Acoustic invisibility in turbulent fluids by optimised cloaking

2014 ◽  
Vol 749 ◽  
pp. 460-477 ◽  
Author(s):  
Xun Huang ◽  
Siyang Zhong ◽  
Xin Liu
Keyword(s):  
Theoretical Model ◽  
Computational Methods ◽  
Physical Mechanism ◽  
Far Field ◽  
Turbulent Wakes ◽  
Scattered Fields ◽  
Turbulent Fluids

AbstractAcoustic invisibility of a cloaking system in turbulent fluids is poorly understood. Here we show that evident scattering would appear in turbulent wakes due to the submergence of a classical cloaking device. The inherent physical mechanism is explained using our theoretical model, which eventually inspires us to develop an optimised cloaking approach. Both the near- and far-field scattered fields are examined using computational methods. The remarkably low scattering demonstrates the effectiveness of the proposed approach, in particular for acoustic cloaking in turbulent fluids.

scholarly journals Violent expiratory events: on coughing and sneezing

2014 ◽  
Vol 745 ◽  
pp. 537-563 ◽  
Author(s):  
Lydia Bourouiba ◽  
Eline Dehandschoewercker ◽  
John W. M. Bush
Keyword(s):  
Fluid Dynamics ◽  
Theoretical Model ◽  
Respiratory Diseases ◽  
Theoretical Models ◽  
Respiratory Pathogens ◽  
Respiratory Events ◽  
Continuous Models ◽  
Analogue Experiments ◽  
Settling Speed ◽  
Turbulent Fluids

AbstractViolent respiratory events such as coughs and sneezes play a key role in transferring respiratory diseases between infectious and susceptible individuals. We present the results of a combined experimental and theoretical investigation of the fluid dynamics of such violent expiratory events. Direct observation of sneezing and coughing events reveals that such flows are multiphase turbulent buoyant clouds with suspended droplets of various sizes. Our observations guide the development of an accompanying theoretical model of pathogen-bearing droplets interacting with a turbulent buoyant momentum puff. We develop in turn discrete and continuous models of droplet fallout from the cloud in order to predict the range of pathogens. According to the discrete fallout model droplets remain suspended in the cloud until their settling speed matches that of the decelerating cloud. A continuous fallout model is developed by adapting models of sedimentation from turbulent fluids. The predictions of our theoretical models are tested against data gathered from a series of analogue experiments in which a particle-laden cloud is ejected into a relatively dense ambient. Our study highlights the importance of the multiphase nature of respiratory clouds, specifically the suspension of the smallest drops by circulation within the cloud, in extending the range of respiratory pathogens.

Physical mechanism of the Schwarzschild effect in film dosimetry—theoretical model and comparison with experiments

2006 ◽  
Vol 51 (17) ◽  
pp. 4345-4356 ◽  
Author(s):  
A Djouguela ◽  
R Kollhoff ◽  
A Rühmann ◽  
K C Willborn ◽  
D Harder ◽  
...  
Keyword(s):  
Theoretical Model ◽  
Physical Mechanism ◽  
Film Dosimetry ◽  
Comparison With Experiments

Accurate decomposition method and identification technology of drag

2020 ◽  
Vol 34 (14n16) ◽  
pp. 2040114
Author(s):  
Jia-Lei Yu ◽  
Jing Jin ◽  
Chun Shao ◽  
Miao Zhang ◽  
Tie-Jun Liu
Keyword(s):  
Shock Wave ◽  
Integral Method ◽  
Physical Mechanism ◽  
Wave Drag ◽  
Irreversible Processes ◽  
Viscous Drag ◽  
Numerical Dissipation ◽  
Far Field ◽  
Induced Drag ◽  
Field Integral

Aiming at accurate decomposition and identification of drag, the drag prediction technology based on the mid/far-field integral method is developed. The method decomposes the far-field drag into entropy drag and induced drag according to its physical mechanism, and introduces an appropriate entropy correction to eliminate the numerical dissipation by analyzing the influence of the trailing integral section position on the entropy drag calculation. Based on the analysis of thermodynamic reversible processes and irreversible processes, the drag is refined into viscous drag, shock wave drag, induced drag and pseudo-drag. The mid-field integral method is used to calculate the separate contribution of viscous drag, shock wave drag and induced drag by calculating the limited integral domain. Numerical results show that the developed method is feasible in accurately reflecting the physical mechanism and predicting the drag ratio. Thus, it provides a reliable tool for drag reduction of large passenger aircraft.

