Numerical Analysis of Electromagnetic Control of the Boundary Layer Flow on a Ship Hull

In this article, electromagnetic control of turbulent boundary layer on a ship hull is numerically investigated. This study is conducted on the geometry of tanker model hull. For this purpose, a combination of electric and magnetic fields is applied to a region of boundary layer on stern so that produce wall parallel Lorentz forces in streamwise direction as body forces in stern flow. The governing equations including RANS equations with SST k-ω?turbulent model coupled with electric potential equation are numerically solved by using Ansys Fluent codes. Accuracy of this turbulent model of Fluent in predicting Turbulent flow around a ship is also tested by comparing with available experimental results that it shows a good agreement with experimental data. The results obtained for ship flow show that by applying streamwise Lorentz forces that are large enough, flow is accelerated. The results are caused to delay or avoid the flow separation in stern, increase the propeller inlet velocity, create uniform flow distribution behind the ship’s hull in order to improve the propeller performance, and finally decrease the pressure resistance and total resistance.

Resistive Factors of the Blood Flow and Energy Distribution in the Body

Purpose of the study is to identify the reason for the formation of the resistive factors in blood flow: inertial flow and turbulence in large arteries and increasing viscosity in the venous blood. Methods and Materials: Blood flow velocities were studied in the different sites of the large vessels in 35 normal adults (15 men, 20 women, age 21 - 49 years) with the use of Magnetic Resonance Angiography. Blood radiodensity (HU) was measured by the CT scanner. Blood flow pulsatility, resistivity indexes were carried out with the Duplex US. Results: Resistive and pulsatility indexes for the ascending aorta are 0.96 ± 0.07 and 3.14. ± 1.7, abdominal aorta 0.91 ± 0.07 and 2.7 ± 1.3, carotid artery 0.74 ± 0.07 and 2.04 ± 0.53, pulmonary trunk 0.74 ± 0.11 and 1.49 ± 0.37, inferior vena cava 0.32 ± 0.21 and 0.69 ± 0.37. Blood radio density (in HU) in the ascending aorta is 57.3 ± 3.5, distal thoracic aorta 25.7 ± 3.1, and inferior vena cava 59.3 ± 3.3. Pulsation of the peak velocity is expressed at the external wall of the isthmus of aorta at the end of systole. Conclusion: Heart energy is stored in the elastic deformation of the blood cells and arterial walls, in kinetic energy of the blood flow, entropy of the system. Inertial blood flow due to the frequency dispersion in the arteries, transforms to the flow with the high fluidity in capillaries. Gibbs free energy increases, enabling spontaneous chemical reaction to proceed across the cell membrane. Process is altered in the venous blood. Changes in resistance express transformation of the energy in the substance.

Theory of Resistivity-Gradient-Driven Turbulence in the Presence of Flow

The resistivity-gradient-driven turbulence is investigated in the presence of both poloidal and parallel/toroidal flows.At saturation,the poloidal flow shear reduces fluctuations at the edge of plasmas,the parallel flow and its shear both reduce fluctuations in the core of plasmas.This indicates that the mechanism for a high confinement regime(H mode)may be the turbulence suppression due to the poloidal flow shear at the edge of plasmas,and the mechanism for a very high confinement(VH mode)may be the turbulence suppression due to the toroidal flow and its shear in the core of plasmas.

MODIFIED LOG-WAKE LAWS FOR TURBULENT FLOW OF THE OUTER AND INNER REGIONS IN SMOOTH PIPES

Based on theoretical analysis two modified log-wake laws for turbulent flow in smooth pipes are obtained, one is applicable to the outer region and other one to inner region, the new law for the outer region fits the velocity profile measured in smooth pipes by Zagarola very well and the effect of Reynolds number can be taken into consideration, the velocity profile for inner region satisfies the wall boundary conditions ,equals zero at the pipe wall and smoothly joins up with the velocity profile in outer region, the adopted eddy viscosity model is consistent with Laufer＇s, Nunner＇s and Reichardt＇s experimental data.

