Laminar sublayer

The laminar sublayer, also called the viscous sublayer, is the region of a mainly-turbulent flow that is near a no-slip boundary and in which the flow is laminar. As such, it is a type of boundary layer. The existence of the laminar sublayer can be understood in that the flow velocity decreases towards the no-slip boundary. Because of this, the Reynolds number decreases until at some point the flow crosses the threshold from turbulent to laminar.