AN EXPERIMENTAL STUDY ON TURBULENT COHERENT STRUCTURES NEAR A SHEARED AIR-WATER INTERFACE

The turbulence structures near a sheared air-water interface were experimentally investigated with the hydrogen bubble visualization technique. Surface shear was imposed by an airflow over the water flow which was kept free from surface waves. Results show that the wind shear has the main influence on coherent structures under air-water interfaces. Low- and high- speed streaks form in the region close to the interface as a result of the imposed shear stress. When a certain airflow velocity is reached, “turbulent spots” appear randomly at low-speed streaks with some characteristics of hairpin vortices. At even higher shear rates, the flow near the interface is dominated primarily by intermittent bursting events. The coherent structures observed near sheared air-water interfaces show qualitative similarities with those occurring in near-wall turbulence. However, a few distinctive phenomena were also observed, including the fluctuating thickness of the instantaneous boundary layer and vertical vortices in bursting processes, which appear to be associated with the characteristics of air-water interfaces.

SOME CHARACTERISTICS OF LOW-SPEED STREAKS UNDER SHEARED AIR-WATER INTERFACES

The characteristics of low-speed fluid streaks occurring under sheared air-water interfaces were examined by means of hydrogen bubble visualization technique. A critical shear condition under which the streaky structure first appears was determined to be u(tau) approximate to 0.19 cm/s. The mean spanwise streak spacing increases with distance from the water surface owing to merging and bursting processes, and a linear relationship describing variation of non-dimensional spacing <(<lambda>+)over bar> versus y(+) was found essentially independent of shear stress on the interface. Values of <(<lambda>+)over bar>, however, are remarkably smaller than their counterparts in the near-wall region of turbulent boundary layers. Though low-speed streaks occur randomly in time and space, the streak spacing exhibits a lognormal probability distribution behavior. A tentative explanation concerning the formation of streaky structure is suggested, and the fact that <(<lambda>+)over bar> takes rather smaller values than that in wall turbulence is briefly discussed.

PREPARATION OF PERFLUOROOCTANOYL-MODIFIED POLY(VINYL-ALCOHOL)S AND THEIR ADSORPTION AT AN AIR-WATER INTERFACE

Perfluorooctanoyl modified poly(vinyl alcohol)s (FPVA) were prepared by means of substituting a small amount of hydroxyl groups on the backbone of poly(vinyl alcohol), for which the initial degree of polymerization is equal to 1750. The substitution extent, defined by the number of substituting units in a chain, for the four FPVA samples was in the range of 0.5-5 perfluorooctanoyl groups per chain. The FPVA samples with the highest substitution extent still had good solubility in water. It was shown by experimental measurement at 30.0 +/- 0.1 degreesC that the surface tension of the aqueous solution of the highest substituted FPVA decreased to 16.6 mN/m at a higher concentration, e.g. about 0.1 g/mL. Obviously, macromolecules of FPVA exhibit a very strong tendency to adsorb at the air-water interface, because the hydrophobic perfluorooctanoyl groups in FPVA have a very high surface activity as they are in small molecular fluorinated surfactants. The chain conformation of such a model polymer adsorbed on the air-water interface was also discussed.

A COUPLING THREE DIMENSIONAL BAROCLINIC MODEL OF BIWA LAKE AIR-WATER SYSTEM

A numerical model coupling atmosphere with hydrodynamics is set up in this paper, and is applied in the experimental study of Lake Biwa. Some results are obtained as : (1) whatever (SSW) in sunimer or (NNW) in winer, there exists a positive wind-stress curl over a lake; (2) in summer the positive wind-stress curi plays an important role to form circulation in a lake and produces a special temperature field corresponding to circulation, lower in the deep water, higher in the shallow water; (3) in summer, the hypothesis of initial horizontal inhomogeneous water temperature has little effiect on the results of simulation, and (4) in winter,there is no obvious circulation formed in the lake.

Characteristics of air-water flow in an emptying tank under different conditions

This paper experimentally studied the features of air-water flow during the emptying of a water-filled prismatic tank with a bottom orifice under different conditions.The experiments were conducted with both circular and elliptical orifices,with and without ventilation.The evolution of bubbles,water pressure variation,and water level change with time were recorded in the experiments and analyzed.Based on the results,the evolution of bubbles could be mainly divided into three stages of formation,deformation,and decomposition.Ventilation was found important to the emptying process,with which the drainage efficiency was much higher than that under the unventilated condition.Additionally,under the unventilated condition,the drainage efficiency with the circular orifice was slightly higher than that with the elliptical orifice.

