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At least 703 records · Page 39Linked to original sources
arXiv2014

Fluid dynamics of aortic root dilation in Marfan syndrome

Aortic root dilation and propensity to dissection are typical manifestations of the Marfan Syndrome (MS), a genetic defect leading to the degeneration of the elastic fibres. Dilation affects the structure of the flow and, in turn, altered flow may play a role in vessel dilation, generation of aneurysms, and dissection. The aim of the present work is the investigation in-vitro of the fluid dynamic modifications occurring as a consequence of the morphological changes typically induced in the aortic root by MS. A mock-loop reproducing the left ventricle outflow tract and the aortic root was used to measure time resolved velocity maps on a longitudinal symmetry plane of the aortic root. Two dilated model aortas, designed to resemble morphological characteristics typically observed in MS patients, have been compared to a reference, healthy geometry. The aortic model was designed to quantitatively reproduce the change of aortic distensibility caused by MS. Results demonstrate that vorticity released from the valve leaflets, and possibly accumulating in the root, plays a fundamental role in redirecting the systolic jet issued from the aortic valve. The altered systolic flow also determines a different residual flow during the diastole.

physics.flu-dyn↗
arXiv2014

High order numerical simulations of the Richtmyer Meshkov instability in a relativistic fluid

We study the Richtmyer--Meshkov (RM) instability of a relativistic perfect fluid by means of high order numerical simulations with adaptive mesh refinement (AMR). The numerical scheme adopts a finite volume Weighted Essentially Non-Oscillatory (WENO) reconstruction to increase accuracy in space, a local space-time discontinuous Galerkin predictor method to obtain high order of accuracy in time and a high order one-step time update scheme together with a "cell-by-cell" space-time AMR strategy with time-accurate local time stepping. In this way, third order accurate (both in space and in time) numerical simulations of the RM instability are performed, spanning a wide parameter space. We present results both for the case in which a light fluid penetrates into a higher density one (Atwood number $A>0$), and for the case in which a heavy fluid penetrates into a lower density one (Atwood number $A<0$). We find that, for large Lorentz factors γ_s of the incident shock wave, the relativistic RM instability is substantially weakened and ultimately suppressed. More specifically, the growth rate of the RM instability in the linear phase has a local maximum which occurs at a critical value of γ_s ~ [1.2,2]. Moreover, we have also revealed a genuine relativistic effect, absent in Newtonian hydrodynamics, which arises in three dimensional configurations with a non-zero velocity component tangent to the incident shock front. In this case, the RM instability is strongly affected, typically resulting in less efficient mixing of the fluid.

physics.flu-dyn↗
arXiv2014

Evaluation of the rate constant and deposition velocity for the escape of Brownian particles over potential barriers

We analyze the escape of Brownian particles over potential barriers using the Fokker-Planck equation in a similar way to that of Chandrasekhar (Rev. Modern Phys., 1943), deriving a formula for the particle deposition velocity to a surface as a function of the particle response time. For very small particle response times, the particle deposition velocity reduces to that obtained using a quasi-steady solution of Smoluchowski's equation and for very large particle response times, the deposition velocity corresponds to that based on the transition state method (E. Wigner, Trans. Faraday Soc., 1938).

cond-mat.stat-mech↗
arXiv2014

Stretching vortices as a basis for the theory of turbulence

Turbulent flows play an important role in many aspects of nature and technics from sea storms to transport of particles or chemicals. Transport of energy from large scales to small fluctuations is the essential feature of three-dimensional turbulence. What mechanism is responsible for this transport and how do the small fluctuations appear? The conventional conception implies a cascade of breaking vortices. But it faces crucial problems in explaining the mechanism of the breaking, and fails to explain the observed long-living structures in turbulent flows. We suggest a new concept based on recent analysis of stochastic Navier-Stokes equation: stretching of vortices instead of their breaking may be the main mechanism of turbulence. This conception is free of the disadvantages of the cascade paradigm; it also does not need finite-time singularities to explain the observed statistical properties of turbulent flows. Moreover, the introduction of the new conception allows immediately to get velocity scaling parameters well consistent with experimental data.

