scholarly journals Experimental Resonances in Viscoelastic Microfluidics

2021 ◽  
Vol 9 ◽  
Author(s):  
Pamela Vazquez-Vergara ◽  
Ulises Torres-Herrera ◽  
Gabriel A. Caballero-Robledo ◽  
Luis F. Olguin ◽  
Eugenia Corvera Poiré
Keyword(s):  
Relaxation Times ◽  
Viscoelastic Fluids ◽  
Single Mode ◽  
System Size ◽  
Fine Tuning ◽  
Fluid Interfaces ◽  
Mean Flow ◽  
Periodic Pressure ◽  
Pulsatile Flows ◽  
Experimental Findings

Pulsatile flows of viscoelastic fluids are very important for lab-on-a-chip devices, because most biofluids have viscoelastic character and respond distinctively to different periodic forcing. They are also very important for organ-on-a-chip devices, where the natural mechanical conditions of cells are emulated. The resonance frequency of a fluid refers to a particular pulsatile periodicity of the pressure gradient that maximizes the amplitude of flow velocity. For viscoelastic fluids, this one has been measured experimentally only at macroscales, since fine tuning of rheological properties and system size is needed to observe it at microscales. We study the dynamics of a pulsatile (zero-mean flow) fluid slug formed by a viscoelastic fluid bounded by two air-fluid interfaces, in a microchannel of polymethyl methacrylate. We drive the fluid slug by a single-mode periodic pressure drop, imposed by a piezoactuator. We use three biocompatible polymer solutions of polyethylene oxide as model viscoelastic fluids, and find resonances. We propose a model accounting for surface tension and fluid viscoelasticity that has an excellent agreement with our experimental findings. It also provides an alternative way of measuring relaxation times. We validate the method with parameters reported in the literature for two of the solutions, and estimate the relaxation time for the third one.

scholarly journals Neural Network DPD for Aggrandizing SM-VCSEL-SSMF-Based Radio over Fiber Link Performance

Photonics ◽  
2021 ◽  
Vol 8 (1) ◽  
pp. 19
Author(s):  
Muhammad Hadi ◽  
Muhammad Awais ◽  
Mohsin Raza ◽  
Kiran Khurshid ◽  
Hyun Jung
Keyword(s):  
Neural Network ◽  
Single Mode ◽  
Single Mode Fiber ◽  
Radio Over Fiber ◽  
Error Vector Magnitude ◽  
Computational Overhead ◽  
Link Performance ◽  
Modulation Signal ◽  
Experimental Findings ◽  
Memory Polynomial

This paper demonstrates an unprecedented novel neural network (NN)-based digital predistortion (DPD) solution to overcome the signal impairments and nonlinearities in Analog Optical fronthauls using radio over fiber (RoF) systems. DPD is realized with Volterra-based procedures that utilize indirect learning architecture (ILA) and direct learning architecture (DLA) that becomes quite complex. The proposed method using NNs evades issues associated with ILA and utilizes an NN to first model the RoF link and then trains an NN-based predistorter by backpropagating through the RoF NN model. Furthermore, the experimental evaluation is carried out for Long Term Evolution 20 MHz 256 quadraturre amplitude modulation (QAM) modulation signal using an 850 nm Single Mode VCSEL and Standard Single Mode Fiber to establish a comparison between the NN-based RoF link and Volterra-based Memory Polynomial and Generalized Memory Polynomial using ILA. The efficacy of the DPD is examined by reporting the Adjacent Channel Power Ratio and Error Vector Magnitude. The experimental findings imply that NN-DPD convincingly learns the RoF nonlinearities which may not suit a Volterra-based model, and hence may offer a favorable trade-off in terms of computational overhead and DPD performance.

scholarly journals Characteristics, Energetics, and Origins of Agulhas Current Meanders and Their Limited Influence on Ring Shedding

2015 ◽  
Vol 45 (9) ◽  
pp. 2294-2314 ◽  
Author(s):  
Shane Elipot ◽  
Lisa M. Beal
Keyword(s):  
Kinetic Energy ◽  
Single Mode ◽  
Cyclonic Circulation ◽  
Recent Analysis ◽  
Altimeter Data ◽  
Western Boundary ◽  
Mean Flow ◽  
Current Time ◽  
Agulhas Current ◽  
The Mean

