Turbulent Prandtl Number in Neutrally Buoyant Turbulent Round Jet

A method which combines two nonintrusive imaging techniques, particle tracking velocimetry and laser induced fluorescence, was used to make simultaneous measurements of velocity and concentration in a neutrally buoyant turbulent round jet. The measurements were made at two different Reynolds numbers (R), 360 and 4,210, at a Schmidt number of 1,930. The mean velocity 〈u〉, mean concentration 〈c〉, Reynolds stress ?〈u′v′〉, and turbulent scalar flux 〈v′c′〉 were obtained and the eddy viscosity, eddy diffusivity, and turbulent Prandtl number (Prt) calculated from these measurements. Both the low and high Reynolds number results show self-similar characteristics that are dependent on R with Prt a function of radial position. For the R=4,210 case, it was found that 0.70.12. For the R=360 case, it was found that Prt ≈ 0.4 for 0.06相似文献   

Maximum Velocity and Regularities in Open-Channel Flow

Maximum velocity in a channel section often occurs below the water surface. Its location is linked to the ratio of the mean and maximum velocities, velocity distribution parameter, location of mean velocity, energy and momentum coefficients, and probability density function underpinning a velocity distribution equation derived by applying the probability and entropy concepts. The mean value of the ratio of the mean and maximum velocities at a given channel section is stable and constant, and invariant with time and discharge. Its relationship with the others in turn leads to formation of a network of related constants that represent regularities in open-channel flows and can be used to ease discharge measurements and other tasks in hydraulic engineering. Under the probability concept, the ratio of mean and maximum velocities being constant means that the probability distribution underpinning the velocity distribution and other related variables is resilient, and that the same probability distribution is governing various phenomena observable at a channel section and explains the regularities in open-channel flows.     

Field and Laboratory Validation of High-Flow Air Bubbler Mechanics

Recent physical model studies have refined designs for high-flow air diffusers for managing accumulations of broken ice at navigation projects. Although these solutions are successful in the model, implementing them in the field can be difficult because of uncertainties in airflow scaling. This study uses field and laboratory data to test theoretical relationships between airflow from the diffuser and the resulting near-surface water velocity. In the experiments, water velocities were measured adjacent to bubbler plumes for depths ranging from 0.52?to?6.5?m and airflow rates ranging from 0.015?to?0.68 standard cubic meters per minute/meter. The observed vertical and horizontal water velocity data compared moderately well to theoretical curves based on the equations of Kobus and Ashton. In addition, a reasonably linear relationship was found between the average velocity of the horizontal, near-surface flow field V and unit airflow from the diffuser Qa.     

Numerical Simulation of Street Canyon Flows with Simple Building Geometries

The velocity and pressure fields of the flow over street canyons formed by groups of buildings are studied numerically. The flow fields are computed by solving the time-dependent incompressible Navier–Stokes equations using the fractional step method. The numerical model is validated by simulating flows over a square cylinder at different Reynolds numbers. The Strouhal numbers, which reflect the dynamic flow characteristics, agree well with published experimental data over a wide range of Reynolds numbers. The wind field model is then applied to two street canyon configurations. First, flows inside street canyons formed by four identical buildings are simulated. The incidental flow is raised by the most upstream building and becomes parallel to the ground at the rooftop level of the fourth building downstream, resulting in a clockwise rotating vortex in downstream street canyons with an inflow from left to right. Second, flows inside street canyons formed by two identical buildings are simulated. In this case, a primary eddy that is counterclockwise rotating may be formed due to flow separation at the front corner of the upstream building. A clockwise rotating primary eddy is formed in the wake area of the separate zone above the street canyon, which drives the counterclockwise rotating eddy in the street canyon. The result indicates that rooftop level flows cannot be assumed parallel to the ground as some modelers have done in their studies. Studies also show that flow regimes in the street canyon will remain unchanged while the inflow velocity is greatly increased from 0.1 to 6.0?m/s. In addition, the wind velocities in the street canyon have a linear response to the inflow velocity.     

