Liquid crystal studies of sliding vapour bubbles

The paper discusses localized measurements that may be used to validate individual sub-models in mechanistic models of nucleate boiling on inclined surfaces with sliding bubbles. A previous study of wall temperature variations near isolated sliding bubbles by Kenning et al. (Multiphase Sci Technol 14:75–94, 2002), employing liquid crystal thermography, has been expanded in range and compared with recent studies by Qiu and Dhir (Exp Therm Fluid Sci 26:605–616, 2002) and Bayazit et al. (J Heat Transfer 125:503–509, 2003). The implications for modelling nucleate boiling are discussed.     

Observations of large-scale structures in wakes behind axisymmetric bodies

Wakes behind disk-shaped axisymmetric bodies of varying solidity are studied using flow visualization and two-dimensional Fourier decomposition of velocity measurements. Evidence of a reverse flow region behind some of the bodies is observed to coincide with the presence of large-scale structures in the near and far wake. Fourier analysis shows that these large-scale structures are predominately helical (m= ±1) and occur at a characteristic frequency which corresponds to their wavelength as observed from flow visualization. Our measured value for this characteristic frequency agrees with vortex shedding frequencies observed for these types of wakes.     

Transition mechanisms in laminar separation bubbles with and without incoming wakes and synthetic jet effects

Laminar separation and transition processes of the boundary layer developing under a strong adverse pressure gradient, typical of Ultra-High-Lift turbine profiles, have been experimentally investigated for a low Reynolds number case. The boundary layer development has been surveyed for different conditions: with steady inflow, with incoming wakes and with the synchronized forcing effects due to both incoming wakes and synthetic jet (zero net mass flow rate jet). In this latter case, the jet Strouhal number has been set equal to half the wake-reduced frequency to synchronize the unsteady forcing effects on the boundary layer. Measurements have been taken by means of a single-sensor hot-wire anemometer. For the steady inflow case, particle image velocimetry has been employed to visualize the large-scale vortical structures shed as a consequence of the Kelvin?CHelmholtz instability mechanism. For the unsteady inflow cases, a phase-locked ensemble averaging technique, synchronized with the wake and the synthetic jet frequencies, has been adopted to reconstruct the boundary layer space-time evolution. Results have been represented as color plots, for several time instants of the forcing effect period, in order to provide an overall view of the time-dependent transition and separation processes in terms of ensemble-averaged velocity and unresolved unsteadiness distributions. The phase-locked distributions of the unresolved unsteadiness allowed the identification of the instability mechanisms driving transition as well as the Kelvin?CHelmholtz structures that grow within the separated shear layer during the incoming wake interval and the synthetic jet operating period. Incoming wakes and synthetic jet effects in reducing and/or suppressing flow separation are investigated in depth.     

The effect of the VOF–CSF static contact angle boundary condition on the dynamics of sliding and bouncing ellipsoidal bubbles

The static contact angle is the only empiricism introduced in a Volume of Fluid–Continuum Surface Force (VOF–CSF) model of bubbly flow. Although it has previously been shown to have a relatively limited effect on the accuracy of velocity and shape predictions in the case of large gas bubbles sliding under inclined walls (e.g. Cook and Behnia, 2001), it may have a more determining influence on the numerical prediction of the dynamics of smaller ellipsoidal bubbles which were shown by Tsao and Koch (1997) to bounce repeatedly when sliding under inclined walls at certain wall inclinations. The present paper reports on the influence of surface tension and the static contact angle on the dynamics of an ellipsoidal air bubble of equivalent diameter D_e = 3.4 mm. The bubble Eötvös and Morton numbers are Eo = 1.56 and Mo = 2 × 10⁻¹¹ respectively. The computational results are achieved with a Piecewise Linear Construction (PLIC) of the interface and are reviewed with reference to experimental measurements of bubble velocity and interface shape oscillations recorded using a high speed digital camera. Tests are performed at plate inclination angles θ ∈ {10°, 20°, 30°, 45°} to the horizontal and computational models consider three static contact angles θ_c ∈ {10°, 20°, 30°}. The static contact angle has been found to have a significant effect on the bubble dynamics but to varying degree depending on the plate inclination. It is shown to promote lift off and bouncing when the plate inclination angle reaches 30° in a way that does not necessarily reflect experimental observations.     

