EXPERIMENTAL INVESTIGATION OF THE REYNOLDS-NUMBER EFFECT ON THE STEADY AND UNSTEADY LOADING ON A SLIGHTLY ROUGH CIRCULAR CYLINDER AT REYNOLDS NUMBERS UP TO 12 MILLION

Abstract

This paper presents the Reynolds-number dependent unsteady aerodynamic force and time-averaged local surface pressure distributions acting on a 2D slightly rough circular cylinder up to high transcritical Reynolds numbers. The research was carried out in the High-Pressure Wind Tunnel (DNW-HDG) over a wide range of Reynolds numbers between 15x10^3 < Re < 12x10^6. The cylinder had a relative surface roughness of ks/D = 1.2x10^-3. The results demonstrated that especially in the critical regime the spanwise flow was strongly three-dimensional. Here, two discontinuities at two closely-spaced Reynolds numbers were observed, coupled with critical fluctuations in the lift force and the time-lagged formation of a separation bubble at each side of the cylinder. The presence of a slight surface roughness led to the disappearance of the supercritical regime, a subsequent strong recovery of the drag coefficient in the upper transition and intermediate steady plateaus for all aerodynamic properties in the transcritical state.