Revisiting the Den Hartog instability of a circular cylinder with surface protrusion: Effects of elevated inflow free-stream turbulence

Experimental studies that study Den Hertog instability criterion from vortex-induced vibration to galloping instability use a laminar inflow with low free-stream turbulence levels. This study explores a modified Den Hartog criterion in a free stream with elevated inflow turbulence levels. Results from experiments that explore the stability assessment of altered circular cross-sections will be discussed. The surface protrusion thickness for the circular cylinder that gives rise to the galloping instability is varied from 0% (smooth cylinder) to 12.8% of the cylinder diameter. Den Hartog instability maps, lift and drag frequencies (due to vortex shedding) as functions of the incident angle, and the frequency power spectrums will be estimated from lift-drag measurements at both elevated turbulence intensity levels and compared to cases without inflow turbulence. It is conjectured that the inflow turbulence may reduce the magnitude of lift but will have no effect on measured drag. Free-stream turbulence also reduces the frequency of vortex shedding and eliminates the abrupt changes in lift observed in the laminar case.