| Flow around a circular cylinder or a group of circular cylinders is one of the classical subjects of fluid mechanics. Therefore, this subject has been studied extensively by numerous researchers. When a circular cylinder or a group of circular cylinders are placed near a wall, the wake structures and the vortex shedding frequencies for the cylinders can be modified remarkably due to the effects of the wall, and the degree of the effects depends on the gap height (G) that between the cylinder and the wall, and the characteristics of the incident boundary layer, etc. As the influence of the wall, the wake structures will be more complex, making the numerical and theoretical studies to be restricted, so the experimental study is essential to provide the basis of the study. In this paper, a circular cylinder, two side-by-side circular cylinders and two tandem circular cylinders in the proximity of a wall have been investigated experimentally by PIV. At the same time, the method to improve the stability and accuracy of iterative PIV image analysis method is investigated.Among the various PIV image processing methods the correlation-based interrogation algorithm plays a major role, and to enhance the precision, iterative image deformation methods are nowadays a widely adopted choice, but these methods are unstable and the result will diverge after several iteration. To overcome this problem, a new method has been proposed:First, an error compensation correction method that according to the error variation law in the evaluation of PIV recordings by using the correlation-based interrogation algorithm is presented, to obtain a predictor displacement field with higher precision, then start the iterative process, the moving least square (MLS) method is used to smooth the vector field during the iterative process. This method can achieve higher evaluation accuracy and reduce the number of iterations, so reduce the computing time. The assessment of Synthetic images and the experimtntal images showed that this method is effective, and the experimtntal images are got from the experiments of flow around a single near wall cylinder.By applying the PIV technique to the experiments of single near wall cylinder, two side-by-side near wall cylinders and two tandem near wall cylinders, this paper reached the following conclusions:1. For the single cylinder, the vortex shedding appears when G/D≥0.3 (D is the diameter of the cylinder), and a vortex pair appear in the wake, this vortex pair getting more symmetric as the increase of GID.2. For two side-by-side cylinders, for T/D≤1.151(T is the cylinder center-to-center spacing), the base-bleed flow pattern appears behind two cylinders when G/D≥0.302. For 1.302≤T/D≤1.906, the biased flow pattern appears when G/D arrives critical values for different T/D. For all the experiments, the two cylinders are entirely embedded in the boundary layer. The flow around the upper cylinder is found to be stronger than the flow around the lower cylinder, causing the gap flow between two cylinders always deflected toward the lower cylinder and neither the classical base-bleed flow pattern nor the classical biased flow pattern is observed.3. For two tandem cylinders, for all the values of T/D that appear in this work, the vortex shedding appears when G/D≥0.3, for the downstream cylinder, and the extended-body flow pattern appears when T/D≤1.302, the reattachment flow pattern appears when T/D≥1.453.The researeh in this paper has contributed to understand the flow characteristics for two cylinders close to a wall, and provides guidance to engineering design. It also provides valuable reference data for numerical simulation.
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