简介：在低Reynolds数条件下,翼型绕流的上表面边界层由于抗逆压梯度能力变差容易发生流动分离,从而形成长层流分离泡.分离泡通常是非定常的,会诱发边界层的转捩、再附并形成湍流边界层.这个过程会使翼型的气动性能急剧下降,并伴随着强非线性效应.转捩后形成的湍流边界层也会产生高摩擦阻力.针对这种现象,文章以NACA0012翼型为例,通过隐式大涡模拟研究了有效的主动控制方案.为了统一分离控制技术和湍流边界层减阻技术,研究了在平板或槽道湍流中取得较好控制效果的壁面垂向反向控制方案.首先利用隐式大涡模拟研究了低Reynolds数条件下NACA0012翼型绕流的流场特征.其次分析并验证了反向控制方案在分离区控制流场的可行性,发现反向控制在分离区的作用相当于基于流场信息的壁面抽吸控制,且控制具有实时性和高效性,控制抽吸了前缘的低能流体,使得翼型前缘附面层变薄,并增强了其抗逆压梯度的能力,较大程度提高了翼型的气动性能.最后在湍流边界层验证了其减阻控制效果,发现反向控制阻断了流向涡的法向输运,抑制了涡结构的发展,并减弱了猝发过程,使得湍流的高摩阻力得到了有效降低.

简介：低Reynolds数流动由于自身特点导致气动特性严重恶化，非定常、非线性效应突出且预测困难，加之相关基础理论研究不足，给以临近空间低速飞行器和高性能微小型飞行器为代表的低Reynolds数飞行器的开发和研制带来了瓶颈和挑战.首先概述了飞行器低Reynolds数的范畴、低Reynolds数空气动力学的主要问题与挑战.随后从低Reynolds数层流分离基础理论出发，依次介绍了低Reynolds数层流分离经典理论、低Reynolds数层流分离非定常流动特性、低Reynolds数后缘层流分离泡.在此基础上，通过对经典长层流分离泡与后缘层流分离泡力学特性的差异以及随攻角和Reynolds数的演化规律的详细分析，逐步揭示了一些低Reynolds数复杂气动效应的本质，如小攻角升力系数的非线性效应，翼型随Reynolds数下降气动特性的二次恶化效应等.最后对低Reynolds数流动基础理论的发展过程进行了总结，并对层流分离诱导转捩及再附效应等复杂流动问题进行了展望.

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简介：方柱绕流是典型的钝体绕流问题,蕴含了丰富的流体力学现象,对这类流动的准确预测面临着诸多挑战.采用自主发展的大涡模拟程序,对来流Mach数M=0.3,Reynolds数尺eD=22000的绕孤立方柱流动进行了细致模拟,亚格子模型使用动力涡黏模型.对计算结果的分析表明,大涡模拟所得的平均流场及Reynolds应力分布与已有实验数据和直接数值模拟结果均吻合较好,验证了预测结果的可靠性;在此基础上对瞬态流场进行了研究,展示了计算条件下方柱绕流分离转捩及尾迹区旋涡交替脱落形成Karman涡街的全过程,为更细致的流动机理探索奠定了基础.

简介：目前,随着相关项目研究的不断推进,如何在高Reynolds数下研究其对气动光学效应的影响成为重要命题.通过设计变Reynolds数气动光学效应实验平台,模拟的单位Reynolds数可以在7.2×10^6-2.2×10^8m^-1范围内变化.搭建的基于背景纹影（backgroundorientedschlieren,BOS）的波前测试系统可以达到6ns的时间分辨率.此系统测量的平凸透镜波前结果表明：实验测量结果与理论计算结果的误差在±4%以内.通过测量9种不同Reynolds数下的超声速气膜瞬态波前数据,分析结果表明：在高Reynolds数条件下,Reynolds数对于超声速气膜气动光学效应的影响比较明显,通过对实验数据进行函数拟合发现OPDrmsRe088,与推导结果OPDrmsRe09十分接近;利用小波分析方法研究了高Reynolds数条件下气动光学效应沿流向的分布特征,发现OPDrms的低频部分（信号的主体）先降低后升高,但是高频部分的震荡幅度先升后降.分析认为OPD的低频部分主要受到流场整体结构的影响,而高频部分更多地受到涡的空间分布影响.

