王文娥,劉海強(qiáng),胡笑濤
·農(nóng)業(yè)水土工程·
側(cè)渠底高對(duì)分水口水力特性影響數(shù)值模擬研究
王文娥,劉海強(qiáng),胡笑濤
(西北農(nóng)林科技大學(xué)旱區(qū)農(nóng)業(yè)水土工程教育部重點(diǎn)實(shí)驗(yàn)室,楊凌 712100)
目前對(duì)灌區(qū)分水口水力性能的研究多集中在主渠和側(cè)渠底部高程相等的情況下,對(duì)于普遍存在的側(cè)渠底部高程高于主渠時(shí)的分流特性缺乏系統(tǒng)研究。該文在試驗(yàn)基礎(chǔ)上,利用FLOW-3D軟件對(duì)側(cè)渠不同底高、主渠來流量的矩形渠道分水口進(jìn)行了數(shù)值模擬研究,將主渠各斷面水深、流速的模擬值與實(shí)測(cè)值進(jìn)行對(duì)比,發(fā)現(xiàn)流速變化與實(shí)測(cè)值變化規(guī)律基本一致,相對(duì)誤差均小于10%,利用FLOW-3D對(duì)分水口進(jìn)行數(shù)值計(jì)算具有合理可信性。結(jié)果表明:分水口處的水面波動(dòng)受主渠來流的影響,流量越大,波動(dòng)越大;高于側(cè)渠底高的水流會(huì)對(duì)低于側(cè)渠底高的下層水流產(chǎn)生影響,使下層水流具有向上的流速分量,參與分水口分流;同一主渠來流量下,隨側(cè)渠底高的增加,側(cè)渠進(jìn)口斷面最大流速和水深逐漸減??;側(cè)渠進(jìn)口斷面靠近上游端的區(qū)域湍動(dòng)較大,而在下游端靠近底部湍動(dòng)能值較小。研究為灌區(qū)配水及水量控制提供了參考依據(jù)。
數(shù)值分析;流量;流速;矩形渠道;分水口;渠寬比
灌區(qū)渠道量水是實(shí)現(xiàn)灌區(qū)現(xiàn)代化的重要途徑[1],分水口是灌區(qū)灌溉渠系常見的過水建筑物[2-3],為方便灌溉,通常直接在經(jīng)過田間的渠道一側(cè)開設(shè)分水口引水入田。若直接在分水口處進(jìn)行量水,就可獲得進(jìn)入田間的流量,同時(shí)避免因修建特設(shè)量水設(shè)施造成的二次水頭損失。對(duì)于分水口的水力性能的研究,最初是理論分析與試驗(yàn)相結(jié)合進(jìn)行研究。Taylor等[4-7]分別研究了分水口處的分流規(guī)律,Hsu等[8-9]研究了矩形渠道主側(cè)渠道等寬時(shí)的分水規(guī)律,建立了主渠上下游水深比、流量比與傅汝德數(shù)關(guān)系等一維理論模型。Hayes等[10-13]利用湍流模型對(duì)明渠分水口處的流場(chǎng)變化進(jìn)行了二維分析研究。Neary等[14-15]選用湍流模型對(duì)渠道分流問題進(jìn)行了三維數(shù)值模型模擬,數(shù)值模擬結(jié)果與試驗(yàn)數(shù)據(jù)的對(duì)比結(jié)果表明,兩者平均速度變化趨勢(shì)之間存在一定的對(duì)應(yīng)關(guān)系。Huang等[16]根據(jù)匯流試驗(yàn)數(shù)據(jù)和數(shù)值模擬數(shù)據(jù)驗(yàn)證了基于Reynolds-Averaged NavierStokes方程和湍流模型的三維數(shù)值模型。近年來,隨著計(jì)算機(jī)的廣泛應(yīng)用及CFD理論的發(fā)展和完善,數(shù)值模擬技術(shù)開始廣泛應(yīng)用到渠道量水方面。楊帆等[17-18]利用FLUENT軟件對(duì)取水角為45°和30°的明渠岸邊側(cè)向取水口進(jìn)行了三維數(shù)值模擬,分析了分水口處的流速分布、分水寬度及湍動(dòng)能。孟文等[19-21]利用FLOW-3D軟件對(duì)彎道及明渠分水口水流進(jìn)行了數(shù)值模擬,分析了分水口水力特性的變化規(guī)律和影響因素。
