Experiments on waves induced in the hollow core of vortices

Recent experimental findings with respect to wave activity in the core of a hollow disk-shaped vortex generated in a cylindrical container with a flat disk rotating at the bottom are reported herein. For relatively low viscosity fluid, several stationary states of the core exist within a certain range of disk speeds. This range becomes narrow as the wave number grows. Between the stationary states, mixed, time dependent states were found to occur. Their interval of endurance decrease with the wave number. The disk speed at which the static state first appears and ends increases linearly with the original height. The phase velocity of the stationary waves rises with the angular velocity of the disk. For a liquid with an intermediate value of viscosity, abrupt transitions from one equilibrium state to the other is taking place. Steady vortex core patterns with wave numbers from one to eleven are observed. States were the basic pattern is subharmonically modulated and states where a wave packet encircles periodically the core are also encountered. Hysterisis is clearly evident. The core of a highly viscous liquid is stable. A suppressed form of instability appears at very large liquid heights and disk speeds.     

Simulation of ship breaking bow waves and induced vortices and scars

An unsteady single‐phase level set RANS method is used to resolve and investigate bow wave breaking around a surface combatant advancing in calm water, including induced vortices and free surface scars. A level set free surface capturing approach was extended and combined with local overset grid refinement for resolution of complex interfacial topologies and small‐scale free surface features. Although the focus of the paper is on wave breaking at Fr=0.35, results over three speeds (Fr=0.28, 0.35, and 0.41) show that the method can accurately predict the changes in resistance and free surface topology, with the two highest speeds showing bow wave breaking. For the Fr=0.35 case, comparison of wave elevation results shows good agreement with the data, including the development and thickening of the bow wave sheet, sequential formation of two overturning plungers with reconnections, and the formation of two free surface scars at the reconnection sites. The computational fluid dynamics (CFD) solution shows a steep shoulder wave, similar to the experiment, but does not predict the experimentally observed weak spilling breaking shoulder wave. Although the current predictions converge to steady state, the region of unsteady free surface measured experimentally can be reasonably well predicted from the region of the simulation where the wave slope exceeds 17°. Comparisons of velocity components and axial vorticity at four cross planes show that the method can accurately predict the wake of low axial velocity and vortical cross flow associated with the breaking bow wave. In addition, the simulation is used to explain the initial development of the overturning bow wave, induced vortices and scars and to fill in the relatively sparse experimental data set by providing a global picture of the axial vortex structure near the free surface. Copyright © 2006 John Wiley & Sons, Ltd.     

Internal waves induced by moving periodic perturbations applied to the surface of a stratified fluid

Investigations of internal wave generation by moving perturbations are of considerable interest for submarine navigation, hydroacoustics, ocean seismology, etc. The main results for perturbations of constant intensity were published in [1–3]. In the present paper we continue the investigations and study moving perturbations whose intensity varies periodically in time. The perturbations are approximated by surface shape variations or an external pressure on the surface. The vertical displacement of the water particles relative to the equilibrium position is obtained in the form of a series in terms of waves modes for a given density stratification. A calculation algorithm and a program for computing each of the wave modes have been compiled. The boundaries of the wave regions and constant-phase lines are constructed and the displacement amplitudes are calculated. It is shown that there are resonance relations between the oscillation frequency and the perturbation velocity for which the displacement for a given mode becomes infinite (in the linear theory). Rostov-on-Don. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 4, pp. 130–135, July–August, 1994.     

Internal waves of finite amplitude

Much research has been devoted to unsteady fluid flow with a free boundary. For example, Ovsyannikov [1] and Nalimov [2] have proven theorems on the existence and uniqueness of a solution, and a number of papers have proposed algorithms for numerical solution, based on various chain methods [3–6] or potential-theory methods [7–9]. In the present article we consider two-dimensional potential waves of finite amplitude on the interface between two heavy fluids of different densities. The initial problem is reduced to the Cauchy problem for a system of two integrodifferential equations. An algorithm for the numerical solution of this system is constructed, and the results of calculations are presented.     

