交叉剪切混合层中拟序结构的数值研究

采用拟谱方法对时间模式的交叉剪切混合层进行了直接数值模拟。计算结果表明:与平面混合层一样,展向涡的拉伸作用是交叉剪切混合层中流向涡形成的主要机制。当展向剪切强度较大时(如两主流交叉角为40°),与初期展向KelvinHelmholtz相关的单向旋转流向涡在拉伸作用下很快增长起来,并“坍缩”成“肋状”涡。当交叉角为40°时,涡核区存在类似平面混合层中“方块状”涡的流向涡结构,展向涡辫区还存在一组符号相反的流向涡,不过与“肋状”涡对应的涡结构呈扁平状,始终没有“坍缩”。当交叉角为60°时,“肋状”涡非常强,以致完全抑制了平面混合层“对称模式”的发展。当交叉角小到20°时,流向涡结构更接近于对称分布,然而“肋状”涡却没有形成。另外,计算结果还证实:与二维混合层相比,大强度展向剪切的引入能够加强流场的混合,同时,适当增加展向扰动波初始强度和波数也是提高混合效率的有效手段。

NUMERICAL RESEARCH ON COHERENT STRUCTURES IN A MIXING LAYER WITH CROSS-SHEAR

The evolution of a time\|developing mixing layer with cross\|shear is simulated numerically using the pseudo\|spectral method. The results indicate that stretching by the rollers is responsible for the formation of the streamwise vortices in a mixing layer with cross\|shear.When the cross\|shear is relatively strong (the cross\|angle of two streams is equal to 40 degrees, for example), the co\|rotating streamwise vortices related to the early spanwise Kelvin\|Helmholtz instability grow rapidly by stretching and collapse into rib\|shaped vortices, which are very similar to the ribs in a plane mixing layer. When the cross\|angle is 40 degrees, the vortex corresponding to the "quadrupole" in a plane mixing layer is also observed in the core region, and a set of streamwise vortices with signs opposite to those of the vortices containing the ribs lie in the spanwise braid region.The counterparts of the ribs, however, are in flat shape and much weaker. When the cross\|angle is up to 60 degrees, the ribs are so strong that their counterparts can not develop. While the cross\|angle is down to 20 degrees, the symmetry of the streamwise vortices is more obvious , but the ribs do not form. Additionally, it is revealed that the introduction of the strong cross\|shear results in enhanced mixing compared to a two\|dimensional mixing layer and that increases of initial intensity and wavenumber of the spanwise disturbance are also conducive to mixing.