Experimental Study on Two Flexible Hanging-off Circular Cylinders undergoing Wake Interference

This paper presents experimental results of three different configurations of two flexible circular cylinders were towed in still water. Each cylinder has length-to-diameter ratio of 34.4 with low mass ratio of 1.24. The cylinders were submerged partially into the still water from towing carriage as hanging vertical cantilever and towed by various speeds that give Reynolds number of 1.08 times 10⁴ up to 3.78 times 10⁴. For side-by-side arrangement, two distinct center-to-center distances between the two cylinders were considered, while for the tandem arrangement the relative position of the cylinders is only 4D. For staggered arrangement, a distance between upstream and downstream cylinders (L_UD) was fixed by 4D, and the position of the downstream cylinder in transverse direction (L_T) was varied by 1D, 1.5D, 2D, and 2.5D, respectively. Measurement of the hydrodynamics forces in two, in-line (drag force) and transverse (lift force) directions are conducted simultaneously with two- component load cells. For all test cases, the characteristics of the time-dependent forces are strongly dictated by the reduced velocity (Ur) parameter. Frequency of the drag force was found approximately twice the corresponding lift force frequency at the related Ur. For all cases, the C_D and C_L values lie in the range of approximately 0.03 up to 0.25. The tandem case became a very special case due to an extreme drop of the CD for the downstream cylinder compared with all other cases. In addition, for staggered configuration, the wake interference only has strong influence on the forces fluctuation until the L_T = 1D, especially for the downstream cylinder.