郄志红,郭丽云,吴鑫淼,冉彦立
太极式鱼道水力特性试验研究及数值模拟
郄志红,郭丽云,吴鑫淼,冉彦立
(河北农业大学城乡建设学院,保定 071001)
针对传统鱼道一经建成,其固体边界条件已经固定,池室内水流流速分布变化不大而难以适应多种鱼类通过的缺点,该文研究一种通过太极圆盘和八卦爻条消减水流能量,形成多态流速场以适应多种鱼类洄游的太极式新型鱼道,并对其进行了水力模型试验和数值分析。首先通过模型试验得到了鱼道在不同工况下的流态和沿程水深变化,然后通过数值计算得到了与模型试验相近的结果,并进一步分析了太极式鱼道的表面流速、近底流速及关键横断面流速分布。结果表明:太极式新型鱼道具有显著的消能减速效果,最浅处水深达到无太极圆盘时相应最浅水深的2倍左右,断面平均最大流速为0.95 m/s,相对于无太极圆盘的情况降低50%左右,池室内水流呈现多态化,另外随太极圆盘方位不同流速场亦有明显变化,该种鱼道可为鱼类提供更多适宜的洄游条件。
流速;模型;流体力学;太极式;鱼道;数学仿真模拟;流态
水利工程在兴利除害的同时有时也带来一定的负面影响,如拦河工程会在一定程度上破坏河流的连通性,阻断鱼类的洄游通道[1-5]。鱼道是改善河流连通性、保护物种多样性的有效措施之一[6-11]。鱼道在国外已有数百年的研究历史,而在国内,鱼道的研究历史只有50多年。文献检索表明,国内外诸多学者通过对鱼道进行大量的室内试验研究与数值模拟计算研究,优化了鱼道内部的水流流态与结构布置尺寸,规范了鱼道设计方法,对于旧式鱼道的改建与新型高效鱼道的建设起到了关键的技术指导作用。Rajaratnam等[12-13]通过对竖缝式鱼道池室流场的试验研究发现,在池室的长宽比为/=10:8时,池室的流态稳定。Liu等[14-15]对竖缝式鱼道进行了模型试验研究,测量了池室内流速分布与紊动特性等水力指标。董志勇等[16-17]对同侧、异侧竖缝式鱼道进行了水力特性试验和放鱼试验,发现同侧竖缝式鱼道适用于中等流量情形,异侧竖缝式鱼道主流流速的沿程变化可用1条二次曲线描述。随着计算机模拟技术的发展,徐体兵等[18]通过数值模拟计算研究发现鱼道水池长宽比在8:8~10.5:8的范围内,可以获得较好的流态。张国强等[19]研究了竖缝宽度对水池内水流结构的影响,并给出竖缝宽度的合理取值范围为/=0.15~0.20(、分别为竖缝宽度和池室宽度)。边永欢等[20]的研究结果表明,竖缝断面平均流速值在0.8~2.0 m/s范围内时对于各级水池内主流区分布、主流流速的沿程衰减规律以及竖缝断面流速分布并无显著影响,并进一步研究了竖缝断面流速分布与各级水池内主流流速分布的变化规律。除关于竖缝式鱼道研究外,Yagci[21]和Ead等[22]分别对池堰式鱼道的水力特性进行了试验及理论研究;孙双科等[23]对近自然鱼道的设计方法和设计理念等进行了分析阐述。
目前国内仍有不少鱼道运行效果不理想,有的鱼道甚至建成即遭废弃,如七里垄电站的鱼道自建成后就从未有鱼、虾、蟹通过,湖南洋塘鱼道1980年建成,自1984年就处于废弃状态[24-27]。传统鱼道的设计大多针对河流中数量较多的某类鱼群,但天然河流存在着不同的洄游时期,并且在每个洄游时期里相应洄游鱼类所适应的流速不同,由于传统鱼道一经建成,固体结构形状也就相对固定,尽管水流会因流量大小等因素有一定变化,但其流速分布等变化不大,因而难以适应多种鱼类通过,这便使得传统鱼道具有一定的局限性,可能导致过鱼种类单一。因此本文将水力学与中国古代哲学思想相结合,提出一种通过太极圆盘和八卦爻条消减水流能量,且可形成多态流速场以适应多种鱼类洄游的太极式新型鱼道,并采用室内模型试验与数值模拟相结合的方法对该新型鱼道的水力特性进行初步分析。
太极式新型鱼道是一种基于古代道家哲学,以“太极八卦”变化的思想为基础,通过阴阳八卦运动营造多变水流环境,改善传统鱼道过鱼种类单一的缺点,形成多态流速场以适应不同鱼类及其他水生生物的生活习性。
池室是鱼道的基本结构单元。目前研究较多的竖缝式鱼道,有同侧竖缝和异侧竖缝,通过池室的束窄和变宽改变水流的流态,消减能量。而太极式鱼道也是通过池室的收缩和扩大,不同的是,太极式鱼道池室边壁采用圆弧曲线,池室之间相连的收缩区由反向圆弧平顺衔接,通过水流的扩散、汇聚消能。池室边壁设置丁坝式隔板,相当于外八卦爻条,上游来流经过外八卦爻条时会受到阻力,消减能量,外八卦爻条亦可为洄游鱼类提供休憩场所。池室中设置太极圆盘,圆盘上阴阳鱼间的过流通道具有分散水流、减弱水流能量的作用,阴阳鱼周围设置与其一起固定在圆盘上的内八卦爻条,本文为简化起见,内外八卦均仅采用单个阳爻。转动起来的太极圆盘与边壁上的丁坝式隔板在不同的时刻组合成不同的运行工况,为水流的多态性创造条件。图1为太极式鱼道设计概念和池室细部结构图。
图1 太极式鱼道设计概念和细部结构
