  Flow control with the goal of reducing the skin friction drag on the
fluid-solid interface is an active fundamental research area, motivated by its
potential for significant energy savings and reduced emissions in the transport
sector. Customarily, the performance of drag reduction techniques in internal
flows is evaluated under two alternative flow conditions, i.e. at constant mass
flow rate or constant pressure gradient. Successful control leads to reduction
of drag and pumping power within the former approach, whereas the latter leads
to an increase of the mass flow rate and pumping power. In practical
applications, however, money and time define the flow control challenge: a
compromise between the energy expenditure (money) and the corresponding
convenience (flow rate) achieved with that amount of energy has to be reached
so as to accomplish a goal which in general depends on the specific
application. Based on this idea, we derive two dimensionless parameters which
quantify the total energy consumption and the required time (convenience) for
transporting a given volume of fluid through a given duct. Performances of
existing drag reduction strategies as well as the influence of wall roughness
are re-evaluated within the present framework; how to achieve the
(application-dependent) optimum balance between energy consumption and
convenience is addressed. It is also shown that these considerations can be
extended to external flows.
