Aerodynamic characteristics and instabilities of catenary-suspended roadway luminaires

Abstract

This paper describes work resulting from studies of catenary-suspended road lamps. Wind-tunnel tests on one design of lamp revealed that the shape of the lamp, which was typical for this type of lighting, could suffer from aerodynamic instability at positive angles of attack in low turbulence flow due to separation from the upper surface. Violent oscillations were generated in the wind tunnel. Tests in a turbulent flow showed no such tendency, which leads to the unusual possibility that sustained low-speed low-turbulence flows could prove to be more critical than extreme strong winds. Site observations of full-scale lamps of broadly similar design in windy conditions revealed a complex mode of oscillation within the array of suspended lamp units. The subsequent failure of a number of lamps in wind speeds that were almost certainly below the level that would have caused a purely static load failure indicate fretting and fatigue due to oscillations as being the likely primary cause of failure. Tests were carried out in order to determine means of reducing down-force, and suppressing the instability. Design modifications to improve the stiffness of the suspension are suggested.