Abstract
The paper presents a computational study of three-dimensional flow past a cylinder forced to oscillate in a uniform stream, following a figure-eight trajectory. Flow simulations were performed for Re = 400, for different cases, defined in terms of the oscillation mode (‘counter-clockwise’ or ‘clockwise’), for values of the ratio, F, of the transverse oscillation frequency to the Strouhal frequency close to 1.0. The results demonstrate that, for F ≤ 1.0, counter-clockwise cylinder motion is associated with positive power transfer from the flow to the cylinder, corresponding to excitation; for the clockwise motion, power transfer is negative at intermediate to high amplitudes, corresponding to damping. For the clockwise mode, in the range F = 0.9–1.1, a transition to two-dimensional vortex street is identified for transverse oscillation amplitude exceeding a critical value. This results from the induced suction of vortices, which moves vortex formation and shedding closer to the cylinder surface, thus resulting in a narrower wake, characterized by an effective lower Reynolds number. Both oscillation modes are characterized by higher harmonics in the lift force spectrum, with the third harmonic being very pronounced, while even harmonics are present for the case of clockwise mode, resulting from a wake transition to a “S + P” mode.
Highlights
Flows past oscillating bluff studies have been the subject of extensive research, mainly motivated by the practical problem of flow-induced vibrations that cylindrical structures undergo
The results presented for F = 1.0 verify the presence of both odd and even harmonics (Figure 9), to the results obtained for F = 0.9
For all frequencies considered for the case of the clockwise mode, even harmonics are present in the lift spectrum at sufficiently high values of transverse oscillation amplitude
Summary
Flows past oscillating bluff studies have been the subject of extensive research, mainly motivated by the practical problem of flow-induced vibrations that cylindrical structures undergo. The present work follows initial computational studies of the authors, performed for a Reynolds number Re = 400, both for two-dimensional [21] and three-dimensional flow [22], the latter for transverse-only or counter-clockwise oscillation at the Strouhal frequency These studies have shown that, in comparison to transverse-only oscillation, the presence of in-line oscillation in general increases the magnitude of forces acting on the cylinder, and can increase the power transfer from the flow to the structure. The objectives of the present study are to present a complete set of computational results in a range of oscillation frequency and amplitude relevant for VIV, including important global parameters as power transfer and force coefficients, and to relate them to wake structure and flow transitions.
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