简介：ThemeasurementresultsbyLaserDopplerVelocimetry(LDV)arecomparedwiththedirectnumericalsimulationresultbyEggelsetal.^[1]foracylindricalpipeflow.Inthecaseofapipeflow,thebiaserrorformeanvelocityisverysmall,becausethelocalturbulentintensityisverysmallalloverthepipecrosssection.Howeverthedifferenceofthecombinationofu'andv'haveconsiderableeffectsonReynoldsshearstress.Fromourinvestigation,itisfoundthattheselectionofcoincidencetimethatisanecessaryparameterforcombinationofu'andv'ismoreimportantinobtainingtheaccurateReynoldsshearstress.Thesuitablecoincidencetimeisselectedforajetflowandtheeffectivenessofcoincidenttimemethodorequaltimeintervalmethodwithcoincidencedataisshown.

简介：批评转变雷纳兹数字的决心具有为一些设计问题的实际重要性。然而，它不与当前的理论方法是可得到的，并且不得不依靠实验。为超声/极超音速的边界层流动，为决心的试验性的方法也不是可行的。在这份报纸，因此，为为不可压缩的飞机隧道流动的批评转变雷纳兹数字的决心的一个数字方法被建议。测试方法的可行性基本上被瞄准。建议方法被扩大在随后的报纸决定超声/极超音速的边界层流动的批评雷纳兹数字。

简介：TheeffectsofReynoldsnumberonbothlarge-scaleandsmall-scaleturbulencepropertiesareinvestigatedinasquarejetissuingfromasquarepipe.ThedetailedvelocityfieldsweremeasuredatfivedifferentexitReynoldsnumbersof8×103190,thevalueofA=εL/〈u2)3/2inthepresentsquarejetconvergesto0.5,whichisconsistentwiththeobservationindirectnumericalsimulationsofboxturbulence,butlowerthanthatincircularjet,platewakeflows,andgridturbulence.ThediscrepanciesincriticalReynoldsnumberandA=εL/〈u2〉3/2amongdifferentturbulentflowsmostlikelyresultfromtheflowtypeandinitialconditions.

简介：TheaerodynamicforcesandflowstructureofamodelinsectwingisstudiedbysolvingtheNavier-Stokesequationsnumerically.Afteraninitialstartfromrest,thewingismadetoexecuteanazimuthalrotation(sweeping)atalargeangleofattackandconstantangularvelocity.TheReynoldsnumber(Re)consideredinthepresentnoteis480(Reisbasedonthemeanchordlengthofthewingandthespeedat60%winglengthfromthewingroot).Duringtheconstant-speedsweepingmotion,thestallisabsentandlargeandapproximatelyconstantliftanddragcoefficientscanbemaintained.Themechanismfortheabsenceofthestallorthemaintenanceoflargeaerodynamicforcecoefficientsisasfollows.Soonaftertheinitialstart,avortexring,whichconsistsoftheleading-edgevortex(LEV),thestartingvortex,andthetwowing-tipvortices,isformedinthewakeofthewing.Duringthesubsequentmotionofthewing,abase-to-tipspanwiseflowconvertsthevorticityintheLEVtothewingtipandtheLEVkeepsanapproximatelyconstantstrength.ThispreventstheLEVfromshedding.Asaresult,thesizeofthevortexringincreasesapproximatelylinearlywithtime,resultinginanapproximatelyconstanttimerateofthefirstmomentofvorticity,orapproximatelyconstantliftanddragcoefficients.Thevariationoftherelativevelocityalongthewingspancausesapressuregradientalongthewingspan.Thebase-to-tipspanwiseflowismainlymaintainedbythepressure-gradientforce.

