  We consider the nonlinear optimal control of bypass transition in a boundary
layer flow subjected to a pair of free stream vortical perturbations using a
receding horizon approach. The optimal control problem is solved using the
Lagrange variational technique that results in a set of adjoint equations, used
to obtain the optimal wall actuation (blowing and suction from a control slot
located in the transition region). The receding horizon approach enables the
application of control action over a longer time period, and allows the
extraction of time-averaged statistics as well as investigation of the control
effect downstream of the control slot. The results show that the controlled
flow energy is initially reduced in the streamwise direction and then increased
because transition still occurs. The distribution of the optimal control
velocity responds to the flow activity above and upstream of the control slot.
The control effect propagates downstream of the slot and the flow energy is
reduced up to the exit of the computational domain. The mean drag reduction is
55% and 10% in the control region and downstream of the slot, respectively. The
control mechanism is investigated by examining the second order statistics and
the two-point correlations. It is found that in the upstream (left) side of the
slot, the controller counteracts the near wall high speed streaks and reduces
the turbulent shear stress; this is akin to opposition control in channel flow,
and since the time-average control velocity is positive, it is more similar to
blowing-only opposition control. In the downstream (right) side of the slot,
the controller reacts to the impingement of turbulent spots that have been
produced upstream and inside the boundary layer (top-bottom mechanism). The
control velocity is positive and increases in the streamwise direction, and the
flow behavior is similar to that of uniform blowing.