AbstractThe Agulhas Current intermittently undergoes dramatic offshore excursions from its mean path because of the downstream passage of mesoscale solitary meanders or Natal pulses. New observations and analyses are presented of the variability of the current and its meanders using mooring observations from the Agulhas Current Time-Series Experiment (ACT) near 34°S. Using a new rotary EOF method, mesoscale meanders and smaller-scale meanders are differentiated and each captured in a single mode of variance. During mesoscale meanders, an onshore cyclonic circulation and an offshore anticyclonic circulation act together to displace the jet offshore, leading to sudden and strong positive conversion of kinetic energy from the mean flow to the meander via nonlinear interactions. Smaller meanders are principally represented by a single cyclonic circulation spanning the entire jet that acts to displace the jet without extracting kinetic energy from the mean flow. Synthesizing in situ observations with altimeter data leads to an account of the number of mesoscale meanders at 34°S: 1.6 yr−1 on average, in agreement with a recent analysis by Rouault and Penven (2011) and significantly less than previously understood. The links between meanders and the arrival of Mozambique Channel eddies or Madagascar dipoles at the western boundary upstream are found to be robust in the 20-yr altimeter record. Yet, only a small fraction of anomalies arriving at the western boundary result in meanders, and of those, two-thirds can be related to ring shedding. Most Agulhas rings are shed independently of meanders.

scholarly journals Sequential Activity in Asymmetrically Coupled Winner-Take-All Circuits

2014 ◽  
Vol 26 (9) ◽  
pp. 1973-2004 ◽  
Author(s):  
Hesham Mostafa ◽  
Giacomo Indiveri
Keyword(s):  
Network Architecture ◽  
External Input ◽  
Spike Timing ◽  
Fine Tuning ◽  
Learning Mechanisms ◽  
State Dependent ◽  
Synaptic Dynamics ◽  
Experimental Findings ◽  
Winner Take All ◽  
Take All

Understanding the sequence generation and learning mechanisms used by recurrent neural networks in the nervous system is an important problem that has been studied extensively. However, most of the models proposed in the literature are either not compatible with neuroanatomy and neurophysiology experimental findings, or are not robust to noise and rely on fine tuning of the parameters. In this work, we propose a novel model of sequence learning and generation that is based on the interactions among multiple asymmetrically coupled winner-take-all (WTA) circuits. The network architecture is consistent with mammalian cortical connectivity data and uses realistic neuronal and synaptic dynamics that give rise to noise-robust patterns of sequential activity. The novel aspect of the network we propose lies in its ability to produce robust patterns of sequential activity that can be halted, resumed, and readily modulated by external input, and in its ability to make use of realistic plastic synapses to learn and reproduce the arbitrary input-imposed sequential patterns. Sequential activity takes the form of a single activity bump that stably propagates through multiple WTA circuits along one of a number of possible paths. Because the network can be configured to either generate spontaneous sequences or wait for external inputs to trigger a transition in the sequence, it provides the basis for creating state-dependent perception-action loops. We first analyze a rate-based approximation of the proposed spiking network to highlight the relevant features of the network dynamics and then show numerical simulation results with spiking neurons, realistic conductance-based synapses, and spike-timing dependent plasticity (STDP) rules to validate the rate-based model.

Critical layers in accelerating two-layer flows

1988 ◽  
Vol 197 ◽  
pp. 429-451 ◽  
Author(s):  
Donald B. Altman
Keyword(s):  
Laboratory Experiments ◽  
Single Mode ◽  
Shear Instability ◽  
Mean Flow ◽  
Velocity Measurements ◽  
Interfacial Wave ◽  
Flow Acceleration ◽  
Critical Layers ◽  
The Mean ◽  
Processing Software

A series of laboratory experiments on accelerating two-layer shear flows over topography is described. The mean flow reverses at the interface of the layers, forcing a critical layer to occur there. It is found that for a sufficiently thin interface, a slowly growing recirculating region, the ‘acceleration rotor’, develops on the interfacial wave at mean-flow Richardson numbers of O(0.5). This, in turn, can induce a secondary dynamical shear instability on the trailing edge of the wave. A single-mode, linear, two-layer numerical model reproduces many features of the acceleration rotor if mean-flow acceleration and bottom forcing are included. Velocity measurements are obtained from photographs using image processing software developed for the automated reading of particle-streak photographs. Typical results are shown.

