李甲振+郭永鑫+甘明生+許金鵬+付輝+黃偉
摘要:原模型阻力相似是開展河工模型試驗的基本要求,模型試驗的糙率復核受目標值、波動幅值、模型材質及研究重點等多因素的影響,選用合理的加糙手段可大幅提升試驗效率和精度。為解決現(xiàn)階段河工模型試驗中存在的加糙方法種類繁多、技術條件復雜、難以快速選擇適宜加糙方法的問題,對國內外常用的加糙方法進行了歸納總結,將加糙方法劃分為點塊型加糙、條帶型加糙和膜片型加糙三種方法。介紹了每種方法常用的加糙體,詳細說明了各加糙方法的實現(xiàn)手段、優(yōu)點、不足以及糙率值的可調整范圍。草墊為代表的膜片型加糙可實現(xiàn)的糙率峰值達0.08以上,梅花十字板加糙可實現(xiàn)的糙率峰值約0.06,條帶型加糙可實現(xiàn)的糙率峰值約為0.04,粘貼尼龍網(wǎng)、玻璃膜以及密實加糙可實現(xiàn)的糙率峰值約為0.02,刨坑加糙方法基本不能提高模型表面糙率。給出了相應的適用條件、加糙范圍及對水流結構的影響,以及部分糙率計算方法,以期研究者可根據(jù)試驗目的快速準確地選取適宜的加糙方法,提高研究效率。
關鍵詞:模型試驗;加糙方法;點塊型加糙;條帶型加糙;膜片型加糙
中圖分類號:P21 文獻標識碼:A 文章編號:1672-1683(2017)04-0129-07
Abstract:Resistance similarity is an essential requirement in river model experiments.Roughness recheck is affected by the target value,fluctuation amplitude,model material,and research emphasis.A reasonable roughening method will substantially improve the experiment efficiency and accuracy.In order to help researchers quickly choose a reasonable roughening way from the various existing methods,we sorted and reviewed the commonly used roughening methods,which were classified into three types:block-type roughening,strip-type roughening,and patch-type roughening.We introduced the commonly used roughening elements of each method and described in detail the realization means,advantages,drawbacks,and adjustable roughness range of each method.The patch-type roughening methods represented by plastic grass cushions can achieve a roughness value larger than 0.08.The block-type roughening methods represented by cross plates in the form of plum blossom can obtain a peak roughness value of about 0.06.The peak roughness value of strip-type roughening methods is 0.04.Sticking nylon nets or glass film and compact roughening can generally realize a roughness value no larger than 0.02.Digging pits on the model can hardly increase its surface roughness.In addition,this paper presents the methods′ application conditions,roughening ranges,and influence on flow structure,and some roughness calculation methods.It is hoped that with this paper,researchers can determine roughening methods quickly and exactly according to their experiment goals,and improve their research efficiency.