Internal waves generated by the turbulent wake of a sphere

Internal waves generated by the turbulent wake of a sphere travelling horizontally through a linearly stratified fluid were studied using shadowgraph and particle-streak photography. The Reynolds and internal Froude number ranges considered were 2,000 Re 12,900 and 2.0 Fi 28.0, respectively. Two quite distinct flow regimes based on the structure of the turbulent wake were identified. In one, the wake is characterized by large-scale coherent structures. In the other, the wake, as viewed on a side-view shadowgraph, grows in a roughly symmetric fashion to a maximum height and then collapses slowly; such flows are termed the smallscale structures regime.Wave lengths and maximum wave heights of the internal waves were measured as functions of Nt and Fi, where N is the Brunt-Väisälä frequency and t the time. It was found that the wave lengths scale well with the streamwise dimension of the spiralling coherent structures. The maximum amplitude of the internal waves were found to scale with the vertical dimension of the turbulent wake, upon varying the internal Froude number.     

Effects of tip perturbation and wing locations on rolling oscillation induced by forebody vortices

The wing rock motion is frequently suffered by a wing-body configuration with low swept wing at high angle of attack. It is found from our experimental study that the tip perturbation and wing longitudinal locations affect significantly the wing rock motion of a wing-body. The natural tip perturbation would make the wing rock motion of a nondeterministic nature and an artificial mini-tip perturbation would make the wing rock motion deterministic. The artificial tip perturbation would, as its circumferential location is varied, generate three different types of motion patterns： （1） limit cycle oscillation, （2） irregular oscillation, （3） equilibrium state with tiny oscillation. The amplitude of rolling oscillation corresponding to the limit cycle oscillatory motion pattern is decreased with the wing location shifting downstream along the body axis.     

Columnar heat transport via advection induced by inertial waves

Inertial waves are oscillations in a rotating fluid that arise due to the restoring action of the Coriolis force. Low-frequency inertial waves are known to create columnar flow structures inrapidly rotating systems. Columnar heat transport away from the equator has been observed in some strongly forced, rapidly-rotating geodynamo simulations of the Earth’s core. In this study, we investigate the mechanism governing this heat transport by performing direct numerical simulations of model problems comprising buoyant blobs under rapid rotation in a periodic box. We consider a wide range of Rossby numbers (Ro), the ratio of advection to Coriolis force, and Peclet numbers (Pe), the ratio of thermal advection to thermal diffusion. Columnar flow structures, that comprise inertial wave packets, are observed to emerge from the buoyant regions and travel towards the box boundary. We find that the columnar heat transport occurs by advection governed by the local Pe (for instance, a larger vertical elongation in the blob is observed for larger Pe at the same Ro). The magnitude of the advection velocity is determined by the balance between the buoyancy and Coriolis forces. Moreover, the direction of advection is determined by the direction of the wave-induced flow in the columns above and below the blob. Our results suggest that the local Pe could be important for the columnar heat transport in strongly forced dynamo simulations.     

Thermal waves induced by a rapidly moving line source

For the cases involving a fast moving heat source or extremely short pulses emitted by lasers or short time after the start of transients, the classical theory of heat conduction breaks down since the wave nature of heat transport dominates. In this study, the temperature field due to a fast moving line source was determined analytically using the wave concept. The results are given for different values of thermal Mach number (M=V/C). For M>1 the heat affected zone is confined in a wedge shape region behind the source. The wedge half angle is equal to sin^?1 (1/M). It was confirmed that the difference between the results of diffusion and wave models depends on the corresponding time scale and the relaxation time.     

Internal gravity waves excited by a source in stratified horizontally inhomogeneous media

Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 124–128, January–February, 1991.     

The onset of Taylor-like vortices in the flow induced by an impulsively started rotating cylinder

The onset of instability in the flow by an impulsively started rotating cylinder is analyzed under linear theory. It is well-known that at the critical Taylor number T_c=1695 the secondary flow in form of Taylor vortices sets in under the narrow-gap approximation. Here the dimensionless critical time _c to mark the onset of instability for TT_c is presented as a function of the Taylor number T. Available experimental data of water indicate that deviation of the velocity profiles from the primary flow occurs starting from a certain time 4_c. It seems evident that during _c4_c the secondary flow is very weak and the primary state of time-dependent annular Couette flow is maintained.     