简介：这篇论文论述实验室斜槽的结果流动速度上的银行植被和石子床的相互作用上的试验性的学习(首先在最大的速度的地点上，Umax)并且雷纳兹压力分布。结果表明在水表面下面的最大的速度的剧降直到在Umax之间的35%流动深度和差别，在水表面的速度面对墙上的植被是可观的。木头法律的地区从墙从多达15%流动深度和它不取决于的y/h=2改变距离。没有植被盖子，在在有墙上的植被盖子的一张石子床上的外部层的速度侧面的偏差比在一张石子床上的流动的盒子大得多。墙上的植被的存在把一致流动改变到不一致的流动。这个事实能被越过斜槽在不同距离在每侧面认为非线性的雷纳兹是压力分发和最大的速度的地点解释。在距离的雷纳兹压力分布从墙的0.02m有否定价值并且离开墙，他们改变在更高的地点与顶与特定的凸的形式拿积极价值的符号。为这研究的vonKarman经常的κ的一般水准等于0.16。基于κ=0.16，Clauser的方法和雷纳兹压力为决心是兼容的砍速度。

简介：潮汐的流动是一个周期的运动不稳定、不一致，它显然有加速和减速过程，特别在沿海的浅水里。潮汐的流动雷纳兹应力的垂直分发从线性分发背离了的许多研究表演。潮汐的流动雷纳兹压力的寓言的分发被歌等建议。(2009)。尽管模型更好充满领域观察和室内的试验性的数据，它有潮汐的流动雷纳兹应力的更低的截断的系列扩大，并且对分发的描述不是很全面的。由介绍潮汐的流动的运动方程并且改善，寓言的分发由歌等建立了。(2009)，潮汐的流动雷纳兹压力的立方的分发被建议。立方的分发被领域数据验证很好(Bowden和Fairbairn，1952；Bowden等，1959；Rippeth等，2002)并且试验性的数据(Anwar和阿特金斯，1980)，与Kuo等的数字模型结果一致。(1996)，并且与潮汐的流动雷纳兹压力的寓言的分发相比。这立方的分布比寓言的分发，而且罐头不仅好，这更好被显示出反映与潮汐的流动减速与潮汐的流动加速并且向上向下从线性分发背离的雷纳兹应力的基本特征，为潮汐的流动的速度侧面上的进一步的学习的基础。

简介：ThespecificsignofReynoldsstressintheboundarylayeronaflatplateatzeroincidenceisnewlyinterpretedinpresentpaperbasedonthetheoryofvortex-inducedvortex.Itavoidssomeproblemsappearedinatraditionalexplanation,onthebasisofrelationshipbetweenmeanandfluctuatingflowsduetothetransportofmomentum.Throughtheanalysisoflocalflowfieldintheimmediateneighborhoodofwall,thecharacteristicsofReynoldsstressareidentifiedthroughintroducingturbulence-inducedsmall-scalestreamwiseeddiesabovetheflatplate.ThepositiveReynoldsstressistheoreticallyverified.AndsuchnewinterpretationillustratesthatthegenerationofReynoldsstress,aswellasfluctuatingvelocity,isintrinsicallyindependentofthemeanflow.Butitsspecificsignwouldbedeterminedbythemeanflowduetotheinertialforces.Otherfeatures,suchastheintensityrelationshipamongthreecomponentsoffluctuatingvelocity,arealsopresented.

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简介：Itiswidelyacceptedthatinaturbulentboundarylayer(TBL)withadversepressuregradient(APG)anouterpeakusuallyappearsintheprofileofstreamwiseReynoldsstress.However,theeffectofAPGonthisouterpeakisnotclearlyunderstood.Inthispaper,theeffectofAPGisanalysedusingthenumericalandexperimentalresultsintheliterature.BecausetheeffectofupstreamflowisinherentintheTBL,wefirstanalysethiseffectinTBLswithzeropressuregradientonflatplates.Undertheindividualeffectofupstreamflow,anouterpeakalreadyappearsintheprofileofstreamwiseReynoldsstresswhentheTBLcontinuesdevelopinginthestreamwisedirection.TheAPGacceleratestheappearanceoftheouterpeak,insteadofbeingatrigger.