本文采用物理模型试验和数学仿真模拟相结合的方法研究太极式新型鱼道在不同工况下的水流特性,用物理模型试验研究沿程水位变化和水流基本流态,而对于物理模型难以测量的表面及内部流速场分布情况则采用CFD数值模拟的方法。比较用这2种方法得到的沿程水位变化及水流基本流态,用物理试验结果验证数值模拟的准确性。
2.1.1 室内试验设计
按照鱼道设计相关规范[28],制作物理模型。模型尺寸依据重力相似准则确定,具体尺寸如下:鱼道宽度20 cm,鱼道池室长度(即相邻垭口间距)40 cm,鱼道池室深度15 cm,竖缝宽度4 cm,坡度1:10,物理模型弧形边壁的壁厚为1 cm,见图2a。试验模型坐标原点为休息室和池室1之间的进水口断面位置,测点选在鱼道中心线与典型断面的交点上,即=0.11 m与各个典型断面的交点,见图2b。试验采用潜水泵循环供水系统供水,上游设置泄水孔用以调节上游水位。整体模型制作完成后,首先进行“无太极圆盘和八卦爻条”(以下简称工况0)的过水试验,并用测针测量选点的水深;安装太极圆盘和八卦条后,再分2种工况测量鱼道中心线上沿程水深,即S形通道近似正交或平行于鱼道中心线2种情况,分别简称“工况1”和“工况2”,整体装置、测点位置及3种工况示意图见图2。需要说明的是,为节省篇幅图2c中分别以3个不同池室示出了3种工况,但试验当中3个池室都对应同一工况。
图2 试验装置、测量断面及工况示意图
2.1.2 过水试验
过水试验主要观察水流流态和水深变化,采用水位测针测量中心线上典型测点(图2b)处的水位,进而计算水深。3种工况下的过水试验照片与水深测量结果如图 3所示。在工况0条件下,由于鱼道从垭口到池室过流断面经历了较大由窄到宽的变化,水流从急流过渡到缓流,发生了水跃现象,水深由浅变深,水面起伏较大,见图 3a。安装太极圆盘之后,工况1和工况2条件下,仍然发生水跃现象,但相比于工况0水流状况得到明显改善,水面起伏平缓,见图3b,其中最浅处水深由无太极圆盘时的5 mm增加至10~12 mm,最浅处水深的增加2倍左右,相应地,断面平均最大流速减小50%左右,说明加入太极装置后的鱼道消能减速效果明显。
图3 过水试验及测量水深
2.1.3 过鱼试验
在过水试验的同时也进行了过鱼试验。
在工况0时,将试验鱼放在池室3中,试验鱼在经过奋力游泳之后勉强游过了2个池室到达池室1,随后又被水流冲回池室2,说明鱼道中流速过大,还应采取进一步的消能措施。
在工况1和工况2时,试验鱼轻松游过3个池室到达上游的水箱。试验说明,安装太极圆盘和八卦爻条的消能和改变流态的效果明显,增加了鱼类洄游的舒适性。
2.2.1 数学模型
为进一步分析水流特性,以物理试验模型为基础,对该太极式新型鱼道建立数学模型,并进行数学仿真模拟计算。数学计算模型按照物理试验模型尺寸设计,即鱼道宽度0.20 m,鱼道池室长度(即相邻垭口间距)0.40 m,鱼道池室深度0.15 m,竖缝宽度0.04 m,坡度1:10。
2.2.2 控制方程
对于复杂的流场研究,在一定程度上依赖于准确的数值模拟结果。本次试验流体是水,根据流体力学理论,满足连续介质假设的流体运动可以用Navier-Stokes方程准确计算。采用Navier-Stokes方程,建立太极式新型鱼道三维水流RNG-紊流数学模型,控制方程包括连续性方程、动量方程、紊动能方程和紊动能耗散率方程[29-31]:
连续性方程
动量方程(N-S方程)
紊动能方程
紊动能耗散率方程
2.2.3 计算区域及边界条件
取顺水流方向为轴,宽度方向为轴,高度方向为轴,坐标原点与图2b相同,即取在鱼道进口断面(=0)处。为使来流平稳,在=0~0.10 m是平底坡=0,坡面以下为实体。边界条件:进口断面有一定高度的初始水位,设置为压力进口边界;出口设为自由出流边界;底部及两侧边墙为固壁边界,无滑移边界条件;计算区域上方空气入口设定为对称边界条件,即默认无流体穿过该边界。
2.2.4 网格划分
模型整体用0.5 cm×0.5 cm×0.5 cm大小的网格划分,由于池室结构复杂,为提高模拟计算精度,网格划分时对太极式鱼道池室结构用0.2 cm×0.2 cm×0.2 cm大小的网格进行局部网格加密处理。
以数学计算模型计算得出的鱼道中心线上沿程水深与物理模型试验结果的对比验证数学模型的准确性。图4为3种工况(工况0,1,2)下鱼道中心线上实测与模拟水深的对比情况。结果表明,最大相对误差在13.6%,小于15%的允许值[32],实测断面水深与数值模拟计算断面水深变化趋势基本一致。
图4 3种工况下模拟水深和测量水深
3.2.1 自由表面的流速场分析
模拟计算的工况1和工况2的表面的流速场见图 5a、5b。工况1与工况2的主流部分表面流速分布有所不同,但总体情况相似,池室中水流均出现明显分区现象,水流多态。池室中央区域受主流的影响,从垭口到池室中心,流速范围为0.27~1.0 m/s,最大速度1.0 m/s,出现在太极圆盘的前端,结合断面尺寸推算出断面平均最大流速为0.95m/s。池室两侧的非主流区的流速整体低于0.13 m/s,流向偏向鱼道中心线,特别是垭口附近近壁水流流向与主流流向近乎相反,形成两侧的回流区,低流速和回流区可以为洄游鱼提供较大区域的休憩场所,为冲击垭口处的较大流速区积攒体力。鱼沿池室边壁洄游可有效避开最大流速区,节省体力。
