Experimental study on the instantaneous fluid force acting on two circular cylinders closely arranged in tandem

A number of studies have examined the fluid flow around two circular cylinders arranged in tandem. An integrating phenomenon was found in the range of L/d=1.2–1.3, where L is the distance between the axes of the two cylinders. Namely, the Strouhal number decreases from 0.25 to 0.10 as the Reynolds number increases. In the present study, the behavior of the unique flow and the oscillating fluid force acting on the cylinders were investigated. The diameter, d, of the cylinders was 100 mm, L/d was 1.2, and the free stream velocity, U, ranged from 6 to 20 m/s. The Reynolds number ranged from 3.8×104 to 1.3×105. llowing results were obtained. The separated shear layer from the upper and lower sides of the first cylinder alternately reattach to the upper and lower sides of the second cylinder. The reattachment flow flows into the gap but not through the gap. The gap flow is pushed back by the flow that reattaches to the lower side of the second cylinder. The phase difference between the oscillating drag and lift coefficients of the first and second cylinders are t/T=5/16 and 3/16, respectively.