Baroclinic vorticity generation near the turbulent/non-turbulent interface in a compressible shear layer
Baroclinic vorticity generation near the turbulent/non-turbulent interface in a compressible shear layer
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DOI：
10.1063/1.4933250
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发表时间：
2015-10
影响因子：
4.6
通讯作者：
R. Jahanbakhshi;N. S. Vaghefi;C. K. Madnia
中科院分区：
文献类型：
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作者：
R. Jahanbakhshi;N. S. Vaghefi;C. K. Madnia
为了研究强涡结构（IVSs）与湍流/非湍流界面（TNTI）相互作用所产生的斜压涡度，对时间演化的可压缩混合层进行了直接数值模拟。为了检验这种相互作用，研究了可压缩湍流中TNTI和IVSs的涡度输运。在界面坐标系中，分析了涡度输运方程中不同项对与TNTI的距离的条件平均预算。在高可压缩混合层中，与涡度输运方程中的其他项相比，在TNTI附近斜压转矩对总涡度变化的贡献是不容忽视的。涡度输运方程中斜压转矩的条件平均在厚度约为一个泰勒长度尺度的界面层内达到最大值，距离约等于IVS到TNTI的半径。流动可视化结果表明，强涡度结构在靠近湍流/非湍流界面时产生斜压转矩。为了统计地检验TNTI和IVSs之间有组织的相互作用，在附录中描述了一种算法来检测和研究强涡度结构。结果表明，IVSs产生了一个压力梯度，从涡旋的核心，低压，向涡旋的外部区域。当IVSs与TNTI相互作用时，压力梯度矢量与密度梯度矢量发生错位，而密度梯度矢量与TNTI垂直方向对齐，从而产生斜压扭矩。可压缩性对剪切层内IVSs的结构特征影响较小。
Direct numerical simulations of temporally evolving compressible mixing layers are performed to study the baroclinic vorticity generation due to the interaction between the intense vorticity structures (IVSs) and the turbulent/non-turbulent interface (TNTI). In order to examine this interaction, the vorticity transport across the TNTI and the IVSs in compressible turbulence are studied. The conditional mean budget of different terms in the transport equation of vorticity with respect to the distance from the TNTI is analyzed in the interface coordinate system. In highly compressible mixing layers, it is shown that in proximity of the TNTI, contribution of the baroclinic torque to the total change of vorticity, compared with the other terms in the transport equation of the vorticity, cannot be ignored. The conditional average of the baroclinic torque in the vorticity transport equation reaches a maximum inside the interface layer, with the thickness of approximately one Taylor length scale, at a distance approximately equal to the radius of the IVS from the TNTI. Flow visualization results show that the intense vorticity structures generate a baroclinic torque as they become close to the turbulent/non-turbulent interface. In order to statistically examine the organized interaction between the TNTI and the IVSs, an algorithm described in the Appendix is developed to detect and to study the intense vorticity structures. It is shown that the IVSs generate a pressure gradient from the core of the vortex, low pressure, towards the region outside of the vortex. As the IVSs interact with the TNTI, the pressure gradient vectors become misaligned with the density gradient vectors, which are aligned with the direction normal to the TNTI, and generate a baroclinic torque. It is also observed that compressibility has a small effect on the structural features of the IVSs in the shear layer.