KINETIC STUDY ON PHOTOISOMERIZATION OF AN AZO-CONTAINING POLYMER AT THE AIR-WATER INTERFACE USING A IANGMUIR TROUGH

Photoisomerization of a new polymer: monoesters of polymaleic acid containing naphthalene-azo-anthraquinone groups in side chains at the air-water interface of a Langmuir trough wan detected by recording the development of surface pressure with time. This process wan found to be a first-order reaction and complete within several minutes.

Forces Acting on Submillimeter Spheres at the Air-Water Interface

The forces acting on submillimeter spheres at the air-water interface are investigated theoretically and experimentally. To calculate the capillary force acting on the sphere, an iterative method is used to determine the immersing position of the liquid interface on the sphere for a given distance. Then the total forces acting on the sphere are considered. The scaling effects of the net force acting on the sphere at the air-water interface are demonstrated. For the experiments, the force-position relationship of microspheres is measured with a precise electronic balance. The results show that the evaporation of the liquid in the container affects the measuring results greatly under ambient conditions. After considering the evaporation compensation, there is a great agree- ment between the theoretical and experimental results. Obvious hysteresis phenomena of the force-distance curve during the emersion processes are also observed and explained.

Tuning Polymer-Grafted Particle Monolayer Structure at the Air-Water Interface by Introducing Anisotropic Features

Fabrication of anisotropic material is one of the important topics and we examined to introduce “anisotropic” nature by spreading polymer-grafted particle on the medium with polymer-reactive potential. Poly (tert-butyl methacrylate) (PtBMA) was polymerized from polystyrene latex (PSL) surface by ATRP to give PtBMA-grafted PSL (PSL-PtBMA). Particle monolayer was formed at air-water and air-acidic water interfaces and the monolayer characteristics were compared by π-A isotherm measurements, SEM observations, and contact angle measurements. π-A isotherms, in particular, indicates that the interaction between polymer chains become stronger by changing the subphase condition, which suggests that anisotropicparticle monolayer formation.

Physical modelling and scale effects of air-water flows on stepped spillways

During the last three decades, the introduction of new construction materials (e.g. RCC (Roller Compacted Concrete), strengthened gabions) has increased the interest for stepped channels and spillways. However stepped chute hydraulics is not simple, because of different flow regimes and importantly because of very-strong interactions between entrained air and turbu- lence. In this study, new air-water flow measurements were conducted in two large-size stepped chute facilities with two step heights in each facility to study experimental distortion caused by scale effects and the soundness of result extrapolation to pro- totypes. Experimental data included distributions of air concentration, air-water flow velocity, bubble frequency, bubble chord length and air-water flow turbulence intensity. For a Froude similitude, the results implied that scale effects were observed in both facilities, although the geometric scaling ratio was only Lr=2 in each case. The selection of the criterion for scale effects is a critical issue. For example, major differences (i.e. scale effects) were observed in terms of bubble chord sizes and turbulence levels al- though little scale effects were seen in terms of void fraction and velocity distributions. Overall the findings emphasize that physical modelling of stepped chutes based upon a Froude similitude is more sensitive to scale effects than classical smooth-invert chute studies, and this is consistent with basic dimensional analysis developed herein.

Carbon dioxide partial pressure and carbon fluxes of air-water interface in Taihu Lake, China

To obtain carbon dioxide (CO2) flux between water-air interface of Taihu lake, monthly water samplers at 14 sites and the local meteorological data of the lake were collected and analyzed in 1998. Carbon dioxide partial pressures (pCO2) at air-water interface in the lake were calculated using alkalinity, pH, ionic strength, active coefficient, and water temperature. The carbon fluxes at different sublakes and areas were estimated by concentration gradient between water and air in consideration of Schmidt numbers of 600 and daily mean windspeed at 10 m above water surface. The results indicated that the mean values of pCO2 in Wuli Lake, Meiliang Bay, hydrophyte area, west littoral zone, riverine mouths, and the open lake areas were 1 807.8±1 071.4 (mean±standard deviation) μatm (1atm=1.013 25×105Pa), 416.3±217.0 μatm, 576.5±758.8 μatm, 304.2±243.5 μatm, 1 933.6±1 144.7 μatm, and 448.5±202.6 μatm, respectively. Maximum and minimum pCO2 values were found in the hypertrophic (4 053.7 μatm) and the eutrophic (3.2 μatm) areas. The riverine mouth areas have the maximum fluxes (82.0±62.8 mmol/m2a). But there was no significant difference between eutrophic and mesot- rophic areas in pCO2 and the flux of CO2. The hydrophyte area, however, has the minimum (-0.58±12.9 mmol/m2a). In respect to CO2 equilibrium, input of the rivers will obviously influence inorganic carbon distribu- tion in the riverine estuary. For example, the annual mean CO2 flux in Zhihugang River estuary was 19 times of that in Meiliang Bay, although the former is only a part of the latter. The sites in the body of the lake show a clear seasonal cycle with pCO2 higher than atmospheric equilibrium in winter, and much lower than atmospheric in summer due to CO2 consumption by photosynthesis. The CO2 amount of the net annual evasion that enters the atmosphere is 28.42×104 t/a, of which those from the west littoral zone and the open lake account for 53.8% and 36.7%, respectively.

