Experimental analysis of the flow around a cylinder with a square cross-section

The present work investigates the flow around a two-dimensional square section object at varying angles of incidence. The experimental approach employs Particle Image Velocimetry for the determination of the time-averaged velocity field and the unsteady large scale coherent variations (e.g. vortex shedding) as well as turbulent fluctuations. The angle of incidence is varied from 0° to 45° covering the entire range of possible wind directions. The investigation constitutes a further step within the ongoing research for the assessment of the quasi-steady theory for the low-frequency oscillation (galloping) phenomenon of bluff bodies. A specific aspect of the set-up is the use of a transparent hollow model, which allows light transmission and minimizes internal total reflection. Above 10° incidence the flow may reattach on the lower surface and produce a separation bubble. Time-mean averaged velocity fields reveal that the near-wake length increases monotonically, peaking at 10°. Beyond, the distance from the vorticity centres to the nearest corner of the model drops significantly, caused by the shift of the separation point location. The vortex shedding is influenced hereby to a large extent. The flow organisation is further analysed with Proper Orthogonal Decomposition (POD) to investigate the unsteady flow structures associated with vortex shedding. The POD modes are used to reconstruct a low-order flow model that provides a phase reconstruction of the vortex shedding sequence for all angles of incidence. In addition to the velocity field characterisation, a method to determine the mean (timeaveraged) lift and drag coefficient of the model from PIV velocity data is proposed. The Reynolds-averaged momentum equations are employed to calculate the pressure gradient field, yielding subsequently the pressure field by means of a two-dimensional integration technique. The estimated pressure field is introduced in an integral momentum balance, which is solved on a contour around the model. The method has been successfully applied to the experimental data of the flow around a square cylinder. The force data show satisfactory agreement with reference data published in literature.AerodynamicsAerospace Engineerin