Experimental study of the very slow flow of a yield stress fluid around a circular cylinder

The slow flow of a yield stress fluid around a circular cylinder was studied by Particle Image Velocimetry (PIV) technique and drag measurement. The fluid used was a Carbopol® 940 gel with shear-thinning elastoviscoplastic behaviour. The case of significant yield stress effects was examined. Special attention was paid to preparing the fluid, controlling slip and the initial stress state in the fluid. In addition to the overall field extending beyond the sheared zone, the field very near to the cylinder was examined in greater detail. Asymmetry was observed between the upstream flow and downstream flow. Examination of the very near field revealed the existence of a recirculation zone upstream of the cylinder. These effects were not predicted numerically with the Herschel–Bulkley viscoplastic model usually used. The sheared zone obtained experimentally is more extensive than that obtained numerically with the Herschel–Bulkley viscoplastic model. A static rigid zone was observed at the downstream stagnation point. Detailed analysis of the results showed that normal stresses could be the cause of this upstream recirculation.