Experimental study on triangular central baffle flume

In this paper the results of the experiments performed to study the flow through a Triangular Central Baffle Flume (TCBF) are reported. The investigated flume consists of a triangular baffle of the apex angle of 75° with a given base width. The theoretical stage-discharge formula was deduced by applying the Buckingham's Theorem and incomplete self-similarity hypothesis and was calibrated using the laboratory measurements carried out in this investigation. The proposed stage-discharge formula is characterized by a mean absolute relative error of 7.4% and 72% of the data points are in an error range of ±5%. The results indicate that TCBF flume is characterized by a flow capacity higher than that of a typical central baffle flume. Experimental observations show that the contraction ratio is a key parameter to distinguish between free and submerged flow regimes through a TCBF. Finally, to identify the flow condition, submergence threshold condition was formulated.     

Experimental study on rectangular cut-throated flume: Effects of flume walls slopes and channel longitudinal slope

This research conducted to study the flow through a rectangular cut-throated flume (RCTF). The flume is simply formed by placing two vertical triangular prisms (two vertical folded plates) on either side of a rectangular open channel. Both channel and flume cross-sections are rectangular. The investigated flume is inexpensive, easy to install and does not require high maintenance. A wide experimental investigation, carried out under free outflow conditions and under upstream subcritical flow regime to investigate the effects of the channel longitudinal slope, the flume throat width, and slopes of upstream and downstream flume walls on the stage-discharge relationship. The stage-discharge relationships were deduced by applying the dimensional analysis and were calibrated using the data of this study. The proposed stage-discharge equation for horizontal channel has an average absolute relative error of 2.97% with the relative errors restricted in the range of ±10%, and 80% of the data points are in the error range of ±5%. The proposed stage-discharge equation for sloping channel has an average absolute relative error of 3.97% with the relative errors restricted in the range of ±10%, and 66% of the data points are in the error range of ±5%. The measurements indicate that slopes of upstream and downstream walls affect the stage-discharge relationship of the CTFs only in sloping channels and flow discharge is not influenced by the flume walls slopes in a horizontal channel. The proposed unified stage-discharge equation for both horizontal and sloping channels has an average absolute relative error of 3.38% with the relative errors restricted in the range of ±10%, and 74% of the data points are in the error range of ±5%. The proposed stage-discharge model demonstrates favorable accurate and convenient estimation of discharge for flows through the CTFs.     

Comments on “Overflow characteristics of streamlined weirs based on model experimentation” by Bagheri S. and Kabiri-Samani A

In this paper the stage-discharge equation of a streamlined weir is theoretically deduced applying the Π-Theorem of dimensional analysis and the self-similarity theory. The coefficients of the new stage-discharge relationship are estimated using the experimental results by Bagheri and Kabiri-Samani.     

Comments on “Mean velocity and turbulent characteristics of flow over half-cycle cosine sharp-crested weirs” by Salehi S., Esmaili K., Azimi A.H.

In this paper the stage-discharge equation of a half-cycle cosine weir is theoretically deduced applying the Π-Theorem of dimensional analysis and the self-similarity theory. The coefficients of the new stage-discharge relationships are estimated using the results of the experimental runs by Salehi et al..     

Stage-discharge relationship for triangular and curved-edge triangular weirs

A sharp-crested weir with power-law sides is a general form, which reduces to the wildly used rectangular, parabolic, and triangular weirs. This general form allows modeling the weirs with different shapes. Up to now, the hydraulic performance of the power-law weirs with the exponent n in the range of 1 ≤ n ≤ 2 has not been studied. In this study, the triangular (n = 1) and curved-edge triangular (n > 1) weirs are studied experimentally and theoretically. For this, weir and critical flow theories along with Buckingham's theorem of dimensional analysis are used to deduce the stage-discharge relationship of the triangular and curved-edge triangular weirs. A series of laboratory experiments (464 runs) were conducted to calibrate the deduced theoretical stage-discharge relationships under free outflow condition. The proposed general stage-discharge relationship using the weir theory has a mean absolute relative error of 2.51% for 1 ≤ n ≤ 2. For this relationship, the mean absolute relative errors for n = 1, n = 1.5 and n = 2 are respectively as 1.96%, 2.64% and 3.08% whilst for the proposed stage-discharge relationship using the critical flow theory they are as 1.96%, 2.75% and 3.82%, respectively. Thus, the proposed stage-discharge relationship using the weir theory may be preferable due to its accuracy and generality.     

