On the aerodynamic characteristics of inclined stay cable sheaths in the critical flow regime

This paper studies the presence of the bistable flow activity around inclined cable models. It presents results from wind tunnel test on static original High-Density Polyethylene cable covers in a range of Reynolds numbers from the subcritical to the critical regime, inclined at angles of 90° (vertical), 60° and 45°. It has already been shown that, in the critical regime, turbulent transition in the boundary layers around a circular cylinder exhibits bistable behavior at zero inclination, i.e. the boundary layer on either side is intermittently turbulent or laminar, leading to intermittent asymmetry of the flow and resulting aerodynamic loads. The present wind tunnel tests consist in measuring the pressure patterns around inclined circular cylinders using numerous pressure taps. Bifurcation diagrams are created in order to quantify the occurrence of bistability and Proper Orthogonal Decomposition is used to identify the geometrical flow modes that are governed by the bistability. The results show that the bistable phenomenon is only identified in the Reynolds number range corresponding to the first transition between the TrBL0 and TrBL1 regimes. Furthermore, the inclination angle seems to have a significant impact on the energy of the flow mode related to unsteady asymmetric reattachment. This paper also treats the potential sensitivity of the aerodynamic drag and lift forces to the circularity defect of the covers and presents the positive impact of the helical fillet on controlling the bistable activity at a critical inclination angle of 60°.