/pdf/evidence-of-very-long-meandering-features-in-the-logarithmic-2m2f4f0daq.pdf

Evidence of very long meandering features in the logarithmic region of turbulent boundary layers

Coherent structures in wall turbulence transport momentum and provide a means of producing turbulent kinetic energy. Above the viscous wall layer, the hairpin vortex paradigm of Theodorsen coupled with the quasistreamwise vortex paradigm have gained considerable support from multidimensional visualization using particle image velocimetry and direct numerical simulation experiments. Hairpins can autogenerate to form packets that populate a significant fraction of the boundary layer, even at very high Reynolds numbers. The dynamics of packet formation and the ramifications of organization of coherent structures (hairpins or packets) into larger-scale structures are discussed. Evidence for a large-scale mechanism in the outer layer suggests that further organization of packets may occur on scales equal to and larger than the boundary layer thickness.

Hairpin vortex organization in wall turbulencea)

It has been thought that sheets of cells move by traction forces exerted by the cells at the leading edge of the sheet. Using traction microscopy to create a map of physical forces, it is now shown that in fact it is cells many rows from the front that do most of the work. Fundamental biological processes including morphogenesis, tissue repair and tumour metastasis require collective cell motions1,2,3, and to drive these motions cells exert traction forces on their surroundings4. Current understanding emphasizes that these traction forces arise mainly in ‘leader cells’ at the front edge of the advancing cell sheet5,6,7,8,9. Our data are contrary to that assumption and show for the first time by direct measurement that traction forces driving collective cell migration arise predominately many cell rows behind the leading front edge and extend across enormous distances. Traction fluctuations are anomalous, moreover, exhibiting broad non-Gaussian distributions characterized by exponential tails10,11,12. Taken together, these unexpected findings demonstrate that although the leader cell may have a pivotal role in local cell guidance, physical forces that it generates are but a small part of a global tug-of-war involving cells well back from the leading edge.

/pdf/physical-forces-during-collective-cell-migration-3s53rslx6s.pdf

Physical forces during collective cell migration

The Structure of Turbulent Shear Flow By Dr. A. A. Townsend. Pp. xii + 315. 8¾ in. × 5½ in. (Cambridge: At the University Press.) 40s.

/pdf/the-structure-of-turbulent-shear-flow-2yf3szhaqb.pdf

The Structure of Turbulent Shear Flow

A new numerical simulation of a turbulent channel in a large box at Reτ=2003 is described and briefly compared with simulations at lower Reynolds numbers and with experiments. Some of the fluctuation intensities, especially the streamwise velocity, do not scale well in wall units, both near and away from the wall. Spectral analysis traces the near-wall scaling failure to the interaction of the logarithmic layer with the wall. The present statistics can be downloaded from http://torroja.dmt.upm.es/ftp/channels. Further ones will be added to the site as they become available.

Scaling of the velocity fluctuations in turbulent channels up to Reτ=2003

Dynamics of a Microsphere in an Anisotropic Gel: a Frontier in Intracellular Microrheology

It has long been realized that turbulent flows contain a wide range of scales, from small viscous ones responsible for the viscous dissipation, to large ones which carry the turbulent energy and the Reynolds stresses. The former are believed to be roughly universal among different flows, while the latter vary with the geometry and with the flow conditions. Evidence has accumulated for some time that in an intermediate layer of wall-bounded shear flows, including the logarithmic region and part of the outer layer, these large scales are very anisotropic and very large, with streamwise lengths that may be of the order of 100 times their distance to the wall [2,4,8]. At their longest, somewhat above the top of the logarithmic layer, this amounts to 20–30 times the boundary layer thickness. The earliest detailed study of these structures was done by Perry [9,10], who identified them as an E uu ~ k -1, long-wavelength, spectral range. Since the turbulent energy is proportional to ∫ k E uu d (log k), a k -1 spectral range essentially contains most of the fluctuating energy in the flow (see figure 1). Moreover, because the size of these structures requires either very large computational boxes or very long experiments, relatively little was known about them until recently. There is for example very little information on their spanwise dimensions, or on the relation of the different velocity components. The goal of the simulations discussed here is to study the large anisotropic scales in turbulent channels, their origin and structure, and their possible influence on other flow properties.

Very Large Anisotropic Scales in Turbulent Wall-Bounded Flows

The eggs of the parasitic blood fluke, Schistosoma, are the main drivers of the chronic pathologies associated with schistosomiasis, a disease of poverty afflicting approximately 220 million people worldwide. Eggs laid by Schistosoma mansoni in the bloodstream of the host are encapsulated by vascular endothelial cells (VECs), the first step in the migration of the egg from the blood stream into the lumen of the gut and eventual exit from the body. The biomechanics associated with encapsulation and extravasation of the egg are poorly understood. We demonstrate that S. mansoni eggs induce VECs to form two types of membrane extensions during encapsulation; filopodia that probe eggshell surfaces and intercellular nanotubes that presumably facilitate VEC communication. Encapsulation efficiency, the number of filopodia and intercellular nanotubes, and the length of these structures depend on the eggs vitality and, to a lesser degree, its maturation state. During encapsulation, live eggs induce VEC contractility and membranous structures formation, in a Rho/ROCK pathway-dependent manner. Using elastic hydrogels embedded with fluorescent microbeads as substrates to culture VECs, live eggs induce VECs to exert significantly greater contractile forces during encapsulation than dead eggs, which leads to 3D deformations on both the VEC monolayer and the flexible substrate underneath. These significant mechanical deformations cause the VEC monolayer tension to fluctuate with eventual rupture of VEC junctions, thus facilitating egg transit out of the blood vessel. Overall, our data on the mechanical interplay between host VECs and the schistosome egg improve our understanding of how this parasite manipulates its immediate environment to maintain disease transmission.

https://biorxiv.org/content/10.1101/2021.09.16.459846v1.full.pdf

Juan C. del Álamo

Papers

Dynamics of a Microsphere in an Anisotropic Gel: a Frontier in Intracellular Microrheology

Very Large Anisotropic Scales in Turbulent Wall-Bounded Flows

Biomechanical interactions of Schistosoma mansoni eggs with vascular endothelial cells facilitate egg extravasation