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May 2012

Volume 24, Issue 5, Articles (05xxxx)

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Phys. Fluids 24, 056603 (2012); http://dx.doi.org/10.1063/1.4719147 (13 pages)

Rotem Aharon, Vered Rom-Kedar, and Hezi Gildor
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back to top Biofluid Mechanics

Dynamics of freely swimming flexible foils

Silas Alben, Charles Witt, T. Vernon Baker, Erik Anderson, and George V. Lauder

Phys. Fluids 24, 051901 (2012); http://dx.doi.org/10.1063/1.4709477 (25 pages) | Cited 2 times

Online Publication Date: 1 May 2012

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We use modeling and simulations guided by initial experiments to study thin foils which are oscillated at the leading edge and are free to move unidirectionally under the resulting fluid forces. We find resonant-like peaks in the swimming speed as a function of foil length and rigidity. We find good agreement between the inviscid model and the experiment in the foil motions (particularly the wavelengths of their shapes), the dependences of their swimming speeds on foil length and rigidity, and the corresponding flows. The model predicts that the foil speed is proportional to foil length to the −1/3 power and foil rigidity to the 2/15 power. These scalings give a good collapse of the experimental data.
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47.11.-j Computational methods in fluid dynamics

Self-propulsion in viscoelastic fluids: Pushers vs. pullers

Lailai Zhu, Eric Lauga, and Luca Brandt

Phys. Fluids 24, 051902 (2012); http://dx.doi.org/10.1063/1.4718446 (17 pages) | Cited 4 times

Online Publication Date: 22 May 2012

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We use numerical simulations to address locomotion at zero Reynolds number in viscoelastic (Giesekus) fluids. The swimmers are assumed to be spherical, to self-propel using tangential surface deformation, and the computations are implemented using a finite element method. The emphasis of the study is on the change of the swimming kinematics, energetics, and flow disturbance from Newtonian to viscoelastic, and on the distinction between pusher and puller swimmers. In all cases, the viscoelastic swimming speed is below the Newtonian one, with a minimum obtained for intermediate values of the Weissenberg number, We. An analysis of the flow field places the origin of this swimming degradation in non-Newtonian elongational stresses. The power required for swimming is also systematically below the Newtonian power, and always a decreasing function of We. A detail energetic balance of the swimming problem points at the polymeric part of the stress as the primary We-decreasing energetic contribution, while the contributions of the work done by the swimmer from the solvent remain essentially We-independent. In addition, we observe negative values of the polymeric power density in some flow regions, indicating positive elastic work by the polymers on the fluid. The hydrodynamic efficiency, defined as the ratio of the useful to total rate of work, is always above the Newtonian case, with a maximum relative value obtained at intermediate Weissenberg numbers. Finally, the presence of polymeric stresses leads to an increase of the rate of decay of the flow velocity in the fluid, and a decrease of the magnitude of the stresslet governing the magnitude of the effective bulk stress in the fluid.
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47.50.Cd Modeling
47.11.Fg Finite element methods
02.70.Dh Finite-element and Galerkin methods

Red blood cell clustering in Poiseuille microcapillary flow

Giovanna Tomaiuolo, Luca Lanotte, Giovanni Ghigliotti, Chaouqi Misbah, and Stefano Guido

Phys. Fluids 24, 051903 (2012); http://dx.doi.org/10.1063/1.4721811 (8 pages) | Cited 5 times

Online Publication Date: 29 May 2012

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Red blood cells (RBC) flowing in microcapillaries tend to associate into clusters, i.e., small trains of cells separated from each other by a distance comparable to cell size. This process is usually attributed to slower RBCs acting to create a sequence of trailing cells. Here, based on the first systematic investigation of collective RBC flow behavior in microcapillaries in vitro by high-speed video microscopy and numerical simulations, we show that RBC size polydispersity within the physiological range does not affect cluster stability. Lower applied pressure drops and longer residence times favor larger RBC clusters. A limiting cluster length, depending on the number of cells in a cluster, is found by increasing the applied pressure drop. The insight on the mechanism of RBC clustering provided by this work can be applied to further our understanding of RBC aggregability, which is a key parameter implicated in clotting and thrombus formation.
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47.85.Np Fluidics
47.60.Dx Flows in ducts and channels
47.15.Rq Laminar flows in cavities, channels, ducts, and conduits
47.55.nb Capillary and thermocapillary flows
47.61.-k Micro- and nano- scale flow phenomena
47.63.Jd Microcirculation and flow through tissues

Transport of particles by magnetic forces and cellular blood flow in a model microvessel

J. B. Freund and B. Shapiro

Phys. Fluids 24, 051904 (2012); http://dx.doi.org/10.1063/1.4718752 (12 pages)

Online Publication Date: 29 May 2012

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The transport of particles (diameter 0.56 μm) by magnetic forces in a small blood vessel (diameter D = 16.9 μm, mean velocity U = 2.89 mm/s, red cell volume fraction Hc = 0.22) is studied using a simulation model that explicitly includes hydrodynamic interactions with realistically deformable red blood cells. A biomedical application of such a system is targeted drug or hyperthermia delivery, for which transport to the vessel wall is essential for localizing therapy. In the absence of magnetic forces, it is seen that interactions with the unsteadily flowing red cells cause lateral particle velocity fluctuations with an approximately normal distribution with variance σ = 140 μm/s. The resulting dispersion is over 100 times faster than expected for Brownian diffusion, which we neglect. Magnetic forces relative to the drag force on a hypothetically fixed particle at the vessel center are selected to range from Ψ = 0.006 to 0.204. The stronger forces quickly drive the magnetic particles to the vessel wall, though in this case the red cells impede margination; for weaker forces, many of the particles are marginated more quickly than might be predicted for a homogeneous fluid by the apparently chaotic stirring induced by the motions of the red cells. A corresponding non-dimensional parameter Ψ′, which is based on the characteristic fluctuation velocity σ rather than the centerline velocity, explains the switch-over between these behaviors. Forces that are applied parallel to the vessel are seen to have a surprisingly strong effect due to the streamwise-asymmetric orientation of the flowing blood cells. In essence, the cells act as low-Reynolds number analogs of turning vanes, causing streamwise accelerated particles to be directed toward the vessel center and streamwise decelerated particles to be directed toward the vessel wall.
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47.63.Cb Blood flow in cardiovascular system
47.60.Dx Flows in ducts and channels
47.52.+j Chaos in fluid dynamics
87.19.U- Hemodynamics
87.85.gf Fluid mechanics and rheology
87.17.-d Cell processes
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