王漢封+鄒超+張運平
文章編號:16742974(2014)04009407
收稿日期:20130820
基金項目:國家自然科學(xué)基金資助項目(51108468)
作者簡介:王漢封(1976-),男,河南開封人,中南大學(xué)副教授,博士
通訊聯(lián)系人,E-mail:wanghfme@gmail.com
摘 要:通過風(fēng)洞實驗,研究了尾部導(dǎo)流板對25°傾角Ahmed類車體尾流與氣動阻力的影響規(guī)律.對比了斜面兩側(cè)與斜面上邊緣寬度分別為5 mm,10 mm和15 mm導(dǎo)流板的減阻效果.試驗中模型縮尺比為1∶2,基于來流風(fēng)速與模型長度的雷諾數(shù)為8.7×105.研究結(jié)果表明,模型尾流中存在一對規(guī)則的拖曳渦,并伴隨有強烈下掃流,尾部斜面上存在D形流動分離區(qū).斜面兩側(cè)5 mm寬導(dǎo)流板對尾流的影響很小,對應(yīng)的氣動阻力會增大約2.1%;斜面兩側(cè)10 mm,15 mm寬導(dǎo)流板以及不同寬度的水平導(dǎo)流板可顯著削弱尾流中的拖曳渦.水平導(dǎo)流板能夠消除斜面上的流動再附著并破壞D形分離區(qū),其減阻效果明顯高于兩側(cè)導(dǎo)流板,最大減阻率可達11.8%.
關(guān)鍵詞:Ahmed模型;尾流;拖曳渦;氣動阻力;流動控制
中圖分類號:O355;U461.1 文獻標(biāo)識碼:A
Control of the Wake and Aerodynamic Drag of an Ahmed
Model with 25° Slant Angle by Using Deflectors
WANG Hanfeng1,2, ZOU Chao1, ZHANG Yunping1
(1.School of Civil Engineering, Central South Univ, Changsha, Hunan 410075,China;
2. National Laboratory for HighSpeed Railway Construction,Changsha, Hunan 410075,China)
Abstract: This paper investigated the effect of rear end deflectors on the near wake and aerodynamic drag of an Ahmed model with 25° slant angle. Drag reduction was compared for deflectors mounted on the two side faces and on the upper edge of the rear slant. The width of deflectors is 5mm, 10mm and 15 mm, respectively. The model scale is 1∶2, and the Reynolds number based oncoming flow velocity and model length is 8.7×105. The results have revealed that there is a pair of organized trailing vortices in the wake, which are accompanied by strong downwash flow. There is a Dshape flow separation zone on the slant face. Deflectors with a width of 5 mm at the two sides of the slant have negligible effect on the near wake, and slightly increase the aerodynamic drag by about 2.1%. On the other hand, all tested horizontal deflectors at the top edge of the slant and deflectors with a width of 10 mm and 15 mm at both sides of the slant considerably weaken the trailing vortices. The horizontal deflectors avoid flow reattachment on the slant and suppress the Dshape separation zone, corresponding to the maximum drag reduction rate of 11.8%, which is higher than that of the deflectors on both sides of the slant.
Key words: Ahmed model; wakes; tailing vortex; aerodynamic drag; flow control
車輛的氣動阻力近似與其行駛速度的平方成正比.當(dāng)時速為90 km時,發(fā)動機功率的80%左右將用于克服氣動阻力[1].通??烧J為車輛氣動阻力是
由壓差阻力與摩擦阻力兩部分構(gòu)成,前者在氣動阻力中占絕大部分[2].為提高燃油經(jīng)濟性,圍繞車輛氣動阻力的主、被動控制方法已開展了廣泛的研究,如導(dǎo)流板[2-5]、漩渦發(fā)生器[6-7]、微射流[8-10]和尾部附加隔板[2, 11]等.