R-C系统消杂散光设计与效果评估

研究R-C系统的消杂散光以提高其成像质量。从杂散光的能量传输方程出发,分析总结了杂散光消除的基本途径。按照R-C系统杂散光消除的具体要求,结合作图法,推导出初始中心遮光比确定时的内遮光罩顶点坐标公式;为了减小初始中心遮光比,将次镜锥状遮光罩改进为百叶窗式结构。设计了杂散光源抑制角已知时的外遮光罩。最后,给出在Tracepro中利用仿真模拟测定杂光系数的方法及R-C系统遮光罩的应用实例。     

Reduction of sloshing effects in a rectangular tank through an air-trapping mechanism – a numerical study

A 2D rectangular tank subjected to horizontal excitations is used to analyze the effects of sloshing. The tank is fitted with horizontal baffles on two sides to suppress the impact pressure of sloshing by using an air-trapping mechanism. The volume of fluid method is adopted to create sloshing phenomena. Five cases with fixed baffle gaps and various baffle lengths are used to analyze the effects of suppressing the sloshing impact pressure in the tank. The peak pressure values of the cases with baffles are compared with those of the cases without baffles. Results show that the maximum suppression rate achieved is 63.6 % due to air trapping and baffle effects. Baffle ratio (Defined by G/L), a geometrical dimensionless factor, is considered to analyze the effects of baffle length and gap. A baffle ratio of 0.5 results in a 14.2 % reduction in the sloshing impact rate. An increasing amount of air is trapped within the baffle array as BR decreases.     

A generalized stage-discharge relationship for sharp-crested power-law weirs by dimensional analysis and self-similarity

Weirs are characterized by a stage-discharge relationship which mainly depends on the shape and dimensions of the hydraulic structure. A weir with symmetrical power-law sides is a versatile weir that can produce some known weir types (rectangular, triangular, parabolic) as special cases. In this paper, the outflow process of sharp-crested power-law weirs is investigated using the dimensional analysis and the incomplete self-similarity theory. A new generalized theoretical stage-discharge relationship is proposed, and its applicability is tested using measurements available in the literature for different weir types.     

Structure and dynamics of the turbulent flow through a central baffle

Central baffle flume (CBF) can be utilized as a control structure to measure flow discharge in irrigation channels under free and submerged flow conditions. Stage-discharge relationship has been extensively studied for various geometrical parameters and flow conditions, whereas internal structure of the flow around a baffle has not been investigated in the literature. To address this need, the present work investigates the turbulent flow around a central baffle through high-resolution numerical simulations using an open source computational model. Velocity measurements were conducted in a laboratory flume to setup and validate the numerical model. Comparison of the numerical results with the experimental measurements proves that the present numerical model can predict water depth and velocity field. Longitudinal distance from the apex to the intersection point of water and critical depths can be estimated as L_xc = 2L_e, where L_e is the longitudinal length of the guide walls. A horseshoe vortex system identified in front of the baffle produces a significant bump on the free-surface and rib vortices generated from the baffle extend up to the sidewalls of the channel. The vertical separation layer observed downstream of the baffle results in a reverse flow and a vortex pair is formed by the impingement of the resulting reverse flow on the back of the baffle. Reverse flow, plunging flow structure, splash and rebounding wave events observed at the downstream produce substantial hydrodynamic effects on the baffle. Geometry of the central baffle was modified to suppress recirculation effects based on the insights into the complete flow structure around the baffle. Eventually, vortex structures were suppressed and the length of the recirculation zone was reduced by 76%.     

无级定尺挡板设计

无级定尺挡板是在原多级定尺挡板的基础上设计而成，它把先进的伺服控制理论和普通液压系统相结合．实现对任意定尺的工艺要求。所受冲击力由原多级定尺挡板承受，并在有级定尺的基础上实现了无级定尺。     

A new approach for deducing the stage-discharge relationship of a triangular broad-crested device

In this paper, the outflow process of a triangular broad-crested device is examined using the dimensional analysis and the incomplete self-similarity theory. A new theoretical stage-discharge relationship is proposed, and its applicability is verified using measurements available in literature.The proposed power equation is characterized by a coefficient depending on device apex angle and a constant value of the exponent.     

