Bubble migration in a turbulent boundary layer

The wall void peaking distribution observed in an upward turbulent bubbly boundary layer along a flat plate is generated by bubbles that move towards the plate, come into contact with the wall and then slide along it. This transverse ‘migration’ has been studied using flow visualization, high speed video and particle tracking techniques to measure the trajectories of mono-disperse air bubbles at very low void fractions. Investigations have been performed at four Reynolds numbers in the range 280 < Re_θ < 3000, covering both the laminar and turbulent regimes, with mono-disperse bubbles of mean equivalent diameter between 2 mm and 6 mm. Lagrangian statistics calculated from hundreds of trajectories show that the migration only occurs in the turbulent regime and for bubble diameters below some critical value: 3.5 mm < d_eqcrit < 4 mm. Above this size (We > 3), the interface deformation is such that bubbles do not remain at the wall, even when they are released at the surface. Also, bubble migration is shown to be non-systematic, to have a non-deterministic character in the sense that trajectories differ significantly, to increase with Reynolds number and to take place on a short time scale. A series of experiments with isolated bubbles demonstrates that these results are not influenced by bubble–bubble interactions and confirm that two-way coupling in the flow is limited. Flow visualizations show that the migration originates with the capture of bubbles inside the large turbulent structures of the boundary layer (‘bulges’). The bubbles begin to move towards the wall as they cross these structures, and the point at which they reach the wall is strongly correlated with the position of the deep ‘valleys’ which separate the turbulent ‘bulges’. The analysis of the mean Lagrangian trajectories of migrating bubbles confirms these observations. Firstly, the average time of migration calculated from these trajectories coincides with the mean transit time of the bubbles across the structures. Secondly, once the trajectories have been scaled by this transit time and the boundary layer thickness δ, they all have the same form in the region y/δ < 0.4, independent of the Reynolds number.     

An experimental study on the effect of air bubble injection on the flow induced rotational hub

Modification of shear stress due to air bubbles injection in a rotary device was investigated experimentally. Air bubbles inject to the water flow crosses the neighbor of the hub which can rotate just by water flow shear stresses, in this device. Increasing air void fraction leads to decrease of shear stresses exerted on the hub surface until in high void fractions, the hub motion stopped as observed. Amount of skin friction decrease has been estimated by counting central hub rotations. Wall shear stress was decreased by bubble injection in all range of tested Reynolds number, changing from 50,378 to 71,238, and also by increasing air void fraction from zero to 3.06%. Skin friction reduction more than 85% was achieved in this study as maximum measured volume of air fraction injected to fluid flow while bubbles are distinct and they do not make a gas layer. Significant skin friction reduction obtained in this special case indicate that using small amount of bubble injection causes large amount of skin friction reduction in some rotary parts in the liquid phases like as water.     

用平均速度剖面法测量壁湍流摩擦阻力

用IFA300恒温热线风速仪精细测量风洞中不同雷诺数流动条件下的平板湍流边界层近壁区域对数律平均速度剖面．利用平板湍流边界层近壁区域的对数律平均速度剖面与壁面摩擦速度、流体黏性系数等内尺度物理量的关系和壁面摩擦速度与壁面摩擦切应力的关系,在准确测量平板湍流边界层近壁区域对数律平均速度剖面的基础上,测量平板湍流边界层的壁面摩擦阻力．实现了平板湍流边界层壁面摩擦阻力的无干扰或微小干扰测量．该种方法操作简便,不需要在流场中安装测力天平、传感器等复杂的测量装置,不需要对湍流边界层的壁面进行破坏,不会影响湍流边界层壁面附近区域原有的流场条件,是一种切实可行的测量平板湍流边界层壁面摩擦阻力的简便方法．     