實際車輛的外形復(fù)雜,不利于相關(guān)研究的對比.Ahmed模型[12]是目前研究最廣泛的類車體模型之一.該模型頭部由4個1/4圓柱面過渡,其尾部傾角α可根據(jù)實際情況而選擇.研究表明,Ahmed類車體尾流及其氣動力特性與尾部傾角α有密切聯(lián)系[12-13].依據(jù)模型尾部斜面上的流動特性可分為3個典型狀態(tài),當(dāng)α< 12.5°時,流動在尾部斜面上不會發(fā)生分離,此時尾流中會形成一對旋向相反的拖曳渦;當(dāng)12.5° <α< 30°時,流動在斜面上會發(fā)生分離與再附著,并形成分離泡,此時在模型尾流中仍會出現(xiàn)拖曳渦,但其強度將明顯大于第1種情況,且此時對應(yīng)的阻力系數(shù)Cd顯著增大;而在α> 30°時,流動在斜面上邊沿發(fā)生分離且無再附著發(fā)生,尾流中拖曳渦顯著減弱,模型上的壓力分布變得非常均勻,Cd顯著減小.由此可知,斜面上是否出現(xiàn)分離泡、以及拖曳渦強度與模型氣動阻力有密切的聯(lián)系,拖曳渦強度越大對應(yīng)的模型氣動阻力也較大[6, 12].
湖南大學(xué)學(xué)報(自然科學(xué)版)2014年
第4期王漢封等:利用導(dǎo)流板控制25°傾角Ahmed類車體尾流與氣動阻力
對于30°傾角Ahmed模型,斜面兩側(cè)導(dǎo)流板的減阻效果最為顯著,最高可達17.7%[3].而對于尾部流動狀態(tài)完全不同的25°傾角Ahmed模型,不同位置導(dǎo)流板對氣動阻力的影響仍缺乏系統(tǒng)的研究.依據(jù)文獻[3]中所提出的2種減阻效果較好的導(dǎo)流板布置方式,本文通過風(fēng)洞實驗系統(tǒng)研究了布置于斜面兩側(cè)和斜面上邊緣的不同寬度的導(dǎo)流板對25°傾角Ahmed模型尾流與氣動力的控制效果.實驗運用壓力掃描閥、眼鏡蛇探針與表面油膜流動顯示等方法,比較了不同工況下模型氣動阻力、尾部壓力分布以及尾流場的變化規(guī)律,揭示了減阻機理.
1 試驗方法
1.1 風(fēng)洞模型
本試驗在中南大學(xué)高速鐵路建造技術(shù)國家工程實驗室的風(fēng)洞高速試驗段內(nèi)完成.該風(fēng)洞為回流式風(fēng)洞,具有低速與高速兩個試驗段,其中低速試驗段寬12 m,高3.5 m,長18 m,風(fēng)速范圍為0~18 m/s,湍流度小于2%;高速試驗段寬3 m,高3 m,長15 m,風(fēng)速范圍為5~90 m/s,湍流度小于0.5%.試驗裝置如圖1(a)所示.試驗中Ahmed模型傾角為25°,縮尺比為1∶2,對應(yīng)的長(l)、寬(w)、高(h)分別為522,194.5和144 mm.模型安裝在一個距風(fēng)洞底面約500 mm的水平板上,避免了風(fēng)洞壁面邊界層的影響.為防止流動分離,水平板前邊緣加工成光滑的橢圓形.模型與水平板間隙為25 mm,距水平板前邊緣約550 mm.可以估算模型處平板邊界層厚度約為13.5 mm,即試驗中模型完全處于均勻來流中.本試驗裝置與文獻[3, 4, 6, 7]中所述的實驗裝置類似.坐標(biāo)原點定義在水平板上模型尾部中點所對應(yīng)的位置上,流動方向為x,側(cè)向為y,高度方向為z.試驗中自由來流風(fēng)速為U∞ = 25 m/s,對應(yīng)的基于模型長度的雷諾數(shù)為8.7×105.模型所造成的風(fēng)洞阻塞率約為0.4%,其影響可忽略不計.
圖1 試驗裝置
Fig.1 Experimental facility
本文研究了兩類不同位置導(dǎo)流板對模型尾流與氣動阻力的影響.對無導(dǎo)流板的工況Case1也進行了測量,以方便結(jié)果的對比.Case2,Case3和Case4中導(dǎo)流板安裝在模型尾部斜面兩側(cè),導(dǎo)流板寬度分別為5,10和15 mm,約相當(dāng)于模型長度的1%,2%和3%;Case5,Case6和Case7中導(dǎo)流板安裝于斜面上邊緣處,寬度分別為5,10和15 mm,如圖1(b)所示.
