共查询到19条相似文献,搜索用时 218 毫秒
1.
本文介绍了床面固体颗粒随水流绕过圆柱体时,将在圆柱周围的床面上形成一个无粒子运动区的试验现象。水槽试验结果表明,当固体颗粒的粒径减小时,无粒子区的范围将增大;无粒子区的范围随圆柱直径的增大而增大;水流条件的变化直接影响着床面固体颗粒的运动情况,同无粒子区的形成、消失及范围大小有密切的关系。根据试验资料,结合量纲分析,建立了无粒子区的无量纲经验关系式 相似文献
2.
3.
4.
单颗泥沙运动力学及统计规律 总被引:1,自引:0,他引:1
我们采用力学与概率论相结合的途径,对单颗泥沙运动进行了研究。1.单颗泥沙运动的机理和概化图形单颗泥沙具有静止、滚动、跳跃和悬浮四种运动状态。当水流强度不很大时,床面颗粒起动之后,常常是滚动。当水流强度较大,或具有一定速度的运动颗粒与床面颗粒碰撞后,产生了一定的竖向分速,颗粒就会离开床面,作短暂的跳跃。当水流强度很大时,在紊动水流的涡体携带下,泥沙颗粒可以在水中悬浮一定的距离。跳跃颗粒与滚动颗粒的区... 相似文献
5.
6.
波浪破碎卷入气体易对建筑物受力产生压力振荡,了解波浪作用下建筑物附近掺气水流的运动特性是精确计算建筑物受力的前提.基于OpenFOAM开源程序包和修正速度入口造波方法建立三维数值波浪水槽,模型采用S-A IDDES湍流模型进行湍流封闭,并采用修正的VOF方法捕捉自由液面,数值模拟了规则波在1:10的光滑斜坡上与直立结构物的相互作用过程,重点分析了结构物附近的水动力和掺气水流运动特性.结果表明,建立的数值模型能精确地捕捉波浪作用下直立结构物附近的自由液面的变化以及气泡输运过程,较好地描述气体卷入所形成的气腔形态以及多气腔之间的融合、分裂等过程;波浪与直立结构物相互作用产生强湍动掺气水流,其运动过程十分复杂;掺气流体输运过程中水气界面周围一直伴随着涡的存在,其中,气泡的分裂与周围正负涡量剪切作用密切相关,且其输运轨迹主要受周围流场的影响;研究揭示了结构物附近湍动能与掺气特性的关系,发现波浪作用下直立结构物附近湍动能的分布与掺气水流特征参数(气泡数量、空隙率)整体呈现一定的线性关系. 相似文献
7.
波浪破碎卷入气体易对建筑物受力产生压力振荡, 了解波浪作用下建筑物附近掺气水流的运动特性是精确计算建筑物受力的前提. 基于OpenFOAM开源程序包和修正速度入口造波方法建立三维数值波浪水槽, 模型采用S-A IDDES湍流模型进行湍流封闭, 并采用修正的VOF 方法捕捉自由液面, 数值模拟了规则波在1:10的光滑斜坡上与直立结构物的相互作用过程, 重点分析了结构物附近的水动力和掺气水流运动特性. 结果表明, 建立的数值模型能精确地捕捉波浪作用下直立结构物附近的自由液面的变化以及气泡输运过程, 较好地描述气体卷入所形成的气腔形态以及多气腔之间的融合、分裂等过程; 波浪与直立结构物相互作用产生强湍动掺气水流, 其运动过程十分复杂; 掺气流体输运过程中水气界面周围一直伴随着涡的存在, 其中, 气泡的分裂与周围正负涡量剪切作用密切相关, 且其输运轨迹主要受周围流场的影响; 研究揭示了结构物附近湍动能与掺气特性的关系, 发现波浪作用下直立结构物附近湍动能的分布与掺气水流特征参数(气泡数量、空隙率)整体呈现一定的线性关系. 相似文献
8.
泥沙颗粒受到的拖曳力是泥沙运动的主要驱动力,而当前应用于计算流体力学-离散颗粒法(CFD-DPM)耦合模型进行水沙运动模拟的泥沙颗粒拖曳力公式均没有考虑明渠流底床边壁作用的影响。求解不可压缩Navier-Stokes方程,对明渠层流不同雷诺数条件下床面附近不同高度处颗粒所受拖曳力进行了模拟,根据模拟结果变化规律,提出了综合考虑床面和水流惯性对标准拖曳力影响的修正拖曳力计算公式。与常用的单颗粒标准拖曳力公式和考虑遮蔽效应的多颗粒拖曳力公式相比,采用本文修正公式得到的水沙作用力更接近高精度数值解,应用于CFD-DPM输沙模拟获得的输沙结果与输沙率公式结果一致,应用分析表明输沙模拟应当采用粗糙底床边界。 相似文献
9.
