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Experimental And Numerical Investigation On Flow Past Four Square-arranged Cylinders

Posted on:2018-08-10Degree:MasterType:Thesis
Country:ChinaCandidate:C S YinFull Text:PDF
GTID:2310330518481191Subject:Port, Coastal and Offshore Engineering
Abstract/Summary:PDF Full Text Request
Four square-arranged cylinders could be found in many coastal and offshore engineering applications,such as wharf,offshore platforms and offshore wind turbine,risers,pipelines etc.When the multiple cylinders are in close proximity configuration,the cylinders could be easily damaged by the combined effects of wave and current under the harsh marine environment.Especially,when the forcing frequency is close to the natural frequency of structure,flow induced vibration arises.The resulting vibration has been reported to cause practical concern and increased risk of fatigue failure for the cylindrical structure.The purpose of this study is to investigate the interactions mechanism among the flow field and multiple cylinders.It is believed that this study will be of significant benefit to optimize the design and construction of the multiple-cylinder foundations.A series of numerical and experimental studies were carried out to investigate flow past four square-arranged cylinders at various spacing ratios,incidence angles and Reynolds numbers.The particle image velocimetry(PIV)technique was employed to capture the wake flow fields behind the four cylinders in the water channel.Instantaneous and time-averaged vorticity contours and streamline topologies were investigated with variation of spacing ratios,incidence angles and Reynolds numbers.The non-dimensional vortex shedding frequencies St of four cylinders were obtained from the power spectral of stream-wise velocity obtained from the instantaneous flow fields.The wake flow changes significantly with the increasing of incidence angle.The results indicate that the spacing ratio L/D has a significant effect on the instantaneous and time-averaged wake flow pattern.Three basic flow regimes are found,namely,"the shielding flow pattern" at small spacing ratios(L/D?2.0),"shear reattachment flow pattern" at intermediate spacing ratios(2.5?L/D?3.0)and "vortex shedding flow pattern" at large spacing ratios(L/D?3.5).Moreover,the value of St is found to be changed significantly when the switching phenomenon of flow patterns occurs.When the incidence angle is not equal to zero,the downstream cylinders obviously affect the wake flow of upstream cylinders in stream-wise and transverse derection.Based on the ANSYS Fluent software,numerical simulations were carried out to investigate the wake flow pattern,vortex shedding frequency and dynamic response of the structure at different spacing ratios and Reynolds numbers.Concerning the combined effects of spacing ratio and Reynolds number,the classification of different flow pattern regions was presented.It was found that the phase difference of shedding vortices is only related with the spacing ratio regardless of the Reynolds number.The mean drag and lift coefficients CD and Cl are varied with spacing ratios,and the values will be changed significantly when the wake flow pattern is switched.Meanwhile with increasing of Reynolds number,the wake flow patterns will be switched and the root mean square value of lift coefficient Cl could become larger.
Keywords/Search Tags:Four square-arranged cylinders, Particle image velocimetry, Numerical simulation, Wake flow pattern, Vortex shedding frequency, Force analysis
PDF Full Text Request
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