在對(duì)分水口的數(shù)值模擬研究中,大多數(shù)研究主要針對(duì)主渠與側(cè)渠底部高程一致的情況下進(jìn)行模擬,對(duì)側(cè)渠底部高程高于主渠時(shí)的分水特性還缺少深入系統(tǒng)地研究。分水口處的流場(chǎng)分布復(fù)雜,試驗(yàn)方法可以測(cè)定一定數(shù)量的斷面流速、水深數(shù)據(jù),采用數(shù)值模擬的方法可以更全面了解分水口附近的流場(chǎng)分布,為系統(tǒng)分析分水口水力性能提供全面的流場(chǎng)分布資料。本文基于矩形渠道分水口水力性能試驗(yàn)研究,利用FLOW-3D軟件對(duì)不同側(cè)渠底高的分水口進(jìn)行了數(shù)值模擬計(jì)算,并將模擬結(jié)果與試驗(yàn)結(jié)果進(jìn)行對(duì)比,以期深入了解側(cè)渠底高對(duì)分水口處水流特性的影響,為灌區(qū)配水及水量控制提供依據(jù)。
分水口水流流動(dòng)控制方程包括連續(xù)性方程、運(yùn)動(dòng)方程及湍流方程[12]。
連續(xù)性方程
運(yùn)動(dòng)方程
式中為流體密度,研究對(duì)象為水,取值1 000 kg/m3;為流動(dòng)時(shí)間,s;u、u分別為流速矢量在x、x方向的分量(=1,2,3;=1,2,3),m/s;為流體動(dòng)力黏滯系數(shù),kg·/(m·s);為流體壓強(qiáng),Pa;f為流體所受的質(zhì)量力,m/s2。
灌區(qū)輸水渠道及末級(jí)渠道中雷諾數(shù)均大于2 000,渠道中的水流基本都屬于紊流粗糙區(qū)(阻力平方區(qū)),本文研究的渠道尺寸接近灌區(qū)農(nóng)渠及毛渠,渠道分水口處的水流狀態(tài)為湍流,所以選用湍流模型[22]。目前常用的3種-湍流模型中,RNG模型是基于重整化群(renormalization group)的理論提出來的[23],經(jīng)過對(duì)標(biāo)準(zhǔn)湍流模型的改進(jìn)和實(shí)用化處理,RNG模型考慮了旋轉(zhuǎn)效應(yīng),因此提高了強(qiáng)旋轉(zhuǎn)流動(dòng)計(jì)算精度。水流流經(jīng)分水口后,一部分水流流向下游,而另一部分通過分水口流向側(cè)渠,因此分水口處的水流變化強(qiáng)烈,呈現(xiàn)復(fù)雜的三維特性,故選用RNG模型。對(duì)于不可壓縮流體流動(dòng),其相應(yīng)的和方程[23]為
方程
方程
目前,大部分處理水氣兩相流的自由表面都是采用流體體積法[24],該方法往往會(huì)增加計(jì)算時(shí)間或是計(jì)算結(jié)果有較大偏差,F(xiàn)LOW-3D軟件采用了TruVOF方法,只計(jì)算含有液體單元,因此在很大程度上減少了模型收斂所需的時(shí)間,同時(shí)精確模擬具有自由界面的流動(dòng)問題[22]。TruVOF法對(duì)流體界面的三維重構(gòu)控制方程[17]為
為了驗(yàn)證數(shù)值分析模型及模擬結(jié)果,首先進(jìn)行了原型試驗(yàn),在西北農(nóng)林科技大學(xué)水工廳進(jìn)行,試驗(yàn)系統(tǒng)的平面布置如圖1a所示。試驗(yàn)系統(tǒng)由水泵、供水管道、蓄水池、穩(wěn)水池、矩形渠道、排水池及回水管道組成。試驗(yàn)中主渠和側(cè)渠均為矩形渠道,主渠長12 m,寬0.46 m,側(cè)渠長2.5 m,寬度在0.14~0.46 m范圍可調(diào),側(cè)渠較主渠底部高程分別高出0.06、0.08、0.1 m。共設(shè)置8個(gè)測(cè)流斷面,斷面測(cè)點(diǎn)具體布置如圖1b所示,距分水口上游端和下游端0.5 m處設(shè)斷面Ⅰ和Ⅶ,試驗(yàn)中側(cè)渠寬度值非定值,為了更好地分析主渠在分水口處水面變化規(guī)律,在分水口處共設(shè)置5個(gè)斷面,分別為斷面Ⅱ、Ⅲ、Ⅳ、Ⅴ、Ⅵ,各斷面間隔相同,每個(gè)斷面設(shè)置3個(gè)測(cè)點(diǎn),位于主渠中心處()、距主渠邊壁5 cm處(和,其中主渠無分水口一側(cè),為靠近分水口一側(cè))。使用SCM60型水位測(cè)針測(cè)量,精度±0.1 mm。
