Osborne Reynolds investigated the effects of a fluid's viscous forces in relation to the fluid's inertia forces and how these interact to effect the resultant flow field around a body moving through the fluid. The ratio of inertia forces to viscous forces is referred to as a Reynolds number. When the viscous forces overshadow the inertia forces, the Reynolds number is small. In general, the Reynolds number is a function of the viscosity of the fluid and also the speed and size of the object that is moving through it. The DOL-Fin operates at Reynolds numbers low enough for the viscous forces to have a noticeable influence on the fin's boundary layer. The boundary layer is the fluid's sheer zone where fluid at the surface of the fin must travel along with the fin, and fluid further away must flow with the free stream.
Viscous forces and the unsteady flow patterns of the DOL-Fin's oscillating movement are major contributors to the boundary layer formation of fluid on the fin. Boundary layer formation and separation are critical influences on the efficiency of the fin. When the boundary layer separates from the fin, the fin can no longer force the fluid to move in the direction necessary for producing efficient thrust for the swimmer. The DOL-Fin design balances all of these considerations into a practical and robust solution for swimmer propulsion.