scholarly journals Flow-accelerated platelet biogenesis is due to an elasto-hydrodynamic instability

2020 ◽  
Vol 117 (32) ◽  
pp. 18969-18976 ◽  
Author(s):  
Christian Bächer ◽  
Markus Bender ◽  
Stephan Gekle
Keyword(s):  
Hydrodynamic Instability ◽  
Blood Platelets ◽  
Three Dimensional ◽  
Immersed Boundary ◽  
Fusion Event ◽  
Specific Flow ◽  
Actomyosin Contractility ◽  
Biophysical Mechanism ◽  
Three Dimensional Simulations

Blood platelets are formed by fragmentation of long membrane extensions from bone marrow megakaryocytes in the blood flow. Using lattice-Boltzmann/immersed boundary simulations we propose a biological Rayleigh–Plateau instability as the biophysical mechanism behind this fragmentation process. This instability is akin to the surface tension-induced breakup of a liquid jet but is driven by active cortical processes including actomyosin contractility and microtubule sliding. Our fully three-dimensional simulations highlight the crucial role of actomyosin contractility, which is required to trigger the instability, and illustrate how the wavelength of the instability determines the size of the final platelets. The elasto-hydrodynamic origin of the fragmentation explains the strong acceleration of platelet biogenesis in the presence of an external flow, which we observe in agreement with experiments. Our simulations then allow us to disentangle the influence of specific flow conditions: While a homogeneous flow with uniform velocity leads to the strongest acceleration, a shear flow with a linear velocity gradient can cause fusion events of two developing platelet-sized swellings during fragmentation. A fusion event may lead to the release of larger structures which are observable as preplatelets in experiments. Together, our findings strongly indicate a mainly physical origin of fragmentation and regulation of platelet size in flow-accelerated platelet biogenesis.

Dynamic pitching of an elastic rectangular wing in hovering motion

2012 ◽  
Vol 693 ◽  
pp. 473-499 ◽  
Author(s):  
Hu Dai ◽  
Haoxiang Luo ◽  
James F. Doyle
Keyword(s):  
Three Dimensional ◽  
Immersed Boundary ◽  
Mass Ratio ◽  
Flow Solver ◽  
Thin Walled Structures ◽  
Insect Wings ◽  
Passive Deformation ◽  
Comparable Magnitude ◽  
Wing Deformation

AbstractIn order to study the role of the passive deformation in the aerodynamics of insect wings, we computationally model the three-dimensional fluid–structure interaction of an elastic rectangular wing at a low aspect ratio during hovering flight. The code couples a viscous incompressible flow solver based on the immersed-boundary method and a nonlinear finite-element solver for thin-walled structures. During a flapping stroke, the wing surface is dominated by non-uniform chordwise deformations. The effects of the wing stiffness, mass ratio, phase angle of active pitching, and Reynolds number are investigated. The results show that both the phase and the rate of passive pitching due to the wing flexibility can significantly modify the aerodynamics of the wing. The dynamic pitching depends not only on the specified kinematics at the wing root and the stiffness of the wing, but also greatly on the mass ratio, which represents the relative importance of the wing inertia and aerodynamic forces in the wing deformation. We use the ratio between the flapping frequency, $\omega $, and natural frequency of the wing, ${\omega }_{n} $, as the non-dimensional stiffness. In general, when $\omega / {\omega }_{n} \leq 0. 3$, the deformation significantly enhances the lift and also improves the lift efficiency despite a disadvantageous camber. In particular, when the inertial pitching torque is assisted by an aerodynamic torque of comparable magnitude, the lift efficiency can be markedly improved.

scholarly journals Hydrodynamics of diatom chains and semiflexible fibres

2014 ◽  
Vol 11 (96) ◽  
pp. 20140314 ◽  
Author(s):  
Hoa Nguyen ◽  
Lisa Fauci
Keyword(s):  
Three Dimensional ◽  
Beam Theory ◽  
Immersed Boundary ◽  
Aggregate Formation ◽  
Three Dimensional Model ◽  
Compressive Stresses ◽  
Parabolic Flow ◽  
Recovery Dynamics ◽  
Diatom Chain

Diatoms are non-motile, unicellular phytoplankton that have the ability to form colonies in the form of chains. Depending upon the species of diatoms and the linking structures that hold the cells together, these chains can be quite stiff or very flexible. Recently, the bending rigidities of some species of diatom chains have been quantified. In an effort to understand the role of flexibility in nutrient uptake and aggregate formation, we begin by developing a three-dimensional model of the coupled elastic–hydrodynamic system of a diatom chain moving in an incompressible fluid. We find that simple beam theory does a good job of describing diatom chain deformation in a parabolic flow when its ends are tethered, but does not tell the whole story of chain deformations when they are subjected to compressive stresses in shear. While motivated by the fluid dynamics of diatom chains, our computational model of semiflexible fibres illustrates features that apply widely to other systems. The use of an adaptive immersed boundary framework allows us to capture complicated buckling and recovery dynamics of long, semiflexible fibres in shear.

scholarly journals Numerical simulations of Hall MHD small-scale dynamos

2009 ◽  
Vol 5 (H15) ◽  
pp. 436-437
Author(s):  
Daniel O. Gómez ◽  
Pablo D. Mininni ◽  
Pablo Dmitruk
Keyword(s):  
Numerical Simulations ◽  
Hall Effect ◽  
Three Dimensional ◽  
Theoretical Framework ◽  
Mhd Equations ◽  
Small Scale ◽  
Hall Parameter ◽  
Hall Mhd ◽  
Three Dimensional Simulations

AbstractMuch of the progress in our understanding of dynamo mechanisms, has been made within the theoretical framework of magnetohydrodynamics (MHD). However, for sufficiently diffuse media, the Hall effect eventually becomes non-negligible. We present results from three dimensional simulations of the Hall-MHD equations subjected to random non-helical forcing. We study the role of the Hall effect in the dynamo efficiency for different values of the Hall parameter, using a pseudospectral code to achieve exponentially fast convergence.