Measurement and analysis of unsteady flow structures in rotor blade wakes

Time-variant data are obtained to investigate the exit flow field from a rotor in a research compressor. In the free-stream region, the instantaneous data are analogous to one another and to the ensemble averaged free-stream results. However, in the wake region, some of the instantaneous signals are similar to one another and to the ensemble averaged wake, but others differ significantly. These variations in the instantaneous data are interpreted and shown to be due to a vortex street structure in the wake. This is accomplished by: (1) developing a mathematical model of the rotor blade exit flow field based on a wake vortex street structure analogous to the unsteady flow field behind bluff bodies due to classical von Karman vortex shedding; and (2) correlating predictions of both the ensemble averaged and instantaneous rotor blade exit flow fields as well as the velocity probability density distributions from this vortex wake flow field model with the corresponding data. The correlation of the ensemble averaged rotor blade exit flow fields is very good and the flow angle distribution correlation excellent. The predicted instantaneous rotor blade exit flow field exhibits many of the flow features found in the data. Also, the probability density distributions for the data and the vortex wake flow field model are analogous to one another.List of symbols N number of rotor revolutions - S _w rotor blade wake width - S _x vortex core horizontal spacing - S _y vortex core vertical spacing - u velocity component parallel to vortex street motion - v velocity component normal to vortex street motion - W instantaneous relative velocity - W _i velocity induced by vortex street - W free-stream relative velocity - W _s velocity of vortex street - x coordinate parallel to vortex street motion - y coordinate normal to vortex street motion - free-stream relative flow angle - _inst instantaneous relative flow angle - vortex strength     

Current topics in the dynamics of gas and vapor bubbles

Summary A review of topics of current interest in the dynamics of gas and vapor bubbles in liquids under conditions of no relative motion is presented. The aspects of gas bubble dynamics considered include: radiation pressure (Bjerknes) forces, analytical and numerical studies of transient forced oscillations, connection of the subharmonic resonance with the onset of acoustic cavitation, thermal behaviour of the gas in small-amplitude and large-amplitude oscillations. For vapour bubbles a detailed discussion of the growth process is given. A scaled formulation of this process is also presented and phenomena connected to the forced oscillations are discussed. Some aspects of the dynamics of vapor bubbles in «cold» liquids (i.e. cavitation bubbles) are also included. Finally several phenomena encountered in the dynamics of non-spherical bubbles are reviewed, namely the effect of viscosity on the stability of the spherical shape, a variational formulation, and phase-change effects.

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Sommario Si presenta una rassegna di argomenti attualmente oggetto di ricerca nella dinamica di bolle di gas e di vapore nei liquidi. La discussione è limitata al caso in cui non esista moto di traslazione relativo fra le fasi. Gli aspetti considerati della dinamica di bolle di gas includono: forze di pressione di radiazione (o di Bjerknes), studi analitici e numerici dei transitori nelle oscillazioni forzate, rapporto fra la risonanza subarmonica e l'apparizione di fenomeni di cavitazione acustica, comportamento termico del gas contenuto nella bolla durante oscillazioni di piccola ampiezza e di grande ampiezza. Per le bolle di vapore si presenta una discussione dettagliata del processo di crescita, una formulazione di similarità di tale processo, e alcuni fenomeni connessi alle oscillazioni forzate. Vengono inoltre discussi alcuni aspetti della dinamica di bolle di vapore in liquidi «freddi» (cioè bolle di cavitazione). Infine si considerano alcune questioni connesse alla dinamica di bolle non sferiche, e cioè l'effetto della viscosità sulla stabilità della forma sferica, una formulazione variazionale, e fenomeni relativi al cambiamento di fase.

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This paper is based in part on an invited review lecture given at Euromech Colloquium 98 «Mechanics and Physics of Gas Bubbles in Fluids» held at Eindhoven, 22 – 24 November 1977. The study reported herein has been supported in part by Gruppo Nazionale di Fisica Matematica of C.N.R.     

