Abstract
AbstractAn experimental study has been performed manipulating a fully developed turbulent channel flow, by means of blade manipulators known as outer layer devices (Olds). A large amount of investigation has been performed for boundary layer flow, whereas little research is available for internal flow manipulation. The influence on drag reduction for different configurations (single and tandem) and for some geometric parameters (distance from the wall and blade gap), have been analysed. Reynolds number effect has also been investigated. Some of the mechanisms involved in the skin friction reduction process are examined, in terms of time-space integral scales, Reynolds stresses and spectral analysis. Results show better efficiency for tandem configurations. These give high local skin friction reductions, up to 20%, which are strongly dependent on the manipulator distance from the wall, but have very little dependence on the blade gap and Reynolds number. Turbulence analysis displays reduced fluctuations in all three directions — as with space-time integral scales. Evidence of suppression for the large structures is also shown by spectral analysis. From a global balance it has been verified that no net drag reduction is obtained due to the additional drag introduced by the manipulators.
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