1.2 測試方法
試驗中采用眼鏡蛇探針測量模型尾流中的總壓與速度分布,所用探針響應(yīng)頻率為2.5 kHz,并已成功運用于多種湍流場的測量[14-15].實驗中探針采樣頻率為2 kHz,每一測點采樣時間為15 s.測量分別在模型下游0.5l與l的流向截面內(nèi)進行,以觀察尾流中的拖曳渦結(jié)構(gòu).探針固定于計算機控制的二維移測架上在測量平面內(nèi)逐點進行測量.移測架位移精度為0.02 mm.考慮到Ahmed模型尾流的對稱性,測量僅在y > 0的范圍內(nèi)進行.
為研究不同工況下模型尾部壓力的變化情況,運用電子壓力掃描閥對模型尾部斜面與垂面上壓力分布進行了測量.壓力測點的布置與文獻[7, 9]相同.試驗中每測點掃描12 000次,以獲得各點平均壓力系數(shù)Cp,其定義式為Cp = -P
SymboleB@
/0.5ρU2
SymboleB@
,其中為各測點平均壓力,P
SymboleB@
為風(fēng)洞靜壓力,ρ為空氣密度.本文中上橫線“ˉ”表示時間平均量.
試驗還采用了表面油膜法對模型尾部斜面上的流動分離情況進行了研究.用二甲基硅油、煤油和鈦白粉按一定比例混合拌勻[16-17],并均勻地涂抹在模型尾部斜面上.在25 m/s風(fēng)速下,約10 min,油膜可達到穩(wěn)定狀態(tài).
2 結(jié)果及分析
2.1 流場測量結(jié)果
Case4和Case7的對應(yīng)結(jié)果分別與Case3和Case6非常類似,限于篇幅,它們在后續(xù)討論中未予給出.圖2給出了當(dāng)x=0.5l和l時,各工況以時均流向渦量ω*x為背景的流線圖.本文中上標(biāo)“*”表示用U∞與l進行無量綱化.由圖2可知,各工況模型尾流中均存在一對規(guī)則的流向拖曳渦(y < 0沒有顯示),并總是伴隨著尾流中心線附近的強烈下掃流,這與文獻[3, 4, 6, 18]等報道的結(jié)果是一致的.
圖2 x=0.5l和x=l截面內(nèi)的時均渦量與流線圖
Fig.2 Timeaveraged vorticity and stremlines in the streamwise planes at x=0.5l and l
在x=0.5l處,Case1的拖曳渦中心ω*x最大值約為13.0.對于Case2,拖曳渦強度相對于Case1無明顯變化.而對于Case 3, ω*x的最大值僅為7.6,相對于Case1減弱了約41.5%,且拖曳渦的尺寸也有明顯減小.而對于導(dǎo)流板水平布置在斜面上邊緣的2種情況Case5和Case6,ω*x最大值分別為7.5和7.4,相對于Case1分別減弱了42.3%和43.1%.如圖2所示各工況的流線圖也可反映拖曳渦的結(jié)構(gòu)與強度.對于Case1和Case2,流線在拖曳渦范圍內(nèi)存在強烈的螺旋結(jié)構(gòu);而在Case3,Case5和Case6中,拖曳渦中心附近流線的螺旋結(jié)構(gòu)相對較弱.與x=0.5l的情況類似,在x=l截面內(nèi),Case1與Case2的拖曳渦強度基本相同,而Case3,Case5和Case6的對應(yīng)值則明顯較小.5種工況在x=l截面內(nèi)ω*x的最大值分別是0.5l截面內(nèi)對應(yīng)值的61%,66%,95%, 55%和54%,這說明Case3中拖曳渦衰減速率最慢,而Case5與Case6的衰減速率相對較快.從圖2還可看出,Case1與Case2對應(yīng)的拖曳渦中心位置也基本相同,而Case3,Case5和Case6的渦團中心更靠近尾流中心線,這表明后3種工況下掃流向外側(cè)排開拖曳渦的作用相對較弱.
為定量比較尾流中下掃流的變化規(guī)律,圖3給出了x=0.5l和l截面內(nèi)的z方向時均速度W*的分布.各工況拖曳渦中心位置在圖3中用“×”標(biāo)出,以方便對比.由圖3可知,在各工況下,拖曳渦中心內(nèi)側(cè)均存在著明顯的下掃流,即W*< 0.Case1和Case2對應(yīng)的W*定性與定量上都非常類似,而Case3相對于Case1也僅略有減小,這表明斜面兩側(cè)導(dǎo)流板對下掃流的影響非常有限.相對于Case1,Case5和Case6中下掃流的強度和其影響范圍都明顯減小了,這與圖2中拖曳渦的變化規(guī)律是一致的.總體來看,模型尾部的水平導(dǎo)流板對拖曳渦和下掃流的抑制作用更為顯著.
