【作者】 田砚；

【导师】 姜楠；

【作者基本信息】 天津大学 ， 流体力学， 2004， 硕士

【摘要】 在平板湍流边界层的固壁表面沿展向周期布置一组相互平行的流向加热丝,通过金属丝加热在固壁表面产生沿展向周期分布的温度场,利用温度场引起空气热对流在湍流边界层近壁区产生一组沿展向周期分布的大尺度流向涡。对人为引入流向涡控制壁湍流多尺度相干结构进行实验研究,提出了一种控制壁湍流多尺度相干结构和壁湍流减阻的有效方法。使用平均速度剖面法分析加热金属丝对平板湍流边界层的减阻效果。用IFA300热线风速仪和热线测速技术,分别沿平板湍流边界层的展向和法向方向,以高于最小湍流尺度的分辨率精细测量了平板湍流边界层近壁区域瞬时速度信号,然后利用湍流边界层近壁区域对数律平均速度剖面迭代计算壁面摩擦速度,进而得到湍流边界层的壁面摩擦阻力和壁面摩擦系数。再根据壁面摩擦系数计算减阻率,评价减阻效果。提出了用子波系数的分尺度平坦因子检测平板湍流边界层中的多尺度相干结构的条件采样方法,用条件相位平均法提取不同尺度相干结构的条件相位平均波形。对比研究平板湍流边界层固壁表面形成流向涡前后,壁湍流多尺度相干结构条件相位平均波形、多尺度相干结构的能量分布、相对强度、间歇性、发生概率等湍流统计性质。对平板湍流边界层固壁表面形成流向涡后的壁湍流减阻机理进行了实验研究。研究结果表明,湍流边界层近壁区人为引入的规则流向涡后,多尺度相干结构的相位平均波形的前半周期的负相速度幅值显著变小,后半周期正相速度幅值显著变大。相干结构的后半周期压缩——拉伸强度更大,呈现较为典型的缓冲层相位平均波形,说明缓冲层增厚。加热还使得近壁区域相干结构的发生概率提高,各尺度流动结构的平坦因子显著增加,多尺度相干结构的湍动能占有率明显提高,流动结构的相干性增强;子波系数的PDF明显变宽,因而增加了近壁区中湍流多尺度流动结构的间歇性和有序性,产生减阻效果。更多还原

【Abstract】 An experimental study is carried out to investigate multi-scale eddystructure control in wall turbulence. The local heating generates a groupof organized stream-wise vortex structures. Local heating is given to theboundary layer flow by means of a set of longitudinal parallel heatingwires fixed on the surface of flat plate.In order to study the effects of drag reduction, distributions of thestream-wise mean velocity profile are measured. First longitudinalfluctuating velocity measurements have been finely carried out using hotwire anemometer at different vertical locations in the near wall region ofa turbulent boundary layer ,at two span locations. Skin friction velocityhas been calculated through fitting the logarithmic law of mean velocityprofile. According as formula τ_w=ρu~2 , the skin friction shear stress iscalculated.A phase-averaging technique is employed to capture the multi-scalecoherent structures. It bases on conditional sampling 、 wavelettransformation and local average technique.It is found that with locally heating, the phased-average evolutionwaveforms show a typical shape in buffer layer. At the same time, thedetected coherent structures and the flatness factors for multi-scale eddystructure are increased, the turbulent flow fluctuating kinetic energy andcoherent structure energy for main scale enhanced, the shape of PDF forwavelet coefficient broaden .The correlativity of flow structures areobviously enhanced and the flow trends to be regularity. Add up to localheating on the wall organized the multi-scale eddy structure orderly, withthe result that skin friction shear stress is reduced, in the center of twowires.更多还原