fluids Article A Swing of Beauty: Pendulums, Fluids, Forces, and Computers Michael Mongelli 1 and Nicholas A. Battista 2,* 1 Department of Computer Science, 2000 Pennington Road, The College of New Jersey, Ewing Township, NJ 08628, USA;
[email protected] 2 Department of Mathematics and Statistics, 2000 Pennington Road, The College of New Jersey, Ewing Township, NJ 08628, USA * Correspondence:
[email protected]; Tel.: +1-609-771-2445 Received: 27 January 2020; Accepted: 5 April 2020; Published: 12 April 2020 Abstract: While pendulums have been around for millennia and have even managed to swing their way into undergraduate curricula, they still offer a breadth of complex dynamics to which some has still yet to have been untapped. To probe into the dynamics, we developed a computational fluid dynamics (CFD) model of a pendulum using the open-source fluid-structure interaction (FSI) software, IB2d. Beyond analyzing the angular displacements, speeds, and forces attained from the FSI model alone, we compared its dynamics to the canonical damped pendulum ordinary differential equation (ODE) model that is familiar to students. We only observed qualitative agreement after a few oscillation cycles, suggesting that there is enhanced fluid drag during our setup’s initial swing, not captured by the ODE’s linearly-proportional-velocity damping term, which arises from the Stokes Drag Law. Moreover, we were also able to investigate what otherwise could not have been explored using the ODE model, that is, the fluid’s response to a swinging pendulum—the system’s underlying fluid dynamics. Keywords: fluid dynamics education; damped pendulums; fluid drag; fluid-structure interaction; computational fluid dynamics 1.