Flow in near-critical fluids induced by shock and expansion waves

Unsteady shock and expansion waves are proposed as means to produce flows near the liquid-vapor critical-point without imposing pressure gradients. By choosing appropriate initial conditions and wave speeds, near-critical post-wave conditions can be obtained. The post-shock conditions are shown to be stable with respect to perturbations in the pre-shock conditions. The initial conditions are sufficiently far from the critical-point to allow fast thermal equilibration, permitting the use of larger fluid volumes. Example calculations for the cases of an impulsively accelerated piston, of a shock tube, and of a Ludwieg-like tube are presented yielding flows up to 20 m/s in sulfur hexafluoride (SF₆), where the limit is due to the region of validity of the equation of state. The proposed setup also allows one to study shock wave propagation into near-critical fluids.Received: 13 August 2003, Revised: 7 October 2004, Published online: 4 February 2005[/PUBLISHED]PACS: 47.40.Nm, 47.50. + d, 47.55.Kf, 64.70.Fx, 64.60.HtCorrespondence to: S. Schlamp     

激波诱导下的气体界面不稳定性实验研究

界面不稳定性, 特别是Richtmyer–Meshkov (RM) 不稳定性, 是流体
力学中一项重要的研究内容, 无论在学术研究领域还是工程应用领域都有着
重要的研究价值和应用背景. RM 不稳定性问题自提出以来, 得到了学术界
广泛的关注, 其研究无论是在实验方法、数值模拟还是在理论分析方面都取
得了很大的进展. 在激波管中开展激波与界面相互作用的实验研究, 即研究
界面初始扰动在激波诱导下的演化规律, 是目前研究RM 不稳定性的重要手
段. RM 不稳定性实验研究包括3 个部分, 分别是激波的产生、界面的形成
以及流场的观测. 综述了RM 不稳定性的实验研究进展, 并针对目前研究的
局限性提出了RM 不稳定性今后实验研究的重点和方向: 汇聚激波作用下界
面不稳定性的发展规律; 激波冲击下多种形状及大振幅界面的演化机理; 三
维界面的RM 不稳定性发展规律; 可压缩湍流的形成与混合机理.      

爆炸冲击波作用下人体肺部的损伤

为探究肺部爆炸伤的致伤机制与评价指标,构建了人体-爆炸流场有限元模型,通过与爆炸事故中人员损伤情况比对,验证了模型的有效性。共进行39个爆炸工况的数值模拟,通过改变爆炸当量与距离,使得胸部受到不同量级爆炸载荷作用,肺部损伤等级从无损伤到严重损伤。通过分析爆炸流场分布、胸腔动力学响应、肺部应力分布等阐明肺部爆炸伤的力学机制。基于人体有限元模型输出的损伤响应,提出肺部爆炸伤的评价指标。研究结果表明：在爆炸载荷作用下,胸前壁高速撞击胸腔脏器,导致肺部产生应力波。随后在惯性作用下,胸前壁持续挤压胸腔脏器,并造成胸腔变形。应力波是造成肺部损伤的主要原因,胸腔变形挤压肺部造成的损伤风险较低。肺部损伤集中在靠近胸前壁及心脏的区域。胸骨速度峰值和胸骨加速度峰值可作为肺部爆炸伤的评价指标。胸部压缩量及黏性响应系数不能反映应力波对肺部造成的损伤,不适合评价肺部爆炸伤。

     

Internal waves generated in an exponentially stratified fluid by an arbitrarily moving source

A theoretical investigation is made into the development of linear internal waves in an exponentially stratified flow of an ideal incompressible fluid in the Boussinesq approximation. The waves are generated by an arbitrarily moving point mass source. The obtained solution is used to investigate three special cases of motion: uniform motion at an angle to the horizontal, nonstationary motion during a finite interval of time, and uniform motion in a circular path. The method of solution of this problem is similar to that used by Wolfe and Lewis [1], who studied the generation of acoustic waves.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 67–74, May–June, 1980.I thank V. V. Sazonov for assistance in the calculations.     