圖3 x=0.5l和x=l截面內(nèi)的時均z方向速度
Fig.3 Timeaveraged velocity in z direction in the streamwise planes at x=0.5l and l
圖4給出了各工況拖曳渦中心處y方向速度v的能譜.Case1的能譜Ev存在顯著的峰值,其對應(yīng)的基于l和U
SymboleB@
的斯托羅哈數(shù)St=1.55,與文獻[13]的結(jié)果非常吻合.這表明Case1中拖曳渦強度和周期性均較顯著.Case2中,盡管能譜峰值略有減小,但其St數(shù)與Case1相同.與前2種工況不同,Case3,Case5和Case6的能譜中已沒有明顯峰值出現(xiàn),表明這些工況中拖曳渦已無顯著的周期性.在x=l截面內(nèi),Ev所表現(xiàn)的規(guī)律與x=0.5l截面內(nèi)完全一致.
圖4 拖曳渦中心處速度v的能譜
Fig.4Power spectra density function of v at tailing vortex center
圖5給出用表面油膜法獲得的模型尾部斜面上的流動結(jié)構(gòu).由于模型尾流的對稱性,圖中僅給出了表面油膜流動顯示的右半部分,而在左半部分給出了相應(yīng)的流動示意圖(Case6大部分區(qū)域流動結(jié)構(gòu)已不明顯,故未給出).Case1,Case2和Case3中流動分離并非發(fā)生在斜面上邊緣,而是上邊緣略下游
的實線所示位置上,如圖5所示.Case1中,上邊緣附近的分離流在斜面上發(fā)生再附,并在斜面上形成一個D形分離泡,如圖5中流動分離線與虛線所圍成范圍,這與文獻[9, 12, 19, 20]中的結(jié)果是完全一致的.Case2中,尾部斜面上的流動結(jié)構(gòu)沒有明顯改變,仍可清晰地觀察到D形流動分離區(qū).Case3和Case1相比,D形分離區(qū)仍然存在,但略有減小.對于Case5,斜面上流動結(jié)構(gòu)相對于Case1發(fā)生了顯著的變化.在水平導(dǎo)流板的作用下,斜面上邊緣附近的流動分離線消失了,且分離流在斜面上不會發(fā)生再附,因而斜面上不再出現(xiàn)封閉的分離泡.Case5所對應(yīng)的流動狀態(tài),非常類似于文獻[13,16,21]中所
圖5 模型尾部斜面上流動顯示結(jié)果
Fig.5 Surface flow pattern on the slant face
給出的30°或35°傾角Ahmed模型的尾流結(jié)構(gòu).隨著水平導(dǎo)流板的寬度增加到10 mm,Case6中斜面兩側(cè)分離流的影響也基本消失,除斜面左右兩個角部區(qū)外,整個斜面基本上都處于分離區(qū)內(nèi),斜面上流動較為均勻.
綜合圖2~圖5可知,對于斜面兩側(cè)導(dǎo)流板的情況,其尾流特性與Case1是類似的.隨著導(dǎo)流板寬度的增加,斜面上D形分離泡逐漸減小,尾流中拖曳渦強度有所減弱.而斜面上邊緣的水平導(dǎo)流板,可破壞斜面上的D形分離泡,并能夠更為顯著地抑制尾流拖曳渦強度,其作用類似于增大了25°Ahmed模型的尾部傾角.
2.2 氣動阻力試驗結(jié)果
2.2.1 氣動阻力
定義一個包括模型在內(nèi)的控制體積(如圖6所示)[22],并將動量守恒方程應(yīng)用于該控制體積,可以獲得模型氣動阻力的精確表達式[4].當(dāng)控制體足夠大時,可認為控制體側(cè)面與頂面上沒有動量輸運.此外,雷諾應(yīng)力、氣體粘性力等對氣動阻力的貢獻比其他項小一個數(shù)量級以上,通常也可忽略[4, 22].模型氣動阻力表達式可簡化為:
Fx=-12ρU2
SymboleB@
∫S1-xU
SymboleB@
2dS+
12ρU2
SymboleB@
∫S2yU2
SymboleB@
+2zU2
SymboleB@
dS+∫S(Pi0-Pi)dS.(1)
式中:Pi0為來流總壓;Pi為控制體積出口截面上各點總壓;x,y與z分別為出口截面上3個方向速度的時均值;S為控制體的出口面積.式(1)右側(cè)三項分別表示流向速度損失、側(cè)向速度變化以及總壓損失對氣動阻力的貢獻.已有文獻[4, 22, 23]成功運用式(1)獲得了類車體的氣動阻力,本文也將采用此方法估算不同工況下模型氣動阻力.
