Comparative analysis of flow patterns in aquaculture rectangular tanks with different water inlet characteristics

The objective of the work is to improve the design rules of rectangular aquaculture tanks in order to achieve better culture conditions and improve water use efficiency. Particle tracking velocimetry techniques (PTV) are used to evaluate the flow pattern in the tanks. PTV is a non-intrusive experimental method for investigating fluid flows using tracer particles and measuring a full velocity field in a slice of flow. It is useful for analysing the effect of tank geometries and water inlet and outlet emplacements. Different water entry configurations were compared, including single and multiple waterfalls and centred and tangential submerged entries.

The appearance of dead volumes is especially important in configurations with a single entry. Configuration with a single waterfall entry shows a zone of intense mixing around the inlet occupying a semicircular area with a radius around 2.5 times the water depth. A centred submerged entry generates a poor mixing of entering and remaining water, promoting the existence of short-circuiting streams. When multiple waterfalls are used, the distance between them is shown to have a strong influence on the uniformity of the velocity field, increasing noticeably when the distance between inlets is reduced from 3.8 to 2.5 times the water depth. The average velocities in configurations with multiple waterfalls are very low outside the entrance area, facilitating the sedimentation of biosolids (faeces and non-ingested feed) on the tank bottom. The horizontal tangential inlet allows the achievement of higher and more uniform velocities in the tank, making it easy to prevent the sedimentation of biosolids.     

Comparative study of flow field and drag coefficient of model and small natural trees in a wind tunnel

The influence of trees in urban areas is typically assessed using urban microclimate models. These models rely on wind tunnel experiments using small-scale tree models to verify and validate their predictions of the flow field. However, it is not known sufficiently to which extent small model trees used in wind tunnel studies can recreate the behavior of large trees found in cities. In the present study, the drag coefficient and the turbulent flow downstream of model trees are compared with the ones of natural trees of a similar size to determine whether both types of tree provide similar aerodynamic characteristics. Therefore, measurements of the drag force and the flow field, using particle image velocimetry, are performed. The aerodynamic characteristics of the small trees are compared with the ones measured on larger mature trees from previous studies. The present study shows that the drag coefficients of model and natural trees are similar only if both types have a similar aerodynamic porosity and if the model tree can undergo an aerodynamic reconfiguration similar to that of a natural tree. Such reconfiguration implies the reorientation of the branches and leaves due to wind. A study on the influence of seasonal foliar density variation shows that the foliage configuration plays a critical role on the drag coefficient and the flow field. A defoliated tree, such as a deciduous tree in winter, is shown to have a substantially lower drag coefficient and a negligible influence on the flow.     

Noninvasive stroke volume variation using electrical velocimetry for predicting fluid responsiveness in dogs undergoing cardiac surgery

Objective

To evaluate the ability of a noninvasive cardiac output monitoring system with electrical velocimetry (EV) for predicting fluid responsiveness in dogs undergoing cardiac surgery.

不同微纳米曝气滴灌入口压力下迷宫流道沿程微气泡行为特征

微纳米曝气滴灌时,微气泡在迷宫流道内的行为特征不明确导致系统的运行缺少科学依据.针对该问题,采用粒子跟踪测速技术(Particle Tracking Velocimetry,PTV),对0.02、0.06、0.10 MPa入口压力下迷宫流道内沿程连续5个位置处的气泡液体进行视频拍摄,分析微气泡在流道内的行为特征和入口压...     

过渡流下近壁双圆柱倾角对绕流结构影响

为了研究过渡流下近壁双圆柱倾角对绕流结构的影响,在雷诺数为300(基于圆柱直径D=15 mm)时,采用开式循环水槽试验台,结合粒子图像测速系统PIV对不同倾角的近壁双圆柱模型进行了试验研究.结果表明:当Re 为 300时,倾角γ改变对圆柱绕流结构影响显著.随着倾角γ增大,上游圆柱尾流由下侧剪切层发展被抑制逐渐演变为上侧剪切层发展被抑制,上游近壁区域加速效果先增强后减弱;下游圆柱尾流由上侧剪切层主导逐渐演变为下侧剪切层主导,下游近壁区域的加速效果和范围不断增大至稳定.当γ<7.6°时,倾角γ对绕流结构影响逐渐增大,近壁区域扰动逐渐增强;当γ>7.6°时, 倾角γ对绕流结构影响逐渐减弱,近壁区域扰动趋于稳定.详细分析了γ=-7.6°工况下的瞬时流动特性,发现近壁双圆柱尾流形成周期性的旋涡交替脱落现象,增强了壁面附近流动不稳定性.     