Key words:model experiment;roughening method;block-type roughening;strip-type roughening;patch-type roughening
進行河工模型試驗時,一般按照重力相似準則進行設計,原模型需滿足阻力相似要求。選定幾何比尺后,即可根據(jù)原型糙率和比尺確定模型表面所需要的糙率值。通常,利用混凝土、有機玻璃材質進行放樣所制作模型的糙率小于需求值,因此,需要對模型表面進行加糙[1]。對于寬深比較大的河道,一般采用變態(tài)模型,變態(tài)率越大,所需的河床糙率也越大[2]。如何對模型表面進行加糙,反映原型的水力特性,成為了試驗需要解決的首要問題。一般情況下,模型加糙需要考慮以下問題:(1)目標糙率的大小,據(jù)此確定可供選擇的加糙方式;(2)試驗研究的重點,考慮加糙對水流結構及流速分布的影響;(3)模型制作的材質,使加糙易于實現(xiàn);(4)加糙的可調節(jié)性,糙率復核通常經過多次調整后才能實現(xiàn),因此,加糙方式需易于調整。
本文對常用的加糙方式進行了概述,將其劃分為點塊型加糙、條帶型加糙和膜片型加糙,分析了不同加糙方式的適用條件、實現(xiàn)方式、優(yōu)缺點以及糙率計算方式,指出了研究需要解決的難點問題和關鍵技術。
1 點塊型加糙
點塊型加糙,是將加糙體按照一定的排列方式粘貼在河床上實現(xiàn)加糙的。常見的點塊型加糙方式有密實加糙、梅花加糙、刨坑加糙等。
1.3 刨坑加糙
刨坑加糙,是在模型表面刨制小坑進行加糙的一種方式。經過實踐,刨坑對河床糙率的影響并不明顯,這主要是因為刨坑的方法不能顯著提高模型表面的當量絕對粗糙度,坑內水體以自旋為主,不能顯著增加糙率。李純良[16]在混凝土表面每隔3.37 cm布置一排間距為3.20 cm的坑;坑的形狀近似為四面體,底面為邊長1.5 cm的等腰三角形,深0.9 cm,體積為0.29 cm3,所獲得的糙率近似于混凝土抹面的糙率0.011。
2 條帶型加糙
2.1 凹槽加糙
利用混凝土材質制作的模型,表面抹光的糙率一般在0.010~0.011;在混凝土未干結前,可利用掃帚等對表面進行打毛,以增加模型糙率,但數(shù)值一般不超過0.014。長江口處河道模型試驗中,通過在模型表面刮制W型波紋凹槽進行加糙;凹槽的尺寸是寬0.8 cm、深0.8 cm,模型達到的糙率為0.019[17-18]。筆者在其模型試驗中,也曾通過表面刮制寬深0.6~0.7 cm的W型波紋凹槽進行加糙,實現(xiàn)的糙率值為0.018。該加糙方法操作簡單,容易實現(xiàn)??紤]混凝土特性及W型凹槽的制作,建議的加糙深度一般不超過1 cm,糙率值一般小于0.020。
2.2 水中拉線加糙
對于河床平順的渠道,水中拉線是一種較為便捷的加糙方式。水中拉線加糙是在渠底兩側預先密集埋設鐵釘,間距一般為5~10 cm;將繩索拴在鐵釘上,通過增加對水流的擾動實現(xiàn)加糙目的。該加糙方式可實現(xiàn)的糙率值達0.04,隨繩距的增大而減小,當繩距大于80 cm時,加糙效果已經不明顯了。M.Asim[19]對1.0 mm和1.5 mm繩索的試驗表明,繩索直徑對糙率的影響并不明顯。前文提及的點塊型加糙方式在改變糙率時,往往需要停水,去掉已粘貼的加糙體并調整間距,或對加糙體進行修剪、拔除,干結后再次進行試驗;糙率調整周期長,通常至少需要十幾天的時間。水中拉線可以在不停水的情況下改變糙率,操作簡便,糙率復核工作可在兩三天時間完成。但水中拉線加糙破壞了水體的流場結構,影響斷面流速分布和河道輸沙,一般在研究水面線問題時應用,在渾水和動床試驗糙率不足時使用[20]。
2.3 粘貼條帶加糙
