Frequency and Amplitude Modulations of a Moving Structure in Unsteady Non-Homogeneous Density Fluid Flow

Abstract

A fluid-structure interaction’s effects on the dynamics of a hydrofoil immersed in a fluid flow of non-homogeneous density is presented and analyzed. A linearized model is applied to solve the fluid-structure coupled problem. Fluid density variations along the hydrofoil upper surface, based on the sinusoidal cavity oscillations, are used. It is shown that for the steady cavity case, the value of cavity length Lp does not affect the amplitude of the hydrofoil displacements. However, the natural frequency of the structure increases according to Lp. In the unsteady cavity case, the variations of the added mass and added damping (induced by the fluid density rate of change) generate frequency and amplitude modulations in the hydrofoil dynamics. To analyse this phenomena, the empirical mode decomposition, a well established data-driven method to handle such modulations, is used.

The influence of fluid–structure interaction on cloud cavitation about a rigid and a flexible hydrofoil. Part 3

“…The actual frequency response may contain other frequencies, as the cavities tend to shed in multiple small clouds and the cavity length is typically not uniform along the span of the body owing to three-dimensional (3-D) effects. After the collapse of cavities, remnant micro-bubbles remain and can be carried upstream in the re-entrant jet owing to the high adverse pressure, which serve as re-nucleation sites for the development of new attached cavities (Barbaca et al 2020;Ram, Agrawal & Katz 2020;Russell & Brandner 2021). The leading edge cavity grows to the maximum cavity length to repeat the process of re-entrant jet formation and cavity shedding.…”

“…Fluid added mass in fully ventilated and partially cavitating regimes is generally lower than that in a fully wetted regime owing to a drop in the local fluid density (Akcabay & Young 2015;Harwood et al 2020). Through empirical mode decomposition, it has also been shown that oscillating cavity length along the fluid-structure interface can cause fluctuations in the added mass (Rajaomazava et al 2021). The system natural frequency is dependent on the fluid added mass, so system natural frequencies are also altered in multiphase flows (De La Torre et al 2013;Harwood et al 2020;Young et al 2020).…”

“…Hydrodynamic damping is typically much higher than structural damping, particularly for the lower-ordered modes (Blake & Maga 1975;Chae et al 2017;Harwood et al 2020). The rate of change of the fluid density also induces added damping on the structure (Rajaomazava et al 2021). Moreover, both the fluid added mass and damping depend on the direction of motion, i.e.…”