Escape trajectories are deflected when fish larvae intercept their own C-start wake

Fish larvae may intercept their own wake during sharp turns, which might affect their escape performance. We analysed C-starts of larval zebrafish (Danio rerio, Hamilton, 1822) using a computational fluid dynamics approach that simulates free swimming (swimming trajectory is determined by fluid forces) by coupling hydrodynamics and body dynamics. The simulations show that fish may intercept their own wake when they turn by 100–180°. During stage 1 of a C-start, the fish generates a strong jet at the tail that is shed into the wake. During stage 2, the fish intercepts this wake. Counterfactual simulations showed that wake interception increased the lateral force on the fish and reduced the fish''s turning angle by more than 5°. Wake interception caused no significant acceleration tangential to the trajectory of the fish and did not affect total power output. While experimental and simulation evidence suggests that fish larvae can either undershoot or intercept but not overshoot their wake, our simulations show that larger fish might be able to avoid intercepting their wake by either under- or overshooting. As intercepting its own wake modifies the fish''s escape trajectory, fish should account for this effect when planning their escape route.     

Nanoscale jet collision and mixing dynamics

Micro- and macroscale investigations have shown that colliding drops always coalesce for small values of the Weber number We = rhoU2d/sigma. Our molecular dynamic simulations show that nanojets always recoil following head-on collision even though We --> 0. The duration between collision and recoil is a function of the nanojet impact velocity Uo and the nature of intermolecular interactions. Evaporation, which promotes mixing, occurs during recoil and is enhanced by reducing intermolecular interactions. Thereafter, mixing occurs through diffusion. The mixing dynamics are independent of Uo and the orifice shape. Consistent with a continuum analysis, the characteristic nanojet diameter at stagnation ds,1 proportional to Uo, recoil time following collision tau proportional to Uo-2, and the number of evaporating molecules N proportional to Uo.     

Detection of the electromagnetic field induced by the wake of a ship moving in a moderate sea state of finite depth

The wake of a ship and ambient sea waves induce disturbances in the Earth??s geomagnetic field. Numerical simulations are used to examine the feasibility of detecting these disturbances for moderate sea states. It is assumed that the electromagnetic disturbances are sampled by an air-borne magnetometer moving steadily along a rectilinear path. Spectral analysis of the samples is performed. Numerical simulations indicate that the magnetic field induced by the noisy sea environment and that by the wake of a moving ship have different spectral characteristics. Typically, the peak of the body-induced magnetic-field spectrum is located in the range of frequencies where the corresponding value of the wind wave??s spectrum is less significant. It is illustrated that the feasibility of electromagnetic detection of a ship??s wake depends strongly on the ship??s depth-based Froude number.     

Detection of nuclear recoils in prototype dark matter detectors, made from Al, Sn and Zn superheated superconducting granules

This work is part of an ongoing project to develop a superheated superconducting granule (SSG) detector for cold dark matter and neutrinos. The response of SSG devices to nuclear recoils has been explored irradiating SSG detectors with a 70 MeV neutron beam. The aim of the experiment was to test the sensitivity of Sn, Al and Zn SSG detectors to nuclear recoil energies down to a few keV. The detector consisted of a hollow teflon cylinder (0.1 cm³ inner volume) filled with tiny superconducting metastable granules embedded in a dielectric medium. The nuclear recoil energies deposited in the SSG were determined measuring the neutron scattering angles with a neutron hodoscope. Coincidences in time between the SSG and the hodoscope signals have been clearly established. In this paper the results of the neutron irradiation experiments at different SSG intrinsic thresholds are discussed and compared to Monte Carlo simulations. The results show that SSG are sensitive to recoil energies down to 1 keV. The limited angular resolution of the neutron hodoscope prevented us from measuring the SSG sensitivity to even lower recoil energies.     