physics.flu-dyn↗
arXiv2014

How the velvet worm squirts slime

The rapid squirt of a proteinaceous slime jet endows the ancient velvet worms (Onychophora) with a unique mechanism for defense from predators and for capturing prey by entangling them in a disordered web that immobilizes their target. However, to date neither qualitative nor quantitative descriptions have been provided for this unique adaptation. Here we investigate the fast oscillatory motion of the oral papillae and the exiting liquid jet that oscillates with frequencies $f\sim 30-60$ Hz. Using anatomical images, high speed videography, theoretical analysis and a physical simulacrum we show that this fast oscillatory motion is the result of an elastohydrodynamic instability driven by the interplay between the elasticity of oral papillae and the fast unsteady flow during squirting. Our results demonstrate how passive strategies can be cleverly harnessed by organisms, while suggesting future oscillating micro-fluidic devices as well as novel ways for micro and nano fiber production using bioinspired strategies.

physics.flu-dyn↗
arXiv2014

Vortex annihilation and inverse cascades in two dimensional superfluid turbulence

We study two dimensional superfluid turbulence by employing an effective description valid in the limit where the density of superfluid vortices is parametrically small. At sufficiently low temperatures the effective description yields an inverse cascade with Kolmogorov energy spectrum $E(k) \sim k^{-5/3}$. Denoting the number of vortices as a function of time by $N(t)$, we find that the vortex annihilation rate scales like $\dot N \sim N^{5/3}$ in states with an inverse cascade and $\dot N \sim N^2$ for laminar flow.

cond-mat.quant-gas↗
arXiv2014

Circularly-confined microswimmers exhibit multiple global patterns

Geometric confinement plays an important role in the dynamics of natural and synthetic microswimmers from bacterial cells to self-propelled particles in high-throughput microfluidic devices. However, little is known about the effects of geometric confinement on the emergent global patterns in such self-propelled systems. Recent experiments on bacterial cells give conflicting reports of cells spontaneously organizing into a spiral vortex in a thin cylindrical droplet and cells aggregating at the inner boundary in a spherical droplet. We investigate here, in an idealized physical model, the interplay between geometric confinement and level of flagellar activity on the emergent collective patterns. We show that decreasing flagellar activity induces a hydrodynamically-triggered transition in confined microswimmers from swirling to global circulation (vortex) to boundary aggregation and clustering. These results highlight that the complex interplay between confinement, flagellar activity and hydrodynamic flows in concentrated suspensions of microswimmers could lead to a plethora of global patterns that are difficult to predict from geometric consideration alone.

physics.flu-dyn↗
arXiv2014

Odor Landscapes in Turbulent Environments

The olfactory system of male moths is exquisitely sensitive to pheromones emitted by females and transported in the environment by atmospheric turbulence. Moths respond to minute amounts of pheromones and their behavior is sensitive to the fine-scale structure of turbulent plumes where pheromone concentration is detectible. The signal of pheromone whiffs is qualitatively known to be intermittent, yet quantitative characterization of its statistical properties is lacking. This challenging fluid dynamics problem is also relevant for entomology, neurobiology and the technological design of olfactory stimulators aimed at reproducing physiological odor signals in well-controlled laboratory conditions. Here, we develop a Lagrangian approach to the transport of pheromones by turbulent flows and exploit it to predict the statistics of odor detection during olfactory searches. The theory yields explicit probability distributions for the intensity and the duration of pheromone detections, as well as their spacing in time. Predictions are favorably tested by using numerical simulations, laboratory experiments and field data for the atmospheric surface layer. The resulting signal of odor detections lends to implementation with state-of-the-art technologies and quantifies the amount and the type of information that male moths can exploit during olfactory searches.

q-bio.QM↗
arXiv2017

Investigation of the formation process of soap bubbles from soap film

In this paper, we study the formation process of a soap bubble by blowing soap film. Both bubble diameter and formation position were investigated in experiments. We found that the ratio between bubble size and soap film column is constant, and that the formation position increases linearly within a critical length-range. We used the theory of Rayleigh-Plateau instability to explain these findings. The theoretical explanations are consistent with the experimental data.

