不同外罩型线短舱边界层的转捩位置预测

Transition prediction of boundary layers over an engine nacelle with different outer covers

  • 摘要: 在设计短舱外罩型线时,希望尽可能使其表面边界层保持层流状态,以减小摩阻,提高有效载荷。理解边界层的转捩机制,并准确预估转捩发生的位置,有利于有效评估不同的外罩型线设计方案。以巡航状态下三种不同外罩型线的通气短舱为研究对象,采用线性稳定性理论分析了边界层稳定性特征,并综合考虑T-S波和激波诱导分离泡这两种转捩机制,给出了不同外罩型线下短舱边界层的转捩位置。结果表明,逆压梯度越大,T-S波越不稳定,流动分离的位置也越靠前。因此,设计时尽可能保持大范围的顺压梯度及减小逆压梯度将有利于推迟转捩发生。此外,针对风洞实验中的短舱缩比模型,给出了预测转捩位置与实验转捩结果的比较,发现两者的转捩线在形态上非常一致,从而进一步证实了预测方法的有效性。

     

    Abstract: In the design of the nacelle outer cover profile, it is crucial to maintain the boundary layer's laminar state to minimize wall friction and enhance payload capacity. Understanding the boundary-layer transition mechanisms and accurately predicting the transition front is fundamental to effectively evaluate different designs of the nacelle. In this work, we focus on the stability characteristics of boundary layers over nacelle with three different outer covers under cruise conditions. Considering transition mechanisms correlated with the T-S waves and the shock wave-induced separation bubbles, the transition fronts are predicted. The results indicate that as the unfavorable pressure gradient increases, T-S waves become more unstable, leading to earlier boundary-layer separation. Therefore, maintaining a relatively large range of favourable pressure gradient as well as reducing unfavorable pressure gradient will delay the boundary-layer transition. In addition, for a scaled nacelle model, the predicted transition front and that obtained in wind tunnel tests are morphologically similar, confirming the validity of the prediction method.

     

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