Application of turbulent pulsating flows to the bacterial removal during a cleaning in place procedure. Part 1: Experimental analysis of wall shear stress in a cylindrical pipe

The effects of pulsating turbulent flows on wall shear stress components were investigated in a straight pipe using the non-intrusive electrochemical method. Experiments were made using a new pulsation generator system which allows high amplitude pulsations in addition to a perfect stability of the installation. Maximum pulsation frequency equal to 2.86 Hz is used, above which fluid inertia dominates over most part of the flow field.Analysis showed that pulsating flows induce an increase of local velocity gradient at the wall pipe. This result is explained by the periodic renewal of the boundary layers. Spectral analysis showed high increasing rates of the fluctuation energy for the different pulsating conditions in comparison with a steady flow. The tested condition involving a recirculation flow induced a modification in the energy dissipation cascade, which can be explained by the redistribution of eddies size near the wall.