粒子速度场仪在泵装置内流测量研

介绍国内外利用粒子速度场仪（PIV）对泵装置内部复杂流动测量的研究进展，分析了针对泵装置内流测量的PIV系统的特点，概括了目前在泵装置测量方面已取得的研究成果，最后预期了PIV技术在泵装置测量应用方面未来的发展方向。     

数字图象技术在流速测量中的应用

分析了激光散斑测速及微粒图象测速法在非稳流的瞬时速度场测量中的缺陷和局限性。针对这些局限，从理论上探讨了采用数字图象技术的方法测速的优越性，它可以消除目前采用光学方法的局限；包括非常有限的动态速度范围的测量，速度失量方向的不明确及实现实时测量的困难。     

基于PIV测试的螺旋离心泵内部流动特性研究

为了研究螺旋离心泵内部流体流动机理,通过对螺旋离心泵改造和透明化处理,应用PIV测试技术,取螺旋离心泵轴截面和径向截面分阶段获得各截面上速度变化,进而得到各个截面上的流动信息,揭示螺旋离心泵内部流动状态。结果表明:整个轴向截面中,物理参量扰动大于径向截面,尤其在叶轮流道的轴截面,有明显的涡旋出现。在深入蜗壳的叶轮流道中,涡旋数量明显增加,这是因为该区域的流体方向在叶轮旋转和蜗壳使其变化过程中,同时受力分配促进了这种变化;在径向截面上,能明显看到速度方向和流线沿一个方向旋转,同时,流体均有由中心向外运动的趋势,这是叶轮螺旋段螺旋推进作用后又一个重要组成部分离心段的离心力作用决定的,叶轮的螺旋段螺旋推进作用和离心段的能量转换相互配合,构成了螺旋离心泵工作的过程。     

动态水压下迷宫流道水流运动特性研究

采用粒子跟踪测速(PTV)技术,通过观测动态水压及恒定水压下迷宫流道内示踪粒子的运动规律,分析动态水压下迷宫流道内水流的运动特性,揭示动态水压滴灌抗堵机理。结果表明:相对于恒定水压,动态水压下流道内流量未出现明显下降,动态水压对滴灌系统的供水能力影响较小;动态水压下流道主流区水流流速始终保持大幅度上下波动,水流紊动更加强烈,大量粒子可以快速经主流区通过流道,粒子在流道内停留时间大大缩短;滞止区水流流速仍较低,但紊动强烈,进入滞止区的粒子随水流不断运动,不易发生沉积,部分粒子在水流紊动作用下能够迅速重返主流区,并最终通过流道,粒子在流道内沉积的概率明显降低。采用动态水压可以有效提高流道抗堵能力。     

不同比转数前伸式双叶片离心泵内部流动规律研究

采用k-ε模型、SST模型和DES模型分别对比转数为70的前伸式双叶片离心泵进行非定常数值计算,获得不同流量工况下(Q/Qd分别为0.6、1.0、1.6)叶轮内部流场的相对速度分布,将不同湍流模型的内部流动模拟结果与PIV试验结果进行对比分析,发现基于k-ε模型的模拟结果与PIV试验测量结果较为吻合。采用k-ε模型对比转数为157的前伸式双叶片离心泵进行非定常数值模拟,研究发现叶轮内部流动规律与比转数为70的离心泵叶轮内部流动规律具有相似性:在流道中部靠近叶片工作面上存在低速区及与叶轮旋转方向相反的轴向旋涡,随着流量的增大,低速区与轴向旋涡逐渐减小;引入少叶片数离心泵内部流动理论,揭示了低速区和轴向旋涡存在和发展的内在机理。