In the latter lectures, we will see that a parameter called Reynolds number   is used to characterize laminar flow vis a vis turbulent flow. If  for a tubular flow, the flow- characteristic is observed to be laminar and , the flow is turbulent. Re- visiting the Hagen–Poiseuille equation for pressure- drop: or, It follows that, if we apply the mechanical energy balance equation between two sections of a horizontal pipe through which there is a laminar flow under steady-state conditions, we can show that

where,