  Research on active control for the delay of laminar-turbulent transition in
boundary layers has made a significant progress in the last two decades, but
the employed strategies have been many and dispersed. Using one framework, we
review model-based techniques, such as linear-quadratic regulators, and
model-free adaptive methods, such as least-mean square filters. The former are
supported by a elegant and powerful theoretical basis, whereas the latter may
provide a more practical approach in the presence of complex disturbance
environments, that are difficult to model. We compare the methods with a
particular focus on efficiency, practicability and robustness to uncertainties.
Each step is exemplified on the one-dimensional linearized Kuramoto-Sivashinsky
equation, that shows many similarities with the initial linear stages of the
transition process of the flow over a flat plate. Also, the source code for the
examples are provided.
