Evaluating 2D and 3D VIV Wake Modes and Forces through Forced Motion Simulations

Simulation based approaches to studying and modeling vortex induced vibrations (VIV) can offer additional understanding to fluid structure interactions that may be difficult to study with experimental approaches, however simplified numerical approaches may lack the proper physics to capture all real effects. 2D simulations of a cylinder in a free stream forced to oscillate perpendicular to the free stream cannot capture spanwise effects that occur on finite span cylinders, resulting in less accurate solutions when compared to 3D, however they are considerably faster computationally. Using the verified boundary data immersion method (BDIM) fluid model ``WaterLily", 2D and 3D forced cylinder motion simulations at a Reynolds number of 4000 are performed and compared with measured experiments with equivalent parameters. WaterLily is written in ``Julia" and solves the unsteady incompressible Navier-Stokes equations on a Cartesian grid. Several combinations of reduced velocity (Vr) and non-dimensional transverse amplitude (Ay/D) from expected wake regimes (e.g., 2 single vortices shed per cycle, 2 pairs of vortices shed per cycle) are selected and both the wake formation and magnitude and phasing of the lift forces are compared. Results show that 3D simulations agree well with measured data whereas 2D simulations show better accuracy in low amplitude regimes and are in many cases unable to produce widely observed wake mode regimes at higher reduced velocity. This comparison provides insight as to where 2D simulations are appropriate in modeling VIV and where 3D effects play an important role in properly modeling the physics. This study provides a first step towards developing 2D forced motion models that can be corrected for 3D effects to yield a model with 3D accuracy and 2D computational speed.