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To investigate the mechanism of drag reduction by addition of small elements in turbulent flows of Newtonian fluid, a direct numerical simulation (DNS) of turbulent channel flow has been carried out. Three types of element model (single sphere, rigid dumbbell (two beads are connected by a rigid rod), viscoelastic dumbbell (two heads are connected by a spring and a dashpot)) are used to know the essential function of the small element to reduce the drag in turbulent channel flow. The DNS results with dumbbell models show the drag reduction, while the drag increases with the single sphere model. The effects of parameters including spring constant, damping coefficient, length of the element on the flow features are investigated. In addition the mechanism of the stress deficit by the elements is discussed. Finally, it is shown the primary mechanism of drag reduction by a dumbbell element is to suppress the vortex stretching in the streamwise direction.

Mechanism of Drag Reduction by Addition of Dumbbell-like Small Elements in Turbulent Channel Flow