Fluid Dynamics Around Two Tandem Cylinders Of Different Diameters

The paper associated with two tandem cylinders presents the upstream cylinder size (diameter d) effect on global parameters of the downstream cylinder including time-mean drag coefficient (CD), fluctuating drag and lift coefficients (CD and CL), shedding frequencies and flow structures at the spacing ratio Ud=1.0～8.0, where L is the distance between the center of the upstream cylinder and the forward stagnation point of the downstream cylinder. d is varied as 8,16,24,32 and 40 mm, while the downstream cylinder diameter D is fixed at 40 mm, corresponding to diameter ratio d/D ranging from 0.2 to 1.0. The Reynolds number is kept constant at 4.27×104 based on D. CD, CD and CL are measured using a sectional load cell, while the shedding frequency is estimated from hotwire-measured fluctuating velocity in the wake. Flow structures are obtained using particle image velocimetry (PIV) and smoke visualization techniques. The critical L/d dividing the reattachment and co-shedding flows is larger at smaller d/D. CD, CD and CL at the critical spacing leap for d/D=1.0-0.4 and decline for d/D= 0.2 only. In the co-shedding regime, the downstream-cylinder shedding frequency locks-in with the shedding frequency of the upstream cylinder for d/D=1.0,0.8 and 0.6, in addition to a sub-harmonic lock-in for d/D=0.6 only. The lock-in however does not occur at d/D=0.4 and 0.2, the shedding frequency of the upstream and downstream cylinders being much higher and smaller, respectively, than that of a single cylinder. CD n general increases with d/D. CD and CL, on the other hand, generally wane and grow as d/D decreases from 1.0 to 0.6 and 0.4 to 0.2, respectively. While the former phenomenon is dominantly influenced by impaired vortices/shear-layer with the decreases in d/D, the later by increased flow velocity in the gap.