An experimental modal testing method for subcritical flow around a cylinder

Modal analysis of fluid flows is essential to understanding flow physics and fluid-solid interaction mechanisms, and to implementing flow control. Unlike unstable flows, the intrinsic stability of subcritical flows has led to failures in experimentally extracting the clear structure of the subcritical flow modes. To this end, this paper proposes an experimental modal testing method for subcritical flows. Using Dynamic Mode Decomposition (DMD), dominant modes of flow around a cylinder at subcritical Reynolds numbers are extracted experimentally for the first time. The extracted structure and parameters of the modes are in agreement with the numerical results in the literature. It is found that the first-order mode is the stable von Kármán mode and can be observed at a Reynolds number as low as 19.3, which is nearly identical to the lower boundary of subcritical vortex-induced vibration (VIV). This finding provides the first experimental evidence of the correlation between the von Kármán mode and fluid-solid interaction instability in subcritical flows.