Добірка наукової літератури з теми "Interference VIV-galloping"

Flow-induced vibration (FIV) of twin square cylinders in a tandem arrangement was numerically investigated at Reynolds numbers 200 and gap L/D = 2.0, 4.0 and 6.0, ( D is the side length of the cylinders). Fluid-structure-coupled Koopman mode analysis method was developed to synchronously identify the coherence flow and structural modes. Then, the energy transfer between cylinders and Koopman modes was analyzed to uncover the underlying mechanism of FIV. The results showed that at L/D = 2.0 and 4.0, only soft lock-in vortex-induced vibration (VIV) was observed. The oscillating amplitude for L/D = 4.0 was much higher than that of L/D = 2.0, due to the interference effects induced by fully developed gap vortices. As L/D = 6.0, VIV and galloping coexisted. For the coherence mode, the primary flow mode induced by vortex shedding dominated the flow field at L/D = 2.0 and 4.0. The direct mode energy dominated the energy transfer process. The upstream cylinder (UC) contributed to the negative work done and thus tended to stabilize the vibration; in contrast, the downstream cylinder (DC) exhibited opposite behavior. In the galloping branch at L/D = 6.0, both the flow field and structural response contained three main modes: one vortex-shedding-induced mode and two vibration-induced modes. For the direct mode energy, owing to the interference effects, DC contributed to more positive work done than UC by the vibration-induced modes. The vortex-induced mode was governed by DC and afforded negative work done. Moreover, all the coupled mode energy was almost equal to zero.

The VIVACE converter was introduced at OMAE2006 as a single, smooth, circular-cylinder module. The hydrodynamics of VIVACE is being improved continuously to achieve higher density in harnessed hydrokinetic power. Intercylinder spacing and passive turbulence control (PTC) through selectively located roughness are effective tools in enhancement of flow induced motions (FIMs) under high damping for power harnessing. Single cylinders harness energy at high density even in 1 knot currents. For downstream cylinders, questions were raised on energy availability and sustainability of high-amplitude FIM. Through PTC and intercylinder spacing, strongly synergetic FIMs of 2/3/4 cylinders are achieved. Two-cylinder smooth/PTC, and three/four-cylinder PTC systems are tested experimentally. Using the “PTC-to-FIM” map developed in previous work at the Marine Renewable Energy Laboratory (MRELab), PTC is applied and cylinder response is measured for inflow center-to-center distance 2D-5D (D = diameter), transverse center-to-center distance 0.5–1.5 D, Re ε [28,000–120,000], m* ε [1.677–1.690], U ε [0.36–1.45 m/s], aspect ratio l/D = 10.29, and m*ζ ε [0.0283–0.0346]. All experiments are conducted in the low turbulence free surface water (LTFSW) channel of MRELab. Amplitude spectra and broad field-of-view (FOV) visualization help reveal complex flow structures and cylinder interference undergoing VIV, interference/ proximity/wake/soft/hard galloping. FIM amplitudes of 2.2–2.8D are achieved for all cylinders in steady flow for all parameter ranges tested.

The thesis deals with the interaction between vortex-induced vibrations (VIV) and galloping for rectangular cylinders of low side ratio, which is defined as the body width on the body depth facing the fluid flow (SR=B/D). In particular, the interaction mechanism has been characterized for a wide range of Reynolds numbers (Re) and mass ratios (m*), aiming to provide a complete description of the response in several flow situations (smooth and turbulent, air and water) for bodies which exhibited, or were known to have, a pronounced proneness to this type of instability. This type of flow-induced vibrations phenomenon occurs for particular combinations of both aerodynamic and dynamic characteristics of a system. The study consists in two main parts, both of them aimed at proposing a complete framework for scientific and designing purposes. The first one investigates the phenomenon occurring in sectional models purposely designed and experimentally tested, whereas the second one is devoted to the implementation of a predictive model for the interaction, implying also further experimental measures to assess the model key-parameter. Several sectional models of a SR=1.5 have been tested given that this section demonstrated to be particularly prone to the interaction between VIV and galloping. Nevertheless, the majority of former literature investigations were performed on the square section. The response features of such a phenomenon are still not fully understood. In order to have a deeper insight and to give a complete description of the interaction, the present investigation was conducted focusing particularly on the SR=1.5 rectangular section: this is a soft oscillator respect to the incipient instability, while the same rotated section with an angle of attack of 90°, that is SR=0.67, is generally referred to as a hard-type one. Results in air flow showed peculiar amplitude response curves differently shaped depending on Re, m* and corners sharpness accuracy. Results in water flow showed the response in amplitude and frequency to be strongly influenced by the abrupt change of m*, recalling the different responses in air and water flow regime reported in literature for a circular cylinder, though related to VIV only. SR=0.67 shows a completely different response, although remaining, differently from air flow measurements, a soft oscillator. Further tests on m* variation constituted an integration for the data so far available in literature about these sections.

