Flow around two side-by-side closely spaced circular cylinders

The flow structure around two side-by-side circular cylinders is investigated at T/d=0.1–0.2T/d=0.1–0.2 (T is the gap spacing between the cylinders, and d   is the cylinder diameter), where the gap flow between the cylinders has a significant influence on the time-averaged lift on the cylinders. Two distinct flow structures are identified at T/d=0.1T/d=0.1 and 0.2, based on time-averaged pressure measurement and surface oil-flow patterns. The gap flow forms a separation bubble in the base region of one cylinder at T/d=0.1T/d=0.1 but separates from the cylinders without further reattachment at T/d=0.2T/d=0.2. At T/d=0.13T/d=0.13, this bubble occurs on and off spontaneously and abruptly, causing the corresponding time-averaged lift force on the cylinder to jump among different discrete values.