scholarly journals The disappearance of laminar and turbulent wakes in complex flows

2002 ◽  
Vol 457 ◽  
pp. 111-132 ◽  
Author(s):  
J. C. R. HUNT ◽  
I. EAMES
Keyword(s):  
Flow Field ◽  
Rigid Body ◽  
The Body ◽  
Viscous Drag ◽  
Total Force ◽  
Far Field ◽  
Volume Flux ◽  
Wake Region ◽  
Complex Flows ◽  
Turbulent Wakes

The singular effects of steady large-scale external strain on the viscous wake generated by a rigid body and the overall flow field are analysed. In an accelerating flow strained at a positive rate, the vorticity field is annihilated owing to positive and negative vorticity either side of the wake centreline diffusing into one another and the volume flux in the wake decreases with downwind distance. Since the wake disappears, the far-field flow changes from monopolar to dipolar. In this case, the force on the body is no longer proportional to the strength of the monopole, but is proportional to the strength of the far field dipole. These results are extended to the case of strained turbulent wakes and this is verified against experimental wind tunnel measurements of Keffer (1965) and Elliott & Townsend (1981) for positive and negative strains. The analysis demonstrates why the total force acting on a body may be estimated by adding the viscous drag and inviscid force due to the irrotational straining field.Applying the analysis to the wake region of a rigid body or a bubble shows that the wake volume flux decreases even in uniform flows owing to the local straining flow in the near-wake region. While the wake volume flux decreases by a small amount for the flow over streamline and bluff bodies, for the case of a clean bubble the decrease is so large as to render Betz's (1925) drag formula invalid.To show how these results may be applied to complex flows, the effects of a sequence of positive and negative strains on the wake are considered. The average wake width is much larger than in the absence of a strain field and this leads to diffusion of vorticity between wakes and the cancellation of vorticity. The latter mechanism leads to a net reduction in the volume flux deficit downstream which explains why in calculations of the flow through groups of moving or stationary bodies the wakes of upstream bodies may be ignored even though their drag and lift forces have a significant effect on the overall flow field.

scholarly journals Theoretical substantiation of optimum energy efficiency of ultrasonic wet dispersing

2021 ◽  
Vol 2094 (2) ◽  
pp. 022068
Author(s):  
Roman N Golykh
Keyword(s):  
Energy Efficiency ◽  
Theoretical Model ◽  
Physical Mechanism ◽  
Maximum Energy ◽  
Solid Particles ◽  
Experimental Results ◽  
Fraction Composition ◽  
Theoretical Substantiation ◽  
Dispersed Particles ◽  
Optimum Energy

Abstract The known preliminary experimental results of ultrasonic dispersing of suspension were presented and analyzed. The need for determination optimum modes providing maximum energy efficiency of ultrasonic wet dispersing is justified. The physical mechanism and the theoretical model of ultrasonic dispersing were proposed. The model allows to calculate fraction composition of dispersed particles and to evaluate, that the maximum of energy efficiency exists at fixed intensity for different materials of solid particles.

scholarly journals The bubble-induced population dynamics of fermenting yeasts

2020 ◽  
Vol 17 (172) ◽  
pp. 20200735
Author(s):  
Atul Srivastava ◽  
Kenji Kikuchi ◽  
Takuji Ishikawa
Keyword(s):  
Cell Wall ◽  
Population Dynamics ◽  
Theoretical Model ◽  
Transport Mechanism ◽  
Physical Mechanism ◽  
Anaerobic Fermentation ◽  
Surface Hydrophobicity ◽  
Leading Role ◽  
General Conservation

Bubble-induced transport is a ubiquitous natural and industrial phenomenon. In brewery, such transport occurs due to gas bubbles generated through anaerobic fermentation by yeasts. Two major kinds of fermentation viz. top (ale) and bottom (lager) fermentation, display a difference in their yeast distributions inside a sugar broth. The reason for this difference is believed to be yeast–bubble adhesion arising due to surface hydrophobicity of the yeast cell wall; however, the physical mechanism is still largely a mystery. In this report, through in vivo experiments, we develop a novel theoretical model for yeast distribution based on the general conservation law. This work clarifies that bubble-induced diffusion is the dominant transport mechanism in bottom-fermentation by lagers whereas, yeast–bubble adhesion plays a leading role in transporting ales in top-fermentation, thereby corroborating the centuries-old belief regarding distribution difference in yeast population in two kinds of fermentation.

