Coherent Structures Analysis Across Cavity Interface in Cloud Cavitating Flows Using Different Vortex Identification Methods

Cavitating flows are characterized by strong compressibility inside the cavity and weak compressibility outside the cavity. The transition between these two regions with distinct acoustic features is defined as the acoustic shear layer (i.e., from cavity interface to 0.99 sonic seed in pure liquid, about 1 × 10^–7 < α_v < 1 × 10^–1) which has been preliminarily studied in the recent paper (Ocean Eng. 209(2020): 107025). The acoustic shear layer is characterized by the large sonic speed gradient which is of great acoustics importance to understand how the disturbance (i.e., shockwave) propagates between these two regions. With varying cavitation numbers and Reynold numbers, cavitating flows present different flow regimes where cloud cavitation is supposed to be the most destructive cavity regime and of great fundamental interest and engineering applications, mainly consisting of two kinds of cavity structures, namely attached sheet cavity and shedding cloud cavity. In this work, to improve the understanding of cavity instabilities associated with wave dynamics, we examine in detail the coherent structures inside the acoustic shear layer of both the attached sheet cavity and the shedding cloud cavity, in particular that across the cavity interface. Numerical simulation of cavitating flows around a NACA66 (mod) hydrofoil was conducted using computational fluid dynamics (CFD) tool. Vortex identification methods including vorticity, Q-criteria, and the Liutex method, are employed to identify the flow structures within the acoustics shear layer. Results show that acoustic shear layer across the attached sheet cavity is thinner than that across the shedding cloud cavity. The acoustic shear layer consists of two regions, i.e., turbulence dominated region and the acoustics dominated region. Specifically, the turbulence dominated region which is identified by the turbulent/non-turbulent interface near the cavity boundary is important for the mass, momentum, and energy transfer characteristics. The acoustics dominated region is significant for the disturbance propagation (i.e., shockwave) between the compressible cavitation region and incompressible pure liquid region. Different vortex identification methods show different performances in terms of identifying the flow structures in these two regions. It is suggested that further work could be done to implement the acoustics characteristics into the vortex identification method to improve the identification performance in acoustics dominated regions, i.e., low void fraction region in gas–liquid flows.