The Reynolds number dependency of the steady and unsteady loading on a slightly rough circular cylinder: From subcritical up to high transcritical flow state

•Wind tunnel experiment on slightly rough circular cylinder from sub- to transcritical.•Simultaneous measurements of spanwise unsteady forces and local surface pressures.•Strong three-dimensional and asymmetric flow along cylinder's span in critical regime.•Constant forces and surface pressures in full transcritical regime up to Re=2×107.•Disappearance of supercritical regime due to dimensionless roughness of O(10−3).

Simultaneous measurements of the spanwise-integrated unsteady aerodynamic forces and time-averaged local surface pressures on a 2D slightly rough circular cylinder were carried out over a wide range of Reynolds numbers in the high-pressure wind tunnel in Göttingen. The Reynolds numbers of 15×103≤Re≤12×10615×103≤Re≤12×106 spanned the known flow state regimes from subcritical up to high transcritical. The surface of the cylinder had a mean relative roughness of ks/D=1.2×10−3ks/D=1.2×10−3. The results demonstrated that especially in the critical flow regime the spanwise flow was strongly three-dimensional. In this regime two discontinuities at two closely spaced Reynolds numbers were observed, coupled with critical fluctuations in the lift force and the formation of a separation bubble at each side of the cylinder. The narrowing of the wake led to a sharp decrease of the drag coefficient and a steep increase of the values for the minimum pressure coefficients and the base pressure coefficient. In the upper transition state the boundary layer separation wandered upstream, resulting in an increase of the drag coefficient and a decrease of the base pressure coefficient and the Strouhal number. All measured flow field properties reached intermediate steady plateaus in the transcritical state for Re≥1.3×106Re≥1.3×106. It was also shown that the surface roughness had a strong effect on the cylinder flow; in particular the disappearance of the supercritical regime and the subsequent strong recovery of the drag coefficient in the upper transition.