表1給出了基于x=0.5l和l截面測量結(jié)果,依據(jù)式(1)估算的模型氣動阻力.可以看出,由上述兩個截面測量數(shù)據(jù)所得到的氣動阻力是非常接近
的.對比式(1)右側(cè)三項對氣動阻力的貢獻可發(fā)現(xiàn),尾流中的總壓損失占氣動阻力的絕大部分,而流向速度損失與側(cè)向速度變化對阻力的貢獻則相對較小.Case1中模型阻力系數(shù)Cd=0.432,與文獻[19, 24]的結(jié)果非常接近,這也驗證了本試驗結(jié)果的可靠性.由表1可知,Case2中Cd=0.441,與Case1非常接近,相對于Case1略微增大約2.1%.這一結(jié)果與3.1節(jié)所述流場變化規(guī)律是吻合的.Case3中Cd=0.415,Case4中Cd=0.399,相對于Case1的減阻率分別為3.9%和7.6%.而斜面上邊緣水平導(dǎo)流板工況Case5,Case6和Case7,對應(yīng)的減阻率可達10.9%,11.6%和11.8%,減阻效果十分顯著,明顯優(yōu)于斜面兩側(cè)導(dǎo)流板各工況.圖7給出了減阻率隨導(dǎo)流板寬度的變化情況.對于水平導(dǎo)流板,減阻率隨導(dǎo)流板寬度的增加變化很??;而對于斜面兩側(cè)導(dǎo)流板,減阻率隨著導(dǎo)流板寬度的增加而逐漸增大,但始終低于水平導(dǎo)流板的減阻率.結(jié)合3.1節(jié)中流場測量
結(jié)果可知,Cd的減小與尾流中拖曳渦強度的減弱是相關(guān)的.總體來看,斜面上邊緣導(dǎo)流板對尾流拖曳渦與氣動阻力的抑制作用明顯強于斜面兩側(cè)導(dǎo)流板,這與30°傾角Ahmed模型的對應(yīng)規(guī)律[3]是截然不同的.
圖6 以動量守恒法計算模型氣動阻力的控制體積[22]
Fig.6 Control volume for drag estimation using
momentum conservation[22]
表1 由式(1)計算的氣動阻力
Tab.1 Aerodynamic drag estimated based on Eq(1)
工況
測量截面
第1項
阻力/N
第2項
阻力/N
第3項
阻力/N
總阻力/N
總阻力均值 /N
阻力系數(shù)
減阻效果/%
Case1
x=0.5l
-0.972
1.368
4.142
4.538
x=l
-0.684
1.172
4.040
4.528
4.533
0.432
-
Case2
x=0.5l
-0.814
0.988
4.502
4.676
x=l
-0.612
1.128
4.090
4.606
4.641
0.441
2.1
Case3
x=0.5l
-1.462
0.548
5.278
4.364
x=l
-0.786
0.508
4.636
4.358
4.361
0.415
-3.9
Case4
x=0.5l
-1.392
0.446
5.214
4.268
x=l
-0.878
0.454
4.544
4.120
4.194
0.399
-7.6
Case5
x=0.5l
-1.702
0.526
5.244
4.068
x=l
-1.150
0.272
4.928
4.050
4.059
0.385
-10.9
Case6
x=0.5l
-1.474
1.032
4.522
4.080
x=l
-1.094
0.410
4.618
3.934
4.007
0.382
-11.6
Case7
x=0.5l
-1.426
1.096
4.342
4.012
x=l
-0.980
0.406
4.558
3.984
3.998
0.381
-11.8
圖7 各工況的減阻率
Fig.7 Drag reduction rate
2.2.2 尾部壓力分布
圖8給出了5種工況中模型尾部斜面與垂面上的壓力分布.總的來看,尾部垂面壓力分布受導(dǎo)流板的影響較小,壓力系數(shù)Cp均為-0.25~-0.35.然而,尾部斜面的壓力分布與導(dǎo)流板位置及導(dǎo)流板寬度密切相關(guān).Case1中斜面的上邊緣與右邊緣附近均出現(xiàn)了較強的負壓,與文獻[7, 9]的測量結(jié)果是
