Pipe Flow Classifications

1.Reynolds Experiment

Osborne Reynolds ( 1842-1912) performed a famous experiment which helped characterise fluid motion. He injected a dye into a flowing fluid and observed its path as it was carried by the fluid.


STEP 1 :

A tank is filled with water. An outlet pipe is fitted at the side of the tank, and a valve can be used to control the rate at which water flows from the tank to through pipe and out into atmosphere.

STEP 2 :

A smaller tank, filled with coloured dye, has an outlet at its bottom. The outlet pipe opens into the middle of the tank's outlet pipe.

                         

RESULT OF EXPERIMENT :

TIME DEPENDENCE OF FLUID VELOCITY AT A POINT

2. Types of flows in pipes and how they influence energy losses in pipes :-

      a) Laminar Flow

Where the fluid moves slowly in layer in a pipe, without much mixing among the layers.

Typically occurs when the velocity is low or the fluid is very viscous.

           b) Turbulent Flow

Opposite of laminar, where considerable mixing occurs, velocities are high.

Laminar and Turbulent flows can be characterized and quantified using Reynolds Number established by Osborne Reynolds.

   and is given as : 

Where NR – Reynolds number

V – velocity of flow (m/s)

D – diameter of pipe (m)

ρ – density of water (kg/m3)

ή – dynamic viscosity (kg/m.s)

ν – kinematic viscosity (m2/s)

when – units are considered – NR is dimensionless.

NOTE – Reynolds number directly proportional to velocity

& inversely proportional to viscosity!

1. NR < 2000 – laminar flow

2. NR > 4000 – Turbulent flow

3. For 2000 < NR < 4000 – transition region or critical region –

                                            flow can either be laminar of turbulent – difficult to pin down exactly.

In summary:

Laminar flow

• Re < 2000
• 'low' velocity
• Dye does not mix with water
• Fluid particles move in straight lines
• Simple mathematical analysis possible
• Rare in practice in water systems.

Transitional flow

• 2000 > Re < 4000
• 'medium' velocity
• Dye stream wavers in water - mixes slightly.

Turbulent flow

• Re > 4000
• 'high' velocity
• Dye mixes rapidly and completely
• Particle paths completely irregular
• Average motion is in the direction of the flow
• Cannot be seen by the naked eye
• Changes/fluctuations are very difficult to detect. Must use laser.
• Mathematical analysis very difficult - so experimental measures are used
• Most common type of flow.