超短型内嵌式遮光罩设计

为了在不影响杂散光抑制效果的同时减少空间遥感器的结构尺寸,方便其姿态控制,提出了一种超短型内嵌式遮光罩。介绍了超短型内嵌式遮光罩的基本结构形式及其优化设计方法。重新设计了遮光罩的形状,采用超短型多层遮光筒结构代替了传统设计中过长的外遮光罩。改变了遮光罩与主体结构的安装方式,将遮光罩嵌入式安装于空间遥感器的主体结构,最大限度地压缩了结构尺寸。以某航空相机光学载荷为例,分析了该种遮光罩的可行性和消杂散光效果。采用Light-tools软件优化设计了双层同心圆柱筒结构的遮光罩,并对该遮光罩的杂散光抑制效果进行了评估。模拟计算结果表明,外遮光罩采用超短型内嵌式结构后,遮光罩总长度和重量均减小为传统设计结果的1/3。系统的点源透过率(PST)曲线显示其整体呈下降趋势,且在离轴角大于25°后,光学系统的PST降低到10~(-7)以下。另外,设计的遮光罩能够有效抑制视场外杂散光,其杂散光抑制能力与其他离轴、同轴系统大体相当,满足使用需求。     

Experimental study and numerical simulation of flow over the inclined airfoil-shaped weir in rectangular channel

In order to find an integrated flow measurement and control facility with stable outflow form, a series of experiments and numerical simulations were performed for the inclined airfoil-shaped weir. Based on the experimental and simulated data, the stage-discharge relationship was deduced by using dimensional analysis and incomplete self-similarity theory. And two flow measurement formulas with inclination angle of 0°–45° were obtained through different fitting schemes. The variation law of discharge ratio with inclination angle and the accuracy comparison of flow measurement formulas were analyzed. Moreover, the Froude number, critical submergence degree, head loss and velocity distribution of the flow field over the weir were studied. The results show that streamlined airfoil improves the stability of flow behind the weir. The average relative error of the free water surface line data between experiments and simulations is 5.05%, which indicates the reliability of the numerical simulation. The discharge ratio proposed in the fitting scheme I takes the 25° inclination angle as the limit, which increases slightly at first and then decreases gradually, and the flow measurement relative errors of the two fitting formulas are all within ±7%. The average Froude number of the upstream reference section at each inclination angle is less than 0.25, the critical submergence range is between 0.87 and 0.96, and the relative head loss ranges from 3.54% to 12.16%, so as to ensure the accuracy of flow measurement.     

Flow characteristics and entrance length effect for MEMS nozzles

The critical back pressure ratio (CBPR) and the discharge coefficients were measured for traditional ISO nozzles with circular throat sections and sonic MEMS nozzles with rectangular throat sections manufactured using Micro-Electro-Mechanical systems techniques. The measurements show that critical flow can be reached in the MEMS type nozzles, but that the CBPR is much smaller than for traditional ISO nozzles and is almost constant for the same MEMS nozzle with different Reynolds numbers. The CBPR for MEMS nozzles with similar shapes increased as the throat diameter increased with different CBPR values for different shapes. For MEMS nozzles with similar geometries, the discharge coefficients in the backward direction with the longer entrance length were always higher than in the forward direction with the shorter entrance length.The measurement results were used to analyze the effect of the entrance length effect on the discharge coefficient with comparisons to theoretical values. As with the Vena Contracta for subsonic flow, the longer entrance section resulted in larger effective throat diameters for the same MEMS nozzle, which resulted in larger discharge coefficients.     

Experimental and numerical investigation on the performance of small-sized cavitating venturis

Cavitating venturis (CVs) are simple devices which can be used in different industrial applications to passively control the flow rate of fluids. In this research the operation of small-sized CVs is characterized and their capabilities in regulating the mass flow rate were experimentally and numerically investigated. The effect of upstream and downstream pressures, as well as geometrical parameters such as the throat diameter, throat length, and diffuser angle on the mass flow rate and critical pressure ratio were studied. For experimental data acquisition, three CVs with throat diameters of 0.7, 1 and 1.5 mm were manufactured and tested. The fabricated CVs were tested at different upstream and downstream pressures in order to measure their output mass flow rate and to obtain their characteristic curves. The flow inside the CVs was also simulated by computational fluid dynamics. The numerical results showed agreement with the experimental data by a maximum deviation of 5–10% and confirmed that the numerical approach can be used to predict the critical pressure ratio and mass flow rate at cavitaing condition. It is found that despite the small size of venturis, they are capable of controlling the mass flow rate and exhibit the normal characteristics. By decreasing the throat diameter, their cavitating mode became more limited. Results also show that increasing the diffuser angle and throat length leads to a decrease in critical pressure ratio.     