一致的.這說明Case1中,斜面上邊緣與側(cè)邊緣均存在較強的流動分離.Case2中斜面上壓力分布無明顯變化,僅上邊緣附近的負壓極值略有增大.這與表1所示Case2中氣動阻力的變化規(guī)律是吻合的.Case3的斜面壓力分布與前2種工況截然不同,兩側(cè)與上邊緣附近的負壓極值明顯減小,Cp的極小值為-0.5左右,僅相當(dāng)于Case1對應(yīng)值的一半.而對于水平導(dǎo)流板的工況Case5和Case6,其上邊緣與側(cè)邊緣附近的壓力極值消失,整個斜面的壓力分布變得非常均勻,且斜面上的負壓明顯減弱了.這表明Case5和Case6中模型尾部斜面上邊緣與側(cè)邊緣附近的流動分離被顯著削弱.Case4和Case7的壓力分布情況分別與Case3和Case6非常類似,圖8中未給出.綜上所述,當(dāng)導(dǎo)流板寬度分別為10和15 mm時,無論是布置在斜面兩側(cè)還是水平布置在斜面上邊緣處,均能起到減小模型氣動阻力的作用,但水平布置在斜面邊上緣處的導(dǎo)流板的減阻效果更優(yōu).
圖8模型尾部斜面和垂面壓力分布
Fig.8 Pressure distributions on the slant and rear surfaces of the mode
Ahmed模型氣動阻力主要由頭部的正壓、尾部斜面與垂面上的負壓、以及其他各表面的摩擦阻力構(gòu)成,其中尾部斜面和垂面上的負壓占據(jù)了總氣動阻力的絕大部分[2].對于傾角為25°的Ahmed模型,當(dāng)雷諾數(shù)為7.0×105時,尾部斜面和垂面負壓對總氣動阻力的貢獻約為80%[25].將斜面與垂面的壓力投影至x方向并積分,可獲得各面對應(yīng)的阻力系數(shù),如表2所示.對于Case1,斜面與垂面阻力系數(shù)分別為0.187和0.158,可估算此時對應(yīng)的Cd約為0.431,這與表1所示結(jié)果非常吻合.對比表2中數(shù)據(jù)可知,Case2中,尾部垂面阻力系數(shù)基本沒有變化.造成Case2中Cd增大的原因主要是斜面阻力系數(shù)增大了,因為如圖8所示斜面上邊緣附近的負壓相對于Case1有所增強.對于Case3~Case7,其尾
表2 尾部斜面與垂面阻力系數(shù)
Tab.2 Drag coefficients of slant and rear surfaces
工況
尾部斜面
阻力系數(shù)
尾部垂面
阻力系數(shù)
尾部總
阻力系數(shù)
模型總
阻力系數(shù)
Case1
0.187
0.158
0.345
0.431
Case2
0.199
0.159
0.358
0.444
Case3
0.139
0.191
0.330
0.416
Case4
0.121
0.193
0.314
0.400
Case5
0.121
0.184
0.305
0.391
Case6
0.110
0.189
0.299
0.385
Case7
0.109
0.189
0.298
0.384
部垂面阻力系數(shù)不僅沒有減小,相對于Case1反而略有增大,由此可知,各工況減阻效果主要來源于斜面上負壓的減弱.即如圖8所示,導(dǎo)流板顯著抑制了斜面上邊緣與側(cè)邊緣附近強烈的流動分離,并削弱了斜面上負壓.
3 結(jié) 論
通過風(fēng)洞試驗研究了25°傾角Ahmed類車體尾部斜面兩側(cè)和斜面上邊緣處不同寬度導(dǎo)流板對模型尾流與氣動力的影響規(guī)律.導(dǎo)流板寬度分別為5,10和15 mm,約相當(dāng)于車長的1%,2%和3%.主要結(jié)論如下:
1) 當(dāng)模型尾部斜面兩側(cè)導(dǎo)流板寬為5mm時(Case2),其對拖曳渦與下掃流的影響可以忽略.斜面兩側(cè)寬分別為10,15 mm導(dǎo)流板(Case3,Case4)和上邊緣寬分別為5,10,15 mm的水平導(dǎo)流板(Case5~Case7)均能夠明顯削弱尾流中拖曳渦與下掃流強度.
