  We study the mechanism of energy injection from the mean flow to the
fluctuating velocity necessary to maintain wall turbulence. This process is
believed to be correctly represented by the linearized Navier--Stokes
equations, and three potential linear mechanisms have been considered, namely,
modal instability of the streamwise mean cross-flow $U(y,z,t)$, non-modal
transient growth, and non-modal transient growth supported by parametric
instability. We have designed three numerical experiments of plane turbulent
channel flow with additional forcing terms aiming to neutralize one or various
linear mechanisms for energy extraction. From our preliminary experiments, only
cases with mean cross-flows capable of supporting modal instabilities were
found to sustain turbulence. However, the question whether such a new
turbulence complies with the same physical mechanisms as those occurring in
actual (unforced) turbulence remains unanswered. On the other hand, cases
exclusively supported by transient growth decayed until laminarization.