注:a、b、c分別主渠無分水口一側(cè)、主渠中心處、靠近分水口一側(cè);Q為主渠來流量,L·s-1。Ⅰ到Ⅷ分別為各測(cè)點(diǎn)所在斷面。
為了更好地對(duì)比分析模擬結(jié)果與試驗(yàn)結(jié)果,同時(shí)消除邊界條件的影響,本研究對(duì)試驗(yàn)渠道進(jìn)行了簡化,如圖2所示。模擬主、側(cè)渠寬度跟實(shí)際主、側(cè)渠寬度B、b一致,主渠道為0.46 m,側(cè)渠道分別設(shè)置了0.14、0.22、0.30、0.38、0.46 cm,即主側(cè)渠寬比R(R=b/B)為0.3、0.48、0.65、0.83、1;側(cè)渠底高(以主渠底部高程為0點(diǎn))取0.06、0.08、0.1 m,主渠和側(cè)渠長度在實(shí)際渠道長度基礎(chǔ)上相應(yīng)的延伸,分別取12和5 m。分水口附近流場(chǎng)變化劇烈,因此對(duì)分水口處的網(wǎng)格進(jìn)行了加密,單元格長度為0.01 m,其余區(qū)域單元網(wǎng)格為0.02 m,網(wǎng)格總量約為90萬。渠道模型中主渠進(jìn)口邊界設(shè)定為流量進(jìn)口(volume flow rate),進(jìn)口流量與試驗(yàn)相同,分別為19.96、25.2、30.2、36.5、45.32 L/s;主渠、側(cè)渠末端出口設(shè)為自由出流(outflow);渠道邊壁選擇無滑移固壁邊壁(wall);自由水面以上為空氣,相對(duì)壓強(qiáng)為0。
圖2 體型圖及邊界設(shè)置
為了更好地將數(shù)學(xué)模型計(jì)算水深與實(shí)測(cè)水深進(jìn)行對(duì)比,主渠斷面選取與試驗(yàn)位置相同(圖1b),模型區(qū)域坐標(biāo)原點(diǎn)取在主渠進(jìn)口渠底,方向?yàn)閭?cè)渠水流流向,方向?yàn)橹髑鞣较?,方向垂直向上?/p>
根據(jù)矩形渠道分水口試驗(yàn)和數(shù)值模擬結(jié)果,對(duì)主渠各個(gè)來流量、不同側(cè)渠底高下的分水口各斷面水深進(jìn)行了對(duì)比分析,表1和表2分別為主渠來流量為30.2 L/s、渠寬比為1時(shí)各側(cè)渠底高條件下,以及主渠來流量為45.32 L/s、側(cè)渠底高0.06 m時(shí)5種渠寬比的模型計(jì)算結(jié)果與試驗(yàn)結(jié)果的對(duì)比分析結(jié)果。
從表1和表2中可以看出,模擬值與實(shí)測(cè)值相差較小,主渠無分水口側(cè)處的相對(duì)誤差值最大為8.76%,主渠中心線處的相對(duì)誤差值最大為8.58%,主渠靠近分水口側(cè)處的相對(duì)誤差值最大為8.45%,相對(duì)誤差值都在9%以內(nèi),表明利用FLOW-3D對(duì)矩形渠道分水口的模擬水深結(jié)果可信。
表1 來流量30.2 L·s-1和渠寬比1時(shí)不同斷面水深相對(duì)誤差分析
注:相對(duì)誤差=(模擬值?實(shí)測(cè)值)/實(shí)測(cè)值×100%,下同。
Note: Relative error = (analog value-measured value)/measured value×100%, same as below.