On the interaction of Taylor bubbles rising in two-phase co-current slug flow in vertical columns: turbulent wakes

An experimental study on the interaction between Taylor bubbles rising through a co-current flowing liquid in a vertical tube with 32 mm of internal diameter is reported. The flow pattern in the bubble's wake was turbulent and the flow regime in the liquid slug was either turbulent or laminar. When the flow regime in the liquid slug is turbulent (i) the minimum distance between bubbles above which there is no interaction is 5D-6D; (ii) the bubble's rising velocity is in excellent agreement with the Nicklin relation; (iii) the experimental values of the bubble length compare well with theoretical predictions (Barnea 1990); (iv) the distance between consecutive bubbles varied from 13D to 16D and is insensitive to the liquid Reynolds number. When the flow regime in the liquid slug is laminar (i) the wake length is about 5D-6D; (ii) the minimum distance between bubbles above which there is no interaction is higher than 25D; (iii) the bubble's rising velocity is significantly smaller than theoretical predictions. These results were explained in the light of the findings of Pinto et al. (1998) on coalescence of two Taylor bubbles rising through a co-current liquid. Received: 2 February 2000 / Accepted: 15 March 2001     

On the dynamics of bubbles in polymer aqueous solutions

By analysing experimentally the rheological behaviors of polyethylene oxide solutions, polyacrylamide solutions, carboxymethylcellulose solutions, and hydroxyethylcellulose solution, a rheological model is proposed. The effect of various polymer additives on an air-liquid surface tension is made clear. The equation of motion for a bubble and the pressure equation are derived by using the new rheological expression, and then the effects of a kind of polymer and a polymer concentration on the bubble radius-time history and impulse pressure are numerically examined.     

An interferometric study of organized structures in wakes of circular cylinders

Double-pulse, diffuse-illumination interferometry has been used to study the organized structures in the wake of a circular cylinder (M = 0.6,Re _d = 300,000). The interferograms show striking similarities to the flow visualization pictures of the incompressible wakes of circular cylinders as obtained by other investigators. In particular, the interferograms taken with very short pulse intervals show concentrations of fine scale turbulence in the von Karman vortices and the existence of streamwise vortices between these counter-rotating largescale vortices. The fact that most of the features observed in low speed wakes using a variety of techniques can be observed in the interferograms illustrated the potential of interferometry for the study of turbulent structures in compressible flows.One of the authors, S. Zhong wishes to thank Dr C-W Hustad of EPFL at Lausanne, Switzerland for his invaluable advice on the experimental set-up of the interferometer. Financial support to her Ph.D study from Churchill College at Cambridge, the British government and the Zonta International Foundation is also gratefully acknowledged. Lastly, the authors would like to express their thanks to Professor A. Roshko at California Institute of Technology and Dr A. Ahmed at Texas A & M University for making available copies of their experimental photographs used in this paper.     

Measurement of the flow structures in the wakes of different types of parachute canopies

We measured flow structures with stereoscopic particle image velocimetry (stereo-PIV) in the turbulent wakes of three parachute canopies, which had the same surface area, but different geometries. The tested parachute canopies included ribbon canopy, 8-branches canopy, and cross canopy. The obtained results showed that the geometry of the parachute canopies had significant influences on the flow structures in the wakes of these three canopies. In addition, the variation of Reynolds number did not lead to a dramatic change in the distributions of velocity, vorticity, Reynolds stress, and turbulent kinetic energy.     

Fusion dynamics of underwater explosion bubbles

Fusion of bubbles is a common and significant phenomenon in the nature. In this work, the potential flow theory and the boundary integral method are employed to simulate the fusion of underwater explosion bubbles. Based on the numerical and experimental results in the literature, a three-dimensional model of bubble fusion is established. A good agreement is shown between the numerical results and experimental data. Influences of characteristic parameters such as distance and depth are specifically investigated by using the developed three-dimensional program, resulting in favorable curves and conclusions. This work provides references for the relevant research on fusion mechanics and dynamic characteristics of bubbles.     