Bubble spreading during the boiling crisis: modelling and experimenting in microgravity

Boiling is a very efficient way to transfer heat from a heater to the liquid carrier. We discuss the boiling crisis, a transition between two regimes of boiling: nucleate and film boiling. The boiling crisis results in a sharp decrease in the heat transfer rate, which can cause a major accident in industrial heat exchangers. In this communication, we present a physical model of the boiling crisis based on the vapor recoil effect. Under the action of the vapor recoil the gas bubbles begin to spread over the heater thus forming a germ for the vapor film. The vapor recoil force not only causes its spreading, it also creates a strong adhesion to the heater that prevents the bubble departure, thus favoring the further spreading. Near the liquid-gas critical point, the bubble growth is very slow and allows the kinetics of the bubble spreading to be observed. Since the surface tension is very small in this regime, only microgravity conditions can preserve a convex bubble shape. In the experiments both in the Mir space station and in the magnetic levitation facility, we directly observed an increase of the apparent contact angle and spreading of the dry spot under the bubble. Numerical simulations of the thermally controlled bubble growth show this vapor recoil effect too thus confirming our model of the boiling crisis.     

Sub-recoil-limit laser cooling via interacting dark-state resonances

We propose a laser cooling mechanism based on velocity-selective double-dark resonance that leads to a temperature significantly lower than the one-photon recoil limit. This mechanism benefits from sharp and high-contrast spectra which are induced by the two electromagnetically induced transparency windows due to the interacting dark-state resonances. It is theoretically demonstrated that four-level atoms illuminated by two counter-propagating probe beams and two additional beams directed perpendicularly to the other two exhibit new cooling effects; for red detuned probe lasers, atoms can be subject to a strong viscous force with an extremely small diffusion, characteristic of heating caused by the stochastic nature of spontaneous emission processes. By quantum mechanical simulations, we then find that the lowest temperature in EIT windows approaches 1?nK for the case of mercury. However, the different broadening mechanisms can destroy the cooling, so that the lowest temperature can increase to the recoil temperature.     

基于模型试验和数值模拟的柔性串列圆柱体涡激振动研究

采用物理模型试验和CFD数值模拟方法研究了大长径比、低质量比的柔性串列圆柱体涡激振动现象。通过分析串列圆柱振幅、振动频率、受力特性和流场结构等特性,着重研究流速和圆柱间距对下游圆柱涡激振动特性影响。研究发现,上、下游圆柱涡激振动幅值差别较大,并且当流速大于某个值后,两者主导频率也不相同,由此提出分离约化速度U r。流速和间距都会影响上游尾流对下游圆柱的作用,其中流速会影响上游尾涡强度及其发展程度,间距会影响上游尾涡发展空间及其与下游圆柱的接触位置。     

火炮磁流变后坐阻尼器的设计与可控性分析

以高冲击、高速环境下的某口径火炮反后坐装置为研究对象,讨论了后坐阻力对火炮静止性和射击稳定性的影响,研究了磁流变阻尼器对火炮后坐运动的控制作用。设计了火炮磁流变后坐阻尼器及冲击试验平台;考虑冲击载荷激励下惯性力的影响,建立了火炮反后坐装置动力学模型并进行了计算仿真,对所设计长行程磁流变阻尼器进行了冲击试验,测试其在冲击载荷下的动态特性,并进行了后坐过程半实物仿真控制。试验结果表明设计的长行程火炮用磁流变阻尼器可有效抑制冲击载荷激励作用,为进一步实现火炮后坐力与行程控制的一体化设计和工程应用奠定了基础。     

Measuring felt recoil of sporting arms

For competitors in the shooting sports, gun recoil can affect performance and create discomfort. An objective method of measuring felt recoil forces would be useful to evaluate methods of recoil reduction. An experimental test apparatus was developed to measure recoil forces of sporting rifles and shotguns. No modifications to the firearms themselves were necessary to make the measurements. Through both inertial and dissipative restraint mechanisms, the apparatus reproduced gun movement that was representative of that experienced by a particular shooter. This should result in measured recoil forces similar to those experienced by the shooter. The apparatus measures the horizontal shoulder force and the vertical force component exerted at the stock comb.     

