LAMINAR FLOW

# When watching a steadily flowing river, the flow of water may be seen to be fastest in the middle, while near the banks of the river the water flows more slowly. 

# This behaviour is also observed in fluid travelling slowly along a wide straight cylindrical tube, where the fastest velocity occurring in the centre of the tube and the slowest at the edge where there is friction between the wall of the tube and the fluid. This is known as laminar flow.

# Viewed from the side as it is passing through a tube, the leading edge of a column of fluid undergoing laminar flow appears parabolic. The fluid flowing in the centre of this column moves at twice the average speed of the fluid column as a whole. The fluid flowing near the edge of the tube approaches zero velocity.

#  #Hagen (in 1839) and #Poiseuille, a surgeon (in 1840) discovered the laws governing laminar flow through a tube. If a pressure P is applied across the ends of a tube of length, l, and radius, r. Then the flow rate, Q, produced is proportional to:
*The pressure gradient (P/l) *The fourth power of the tube radius *The reciprocal of fluid viscosity . This is often combined as: (see the figure for the equation)

where Q is flow, ΔP is pressure gradient, r is radius, η is fluid viscosity and l is length

# Also note: Viscosity is the important property for laminar flow, whereas density is the important property for turbulent flow. Reynold’s number of 2000 delineates laminar from turbulent flow