Effects of axial rectangular groove on turbulent Taylor-Couette flow from analysis of experimental data

The effects of the number of surface grooves on the hydraulic resistance are experimentally investigated in the case of fully turbulent Taylor-Couette flow at a fixed radius ratio η=0.9375 and for Reynolds numbers Rei from 2×104 to 13×104. Three configurations of inner cylinder having 6,12 and 24 grooves of the same rectangular shape and regularly distributed are compared to a reference smooth cylinder case. Such configurations are common in electric motors of high power density and of very high rotation rate, for which better understanding and knowledge of the flow patterns and aerodynamic losses are essential to enhance their design and to develop appropriate cooling systems. Torque and pointwise velocity measurements are performed. The effects of the flow modulations induced by the grooves on the friction coefficient and the flow features are examined. The results show significant difference in the average flow pattern, the presence of the grooves suppressing the Taylor Vortices. An increase of the friction coefficient with the groove number is reported. However, the individual drag-increasing contribution of each groove is reduced when increasing the groove number. It is also found that the local scaling exponent of the friction coefficient as a function of the Reynolds number is insensitive to the present surface roughness, suggesting that the bulk flow contribution is similar for all the four configurations.