Numerical Investigation of Transmission of Low Frequency Sound Through a Smooth Air-water Interface

到空气的从水的健全传播由于在空气和水阻抗之间的一个大对比是很弱的，这是传统的信仰。最近，提高的健全传播和空气水接口的异常透明性被介绍了。声音由在深度局部性的沉没浅深度单极点来源产生了的空气水接口状态的异常透明性不到 1/10 声音波长，能与全向的模式被播送进空气。产生声音与古典光线理论预言相比有高 35 倍的力量。在这份报纸，通过空气水接口鸣传播为一局部性在水下浅深度来源被检查。完成这，在空气水的二阶段的媒介的二阶段的联合 Helmholtz 波浪方程被商业有限元素解决基于的 COMSOL Multiphysics 软件。不同健全来源在的压力振幅的比率二不同在水下并且空气坐标对来源深度(D) 的非维的比率被计算并且分析到健全波长() 。获得的结果与试验性的数据相比，好同意被显示。

Experimental and numerical simulation of air-water two-phase flow in the rod bundle with grid spacer

Experimental and numerical simulation were carried out on vertically upward air-water two-phase flow in the rod bundle with grid spacer. The related numerical simulation has been performed by using the Computational Fluid Dynamics code-CFX4.2, in which lateral interfacial effects based on a two-fluid model are accounted for. This model has been used to evaluate the velocity fields of gas and liquid phases, as well as phase distribution between elements in rod bundle by simulating 1/4 zone of experimental model, and mixing vanes of spacer in this area. Fur- thermore, this model has been used to predict the effects of spacer on flow and pressure drop along the rod bundle. The calculation results show that the mixing vane has significant influence on axial and lateral velocity. In order to obtain some experimental data to verify the numerical solutions, a series of tests, using a specially designed 3×3 rod bundle test section with AFA-2G structure spacer have been performed. An optical probe was used to measure local void fractions. At the same time, the pressure loss has been measured. A comparison between the calculated void pro- file and pressure loss and the measured results shows that the predicted void profiles are consistent at low gas appar- ent velocity. This research shows that the code CFX4.2 can be used to describe the 3-D air-water two-phase flow in the rod bundle channel with grid spacer.

Liquid mean velocity and turbulence in a horizontal air-water bubbly flow

The liquid phase turbulent structure of an air-water bubbly horizontal flow in a circular pipe has been investigated experimentally. Three-dimensional measurements were implemented with two "X" type probes oriented in different planes, and local liquid-phase velocities and turbulent stresses were simultaneously obtained. Systematic measurements were conducted covering a range of local void fraction from 0 to 11.7%. The important experiment results and parametric trends are summarized and discussed.

Multiple evaluations of atmospheric behavior between Criegee intermediates and HCHO: Gas-phase and air-water interface reaction

Given the high abundance of water in the atmosphere,the reaction of Criegee intermediates(CIs)with(H_(2)O)_(2) is considered to be the predominant removal pathway for CIs.However,recent experimental findings reported that the reactions of CIs with organic acids and carbonyls are faster than expected.At the same time,the interface behavior between CIs and carbonyls has not been reported so far.Here,the gas-phase and air-water interface behavior between Criegee intermediates and HCHO were explored by adopting high-level quantum chemical calculations and Born-Oppenheimer molecular dynamics(BOMD)simulations.Quantum chemical calculations evidence that the gas-phase reactions of CIs+HCHO are submerged energy or low energy barriers processes.The rate ratios speculate that the HCHO could be not only a significant tropospheric scavenger of CIs,but also an inhibitor in the oxidizing ability of CIs on SO_(x) in dry and highly polluted areas with abundant HCHO concentration.The reactions of CH_(2)OO with HCHO at the droplet’s surface follow a loop structure mechanism to produce i)SOZ(■),ii)BHMP(HOCH_(2)OOCH_(2)OH),and iii)HMHP(HOCH_(2)OOH).Considering the harsh reaction conditions between CIs and HCHO at the interface(i.e.,the two molecules must be sufficiently close to each other),the hydration of CIs is still their main atmospheric loss pathway.These results could help us get a better interpretation of the underlying CIs-aldehydes chemical processes in the global polluted urban atmospheres.

