  Motivated by the results of recent experiments (K\"uhnen et al., Flow Turb.
Combust., vol. 100, 2018, pp. 919-943), we consider the problem of designing a
baffle (an obstacle to the flow) to relaminarise turbulence in pipe flows.
Modelling the baffle as a spatial distribution of linear drag
$\mathbf{f}(\mathbf{x},t)=-\chi(\mathbf{x})\mathbf{u}_{tot}(\mathbf{x},t)$
within the flow ($\mathbf{u}_{tot}$ is the total velocity field and $\chi \ge
0$ a scalar field), two different optimisation problems are considered to
design $\chi$ at a Reynolds number $Re=3000$. In the first, the smallest baffle
defined in terms of a $L_1 $ norm of $\chi$ is sought which minimises the
viscous dissipation rate of the flow. In the second, a baffle which minimises
the total energy consumption of the flow is treated. Both problems indicate
that the baffle should be axisymmetric and radially localised near the pipe
wall, but struggle to predict the optimal streamwise extent. A manual search
finds an optimal baffle one radius long which is then used to study how the
amplitude for relaminarisation varies with $Re$ up to $15\,000$. Large stress
reduction is found at the pipe wall, but at the expense of an increased
pressure drop across the baffle. Estimates are then made of the break even
point downstream of the baffle where the stress reduction at the wall due to
the relaminarised flow compensates for the extra drag produced by the baffle.