流速是分析分水口處水流水力性能和運(yùn)動(dòng)規(guī)律的基本要素。為了保證利用數(shù)值模擬研究分水口處流速分布的合理性,以流量45.32 L/s、側(cè)渠底高為0.06 m、渠寬比為1時(shí)為例,對(duì)斷面Ⅰ、斷面Ⅱ和斷面Ⅶ和距分水口1 m處的側(cè)渠斷面流速模擬值和實(shí)測(cè)值進(jìn)行了比較,在流量45.32 L/s時(shí),主渠和側(cè)渠水深分別小于0.15和0.09 m,因此提取了主渠各斷面測(cè)點(diǎn)分別在0.4和0.7處(為主渠流量45.32 L/s時(shí)斷面Ⅰ的水深,m)、側(cè)渠各斷面0.5處的流速。由圖3a~圖3d可知,各斷面流速實(shí)測(cè)值與模擬值分布規(guī)律相似,且模擬值與實(shí)測(cè)值的最大相對(duì)誤差均在10%以內(nèi),表明利用FLOW-3D對(duì)分水口進(jìn)行流速研究和分析合理可信。
同一來流量和側(cè)渠底高、渠寬比不同時(shí)分水口附近水面線、流速分布變化趨勢(shì)一致[25],本文主要討論側(cè)渠底高對(duì)分水口水力性能的影響,因此以渠寬比=1時(shí)分水口附近流場(chǎng)分布為例,分析側(cè)渠底高對(duì)分水口水力特性的影響。
表2 來流量45.32L·s-1和側(cè)渠底高0.06 m時(shí)各渠寬比下斷面水深相對(duì)誤差
注:H為主渠流量45.32 L·s-1時(shí)斷面Ⅰ的水深,m。
2.2.1 水面線
主渠水流在分水口處,受到分流的影響,水面變化劇烈,具有復(fù)雜的三維特性[19],如圖4a所示。為了研究分水口處的水面變化,提取了模擬結(jié)果中的水深。受篇幅所限,而且各側(cè)渠底高下的主渠在分水口處的水面變化規(guī)律基本相似,圖4b給出了側(cè)渠底高為0.06 m時(shí)不同來流量下的主渠在分水口處的水面線變化圖。由圖4b可以看出,主渠遠(yuǎn)離分水口一側(cè)、主渠中心線和主渠靠近分水口一側(cè)的水面線變化各不相同。水流靠近分水口時(shí),和處水深呈現(xiàn)逐漸上升的趨勢(shì),隨主渠來流量的增大,上升趨勢(shì)逐漸增加,當(dāng)主渠來流量較小時(shí),和處水面線基本重合,當(dāng)流量較大時(shí),處水面要低于處水面,即越靠近分水口,水面受分水口的影響越大;而處水深在分水口段呈先減小后增加的趨勢(shì),水深最小點(diǎn)的位置隨主渠來流量的變化而變化,水深在分水口下游端達(dá)到最大值,且最大值得位置點(diǎn)保持不變;當(dāng)主渠來流量較小時(shí),、和處的水面波動(dòng)較小,尤其是和處水面線近似于直線,當(dāng)主渠來流量增加時(shí),三處的水面波動(dòng)程度增加,處的水面波動(dòng)最大,主渠來流量越大,分水口處的水深變化越劇烈,水面越不穩(wěn)定。
注:Q為主渠來流量,L·s-1;P為側(cè)渠底高,m。
2.2.2 主渠斷面流速
主渠水流流經(jīng)分水口時(shí),受分水口的影響,主渠各層流線急劇變化,各斷面流速分布不同,故提取了數(shù)值模擬結(jié)果中不同水深處的流速矢量圖以及各斷面不同水深處的流速。圖5為流量為45.32 L/s,側(cè)渠底高為0.06 m,水深分別為0.5、、1.5、2下的流速矢量分布圖。
注:流量為45.32 L·s-1;側(cè)渠底高0.06 m。圖6~圖7同。
由圖5可以看出,當(dāng)主渠中水流未流經(jīng)分水口時(shí),水流不受分水口的影響,流線較為平順,流速分布均勻;當(dāng)水流運(yùn)動(dòng)到距分水口上游端一定距離時(shí),受到分水口的影響,主渠在靠近分水口一側(cè)的流線發(fā)生偏轉(zhuǎn),而遠(yuǎn)離分水口處的流線偏轉(zhuǎn)角度較小,且隨著距離的增加,偏轉(zhuǎn)角度逐漸減小,即分水口對(duì)主渠水流的影響隨著距離的增加逐漸減??;主渠水流流經(jīng)分水口時(shí),受到水流側(cè)向離心力的影響,部分水流流線發(fā)生偏折,進(jìn)入側(cè)渠。當(dāng)主渠水深小于側(cè)渠底高時(shí),側(cè)渠未分流,主渠水流流線將不受分水口的影響,流線較為平順,當(dāng)側(cè)渠分流時(shí),由圖5a和圖5b可以看出,分水口處的流線較主渠上游端發(fā)生了明顯的偏轉(zhuǎn),說明高于側(cè)渠底高的水流即參與分流的水流對(duì)下層水流產(chǎn)生了影響,使低于側(cè)渠底高下的部分水流產(chǎn)生了趨向于側(cè)渠的流動(dòng)趨勢(shì)。
為了更好地了解主渠各斷面不同水深處各方向流速和大小變化,分析了各工況下的流速。圖6給出了流量為45.32 L/s,側(cè)渠底高為0.06 m時(shí)0.5、、1.5、2水深下的主渠斷面Ⅰ~Ⅶ的沿主渠方向()、垂直于主渠方向()和垂直于渠道方向()流速分布。
