Virtual Spring–Damping System for Flow-Induced Motion Experiments

Hai Sun,Marinos P. Bernitsas,Michael M. Bernitsas,Eun Soo Kim

doi:10.1115/1.4031327

Abstract

Flow-induced motion (FIM) experiments of a single circular cylinder or multiple cylinders in an array involve several configuration and hydrodynamic parameters, such as diameter, mass, damping, stiffness, spacing, Reynolds number, and flow regime, and deviation from circular cross section. Due to the importance of the FIM both in suppression for structural robustness and in enhancement for hydrokinetic energy conversion, systematic experiments are being conducted since the early 1960s and several more decades of experimentation are required. Change of springs and dampers is time consuming and requires frequent recalibration. Emulating springs and dampers with a controller makes parameter change efficient and accurate. There are two approaches to this problem: The first involves the hydrodynamic force in the closed-loop and is easier to implement. The second called virtual damping and spring (Vck) does not involve the hydrodynamic force in the closed-loop but requires an elaborate system identification (SI) process. Vck was developed in the Marine Renewable Energy Laboratory (MRELab) of the University of Michigan for the first time in 2009 and resulted in extensive data generation. In this paper, the second generation of Vck is developed and validated by comparison of the FIM experiments between a Vck emulated oscillator and an oscillator with physical springs and dampers. The main findings are: (a) the Vck system developed keeps the hydrodynamic force out of the control-loop and, thus, does not bias the FIM, (b) The controller-induced lag is minimal and significantly reduced compared to the first generation of Vck built in the MRELab due to use of an Arduino embedded board to control a servomotor instead of Labview, (c) The SI process revealed a static, third-order, nonlinear viscous model but no need for dynamic terms with memory, and (d) The agreement between real and virtual springs and dampers is excellent in FIM including vortex-induced vibrations (VIVs) and galloping measurements over the entire range of spring constants and velocities tested (16,000 < Re < 140,000).

Highlights

Flow-induced motion (FIM) present a source of challenge for diverse structures in steady flows such as heat exchangers, bridges, buildings, offshore structures, or power-transmission cables
For a single rigid cylinder on elastic supports in a cross-flow, FIM is initiated as vortex-induced vibrations (VIVs) due to vortex shedding at low speeds, when the reduced velocity reaches the synchronization range starting with the initial branch
The hydrodynamic force is not affected, as it is not included in the control loop

Summary

Introduction

FIMs present a source of challenge for diverse structures in steady flows such as heat exchangers, bridges, buildings, offshore structures, or power-transmission cables. For a single rigid cylinder on elastic supports in a cross-flow, FIM is initiated as VIV due to vortex shedding at low speeds, when the reduced velocity reaches the synchronization range starting with the initial branch. This response amplitude increases with the velocity of the flow in the upper branch. All FIM experiments are conducted in the low turbulence free surface water (LTFSW) channel of the University of Michigan at 16,000 < Re < 140,000 The drawback of this approach is that it requires extensive SI to identify accurately the spring constant and system damping.

System Description

SI for Vck

Controlled FIM Experiments

Findings

Conclusions