The development of the structure of water – air bubble grid turbulence

The development of the turbulent flow field generated due to the interaction of grid turbulence with a swarm of bubbles is investigated experimentally, in a vertical channel of rectangular cross section. Void fraction and streamwise mean and rms velocity distributions have been measured at several distances from the grid, with an optical probe and Laser Doppler Velocimetry, in relation to the air flow rate ratio. The obtained results indicate that close to the grid the void fraction and velocity distributions are dictated by the bubble injectors’ location on the grid. Downstream the void fraction distribution changes to a double peak pattern. The velocity distribution is characterized by a shear layer between the wall area and the central area of the channel. The extend of this shear layer is increasing as the distance from the grid and the gas flow rate ratio are increasing, and is associated with a corresponding increase of the turbulence fluctuations. Autocorrelation and spectra measurements at the centre of the channel show a reduction of the flow scales for low void fraction. Consistently, power spectra distributions indicate that bubbles cause a redistribution of energy manifested by the relative enhancement of the intermediate scales’ energy content and a consequent reduction in the larger scales. These trends are gradually alleviated and reversed at large distances from the grid, as the air flow rate is increased.     

Robust design and numerical simulation on drag reduction by a mixture film for liquid turbulence in a channel

An important way of increasing the speed and lowering the fuel consumption of ships is by decreasing the frictional drag. One of the most promising techniques for reducing drag is the use of air bubbles. The goal of this investigation is to establish a set of optimum robust parametric levels for drag reduction by a mixture (air–water) film in turbulent channel flow. Based on the conditions laid out by the Taguchi orthogonal array method, turbulent flows, with air bubbles injected into a channel, are simulated using commercial computational fluid dynamics software. The local shear stress on the upper wall is computed to evaluate the efficiency of drag reduction. Many factors can affect drag reduction. The factors investigated in this study are the rate of air injection, bubble size, area of air injection, flow speed, and measured position of the shear stress. These factors have been investigated through the analysis of variance, which has revealed that the rate of air injection and water flow speed dominate the efficiency of drag reduction by a mixture film. According to the results, the drag can be reduced by an average of 83.4%; and when the configuration of the parametric levels is optimum the maximum drag reduction of 88.5% is achieved. Copyright © 2008 John Wiley & Sons, Ltd.     

减阻工况下壁面周期扰动对湍流边界层多尺度的影响

通过在平板壁面施加不同频率振幅的压电陶瓷振子周期性扰动,进行了湍流边界层主动控制减阻的实验研究.在压电陶瓷振子最大减阻工况下(80 V和160Hz),使用单丝边界层探针对压电振子自由端下游2mm处进行测量,得到不同法向位置流向速度信号的时间序列.通过对比施加控制前后的多尺度分析,发现压电振子产生的扰动只对近壁区产生影响,使得近壁区大尺度脉动降低,小尺度脉动强度增大,而对边界层的外区则基本没有影响.进一步对大尺度和小尺度的脉动信号进行条件平均,发现压电振子产生的扰动对小尺度脉动的影响在时间相位上并不均匀,小尺度脉动强度在大尺度脉动为正时比在大尺度脉动为负时具有更明显的增加.这表明壁面周期扰动主要通过使大尺度高速扫掠流体破碎为小尺度结构,来影响相应的高壁面摩擦事件,从而达到减阻效果.      

Turbulent shear stress profiles in a bubbly channel flow assessed by particle tracking velocimetry

Particle tracking velocimetry (PTV) is applied to a bubbly two-phase turbulent flow in a horizontal channel at Re = 2 × 10⁴ to investigate the turbulent shear stress profile which had been altered by the presence of bubbles. Streamwise and vertical velocity components of liquid phase are obtained using a shallow focus imaging method under backlight photography. The size of bubbles injected through a porous plate in the channel ranged from 0.3 to 1.5 mm diameter, and the bubbles show a significant backward slip velocity relative to liquid flow. After bubbles and tracer particles are identified by binarizing the image, velocity of each phase and void fraction are profiled in a downstream region. The turbulent shear stress, which consists of three components in the bubbly two-phase flow, is computed by analysis of PTV data. The result shows that the fluctuation correlation between local void fraction and vertical liquid velocity provides a negative shear stress component which promotes frictional drag reduction in the bubbly two-phase layer. The paper also deals with the source of the negative shear stress considering bubble’s relative motion to liquid.     