scholarly journals The determination of the surface impedance of an obstacle

1993 ◽  
Vol 36 (1) ◽  
pp. 1-15 ◽  
Author(s):  
Andrzej W. Kȩdzierawski
Keyword(s):  
Acoustic Waves ◽  
Surface Impedance ◽  
Scattering Problem ◽  
Three Dimensional ◽  
Inverse Scattering Problem ◽  
Far Field ◽  
Field Patterns ◽  
Time Harmonic ◽  
Scattered Fields

The inverse scattering problem we consider is to determine the surface impedance of a three-dimensional obstacle of known shape from a knowledge of the far-field patterns of the scattered fields corresponding to many incident time-harmonic plane acoustic waves. We solve this problem by using both the methods of Kirsch-Kress and Colton-Monk.

Computational study of optical distortions by separated shear layers and turbulent wakes

2009 ◽  
Vol 625 ◽  
pp. 273-298 ◽  
Author(s):  
ALI MANI ◽  
PARVIZ MOIN ◽  
MENG WANG
Keyword(s):  
Computational Study ◽  
Reynolds Numbers ◽  
Shear Layers ◽  
Scale Model ◽  
Far Field ◽  
Beam Position ◽  
Optical Wavelength ◽  
Eddy Simulation ◽  
Turbulent Wakes ◽  
Field Peak

The flow over a circular cylinder at ReD = 3900 and 10000 and M = 0.4 is considered a platform to study the aero-optical distortions by separated shear layers and turbulent wakes. The flow solution is obtained by large eddy simulation (LES) and validated against previous experimental and numerical results. The fluctuating refractive index obtained from LES is used in a ray-tracing calculation to determine wavefront distortions after the beam passes through the turbulent region. Free-space propagation to the far field is computed using Fourier optics. The optical statistics are analysed for different conditions in terms of optical wavelength, aperture size and the beam position. It is found that there exists an optimal wavelength which maximizes the far-field peak intensity. Optical results at both Reynolds numbers are compared. The optical distortion by the downstream turbulent wake is found to be Reynolds number insensitive. However, due to their different transition mechanisms, distortions by the near wake regions are different in the two flows. The aero-optical effects of different flow scales are examined using filtering and grid refinement. Through a grid convergence study it is confirmed that an adequately resolved LES can capture the aero-optics of highly aberrating flows without requiring additional subgrid scale model for the optics.

scholarly journals Metaphase kinetochore movements are regulated by kinesin-8 motors and microtubule dynamic instability

2018 ◽  
Vol 29 (11) ◽  
pp. 1332-1345 ◽  
Author(s):  
Anna H. Klemm ◽  
Agneza Bosilj ◽  
Matko Gluncˇic´ ◽  
Nenad Pavin ◽  
Iva M. Tolic´
Keyword(s):  
Theoretical Model ◽  
Physical Mechanism ◽  
Dynamic Instability ◽  
Protein Complexes ◽  
Spindle Pole ◽  
Wild Type ◽  
Microtubule Dynamic ◽  
Spindle Poles ◽  
Sister Chromatids ◽  
Pulling Forces

During metaphase, sister chromatids are connected to microtubules extending from the opposite spindle poles via kinetochores to protein complexes on the chromosome. Kinetochores congress to the equatorial plane of the spindle and oscillate around it, with kinesin-8 motors restricting these movements. Yet, the physical mechanism underlying kinetochore movements is unclear. We show that kinetochore movements in the fission yeast Schizosaccharomyces pombe are regulated by kinesin-8-promoted microtubule catastrophe, force-induced rescue, and microtubule dynamic instability. A candidate screen showed that among the selected motors only kinesin-8 motors Klp5/Klp6 are required for kinetochore centering. Kinesin-8 accumulates at the end of microtubules, where it promotes catastrophe. Laser ablation of the spindle resulted in kinetochore movement toward the intact spindle pole in wild-type and klp5Δ cells, suggesting that kinetochore movement is driven by pulling forces. Our theoretical model with Langevin description of microtubule dynamic instability shows that kinesin-8 motors are required for kinetochore centering, whereas sensitivity of rescue to force is necessary for the generation of oscillations. We found that irregular kinetochore movements occur for a broader range of parameters than regular oscillations. Thus, our work provides an explanation for how regulation of microtubule dynamic instability contributes to kinetochore congression and the accompanying movements around the spindle center.

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