風(fēng)粗糙度和風(fēng)切變

RoughnessandWindShear粗糙度和風(fēng)切變Highabovegroundlevel,ataheightofabout1kilometer,thewindishardlyinfluencedbythesurfaceoftheearthatall.Inthelowerlayersoftheatmosphere,however,windspeedsareaffectedbythefrictionagainstthesurfaceoftheearth.Inthewindindustryonedistinguishesbetweentheroughnessoftheterrain,theinfluencefromobstacles,andtheinfluencefromtheterraincontours,whichisalsocalledtheorographyofthearea.Weshallbedealingwithorography,whenweinvestigatesocalledspeedupeffects,i.e.tunneleffectsandhilleffects,later.在距離地面約1公里的高度，風(fēng)幾乎不受地球表面的影響。然而，在大氣的下層，風(fēng)速受到與地球表面的摩擦的影響。在風(fēng)能行業(yè)中，人們區(qū)分地形的粗糙度，障礙物的影響以及地形輪廓的影響，這也稱(chēng)為該地區(qū)的地形。當(dāng)我們稍后研究所謂的加速效應(yīng)，即隧道效應(yīng)和山地效應(yīng)時(shí)，我們將討論山岳學(xué)Orography。Roughness粗糙度Ingeneral,themorepronouncedtheroughnessoftheearth'ssurface,themorethewindwillbesloweddown.一般來(lái)說(shuō)，地球表面粗糙度越明顯，風(fēng)速就越慢。Forestsandlargecitiesobviouslyslowthewinddownconsiderably,whileconcreterunwaysinairportswillonlyslowthewinddownalittle.Watersurfacesareevensmootherthanconcreterunways,andwillhaveevenlessinfluenceonthewind,whilelonggrassandshrubsandbusheswillslowthewinddownconsiderably.森林和大城市會(huì)顯著減緩了風(fēng)速，而機(jī)場(chǎng)的混凝土跑道減緩風(fēng)速的效果有限。水面比混凝土跑道更平滑，對(duì)風(fēng)的影響更小，而長(zhǎng)草，灌木則會(huì)使風(fēng)速大幅減緩。RoughnessClassesandRoughnessLengths粗糙度等級(jí)和粗糙度長(zhǎng)度粗糙度等級(jí)和粗糙度長(zhǎng)度的正確定義可在參考手冊(cè)中找到。術(shù)語(yǔ)粗糙度長(zhǎng)度實(shí)際上是地面以上的距離，其中風(fēng)速理論上應(yīng)該為零。Inthewindindustry,peopleusuallyrefertoroughnessclassesorroughnesslengths,whentheyevaluatewindconditionsinalandscape.Ahighroughnessclassof3to4referstolandscapeswithmanytreesandbuildings,whileaseasurfaceisinroughnessclass0.在風(fēng)能行業(yè)中，人們通常會(huì)在評(píng)估風(fēng)況時(shí)參考粗糙度等級(jí)(roughnessclasses)或粗糙度長(zhǎng)度(roughnesslengths)。具有許多樹(shù)木和建筑物的場(chǎng)地一般是3至4的高粗糙度等級(jí)，而海面的粗糙度等級(jí)為0。Concreterunwaysinairportsareinroughnessclass0.5.Thesameappliestotheflat,openlandscapetotheleftwhichhasbeengrazedbysheep.機(jī)場(chǎng)的混凝土跑道的粗糙度等級(jí)為0.5。這同樣適用于由綿羊放牧的平坦，開(kāi)闊的場(chǎng)地(如下圖所示)。Sheepareawindturbine'sbestfriend.InthispicturefromAkaroaSpit,NewZealand,thesheepkeeptheroughnessofthelandscapedownthroughtheirgrazing.綿羊是風(fēng)力渦輪機(jī)最好的朋友。在這張來(lái)自新西蘭AkaroaSpit的照片中，通過(guò)放牧保持了場(chǎng)地的低粗糙度。TheproperdefinitionofroughnessclassesandroughnesslengthsmaybefoundintheReferenceManual.Thetermroughnesslengthisreallythedistanceabovegroundlevelwherethewindspeedtheoreticallyshouldbezero.粗糙度等級(jí)和粗糙度長(zhǎng)度的正確定義可在參考手冊(cè)中找到。術(shù)語(yǔ)粗糙度長(zhǎng)度實(shí)際上是地面以上的距離，其中風(fēng)速理論上應(yīng)該為零。WindShear風(fēng)切變Thisgraphwasplottedwiththewindspeedcalculatoronthenextpage.Itshowsyouhowwindspeedsvaryinroughnessclass2(agriculturallandwithsomehousesandshelteringhedgerowswithsome500mintervals),ifweassumethatthewindisblowingat10m/sataheightof100metres.該圖使用下一章節(jié)的風(fēng)速計(jì)算器繪制。它向您展示了如果我們假設(shè)高度100米處風(fēng)速10米/秒，那么風(fēng)速如何在粗糙度等級(jí)2(有一些間隔約500米的房屋和籬笆的農(nóng)田)中變化。Thefactthatthewindprofileistwistedtowardsalowerspeedaswemoveclosertogroundlevel,isusuallycalledwindshear.Windshearmayalsobeimportantwhendesigningwindturbines.Ifyouconsiderawindturbinewithahubheightof40metresandarotordiameterof40metres,youwillnoticethatthewindisblowingat9.3m/swhenthetipofthebladeisinitsuppermostposition,andonly7.7m/swhenthetipisinthebottomposition.Thismeansthattheforcesactingontherotorbladewhenitisinitstoppositionarefarlargerthan