INVESTIGATION OF THE MEAN-FLOW SCALING AND TRIPPING EFFECT ON FULLY DEVELOPED TURBULENT PIPE FLOW

Fully developed turbulence measurements in pipe flow were made in theReynolds number range from 10 X 10~3 to 350 X 10~3 with hot-wire anemometer and a Pilot tube.Comparisons were made with the experimental results of previous researchers. The mean velocityprofile and the turbulent intensity in the experiments indicate that for the mean velocity profile,in the fully developed turbulent pipe flow, von Karman's constant κ is a function of the Reynoldsnumber, i. e. κ increases slowly with the Reynolds number. For turbulent pipe flow, the outer limitdepends on whether the Kdrmdn number R^+ is greater or less than 850 in the centerline velocityprofile: a log law exists for 850 < R^+< 1750 in the experiment, and von Karman' s constant κ isshown to be 0. 408. Under the effects of the test trip at the inlet, fully developed turbulence wasobtained in pipe flow at lower Reynolds number when the entrance length (x/D) was larger than 40. Inthe experiment it was also found that turbulence quantities in pipe flow remain independent of theupstream conditions when the trip blockage ratio is higher than 20%, and the comparison with channelwater flow was also performed.

Flow structure around high-speed train in open air

According to the analysis of the turbulent intensity level around the high-speed train, the maximum turbulent intensity ranges from 0.2 to 0.5 which belongs to high turbulent flow. The flow field distribution law was studied and eight types of flow regions were proposed. They are high pressure with air stagnant region, pressure decreasing with air accelerating region, low pressure with high air flow velocity region I, turbulent region, steady flow region, low pressure with high air flow velocity region II,pressure increasing with air decelerating region and wake region. The analysis of the vortex structure around the train shows that the vortex is mainly induced by structures with complex mutation and large curvature change. The head and rear of train, the underbody structure, the carriage connection section and the wake region are the main vortex generating sources while the train body with even cross-section has rare vortexes. The wake structure development law studied lays foundation for the train drag reduction.

Semi-continuous casting of magnesium alloy AZ91 using a filtered melt delivery system

A 3-D numerical simulation of an industrial-sized slab caster for magnesium alloy AZ91 has been carried out for the steady state operational phase of the caster.The simulated model consists of an open-top melt delivery system fitted with a porous filter near the hot-top.The melt flow through the porous filter was modeled on the basis of Brinkmann-Forchimier-Extended non-Darcy model for turbulent flow.An in-house 3-D CFD code was modified to account for the melt flow through the porous filter.Results are obtained for four casting speeds namely,40,60,80,and 100 mm/min.The metal-mold contact region as well as the convective heat transfer coefficient at the mold wall were also varied.In addition to the above,the Darcy number for the porous media was also changed.All parametric studies were performed for a fixed inlet melt superheat of 64℃.The results are presented pictorially in the form of temperature and velocity fields.The sump depth,mushy region thickness,solid shell thickness at the exit of the mold and axial temperature profiles are also presented and correlated with the casting speed through regression analysis.©2015 Production and hosting by Elsevier B.V.on behalf of Chongqing University.

On numerical modeling of low-head direct chill ingot caster for magnesium alloy AZ31

A comprehensive 3D turbulent CFD study has been carried out to simulate a Low-Head(LH)vertical Direct Chill(DC)rolling ingot caster for the common magnesium alloy AZ31.The model used in this study takes into account the coupled laminar/turbulent melt flow and solidification aspects of the process and is based on the control-volume finite-difference approach.Following the aluminum/magnesium DC casting industrial practices,the LH mold is taken as 30 mm with a hot top of 60 mm.The previously verified in-house code has been modified to model the present casting process.Important quantitative results are obtained for four casting speeds,for three inlet melt pouring temperatures(superheats)and for three metal-mold contact heat transfer coefficients for the steady state operational phase of the caster.The variable cooling water temperatures reported by the industry are considered for the primary and secondary cooling zones during the simulations.Specifically,the temperature and velocity fields,sump depth and sump profiles,mushy region thickness,solid shell thickness at the exit of the mold and axial temperature profiles at the center and at three strategic locations at the surface of the slab are presented and discussed.

