A polydisperse two-phase flow model is developed and used to analyze the effect of the bubble size on the radial phase distribution in vertical upward channels. The two-fluid model is evaluated ...considering that the bubble size distribution can be represented with groups of constant mass. The model accounts for interfacial momentum transfer terms arising from drag, lift, turbulent dispersion and wall forces for the different bubble sizes. The turbulence is modeled with the
k–
ε model for bubbly flow. A two-phase wall logarithmic law is developed to evaluate the boundary conditions for the
k–
ε and the two-fluid models. The turbulence in the buffer and laminar near-wall regions is evaluated considering the asymptotic consistency of the
k–
ε model approaching the solid surface. The model is able to predict the transition from the near-wall gas volume fraction peaking to the core peaking beyond a critical bubble size. The double gas volume fraction peak experimentally observed when both, small and big bubbles, are present can be also simulated. The model was numerically solved for fully developed flow by means of a finite difference method and the results were compared against the experimental data measured by others in air/water vertical ducts.
Elevated levels of dissolved gas in the spillway stilling basin, which are responsible for gas bubble disease in fish, constitute an important negative environmental effect of dams. Bubbles, ...entrained when a plunging jet impacts the tailwater pool, plunge beneath the surface and transfer mass to the liquid, causing an increase in the total dissolved gas (TDG) concentration. Most of the numerical studies onTDG downstream of spillways found in the literature are based on experimental correlations for the gas volume fraction.Abetter approach involves the use of a two-phase flowmodel. In this paper, a two-fluid model is used to calculate the gas volume fraction and velocity of the bubbles. A polydisperse model is used in which a Boltzmann transport equation predicts the bubble size distribution, to account for the different bubble sizes found in the flow downstream of spillways. The bubble mass is discretized considering groups of bubbles of variable mass, with the mass of the bubbles changing due to bubble/liquid mass transfer and pressure. A two-phase transport equation for the TDG is presented, whose source is the bubble/liquid mass transfer, which is a function of the gas volume fraction and bubble size distribution. Two-dimensional numerical results of TDG, gas volume fraction, bubble number density, and velocities are presented and discussed. The predictions of TDG downstream of a spillway are compared against field data in the stilling basin ofWanapum Dam, on the Columbia River.
After more than a century of intensive changes in the state's agricultural watersheds, repeated record floods motivated Iowa to innovate in its flood recovery and disaster mitigation efforts ...following the 2008 floods. The state created the Iowa Flood Center (IFC) and authorized the creation of Watershed Management Authorities. With enhanced funding from the federal government for disaster recovery, the groups collaborated in four watersheds as part of the Iowa Watersheds Project. The Watershed Management Authorities brought together governmental organizations and local stakeholders, developed watershed plans, defined potential flood and nutrient reduction projects, identified willing landowners for project locations in one subwatershed, and hired consultants to design and build the projects. The IFC coordinated efforts across the four watershed areas, provided hydrologic assessments for each, carried out monitoring and modelling to evaluate the performance of all the constructed projects, and developed water resources information systems to support ongoing activities. The Iowa Watersheds Project adopted a watershed systems approach, and the resulting synergy between engaged communities and technical professionals demonstrates avenues for implementing elements of an Integrated Water Resources Management framework in a highly decentralized setting.
Results of an experimental study of a two-phase jet are presented, with the jet issued near and below a free surface, parallel to it. The jet under study is isothermal and in fresh water, with air ...injectors that allow variation of the inlet air volume fraction between 0 and 13 %. Measurements of water velocity have been performed using LDV, and the jet exit conditions measured with PIV. Air volume fraction, bubble velocity and chord length distributions were measured with sapphire optical local phase detection probes. The mean free surface elevation and RMS fluctuations were obtained using local phase detection probes as well. Visualization was performed with laser-induced fluorescence. Measurements reveal that the mean free surface elevation and turbulent fluctuations significantly increase with the injection of air. The water normal Reynolds stresses are damped by the presence of bubbles in the bulk of the liquid, but very close to the free surface the effect is reversed and the normal Reynolds stresses increase slightly for the bubbly flow. The Reynolds shear stresses
decrease when bubbles are injected, indicating turbulence attenuation, and are negative at deeper locations, as turbulent eddies shed downward carry high axial momentum deeper into the flow. Flow visualization reveals that the two-phase jet is lifted with the presence of bubbles and reaches the free surface sooner. Significant bubble coalescence is observed, leading to an increase in mean bubble size as the jet develops. The coalescence near the free surface is particularly strong, due to the time it takes the bubbles to pierce the free surface, resulting in a considerable increase in the local air volume fraction. In addition to first explore a bubbly surface jet, the comprehensive dataset reported herein can be used to validate two-phase flow models and computational tools.