Internal gravitational waves and convectional instability in liquids

A study is made of the interconnection between the conditions for convective instability and the condition for existence of internal gravitational waves in a liquid in the presence of a height density gradient due both to a gravitational field and to a temperature and concentration gradient, in particular, in the proximity of the critical point of pure liquids and binary mixtures. The error in measuring the thermal conductivity coefficient close to the critical point, connected with the propagation of internal gravitational waves, is evaluated.Moscow. Translated from Izvestiya Akademii Nauk SSSR. Mekhanika Zhidkosti i Gaza, No. 2, pp. 55–61, March–April, 1972.     

Mass transfer enhancement by longitudinal vortices

From numerical and experimental investigations it has been recently established that convective heat transfer can be dramatically enhanced by the generation of longitudinal vortices in the flow. The phenomenological similarity between heat and mass transfer suggests that longitudinal vortices should increase also mass transfer. The mixing between two parallel streams of two components in a rectangular channel with and without a pair of rectangular winglets as vortex generators has been numerically investigated. The results show that one pair of vortex generators can increase the global mixing by more than 50 for laminar flow. This global mixing has been defined as the sum of the square of the differences of concentrations.     

Numerical investigation of ethylene flame bubble instability induced by shock waves

In this paper, the ethylene/oxygen/nitrogen premixed flame instabilities induced by incident and reflected shock wave were investigated numerically. The effects of grid resolutions and chemical mechanisms on the flame bubble deformation process are evaluated. In the computational frame, the 2D multi-component Navier–Stokes equations with second-order flux-difference splitting scheme were used; the stiff chemical source term was integrated using an implicit ordinary differential equations (ODEs) solver. The two ethylene/oxygen/nitrogen chemical mechanisms, namely 3-step reduced mechanism and 35-step elementary skeletal mechanism, were used to examine the reliability of chemistry. On the other hand, the different grid sizes, Δx × Δy = 0.25 × 0.5mm and Δx × Δy = 0.15 × 0.2mm, were implemented to examine the accuracy of the grid resolution. The computational results were qualitatively validated with experimental results of Thomas et al. (Combust Theory Model 5:573–594, 2001). Two chemical mechanisms and two grid resolutions used in present study can qualitatively reproduce the ethylene spherical flame instability process generated by an incident shock wave of Mach number 1.7. For the case of interaction between the flame and reflected shock waves, the 35-steps mechanism qualitatively predicts the physical process and is somewhat independent on the grid resolutions, while the 3-steps mechanism fails to reproduce the instability of ethylene flame for the two selected grid resolutions. It is concluded that the detailed chemical mechanism, which includes the chain elementary reactions of fuel combustion, describes the flame instability induced by shock wave, in spite of the fact that the flame thickness (reaction zone) is represented by 1–2 grids only.      

A two-fluid model of non-Newtonian blood flow induced by peristaltic waves

The problem of blood flow induced by peristaltic waves in a uniform small diameter tube has been investigated. Blood has been represented by a two-fluid model consisting of a core region of suspension of all the erythrocytes, assumed to be a Casson fluid, and a peripheral layer of plasma as a Newtonian fluid. The expressions for dimensionless pressure drop and friction force have been obtained. The results obtained in the analysis have been evaluated numerically and discussed briefly. The significance of the present model over the existing models has been pointed out by comparing the results with other theories both analytically and numerically.     

Internal waves generated by a moving sphere and its wake in a stratified fluid

The internal gravity waves and the turbulent wake of a sphere moving through stratified fluid were studied by the fluorescent dye technique. The Reynolds number Re=U·2a/v was kept nearly constant at about 3 · 10³ and the Froude number F;U/a N ranged from 0.5 to 12.5. It is observed that waves generated by the body are dominant only when F<4 and are replaced by waves generated by the large scale coherent structures of the wake when F>4.