張晶玲,周麗麗?,黃毅

(1.沈陽農(nóng)業(yè)大學(xué),水利學(xué)院,110866,沈陽;2.沈陽農(nóng)業(yè)大學(xué),土地與環(huán)境學(xué)院,110866,沈陽)

搖臂上噴式降雨模擬器降雨特性分析

張晶玲1,周麗麗1?,黃毅2

(1.沈陽農(nóng)業(yè)大學(xué),水利學(xué)院,110866,沈陽;2.沈陽農(nóng)業(yè)大學(xué),土地與環(huán)境學(xué)院,110866,沈陽)

降雨模擬器是進(jìn)行室內(nèi)或野外土壤侵蝕規(guī)律研究、降雨產(chǎn)流及入滲過程研究等的有效手段,目前野外降雨模擬器存在噴頭安裝高度大、有效降雨區(qū)域較小、與天然降雨有一定差別等問題,本文針對以上問題研制出一種可廣泛使用的搖臂上噴式降雨模擬器。通過率定降雨面積、降雨強(qiáng)度與均勻系數(shù)、雨滴中數(shù)直徑、終點(diǎn)速度與雨滴動能六大降雨特性,探討該降雨裝置的適用性。結(jié)果表明:該裝置的降雨面積可達(dá)到100 m2,降雨均勻系數(shù)為41%;當(dāng)降雨面積為50 m2,供水壓力為0.32 kg/cm2時,平均最大降雨強(qiáng)度為116.25 mm/h,最小降雨強(qiáng)度為61.01 mm/h,降雨均勻系數(shù)最大值為82%,最小值為46%,平均雨滴中數(shù)直徑D50為1.67 mm,平均雨滴終點(diǎn)速度為3.656 m/s,平均雨滴動能為299.4 J/(m2·cm)。該裝置具有能耗小、重量輕、操作簡單、便于移動等優(yōu)點(diǎn),符合天然降雨規(guī)律,可滿足一般田間和野外試驗(yàn)的要求。

搖臂上噴式降雨模擬器;動態(tài)上噴式;降雨面積;降雨均勻性;雨滴中數(shù)直徑

降雨模擬器是模擬天然降雨的一種試驗(yàn)設(shè)備,它可以縮短試驗(yàn)周期,加快雨水入滲規(guī)律和土壤侵蝕規(guī)律的研究過程[1]。對比自然降雨,降雨模擬器可以不受時間和空間的限制,還可以對降雨進(jìn)行有效控制,模擬不同降雨特性[2],具有經(jīng)濟(jì)、便捷、可重復(fù)等優(yōu)點(diǎn)[3]。國內(nèi)外研制的野外模擬降雨設(shè)備包括靜態(tài)下噴式、旋轉(zhuǎn)下噴式、側(cè)噴式、固定上噴式和動態(tài)上噴式等。M.A.Clarke等[4]研制的一種針頭下滴式降雨裝置,屬于靜態(tài)下噴式模擬器,噴出的雨滴分布固定,與天然降雨相比真實(shí)性尚有距離。霍云梅等[5]研制的QYJY503C人工模擬降雨裝置為旋轉(zhuǎn)下噴式,噴出的雨滴為動態(tài),但是這種模擬器噴出的雨滴初速度不為零,不容易計算雨滴落地時的破壞力。陳文亮[6]研制的野外人工模擬降雨裝置為組合側(cè)噴式,水流經(jīng)過供水接管流到噴頭的孔板上,由孔板的錐形面和錐頂上的集流孔進(jìn)行集流形成水柱,水柱射向碎流擋板上,經(jīng)過碎流擋板被分散,形成近似扇形碎流面噴射出去,散落下來形成降雨,降雨雨滴直徑較大,受風(fēng)影響大。M.Esteves等[7]設(shè)計的EMIRE型組合上噴式人工模擬降雨裝置為固定上噴式,這種模擬器將水柱拋向高空,再散落下來形成雨滴,比較接近天然降雨,但該裝置為固定式不具有天然降雨的動態(tài)性。動態(tài)上噴式降雨模擬器通過搖臂將射向空中的水柱以不同的擺動頻率散開,形成大小不等的雨滴,更接近天然降雨的特性。此外,現(xiàn)有野外降雨模擬器還存在噴頭普遍安裝在較高的支架上、耗時費(fèi)力、可移性較差[8]和有效降雨區(qū)域較小[9]等問題。筆者針對以上問題研制出一種可廣泛使用的搖臂上噴式降雨模擬器,此設(shè)備為動態(tài)上噴式,通過搖擺后上噴水流,散落的雨滴初速度為零,上射水流在空中變化更能體現(xiàn)天然降雨的特性。該裝置降雨有效面積較大,體積小,安裝與操作方便,能夠滿足一般環(huán)境條件下野外降雨試驗(yàn)要求。

