Half a million of hard (little) disks, all identical. Colors are only for visualization. The boundary conditions are periodic. So that the incoming flow is laminar, invisible straws quench outgoing turbulence, as in a wind tunnel. The roughness of the large disk is molecular: its circumference is made with the same little hard disks as those of the liquid.
<a href="//commons.wikimedia.org/wiki/User:Thierry_Dugnolle" title="User:Thierry Dugnolle">Thierry Dugnolle</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>