Characterization of hydrofoil damping due to fluid–structure interaction using piezocomposite actuators

Abstract

Hydroelectric power generation is an important non-fossil fuel power source to help meet the world’s energy needs. Fluid–structure interaction (FSI), in the form of mass loading and damping, governs the dynamic response of water turbines, such as Francis turbines. Although the effects of fluid mass loading are well documented, fluid damping is also a critical quantity that may limit vibration amplitudes during service, and therefore help to avoid premature failure of the turbines. However, fluid damping has received less attention in the literature. This paper presents an experimental investigation of damping due to FSI. Three hydrofoils were designed and built to investigate damping due to FSI. Piezoelectric actuation using macrofiber composites (MFCs) provided excitation to the hydrofoil test structure, independent of the flow conditions, to overcome the noisy environment. Natural frequency and damping estimates were experimentally obtained from sine sweep frequency response functions measured with a laser vibrometer through a window in the test section. The results indicate that, although the natural frequencies were not substantially affected by the flow, the damping ratios were observed to increase in a linear manner with respect to flow velocity.

Effect of trailing edge shape on hydrodynamic damping for a hydrofoil

“…The identification of a blade hydrodynamic damping is a difficult task as previously reported by many authors (Seeley et al, 2012;Roth et al, 2009). The vibration due to flowing water makes traditional testing methods of dynamic system identification impractical.…”

“…In these testing situations, the hydrofoil motion under harmonic loading may be modeled by a Single Degree of Freedom linear system (Kaminer and Nastenko, 1976;Seeley et al, 2012). According to the classical analytical approach, the structure displacement, x, measured by the patch close to hydrofoil trailing edge is governed by the following differential equation:…”

“…A curve fit is used to determine the best values of f n , ς, and x 0 according to Eq. (5) to match the experimental data (Seeley et al, 2012).…”

“…He pointed out that hydrodynamic damping varies in a linear way with reduced flow velocity and may be influenced by the profile solidity. Recently, Seeley et al (2012) investigated experimentally the hydrodynamic damping of three hydrofoils. The results indicated that although the natural frequency was not altered by the flow, the damping ratio increased in a linear manner with respect to flow velocity.…”

“…Nevertheless, it was found that beyond a threshold value of 15 m/s flow velocity, such a method is no more appropriate since the flow induced vibration becomes significantly larger than the impulse response. An alternate technique for the mechanical excitation of a hydrofoil is the use of embedded piezoelectric actuators (Seeley et al, 2012;De La Torre, 2013;De La Torre et al, 2013), which provides an interesting and non-intrusive way to excite a specific eigen mode of immersed hydrofoils. In the present paper, we intend to evaluate experimentally the effect of Donaldson trailing edge shape on the hydrodynamic damping of a symmetric hydrofoil at high Reynolds number with the help of two embedded piezoelectric patches.…”