Virtual mass in two-phase bubbly flow

The two-phase flow equations in their usual form are unstable. It is known that the inclusion of the virtual mass of the gas bubbles greatly improves the stability of numerical computations. Since there is some confusion concerning the proper form of the virtual-mass terms, we first present a systematic derivation of the two-fluid equations for a liquid/gas mixture from a generalised form of Hamilton's variational principle. The two-fluid theory of superfluid ⁴He has been derived in a similar way. The resulting equations do not seem to have been presented before in the literature on two-phase flow. The derivation demonstrates how the pressure of the liquid/gas mixture can be defined in a natural way. Two independent vorticities can be distinguished, each having its own law of transportation (Kelvin theorem). A subsequent stability analysis shows that at neutral stability the virtual-mass coefficient m(α) takes the form $\text{m(α)=}\text{1}\text{2}\text{α(1?α)(1?3α)}$ in the case of spherical gas bubbles with void fraction α. The corresponding local distribution of gas bubbles is anisotropic. The vanishing of the virtual mass at $\text{α=}\text{1}\text{3}$ is interpreted as the breakdown of bubbly flow. Dissipative terms are introduced and analysed in an appendix.     

Three-dimensional surface measurement using the confocal scanning microscope

The use of the depth discriminination property of the confocal scanning microscope for surface profiling has been adapted to provide a method of high-resolution three-dimensional surface profilometry. Measurements on a semiconductor specimen demonstrate the technique; depth variations of the order of 0.1 m are clearly resolved.     

应用粒子图像测试技术测量球床多孔介质单相流动的流场

多孔介质在生产生活以及科技发展中的应用十分广泛, 随着能源、化工、冶金和原子能等领域技术的发展, 以及近代工农业生产技术的进步, 大量多孔介质的传热传质问题逐渐出现, 进一步促进了多孔介质学科的形成和发展, 使其成为当今科学技术中令人瞩目的研究热点之一. 通过实验获得准确的实验图像和数据, 并使用相应软件对实验所得数据和流体流动图像进行深度分析, 这样既有真实可靠的实际数据, 又有直观的理论的支持, 使对多孔介质的研究更为完善. 实验结合粒子图像测试技术和折射率匹配技术对叉排排列玻璃球多孔介质填充床内的流体流动转变过程进行流场测试, 并提取数据, 采用Tecplot软件对提取数据进行处理, 得出流体流动机理的转变过程. 实验固相为由直径25 mm水晶玻璃球叉排堆积而成的填充床, 液相为65%苯甲醇和35% 无水乙醇配制的匹配液. 液相与固相的折射率都为1.477, 成功消除由于折射率不匹配引起的激光光线偏折. 实验得到雷诺数为 4.7 ≤ Re ≤ 1000时球床内流场图, 对比不同雷诺数时流场和流线变化得出: 随着雷诺数的增加, 流线变得越来越紊乱; 当雷诺数在220以上时, 球床内漩涡在尺寸变化上出现突跃, 在位置和形态变化表现出随机特征, 预示进入了稳定的湍流.     

Visualization and PIV measurement of the flow around and inside of a falling droplet

Although the phenomena related to the multiphase flow can be found in many kinds of industrial and engineering applications, the physical mechanism of the multiphase flow has not been investigated in detail. The major reason for the lack of data in the multiphase flow lies in the difficulties in measuring the flow quantities of the multiple phases simultaneously. Presently, the visualization and the PIV measurement have been carried out about the both phases of the liquid-liquid two-phase flow. The difference in the refractive indices makes the visualization in the vicinity of the boundary of the multiple phases very difficult. In this study, the refractive index of the aqueous phase has been equalized to that of the oil phase by adjusting the concentration of the aqueous solution. As for the surrounding fluid, silicon oil is chosen and as for the droplet, the aqueous solution of glycerol is prepared whose refractive index matches that of silicon oil. Both phases are seeded with neutrally buoyant particles. The droplet is slightly colored with Rhodamine B so that the position of the invisible droplet can be identified. The difference in the background brightness in both phases helps PIV algorithm in distinguishing the motions in each phases. The results show the details of the flow structures both around and inside of a falling droplet simultaneously.     

Dynamic PIV measurement of a high-speed flow issuing from vent-holes of a curtain-type airbag

A curtain-type airbag is a safety device designed to protect passengers from the side collisions of a car. The curtain-type airbag system consists of an inflator, a fill-hose, and a curtain-airbag. The fill-hose is a passageway and distributor of the exploded gases from the inflator to the airbag through vent-holes. Although the design of vent-holes is important for proper deployment of the airbag, it is very difficult to measure the exceedingly high speed flow issuing from the vent-holes by using conventional measurement methods. In this study, we employed a dynamic PIV technique to measure the temporal evolution of instantaneous velocity fields of the flow ejecting from the vent-holes. From the velocity field data measured at a frame rate of 2000 fps, the temporal variation of the volume flux from vent-holes was also evaluated for the diagnosis of airbag performance. The flows ejecting from the vent-holes showed high velocity fluctuations, and the maximum velocity was about 480 m/s. The instantaneous velocity fields in the initial stage showed a swaying motion of a high-speed jet. The accumulated volume flux from the vent-holes was also compared at each vent-hole region.     

