Multi-scale Flow Dynamics and Instability in a Rotor-Stator Cavity Revealed by Combined PIV and Tomo-PIV

Abstract

This study addresses the inherent limitations of conventional single-plane measurements, which predominantly rely on two-dimensional (2D) data, in fully capturing the three-dimensional (3D) physical essence of rotor-stator cavity flows. To overcome this constraint, the research innovatively integrates 2D particle image velocimetry (PIV) with 3D tomographic PIV (Tomo-PIV), establishing a synergistic measurement framework. Experiments were conducted at speeds of 300–900 r/min and gap ratios (G) of 0.07–0.11. The 2D PIV technique tracked the planar evolution of large-scale coherent structures, while Tomo-PIV enabled observation of 3D vortex stretching and breakup processes, achieving mutual validation between planar observations and volumetric reality. Through integrated analysis using Proper Orthogonal Decomposition (POD) and the Omega vortex identification criterion, key findings reveal under the tested conditions, a small gap ratio imposes strong wall confinement, effectively suppressing 3D instabilities and maintaining quasi-2D vortex ring structures. The 2D POD modes exhibit high fidelity to the actual 3D vortex morphology. Conversely, larger gap ratios allow centrifugal stretching to dominate, leading to vortex distortion, fragmentation, and accelerated transition to turbulence. Energy spectrum analysis further confirms that anisotropic stretching at high rotational speeds inhibits energy transfer to smaller scales. This combined measurement approach not only identifies G=0.07 as an effective condition for suppressing 3D instabilities, but also directly links 2D modal dynamics to 3D vortex structures. The findings provide a validated theoretical framework for diagnosing multi-scale flow instability and optimizing energy efficiency in rotor-stator systems.