Interference of vortex-induced vibration and transverse galloping for a rectangular cylinder Claudio Mannini n , Antonino Maria Marra, Tommaso Massai, Gianni Bartoli CRIACIV/Department of Civil and Environmental Engineering, University of Florence, Via S. Marta 3, 50139 Florence, Italy article info Article history: Received 29 February 2016 Received in revised form 21 June 2016 Accepted 3 August 2016 Keywords: Galloping Vortex-induced vibration Interference Rectangular cylinder Wind tunnel tests abstract The phenomenon of interference between vortex-induced vibration (VIV) and galloping in the transverse degree of freedom was studied in the wind tunnel in the case of a spring- mounted slender rectangular cylinder with a side ratio of 1.5 having the short side per- pendicular to the flow. The tests were carried out in a wide Scruton number range, starting from low values and increasing it in small steps by using eddy-current viscous dampers. This study helped understanding the dynamics of the interaction between the two excitation mechanisms and clearly highlighted the transition through four regimes of VIV-galloping interference. It was found that a high value of the mass-damping parameter is required to decouple the ranges of excitation of vortex-induced vibration and galloping completely, and for the quasi-steady theory to predict the galloping critical wind speed correctly. This conclusion is also relevant from the engineering point of view, as it means that structures and structural elements with ordinary mass-damping properties can ex- hibit sustained vibrations in flow speed ranges where no excitation is predicted by clas- sical theories of vortex-induced vibration and galloping. Although most of the experi- mental tests were conducted in smooth flow at zero angle of attack, the paper also dis- cusses the sensitivity of the results to a small variation of the mean flow incidence and to the presence of a low-intensity free-stream turbulence. & 2016 Elsevier Ltd. All rights reserved. 1. Introduction Slender prismatic and nearly-prismatic bodies characterized by a bluff cross section with sufficient afterbody are prone to both vortex-induced vibration (VIV) and galloping. Galloping instability in the transverse degree of freedom usually occurs at high reduced flow speed and is therefore approached with the quasi-steady theory. By contrast, vortex-induced vibration is a low-reduced-flow-speed phenomenon. Nevertheless, if the oscillating structure is light and low damped, i.e. the dissipation capability of the system is low, three important consequences can ensue: (i) vortex-induced vibrations may occur with large amplitudes in a wide range of flow speeds; (ii) the onset of VIV and galloping may be expected at close flow speeds and the two phenomena may interfere with each other; (iii) the quasi-steady theory could fail to predict the gal- loping instability threshold and the post-critical oscillatory behavior, due to the insufficiently high reduced flow speed. In the case of rectangular cylinders in smooth flow with side ratios ≲ ≲ BD 0.75 / 3 (Parkinson, 1965), B and D being respectively the streamwise and cross-flow section dimensions, a peculiar instability spawned by the interference of VIV Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jfs Journal of Fluids and Structures http://dx.doi.org/10.1016/j.jfluidstructs.2016.08.002 0889-9746/& 2016 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: claudio.mannini@unifi.it (C. Mannini). Journal of Fluids and Structures 66 (2016) 403–423