On the dynamics and instability of bubbles formed during underwater explosions

Dynamics of explosion bubbles formed during underwater detonations are studied experimentally by exploding fuel (hydrogen and/or carbon monoxide)–oxygen mixture in a laboratory water tank. Sub-scale explosions are instrumented to provide detailed histories of bubble shape and pressure. Using geometric and dynamic scaling analyses it has been shown that these sub-scale bubbles are reasonable approximations of bubbles formed during deep sea underwater explosions. The explosion bubble undergoes pulsation and loses energy in each oscillation cycle. The observed energy loss, which cannot be fully explained by acoustic losses, is shown here to be partly due to the excitation of instability at the interface between the gaseous bubble and the surrounding water. Various possible mechanisms for the dissipation of bubble energy are addressed. The analysis of the experimental data gives quantitative evidence (confirmed by recent numerical studies) that the Rayleigh–Taylor instability is excited near the bubble minimum. The dynamics of the bubble oscillation observed in these experiments are in good agreement with experimental data obtained from deep sea explosions     

The dynamics and dissolution of gas bubbles in a viscoelastic fluid

This paper reports an experimental study of the motion of dissolving and non-dissolving gas bubbles in a quiescent viscoelastic fluid. The objective of the investigation was to determine the influence of the abrupt transition in bubble velocity, which had been observed at a critical radius of approx. on the rate of mass transfer. Thus, a range of bubble sizes from an equivalent (spherical) radius of 0.2–0.4 cm was employed using CO₂ gas, and five different fluids, including one Newtonion glycerine/water solution and four viscoelastic solutions of Separan AP30 in water (0.1, 0.5, 1% by weight) and in a water/glycerine mixture.The experimental data on bubble velocity shows that the discontinuous increase with bubble volume observed previously for air bubbles in viscoelastic fluids, does not occur for dissolving CO₂ bubbles—presumably due to the continuous decrease in bubble volume. Instead, a very steep but definitely continuous transition is found. Mass transfer rates are found to be significantly enhanced by viscoelasticity, and comparison with available theoretical results shows that the increase is greater than expected for purely viscous, power-law fluids. We conclude that a fully viscoelastic constitutive model would be necessary for a successful analysis of the dissolution of a gas bubble which is translating through a (high molecular weight) polymer solution.     

Acoustic emission of single laser-produced cavitation bubbles and their dynamics

Cavitation bubble dynamics is investigated by the method of optic cavitation, i.e. the formation of single cavities in liquids by light. From the sound waves radiated upon collapse the pressure-time curve is obtained. Maximum bubble size and shock wave amplitudes are evaluated and an energy balance is considered. Numerical calculations with a modified Gilmore model taking into account the mass loss of the cavity can explain the rapid damping of the bubble oscillation observed in the experiments.     

近场水下爆炸气泡与双层破口结构的相互作用

针对双层结构在水中受到水下爆炸冲击这一问题,利用欧拉有限元数值模型对近场水下爆炸气泡与双层破口结构的相互作用机理进行了研究,分析了舱室涌流及流场演化等规律。首先,通过放电实验对数值模型进行了验证,结果表明,数值结果和实验结果吻合较好;然后,总结了不同破口尺寸、不同起爆位置和不同壳间水位条件下的耦合作用规律。在内部空气、流体惯性以及破口的联合作用下,气泡演化过程中会出现气泡分割现象。当内层破口尺寸系数小于0.5时,内舱室内会出现二次涌流现象,且涌流形态较细长;炸药起爆位置系数小于0.1时,自由液面处会出现破碎和重闭合现象;壳内水位对舱室涌流量的影响作用较为复杂,当水位满舱时,急速涌流会减少船艇的应急时间。

     

On the dynamics of rotating and rolling structures

The dynamics of rotating flexible bodies is influenced by gyroscopic effects, which yields complex-valued eigenvectors and a split of eigenfrequencies in comparison with a resting body depending on rotational speed. These phenomena, which have been well investigated for simple structures like elastic rings, also affect more complicated systems such as rolling drums in printing machines. The aim of this contribution is to give a clear introduction into the physics of rotating bodies, starting with a rather simple analytical model, followed by some basic experimental investigations and ending with a finite element approach of more complicated three-dimensional structures. Special care is taken for the numerical solution of complex eigenvalue problems. The results obtained in each step are discussed in detail regarding their physical meaning.