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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。
The pseudo-spectral method is used to simulate the time-mode cross-shear mixing layer directly. The calculation results show that the stretching effect of vortical vortex is the main mechanism of flow vortex formation in the cross-shear mixing layer, as with the planar mixing layer. When the orientation shear strength is large (eg, the cross angle of the two main streams is 40 °), the unidirectional swirling vortices associated with the initial Kelvin Helmholtz growth rapidly increases under tension and “collapses” into “ribbed ”eddy. When the crossing angle is 40 °, there is a flow vortex structure similar to the “square” vortex in the vortex core region in the vortex core region. There is also a group of flow vortices opposite in sign in the vortex plaiting region. However, Corresponding vortex structure is flat, there is no “collapse.” When the crossing angle is 60 °, the “ribbed” vortex is very strong, so that the development of the “symmetrical mode” of the planar mixed layer is completely suppressed. When the crossing angle is as small as 20 °, the flow vortex structure is closer to the symmetrical distribution, however, the “rib” vortex is not formed. In addition, the calculation results also confirm that the introduction of high-intensity shear-to-shear can enhance the mixing of the flow field compared with the two-dimensional mixing layer. Meanwhile, increasing the initial intensity and wavenumber of the perturbation wave is also an effective way to improve the mixing efficiency.