Stable and efficient mass exchange is crucial to the combustion stability of a dual-mode scramjet. In subsonic mode, the progression of mass exchange between the cavity fluid and the mainstream, which is controlled by the cavity shear layer, is different from the scramjet mode due to the weak compressibility effects. However, mass exchange mode studies in the subsonic mode are rare. In this study, experimental research has been conducted to obtain the flow structure of Mach 0.3 and 0.5 inflow over a rectangular cavity and reveal the properties of mass exchange using particle image velocimetry technology. The results reveal the weak compressibility effects and geometry effects on the mass exchange by analyzing the growth rate and turbulence characteristics of the cavity shear layer. The mass exchange between the cavity fluid and mainstream is determined based on the vertical velocity along the cavity lip line. When the flow Mach number increased from 0.3 to 0.5, the growth rate of the cavity shear layer decreases due to the weak compressibility effects. Also, the growth rate of the cavity shear layer is smaller than that of the compressible free shear layer due to the effects of recirculation within the cavity. For length-to-depth (L/D) ratios ranging from 0.8 to 1.2, the cavity shear-layer growth rate decreases with increasing L/D. However, at an L/D of 1.5, the shear-layer growth rate increases, which changes the mass exchange mode. Finally, the effects of increasing the Mach number (Mach = 0.3 versus 0.5) show that the L/D ratio at the “crossover” point of the mass exchange mode transition is reduced (L/D = 1.5 versus 1.2) due to the weak compressibility at a higher Mach number.

稳定高效的质量交换对于双模超燃冲压发动机的燃烧稳定性至关重要。在亚音速模式下，由腔体剪切层控制的腔体流体与主流之间的质量交换进程与超燃冲压模态不同，因为压缩效应较弱。然而，亚音速模式中的质量交换模式研究很少见。在这项研究中，进行了实验研究，以获得矩形腔上 0.3 和 0.5 马赫的流入流结构，并使用粒子图像测速技术揭示质量交换的特性。结果通过分析腔剪切层的生长速率和湍流特性揭示了对质量交换的弱压缩效应和几何效应。腔流体和主流之间的质量交换基于沿腔唇线的垂直速度确定。当流动马赫数从 0.3 增加到 0.5 时，由于弱压缩效应，腔剪切层的生长速率降低。此外，由于腔内再循环的影响，腔剪切层的生长速率小于可压缩自由剪切层的生长速率。对于从 0.8 到 1.2 的长度与深度 (L/D) 比，腔剪切层生长速率随着 L/D 的增加而降低。然而，在 L/D 为 1.5 时，剪切层生长速率增加，这改变了质量交换模式。最后，增加马赫数（马赫 = 0.3 对 0.5）的影响表明，质量交换模式转变的“交叉”点处的 L/D 比降低（L/D = 1.5 对 1。