Compressive properties of single-filament carbon fibres

Compressive properties of mesophase pitch-based carbon fibres (NT-20, NT-40 and NT-60) were measured using the tensile recoil test and the elastica loop test. The NT-40 fibre with a 400 GPa tensile modulus showed a smaller loop compressive yield strain and a larger recoil compressive strength compared to these values obtained from the longitudinal compression test on its unidirectional composites. Further, the recoil compressive strength of this fibre was higher than that of PAN-based carbon fibre with a corresponding modulus. Under the ideal conditions in the tensile recoil test, the strain energy was conserved before and after recoil, and the initial tensile stress and the recoil compressive stress do not coincide when fibre stress-strain behaviour is non-linear, and the non-linearity in compression and in tension is different. The difference between the composite compressive strength and the recoil compressive strength of NT-40 was quantitatively explained by taking account of the fibre compressive stress-strain non-linear relation. The difference between the loop compressive yield strain and the composite compressive strain to failure was also explained by this non-linearity.     

Wake field analysis and modelling of microwave instability in the ILC damping ring

Wake field simulations are performed for given technical designs of the vacuum chamber components for the 6.4 km ILC damping ring, in order to calculate longitudinal wake functions. Modelling of the microwave instability based on multi-turn tracking with many particles is described. A comparison is presented between the potential well distortion found from solving the Haissinski equation and the results of tracking simulations. The threshold for the microwave instability is found for the given designs and lattice parameters.     

基于二维模型的火炮冲击运动计算

当前火炮后坐与复进冲击运动计算基于一维准定常理论,导致制退机液压阻力求解偏离实际较大,并且无法反映制退机内流场分布与流体流动特性。在实际结构下的制退机二维模型基础上,通过MATLAB与FLUENT协同仿真技术,实现了火炮后坐、复进冲击完整过程的数值模拟。将仿真结果与试验结果进行对比,验证了该方法的可行性和有效性,为改进火炮后坐与复进运动计算提供了一种新思路。     

The recoil compressive strength of pitch-based carbon fibres

The recoil compressive strengths of pitch-based carbon fibres with folded-radial and flat-layer textures were compared. Using the recoil test method, it was shown that the compressive strengths of pitch-based carbon fibres with folded-radial textures are superior to pitch-based carbon fibres with flat-layer textures at all modulus levels. Analysis of the failed fibre ends revealed that the folded-radial texture appeared to inhibit shearing of the basal planes. This may account for the superior compressive strengths of pitch-based carbon fibres with folded-radial textures. Procedural differences in the recoil test were shown to influence the calculated recoil compressive strength of pitch-based carbon fibres. Microscopic analysis of the recoil test specimens revealed that some energy is absorbed in the area where the fibre is secured to the support tab.     

火炮磁流变阻尼器试验分析与动态模型

火炮磁流变阻尼器工作在高速高冲击条件下,其动力特性及其力学模型与常用的磁流变阻尼器存在较大不同。针对某型号火炮设计了反后坐用磁流变阻尼器,并进行了5种不同电流下的动态测试,验证了该阻尼器对反后坐控制的可行性。由试验分析得,火炮磁流变阻尼器的输出力不仅与控制电流和后坐速度有关,还与高冲击条件下的磁流变效应的复杂性、惯性力、腔体内气体压力等因素有关。提出了用改进的多项式模型描述火炮磁流变阻尼器,该模型具有形式简洁,易于求解逆模型,便于实时控制等优点。通过参数辨识后的模型能较好的描述火炮反后坐过程中阻尼器的输出力。     

高耸建筑物爆破拆除引起后座的数值模拟

On the basis of recoil in the process of blasting demolition of high buildings,this thesis applies dynamics theory to predict whether the phenomena of recoil will occur or not by simulation and calculation of the recoil and the pressure suffered from the rudimental supprter in the process of demolition.The eriterion of recoil relies on whether recoil can be overcome by the intensity of reservation or friction.To take the obliquity as increment,and to calculate the recoil and the pressure suffered from the rudimental supporter in correspondence with the every obliquity in the demolition by VC program,the relation curve can be obtained between recoil and obliqity. By means of examples testified with programs,the obtatined results are consistent with those those of practice。     

