Fluidelastic instability produces large amplitude self-excited vibrations close to the natural frequency of the structure. It is now recognised as the excitation mechanism with the greatest potential for causing damage in tube arrays. It can be split into two mechanisms: fluid stiffness controlled and fluid damping controlled instability. The former is reasonably well understood, although a better understanding for fluid damping controlled instability is required. There is a time delay between tube motion and the resulting fluid forces at the root of fluid damping controlled instability. The exact nature of the time delay is still unclear. The current study directly measures the time delay between tube motion and the resulting fluid forces in a normal triangular tube array with a pitch ratio of 1.32 with air cross-flow. The instrumented cylinder has 36 pressure taps with a diameter of 1 mm, located at the mid-span of the cylinder. The instrumented cylinder was forced to oscillate in the lift direction at four excitation frequencies for a range of flow velocities. Unsteady pressure measurements at a sample frequency of 2kHz were simultaneously acquired along with the tube motion which was monitored using an accelerometer. The instantaneous fluid forces were obtained by integrating the surface pressure data. A time delay between tube motion and resulting fluid forces was obtained. The time delay measured was of the order of magnitude assumed in the semi-empirical models of by Price & Paidoussis (1984, 1986), Weaver and Lever et al. (1982, 1986, 1989, 1993), Granger & Paidoussis (1996), Meskell (2009), i.e. t = μd/U, with μ = O(1). Although, further work is required to provide a parameterized model of the time delay which can be embedded in these models, the data already provides some insight into the physical mechanism responsible.

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