THEORETICAL ANALYSIS OF TURBULENT FRICTIONAL RESISTANCE INSIDE DUCTS OF ARBITRARY ANGULAR CROSS SECTIONS

It is recognized that for turbulent flow inside a duct of angular cross section,devi- ation will take place if the hydraulic diameter is taken as the equivalent diameter to calculate the mean friction coefficient of the duct wall.It still remains a question to determine the extent of the deviation as no common formula is available and only a few cross sections have been tested. This paper presents an approach which takes the angle area of the angular cross section as an ad- dition to the angle-less cross section,analyses and calculates the size of laminar flow area of the angle area,quantifies the amount of deviation of using hydraulic diameter as the equivalent di- ameter and finally obtains the formula for calculating the turbulent friction coefficients of the ducts of arbitrary angular cross sections.The formula derived is universally applicable to cross sections with arbitrary number of angles and arbitrary forms of angles,i.e.,arbitrary angular cross sections.Experience shows it is easy to use and the result obtained has a good agreement with physical mechanism and test measurements.

柔性植物影响下水沙运动及物质输运研究进展

水生植物是河流、湖泊、滨海湿地环境中不可缺少的一部分,植物的存在使水动力条件发生了改变,进而影响了泥沙悬浮和污染物的输运。从恒定流和波浪两种水动力条件出发,针对柔性植物影响下的水流阻力、水流结构、紊动特性、特征流速、消波系数、泥沙运动、污染物输运等方面,综述了国内外的主要成果,并指出复杂水动力条件下的植物-水流-泥沙-污染物之间的相互作用规律是今后研究的重点和难点。

Flow characteristics of the wind-driven current with submerged and emergent flexible vegetations in shallow lakes

A pneumatic annular flume is designed to simulate the current induced by the wind acting on the water surface in shallow lakes and the experiments are conducted to investigate the influence of submerged and emergent flexible vegetations of different densities on the flow characteristics （e.g., the flow velocity, the turbulence intensity, the vegetal drag coefficient CD and the equivalent roughness coefficient nb ） at different wind speeds. Vallisneria natans （K natans ） and Acorus calamus （A. calamus） widely distributed in Taihu Lake are selected in this study. It is indicated that the vertical distribution profiles are in logarithmic- curves, The stream-wise velocity rapidly decreases with the increasing vegetation density. The flow at the lower layer of the vegeta- tion sees compensation current characteristics when the vegetation density is the largest. The turbulence intensity in the flume without vegetation is the highest at the free surface and it is near the canopy top for the flume with V. natans. The turbulence intensity near the bottom in the flume with vegetation is smaller than that in the flume without vegetation. A. calamus exerts much larger resistance to the flow than V. natans. The variations of CD and nb caused by the vegetation density and the wind speed are also discussed.

阻力平方区满流糙率影响因素及敏感性分析

糙率在实际应用中通常以经验值取为常数,但其实际值会随着水力学条件的变化而改变。基于ColebrookWhite方程与曼宁公式推导出阻力平方区内满流状态下糙率的计算公式,得到糙率的影响因素包括当量粗糙度、雷诺数及水力半径,为糙率的准确计算提供理论基础,并通过敏感性分析研究糙率随各因素的变化特性。为避免流量未知情况下求解糙率值时的迭代计算,结合所提出的比值系数雷诺糙度比KRek,分析忽略雷诺项时所得糙率值的相对误差,从而得出可忽略雷诺项来计算糙率的条件,为简便计算糙率值提供参考。

微通道中高分子湍流减阻实验研究

主要研究微管道内高分子湍流减阻。介绍了高分子湍流减阻理论发展的基本概述;详细介绍了高分子湍流减阻微通道实验的实验装置及有关做法,并对实验数据进行分析处理;通过分析实验数据,对减阻率进行拟合处理,建立起包含多种因素的实验模型,对影响减阻效果的各因子水平及其相互作用进行分析评价;通过实验值与计算值的对比,发现本模型能够很好地预测减阻率,误差大多在30%以内。

内螺纹管湍流特性的数值模拟

采用数值模拟方法,对不同工况下内螺纹管的湍流特性进行了分析。通过设置不同螺旋角、不同流速、不同肋高和不同肋条数等内螺纹管参数,对不同工况下的流动阻力和阻力系数进行比较,探讨了各参数对流动阻力和阻力系数的影响。研究结果表明,雷诺数和内螺纹管参数对内螺纹管阻力特性的影响显著;采用数值模拟方法可揭示内螺纹管的湍流特性,为内螺纹管的设计和优化提供参考。