從圖6中可以看出各斷面靠近渠道邊壁的和方向的流速,受邊壁影響,流速越靠近邊壁值較小;在分水口處,水深高于側(cè)渠底高時(shí),水流不受側(cè)渠底坎邊壁的影響,和方向的流速隨坐標(biāo)的增加而增大;當(dāng)坐標(biāo)在0到2/3B范圍內(nèi)時(shí),主渠方向流速值沿主渠方向逐漸減?。划?dāng)坐標(biāo)大于2/3B時(shí),靠近分水口上游端的斷面Ⅱ和斷面Ⅲ方向流速高于斷面Ⅰ,分水口下游端的斷面Ⅵ方向流速越靠近分水口下游端值越小。由于遠(yuǎn)離分水口,斷面Ⅰ和斷面Ⅶ方向流速大致呈對(duì)稱分布,且對(duì)稱點(diǎn)隨水深的增加逐漸遠(yuǎn)離分水口一側(cè),斷面Ⅰ和斷面Ⅶ位于分水口上下游一定距離處,分水口對(duì)斷面Ⅰ和斷面Ⅶ在方向的流速影響較小,但隨水深的增加,各斷面方向流速最大值逐漸增大,因?yàn)殡S著水深的增加,渠道底部邊壁和底坎對(duì)水流的影響減小,流速逐漸增加;由圖6a中垂直于主渠方向流失可知,分水口處的各斷面(斷面Ⅱ~Ⅵ)方向流速沿側(cè)渠方向呈先增大后減小的規(guī)律,且流速最大值的點(diǎn)隨斷面的改變而改變,受側(cè)渠進(jìn)口處底坎的影響,方向流速在底坎處的速度驟減為0,受分水口側(cè)向離心力的影響,越靠近分水口,影響越大,即流速變化越大,所以水深低于側(cè)渠底高時(shí),主渠各斷面流速呈先增加后減小的趨勢(shì);圖6b和圖6c(主渠和垂直于主渠方向流速)中,水深高于底坎高度,不再受底坎的阻擋作用,斷面Ⅲ、Ⅳ、Ⅴ流速值沿側(cè)渠方向逐漸增加,斷面Ⅱ、Ⅵ分別為分水口上、下游端,受到主渠垂向邊壁的阻礙作用,流速值先增加后減小,且變化趨勢(shì)較斷面Ⅲ、Ⅳ、Ⅴ要小的多。從4個(gè)垂向流速圖,可以看出方向流速在分水口處變化較大,越靠近分水口,流速值沿主渠方向變化越大;當(dāng)水深低于側(cè)渠底高高時(shí),從圖6a和圖6b垂向流速圖可以看出,分水口處的水流具有向上的方向流速,受上層水流分流的影響,低于側(cè)渠底高的水流也參與分流,且越靠近分水口,方向流速值越大,參與分流量越多。
為了解主渠靠近分水口一側(cè)各斷面沿程流速變化,將各斷面點(diǎn)不同水深下的流速值沿主渠方向繪制成流速變化圖。圖7為流量45.32 L/s,側(cè)渠底高為0.06 m時(shí)主渠斷面Ⅰ~Ⅶ點(diǎn)流速矢量圖。由圖7可以看出,各水深下分水口處方向流速越靠近分水口下游端,流速值越小,即方向流速值在分水口上游端(斷面Ⅱ)達(dá)到最大值,在斷面Ⅵ處流速值最小,過后流速值雖有增加但流速值仍小于主渠上游。各水深下方向流速值呈先增加后減小的變化,在斷面Ⅵ處(即側(cè)渠中心線)達(dá)到最大,距分水口越遠(yuǎn),方向的流速越小,受分水口側(cè)向分水的影響越小。各水深下方向流速在遠(yuǎn)離分水口的兩個(gè)斷面流速值接近于0,而分水口處的流速值呈先增加后減小的趨勢(shì),最大值點(diǎn)在斷面Ⅳ,此時(shí)水深為側(cè)渠底高;高于側(cè)渠底高的水流在分水口處對(duì)下層水流產(chǎn)生了擾動(dòng),使低于側(cè)渠底高的水流也參與上層分流,若主渠中含有泥沙,當(dāng)?shù)卓睬暗乃鞣较蛄魉僦荡笥谇赖撞磕嗌称饎?dòng)流速時(shí),會(huì)攜帶泥沙進(jìn)入側(cè)渠;若方向流速值達(dá)不到泥沙起動(dòng)流速,那么側(cè)渠進(jìn)口處的底坎就會(huì)具有阻礙泥沙進(jìn)入側(cè)渠的作用。
圖6 主渠各斷面流速矢量圖
圖7 主渠各斷面c點(diǎn)流速(v)矢量圖
2.2.3 側(cè)渠進(jìn)口斷面流速
水流流經(jīng)分水口時(shí),在分水口處無側(cè)向邊壁的約束,主渠部分水流發(fā)生偏轉(zhuǎn)進(jìn)入側(cè)渠,由于主渠水流具有沿主渠方向的流速,因此側(cè)渠進(jìn)口處的流速沿渠道斷面方向變化。圖8為流量為30.2 L/s,主渠下游自由出流工況下,各側(cè)渠底高下側(cè)渠進(jìn)口斷面流速分布圖。由圖8可以看出,水流進(jìn)入側(cè)渠后,受側(cè)向分水的影響,水流在分水口上游端與邊壁分離,而在分水口下游端水流碰撞邊壁,流速逐漸減小;側(cè)渠進(jìn)口斷面越靠近上游端,水深和沿側(cè)渠方向流速越小,隨坐標(biāo)的增加水深逐漸增加,在斷面下游端達(dá)到最大值;各水深沿側(cè)渠方向流速值最大點(diǎn)隨水深的增加逐漸靠近側(cè)渠中心,斷面縱向流速等值線向下游傾斜;同一主渠來流量下,堰坎高度越大,側(cè)渠進(jìn)口斷面同一位置處的水深越小,同時(shí)水面沿?cái)嗝孀兓叫?;隨側(cè)渠底高的增加,側(cè)渠進(jìn)口斷面最大流速和水深逐漸減小。