1 材料與方法

1.1 試驗(yàn)裝置與材料

1)搖臂上噴式降雨模擬器主要包括降雨支架、供水系統(tǒng)、萬向直流式降雨噴頭、擺動系統(tǒng)和動力系統(tǒng)等5部分,立體結(jié)構(gòu)如圖1所示。當(dāng)電機(jī)11轉(zhuǎn)動時,通過曲柄連桿機(jī)構(gòu)10的傳動,“十字型”的搖臂7在一定擺幅內(nèi)往復(fù)運(yùn)動,通過安裝在支架上的萬向直流噴頭將水柱噴向空中,水柱上升至20 m左右受空氣阻力影響而散開形成雨滴,噴頭的往復(fù)運(yùn)動決定了降雨區(qū)域的范圍。降雨過程中可以通過調(diào)節(jié)水壓、“十字型”搖臂的擺動頻率、噴頭數(shù)量以及噴頭角度控制降雨強(qiáng)度和區(qū)域。試驗(yàn)采用搖臂上噴式降雨裝置的1,2,5號降雨噴頭。

圖1 降雨模擬器的立體示意圖Fig.1 Three dimensional schematic diagram of rainfall simulator

2)其他材料:涂料濾紙、雨滴取樣器、滑石粉、天平、燒杯、壓力表、曙紅、100 cm直鋼尺、毛刷、研缽、秒表、游標(biāo)卡尺、量杯、水桶、Φ=150 mm定性濾紙。

1.2 試驗(yàn)方法

1.2.1 降雨面積 降雨面積是指一場降雨中,雨滴降落到地面的覆蓋面積。由式(1)計算得出本裝置降雨控制面積。

式中:S為降雨控制面積,m2;L為徑流小區(qū)長,m;W為徑流小區(qū)寬,m。

1.2.2 降雨強(qiáng)度與均勻系數(shù)率定 降雨強(qiáng)度是指單位時段內(nèi)的降雨量。如圖2和表1所示,在10 m× 5 m的實(shí)驗(yàn)小區(qū)內(nèi),以(0,0)為原點(diǎn),X軸為橫坐標(biāo), Y軸為縱坐標(biāo),相鄰縱坐標(biāo)間距為1 m?？v坐標(biāo)為1、3、5的每排安置9個雨量筒,相鄰觀測點(diǎn)間距為1.25 m。0、2、4每排安置8個雨量筒作為觀測點(diǎn),除左右2個點(diǎn)與邊界距離為0.625 m,其余都為1.25 m。當(dāng)壓力控制為31 kPa、降雨時間為5 min、噴頭向前后左右4個方向偏移5種角度時,通過測定每個點(diǎn)的降雨量,求出平均降雨量和降雨強(qiáng)度,然后由式(2)計算降雨均勻系數(shù)

式中:K為降雨均勻系數(shù),%;n為測點(diǎn)數(shù);Hi為各測點(diǎn)雨量為各測點(diǎn)平均雨量,mm。

表1 噴頭角度組合Tab.1 Nozzle angle combination

1.2.3 雨滴中數(shù)直徑、終點(diǎn)速度及雨滴動能測定在10 m×5 m小區(qū)內(nèi),雨滴直徑測定布置如圖3所示。利用已經(jīng)制備的濾紙測得每個點(diǎn)的色斑直徑,并分別統(tǒng)計出直徑雨滴的個數(shù);根據(jù)公式d= 0.356D0.712,由色斑直徑D算出雨滴直徑d,再由雨滴直徑d求出雨滴質(zhì)量m,然后根據(jù)雨滴直徑的大小分為2種情況求出雨滴的速度,進(jìn)而計算降雨動能,由總雨滴質(zhì)量和總雨滴動能,換算為每m2面積

圖2 降雨強(qiáng)度率定布置圖Fig.2 Fixed layout of rainfall intensity rate

上1 cm降雨的動能。

當(dāng)d＜1.9 mm時,雨滴降落速度用修正的沙玉清公式[10]:

式中:

當(dāng)d≥1.9 mm時,雨滴降落速度用修正的牛頓公式:

雨滴動能公式:

式中:d為雨滴直徑,mm;V為雨滴終點(diǎn)速度,m/s;m為雨滴質(zhì)量,mg;E為雨滴動能,J。

圖3 雨滴直徑測定布置圖Fig.3 Layout of measuring the diameters of raindrops

2 結(jié)果與分析

2.1 降雨面積

試驗(yàn)降雨面積見表2。本裝置最大降雨面積可達(dá)5×20 m2,平均降雨均勻系數(shù)為41%,左右兩側(cè)降雨很少,分布不均勻;因此選取降雨裝置正中間部分,降雨面積為5×10 m2作為試驗(yàn)區(qū)。當(dāng)采用1、2、5號噴頭、供水壓力為31 kPa、降雨控制面積為5× 10 m2時,降雨均勻系數(shù)大于80%,能夠滿足試驗(yàn)要求。

表2 降雨面積表Tab.2 Effective rainfall area

2.2 降雨強(qiáng)度與均勻系數(shù)