Pump or coast: the role of resonance and passive energy recapture in medusan swimming performance

2019 ◽  
Vol 863 ◽  
pp. 1031-1061 ◽  
Author(s):  
Alexander P. Hoover ◽  
Antonio J. Porras ◽  
Laura A. Miller
Keyword(s):  
Immersed Boundary Method ◽  
Significant Contribution ◽  
Swimming Performance ◽  
Three Dimensional ◽  
Vortex Rings ◽  
Immersed Boundary ◽  
Cost Of Transport ◽  
The Cost ◽  
Inertial Regime

Diverse organisms that swim and fly in the inertial regime use the flapping or pumping of flexible appendages and cavities to propel themselves through a fluid. It has long been postulated that the speed and efficiency of locomotion are optimized by oscillating these appendages at their frequency of free vibration. In jellyfish swimming, a significant contribution to locomotory efficiency has been attributed to the effects passive energy recapture, whereby the bell is passively propelled through the fluid through its interaction with stopping vortex rings formed during each expansion of the bell. In this paper, we investigate the interplay between resonance and passive energy recapture using a three-dimensional implementation of the immersed boundary method to solve the fluid–structure interaction of an elastic oblate jellyfish bell propelling itself through a viscous fluid. The motion is generated through a fixed duration application of active tension to the bell margin, which mimics the action of the coronal swimming muscles. The pulsing frequency is then varied by altering the length of time between the application of applied tension. We find that the swimming speed is maximized when the bell is driven at its resonant frequency. However, the cost of transport is maximized by driving the bell at lower frequencies whereby the jellyfish passively coasts between active contractions through its interaction with the stopping vortex ring. Furthermore, the thrust generated by passive energy recapture was found to be dependent on the elastic properties of the jellyfish bell.

scholarly journals Simulating the chemistry and dynamics of molecular clouds

2009 ◽  
Vol 5 (H15) ◽  
pp. 405-405
Author(s):  
S. C. O. Glover ◽  
C. Federrath ◽  
M.-M. Mac Low ◽  
R. S. Klessen
Keyword(s):  
Molecular Clouds ◽  
Chemical Structure ◽  
Three Dimensional ◽  
Dynamical Evolution ◽  
Giant Molecular Clouds ◽  
Interstellar Gas ◽  
Density Inhomogeneities ◽  
First Time ◽  
Three Dimensional Simulations

AbstractWe have performed high-resolution three-dimensional simulations of turbulent interstellar gas that for the first time self-consistently follow its coupled thermal, chemical and dynamical evolution. Our simulations have allowed us to quantify the formation timescales for the most important molecules found in giant molecular clouds (H2, CO), as well as their spatial distribution within the clouds. Our results are consistent with models in which molecular clouds form quickly, within 1–2 turbulent crossing times, and emphasize the crucial role of density inhomogeneities in determining the chemical structure of the clouds.

scholarly journals Role of atomic diffusion in the opacity enhancement inside B-type stars

2018 ◽  
Vol 610 ◽  
pp. L15 ◽  
Author(s):  
A. Hui-Bon-Hoa ◽  
S. Vauclair
Keyword(s):  
Mass Loss ◽  
Main Sequence ◽  
Three Dimensional ◽  
Atomic Diffusion ◽  
Parameter Method ◽  
Stellar Models ◽  
Three Dimensional Simulations

Context. The pulsation frequencies of early B-type stars cannot be reproduced using stellar models with homogeneous abundances. A suitable match requires a dedicated enhancement of the opacity in the layers where its main contributors are the iron-peak elements (the so-called Z-bump), which trigger the oscillations in these stars. Aim. Our aim is to test whether the abundance stratification induced by atomic diffusion in these stellar layers is able to modify the local opacity as needed to account for the asteroseismic observations. Methods. Models representing a typical pulsating B-star were evolved during the main sequence using the Toulouse–Geneva evolution code in an improved version. The migration of the chemicals involves radiative accelerations, which were computed with the single-valued parameter method, and fingering mixing with parameters constrained by three-dimensional simulations. The possible effect of mass-loss was also considered. Results. We show that atomic diffusion modifies the abundance profiles inside the star, leading to an overabundance of the iron-peak elements in the upper part of the envelope. The opacities may become as high as required, provided that fingering mixing, which extends the size of the overabundance zone, is taken into account. A zero-flux of the elements at the surface leading to unphysical accumulations, mass-loss is also required to evolve the model until the end of the main sequence.

scholarly journals Three dimensional simulations of Hall magnetohydrodynamics

2010 ◽  
Vol 6 (S274) ◽  
pp. 433-436 ◽  
Author(s):  
Daniel O. Gómez
Keyword(s):  
Magnetic Fields ◽  
Hall Effect ◽  
Turbulent Flows ◽  
Three Dimensional ◽  
Mhd Equations ◽  
Hall Parameter ◽  
Astrophysical Objects ◽  
Hall Mhd ◽  
Three Dimensional Simulations

AbstractTurbulent flows take place in a large variety of astrophysical objects, and often times are the source of dynamo generated magnetic fields. Much of the progress in our understanding of dynamo mechanisms, has been made within the theoretical framework of magnetohydrodynamics (MHD). However, for sufficiently diffuse media, the Hall effect eventually becomes non-negligible.We present results from simulations of the Hall-MHD equations. The simulations are performed with a pseudospectral code to achieve exponentially fast convergence. We study the role of the Hall effect in the dynamo efficiency for different values of the Hall parameter.

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