EXPERIMENTAL STUDY ON EVOLUTION OF WAKE STRUCTURES IN FLOW PAST THE CIRCULAR CYLINDER PLACED NEAR THE WALL

The experimental measurement of the flow field around the circular cylinder near the wall is carried out by using the Particle Image Velocimetry. The characteristics of the flow regime under different Reynolds numbers (Re = 1500 \sim 5540) together with three different gap ratios (G \mathord\left/ \vphantom G D \right. D \;= 0.5, G \mathord\left/ \vphantom G D \right. D\; = 1.0, G \mathord\left/ \vphantom G D \right. D \;= 1.5) are studied. The experiment results shows that for the case of G \mathord\left/ \vphantom G D \right. D \;= 0.5, with the increasing of Reynolds number, the recirculation zone behind the cylinder is gradually symmetrical about the centerline of the cylinder while its size is decreasing, and the size of the separation bubble on the wall also decreases gradually. The experiment reveals that the cylindrical wake and the gap flow perform differently while the Reynolds numbers Re _t between Re = 3000 \sim 3200. When the Reynolds number is smaller than Re _t, a small separation bubble will form on the front wall of the cylinder, which hinders the flow of upstream fluid through the gap and reduces the intensity of the gap flow, and then deviates from the wall. At Re \;= 1500, the vortex shedding frequency increases with the decrease of the gap ratio. And with the decrease of gap ratio, the vortex shedding frequency increases first and then decreases in a small range (0.185 \leqslant St \leqslant 0.227) for Re \geqslant 3000. The Reynolds number has a significant influence on the flow characteristics, especially for the case of small gap ratios. At G \mathord\left/ \vphantom G D \right. D \;= 0.5, the secondary vortex deviates from the wall and moves upward to the position close to the upper wake vortex, and the vortex merging process appears between the upper wake vortex and the secondary vortex for the Re \;= 1500. As the Reynolds number increases to Re \;= 5540, the secondary vortex does not merge with the upper wake vortex, and the secondary vortex directly interacts with the lower wake vortex. At G \mathord\left/ \vphantom G D \right. D \;= 1.0 and G \mathord\left/ \vphantom G D \right. D \;= 1.5, the energy carried by the secondary vortex is decreasing gradually with the increasing of Reynolds number.