Galloping of an elliptical cylinder at the critical Reynolds number and its quasi-steady prediction

Abstract

Dry galloping of inclined cables has been shown to have a strong relation to the critical Reynolds number. This study concerns the occurrence of galloping of an elliptical cylinder at critical Reynolds numbers under normal wind and an assessment of the quasi-steady assumption on predicting these vibrations. A series of static and dynamic wind tunnel tests are carried out to measure the wind pressure on a static cylinder and displacement of a three-degree-freedom vibrating cylinder. The static aerodynamic force on the cylinder shows the occurrence of reattachment which lowers the drag coefficient and increases the lift coefficient at critical Reynolds numbers. This phenomenon gives the possibility of satisfying the across-wind galloping criterion by adversely changing the aerodynamic force with the angle of attack. Meanwhile, unsteady and steady amplitude galloping are observed in dynamic tests in a certain range of Reynolds number and angle of attack. The observations indicate that the galloping is across-wind dominated and strongly depends on the Reynolds number. Finally, quasi-steady predictions of galloping instability are compared with the observed galloping. Most of the observed occurrences of galloping are in the predicted unstable range, but several other cases for which galloping was predicted do not exhibit large vibrations. This implies that the quasi-steady assumption does not work well in predicting the galloping of elliptical cylinders at critical Reynolds numbers.

The effect of surface roughness on aerodynamic forces and vibrations for a circular cylinder in the critical Reynolds number range

“…Large amplitude vibrations in the critical Reynolds number range for a smooth elliptical cylinder under normal wind have also been observed in wind tunnel tests (Ma et al, 2017b;Ma et al, 2017c). The results imply that imperfections and axial flow may not be necessary conditions for vibrations in the critical Reynolds number range.…”

“…However, the flow regime in which dry galloping occurs and the effect of surface roughness on them remains unclear. From the practical point of view, dry galloping has been observed in wind tunnel tests many times, and they have been reproduced by the author's group for circular cylinders under normal and skew winds (MA et al, 2017a;Ma et al, 2017b;Ma et al, 2017c) . However, these vibrations are rarely observed in the field.…”

“…Dry galloping has attracted much interest for several years and has been reproduced in wind tunnel tests (Cheng et al, 2008a;Cheng et al, 2008b;Jakobsen et al, 2012;Ma et al, 2017b;Ma et al, 2017c;Nikitas et al, 2012;Nikitas and Macdonald, 2015).…”

“…Compared with the mean drag coefficient, the lift coefficient experiences even more significant variation in the mean and fluctuation components. The jumps in lift coefficients (Benidir et al, 2015;Jakobsen et al, 2012;Ma et al, 2017c;Nikitas and Macdonald, 2015) represent a transition process between the TrBL0 and TrBL1 regimes. The non-zero mean lift coefficient is induced by a separation bubble on one side of the cylinder in the TrBL1 regime.…”

Aerodynamic characteristics and excitation mechanisms of the galloping of an elliptical cylinder in the critical Reynolds number range

“…The large vibrations observed in this study are termed 'dry galloping' because vibration in the critical Reynolds number range is complicated and similar vibrations of circular cylinders have been named 'dry galloping'. Many large vibrations were observed in the dynamic tests and had been reported in another paper (Ma et al, 2017), but the interactions of the aerodynamic forces and vibration need further detailed analysis. The large vibrations were observed in the ranges Re=1.5×10 5 -1.8×10 5 and α=26.5°-27.5° (note that is the angle of attack without wind and α is the mean angle of attack with the wind).…”