Flow can occur in two different ways: Laminar and Turbulent. We will describe both and see how they differ.
LAMINAR FLOW
This type of flow occurs in smooth tubes and at LOW flow rates.
The flow is streamlined and there is no turbulence. The flow occurs in parallel layers, with minimal disruption between these layers.
The flow is greatest at the centre and diminishes towards the periphery. This makes the laminar flow describe a bullet shaped "velocity profile" shown in red below:
TURBULENT FLOW
This type of flow occurs in rough tubes and at higher flow rates.
The flow is not streamlined. There is a lot of swirling (eddies) of the fluid.
The flow is not greatest at the centre. Thus, as shown in red below, the "velocity profile" of turbulent flow is more flat than that caused by laminar flow.
TURBULENT FLOW is "BAD"
Turbulent flow is sort of the "bad cousin" of laminar flow.
For turbulent flow, the flow is NOT directly proportional to the pressure difference. I.e. You do not get a straight line (linear) graph. Whereas, for laminar flow, the flow is directly proportional to the pressure difference. I.e. You get a straight line (linear) graph:
Turbulent flow needs more pressure to drive it. For a given pressure difference, you will have a lesser flow with turbulent flow than with laminar flow.
Halving the radius reduces flow to a slightly greater amount in turbulent flow than when the flow is laminar. For laminar flow, halving the radius of the tube reduces flow by 16 times. For turbulent flow, it reduces it by slightly more than this.
And remember, turbulent flow is affected by density and not by viscosity. Laminar flow is affected by viscosity, but not by density.
ONSET OF TURBULENT FLOW
Certain factors favour the changing of laminar flow to turbulent flow.
A gentleman named Reynolds studied the factors in which the flow of fluid in pipes changed from laminar to turbulent. He experimented with various factors that affected flow (velocity, density, diameter, viscosity) and came up with an equation and magical number that predicted when a fluid would start to change form laminar to turbulent. The Reynolds number is described by the following equation:
He found with his experiments, that when the Reynolds number was below 2000 that the flow was laminar and that when the Reynolds number was above 2000, the flow was turbulent.
Thus factors that increase Reynolds number promote turbulence. From the equation, you can see that an higher fluid velocity (forward speed), higher density of the fluid, and higher diameter of the pipe favour the flow to change from laminar to turbulent.
Viscosity is in the lower part (denominator) of the equation and therefore, unlike the other factors, a higher viscosity lowers Reynolds number and thus favours laminar flow.
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