Numerical simulation of air-water two-phase flow over stepped spillways

Stepped spillways for significant energy dissipation along the chute have gained interest and popularity among researchers and dam engineers. Due to the com- plexity of air-water two-phase flow over stepped spillways, the finite volume computational fluid dynamics module of the FLUENT software was used to simulate the main characteristics of the flow. Adopting the RNG k-ε turbulence model, the mixture flow model for air-water two-phase flow was used to simulate the flow field over stepped spillway with the PISO arithmetic technique. The numerical result successfully reproduced the complex flow over a stepped spillway of an experiment case, including the interaction between entrained air bubbles and cavity recirculation in the skimming flow regime, velocity distribution and the pressure profiles on the step surface as well. The result is helpful for under-standing the detailed information about energy dissipation over stepped spillways.

Experimental study of air-water interface properties in self-aerated flows

The microscopic air-water structures and the interface area properties in the self-aerated flows are important research issues in the high-speed self-aerated flows.The present experimental study concerns the mean and medium air chord length distributions in the self-aerated chute flows for different flow Reynolds numbers and air concentration conditions.The relationship between the microscopic and the macroscopic aerated properties in the air-water mixture region is analyzed.The distribution of the microscopic specific air-water interface area with the macroscopic air concentration variation is self-similar in the self-aerated region.In view of the difference of the air-water structure in high and low aerated regions,a new relationship is proposed for predicting the distributions of a specific air-water interface area,and the agreement between the measured and predicted results is satisfactory.

In-situ measurement on air-water flux of CH_(4),CO_(2) and their carbon stable isotope in lakes of northeast Tibetan Plateau

Inland waters are important sources of greenhouse gases(GHGs)to the atmosphere that may partially offset the terrestrial carbon sink.However,GHG emissions from high altitude saline lakes on the Tibetan Plateau are currently poorly constrained.In this study,we measured methane(CH_(4))and carbon dioxide(CO_(2))concentrations and their carbon stable isotope(δ13C),and calculated diffusive water-to-air fluxes from two saline high-altitude lakes,Qinghai Lake and Hala Lake,located in northeast Tibetan Plateau in October 2020.The two lakes were mostly supersaturated in CH_(4)(89.8%-4623.9%sat)and acted as sources of CH_(4) to the atmosphere.Conversely,more than 96%investigated area of Qinghai Lake and all investigated area of Hala Lake was a sink of CO_(2).The average diffusive fluxes of CH_(4) and CO_(2) in the surface waters of Qinghai Lake were 34.51μmol m−2 d−1 and−1.29 mmol m−2 d−1,while diffusive fluxes of CH_(4) and CO_(2) in Hala Lake averaged 5.48μmol m−2 d−1 and−5.24 mmol m−2 d−1,respectively.Salinity(Sal),fluorescent dissolved organic matter(fDOM),temperature(Temp)and dissolved oxygen(DO)are key factors for diffusive CH_(4) and CO_(2) flux in Qinghai Lake and Hala Lake.CH_(4) diffusive fluxes across water-air interface from Qinghai Lake and Hala Lake were found to be significantly lower than other freshwater lakes,likely due to their saline nature.A simple isotope mixing model(Miller-Tans plots)revealed a distinctly different isotopic source values of CO_(2) and CH_(4) in Qinghai Lake and Hala Lake.The CH_(4) production pathway of Qinghai Lake was mainly by the acetate fermentation,while that of Hala Lake was mainly by the CO_(2) reduction.The results show that the continuous measurement can help capture the spatial variability of GHGs fluxes in saline lakes.

Research on measurement-device-independent quantum key distribution based on an air-water channel

A measurement-device-independent quantum key distribution(MDI-QKD) method with an air-water channel is researched. In this method, the underwater vehicle and satellite are the legitimate parties, and the third party is at the airwater interface in order to simplify the unilateral quantum channel to water or air. Considering the condition that both unilateral transmission distance and transmission loss coefficient are unequal, a perfect model of the asymmetric channel is built. The influence of asymmetric channel on system loss tolerance and secure transmission distance is analyzed. The simulation results show that with the increase of the channel's asymmetric degree, the system loss tolerance will descend, one transmission distance will be reduced while the other will be increased. When the asymmetric coefficient of channel is between 0.068 and 0.171, MDI-QKD can satisfy the demand of QKD with an air-water channel, namely the underwater transmission distance and atmospheric transmission distance are not less than 60 m and 12 km, respectively.