输气管道壁面涂料减阻机理的实验研究

用IFA-300热线风速仪以高于对应最小湍流时间尺度的分辨率精细测量了风洞中不同壁面涂料的管道湍流边界层不同法向位置流向速度分量的时间序列信号,利用湍流边界层近壁区域对数律平均速度剖面与壁面摩擦速度、流体黏性系数等内尺度物理量的关系和壁面摩擦速度与壁面摩擦切应力的关系,在准确测量湍流边界层近壁区域对数律平均速度剖面的基础上,间接测量湍流边界层的壁面摩擦阻力．对不同壁面涂料的壁湍流脉动速度信号用子波分析进行多尺度分解,用子波系数的瞬时强度因子和平坦因子检测管道湍流边界层中的多尺度相干结构,提取不同尺度相干结构的条件相位平均波形,对比研究输气管道壁面涂料的减阻机理．     

Bubble-size distributions produced by wall injection of air into flowing freshwater,saltwater and surfactant solutions

As air is injected into a flowing liquid, the resultant bubble characteristics depend on the properties of the injector, near-wall flow, and flowing liquid. Previous research has shown that near-wall bubbles can significantly reduce skin-friction drag. Air was injected into the turbulent boundary layer on a test section wall of a water tunnel containing various concentrations of salt and surfactant (Triton-X-100, Union Carbide). Photographic records show that the mean bubble diameter decreased monotonically with increasing salt and surfactant concentrations. Here, 33 ppt saltwater bubbles had one quarter, and 20 ppm Triton-X-100 bubbles had one half of the mean diameter of freshwater bubbles.     

Influence of bubble size on micro-bubble drag reduction

Micro-bubble drag reduction experiments were conducted in a turbulent water channel flow. Compressed nitrogen was used to force flow through a slot injector located in the plate beneath the boundary layer of the tunnel test section. Gas and bubbly mixtures were injected into a turbulent boundary layer (TBL), and the resulting friction drag was measured downstream of the injector. Injection into tap water, a surfactant solution (Triton X-100, 20 ppm), and a salt-water solution (35 ppt) yielded bubbles of average diameter 476, 322 and 254 μm, respectively. In addition, lipid stabilized gas bubbles (44 μm) were injected into the boundary layer. Thus, bubbles with d ⁺ values of 200 to 18 were injected. The results indicate that the measured drag reduction by micro-bubbles in a TBL is related strongly to the injected gas volumetric flow rate and the static pressure in the boundary layer, but is essentially independent of the size of the micro-bubbles over the size range tested.     

Three-dimensional CFD simulation of bubble–melt two-phase flow with air injecting and melt stirring

This paper reports on progress in developing CFD simulations of gas bubble–metallic melt turbulent flows induced by a pitched-blade impeller with an inclined shaft. Foaming process of aluminum foams, in which air is injected into molten aluminum composites and the melt is mechanical stirred by the impeller, has been investigated. A two-fluid model, incorporated with the Multiple Reference Frames (MRF) method is used to predict the three-dimensional gas–liquid flow in the foaming tank, in which a stirring shaft is positioned inclined into the melt. Locally average bubble size is also predicted by additively solving a transport equation for the bubble number density function, which accounts for effects of bubble breakup and coalescence phenomena. The computed bubble sizes are compared with experimental data from our water model measurement and reasonable agreements are obtained. Further, simulated results show that the volume averaged total and local gas fractions are generally increased with rising impeller speed and gas flow rate. The local averaged bubble size increases with increasing gas flow rate and orifice diameter and decreasing liquid viscosity, and decreases also with rising rotation speed of the impeller.     

Influence of local ultrasonic forcing on a turbulent boundary layer

An experimental study was carried out to investigate the effect of local ultrasonic forcing on a turbulent boundary layer. The ultrasonic forcing system was constructed by adhering six ultrasonic transducers to a flat plate over which water was flowed. In this system, the ultrasonic waves projected into the water by the transducers caused cavitation, giving rise to an enormous number of tiny water-vapor bubbles. Stereoscopic particle image velocimetry (SPIV) was used to probe the flow characteristics. The SPIV results showed that imposition of the ultrasonic forcing caused a substantial increase in the mean wall-normal velocity but a decrease in the mean streamwise velocity. The ultrasonic forcing reduced the skin friction coefficient by up to 60% immediately downstream of the transducers; this effect gradually dissipated with moving downstream. The streamwise turbulence intensity was reduced near the wall but increased away from the wall, whereas the wall-normal turbulence intensity was not much affected near the wall but increased away from the wall. The Reynolds shear stress and the production of turbulent kinetic energy were reduced near the wall. Imposition of the ultrasonic forcing shifted the streamwise vortical structures away from the wall, leading to a reduction in skin friction.     