局部冲刷的三维数值模拟可预测水力冲刷的破坏程度和破坏机制,进而提供更加合理的工程措施以减轻或避免局部冲刷造成的工程破坏。基于有限体积法和非结构化的计算网格构建了以水动力学模型、泥沙冲淤和河床变形方程为基础的三维局部冲刷数值模型。水动力学模型中的湍流模型为剪切应力SST k-ω输运模型,泥沙冲淤以底床切应力大小和分布为基础,水沙模型的耦合采用单向弱耦合方式。首先,通过ANSYS-FLUENT软件数值计算水动力学模型后,将水力特性数据单向传递至泥沙模块,并应用UDF函数二次开发实现泥沙模型的数值计算。利用动网格技术重构因河床地形更新引起的变形网格。与动床圆柱冲刷和丁坝局部冲刷的试验结果进行比较,验证了局部冲刷数值模型的可靠性。从平衡冲深时的冲刷深度和冲坑内水流特性等结果的对比可以看出,该数值模型成功地模拟出最终冲刷地形和形态,并能捕捉不同时刻的三维地形变化。根据数值模型的建立及应用结果分析主要得到以下结论,以切应力观点为基础开发该模型时,具有简易性和较强的可靠性;单元体泥沙通量的重构和床面坡度等因素均影响模型的精度;FLUENT软件提供的动网格技术能较好重构小变形网格,但是重构因地形变化引起的大变形网格时略显不足。 相似文献
10.
11.
The present study provides a comparison between the flow pattern around two circular piers in tandem and a single pier set up on a moderately rough flat bed in a laboratory flume. Velocities are measured by an Acoustic Doppler Velocimeter (ADV). The contours of the time-averaged velocity components, Reynolds shear stresses, turbulence intensities and turbulence kinetic energy at different planes are presented. Streamlines and vectors are used to study the flow features. The analysis of power spectra around the piers is also presented. The results show that the presence of downstream pier changes the flow structure to a great extent, particularly in the near-wake region. Within the gap between the two piers, a stronger and substantial upflow is shaped. However, a weaker transverse-deflection is formed in comparison with that in the single pier. Near the bed, the velocity of flow approaching the downstream pier decreases to 0.2–0.3 times of the approach mean velocity (U 0) due to the sheltering effect of the upstream pier. In the wake of downstream pier, the flow structure is completely different from the one in the wake of single pier. In comparison with the single pier, the values of turbulence kinetic energy and turbulence intensities show a considerable decrease around the tandem piers. In tandem piers, the high values of turbulence characteristics are found near the downstream pier. There is a recirculation zones just upstream of the sheltered pier close to the bed and another behind that pier near the free surface. The results show a decrease in the strength of vortical structure in the wake of tandem piers in comparison with single pier. It is shown that the formation of flow with different Reynolds number along the flow depth due to the effect of bed roughness, as well as pier spacing, can influence the type of flow regime of tandem case. In addition to enhancing the flow structure indulgence, the present detailed measurements can also be used for verification of numerical models. 相似文献
12.
This paper introduces an improved formula for the bed‐shear stress by applying the vorticity effect and its application in a 3D flow and sediment model to estimate scouring around bridge piers. Up to now, the sediment transport formulae used for computing pier scour were developed based on the general scouring in unobstructed flow. The capability for numerical models to predict local scour around bridge piers was severely restricted by the sediment transport formulae. The new formula introduced in this paper can take into account vortices that affect the local scour process by adding some terms into the classic bed‐shear stress equation. The 3D numerical model system used in this study consists of three modules: (a) an unsteady hydrodynamic module; (b) a sediment transport module; and (c) a Fation module. The hydrodynamic module is based on the 3D RANS equations. The sediment transport module is comprised of semi empirical models of suspended load and non‐equilibrium bed load. The bed‐deformation module is based on the mass balance for sediment. The model was used to simulate pier scour in tree different test cases: (1) a circular pier; (2) a square pier; and (3) a rectangular pier, by applying the ordinary sediment equation and the newly introduced sediment equation. Results of both numerical simulations were compared against laboratory measured data and also in case 1 with result of Olsen and Melaaen (J. Hydraul. Eng. 1993; 119 (9):1048–1054). Comparisons show that the new sediment formula could predict the scour more accurately than the ordinary one. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
13.
圆柱绕流局部冲刷机制的实验研究 总被引:1,自引:0,他引:1
采用粒子图像测速技术和粒子图像识别技术研究了冲刷坑的演化过程及局部冲刷的流体动力机制。首先研究了动床条件下圆柱周围的床面演化过程,给出垂直对称面输沙率随时间变化以及圆柱下游垂直对称面的沙颗粒浓度和泥沙分布规律;其次针对典型冲刷坑形状固定底床,测量了圆柱下游的瞬时流场,并给出平均流场。结果表明尾流区平均剪应力较低,但垂向涡量和湍流强度较高。分析指出平均剪应力较低的尾流区域,局部冲刷的主要动力机制是尾涡和湍流的综合作用。 相似文献
14.
