Control of the Streaky Structure in Turbulent Boundary Layer by means of Streamwise Vortex Pair

实验研究了湍流边界层与纵向涡对的相互作用过程。自由流动中的三角翼产生了两种类型的纵向涡对，即共同向上和共同向下，并被引入到发展的湍流边界层中。涡旋半径与相互作用开始时的边界层厚度相当。纵向涡旋环流远小于整个边界层环流，但外层环流级别相同。在本研究中，展向振荡被应用于涡流发生器。展向振动的幅值为三角翼翼展的一半。正弦振动的频率分别为0.5赫兹和1.0赫兹，并用自由流中统计量的行为研究了展向振动的影响。在展向振荡作用下，流向涡度等值线变得平坦。一些较强的峰Apea…在雷诺应力的等高线上有更多的r。根据涡康托概率分布的不同，在给定横流位置测得的雷诺应力包含视分量。用两种不同的方法测量了机翼产生的环量，即纵向涡度积分和由升力估算得到的压力。在相互作用过程中，测量了边界层内平均速度和雷诺应力分布的形变和松弛。在共同向上流动的情况下，纵向涡对以更快的速度远离壁面。在同向流情况下，两条纵向涡路径的展向展开率较大。共同下沉时，最大纵向涡度衰减较慢。在两涡对称面上，纵向涡的相互作用使均匀分布的平均速度等高线和较高的壁面摩阻系数在同向下流情况下增大。另外，在同向流动的情况下，相互作用使平均速度等值线分布较广，而壁面摩擦系数较小。应用动量积分方程中附加应变率的影响来解释这两种情况下的边界层行为，考察了附加应变率∂W/∂z和∂V/∂y在三种湍流强度分量分布和雷诺剪应力分布中的影响。沿对称平面的边界层在向下共同流动时可分为“横流”，在共同向上流动时可分为“可识别的流向涡”。在向下共同流动的情况下，额外的应变率改变了内层湍流强度和雷诺剪应力的大小。同时采用多点热线测量和条件检测技术研究了横向发散比跨向收敛对条纹状结构的影响，得出了在同向流动上升时靠近壁面的局部逆压梯度。多点热线传感器有12个单一热线传感器，跨距为2 mm，相当于粘性壁长的40倍。采用变间隔时间平均方法(VITA)作为低速条纹的检测方案。由该方案检测到的低速条纹的平均跨距约为粘性壁长的150倍。在同向流情况下，沿壁面二次流的平均间距较大，向上二次流的平均间距较小。比较表明，跨向振荡使这种影响有所减弱。较少

Interaction process between turbulent boundary layer and longitudinal vortex pair was investigated experimentally. Two-types of longitudinal vortex pair, that is, common-flow up and common-flow down, were generated by delta wing located in free stream and introduced into developed turbulent boundary layer. Radius of vortex is comparable with the boundary layer thickness at the start of the interaction. Circulation of the longitudinal vortex is much less than circulation of whole boundary layer, but, of the same order of circulation in the outer layer. In the present study, spanwise oscillation was applied to vortex generator. Amplitude of spanwise oscillation is a half of wing span of the delta wing. Frequencies of sinusoidal oscillation were chosen as 0.5Hz and 1.0Hz.Effect of spanwise oscillation was investigated with behavior of statistical quantities in free stream. Contours of streamwise vorticity becomes to flattened shape by applying spanwise oscillation. Some strong peaks appea … More r in contours of Reynolds stresses. It is expected that Reynolds stresses measured at a given cross-stream position contain apparent components depending on probability distribution of vortex cantor. Circulation generated by the wing was measured by two different ways, that is, integral of longitudinal vorticity and estimation from lift force obtained pressure measurement. The two results prove that spanwise oscillation has no significant effect on the vorticity generating process.In the interaction process, deformation and relaxation of the mean velocity and Reynolds stresses profile were measured in the boundary layer. The longitudinal vortex pair moves away from the wall in faster rate in case of common-flow up. Spanwise spreading rate of two longitudinal vortex path is larger in case of common-flow down. Decay of maximum longitudinal vorticity is slower in case of common-flow down. At symmetrical plane between two vortex, the interaction between longitudinal vortex makes closely distributed mean velocity contour lines and higher skin-friction coefficient in case of common-flow down. Otherwise, in case of common-flow up the interaction makes widely distributed mean velocity contour lines and low skin-friction coefficient. Effect of extra rate of strain involved in momentum integral equation is applied to explain boundary layer behaviors in the two cases.Effect of the extra rate of strain, ∂W/∂z and ∂V/∂y, was examined in the profiles of three turbulent intensity components and Reynolds shear stress profiles. The boundary layer along symmetrical plane is classified into "cross flow" in case of common-flow down and "identifiable streamwise vortices" in case of common-flow up. The extra rates of strain modify magnitude of turbulent intensity and Reynolds shear stress in the inner layer in case of common-flow down. Otherwise, close to the wall in the case of common-flow up, local adverse pressure gradient is deduced from the fact that magnitude of transverse divergence is grater than that of spanwise convergence.Effect of the interaction on the streaky structure was investigated by simultaneous multi point hot-wire measurement and conditional detection technique. The multi point hot-wire sensor has 12 single hot-wire sensors with 2mm spanwise separation that corresponds to 40 times viscous wall length. The Variable Interval Time Averaging method (VITA) was employed as a detection scheme for low speed streaks. Averaged spanwise spacing of the low speed streaks detected by the scheme is about 150 times viscous wall length. In the common-flow down case, the average spacing is larger at wall-ward secondary current and smaller at up-ward secondary current. The comparison shows that the spanwise oscillation makes the effect somewhat weaker. Less