CFG桩复合地基沉降影响因素分析

对绕2D水翼无分离流边界层内的初生空化形态进行了实验研究. 采用高速摄像机观测 了空化初生结构的形态，应用2D-LDV测量了空化初生时翼型周围的流动速度分布，并对实 验结果进行了分析. 结果表明：绕水翼无分离边界层内，初生空化结构中空泡伴随 着近壁湍流拟序结构的发生而出现，在初生空化条件下，形成空化涡结构，大量的微空泡产生于发夹 涡结构中，并在涡结构的猝发过程中出现生成---长大---溃灭---反弹---再溃灭的过程. 初生空 化涡结构具有空泡和近壁拟序结构双重特性.     

双振子同异步振动主动控制湍流边界层减阻实验研究

本文以镶嵌在平板上沿展向对放的两个压电陶瓷振子为主动控制激励器,自主设计了零质量射流主动控制湍流边界层减阻实验方案.在风洞中开展了双压电振子同步和异步振动主动控制湍流边界层减阻的实验研究,实现了压电振子的周期扰动对湍流边界层多尺度相干结构的干扰和调制,施加控制后减小了壁面摩擦阻力,获得减阻效果.当异步控制100 V, 160 Hz工况时得到最大减阻率为18.54%.小波多尺度分析结果表明,施加控制工况中PZT振子的周期性扰动使得小尺度结构的湍流脉动强度增强,改变了近壁区大尺度和小尺度结构的含能分布,且异步控制工况比同步控制工况的减阻效果好.当双振子振动频率为160 Hz时,流向脉动速度的小波系数PDF曲线呈现出波动特征,尾部变宽显著,近壁湍流脉动更加有序和规则,湍流间歇性减弱.对小尺度脉动进行条件相位平均的结果表明,施加PZT周期扰动后使得大尺度结构破碎成为小尺度结构,小尺度脉动强度增强,实现减阻.随着流向位置离PZT振子越来越远,周期性扰动对相干结构的调制作用逐渐减弱.     

Simulation of a Large-Eddy-Break-up Device (LEBU) in a Moderate Reynolds Number Turbulent Boundary Layer

A well-resolved large eddy simulation (LES) of a large-eddy break-up (LEBU) device in a spatially evolving turbulent boundary layer is performed with, Reynolds number, based on free-stream velocity and momentum-loss thickness, of R e _θ ≈ 4300. The implementation of the LEBU is via an immersed boundary method. The LEBU is positioned at a wall-normal distance of 0.8 δ (δ denoting the local boundary layer thickness at the location of the LEBU) from the wall. The LEBU acts to delay the growth of the turbulent boundary layer and produces global skin friction reduction beyond 180δ downstream of the LEBU, with a peak local skin friction reduction of approximately 12 %. However, no net drag reduction is found when accounting for the device drag of the LEBU in accordance with the towing tank experiments by Sahlin et al. (Phys. Fluids 31, 2814, 1988). Further investigation is performed on the interactions of high and low momentum bulges with the LEBU and the corresponding output is analysed, showing a ‘break-up’ of these large momentum bulges downstream of the LEBU. In addition, results from the spanwise energy spectra show consistent reduction in energy at spanwise length scales for \(\lambda _{z}^{+} > 1000\) independent of streamwise and wall-normal location when compared to the corresponding turbulent boundary layer without LEBU.     

Determination of skin friction in strong pressure-gradient equilibrium and near-equilibrium turbulent boundary layers

The conventional Clauser-chart method for determination of local skin friction in zero or weak pressure-gradient turbulent boundary layer flows fails entirely in strong pressure-gradient situations. This failure occurs due to the large departure of the mean velocity profile from the universal logarithmic law upon which the conventional Clauser-chart method is based. It is possible to extend this method, even for strong pressure-gradient situations involving equilibrium or near-equilibrium turbulent boundary layers by making use of the so-called non-universal logarithmic laws. These non-universal log laws depend on the local strength of the pressure gradient and may be regarded as perturbations of the universal log law. The present paper shows that the modified Clauser-chart method, so developed, yields quite satisfactory results in terms of estimation of local skin friction in strongly accelerated or retarded equilibrium and near-equilibrium turbulent boundary layers that are not very close to relaminarization or separation.     

