  A numerical investigation of two locally applied drag reducing control
schemes is carried out in the configuration of a spatially developing turbulent
boundary layer (TBL). One control is designed to damp near-wall turbulence and
the other induces constant mass flux in the wall-normal direction. Both control
schemes yield similar local drag reduction rates within the control region.
However, the flow development downstream of the control significantly differs:
persistent drag reduction is found for the uniform blowing case whereas drag
increase is found for the turbulence damping case. In order to account for this
difference the formulation of a global drag reduction rate is suggested. It
represents the reduction of the streamwise force exerted by the fluid on a
finite length plate. Furthermore, it is shown that the far downstream
development of the TBL after the control region can be described by a single
quantity, namely a streamwise shift of the uncontrolled boundary layer, i.e. a
changed virtual origin. Based on this result, a simple model is developed that
allows relating for the local drag reduction rate to the global one without the
need of conducting expensive simulations or measurements far downstream of the
control region.