physics.flu-dyn↗
arXiv2017

A Calculator for Sediment Transport in Microchannels Based on the Rouse Number

The Rouse number is commonly used to estimate the mode of the sediment transports in turbulent flows with large Reynolds number. However, in microchannels such as in modern inkjet systems, the liquid flows are usually laminar. In this paper, I modify the Rouse number by expanding it to the case of weakly turbulent and laminar flows and construct a calculator to estimate the modes of sediment transport in microchannels. To illustrate the applicability of the modified Rouse number, I apply it to the transport of sediments in an inkjet system and compare theoretical results with experimental observations. The modified Rouse number constructed in this paper can be used in other application as well.

physics.flu-dyn↗
arXiv2018

Microfluidics Development at Berkeley

Richard A. Mathies is professor emeritus of Chemistry at the University of California, Berkeley. In this contribution he summarizes his journey through microfluidics over the past 30 years from the invention of Capillary Array Electrophoresis in the 1990s to recent attempts to launch a microfluidic system into space.

physics.flu-dyn↗
arXiv2018

Microfluidics for Biofabrication

Shoji Takeushi is professor in and the Director of the Center for International Research on Integrative Biomedical Systems (CIBiS), Institute of Industrial Science (IIS) at the University of Tokyo. Here he describes his attempts to use inherent function of living materials in engineered systems by reconstructing the biological structure using microfluidic technology.

physics.flu-dyn↗
arXiv2018

Microfluidic tools for assaying immune cell function

Joel Voldman is a professor in the Electrical Engineering and Computer Science Department at MIT. Here he describes his labs efforts to develop microfluidic devices for cell manipulation and analysis.

physics.flu-dyn↗
arXiv2018

Centrifugal Microfluidics for Biomedical Applications

Yoon-Kyoung Cho is a full professor in Biomedical Engineering at UNIST. In this contribution she describes a fully integrated and automated lab-on-a-disc using centrifugal microfluidics to provide a "sample-in and answer-out" type of biochemical analysis solution with simple, size-reduced, and cost-efficient instrumentation. She also gives various examples of the fully integrated "lab-on-a-disc" developed for broad applications ranging from medical diagnostics to food, environment, and energy applications.

physics.flu-dyn↗
arXiv2018

Microfluidics for Chemical Synthesis: Flow Chemistry

Klavs F. Jensen is Warren K. Lewis Professor in Chemical Engineering and Materials Science and Engineering at the Massachusetts Institute of Technology. Here he describes the use of microfluidics for chemical synthesis, from the early demonstration examples to the current efforts with automated droplet microfluidic screening and optimization techniques.

physics.flu-dyn↗
arXiv2018

When Streams of Optofluidics Meet the Sea of Life

Luke P. Lee is a Tan Chin Tuan Centennial Professor at the National University of Singapore. In this contribution he describes the power of optofluidics as a research tool and reviews new insights within the areas of single cell analysis, microphysiological analysis, and integrated systems.

physics.flu-dyn↗
arXiv2018

A Viscoelastic Catastrophe

We use a differential constitutive equation to model the flow of a viscoelastic flow in a cross-slot geometry, which is known to exhibit bistability above a critical flow rate. The novelty lies in two asymmetric modifications to the geometry, which causes a change in the bifurcation diagram such that one of the stable solutions becomes disconnected from the solution at low flow speeds. First we show that it is possible to mirror one of the modifications such that the system can be forced to the disconnected solution. Then we show that a slow decrease of the flow rate, can cause the system to go through a drastic change on a short time scale, also known as a catastrophe. The short time scale could lead to a precise and simple experimental measurement of the flow conditions at which the viscoelastic catastrophe occurs. Since the phenomena is intrinsically related to the extensional rheology of the fluid, we propose to exploit the phenomena for in-line extensional rheometry.

physics.flu-dyn↗