Interaction between longitudinal vortices and flat plate boundary layer

The interaction between longitudinal vortices and flat plate boundary layer has been studied numerically for both laminar and turbulent flow situations. The vortices are assumed to be placed in an otherwise two-dimensional boundary layer flow. The flow is assumed to be incompressible and steady. Considering the fact that the velocity, vorticity and temperature gradients in the transverse directions are much larger than the longitudinal (streamwise) gradients for these flows, the original Navier Stokes equations are parabolized in the streamwise direction. A simple model, based on Boussinesq hypothesis, is used for turbulent flow. The discretized equations are then solved step by step in the streamwise direction, using an iterative procedure at each station. Numerical solutions have been obtained for different parameters, such as the Reynolds number, the circulation and the initial position of the vortices. The computed flow patterns and the skin friction coefficient and Stanton number are found to be qualitatively consistent with available experimental results. It is shown that the interaction between the vortices and the boundary layer may severely disturb the boundary layer flow field and thus considerably increase the local skin friction and heat transfer rate on surface of an aircraft.     

Effect of local forcing on a turbulent boundary layer

An experimental study is performed to analyze flow structures behind local suction and blowing in a flat-plate turbulent boundary layer. The local forcing is given to the boundary layer flow by means of a sinusoidally oscillating jet issuing from a thin spanwise slot at the wall. The Reynolds number based on the momentum thickness is about Re _θ =1700. The effects of local forcing are scrutinized by altering the forcing frequency (0.011 ≤ f ⁺≤ 0.044). The forcing amplitude is fixed at A ₀=0.4. It is found that a small local forcing reduces the skin friction and the skin friction reduction increases with the forcing frequency. A phase-averaging technique is employed to capture the large-scale vortex evolution. An organized spanwise vortical structure is generated by the local forcing. The cross-sectional area of vortex and the time fraction of vortex are examined by changing the forcing frequency. An investigation of the random fluctuation components reveals that turbulent energy is concentrated near the center of vortical structures. Received: 17 March 2000/Accepted: 3 April 2001     

Simulating air entrainment and vortex dynamics in a hydraulic jump

The air entrainment characteristics of three separate Froude number hydraulic jumps are investigated numerically using an unsteady RANS, realizable k–ε turbulence model, with a Volume of Fluid treatment for the free surface. Mean velocity profiles, average void fraction, and Sauter mean diameter compare favorably with experimental data reported in literature. In all simulations, time-averaged void fraction profiles show good agreement with experimental values in the turbulent shear layer and an accurate representation of interfacial aeration at the free surface. Sauter mean diameter is well represented in the shear layer, and free surface entrainment results indicate bubble size remains relatively unchanged throughout the depth of the jump. Several different grid resolutions are tested in the simulations. Significant improvements in void fraction and bubble size comparison are seen when the diameter to grid size ratio of the largest bubbles in the shear layer surpasses eight. A three-dimensional simulation is carried out for one Froude number jump, showing an improvement in the prediction of entrained air and bubble size compared with two-dimensional results at a substantial increase in computation time. An analysis of three-dimensional vorticity shows a complex interaction between spanwise and streamwise vortical structures and entrained air bubbles. The jump is similar to a turbulent mixing layer, constrained by the free surface, with vortex pairing and subsequent fluctuations in free surface elevation. Downstream fluctuations of the toe are associated with a roll up of the primary spanwise vortex, fluctuations of the free surface, and counter-rotating streamwise vortex pairs. The action of these flow structures is likely responsible for the improvement in three-dimensional results.     

平板湍流边界层整体摩擦阻力的振荡性

本文分别用流场显示和阻力测量两种实验方法,证明了平板整体摩擦阻力存在着振荡性,并得到了其振荡的一些规律和特点。另外,提出了一个与时间相关联的内层速度分布模型。并对其合理性进行了研究,从而导出一个与时间相关联的整体摩擦阻力模型。