Triggering of galloping in structures at low Reynolds numbers

Abstract

The fundamental mechanisms that are important for the triggering of galloping in a flow-induced vibration (FIV) system consisting of the flow past an elastically-mounted body (D-section, isosceles-triangular, rectangular) is investigated in this paper using three key-enabling methodologies: namely, high-fidelity full-order model/computational fluid dynamics simulations, modal analysis based on the determination of an data-driven model using the eigensystem realization algorithm, and use of the Den Hartog stability (galloping) criterion for the assessment of the aerodynamically unstable behavior of the system obtained from classical quasi-steady theory. The synthesis of the results from the application of these three key-enabling technologies is used to study the effect of the Reynolds number Re, the mass ratio m∗, the cross-sectional geometry and the angle of attack of the incident wind direction relative to the orientation of the elastically-mounted body on the suppression or initiation of galloping. In the application of data-driven modal analysis to a FIV system with coupled modes, the importance of identifying correctly which of the coupled modes correspond to the structure mode (SM) possibly taking into account mode switching is stressed, and the failure to do so is shown to lead to a significant underestimation of the value of the reduced velocity Ur associated with the onset of galloping. The range of values of Ur where the SM is positive is related to the flutter lock-in regime for smaller values of Ur (typically for Ur≤7) and to the galloping regime for the larger values of Ur (typically for Ur≥10). The application of data-driven modal analysis to a FIV system shows that increasing Re and decreasing m∗ has two effects: namely, enhancement of the coupling between the structure and wake modes and increases of the range of Ur values where the SM exhibits a positive growth rate. Data-driven modal analysis and the Den Hartog stability criterion is applied to explain the recently observed phenomenon of the collapse of galloping for a rectangular cylinder when the side ratio is decreased below a critical threshold value (between about 0.1 and 0.2). It is shown that galloping appears and disappears when the flat face of a body in directed into (windward of) or away from (leeward of) the incident wind direction. An investigation of the effects of the cross-sectional geometry of the body on the triggering of soft- or hard-galloping is undertaken.