图5a、5b还显示顺水流向下级池室的水流更加分散,主流区范围扩大,最大流速由1.0 m/s减小到0.9 m/s,流速减少0.1 m/s,水流自上而下逐级改善趋势明显。
3.2.2 临底流速场分析
在数值模拟计算模型中,紧贴太极圆盘上表面做平行于底坡的剖面,该剖面与池室中心太极圆盘相交,观察顺水流方向上临近底坡的流速场(图5c、5d)。由于内八卦爻条均采用单个阳爻,故工况1和工况2的主要区别是S型通道内的水流流速,工况2中有水流通过,且流道前部流速较大。流速场的整体分布规律和趋势与自由表面流速场大致相同,但与图5a、5b相比,在太极圆盘后部的临底流速小于0.25m/s,明显小于表面流速,且在平面上分布较为均匀,这有赖于池室内形成的水跃及太极装置产生的底流消能作用。
图5 工况1、2的表面流速场和临底流速场
3.2.3 横断面的流速场分析
在池室1、池室2中圆盘上各选取一个横断面,其横坐标分别为=0.52 m、=0.80 m。断面位置见图6a,不同工况下横断面流速场分布见图6b至图6e。
在横断面=0.52 m处,工况1的断面流速范围0~0.40 m/s,最大流速0.40 m/s在断面中心;工况2时的断面流速范围0~0.50 m/s,最大流速0.50 m/s在断面中心,2种工况下,该横断面两侧的八卦爻条内侧流速均可达到0.30 m/s。在横断面=0.80 m处,工况1的断面流速范围0~0.33 m/s,最大流速0.33 m/s在两侧的八卦爻条内侧,断面中心流速较低;工况2的断面流速范围0~0.40 m/s,最大流速0.40 m/s在断面中心及八卦爻条边缘。各断面流速场均呈现较强的多态性,且不同工况的横断面最大流速及流速分布均有明显差异。另外,比较图6b和图6d(或者图6c和图6e),表明在同一种工况下,下级池室断面最大流速小于上游断面最大流速,说明水流得到了进一步调整,使过鱼阻力减小。
图6 横断面位置及流速分布
太极式鱼道的池室弧形边壁和池室的设计与传统鱼道的相似之处在于通过过流通道的宽窄变化雍高垭口上游水位,在下游形成水跃,消减水流动能,降低流速,以利过鱼。不同的是融入了中国古代哲学思想,通过池室中的太极圆盘(含内八卦爻条)转动及与池壁上的外八卦爻条(丁坝式隔板)的组合,营造空间上“多态”和“应时而变”的水流,从而提高鱼道的适用性。文章通过对无圆盘的鱼道、太极圆盘的S形通道与水流近似正交与平行3种工况进行过水试验、过鱼试验及数学仿真模拟,得出以下主要结论:
1)与无太极装置的情况相比,太极圆盘和内外八卦条的组合显著增强了鱼道消能减速效果,最浅处水深达到无太极装置时相应最浅水深的2倍左右,断面最大平均流速减小50%左右。
2)太极式鱼道池室中的表面水流有明显分区现象,水流多态。池室自中间向两侧依次形成主流区(流速变化范围为0.27~1.0 m/s,最大流速出现在太极圆盘的前端)、低速区(流速低于0.13 m/s)和回流区。低速区和回流区可以为洄游鱼提供休憩场所,鱼沿池室边壁洄游可有效避开最大流速区,节省体力。
3)沿水流方向下级池室的水面最大流速低于上一级池室,流速减小0.1 m/s,水流自上而下主流区范围扩大、流态改善。
4)池室内太极圆盘下游水流临底流速分布均匀,小于0.25 m/s,且低于表面主流流速,水跃消能及太极装置的辅助消能效果明显。
5)池室内典型横断面流速场呈现较强的多态性。不同工况下,=0.52 m断面的流速最大变动范围为0~0.50 m/s,=0.80 m断面的流速最大变动范围为0~0.40 m/s,横断面最大流速及流速分布均有明显差异,说明池室中太极圆盘的S形通道处于不同方位可以使流速场分布相应发生变化,即通过太极圆盘的转动可以使鱼道的流速场随时间而变化。
太极式新型鱼道是一种新型鱼道,目前尚处于研究和探索阶段。由于影响流态的因素众多,比如八卦爻条、S形通道、阴阳鱼、丁坝式隔板的尺寸等,本文进行的试验将八卦简化为一圈隔墙,即只有一个阳爻,未形成真正的八卦,今后将进一步研究这种新型鱼道的结构、尺寸、转动机构等问题。
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Experimental study and numerical simulation of hydraulic characteristics of Tai Chi fishway
Qie Zhihong, Guo Liyun, Wu Xinmiao, Ran Yanli
(071001,)