A novel approach for designing the next generation of vertex detectors foresees to employ wafer-scale sensors that can be bent to truly cylindrical geometries after thinning them to thicknesses of ...20–40 μm. To solidify this concept, the feasibility of operating bent MAPS was demonstrated using 1.5cm×3cm ALPIDE chips. Already with their thickness of 50µm, they can be successfully bent to radii of about 2cm without any signs of mechanical or electrical damage. During a subsequent characterisation using a 5.4GeV electron beam, it was further confirmed that they preserve their full electrical functionality as well as particle detection performance.
In this article, the bending procedure and the setup used for characterisation are detailed. Furthermore, the analysis of the beam test, including the measurement of the detection efficiency as a function of beam position and local inclination angle, is discussed. The results show that the sensors maintain their excellent performance after bending to radii of 2cm, with detection efficiencies above 99.9% at typical operating conditions, paving the way towards a new class of detectors with unprecedented low material budget and ideal geometrical properties.
Water entrainment due to spillway surface jets Turan, C.; Politano, M. S.; Carrica, P. M. ...
International journal of computational fluid dynamics,
20/3/1/, Letnik:
21, Številka:
3-4
Journal Article
Recenzirano
Strong flow entrainment has been observed downstream of spillways constructed with flow deflectors. This water entrainment has important environmental and ecological impacts because it improves the ...mixing of powerhouse and spillway flows, but may negatively impact fish migration or create adverse flow conditions.
Most studies found in the literature attempt to explain this entrainment with turbulent mixing. Both reduced-scale hydraulic models and single-phase, isotropic RANS models grossly under-predict the degree of entrainment observed in prototypes. In this paper, an anisotropic model that accounts for the bubble volume fraction and attenuation of the normal velocity fluctuations at the free surface is presented. The model adequately predicts the main mechanisms causing water entrainment and compares well against experimental data for a round surface jet and for Brownlee Dam at model scale. It is shown that appropriate entrainment can only be captured if the turbulence anisotropy and the two-phase nature of the flow are modelled.
Bismuth teUuride （Bi2Te3） is one of the most important commercial thermoelectric materials. In recent years, the discovery of topologically protected surface states in Bi chalcogenides has paved the ...way for their application in nanoelectronics. Determination of the fracture toughness plays a crucial role for the potential application of topological insulators in flexible electronics and nanoelectro- mechanical devices. Using depth-sensing nanoindentation tests, we investigated for the first time the fracture toughness of bulk single crystals of Bi2Te3 topological insulators, grown using the Bridgmantockbarger method. Our results highlight one of the possible pitfalls of the technology based on topological insulators.
Peak expiratory flow (PEF) variation was measured in 18 healthy children (Group I) aged 9.25 to 13.3 years (mean, 11 years) and in 17 asymptomatic asthmatic children (Group II) aged 8.75 to 14.25 ...years (mean, 11.9 years) receiving no bronchodilators. PEF was measured four times a day for a period of 10-14 days. There were no differences between Group I and Group II regarding age, height, weight, sex, duration of the study, and overall mean PEF values. Mean daily PEF variation amplitude (difference between the highest and the lowest daily values as a percent of the mean) was significantly higher in Group II than in Group I subjects (15.2 vs. 9.9%), and so was cosinor amplitude (9.6 vs. 5.9%). Significant circadian rhythm in PEF was detected by cosinor analysis in nine subjects of Group I and in eight subjects of Group II. Mean PEF bathyphase calculated by cosinor analysis (trough time in a 24 h scale) was similar in the two groups (0503 h, in Group I and 0434 h in Group II). The results of this study show that the amplitude of circadian variation in PEF tends to be higher in asymptomatic asthmatic children than in normal children, but there is too much overlap to use PEF monitoring to identify an individual asymptomatic asthmatic child. Therefore, diurnal variation of PEF is not suitable for diagnosing mild asthmatics.
We present results of a study of the equilibrium between coalescence and breakup of bubbles in homogeneous media with isotropic turbulence. The Boltzmann equation for the particle distribution ...function (pdf) was evaluated in steady state, using a multigroup approach. Binary bubble breakup was assumed. We used uniform function, delta function, and the model proposed by Luo and Svendsen (1996) for the bubble size distributions resulting from a breakup. The bubble breakup rate was calculated with Luo and Svendsen (1996) and Prince and Blanch (1990) models. Significant differences in bubble breakup rate, and therefore in bubble size distribution, are predicted by both models. The models were compared to the bubble size distributions measured by Boyd and Varley (1998) in air-water flow. The transient response of the bubble size distribution and interfacial area density was also analyzed. This work is of significance in the prediction of reaction rates when they are dependent on bubble size distribution.
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