2) 無導(dǎo)流板時,模型尾部斜面上邊緣附近分離流會發(fā)生再附著并形成D形流動分離泡.對于斜面兩側(cè)導(dǎo)流板,分離泡隨導(dǎo)流板寬度的增加而有所減小,但不會消失;而斜面上邊緣導(dǎo)流板能夠抑制斜面上的流動再附著,破壞分離泡的形成.隨著上邊緣導(dǎo)流板寬度的增加,斜面上流動變得非常均勻,類似于增大了Ahmed模型的尾部傾角.
3) 基于尾流中總壓與時均速度的測量結(jié)果,依據(jù)Onorato等[22]給出的方法估算氣動阻力是可行的.斜面兩側(cè)布置5 mm寬導(dǎo)流板(Case2)不僅無減阻效果,反而使氣動阻力增大約2.1%.當(dāng)兩側(cè)導(dǎo)流板寬度分別增加到10和15 mm時(Case3,Case4),減阻效率分布為3.9%和7.6%.Case5~Case7的減阻效率分別可達10.9%,11.6%和11.8%,基本不隨導(dǎo)流板寬度而變化.斜面兩側(cè)導(dǎo)流板的減阻效率隨導(dǎo)流板寬度增加而逐漸增大,但始終小于水平導(dǎo)流板的減阻效率.
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ZHU Hui, YANG Zhigang. Experimental study on the flow field in the wake of Ahmed model[J]. Journal of Experiments in Fluid Mechanics,2010,24(2):24-27.(In Chinese)
[19]GILLIERON P, CHOMETON F. Modelling of stationary threedimensional separated air flows around an Ahmed reference model[C]//3rd International Workshop on Vortex, ESAIM Proceedings. Amsterdam:Elsevier, 1999,7:173-182.
[20]KRAJNOVIC S, DAVIDSON L. Flow around a simplified car, part 2: understanding the flow[J]. J Fluids Eng,2005, 127: 919-928.
[21]GILLIERON P, KOURTA A. Aerodynamic drag control by pulsed jets on simplified car geometry[J]. Exp Fluids, 2013, 54:1-16.
[22]ONORATO M, COSTELLI A F, GARONNE A. Drag measurement through wake analysis[C]//SAE Technical Paper Series 840302,1984.
[23]VANDAM C. Recent experience with different methods of drag prediction[J]. Prog Aerosp Sci,1999, 35(8):751-798.
[24]BRUNN A, WASSEN E, SPERBER D, et al. Active drag control for a generic car model [C]// Notes on Numerical Fluid Mechanics and Multidisciplinary Design.Berlin Heidelberg:Springer, 2007: 247-259.
[25]KRAJNOVIC S, BASARA B. LES of the flow around Ahmed body with active flow control [C]//Turbulence and Interactions. Berlin Heidelberg: Springer, 2010: 247-254.
[5] 吳志剛,魏琪,加藤征三,等. 導(dǎo)流翼片的傾角和長度在降低大后壁車輛氣動阻力中的應(yīng)用[J]. 汽車工程,2003,25(6): 634-637.
WU Zhigang, WEI Qi, SEIZO Kato, et al. The effect of length and inclined angle of air deflectors on reducing drag of large bluff end vehicles[J]. Automotive Engineering,2003,25(6): 634-637.(In Chinese)
[6] AIDERD J, BEAUDOIN J, WESFREID J. Drag and lift reduction of a 3D bluff body using active vortex generators[J]. Exp Fluids, 2010, 48:771-789.
[7] PUJALS G, DEPARDON S, COSSU C. Drag reduction of a 3D bluff body using coherent streamwise streaks[J]. Exp Fluids, 2010, 49: 1085-1094.
[8] 谷正氣,李學(xué)武,何憶斌. 汽車減阻新方法[J]. 汽車工程,2008,30(5):441-443.
