Abstract: In order to probe into the mechanism and behavior of drag reducing by coupling flexible tubes with turbulent flow, experimental investigation is performed on behavior of turbu- lent drag reduction using flexible tubes with sleeve-structure, and the fluctuating displacement characters of tubes at the outer wall and their relations with the drag reduction rates are studied by using laser displacement sensor. The results are found as follows. The thinner the flexible tubes are or the bigger the Reynolds number is, the bigger the drag reduction rates by coupling flexible tubes with turbulent flow are. At the Reynolds number of about 3×104 , the rates of drag reduction of the sleeve-structured flexible tubes with thickness of 2, 3, 4 mm are about 16%, 11%, 9% respectively. A positive correlation exists between the drag reduction rates and the root mean square of fluctuating displacement in low frequency band at the outer wall of simple tubes. With simple tubes and at the big Reynolds number, the rates of drag reduction of the flexible tubes are bigger than their true values, as the drag reduction rates, which are caused by the inside expansion of the tubes and obtained by using the theoretical formula, are much lower.

Key words: Coupling, Flexible tubes, Fluctuating displacement, Tubes with sleeve-structure, Turbulent drag-reduction