Self-sustained oscillation of the flow in a double-cavity channel: a time-resolved PIV measurement

Journal of Visualization - This study investigates self-sustained oscillation of the flow in a double-cavity channel with cavity length–width ratio L/H?=?3 using a time-resolved...     

Flow visualization and PIV measurement of flow field around a darrieus rotor in dynamic stall

The flow field around a Darrieus rotor in dynamic stall is studied by flow visualization and PIV measurements. The visualization is carried out by dye injection technique while the phase averaged velocity distributions around the blade are measured by PIV combined with a conditional imaging technique. The results indicate the appearance of dynamic stall phenomenon due to the shedding of two pairs of vortices from the blade during one rotation of the rotor. These stall vortices are produced by the separation of flow over the inner surface of the blade and the formation of roll-up vortices from the outer surface. The second stall vortices develop along the blade and strongly interact with the flow field near the blade, affecting the aerodynamic performance of the rotor.     

SPRITE MRI of bubbly flow in a horizontal pipe

Bubble flow is characterised by numerous phase interfaces and turbulence, leading to fast magnetic resonance signal decay and artefacts in spin-warp imaging. In this paper, the SPRITE MRI pulse sequence, with its potential for very short encoding times, is demonstrated as an ideal technique for studying such dynamic systems. It has been used to acquire liquid velocity and relative intensity maps of two-phase gas–liquid dispersed bubble flow in a horizontal pipe at a liquid Reynolds number of 14,500. The fluids were air and water and a turbulence grid was used to generate a dispersed bubble flow pattern. The SPRITE technique shows promise for future research in gas–liquid flow.     

Gradient-based PIV using neural networks

This paper proposes a new gradient-based PIV using an artificial neural network for acquiring the characteristics of a two-dimensional flow field. The neural network can effectively realize an accurate approximation of the vector field by introducing some knowledge on the characteristic property. The neural network is trained by using spatial and temporal image gradients so that the basic equation of the gradient-based method is satisfied. Since the neural network itself learns the stream function, the continuity equation of flow is consequently satisfied in the measured velocity vector field. The new gradient-based PIV can be applied to even partly lacking visualized images.     

An experimental study on swirling flow in a cylindrical annuli using the PIV technique

Abstract  

An experimental investigation was conducted to study the characteristics of turbulent swirling flow in an axisymmetric annuli. The swirl angle measurements were performed using a flow visualization technique using smoke and dye liquid for Re = 60,000–80,000. Using the two-dimensional particle image velocimetry method, this study found the time-mean velocity distribution and turbulent intensity in water with swirl for Re = 20,000, 30,000, and 40,000 along longitudinal sections. There were neutral points for equal axial velocity at y/(R − r) = 0.7–0.75, and the highest axial velocity was recorded near y/(R − r) = 0.9. Negative axial velocity was observed near the convex tube along X/(D − d) = 3–23.     

Improvement of ventilation flow inside a large factory building using PIV velocity field measurements

Air movement in workplaces, whether resulting from a forced ventilation system or natural airflow, has a significant impact on occupational health. In a huge building of shipbuilding factory, typical harmful factors such as fume or vaporized gas from welding and cutting of steel plates give an unpleasant feeling. From field data survey, the yearly dominant wind directions around the factory building tested were north-west, north-east and south-east. Among the three wind directions, the ventilation improvement was the worst for the north-eastern wind. This study was focused on modification of opening vents in order to utilize the natural ventilation flow effectively. Instantaneous velocity fields inside the 1/1000 scale-down factory building model were measured using a 2-frame cross-correlation PIV method. The factory model was embedded in an atmospheric boundary layer simulated in a wind tunnel. The modified vents improved the internal ventilation flow with increasing the flow speed more than two times, compared with that of present vents.     

Investigations of three-dimensional flow characteristics in a liquid ramjet combustor using the PIV method

Three-dimensional flow characteristics in a liquid fuel ramjet combustor were investigated using the PIV method. The combustor had two rectangular inlets that form a 90-degree angle with each other, with intake angles of 30 degrees. Three guide vanes were installed in each rectangular inlet to improve flow stability. The experiments were performed in a water tunnel test with the same Reynolds number as Mach 0.3 at the inlet. PIV software was developed to measure the characteristics of the flow field in the combustor. Accuracy of the developed PIV program was verified with a rotating disk experiment and standard data. The experimental results showed that the two main streams from the rectangular intakes collided near the plane of symmetry and generated two large longitudinal vortices, which was in agreement with three dimensional computational results. A large and complex threedimensional recirculating flow was measured behind the intakes.     

Underwater CCD cameras for PIV flow mapping

Accurate estimation of the forces imposed on offshore structures due to wave and current loading has become more critical due to the introduction of floating production platforms. Work being carried at both Edinburgh and Glasgow Universities aims to measure these effects by force transducer measurements and Particle Image Velocimetry (PIV). A multiple CCD array system is presented which can directly measure accelerations and hence forces acting on offshore structures.