A simplified model for red cell dynamics in small blood vessels

A simple mechanism for the confinement of red cells in the middle of narrow blood vessels is proposed. In the presence of a quadratic shear, red cells deform in such a way to loose fore-aft symmetry and to achieve a fixed orientation with respect to the flow. This leads to a drift away from the vessel walls, when the vessel diameter goes below a critical value depending on the viscoelastic properties and the dimensions of the cell.Comment: 7 pages, 3 figures; to be published on Phys. Rev. Lett.; various modifications to text and figure

A closure model for intermittency in three dimensional incompressible turbulence

A simplified Lagrangean closure for the Navier-Stokes equation is used to study the production of intermittency in the inertial range of three dimensional turbulence. This is done using localized wavepackets following the fluid rather than a standard Fourier basis. In this formulation, the equation for the energy transfer acquires a noise term coming from the fluctuations in the energy content of the different wavepackets. Assuming smallness of the intermittency correction to scaling allows the adoption of a quasi-gaussian approximation for the velocity field, provided a cutoff on small scales is imposed and a finite region of space is considered. In this approximations, the amplitude of the local energy transfer fluctuations, can be calculated self consistently in the model. Definite predictions are obtained on the scaling of the wavepacket energy moments.Comment: Plain tex source code; file in ascii format; 24 pages 6 figures and 1 table (not included); table and figures available directly from author; send e-mail to: [email protected]

Passive swimming in low Reynolds number flows

The possibility of microscopic swimming by extraction of energy from an external flow is discussed, focusing on the migration of a simple trimer across a linear shear flow. The geometric properties of swimming, together with the possible generalization to the case of a vesicle, are analyzed.The mechanism of energy extraction from the flow appears to be the generalization to a discrete swimmer of the tank-treading regime of a vesicle. The swimmer takes advantage of the external flow by both extracting energy for swimming and "sailing" through it. The migration velocity is found to scale linearly in the stroke amplitude, and not quadratically as in a quiescent fluid. This effect turns out to be connected with the non-applicability of the scallop theorem in the presence of external flow fields.Comment: 4 pages, 4 figure