摇臂上喷式降雨模拟器降雨特性分析

张晶玲,周丽丽†,黄毅

(1.沈阳农业大学,水利学院,110866,沈阳;2.沈阳农业大学,土地与环境学院,110866,沈阳)

摇臂上喷式降雨模拟器降雨特性分析

张晶玲1,周丽丽1†,黄毅2

(1.沈阳农业大学,水利学院,110866,沈阳;2.沈阳农业大学,土地与环境学院,110866,沈阳)

降雨模拟器是进行室内或野外土壤侵蚀规律研究、降雨产流及入渗过程研究等的有效手段,目前野外降雨模拟器存在喷头安装高度大、有效降雨区域较小、与天然降雨有一定差别等问题,本文针对以上问题研制出一种可广泛使用的摇臂上喷式降雨模拟器。通过率定降雨面积、降雨强度与均匀系数、雨滴中数直径、终点速度与雨滴动能六大降雨特性,探讨该降雨装置的适用性。结果表明:该装置的降雨面积可达到100 m2,降雨均匀系数为41%;当降雨面积为50 m2,供水压力为0.32 kg/cm2时,平均最大降雨强度为116.25 mm/h,最小降雨强度为61.01 mm/h,降雨均匀系数最大值为82%,最小值为46%,平均雨滴中数直径D50为1.67 mm,平均雨滴终点速度为3.656 m/s,平均雨滴动能为299.4 J/(m2·cm)。该装置具有能耗小、重量轻、操作简单、便于移动等优点,符合天然降雨规律,可满足一般田间和野外试验的要求。

摇臂上喷式降雨模拟器;动态上喷式;降雨面积;降雨均匀性;雨滴中数直径

降雨模拟器是模拟天然降雨的一种试验设备,它可以缩短试验周期,加快雨水入渗规律和土壤侵蚀规律的研究过程[1]。对比自然降雨,降雨模拟器可以不受时间和空间的限制,还可以对降雨进行有效控制,模拟不同降雨特性[2],具有经济、便捷、可重复等优点[3]。国内外研制的野外模拟降雨设备包括静态下喷式、旋转下喷式、侧喷式、固定上喷式和动态上喷式等。M.A.Clarke等[4]研制的一种针头下滴式降雨装置,属于静态下喷式模拟器,喷出的雨滴分布固定,与天然降雨相比真实性尚有距离。霍云梅等[5]研制的QYJY503C人工模拟降雨装置为旋转下喷式,喷出的雨滴为动态,但是这种模拟器喷出的雨滴初速度不为零,不容易计算雨滴落地时的破坏力。陈文亮[6]研制的野外人工模拟降雨装置为组合侧喷式,水流经过供水接管流到喷头的孔板上,由孔板的锥形面和锥顶上的集流孔进行集流形成水柱,水柱射向碎流挡板上,经过碎流挡板被分散,形成近似扇形碎流面喷射出去,散落下来形成降雨,降雨雨滴直径较大,受风影响大。M.Esteves等[7]设计的EMIRE型组合上喷式人工模拟降雨装置为固定上喷式,这种模拟器将水柱抛向高空,再散落下来形成雨滴,比较接近天然降雨,但该装置为固定式不具有天然降雨的动态性。动态上喷式降雨模拟器通过摇臂将射向空中的水柱以不同的摆动频率散开,形成大小不等的雨滴,更接近天然降雨的特性。此外,现有野外降雨模拟器还存在喷头普遍安装在较高的支架上、耗时费力、可移性较差[8]和有效降雨区域较小[9]等问题。笔者针对以上问题研制出一种可广泛使用的摇臂上喷式降雨模拟器,此设备为动态上喷式,通过摇摆后上喷水流,散落的雨滴初速度为零,上射水流在空中变化更能体现天然降雨的特性。该装置降雨有效面积较大,体积小,安装与操作方便,能够满足一般环境条件下野外降雨试验要求。

1 材料与方法

1.1 试验装置与材料

1)摇臂上喷式降雨模拟器主要包括降雨支架、供水系统、万向直流式降雨喷头、摆动系统和动力系统等5部分,立体结构如图1所示。当电机11转动时,通过曲柄连杆机构10的传动,“十字型”的摇臂7在一定摆幅内往复运动,通过安装在支架上的万向直流喷头将水柱喷向空中,水柱上升至20 m左右受空气阻力影响而散开形成雨滴,喷头的往复运动决定了降雨区域的范围。降雨过程中可以通过调节水压、“十字型”摇臂的摆动频率、喷头数量以及喷头角度控制降雨强度和区域。试验采用摇臂上喷式降雨装置的1,2,5号降雨喷头。

