欠膨胀同流射流中的马赫盘形成与激波结构转变
Mach-disk formation and shock-structure transitions in underexpanded coflowing jets
中文总结 AI 辅助
本文研究欠膨胀同流射流的激波结构，发现同流会使马赫盘缩小消失，转变的NPR随同流速度比增大，且可通过含非均匀压力边界条件的特征线法复现，还能线性延长首个激波胞。
中文摘要 AI 辅助
本文针对亚音速飞行中推进喷管的典型工况，在一系列喷管压比（NPR）和同流与喷管出口速度比（U_c）范围内，研究了排入亚音速同流的欠膨胀音速射流的近场激波结构。采用完全解析的轴对称模拟的时间平均统计结果与无粘特征线法（MOC）分析，探究同流如何改变激波胞结构，尤其是马赫盘的形成。已有研究表明，提高NPR会使中心线反射从规则反射（以斜激波为特征）转变为马赫反射（以近法向马赫盘为特征）。本文发现同流具有相反的影响：强同流会缩小马赫盘直至其消失，使马赫反射回归规则反射，因此该转变对应的NPR随U_c增大而提高。此前该效应被归因于喷管唇口处射流边界倾角减小，这会将唇口普朗特-迈耶（Prandtl-Meyer）扇限制在更小角度并削弱嵌入激波。本文则表明，该倾角由同流沿射流边界施加的非均匀压力决定，这是同流射流中激波结构转变的主要驱动因素。非均匀压力会削弱边界反射的压缩波并使其取向更浅，从而使嵌入激波发生规则反射或无法形成。采用该非均匀压力边界条件、经模拟验证的MOC分析可复现随同流增大的转变行为。同流还会线性延长第一个激波胞，这可通过对普朗特经典激波胞长度标度的简单修正准确估算。
英文摘要
The near-field shock structures of underexpanded sonic jets exiting into a subsonic coflow are investigated over a range of nozzle pressure ratio (NPR) and coflow-to-nozzle-exit velocity ratio ($U_c$), representative of a propulsive nozzle in subsonic flight. Time-averaged statistics from fully-resolved axisymmetric simulations and inviscid method-of-characteristics (MOC) analysis are used to understand how coflow alters the shock-cell structures, in particular the Mach-disk formation. It is well established that increasing NPR transitions the centerline reflection from regular (characterized by oblique shocks) to Mach reflection (characterized by a near-normal Mach disk). We find that coflow has the opposite influence: a strong coflow shrinks the Mach disk until it vanishes, reverting Mach reflection to regular reflection, so the NPR for this transition increases with $U_c$. This effect has previously been attributed to a reduction in the jet-boundary inclination at the nozzle lip, which confines the lip Prandtl-Meyer fan to a smaller angle and weakens the embedded shock. We show instead that this inclination is determined by the non-uniform pressure the coflow imposes along the jet boundary, which is the primary driver of the shock-structure transitions in coflowing jets. The non-uniform pressure weakens the boundary-reflected compression waves and orients them at shallower angles, so the embedded shock reflects regularly or fails to form. A simulation-informed MOC analysis with this non-uniform pressure boundary condition reproduces the transition behavior with increasing coflow. Coflow also lengthens the first shock cell linearly, which is accurately estimated by a simple correction to Prandtl classical shock-cell length scaling.