本試驗(yàn)利用各測點(diǎn)的坐標(biāo)來描述降雨強(qiáng)度的最大值和最小值位置,降雨強(qiáng)度與均勻系數(shù)見表3。當(dāng)噴頭向前后移動5°時,在坐標(biāo)(8,5)獲得最大降雨強(qiáng)度為173.16 mm/h,坐標(biāo)(0,5)獲得最小降雨強(qiáng)度為41.84 mm/h,噴頭前后移動均勻性系數(shù)分別為82%和80%。當(dāng)噴頭向左右移動5°時,在(13, 4)獲得最大降雨強(qiáng)度125.92 mm/h,在(15,0)獲得最小降雨強(qiáng)度0,可見該點(diǎn)沒有雨滴降落,為邊界無效點(diǎn)。噴頭左右移動時,均勻系數(shù)為78%和46%,低于80%。在均勻性方面,以降雨模擬器正方向?yàn)闇?zhǔn),相同壓力下,萬向直流噴頭向前移動的均勻系數(shù)比向后移動的均勻系數(shù)大,均在80%以上,符合天然降雨的均勻性要求;但降雨噴頭向左右移動均勻系數(shù)小,為左右兩側(cè)降雨量與中間觀測點(diǎn)分布不均造成。在降雨強(qiáng)度方面,噴頭向前后移動的降雨強(qiáng)度大于向左右兩側(cè)移動的降雨強(qiáng)度,是因?yàn)閲婎^擺動對左右方向降雨量有影響,使左右降雨與中間區(qū)域分布不均勻。

2.3 雨滴中數(shù)直徑、終點(diǎn)速度與雨滴動能

通過雨滴中數(shù)直徑圖獲得不同降雨強(qiáng)度下的雨滴中數(shù)直徑D50,見表4。雨滴中數(shù)直徑均在1.3mm以上,若參照其他學(xué)者的雨滴換算公式,與本研究計算結(jié)果比較,差值在0.1～0.3 mm之間,如果用0.2 mm來修正,其雨滴的中數(shù)直徑在1.5 mm以上。由表4可以看出,當(dāng)平均降雨強(qiáng)度為96.64 mm/h時,雨滴中數(shù)直徑為1.67 mm,雨滴終點(diǎn)速度為3.66 mm/s,雨滴動能為299.4 J/(m2·cm)。雨滴中數(shù)直徑隨著降雨強(qiáng)度的增大而減小,但天然降雨的雨滴直徑卻隨著降雨強(qiáng)度的增加而增大。這可能是由于3個降雨噴頭的孔徑是一定的,而雨滴中數(shù)直徑卻隨著上升高度的增大而降低的緣故,或者是因?yàn)榻涤陱?qiáng)度達(dá)到80～100 mm/h,D50不再增大反而減小。雨滴終點(diǎn)速度隨著降雨強(qiáng)度的增加而降低,雨滴動能隨著降雨強(qiáng)度的增大而增大;但本裝置的1號、2號、5號3個噴頭雨滴的直徑隨著降雨強(qiáng)度的增大而減小,可能是雨滴的密度非常大,導(dǎo)致雨滴的動能仍然隨著降雨強(qiáng)度的增大而增大。

表3 降雨強(qiáng)度與均勻系數(shù)表Tab.3 Rainfall intensity and uniform coefficient

表4 雨滴中數(shù)直徑與終點(diǎn)速度Tab.4 Median diameter and terminal velocity of raindrops

3 結(jié)論

搖臂上噴式降雨模擬器體積小、重量輕,便于運(yùn)輸,尤其適宜進(jìn)行野外模擬降雨試驗(yàn);采用柴油機(jī)動力,將提水、加壓、發(fā)電環(huán)節(jié)合而為一,能耗小,很好地解決了輸水和電力問題;操作簡單,便于控制。最大降雨面積可達(dá)5×20 m2,有效降雨面積為5×10 m2,其降雨均勻系數(shù)大于80%,滿足天然降雨的要求,可用于水土保持及其他相關(guān)領(lǐng)域的教學(xué)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

項(xiàng)目名稱:國家自然科學(xué)基金項(xiàng)目“東北黑土區(qū)壟作農(nóng)田融雪侵蝕過程中氮磷遷移轉(zhuǎn)化機(jī)制研究”(41471225);遼寧省農(nóng)業(yè)領(lǐng)域青年科技創(chuàng)新人才培養(yǎng)計劃“遼寧省土壤侵蝕預(yù)報”(2014054);遼寧省高等學(xué)校優(yōu)秀人才支持計劃“凍融條件下秸稈覆蓋坡面磷素流失研究”(LJQ2013074)

張晶玲(1992—),女,研究生。主要研究方向:土壤侵蝕與徑流泥沙研究。E-mail:2512863471@qq.com

?通信作者簡介:周麗麗(1979—),女,博士,副教授。主要研究方向:土壤侵蝕與流域治理。E-mail:zhoulilia@163.com