圖8 不同側(cè)渠底高下側(cè)渠進(jìn)口斷面流速分布圖
2.2.4 湍動(dòng)能
湍動(dòng)能是表征湍流脈動(dòng)劇烈程度的重要參量[26],脈動(dòng)對(duì)于輸水挾沙有著重要的影響,會(huì)增加水流與邊壁的碰撞,侵蝕邊壁,增加水頭損失。分水口處水流各流層之間液體質(zhì)點(diǎn)相互混摻,所以屬于湍流流動(dòng)。圖9為流量45.32 L/s、側(cè)渠底高0.06 m時(shí)的主渠在分水口處斷面和側(cè)渠進(jìn)口斷面湍動(dòng)能等值線圖。
分水口處的水流受到側(cè)向分水、固體邊壁的雙重影響,存在著分流、潛流和滯流等多種流動(dòng)現(xiàn)象,為高湍動(dòng)區(qū)。從圖中可以看出,在遠(yuǎn)離分水口接近表層處,各斷面湍動(dòng)強(qiáng)度值較大(圖9a~圖9e左側(cè)上部);結(jié)合圖6可以看出,主渠道來流在經(jīng)過分水口時(shí),靠近分水口處的水流進(jìn)入側(cè)渠道;遠(yuǎn)離分水口一側(cè)水流也受到影響,水流流線與邊壁發(fā)生分離,流速方向和大小變化劇烈,湍動(dòng)強(qiáng)度較大(圖9a~圖9e右側(cè)下部)。湍動(dòng)強(qiáng)度值越靠近分水口(側(cè)渠道進(jìn)口)處越大,即水流紊動(dòng)越劇烈,流速變化越快;圖9f為側(cè)渠道進(jìn)口斷面湍動(dòng)能分布圖,在分水口上游端,水流在此處發(fā)生偏轉(zhuǎn)進(jìn)入側(cè)渠道,由于水流具有沿主渠道方向的流速,在發(fā)生偏轉(zhuǎn)時(shí)水流會(huì)脫離邊壁,同時(shí)在側(cè)渠道進(jìn)口處形成回流區(qū),流速變化較大,易形成渦流,所以靠近分水口上游端的區(qū)域湍動(dòng)能值較大,在分水口下游端湍動(dòng)能值較小,實(shí)際運(yùn)行中將對(duì)泥沙淤積的位置及數(shù)量產(chǎn)生影響。
圖9 Q=45.32 L·s-1、P=0.06 m時(shí)各斷面湍動(dòng)能分布圖
灌區(qū)渠道量水是實(shí)現(xiàn)灌區(qū)現(xiàn)代化的重要途徑,利用渠系分水口進(jìn)行流量測(cè)量,可避免由于增設(shè)量水設(shè)施引起的二次水頭損失;目前對(duì)于主渠與側(cè)渠底部高程不相同的情況還缺乏深入系統(tǒng)地研究。本文通過試驗(yàn)和FLOW-3D軟件對(duì)不同側(cè)渠底高的矩形渠道分水口水力性能進(jìn)行了模擬研究,分析了分水口處主渠水面線變化、分水口處各斷面流速分布及分水口處湍動(dòng)能分布,得到了以下結(jié)論:
1)采用RNG模型對(duì)矩形分水口進(jìn)行數(shù)值模擬計(jì)算,模擬水深和流速與實(shí)測(cè)值相對(duì)誤差均在10%以內(nèi),流速變化與實(shí)測(cè)值變化規(guī)律基本一致。
2)主渠在分水口處的水面線波動(dòng)較為劇烈,越靠近分水口,水面線波動(dòng)越劇烈;在分水口下游端水深達(dá)到最大值。當(dāng)主渠來流量增加時(shí),分水口處的水面波動(dòng)程度增加,主渠來流量越大,分水口處的水深變化越劇烈,水面越不穩(wěn)定。
3)主渠各斷面流速分布規(guī)律在不同水深下大致相同,各斷面方向流速分布較為均勻,靠近邊壁處,流速較??;高于側(cè)渠底高的主渠水流對(duì)下層分流產(chǎn)生擾動(dòng),使沿主渠流動(dòng)的水流發(fā)生趨向于側(cè)渠的流動(dòng)趨勢(shì),側(cè)渠底坎前的水流具有向上的流速,與上層水流一起參與分流。
4)側(cè)渠進(jìn)口斷面不同水深下沿側(cè)渠方向流速值最大點(diǎn)隨水深的增加逐漸靠近側(cè)渠中心,斷面縱向流速等值線向下游傾斜;同一主渠來流量下,側(cè)渠底高越大,側(cè)渠進(jìn)口斷面同一位置處的水深越小,同時(shí)水面沿?cái)嗝孀兓叫 ?/p>
5)湍動(dòng)強(qiáng)度值越靠近分水口(側(cè)渠進(jìn)口)處湍動(dòng)值較大;在遠(yuǎn)離分水口接近表層處,各斷面湍動(dòng)強(qiáng)度值較大,受到邊壁和分水口的影響,湍動(dòng)強(qiáng)度較大,流速變化劇烈;側(cè)渠進(jìn)口斷面靠近分水口上游端的區(qū)域湍動(dòng)能值較大,在分水口下游端湍動(dòng)能值較小。
分水口處的水力特性還受渠寬比、主渠與側(cè)渠軸線夾角、分水口結(jié)構(gòu)、水流中的泥沙含量等因素影響,還需進(jìn)一步系統(tǒng)研究,確定各因素對(duì)分流特性的影響。