Incipient breaking of steady waves in the presence of surface wakes

1999 ◽  
Vol 383 ◽  
pp. 285-305 ◽  
Author(s):  
MATTHEW MILLER ◽  
TOBIAS NENNSTIEL ◽  
JAMES H. DUNCAN ◽  
ATHANASSIOS A. DIMAS ◽  
STEPHAN PRÖSTLER
Keyword(s):  
Free Surface ◽  
Wave Height ◽  
Flow Characteristics ◽  
Breaking Wave ◽  
Maximum Height ◽  
Two Dimensional ◽  
Mean Flow ◽  
Experimental Findings ◽  
Drift Layer ◽  
Surface Drift

The effect of free-surface drift layers on the maximum height that a steady wave can attain without breaking is explored through experiments and numerical simulations. In the experiments, the waves are generated by towing a two-dimensional fully submerged hydrofoil at constant depth, speed and angle of attack. The drift layer is generated by towing a plastic sheet on the water surface ahead of the hydrofoil. It is found that the presence of this drift layer (free-surface wake) dramatically reduces the maximum non-breaking wave height and that this wave height correlates well with the surface drift velocity. In the simulations, the inviscid two-dimensional fully nonlinear Euler equations are solved numerically. Initially symmetric wave profiles are superimposed on a parallel drift layer whose mean flow characteristics match those in the experiments. It is found that for large enough initial wave amplitudes a bulge forms at the crest on the forward face of the wave and the vorticity fluctuations just under the surface in this region grow dramatically in time. This behaviour is taken as a criterion to indicate impending wave breaking. The maximum non-breaking wave elevations obtained in this way are in good agreement with the experimental findings.

Laminar mixing of non-Newtonian fluids in static mixers: process intensification perspective

2020 ◽  
Vol 36 (3) ◽  
pp. 423-436 ◽  
Author(s):  
Sinthuran Jegatheeswaran ◽  
Farhad Ein-Mozaffari ◽  
Jiangning Wu
Keyword(s):  
Review Paper ◽  
Relaxation Times ◽  
Process Intensification ◽  
Viscoelastic Fluids ◽  
Industrial Applications ◽  
Newtonian Fluids ◽  
Future Research ◽  
Comprehensive Overview ◽  
Static Mixers ◽  
Laminar Mixing

AbstractStatic mixers are widely used in various industrial applications to intensify the laminar mixing of non-Newtonian fluids. Non-Newtonian fluids can be categorized into (1) time-independent, (2) time-dependent, and (3) viscoelastic fluids. Computational fluid dynamics studies on the laminar mixing of viscoelastic fluids are very limited due to the complexity in incorporating the multiple relaxation times and the associated stress tensor into the constitutive equations. This review paper provides recommendations for future research studies while summarizing the key research contributions in the field of non-Newtonian fluid mixing using static mixers. This review discusses the different experimental techniques employed such as electrical resistance tomography, magnetic resonance imaging, planar laser-induced fluorescence, and positron emission particle tracking. A comprehensive overview of the mixing fundamentals, fluid chaos, numerical characterization of fluid stretching, development of pressure drop correlations, and derivations of generalized Reynolds number is also provided in this review paper.

scholarly journals Persistent dark states in anisotropic central spin models

2020 ◽  
Vol 10 (1) ◽  
Author(s):  
Tamiro Villazon ◽  
Pieter W. Claeys ◽  
Mohit Pandey ◽  
Anatoli Polkovnikov ◽  
Anushya Chandran
Keyword(s):  
Relaxation Time ◽  
Solid State ◽  
Quantum Dot ◽  
Large Deviations ◽  
Thermal Equilibrium ◽  
Relaxation Times ◽  
System Size ◽  
Small Perturbations ◽  
Spin Models ◽  
Spin Bath