Wind tunnel experiments are carried out to investigate interference effect of wake body on cross flow vibration of a square cylinder in uniform flow. The side length d of the square cylinder is 26∼40 mm and the length le = 315 mm. As the wake body, a strip-plate of width w = d is set downstream the square cylinder with a gap s in cruciform arrangement. The length of the plate ld is varied from infinity, i.e. full measuring section height, to ld/d = 1. Both the Karman vortex excitation (K-VIV) and the galloping are suppressed by the ld/d = ∞ plate in the non-dimensional gap range of 1.6<s/d ≦ 4, although the mechanisms are completely different between the two oscillations. The longitudinal vortex excitation (L-VIV) found in the previous work is confirmed to be induced by the plate at around s/d = 1.4 for the systems with various dimensions and structure parameters. The K-VIV suppression effect is virtually the same for the wake plates with ld/d≧10, and becomes less definite for shorter plates when ld/d ≦ 6. The galloping suppression effect persists up to the shortest wake plate of ld/d = 1 at s/d<2. The L-VIV is observed for plates of ld/d≧6, with weaker degree for shorter plates. The K-VIV seems to be enhanced by ld/d = 2, 4, 6 plates at 1≦s/d≦2 (EK-VIV). By setting the wake plate with ld/d = 1 or 2 at s/d around 0.3, a new type of fluid-elastic vibration is induced as an interference effect of wake body (WBI-FEV). A method is presented to predict fluid-elastic vibrations to which the Van der Pol equation does not apply. The prediction by this method agrees well with measured WBI-FEV for the ld/d = 2 plate.

This paper presents experimental results concerning flow-induced oscillations of rigid-circular cylinders in tandem. Preliminary results are presented: new measurements on the dynamic response oscillations of an isolated cylinder and flow interference of two cylinders in tandem are shown. The oscillations are due to vortex-induced vibrations (VIV). Models are mounted on an elastic base fitted with flexor blades and instrumented with strain gages. The base is fixed on the test section of a water channel facility. The flexor blades possess a low damping characteristic [ζ ≈ 0.008 and less] and they are free to oscillate only in the cross-flow direction. The Reynolds number of the experiments is from 3,000 to 13,000 and reduced velocities, based on natural frequency in still water, range up to 12. The interference phenomenon on flow-induced vibrations can be investigated by conducting experiments in two ways: first, the upstream cylinder is maintained fixed and the downstream one is mounted on the elastic base; subsequently, an investigation will be carried out letting both cylinders oscillate transversally. The results for an isolated cylinder are in accordance with other measurements in the literature for m* ≈ 2 and m* ≈ 8. For the tandem arrangement (m* ≈ 2), the trailing cylinder oscillation presents what previous researchers have termed interference galloping behaviour for a centre-to-centre gap spacing ranging from 3·0D to 5·6D. These initial results validate the experimental set up and lead the way for future work; including tandem, staggered and side-by-side arrangements with the two cylinders free to move.

The VIVACE (Vortex Induced Vibration for Aquatic Clean Energy) Converter was introduced at OMAE2006 as a single, smooth, circular-cylinder module. The hydrodynamics of VIVACE is being improved continuously to achieve higher density in harnessed hydrokinetic power. Inter-cylinder spacing and Passive Turbulence Control (PTC) through selectively located roughness are effective tools in enhancement of Flow Induced Motions (FIMs) under high damping for power harnessing. VIVACE Converters consist of multi-cylinder modules. Single cylinders harness energy at high density even in 1knot currents. For downstream cylinders questions are raised on energy availability and sustainability of high-amplitude FIM. Through PTC and inter-cylinder spacing, strongly synergetic FIM of 2/3/4 cylinders is achieved, harnessing hydrokinetic energy with increased footprint density. Two-cylinder smooth/PTC and four-cylinder PTC systems are tested experimentally. Using the “PTC-to-FIM” map developed in previous work at the Marine Renewable Energy Laboratory (MRELab), PTC is applied and cylinder response is measured for the following parameter ranges: In-flow center-to-center distance 1.63•D–5.00•D (D = diameter), transverse center-to-center distance 0.5•D–1.5•,D, Re ∈[28,000–120,000], m* ∈[1.677–1.690], U ∈[0.36m/s–1.45m/s], aspect ratio l/D = 10.29, and m*ζ ∈[0.0283–0.0346]. All experiments are conducted in the Low Turbulence Free Surface Water (LTFSW) Channel of MRELab. Amplitude spectra and broad filed-of-view (FOV) visualization help reveal complex flow structures and cylinder interference undergoing VIV, interference/proximity/wake/soft/hard galloping.