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Numerical simulation of influence of side channel bottom height on hydraulic performance of bleeder
Wang Wen’e, Liu Haiqiang, Hu Xiaotao
(,,,712100,)
In recent years, most researches focus on the same height at the bottom of both main and side canal to study the hydraulic performance of water diversion. However, in practice, the height of main and side canal bottom are different in most irrigation districts. In this paper, the effect of the height of the side canal bottom on the hydraulic performance of bleeder was studied. The prototype test was carried out in Northwest A & F University in Yangling, Shannxi of China. In the prototype test, both the main channel and side channel were rectangular. The length of the main channel was 12-m long and 0.46-m wide. The length of the side channel was 2.5 m, and the width was adjustable in the range of 0.14-0.46 m. The elevation of the side channel was 0.06, 0.08 and 0.1 m higher than the bottom elevation of the main channel. A total of 8 flow sections were set up, 5 of which were at the water inlet. In order to analyze the water surface variation at the water diversion of the main channel, 3 measuring points at section I, section II, section III, section IV, section V, section VI were taken. The 3 measuring points were on the center line and 5 cm away from the wall of the main channel on both sides. The variables in the test were the side channel width and the flow rate. The water depth and velocity were determined at the measuring point. To eliminate the influence of boundary conditions, this study simplified the experimental channels. The width of the simulated main and side channels was the same as the actual width of the main and side channels. However, the lengths of the main and side channels were correspondingly extended on the basis of the actual channel length, which were 12 and 5m respectively. Because the flow field near the water-diversion changed drastically, the grid interval at the area was decreased, the cell length was 0.01 m, the remaining area cell grid was 0.02 m, and the total grid number was about 900 000. In the channel model, the inlet boundary of the