needle chain polymer model. Next, the equivalent generalized coordinate Itô stochastic differential equation is established. Nonlinear transformations of variables finally yield a stochastic differential equation for the needle chain spatial coordinates in the laboratory coordinate system where the coefficients are expressed in terms of the chain constraint conditions. This latter equation constitutes the basis for our needle chain Brownian dynamics ͑BD͒ algorithm. The used needle chain model includes needle translation-translation and rotation-rotation hydrodynamic interactions, a homogeneous solvent flow field, external forces, excluded volume effects, and bending and twisting stiffness between nearest neighbor segments. For this chain model we find that by proper generalization of the involved parameters the mathematical analysis of the polymer dynamics, in great detail, maps onto the analysis of the bead-rod-spring polymer chain model with constraints presented by Ö ttinger in Phys. Rev. E 50, 2696 ͑1994͒. Preliminary numerical simulation data show that for a three segment needle chain, with needle axial ratio equal to five, our new needle chain BD algorithm is, in general, more than about 10 3 times more efficient than the bead-spring polymer chain BD algorithm commonly used as an approximation for studies of such polymer chains. This efficiency ratio increases asymptotically proportional to approximately the fourth power of the needle axial ratio. In addition to this major gain in efficiency, the needle chain model for segmented polymers, in general, incorporates a more realistic hydrodynamic description of the individual segments and, in particular, the joints between the segments than the bead-rod-spring models.

In this paper we review the current understanding of bubble plumes discharged from subsea sources related to oil and gas production, and show how CFD can be applied for risk assessments. A general introduction to causes and risks is given. This is followed by a discussion of the physics that need to be accounted for before giving a brief review of the different modelling approaches employed today. The empirical and experimental knowledge base is also summarized. An example of how CFD can be applied to study gas releases is given. At the end we outline what is needed to advance current understanding of such releases and model their interaction with the surroundings. The scope of the review is limited to the fate of the gas and the flow induced by the ascending bubble plume in the water column. Atmospheric dispersion of surfacing gas is not considered.

No abstract

No abstract