Once the fishway is completed in the traditional style and the flow condition is basically fixed. It will be hard for a variety of fish to go through it at the same time. To solve the problem, in this paper, we developed a Tai Chi fish-way, which can form a rich flow regime and adapt to the migration of many fishes. Based on the ancient Taoist philosophy of “Tai Chi Eight Diagrams”, Tai Chi fishway created a flow changing environment when the eight diagrams rolled, which can offset the general fishway’s shortcoming, “once built, water conditions were basically fixed”. The pool room of the Tai Chi fishway was arc-shaped, which was connected by a reverse arc smoothly. Through the diffusion and aggregation of water, it can help the water flow more smoothly, and reduce the adverse effects of turbulence, vortex and so on. The wall of the pool room was provided with external eight diagrams (groyne type clapboards), which offered resistance to the water flow and helped change its direction when flowing through. The migratory fishes may rest below the external eight diagrams. Tai Chi disk was also set in the pool room and the S shaped channel on the disk had the function of dispersing water flow and dissipating energy. The Tai Chi disk and internal eight diagrams partition can rotate in certain cycles which were combined into different operating conditions at different times to create conditions for the polymorphism of the stream. For Tai Chi fishway had its unique pool room structure, the physical model test alone was difficult to capture the flow structure in detail, and it was time-consuming, too. We used a method combined with the physical model test and numerical simulation to study the flow characteristics of Tai Chi fishway in different conditions. Physical model test was used to study the variation of water level along the course and the basic flow pattern of water flow, and the CFD numerical simulation method was applied to determine the distribution of the surface and internal flow fields which were difficult to be measured by the physical model. Then the water level and flow pattern were compared and verified with each other after being obtained via the two methods. In accordance with the relevant norms