Flow Upstream of Orifices and Sluice Gates

Potential flow solutions of a point/line sink are extended to study the velocity field upstream of a finite-size orifice and sluice gate. It is found that, in the “near field” zones, the iso-velocity surfaces appear to be semiellipsoidal; while in the “far field” zones they become hemispheres. The shape and size of the orifice/sluice gate were found to be of no effect on the flow behavior beyond a certain distance. The development of velocity profile away from the orifice and sluice gate is examined, and the effects of water depth are studied. The results of this study compare very well with other numerical and experimental studies, and provide a general understanding of the flow field upstream of orifices and sluice gates.     

Two Methods for the Computation of Commercial Pipe Friction Factors

Two methods are proposed for the computation of friction factors of commercial pipes. The first method applies the mean value of the zero velocity point (MZVP) to a theoretical friction factor equation, and the other directly computes the mean friction factor (MFF) by averaging the friction factor of both the smooth and rough walls while considering their relative contribution. The MFF method is preferred, because it is simple but covers all the flow characteristics of commercial pipes. Both MFF and MZVP methods consider two parts of a wall with different roughness heights: One part is rough and the other is smooth. A regression analysis was performed to determine optimum values of the roughness height and probability of encountering each part, using several sets of field data, including galvanized iron, wrought iron, cast iron, concrete, riveted steel, and concrete. The analysis showed that both the roughness height and the relative contribution of the rough part are strongly dependent on the pipe diameter. The MFF method gave an average error of less than 3%, whereas the traditional Colebrook–White equation gave an average error of more than 11% when compared with Colebrook’s data.     

FLOW AROUND A CIRCULAR CYLINDER USING A FINITE-VOLUME TVD SCHEME BASED ON A VECTOR TRANSFORMATION APPROACH

Flows around a circular cylinder displaying an unsteady vortex shedding process at the Reynolds numbers of 1000,3900 and 1×104 are studied using a finite-volume Total Variation Diminishing(TVD) scheme for solving the Unsteady Reynolds-Averaged Navier-Stokes(URANS) equations.An Elemental Velocity Vector Transformation(EVVT) approach is proposed for the local normal and tangential velocity transformation at the interfaces of main and satellite elements.The presented method is validated by comparing with the available experimental data and numerical results.It is shown that the two-dimensional TVD finite volume method with the Renormalization Group(RNG) turbulence model can be used to determine hydrodynamic forces and captures vortex shedding characteristics very well.     

阳离子淀粉与阴离子物质的混溶性

为研究阳离子淀粉与阴离子物质的混溶性,对阳离子淀粉与聚丙烯酸类浆料、乳化油的混溶性进行了试验.以分离速度和沉降率两个参数为衡量指标检验混溶效果,发现混合浆料中随着聚丙烯酸类浆料和乳化油的比例增加,分离速度加快,沉降率增加,同时试验了不同浓度的混合浆(处理前阳离子淀粉与处理后阳离子淀粉与阴离子物质的混合浆)在室温和95℃条件下的混溶情况.结果表明:处理后阳离子淀粉浆料在95℃条件下与阴离子物质的混溶效果良好.     

Penetration Length of Confined Counter Flowing Free Jets

Counter flowing jets are effective devices for mixing and dilution of effluents injected into a stream. In the present study, the penetration length of a counter flowing free round jet was experimentally investigated. Jet nozzles with different diameters were used to form a jet located at the center of a closed conduit with a square cross section. Based on the measurement of the axial velocity on the jet axis, penetration lengths were determined. Under certain circumstances where confinement effects are absent, penetration lengths vary linearly with the ratio of the jet velocity to the main flow velocity. This study also provides a criterion for the range of velocity ratios for which this linearity is valid, and explains the disagreement in the existing literature on the determination of critical velocity ratios.