图1 降雨模拟器的立体示意图Fig.1 Three dimensional schematic diagram of rainfall simulator

2)其他材料:涂料滤纸、雨滴取样器、滑石粉、天平、烧杯、压力表、曙红、100 cm直钢尺、毛刷、研钵、秒表、游标卡尺、量杯、水桶、Φ=150 mm定性滤纸。

1.2 试验方法

1.2.1 降雨面积 降雨面积是指一场降雨中,雨滴降落到地面的覆盖面积。由式(1)计算得出本装置降雨控制面积。

式中:S为降雨控制面积,m2;L为径流小区长,m;W为径流小区宽,m。

1.2.2 降雨强度与均匀系数率定 降雨强度是指单位时段内的降雨量。如图2和表1所示,在10 m× 5 m的实验小区内,以(0,0)为原点,X轴为横坐标, Y轴为纵坐标,相邻纵坐标间距为1 m。纵坐标为1、3、5的每排安置9个雨量筒,相邻观测点间距为1.25 m。0、2、4每排安置8个雨量筒作为观测点,除左右2个点与边界距离为0.625 m,其余都为1.25 m。当压力控制为31 kPa、降雨时间为5 min、喷头向前后左右4个方向偏移5种角度时,通过测定每个点的降雨量,求出平均降雨量和降雨强度,然后由式(2)计算降雨均匀系数

式中:K为降雨均匀系数,%;n为测点数;Hi为各测点雨量为各测点平均雨量,mm。

表1 喷头角度组合Tab.1 Nozzle angle combination

1.2.3 雨滴中数直径、终点速度及雨滴动能测定在10 m×5 m小区内,雨滴直径测定布置如图3所示。利用已经制备的滤纸测得每个点的色斑直径,并分别统计出直径雨滴的个数;根据公式d= 0.356D0.712,由色斑直径D算出雨滴直径d,再由雨滴直径d求出雨滴质量m,然后根据雨滴直径的大小分为2种情况求出雨滴的速度,进而计算降雨动能,由总雨滴质量和总雨滴动能,换算为每m2面积

图2 降雨强度率定布置图Fig.2 Fixed layout of rainfall intensity rate

上1 cm降雨的动能。

当d＜1.9 mm时,雨滴降落速度用修正的沙玉清公式[10]:

式中:

当d≥1.9 mm时,雨滴降落速度用修正的牛顿公式:

雨滴动能公式:

式中:d为雨滴直径,mm;V为雨滴终点速度,m/s;m为雨滴质量,mg;E为雨滴动能,J。

图3 雨滴直径测定布置图Fig.3 Layout of measuring the diameters of raindrops

2 结果与分析

2.1 降雨面积

试验降雨面积见表2。本装置最大降雨面积可达5×20 m2,平均降雨均匀系数为41%,左右两侧降雨很少,分布不均匀;因此选取降雨装置正中间部分,降雨面积为5×10 m2作为试验区。当采用1、2、5号喷头、供水压力为31 kPa、降雨控制面积为5× 10 m2时,降雨均匀系数大于80%,能够满足试验要求。

表2 降雨面积表Tab.2 Effective rainfall area

2.2 降雨强度与均匀系数

本试验利用各测点的坐标来描述降雨强度的最大值和最小值位置,降雨强度与均匀系数见表3。当喷头向前后移动5°时,在坐标(8,5)获得最大降雨强度为173.16 mm/h,坐标(0,5)获得最小降雨强度为41.84 mm/h,喷头前后移动均匀性系数分别为82%和80%。当喷头向左右移动5°时,在(13, 4)获得最大降雨强度125.92 mm/h,在(15,0)获得最小降雨强度0,可见该点没有雨滴降落,为边界无效点。喷头左右移动时,均匀系数为78%和46%,低于80%。在均匀性方面,以降雨模拟器正方向为准,相同压力下,万向直流喷头向前移动的均匀系数比向后移动的均匀系数大,均在80%以上,符合天然降雨的均匀性要求;但降雨喷头向左右移动均匀系数小,为左右两侧降雨量与中间观测点分布不均造成。在降雨强度方面,喷头向前后移动的降雨强度大于向左右两侧移动的降雨强度,是因为喷头摆动对左右方向降雨量有影响,使左右降雨与中间区域分布不均匀。

2.3 雨滴中数直径、终点速度与雨滴动能

通过雨滴中数直径图获得不同降雨强度下的雨滴中数直径D50,见表4。雨滴中数直径均在1.3mm以上,若参照其他学者的雨滴换算公式,与本研究计算结果比较,差值在0.1～0.3 mm之间,如果用0.2 mm来修正,其雨滴的中数直径在1.5 mm以上。由表4可以看出,当平均降雨强度为96.64 mm/h时,雨滴中数直径为1.67 mm,雨滴终点速度为3.66 mm/s,雨滴动能为299.4 J/(m2·cm)。雨滴中数直径随着降雨强度的增大而减小,但天然降雨的雨滴直径却随着降雨强度的增加而增大。这可能是由于3个降雨喷头的孔径是一定的,而雨滴中数直径却随着上升高度的增大而降低的缘故,或者是因为降雨强度达到80～100 mm/h,D50不再增大反而减小。雨滴终点速度随着降雨强度的增加而降低,雨滴动能随着降雨强度的增大而增大;但本装置的1号、2号、5号3个喷头雨滴的直径随着降雨强度的增大而减小,可能是雨滴的密度非常大,导致雨滴的动能仍然随着降雨强度的增大而增大。