粘貼條帶加糙是將有機玻璃、塑料材質的加糙條或吸管等按照一定的規(guī)律粘貼在底板上進行加糙的一種方式[21]。卞華[22]對三角形、半圓形、矩形和圓形加糙條的紊流結構進行研究,矩形條帶的加糙效果最好。當加糙條間距λ/高度[WTB1X]k=8時,當量糙率最大。利亞平[23]、D W Knight[24]對不同尺寸加糙條的試驗也證實了間距為加糙條高度的8~9倍時,加糙效果最明顯,粘貼條帶加糙一般用于糙率值不超過0.03的河工模型。將條帶波浪狀或交錯式布置,可進一步提升加糙效果,在泄水工程陡坡段的消能防沖中應用較多[25-26]。需要注意的是,當加糙條間距大于高度的8倍時,為大尺度漩渦的存在和發(fā)展創(chuàng)造了條件,0.6~1.3倍加糙條高度范圍的水體流速分布發(fā)生偏離,不滿足對數(shù)型流速分布[22,27]。
3 膜片型加糙
膜片型加糙,是一種在模型的表面粘貼膜片進行加糙的方式,常用的膜片有尼龍網(wǎng)、玻璃膜、塑料草墊等,見圖8。在香港錦田主排水道雨洪渠道模型試驗中,王濤[30]利用厚0.7 mm,網(wǎng)格間距為1 mm的尼龍網(wǎng)模擬漿砌塊石護坡。為保持有機玻璃模型的可視性,孫東坡[31]在表面粘貼玻璃膜進行加糙。紋理越深,加糙效果越明顯;加糙膜紋理垂直來流方向的糙率最大。尼龍網(wǎng)和玻璃膜可實現(xiàn)的糙率值一般不超過0.020,加糙時,制作成塊狀進行粘貼,便于撕除進行糙率調整。
利用塑料草墊進行加糙,可實現(xiàn)的加糙范圍為0.027 4~0.081 3[32],常用于表面較平整的有機玻璃、混凝土、鋼質和木質模型的加糙;減小糙率時,在草墊格柵的節(jié)點上摘除塑料草即可。其加糙效果的影響因素主要是草間距、葉開度、高度以及單片葉的尺寸。草間距越小、開度越大、草越高、單片葉尺寸越大,加糙效果越明顯;葉片平面舒展型草墊的加糙效果優(yōu)于豎向籠縮型草墊。由于工業(yè)化生產的草墊具有統(tǒng)一的規(guī)格和尺寸,因此,草墊對水流底部流速的干擾也是比較均勻的。通過對底部紊動強度的分析,草墊加糙要求模型水深不小于3.0~3.5 cm。鄔年華[33]對鄱陽湖實體模型采用梅花形石塊加糙和塑料草墊加糙相結合的方法,實現(xiàn)了模型和原型的水流運動條件相似。
(3)加糙體所占水體體積V考慮加糙體后面旋渦的體積,盧漢才[6]認為加糙體后面的旋渦形狀為一平放的錐體,影響距離為顆粒直徑的10~12倍,虞邦義[15]認為旋渦區(qū)為四分之一橢球體(長軸為兩排加糙體間距,短軸為加糙體垂直來流方向寬度,高度為加糙體高度)。
5 主要認識
通過對國內外河工模型試驗加糙方法的概述,將其劃分為點塊型加糙、條帶型加糙和膜片型加糙三種方法,分析了各方法的適用條件、加糙范圍及對水流結構的影響,通過系統(tǒng)分析總結得出如下認識。
(1)考慮河床加糙目標值,以草墊為代表的膜片型加糙可實現(xiàn)的糙率峰值達0.08以上,梅花十字板加糙可實現(xiàn)的糙率峰值約0.06,條帶型加糙可實現(xiàn)的糙率峰值為0.04,粘貼尼龍網(wǎng)、玻璃膜以及密實加糙可實現(xiàn)的糙率峰值約為0.02,刨坑加糙方法基本不能提高模型表面糙率。
(2)從對水流結構的影響考慮,密實加糙更接近天然河床情況,對水流結構的影響最小,膜片型加糙次之;條帶型加糙和梅花加糙一般常用于一維、二維水流問題研究,而在三維問題的研究中,建議使用密實加糙和膜片型加糙方法。
(3)從糙率調整的角度考慮,水中拉線加糙方法最便捷,膜片型加糙和梅花型十字板加糙方法次之,需要調整加糙體間距的點塊型加糙和其他條帶型加糙最為復雜。
(4)河道槽蓄量對水流水力特性影響較大的模型,建議使用梅花形十字板加糙或水中拉線加糙。
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