Pressure distribution, fluctuating forces and vortex shedding behavior of circular cylinder with rotatable splitter plates

Previous studies on the flow around a circular cylinder with fixed splitter plates have shown that the drag and lift can be reduced, and the primary vortex shedding can be suppressed obviously. In this study, a wind tunnel experiment on the flow around a circular cylinder with diameter D (40 mm) attached with ten splitter plates freely rotatable around the cylinder axis has been carried out with different ratios of length to cylinder diameter ( L/ D) from 0.5 to 6.0, in a range of Reynolds number from 3×10 4 to 6×10 4. The influences of the attachment of these rotatable splitter plates on the pressure distribution, fluctuating drag and lift forces and vortex shedding behavior were investigated. It is found that the splitter plates rotate to an off-axis equilibrium angle δ (on either side of the wake with equal probability) rather than align themselves with free stream due to the integrated effect of the pressure difference along the sides of the splitter plates. The plate length L/ D is crucial in determining the equilibrium angle δ. Longer splitter plate causes smaller angle; δ remains zero, i.e., parallel to the flow direction, for L/ D≥4. The mean pressures in the wake near the cylinder are higher than that of a bare cylinder. Further, the mean drag coefficients and the root-mean-square fluctuating lift coefficients, which are also largely determined by δ, are less than those of the corresponding bare cylinder, with a reduction up to about 30% and 90%, respectively. However, freely rotatable splitter plate develops a mean lift force towards the side the plate has deflected. In addition, the Strouhal number of fluctuating forces and correlation analysis are presented. The visualized flow structures show that the freely rotatable splitter plates elongate the vortex formation region, and the communication between the two shear layers on either side of the body is inhibited. For comparison, experiments of attaching fixed splitter plates with the same size were also conducted. The results indicate that the two kinds of splitter plates have their own advantages in force reduction and vortex shedding suppression.