近床面区挟沙气流流速垂线分布特征

Near-Bed Velocity Profiles in Blown-Sand Flow

以测试参数较为齐全的挟沙气流风洞实验资料为基础 ,结合已有各家资料重点对近床面区挟沙气流流速垂线分布进行了系统的分析。在对近床面区沙粒跃移层内气流剪切应力分布的理论分析基础上 ,指出了该区的剪切应力由沙粒临界起动风速、沙粒垂直速度的垂线变化和输沙量垂线分布三者决定 ,并进一步导出了近床面区的挟沙气流流速垂线分布廓线方程 ,由该方程计算得到的结果与实测资料吻合很好。此外 ,研究还表明 ,Bagnold提出的不同风力条件下挟沙气流流速廓线聚焦于一点的观点基本正确 ,但这一点不宜作为跃移层内外风速廓线的分界点 ,实际的分界点高度是沙粒跃移层外气流摩阻速度平方的 0 .0 15倍 ;Owen提出的跃移层外风速廓线方程基本合理 ,但其中有效粗糙度公式中的系数C取值为 0 .0 4时更符合实测资料 ;来流速度相同的情况下 。

TheKeyLaboratoryofWaterandSedimentSciences,MinistryofEducation ,China ; 3 .DepartmentofHydraulicEngineering ,TsinghuaUniversity ,Beijing 10 0 0 84China) Abstract:Thedistributionofwindvelocitieswithheightduringsaltationisdiscussedatlengthbasedonboththe datafromauthors’windtunnelexperiments ,inwhichtherelativecompletemeasurementsoftheparametersrelated tothewindvelocitiesweremade ,andexistingdatafromothers’experiments.Theoreticalanalysesweremadefor thefluidshearstresswithinthesaltationlayer,whichwasfounddependingonthethresholdshearvelocity,thevari ationsofverticalvelocityofgrainswithheight,andtheverticaldistributionofthemassfluxofgrains.Asaresult, ananalyticalexpressionforthewindvelocityprofileduringsaltationwasdevelopedandthepredictedresultswerein goodagreementwiththeexperimentaldata .ItwasindicatedthatBagnold’sviewthatthewindvelocityprofilesfor differentwindstrengthsduringsaltationconvergetoafocusiscorrect,butthatthefocusisnotalwaystransitionfrom profileswithinthesaltationlayertoonesoutsideofthesaltationlayer.Themorereasonabletransitionwasfound , theheightofwhichscaledwith 0 .0 1 5timesofthesecondpowerofthefluidshearvelocityabovethesaltationlayer. TheequationgivenbyOwenforthewindvelocityprofileoutsidethesaltationlayerissound ,butifthecoefficient, C =0 .0 4inthedynamicroughnessformulae ,itismoreconsistentwithexperimentaldata .Foragivenfree stream velocity ,theairfrictionvelocitiesduringsaltationarelargerthantheequivalentcleanairfrictionvelocities.Airki neticenergylossduetograinmovementreachesthemaximumatsomeheightnearthebedsurfacewhiletherateof airenergylossmonotonicallydeclineswithheightabovethebed .