|Title:||Experimental investigations of fluid-structure interaction of bluff bodies subjected to a cross-flow|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Department:||Department of Mechanical Engineering|
|Pages:||xiii, 203 leaves : ill. ; 30 cm|
|Abstract:||This thesis presents an experimental study of fluid-structure interactions of bluff bodies subjected to a uniform cross-flow. The investigation concentrates mainly on the fluid-structure interactions of two side-by-side circular cylinders in a cross-flow. However, the flow separation effects on the structural dynamic response are also investigated. Four topics are covered. First, the flow behind two side-by-side circular cylinders in a cross-flow was studied using laser-illuminated flow-visualization, laser Doppler anemometer and hot-wire techniques. Three typical T/d values, i.e. 3.00, 1.70 and 1.13, were investigated, where T is the centre-to-centre cylinder spacing and d is the cylinder diameter. The vortex formation, interaction and downstream evolution are examined in detail in the three different flow regimes. With the WAG (window average gradient) detection method, the relative probability of the two typical flow pattern, symmetrical and anti-symmetrical vortex arrangement, at large cylinder spacing, Tid = 3.00, is estimated. Specific attention was given to the asymmetrical flow regime, T/d = 1.70, including the dominant frequencies, the stability of the deflected gap flow and its random change over from one side to another. Based on the observation, a mechanism is proposed for the stability of the deflected gap flow. An explanation is put forward for the existence of two different dominant frequencies associated with the narrow and the wide wake, respectively. The role gap bleeding plays in the vortex formation and downstream development for very small spacing between the cylinders, T/d = 1.13, was also investigated in detail. Second, free vibrations and the associated non-linear fluid-structure interactions of two side-by-side elastic cylinders with fixed support at both end splaced in a cross-flow were experimentally investigated using FBG sensors, hot wire technique and flow visualization. Three T/d ratios, identical with those mentioned above, were investigated. The structural vibration behaviour at each typical flow regime, i.e. T/d = 3.00, 1.70 and 1.13, was examined in detail. The characteristics of the system modal damping ratios, including both structural and fluid damping, and natural frequencies were also addressed using an auto-regressive moving average(ARMA) technique. For example, the dependence on T/d and Ur of the system natural frequency and the effective and fluid damping ratios were investigated thoroughly, in particular, at and near resonance, thus throw a new light on the fluid-structure interaction behaviour. Third, the nonlinear interplay between the simultaneous vibrations of two side-by-side elastic cylinders in a cross-flow, at three identical transverse spacing ratios as mentioned above, were experimentally investigated using FBG sensor, laser vibrometer and hot wire technique. The emphasis was on the structural dynamics and the vibration characteristics. The strain-displacement relationship associated with the two interfering cylinders at each T/d values were established and was compared with that of an isolated cylinder. Interference effects on behaviours of the system natural frequencies, the vibration amplitude and the correlation coefficient between the two interfering cylinders at each T/d value were also investigated in detail. It was found that, in general, the correlation coefficient p12 between fluctuating strains measured from the two cylinder increases as T/d decreases, suggesting an increasing interference. Finally, the effect of the nature (fixed or oscillating point) of flow separation on fluid-structure interactions was experimentally investigated. Flow field and structural vibrations of both square and circular cylinders, associated with fixed and moving flow separation points, respectively, were investigated using hot wire and FBG sensors. The vibration characteristics, resonance behaviour, system modal damping ratios and natural frequencies associated both square and circular cylinder cases were investigated in an effort to understand the effect of flow separation on fluid-structure interactions. Furthermore, the incidence angle effects on the fluid-structure interaction behaviour associated with the square cylinder were also studied in detail. Nine publications, including 5 refereed journal papers and 4 refereed conference proceedings, have been produced out of this work.|
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