Rotating free-shear flows. Part 2. Numerical simulations
Rotating free-shear flows. Part 2. Numerical simulations
复制标题
旋转自由剪切流。
DOI：
--
复制
发表时间：
1995
影响因子：
3.7
通讯作者：
M. Lesieur
中科院分区：
文献类型：
--
作者：
O. Métais;C. Flores;S. Yanase;J. Riley;M. Lesieur
通过直接模拟（DNS）和大涡模拟（LES）研究了平行于基本涡度轴的固体旋转作用下周期性平面混合层和尾迹中相干涡的三维动力学。最初，流动受到叠加在基本剪切上的弱随机扰动的强迫，这种扰动要么是准二维的（强迫转变），要么是三维的（自然转变）。对于初始罗斯比数Ro(i)，基于在拐点处的涡度，模数较小，旋转的效果总是使流动更二维，无论旋转的意义（气旋或反气旋）。这与泰勒-普罗德曼定理是一致的。在这种情况下，在没有旋转的强迫过渡中发现的纵向涡被抑制。结果表明，在气旋混合层中，无论罗斯比数的值是多少，旋转都会抑制三维扰动的增长。这种抑制也存在于|Ro(i)|≤1的反气旋情况下。在中等的反气旋旋转速率下（Ro(i) < - 1），气流是强烈不稳定的。|Ro(i)≈2.5时达到最大不稳定，与Yanase等人（1993）进行的线性稳定性分析很好地一致。然后，该层由强大的纵向交替绝对涡管组成，这些绝对涡管被气流拉伸，并相对于流向略微倾斜。由此产生的涡量比非旋转情况下的涡量大。开尔文-亥姆霍兹涡被抑制了。背景速度剖面表现为长范围的近恒定切变，其涡度正好补偿了固体旋转涡度。这与Lesieur， Yanase & msametis（1991）提出的现象学理论是一致的。正如预期的那样，在LES中拉伸比在DNS中更有效。旋转尾流的一面为气旋，另一面为反气旋。当|Ro(i)|≤1时，旋转的作用是使尾迹更加二维化。在中等旋转速率下（|Ro(i)| > 1），气旋侧由Kármán涡旋组成，没有纵向发夹涡旋。反气旋侧卡门涡消失，表现为混合层，具有强烈的纵向绝对发夹涡。因此，适度的旋转在尾迹拓扑中产生了戏剧性的对称性破坏。与线性理论一样，|Ro(i)|≈2.5时仍然观察到最大的不稳定。本文还分析了旋转对平均流与场的二维和三维分量之间能量传递的影响。
The three-dimensional dynamics of the coherent vortices in periodic planar mixing layers and in wakes subjected to solid-body rotation of axis parallel to the basic vorticity are investigated through direct (DNS) and large-eddy simulations (LES). Initially, the flow is forced by a weak random perturbation superposed on the basic shear, the perturbation being either quasi-two-dimensional (forced transition) or three-dimensional (natural transition). For an initial Rossby number Ro(i), based on the vorticity at the inflexion point, of small modulus, the effect of rotation is to always make the flow more two-dimensional, whatever the sense of rotation (cyclonic or anticyclonic). This is in agreement with the Taylor–Proudman theorem. In this case, the longitudinal vortices found in forced transition without rotation are suppressed. It is shown that, in a cyclonic mixing layer, rotation inhibits the growth of three-dimensional perturbations, whatever the value of the Rossby number. This inhibition exists also in the anticyclonic case for |Ro(i)| ≤ 1. At moderate anticyclonic rotation rates (Ro(i) < −1), the flow is strongly destabilized. Maximum destabilization is achieved for |Ro(i) ≈ 2.5, in good agreement with the linear-stability analysis performed by Yanase et al. (1993). The layer is then composed of strong longitudinal alternate absolute vortex tubes which are stretched by the flow and slightly inclined with respect to the streamwise direction. The vorticity thus generated is larger than in the nonrotating case. The Kelvin–Helmholtz vortices have been suppressed. The background velocity profile exhibits a long range of nearly constant shear whose vorticity exactly compensates the solid-body rotation vorticity. This is in agreement with the phenomenological theory proposed by Lesieur, Yanase & Métais (1991). As expected, the stretching is more efficient in the LES than in the DNS. A rotating wake has one side cyclonic and the other anticyclonic. For |Ro(i)| ≤ 1, the effect of rotation is to make the wake more two-dimensional. At moderate rotation rates (|Ro(i)| > 1), the cyclonic side is composed of Kármán vortices without longitudinal hairpin vortices. Karman vortices have disappeared from the anticyclonic side, which behaves like the mixing layer, with intense longitudinal absolute hairpin vortices. Thus, a moderate rotation has produced a dramatic symmetry breaking in the wake topology. Maximum destabilization is still observed for |Ro(i)| ≈ 2.5, as in the linear theory. The paper also analyses the effect of rotation on the energy transfers between the mean flow and the two-dimensional and three-dimensional components of the field.