Abstract Long-lived dark states, in which an experimentally accessible qubit is not in thermal equilibrium with a surrounding spin bath, are pervasive in solid-state systems. We explain the ubiquity of dark states in a large class of inhomogeneous central spin models using the proximity to integrable lines with exact dark eigenstates. At numerically accessible sizes, dark states persist as eigenstates at large deviations from integrability, and the qubit retains memory of its initial polarization at long times. Although the eigenstates of the system are chaotic, exhibiting exponential sensitivity to small perturbations, they do not satisfy the eigenstate thermalization hypothesis. Rather, we predict long relaxation times that increase exponentially with system size. We propose that this intermediate chaotic but non-ergodic regime characterizes mesoscopic quantum dot and diamond defect systems, as we see no numerical tendency towards conventional thermalization with a finite relaxation time.

scholarly journals Essential Role of the cVRG in the Generation of Both the Expiratory and Inspiratory Components of the Cough Reflex

2020 ◽  
pp. S19-S27
Author(s):  
E. Cinelli ◽  
L. Iovino ◽  
F. Bongianni ◽  
T. Pantaleo ◽  
D. Mutolo
Keyword(s):  
Essential Role ◽  
Motor Pattern ◽  
Fine Tuning ◽  
Cough Reflex ◽  
Defensive Responses ◽  
Inspiratory Muscles ◽  
Prominent Component ◽  
Expiratory Neurons ◽  
Inspiratory Activity ◽  
Experimental Findings

As stated by Korpáš and Tomori (1979), cough is the most important airway protective reflex which provides airway defensive responses to nociceptive stimuli. They recognized that active expiratory efforts, due to the activation of caudal ventral respiratory group (cVRG) expiratory premotoneurons, are the prominent component of coughs. Here, we discuss data suggesting that neurons located in the cVRG have an essential role in the generation of both the inspiratory and expiratory components of the cough reflex. Some lines of evidence indicate that cVRG expiratory neurons, when strongly activated, may subserve the alternation of inspiratory and expiratory cough bursts, possibly owing to the presence of axon collaterals. Of note, experimental findings such as blockade or impairment of glutamatergic transmission to the cVRG neurons lead to the view that neurons located in the cVRG are crucial for the production of the complete cough motor pattern. The involvement of bulbospinal expiratory neurons seems unlikely since their activation affects differentially expiratory and inspiratory muscles, while their blockade does not affect baseline inspiratory activity. Thus, other types of cVRG neurons with their medullary projections should have a role and possibly contribute to the fine tuning of the intensity of inspiratory and expiratory efforts.

Mean Flow and Turbulent Stress Estimation Using DPIV in Pulsatile Flow Through Mechanical Heart Valve

2000 ◽  
Author(s):  
Amirthaganesh Subramanian ◽  
Haining Mu ◽  
Jaikrishnan R. Kadambi ◽  
Hiroaki Harasaki ◽  
Mark P. Wernet
Keyword(s):  
Heart Valve ◽  
Pulsatile Flow ◽  
Laser Doppler Anemometry ◽  
Measurement Techniques ◽  
Mechanical Heart Valve ◽  
Accurate Estimation ◽  
Turbulent Stress ◽  
Mean Flow ◽  
Flow Loop ◽  
Pulsatile Flows

Abstract Accurate estimation of turbulent stresses in pulsatile flows is very important in the design of mechanical heart valves to reduce thromboembolism and hemolysis. The flow within the lumen of a fully transparent bileaflet mechanical heart valve model, in the “mitral” position of an in-vitro pulsatile flow loop, is characterized using digital particle image velocimetry (DPIV). A phase-averaging technique is used to obtain the mean velocities and turbulent stresses. The results are compared with those obtained using laser Doppler anemometry which shows good agreement between the two measurement techniques, thereby validating the integrity of the DPIV measurements in the conditionally sampled time dependent flow. Convergence and accuracy issues involving turbulent stress measurements with DPIV in pulsatile flows are addressed in this paper.

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