Experimental and numerical studies of fluid-structure interaction in flow-induced vibration problems

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Experimental and numerical studies of fluid-structure interaction in flow-induced vibration problems


Author: Lau, Yin-lung
Title: Experimental and numerical studies of fluid-structure interaction in flow-induced vibration problems
Degree: Ph.D.
Year: 2003
Subject: Hong Kong Polytechnic University -- Dissertations
Fluid dynamics -- Mathematical models
Department: Dept. of Mechanical Engineering
Pages: xxii, 266 leaves : ill. (some col.) ; 30 cm.
InnoPac Record: http://library.polyu.edu.hk/record=b1733024
URI: http://theses.lib.polyu.edu.hk/handle/200/5481
Abstract: This thesis aims to validate a boundary element numerical method (BEM) and use it to investigate fluid-structure interaction problems occurring in turbomachines, especially problems arising from rotor-stator pairs. Before applying this BEM technique to treat stator blade vibration in a rotor-stator pair, a comprehensive study was carried out to validate it. The procedure involved careful experimental work to measure the structural response, the use of the Particle Image Velocimetry (PIV) technique to measure the wake flow and the use of a finite-element numerical method to calculate the whole flow field in order to determine the vortex shedding pattern and to provide calculations that take into account viscous and turbulence effects. The measured and calculated wake pattern was used as input to the BEM for prediction of the structural response. Thus configured, the inviscid flow assumption made in the BEM can also be verified. Originally, the BEM was developed to analyze different kinds of two-dimensional flow-induced vibration problems due to vortex/blade interactions and has not been formally validated. In this thesis, the BEM is formally validated using recently measured vortex-induced unsteady aerodynamic response and blade vibration data. Three problems were examined and they included the case of single-blade single-vortex interaction, single-blade single vortex-street interaction and single-blade dual vortex-street interaction. In the single-blade single-vortex interaction case, the blade was modelled as rigid; therefore, the response of the structure was purely aerodynamic. The trend of the calculated variation of blade lift coefficient with the horizontal miss distance of the convected vortex compares well with known experimental data. An experiment of a coupled elastic cylinder/blade system was carried out to model the Karman-vortex-street/blade interaction. In the experiment, a circular cylinder placed upstream in tandem with an elastic blade was used to generate the Karman-vortex-street. A NACA0012 airfoil with a constant cross-section along its span was used to model the blade, which was exposed to an oncoming Karman-vortex-street. In order to determine the vortex spacing correctly, three different methods were employed. The first method involved the use of PIV to measure the vorticity distribution in the wake flow, the second method was to use the Karman vortex street theory to calculate the wake properties, and the third method was to interpret the vortex spacing from existing flow visualization data at about the same Reynolds number. All three techniques yielded comparable results in terms of the vortex spacing thus determined. The calculated blade response due to the Karman-vortex-street induced vibration was compared with the measured vibration characteristics of the airfoil placed behind the cylinder at different separation distances. Good agreement between calculations and measured vibration amplitudes of the blade at its mid-span was obtained. In addition, the BEM was also validated against data in the literature for the case of dual Karman vortex streets in the oncoming stream toward an airfoil. In this case too, agreement with data was good. Furthermore, the BEM calculations were also compared with a full finite-element calculation of the elastic cylinder/blade system and good agreement was obtained. These good agreements indicate that the BEM is a viable method for the analysis of the aerodynamic and structural response in vortex/blade interaction problems with complicated rotor-wake/stator interaction included.
After verification, the BEM was applied to analyze the coupling effect of the rotor-wake/stator interaction problem in turbomachines. The turbomachine problem was idealized by treating the stator as a finite cascade and the rotor wake was modelled by Karman vortex streets. The problem of vortex-induced vibration in a rotor-stator pair thus formulated was found to depend on the normalized frequency c/d of the oncoming vortex streets and the stator blade spacing s/c, where c is the chord length of the blade, s is the cascade spacing and d is the vortex spacing. Calculations were carried out to investigate the effect of these ratios on the performance of the stator blades and their life span as a result of the induced vibration. Since the ratios c/d and s/c are related to the operational conditions and geometry of the rotor-stator pair, the data of the aerodynamic and structural dynamic responses of the stator blade could be used to construct design curves related to the aeroelastic stability and fatigue life of stator blades. The calculated data was used to construct these design curves in the c/d versus s/c space. They indicate that optimum s/c and c/d values for the design of a rotor-stator pair could be determined from these curves. Furthermore, if the s/c and c/d values were properly selected, substantial reduction of the induced forces on the stator blade could be achieved with a concomitant increase in the fatigue life of the stator blade.

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