Uniform suction or blowing from the wall is one of the methods to reduce the friction drag. The uniform suction improves the stability of a laminar boundary layer: the transition will be delayed and the overall friction drag will be reduced due to the extended laminar region. In contrast, the uniform blowing is known to reduce the drag in the fully-turbulent regime. Therefore, a combination of uniform suction and blowing is expected to be effective for flows involving transition, such as the flow around an airfoil, by delaying the transition near the trailing edge and by reducing the turbulent drag in the post-transition (i.e., turbulent) region. The objective of this study is to investigate the friction drag reduction effect of such a combined uniform suction and blowing. The ReynoldsAveraged Navier-Stokes simulation is used to deal with a spatially developing boundary layer on a flat plate at a practically high Reynolds number. As a result, the combined control is found to reduce the global skin friction coefficient by 44.1%, whereof the contribution of transition delay by the uniform suction is about 90%, and that of turbulent drag reduction by the uniform blowing is about 10%. It is also found that the position of the blowing region should better be located in the upstream side of the turbulent region because the drag reduction effect is sustained for a while even after the blowing is terminated.
A concept for lift and drag modification on a airfoil using dielectric barrier discharge plasma actuation (DBD-PA) are investigated and proposed through direct numerical simulations. DBD-PA has placed on the wing-tip surface with varying its placement and magnitude of actuation, in order to modify the wing-tip vortex by adding and impeding the momentum. The assessment has been carried out by NACA 0012 with square tip at 10 degrees of angle of attack. Reynolds number is set to 3000. As actuating strength increases, lift force significantly increased in Blow-up case and decreased in Blow-down case, while slightly increased in Suction-up and is nearly unaffected in Suction-down case. Drag is reduced only in Suction-up case and Blow-down case at higher induced velocity, while increased in other cases. Also a large modification near the wing outboard region was confirmed due to a new vortex additionally created by the actuation. The increase of lift around wing outboard to center region of the wing which has been observed. These were attributed to the negative pressure fields which is created by a new vortex additionally created by the actuation.
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