聚氯乙烯球阀水流阻力特性及流动规律分析

为了研究排灌工程管网中PVC球阀水流阻力及流动规律,对2种PVC球阀DN75和DN50进行了试验,采用角位移传感器监测阀门开度,同时采用Realizablek-ε紊流模型对2种球阀进行了数值模拟并分析验证,在此基础上对3种规格球阀DN110、DN90和DN63进行了数值模拟研究,得到5种规格球阀的阻力系数计算表达式。比较试验与数值模拟结果,DN75和DN50球阀模拟的阻力系数值与试验吻合的较好。DN75球阀流场特征显示相对开度为0.91时流态比较稳定,无明显漩涡;相对开度为0.36时压力、速度梯度较大,在球阀YZ截面出现大小相当、方向相反的对称漩涡,造成了较大的水头损失。研究结果可为管网的水力计算提供参考。

扭旋叶片组合对静态混合器流场特性影响

运用FLUENT计算软件,对一个截面上安有一至四个扭旋叶片的静态混合器内流场分别进行模拟计算分析。结果表明:单叶片静态混合器内仅有一个较大旋涡,而多叶片组合静态混合器内有数量为叶片数量二倍的旋涡,且两两旋向相反交替排列;按单叶片、双叶片组合、三叶片组合、四叶片组合顺序静态混合器内湍动能与湍流强度呈递增趋势;在同一流速下按双叶片组合、三叶片组合、单叶片、四叶片组合顺序各混合器流体阻力逐渐增加。

混流式水泵水轮机全特性曲线S形区流动特性

混流式水泵水轮机转轮的离心效应较混流式水轮机明显,形成了全特性曲线上的S形特性。该S形区水泵水轮机流道内流动状况很不稳定,为了详细了解该区域的流动特性,选取等开度下水轮机工况、水轮机飞逸工况、零流量附近水轮机制动工况、零流量附近反水泵工况以及反水泵工况等5个工况点进行全流道定常流和非定常流数值分析。定常流动分析表明:全特性曲线上的S形区转轮和导叶流道内存在大量的涡,消耗了大量的水能,致使机组输出功率很小。非定常流场计算表明:在S形过渡工况区,蜗壳与尾水管直锥段内的压力脉动频率与幅值均相近,且幅值小;而导叶至叶片的无叶区和叶片进口的压力脉动幅值高,主要为高频脉动。

明渠交汇水流阻力对平面二维数值模拟的影响

与顺直明渠水流相比,明渠交汇水流由于存在分离区和自由剪切面,其紊动特性引起的阻力较为复杂,交汇水流除床面阻力外还受到较强的阻力。基于数值模拟与实验比较分析表明,分离流与自由剪切流所产生的阻力(紊动粘性阻力)对于交汇水流数值模拟具有重要影响。同时,由于平面二维模型对二次流影响的忽略,断面环流较强时数值模拟会产生较大的阻力(环流阻力),而断面环流较弱时,交汇水流的三维特性相对较弱,采用平面二维模型和k-ε紊流模型能达到较好的模拟效果。因此,紊动及环流阻力的计算对交汇水流数值模拟的精度至关重要。

不同湍流模型在管道流动阻力计算中的应用和比较

应用几种不同的湍流模型,模拟计算水力光滑管的沿程阻力系数。将计算结果与尼古拉茨试验结果进行比较,可见Spalart-Allmaras湍流模型与试验结果吻合得最好,即在5个不同雷诺数(Re)下,其计算结果的最大误差为1.65%。研究还表明,无论圆管内流体流动是层流还是湍流,流动阻力的大小主要由流体在壁面附近的速度分布决定。

SK型静态混合器流体湍流阻力的研究

为了获得流体在SK型静态混合器中湍流流动时的流动阻力规律,提出一种新的流体阻力的计算模型。在流体不可压缩的假设前提下,将流体在SK型静态混合器中的螺旋形运动分解成轴向直线运动和环向旋转运动。在流体作湍流流动时,运用流体力学理论,分别求解出流体作2种运动时所产生的流体阻力的计算式,然后进行叠加得到SK型静态混合器湍流时流体阻力理论计算式。以水为实验介质,对SK型静态混合器流体湍流阻力进行了实验测量,回归出实验公式。与理论结论进行比较分析,得出流动摩擦系数与雷诺数的负0.25次幂呈线性关系的结论。