Abstract Our study provides the first experimental investigation of the internal flows of ventilated partial cavitation (VPC) formed by air injection behind a backward-facing step. The experiments are conducted using flow visualization and planar particle image velocimetry with fog particles for two different cavity regimes of VPC, i.e., open cavity (OC) and two-branch cavity (TBC), under various ranges of free stream velocity ( U ) and ventilation rates ( Q ). Our experiments reveal similar flow patterns for both OC and TBC, including forward flow region near the air–water interface, reverse flow region, near-cavitator vortex, and internal flow circulation vortex. However, OC internal flow exhibits highly unsteady internal flow features, while TBC internal flow shows laminar-like flow patterns with a Kelvin–Helmholtz instability developed at the interface between forward and reverse flow regions within the cavity. Internal flow patterns and the unsteadiness of OC resemble those of turbulent flow separation past a backward-facing step (BFS flow), suggesting a strong coupling of internal flow and turbulent external recirculation region for OC. Likewise, internal flow patterns of TBC resemble those of laminar BFS flow, with the presence of unsteadiness due to the strong velocity gradient across the forward–reverse flow interface. The variation in the internal flow upon changing U or Q is further employed to explain the cavity regime transition and the corresponding change of cavity geometry. Our study suggests that the ventilation control can potentially stabilize the cavity in the TBC regime by delaying its internal flow regime transition from laminar-like to highly unsteady. Graphic abstract

中文翻译：

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通风局部空化的内部流动

摘要 我们的研究首次对反向台阶后面的空气注入形成的通风部分空化 (VPC) 的内部流动进行了实验研究。实验是使用雾粒子的流动可视化和平面粒子图像测速法进行的，用于 VPC 的两种不同腔体状态，即开放腔 (OC) 和双分支腔 (TBC)，在不同范围的自由流速度 (U) 和通风率 (Q)。我们的实验揭示了 OC 和 TBC 的相似流动模式，包括空气-水界面附近的正向流动区域、反向流动区域、近空化涡和内部流动循环涡。然而，OC 内部流动表现出高度不稳定的内部流动特征，而 TBC 内部流动显示层流模式，在腔内正向和反向流动区域之间的界面处形成开尔文-亥姆霍兹不稳定性。内部流动模式和 OC 的不稳定性类似于通过向后步骤（BFS 流）的湍流分离，表明内部流动和 OC 的湍流外部再循环区域的强耦合。同样，TBC 的内部流动模式类似于层流 BFS 流，由于正向-反向流动界面上的强速度梯度存在不稳定。改变 U 或 Q 时内部流动的变化进一步用于解释腔体状态转变和腔体几何形状的相应变化。我们的研究表明，通风控制可以通过延迟其内部流动状态从层流向高度不稳定的过渡来潜在地稳定 TBC 状态下的空腔。图形摘要