  Dimples are small concavities imprinted on a flat surface, known to affect
heat transfer and also flow separation and aerodynamic drag on bluff bodies
when acting as a standard roughness. Recently, dimples have been proposed as a
roughness pattern that is capable to reduce the turbulent drag of a flat plate,
by providing a reduction of skin friction that compensates the dimple-induced
pressure drag, and leads to a global benefit.
  The question whether dimples do actually work to reduce friction drag is
still unsettled. In this paper, we provide a comprehensive review of the
available information, touching upon the many parameters that characterize the
problem. A number of reasons that contribute to explaining the contrasting
literature information are discussed. We also provide guidelines for future
studies, by highlighting key methodological steps required for a meaningful
comparison between a flat and dimpled surface in view of drag reduction.