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简介：Adirect-forcingfictitiousdomain(DFFD)methodisusedtoperformfullyresolvednumericalsimulationsofturbulentchannelflowsladenwithlargeneutrallybuoyantparticles.Theeffectsoftheparticlesontheturbulence(includingthemeanvelocity,therootmeansquare(RMS)ofthevelocityfluctuation,theprobabilitydensityfunction(PDF)ofthevelocity,andthevortexstructures)atafrictionReynoldsnumberof395areinvestigated.Theresultsshowthatthedrag-reductioneffectcausedbyfinite-sizesphericalparticlesatlowparticlevolumesisnegligiblysmall.TheparticleeffectsontheRMSvelocitiesatReτ=395aresignificantlysmallerthanthoseatReτ=180,despitequalitativelythesameeffects,i.e.,thepresenceofparticlesdecreasesthemaximumstreamwiseRMSvelocitynearthewallviaweakeningthelarge-scalestreamwisevortices,andincreasesthetransverseandspanwiseRMSvelocitiesinthevicinityofthewallbyinducingsmaller-scalevortices.TheeffectsoftheparticlesonthePDFsofthefluidfluctuatingvelocitiesnormalizedwiththeRMSvelocitiesaresmall,regardlessoftheparticlesize,theparticlevolumefraction,andtheReynoldsnumber.

简介：Pipesarewidelyusedtotransportgas,oilandwaterinindustries.Dragreductioninpipesisanincreasinglyconcernedproblemtosaveenergy.Someresearcheshaveindicatedthatthenon-smoothsurfacewithspecialstructurescanreduceflowloss.Inthispaper,anexperimentalinvestigationhasbeenperformedontheeffectsofakindofsurfacegrooveonthedraginbothrectangularandcircularductatdifferentReynoldsnumbers.Intheexperimentoftherectangularduct,totalpressureatbothinletandoutletweremeasured.Staticpressureonthewallwasmeasuredonthesurfacewithsmoothandgroovedfilmrespectively.Inthecircularduct,aboundarylayerpressureprobewasusedtomeasurethetotalpressuredistributionatbothinletandoutlet.Fourtapsatinletandoutletwereusedtomeasurestaticpressure.Thelosscoefficientisusedtoevaluatetheeffectsofthesurfacegrooveondragreduction.TheexperimentwasconductedwiththeReynoldsnumberrangefrom1.28×10~4to2.57×10~4.Theresultshowsamaximumdraglossreductionofapproximately2.4%inrectangularductatReynoldsnumberof2.4×10~4.A10%reductionofpipepressurelossbygroovedsurfaceismeasuredincircularductataReynoldsnumberof3.0×10~5.

简介：Neglectingtheconsumptionofthematerial,asteadyincompressibleflowofanexothermicreactingthird-gradefluidwithviscousheatinginacircularcylindricalpipeisnumericallystudiedforbothcasesofconstantviscosityandReynolds’viscositymodel.Thecoupledordinarydifferentialequationsgoverningtheflowincylindricalcoordinates,aretransformedintodimensionlessformsusingappropriatetransformations,andthensolvednumerically.SolutionsusingMaplearepresentedintabularformandgivenintermsofdimensionlesscentralfluidvelocityandtemperature,skinfrictionandheattransferrateforthreeparametricvaluesintheReynolds’case.Thenumericalresultsforthevelocityandtemperaturefieldsarealsopresentedthroughgraphs.Bifurcationsarediscussedusingshootingmethod.Comparisonsarealsomadebetweenthepresentresultsandthoseofpreviouswork,andthusverifythevalidityoftheprovidednumericalsolutions.Importantpropertiesofthermalcriticalityareprovidedforvariableviscosityparameterandreactionorder.Furthernumericalresultsarepresentedintheformoftablesandgraphsfortransitionofphysicalparameters,whilevaryingcertainflowandfluidmaterialparameters.Also,theflowbehaviourofthereactivefluidofthird-gradeiscomparedwiththoseoftheNewtonianreactivefluid.