GU Zhengqi LI Xuewu HE Yibin. A new method of reducing aerodynamic drag[J]. Automotive Engineering,2008,30(5):441-443.(In Chinese)
[9] JOSEPH P, AMANDOLESE X,AIDER J L.Drag reduction on the 25°slant angle Ahmed reference body using pulsed jets[J]. Exp Fluids, 2011, 52:1169-1185.
[10]AUBRUN S, McNally J, ALVI F,et al. Separation flow control on a generic ground vehicle using steady microjet arrays[J]. Exp Fluids,2011, 51: 1177-1187.
[11]張攀峰,王晉軍,唐青. 氣動附加裝置降低廂式貨車后體阻力[J]. 實驗流體力學(xué),2009,23:12-15.
ZHANG Panfeng,WANG Jinjun, TANG Qing. Experimental investigation on the aft body drag reduction of the tractortrailer truck by aerodynamic addon device[J].Journal of Experiments in Fluid Mechanics,2009,23:12-15.(In Chinese)
[12]AHMED S R, RAMM R, FALTIN G. Some salient features ofthe timeaveraged ground vehicle wake[C]// SAE Technical Paper Series 840300,1984.
[13]VINO G, WATKINS S, MOUSLEY P,et al. Flow structures in the near wake of Ahmed model[J]. J Fluids Struct,2005, 20: 673-695.
[14]SCHNEIDER G, HOOPER J, MUSGROVE A,et al. Velocity and Reynolds stresses in a precessing jet flow [J]. Exp Fluids, 1997, 22: 489-495.
[15]CHEN J, HAYNES B, FLETCHER D. Cobra probe measurements of mean velocities, reynolds stresses and highorder velocity correlations in pipe flow [J]. Exp Therm Fluid Sci, 2000, 21: 206-217.
[16]CONAN B, ANTHOINE J, PLANQUART P. Experimental aerodyanmic study of a cartype bluff body[J]. Exp Fluids, 2011, 50: 1273-1284.
[17]THACKER A, AUBRUN S, LEROY A,et al. Effects of suppressing the 3D separation on the rear slant on the flow structures around an Ahmed body[J]. J Wind Eng Ind Aerodyn,2012,107/108:237-243.
[18]朱暉,楊志剛. 類車體尾跡區(qū)流動的實驗研究[J]. 實驗流體力學(xué),2010,24(2):24-27.
ZHU Hui, YANG Zhigang. Experimental study on the flow field in the wake of Ahmed model[J]. Journal of Experiments in Fluid Mechanics,2010,24(2):24-27.(In Chinese)
[19]GILLIERON P, CHOMETON F. Modelling of stationary threedimensional separated air flows around an Ahmed reference model[C]//3rd International Workshop on Vortex, ESAIM Proceedings. Amsterdam:Elsevier, 1999,7:173-182.
[20]KRAJNOVIC S, DAVIDSON L. Flow around a simplified car, part 2: understanding the flow[J]. J Fluids Eng,2005, 127: 919-928.
[21]GILLIERON P, KOURTA A. Aerodynamic drag control by pulsed jets on simplified car geometry[J]. Exp Fluids, 2013, 54:1-16.
[22]ONORATO M, COSTELLI A F, GARONNE A. Drag measurement through wake analysis[C]//SAE Technical Paper Series 840302,1984.
[23]VANDAM C. Recent experience with different methods of drag prediction[J]. Prog Aerosp Sci,1999, 35(8):751-798.
[24]BRUNN A, WASSEN E, SPERBER D, et al. Active drag control for a generic car model [C]// Notes on Numerical Fluid Mechanics and Multidisciplinary Design.Berlin Heidelberg:Springer, 2007: 247-259.
[25]KRAJNOVIC S, BASARA B. LES of the flow around Ahmed body with active flow control [C]//Turbulence and Interactions. Berlin Heidelberg: Springer, 2010: 247-254.
[5] 吳志剛,魏琪,加藤征三,等. 導(dǎo)流翼片的傾角和長度在降低大后壁車輛氣動阻力中的應(yīng)用[J]. 汽車工程,2003,25(6): 634-637.
WU Zhigang, WEI Qi, SEIZO Kato, et al. The effect of length and inclined angle of air deflectors on reducing drag of large bluff end vehicles[J]. Automotive Engineering,2003,25(6): 634-637.(In Chinese)
[6] AIDERD J, BEAUDOIN J, WESFREID J. Drag and lift reduction of a 3D bluff body using active vortex generators[J]. Exp Fluids, 2010, 48:771-789.
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