The aim of the present study is to investigate, by numerical simulation, the three‐dimensional turbulent flow field around square and circular piers. The numerical model employs a finite volume method based on MacCormack's explicit predictor–corrector scheme to solve weakly compressible hydrodynamic equations for turbulent flow. Computed results are compared with Dargahi's experimental measurements to assess the validity of the proposed model. Very good agreements are obtained. The results of flow simulation indicate that near the upstream face of the pier there exists a downflow, which joins the separated flow to form the horseshoe vortex stretched around the pier. This horseshoe vortex interacts with the wake vortex to create the upflow behind the pier. These phenomena appear to be very important to the mechanism of scouring around the pier. In general, the flow patterns for the square and circular piers are similar. However, the strengths of the downflow and horseshoe vortex are greater in the case of the square pier. The position of the horseshoe vortex around the circular pier is closer to the front face than that around the square pier. In the meantime, the domain of the wake flow in the case of the square pier is greater than that in the case of the circular one. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
15.
Experimental results of detailed flow measurements using an Acoustic-Doppler Velocimeter (ADV) around a complex bridge pier (CBP) are presented. The pier consists of a column, a pile cap (PC) and a 2×4 pile group. The time-averaged velocities, turbulence intensities, and Reynolds stresses are studied and presented at different horizontal and vertical planes. Streamlines obtained from the velocity fields are used to show the complexity of the flow around the pier. It is shown that the main feature of the flow responsible for the entrainment of the bed sediments is a contracted (pressurized) flow below the PC toward the piles. A deflected flow around the PC and a strong down-flow along its sides are observed and have been measured. It is shown that these flow patterns also cause sediment entrainment. Vortex flow behind the PC and amplification of turbulence intensity along its sides near the downstream region can be other reasons for the scour hole (SH) development. Turbulence intensities and Reynolds shear stresses are presented and discussed. A comparison is made between the flow field measured with the equilibrium SH and that measured on the fixed flat-bed. The results show that the flow field around the PC is considerably influenced by the development of the SH. The extent of the wake region at the rear of the PC is about 1.4 times larger for the fixed bed (FB) than for the scoured bed (SB). Moreover, the size of the core of high turbulent kinetic energy K, as well as the maximum values of K behind the column for the FB case is larger than that of the SB case. When a scour hole develops, the flow below the PC around the piles is considered to be the main cause of the scour. This is the first time that these observations about the flow and turbulence field around a complex bridge pier are reported and analyzed. In addition to improving the understanding of the flow structure, the present detailed measurements can also be used for benchmarking and verification of numerical models. 相似文献
16.
17.
The present paper describes the applicability of the active flow control device, mini electromagnetic flap actuators attached on the leading edge of an airfoil, for the flow separation under both the steady and the unsteady flow conditions in the low Reynolds number region. At first, lift and drag have been measured for a wide variety of the wind speed Reynolds numbers and the angles of attack for the steady flow condition. Then, effects of some simple feedback flow controls, where the time-dependent signal of the lift-drag ratio have been used to detect the stall and served as a trigger to start the actuation, have been explored under the unsteady flow condition for evading the stall. In every low Reynolds number ranging from 30 000 to 80 000, the present actuators worked quite well to delay the stall, increasing in the lift and delaying the stall angle of attack. These aerodynamic modifications by the flap actuators obtained from the steady flow were found to be available even if the manipulation of the actuators started after the stall. Activation threshold of the lift-drag ratio as the input for the feedback control was determined from a stall classification map obtained under the steady flow experiment. Effectiveness of this feedback control was then demonstrated under the condition of the wind speed decrease (Reynolds number from 80 000 to 40 000) keeping the angle of attack constant at 11°, at which the stall occurs without the active control. Immediately after the sudden velocity decrease, the decrease in the lift-drag ratio were detected and the dynamic actuations were successfully started, resulting in evading the stall and keeping high and stable lift. An additional operation of the feedback, in which the running actuation is turned off when the lift-drag ratio shows lower than the second threshold value after operation, was revealed to be effective to keep the high lift force under the condition combined with the wind speed increase and decrease within the low Reynolds number range treated in this study. 相似文献
18.
19.
《中国颗粒学报》2008,6(6)
The flow behaviour of powders from a stationary shoe into a moving die, which mimics the die filling process in a rotary tablet press, was analysed using a discrete element method (DEM), in which 2D irregular shaped particles were considered. The influence of the particle shape,size and size distribution, the number of particles used in the simulation, the initial height of powder bed in the shoe, and the filling speed on the average mass flow rate and the critical filling speed (the highest speed at which the die can be completely filled) were explored. It has been found that a maximum flow rate is obtained at the critical filling speed for all systems investigated and poly-disperse systems have higher mass flow rates and higher critical filling speeds than mono-disperse systems. In addition, the powder with particles which can tessellate generally has a lower filling rate and a lower critical filling speed. 相似文献