Direct numerical simulations of forced and unforced separation bubbles on an airfoil at incidence
Direct numerical simulations of forced and unforced separation bubbles on an airfoil at incidence
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DOI：
10.1017/s0022112008000864
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发表时间：
2008-05-10
影响因子：
3.7
通讯作者：
Sandham, N. D.
中科院分区：
文献类型：
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作者：
Jones, L. E.;Sandberg, R. D.;Sandham, N. D.
介绍了 NACA-0012 翼型上的层流分离气泡在 Re-c = 5 x 10(4) 和入射角 5 度时的直接数值模拟 (DNS)。最初引入体积强迫是为了促进向湍流的转变。从这种强制情况获得足够的数据后，明确添加的干扰将被删除，并且模拟将进一步运行。在没有强迫的情况下，观察到湍流是自我维持的，并且在重新附着区域中湍流强度增加。强制和非强制情况的比较表明，强制提高了空气动力性能，同时需要很少的能量输入。对时间平均流场进行经典的线性稳定性分析；然而，没有观察到绝对的不稳定可以解释自持湍流的存在。最后，提出了一系列简化的 DNS，说明了自然发生的二维涡旋脱落的三维绝对不稳定性。三维扰动在发展涡流的辫状区域被放大，随后通过局部逆流区域向上游对流，其中上游速度幅度大大超过时间平均值。扰动被对流到下一个发展的涡流的编织区域，在那里它们被进一步放大，因此循环以不断增加的幅度重复。这一转变过程独立于上游扰动的事实对于分离气泡建模具有重要意义。
Direct numerical simulations (DNS) of laminar separation bubbles on a NACA-0012 airfoil at Re-c = 5 x 10(4) and incidence 5 degrees are presented. Initially volume forcing is introduced in order to promote transition to turbulence. After obtaining sufficient data from this forced case, the explicitly added disturbances are removed and the simulation run further. With no forcing the turbulence is observed to self-sustain, with increased turbulence intensity in the reattachment region. A comparison of the forced and unforced cases shows that the forcing improves the aerodynamic performance whilst requiring little energy input. Classical linear stability analysis is performed upon the time-averaged flow field; however no absolute instability is observed that could explain the presence of self-sustaining turbulence. Finally, a series of simplified DNS are presented that illustrate a three-dimensional absolute instability of the two-dimensional vortex shedding that occurs naturally. Three-dimensional perturbations are amplified in the braid region of developing vortices, and subsequently convected upstream by local regions of reverse flow, within which the upstream velocity magnitude greatly exceeds that of the time-average. The perturbations are convected into the braid region of the next developing vortex, where they are amplified further, hence the cycle repeats with increasing amplitude. The fact that this transition process is independent of upstream disturbances has implications for modelling separation bubbles.