  The no-slip boundary condition results in a velocity shear forming in fluid
flow near a solid surface. This shear flow supports the turbulence
characteristic of fluid flow near boundaries at Reynolds numbers above
$\approx1000$ by making available to perturbations the kinetic energy of the
externally forced flow. Understanding the physical mechanism underlying this
energy transfer poses a fundamental question. Although qualitative
understanding that this transfer involves nonlinear destabilization of the
roll-streak coherent structure has been established, identification of this
instability has resisted analysis. The reason this instability has resisted
analysis is that its analytic expression lies in the Navier-Stokes equations
(NS) expressed using statistical rather than state variables. Expressing NS as
a statistical state dynamics (SSD) at second order in a cumulant expansion
suffices to allow analytical identification of the nonlinear roll-streak
instability underlying turbulence in wall-bounded shear flow. In this nonlinear
instability the turbulent perturbation field is identified by the SSD with the
Lyapunov vectors of the linear operator governing perturbation evolution about
the time-dependent streamwise mean flow. In this work, the implications of the
predictions of SSD analysis (that this parametric instability underlies the
dynamics of turbulence in Couette flow and that the perturbation structures are
the associated Lyapunov vectors) are interpreted to imply new conceptual
approaches to controlling turbulence. It is shown that the perturbation
component of turbulence is supported on the streamwise mean flow, which implies
optimal control should be formulated to suppress perturbations from the
streamwise mean. It is also shown that suppressing only the top few Lyapunov
vectors on the streamwise mean vectors results in laminarization. These results
are verified using DNS.
