Laminar separation bubbles (LSB's) over the suction surface of a wing at low Reynolds number ( based on the airfoil chord length) can significantly affect the aerodynamic performance of the wing, and pose a unique challenge for the predictive capabilities of simulation tools due to their high sensitivity to flow environments and wing surface conditions. In this work a series of two-dimensional (2D) and three-dimensional (3D) low-order, and high-order accurate unstructured-grid-based numerical methods with varying model fidelity levels were used to study LSB physics over a NACA 0012 airfoil both in a clean freestream and in a turbulent freestream at a chord-based Reynolds number of 12,000. Lift production and time-averaged flow fields were compared with available experimental results. A major discovery is that in clean freestream flow a 3D high-order numerical scheme is necessary to capture LSB physics. This is due to the sensitivity of LSB-induced laminar-turbulent transition to flow conditions and boundary geometry at low Reynolds number. In freestream flows with moderate background turbulence (), 2D simulations failed to capture subtle 3D flow physics due to their intrinsic limitation, but can reasonably predict time-averaged airfoil performance. Similarity and distinction between freestream vortex-LSB interaction in 2D and eddy-LSB interaction in 3D were explained. The role of the Kelvin-Helmholtz instability and Klebanoff modes in the transition of 3D airfoils were shown to be critical for understanding laminar-turbulent transition and LSB formation on airfoils in clean and turbulent freestreams.

中文翻译：

---

自由流湍流对低雷诺数 NACA 0012 翼型层流分离气泡和升力产生的影响


低雷诺数（基于翼型弦长）的机翼吸力表面上的层流分离气泡 (LSB) 会显着影响机翼的空气动力学性能，并且由于其对流动环境和机翼表面条件高度敏感。在这项工作中，使用了一系列具有不同模型保真度水平的二维 (2D) 和三维 (3D) 低阶和高阶精确非结构化网格数值方法来研究 NACA 0012 上的 LSB 物理机翼在干净的自由流和湍流自由流中的翼型基于弦的雷诺数为 12,000。将升力产生和时间平均流场与现有的实验结果进行了比较。一个重大发现是，在干净的自由流中，需要 3D 高阶数值方案来捕获 LSB 物理现象。这是由于 LSB 引起的层流-湍流转变对低雷诺数下的流动条件和边界几何形状的敏感性。在具有中等背景湍流 () 的自由流中，2D 模拟由于其固有的局限性而无法捕捉微妙的 3D 流动物理现象，但可以合理地预测时间平均翼型性能。解释了 2D 中的自由流涡旋-LSB 相互作用和 3D 中的涡流-LSB 相互作用之间的相似性和区别。开尔文-亥姆霍兹不稳定性和克莱巴诺夫模式在 3D 翼型转变中的作用被证明对于理解洁净和湍流自由流中翼型的层流-湍流转变和 LSB 形成至关重要。