星载激光通信系统遮光罩的优化设计

为了提高某GEO对地激光通信系统的可通率,改善光学天线主镜、次镜的温度水平和温度不均匀性,对其遮光罩进行了优化设计。结合地球同步轨道外热流特点,在不降低通信信号性能的前提下,提出了内壁等间距排布栅板的遮光罩结构。首先,以能量遮挡率不高于5%为约束条件,讨论栅板、遮光罩内壁对主镜阳光辐射角系数的影响,得出遮光罩优化设计方案;其次,采用IDEAS软件开展系统热设计仿真分析,考核栅板布局对主镜和次镜温度指标的影响;最后,计算采用优化后的遮光罩时的激光通信可通率。从温控的角度分析,遮光罩内壁平行和垂直于赤道面方向都等间距布置4块栅板时,与采用传统空心圆柱形遮光罩相比,可通率从81.51%提高到91.21%。     

新型纵向流油冷却器传热性能及压降的试验研究

对两种不同角度的螺纹翅片管,分别在螺旋折流板及新型壳程挡板支撑下的3种结构的油冷却器进行了传热与阻力性能试验,结果表明,翅片角为5°的螺纹翅片管的传热性能比翅片角为13.5°的螺纹翅片管提高10%～30%,而流动阻力相差很小。对于同角度的螺纹翅片管油冷却器,新型阻流片壳程挡板虽然传热效果不及螺旋折流板,但是其壳程压降较螺旋折流板降低30%～40%,且单位压降下的热交换量提高10%～40%,这种结构对于大流量、长径比大且对壳程压降要求较高的油冷却器有一定的研究意义。     

Deducing the stage-discharge relationship of rectangular broad and sharp-crested contraction devices

Flumes with a local contraction of the channel width are a common way for accurately measuring the flow discharge. In this paper, the outflow process of rectangular broad and sharp-crested contraction devices is modeled using the dimensional analysis and the incomplete self-similarity condition. The proposed theoretical stage-discharge relationship is tested using measurements available in the literature. The proposed power equation is characterized by a value of the exponent close to 1.5 and a coefficient depending on the contraction ratio. The proposed flow-measuring flumes are characterized by a good accuracy of the measured discharge (ranging from −2.71 to 3.28% for the broad-crested contraction device and in the range from −0.24 to 0.31% for the sharp-crested one). Even if both flume types have good accuracy of the measured discharge (less than the limit of ±5% suggested in the literature), the developed analysis demonstrated that the sharp-crested device is characterized by the lowest values of the errors in the estimate of discharge, and this result increases the interest of this device in practical applications. The advantage of the proposed approach is combining an accurate estimate of the flow discharge and working regardless of the discharge coefficient estimate.     

喷射泵系统中的能量分布与效率特性研究

为得出喷射泵系统内各种能量损失的大小及变化规律，提出提高效率的针对性措施，根据能量守恒原理及动力定理，推导出喷射泵系统中各种能量的量纲一形式的表达式，并绘制出各能量及效率随体积流量比变化的对比曲线。根据该对比关系进一步分析了各种摩擦损失系数对效率的影响，提出了影响喷射泵效率的主要因素及提高效率的有效途径。计算结果表明，在最佳效率点处，各种能量损失处于同一量级上，它是决定喷射泵效率较低的根本原因；各种能量损失随流量比的改变而变化；喉管与扩散管的摩擦损失系数是影响效率的主要因素。     

New stage-discharge relationship for cylindrical and semi-cylindrical edged sluice gates

Gates are simple hydraulic structures which have been widely used for flow control and measurement in irrigation networks. In this paper new sluice gates with cylindrical and semi-cylindrical edges were introduced and the flow movement below these types of gates was experimentally investigated for both free- and submerged-flow conditions. The Buckingham theorem of dimensional analysis and the self-similarity theory were applied for developing the stage-discharge relationships for the two investigated flow conditions. For these cylindrical and semi-cylindrical edged gates and a free-flow condition, the proposed stage-discharge relationship was characterized by an estimate error less than or equal to 5% for 96.0% and 93.7% of the calculated values, respectively. Finally, an equation for calculating the maximum tail water depth allowing free flow condition was proposed.