Three dimensional flow around a circular cylinder confined in a plane channel

This paper presents two-and three-dimensional direct numerical simulations of the flow around a circular cylinder placed symmetrically in a plane channel. Results are presented in the Reynolds number range (based on the cylinder diameter and centerline velocity) of 10 to 390 for a blockage ratio (ratio of the cylinder diameter to the channel height) of 0.2. The aim of this work was to investigate in detail the confinement effect due to the channel's stationary walls on the force coefficients and the associated Strouhal numbers, as well as on the generated flow regimes. Present results suggest a transition from a 2-D to a 3-D shedding flow regime between Re = 180 and Re = 210. This transition was found to be dominated by mode A and mode B three dimensional instabilities, similar to those observed in the case of an unconfined circular cylinder. This is the first time that the existence of the two modes, and of naturally occurring vortex dislocations, has been confirmed via full 3-D simulations for the case of a confined circular cylinder in a channel. A discontinuity in the variation of the Strouhal number St, and of the base pressure coefficient C-pb, with Re was also observed. This was found to be associated with the onset of mode A instability and the development of vortex dislocations, and parallels what occurs in the unconfined case, but previous studies could not confirm its existence in the confined case. Furthermore, by analyzing the mechanisms affecting the shape and evolution of these instabilities, it is demonstrated that they are significantly affected by the confinement only in the far wake. (C) 2011 American Institute of Physics. [doi:10.1063/1.3599703]