Observed wake branches from the 2-DOF forced motion of a circular cylinder in a free stream

The combined in-line (IL) and cross flow (CF) vibration of flexibly mounted cylinders in a free stream due to vortex-induced vibration (VIV) demonstrates significantly more complex dynamics than purely CF vibrations. In this study, the forces on a circular cylinder undergoing combined IL and CF motion in a free stream are investigated through an expansive series of forced motion experiments with force measurement and quantitative flow visualization. The motion of the cylinder is parameterized using sinusoidal motions with variation of the non-dimensional IL amplitude, non-dimensional CF amplitude, CF reduced velocity, and phase between IL and CF motion, while the Reynolds number is held constant at 7620. The resulting database of experimental measurements consists of 9555 variations of the governing motion parameters. The decomposed force coefficients are reported and mapped over the range of parameters. We observe a number of interesting wake-dependent parameter space features, such as regions of multiple branch responses, where the wake may exhibit multiple mode behaviors for the same cylinder motion parameters. Multiple branch regions are observed throughout the experimental data space.