表3 降雨强度与均匀系数表Tab.3 Rainfall intensity and uniform coefficient

表4 雨滴中数直径与终点速度Tab.4 Median diameter and terminal velocity of raindrops

3 结论

摇臂上喷式降雨模拟器体积小、重量轻,便于运输,尤其适宜进行野外模拟降雨试验;采用柴油机动力,将提水、加压、发电环节合而为一,能耗小,很好地解决了输水和电力问题;操作简单,便于控制。最大降雨面积可达5×20 m2,有效降雨面积为5×10 m2,其降雨均匀系数大于80%,满足天然降雨的要求,可用于水土保持及其他相关领域的教学和科研。

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Rainfall characteristics of Rocker Arm Top Spray rainfall simulator

Zhang Jingling1,Zhou Lili1,Huang Yi2

(1.College of WaterResource,Shenyang Agricultural University,110866,Shenyang,China; 2.College of Soil and Environment,Shenyang Agricultural University,110866,Shenyang,China)

[Background]Rainfall simulator is commonly used in indoor and outdoor soil erosion research and the process of infiltration and rainfall runoff study.However,tall nozzles,small efficient rainfall area,and differences from natural rainfall constraint some rainfall simulators widely used.Based on all above,we developed a Rocker Arm Top Spray rainfall simulator which is supposed to be used widely.[Methods]The Rocker Arm Top Spray rainfall simulator we developed was composed of three rainfall nozzles(number1,2,and 5,aperture 9 mm),two middle rainfall nozzles(number 3 and 4, aperture 13 mm),Axle tube,“cross type”rocker arm,Axle hole,connecting rod,crank connecting rod mechanism,motor,hose,main water supply pipe,pressure gauge and water well pipeline interface.The precipitation area,rainfall intensity,average rainfall uniformity coefficient,median diameter of raindrops,terminal velocity and the kinetic energy of rain were measured,and the applicability of the rainfall simulator was discussed.[Results]Rainfall uniformity value was 41%,when the rainfall area was 100 m2,which maybe the maximum rainfall area of the simulator.When the rainfall area was 50 m2, water pressure was 0.32 kg/cm2,the rainfall uniformity value was 82%for the maximum rainfall intensity(116.25 mm/h)and 46%for the minimum rainfall intensity(61.01 mm/h).Under the same pressure, the rainfall uniformity value range changed from 46%to 82%.The rainfall uniformity value was 82% when the rainfall nozzles moving forward,while the rainfall uniformity value was 80%when the rainfall nozzles moving backward.That means that when the rainfall nozzles moving forward and backward,the rainfall uniformity value was more than 80%,which accorded with the requirements of natural rainfall uniformity.But the average rainfall uniformity value was smaller than 80%when the rainfall nozzle moving left and right,which might be caused by the uneven rainfall.The rain intensity was related to the rainfall median diameter.The measured average median diameter(D50),the average terminal velocity of raindrops,and the average raindrop kinetic energy was 1.67 mm,3.656 m/s and 299.4 J/(m2·cm) respectively,under the different rainfall intensity.The median diameter value presented decreasing trend with the increase of rainfall intensity,but the median diameter of natural rainfall increased with the increase of rainfall intensity,which may be caused by the definite apertures of the three rainfall nozzle. The median diameter decreased with the height increasing,or because the rainfall intensity reached 80～100 mm/h the measured median diameter no longer increased.With the increase of rainfall intensity,the terminal velocity of raindrops tended to decrease,but the kinetic energy of raindrops showed an increasing trend.That was probably because that the droplet density was very large,the kinetic energy of rainfall increased with the rainfall intensity increasing.[Conclusions]The device was small,light,in simple operation and convenient movement,especially suitable for field rainfall simulation,which can meet the requirements of the general field and outdoor experiments.

Rocker Arm Top Spray rainfall simulator;dynamic spray type;rainfall area;rainfall uniformity coefficient;median diameter

P481;TH765

A

1672-3007(2016)06-0125-06

10.16843/j.sswc.2016.06.016

2016 04 05

2016 06 20

项目名称:国家自然科学基金项目“东北黑土区垄作农田融雪侵蚀过程中氮磷迁移转化机制研究”(41471225);辽宁省农业领域青年科技创新人才培养计划“辽宁省土壤侵蚀预报”(2014054);辽宁省高等学校优秀人才支持计划“冻融条件下秸秆覆盖坡面磷素流失研究”(LJQ2013074)

张晶玲(1992—),女,研究生。主要研究方向:土壤侵蚀与径流泥沙研究。E-mail:2512863471@qq.com

†通信作者简介:周丽丽(1979—),女,博士,副教授。主要研究方向:土壤侵蚀与流域治理。E-mail:zhoulilia@163.com