arXiv 2301.085082023-01-20physics.flu-dyn

翼型上跨音速抖振与不可压缩低频振荡的联系

Connecting transonic buffet with incompressible low-frequency oscillations on aerofoils

Pradeep Moise, Markus Zauner, Neil D. Sandham

备注：: 28 pages, 20 figures

摘要：

在跨音速范围内刚性翼型上的自持低频流动非定常性称为跨音速抖振。虽然这种现象背后的确切物理机制尚不清楚，但通常认为这是跨音速区所特有的。通过对NACA 0012剖面上各种流动条件下的流动进行大涡模拟，证明这种假设是不正确的。在零攻角和足够高的自由飞行马赫数M时，跨音速抖振发生，气流中出现激波。然而，在较低的M下观察到以类似频率发生的自持振荡，其中不存在激波，并且整个流场始终保持亚音速。在更高的入射角下，振荡在逐渐降低的M下持续。振荡被观察到的M低至0.3，压缩性的影响是小的。谱本征正交分解表明这些振荡的空间结构（即，振型）对于所有情况基本相同。这些结果表明，翼型抖振不一定需要激波的存在。此外，随着迎角的增加所看到的趋势表明，翼型上的跨音速抖振和不可压缩状态下的低频振荡（Zaman等人，1989年，《流体力学杂志》，第202卷，第202页。403- 442）有类似的起源。因此，专门依靠激波来解释跨音速抖振的模型是不正确的。这些见解可能有助于理解“跨音速”抖振的起源，并通过将重点从冲击波转移来重新制定减缓战略。

英文摘要：

Self-sustained low-frequency flow unsteadiness over rigid aerofoils in the transonic regime is referred to as transonic buffet. Although the exact physical mechanisms underlying this phenomenon are unclear, it is generally assumed to be unique to the transonic regime. This assumption is shown to be incorrect here by performing large-eddy simulations of flow over a NACA0012 profile for a wide range of flow conditions. At zero incidence and sufficiently high freestream Mach numbers, M, transonic buffet occurs with shock waves present in the flow. However, self-sustained oscillations that occur at similar frequencies are observed at lower M for which shock waves are absent and the entire flow field remains subsonic at all times. At higher incidences, the oscillations are sustained at progressively lower M. Oscillations were observed for M as low as 0.3, where compressibility effects are small. A spectral proper orthogonal decomposition shows that the spatial structure of these oscillations (i.e., mode shapes) are essentially the same for all cases. These results indicate that buffet on aerofoils does not necessarily require the presence of shock waves. Furthermore, the trend seen with increasing incidence angles suggests that transonic buffet on aerofoils and low-frequency oscillations reported in the incompressible regime (Zaman et al., 1989, J. Fluid Mech., vol. 202, pp. 403--442) have similar origins. Thus, models which rely specifically on shock waves to explain transonic buffet are incorrect. These insights could be useful in understanding the origins of ``transonic" buffet and reformulating mitigation strategies by shifting the focus away from shock waves.

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