Flow Characteristics around a Circular Cylinder Placed Horizontally above a Plane Boundary

Flow characteristics around a circular cylinder positioned near a plane boundary (on which laminar boundary layer flow develops in the absence of circular cylinder), are investigated for Reynolds numbers $\mathsf{R}$ ranging from $7.8\times {10}^2$ to $1.15\times {10}^4$ . Particle image velocimetry and fiber laser Doppler velocimetry were used to measure the velocity fields and velocity time histories, respectively. Flow structures are particularly revealed using flow visualization technique at $\mathsf{R}=7.8\times {10}^2$ for gap ratios $G∕D$ (where $G$ is the net gap between the surface of circular cylinder and the plane boundary), varying from 0 to 4. Based on the experimental results, the variation of Strouhal number of shedding vortex (or eddy) with $G∕D$ , the mechanism of vortex shedding suppression, and the streamwise velocity profiles of the upper shear layers and gap flows for small $G∕D$ are all discussed. Although the regular, alternate vortex shedding is suppressed for $G∕D<0.5$ , the periodicity could be detected due to the vortex (or eddy) shedding from the upper shear layer of the circular cylinder. Gap flow switching randomly is found and first put forward to be the main reason of multipeak or broadband spectral characteristics of the shedding event at a certain small gap ratio. It is also found that the streamwise velocity profiles of the upper shear layer, where periodic shedding eddies originate, exhibit well-behaved similarity. In addition, a unique similarity of mean streamwise velocity profiles of the gap flows is demonstrated for $G∕D⩽0.3$ . For $\mathsf{R}<4\times {10}^3$ , the $\mathsf{S}$ increases as $G∕D$ decreases to its maximum around $G∕D\cong 0.5$ and then decreases as $G∕D$ decreases. For $\mathsf{R}⩾4\times {10}^3$ , although most of the previous studies indicate that the $\mathsf{S}$ is insensitive to $G∕D$ , the present study shows that $\mathsf{S}$ still increases as $G∕D$ decreases but the variations of $\mathsf{S}$ are in a small range (i.e., $0.18⩽\mathsf{S}⩽0.22$ ).