Spallation of single crystal nickel by void nucleation at shock induced grain junctions

Molecular dynamics (MD) simulations of spallation in single crystal nickel were performed for a range of system sizes and impact velocities. The initial compressive wave leaves a rich microstructure in its wake. The subsequent tensile waves create multiple grains and grain junctions between regions of differing crystal orientation. These grain junctions serve as void nucleation sites when the reflected tensile waves interact, leading to ductile failure. In this way, the mechanism for failure in an initially single-crystalline sample is similar to that seen experimentally in high-purity, poly-crystalline metals, in which grain boundaries are sites for void nucleation.     

Radiochemical approaches for formation of endohedral fullerenes and MD simulation

The formation of atom-doped fullerenes has been investigated by using several types of radionuclides produced by nuclear reactions. From the trace of the radioactivities after a high performance liquid chromatography, it was found that formation of endohedral fullerenes (or heterofullerenes) of small atoms (Be, Li), noble-gas atoms (Kr, Xe) and 4B–6B elements (Ge, As, Se, Sb, Te, etc.) is possible by a recoil process following the nuclear reaction. In order to show the possibility of creating endohedral fullerenes with a suitably high kinetic energy of foreign atoms, we have carried out large-scale ab initio molecular dynamics simulations on the basis of the all-electron mixed-basis approach with atomic orbitals and plane waves for several atoms.     

Radiochemical approaches for formation of endohedral fullerenes and MD simulation

The formation of atom-doped fullerenes has been investigated by using several types of radionuclides produced by nuclear reactions. From the trace of the radioactivities after a high performance liquid chromatography, it was found that formation of endohedral fullerenes (or heterofullerenes) of small atoms (Be, Li), noble-gas atoms (Kr, Xe) and 4B–6B elements (Ge, As, Se, Sb, Te, etc.) is possible by a recoil process following the nuclear reaction. In order to show the possibility of creating endohedral fullerenes with a suitably high kinetic energy of foreign atoms, we have carried out large-scale ab initio molecular dynamics simulations on the basis of the all-electron mixed-basis approach with atomic orbitals and plane waves for several atoms.     

A novel method for modeling the recoil in W boson events at hadron colliders

We present a new method for modeling the hadronic recoil in W→ℓν events produced at hadron colliders. The recoil is chosen from a library of recoils in Z→ℓℓ data events and overlaid on a simulated W→ℓν event. Implementation of this method requires that the data recoil library describe the properties of the measured recoil as a function of the true, rather than the measured, transverse momentum of the boson. We address this issue using a multidimensional Bayesian unfolding technique. We estimate the statistical and systematic uncertainties from this method for the W boson mass and width measurements assuming of data from the Fermilab Tevatron. The uncertainties are found to be small and comparable to those of a more traditional parameterized recoil model. For the high precision measurements that will be possible with data from Run II of the Fermilab Tevatron and from the CERN LHC, the method presented in this paper may be advantageous, since it does not require an understanding of the measured recoil from first principles.     

Understanding granular media: from fundamentals and simulations to industrial application

Academic and industrial research on the behaviour of granular material is increasingly supported by numerical simulations. Such simulation environments are not only a vital tool in design and analysis, but also a key link between physicists and engineers. This Topical Collection includes a range of papers that use simulation to study granular matter from the micro- and macroscopic behaviour of particles to the behaviour of large industrial or geomechanical systems. These contributions enhance our understanding of basic physical effects and processes such as heat transfer, agglomeration and screening but also give essential parameters for the simulation of industrial systems, or, at least, guide the strategies required to provide realistic simulation results. The collection focuses on the presentation of (a) new coupled simulation methods to consider the interaction of granular bodies with structural or fluid systems (b) new findings for the consideration of particle shape and particle size distributions within simulations, (c) acoustic wake agglomeration, and (d) gravitational flow in geomechanics.