of fishway designs and gravity similarity criteria, the fishway physical test models were made. Before the installation of the Tai Chi disc and external and internal eight diagrams, a water test and a measurement of the water depth along the center line were conducted. After the installation of Tai Chi disc, the test was divided into two groups to measure the water depth along the center line. The S shaped channel was approximately parallel and vertical to the flow. In each case, Tai Chi disk placement angle was consistent. The above three situations were simulated by numerical calculation. The results showed that they had a same trend. The maximum error was 13.6%, within the allowable range of 15%. Therefore, the calculation model was reliable and can be used to simulate the complex flow field of the pool room. Therefore, the surface velocity, bottom velocity and critical cross section velocity were further analyzed. The results showed that the Tai Chi fishway had significant energy dissipation reduction effect. The shallowest water depth was about twice of that of the corresponding shallowest water depth without the Taichi disk. Average maximum velocity of cross section decreased by about 50%, the flow presented polymorphism, Tai Chi disk’s different positions also made flow change, which can provide more suitable conditions for migratory fish.
flow velocity; models; hydrodynamics; Tai Chi; fishway; mathematical simulation; flow condition
10.11975/j.issn.1002-6819.2018.02.025
TV131
A
1002-6819(2018)-02-0182-07
2017-09-03
2017-11-12
河北省水利科研推广计划(2016-63);河北农业大学青年学术带头人基金(2016)
郄志红,男,河北徐水人,教授,博士生导师,研究方向为水工结构优化及管理。Email:qiezhihong@163.com
郄志红,郭丽云,吴鑫淼,冉彦立. 太极式鱼道水力特性试验研究及数值模拟[J]. 农业工程学报,2018,34(2):182-188. doi:10.11975/j.issn.1002-6819.2018.02.025 http://www.tcsae.org
Qie Zhihong, Guo Liyun, Wu Xinmiao, Ran Yanli. Experimental study and numerical simulation of hydraulic characteristics of Tai Chi fishway[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2018, 34(2): 182-188. (in Chinese with English abstract) doi:10.11975/j.issn.1002-6819.2018.02.025 http://www.tcsae.org