Friction Losses in Laminar and Turbulent Flow

When a fluid flows through a pipe, there will be frictional losses.  Theses losses are due to the shear stress between the fluid molecules and the walls of the pipe.

When the flow is laminar
If the flow through the liquid is laminar, the losses will be proportional to the velocity.
The frictional losses are inversely proportional to the temperature of the fluid and to the pipe area.

When the flow is turbulent
In turbulent flow, the losses are proportional to the square of the velocity.  In turbulent flow, a layer of eddies and vortices is formed near the pipe surface.  The roughness of the pipe surface also has a bearing on the losses in turbulent flow.  If the pipe surface is rough, the losses will be more.  In turbulent flow, the losses are proportional to the density of the fluid.