main channel was set to several volume flow rates, which were 19.96, 25.2, 30.2, 36.5 and 45.32 L/s, respectively; the outlets of the main channel and the side channel were set to be free; the side wall of the channel was selected to have no sliding wall; the air above the free surface was air, and the relative pressure was 0. Based on experiments, FLOW-3D was used to simulate and calculate several flow rate and heights on the rectangular channel water diversion, compared to the measured water depths and velocity. The results showed that the variation of velocity was basically consistent with the measured value and relative error was less than 10%. It was reliable to make numerical analysis on water diversion by using FLOW-3D. The numerical simulation showed that the fluctuation of water surface at the water diversion was affected by the flow rates from the main channel. With the increasing of the bottom height, the maximum velocity and water depth of the side channel inlet section gradually decreased. At the entrance of the side channel, the turbulent kinetic energy near the upstream end was large, while at the downstream end of the branch, the turbulent kinetic energy near the bottom was small. The study provides information on water distribution and water-quantity control in irrigation area.
numerical analysis; flow rate; flow velocity; rectangular channel; bleeder; canal width ratio
王文娥,劉海強(qiáng),胡笑濤. 側(cè)渠底高對(duì)分水口水力特性影響數(shù)值模擬研究[J]. 農(nóng)業(yè)工程學(xué)報(bào),2019,35(20):60-68.doi:10.11975/j.issn.1002-6819.2019.20.008 http://www.tcsae.org
Wang Wen’e, Liu Haiqiang, Hu Xiaotao. Numerical simulation of influence of side channel bottom height on hydraulic performance of bleeder[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2019, 35(20): 60-68. (in Chinese with English abstract) doi:10.11975/j.issn.1002-6819.2019.20.008 http://www.tcsae.org
2019-02-22
2019-09-10
“十三五”國家重點(diǎn)研發(fā)計(jì)劃(2016YFC0400203);公益性行業(yè)(農(nóng)業(yè))科研專項(xiàng)(201503125)
王文娥,教授,博士,博士生導(dǎo)師,主要從事流體機(jī)械及工程水力學(xué)研究。Email:wangwene@nwsuaf.edu.cn
10.11975/j.issn.1002-6819.2019.20.008
S274.4
A
1002-6819(2019)-20-0060-09