# Calculate and Understand the Significance of Reynolds Number and Viscosity

Quite often you have to calculate Reynolds number while dealing with hydraulics and pneumatics. Know how to calculate and appreciate its values.

Understanding the following two points will be helpful before going to the actual calculation and formula:

1. Dynamic viscosity and kinematic viscosity: The ratio of the dynamic viscosity and the density is known as kinematic viscosity. More details you can find here.
2. Characteristic length: This is the quantity which represents the geometry of the flow passage. In general, for flow in pipe this is equal to the hydraulic diameter of that pipe. For circular cross section pipe it is equal to the diameter (D) of the pipe.

Now, the required equation:
Re = ρsL/ μ………………………………….eqn.1

Where,
Re – Reynolds number (unit less)
ρ – Density of the flowing fluid (kg/m3, gm/cc, lb/ft3)
s – Velocity of the fluid (m/s, cm/s, ft/s)
L – Characteristic length (m, cm, ft)
μ – Dynamic viscosity (Pa-s, Poise, lb/ft-s)
Importance of Re: If the value of Re is less than 2300 then the flow will be laminar. In case Re is in between 2300 and 4000, then the flow will be transient. For the Re value of more than 4000, the flow will be turbulent.

Example
For,
ρ = 900 kg/m3
s = 2.5 m/s
L = pipe diameter = 0.025 m
μ = 0.35 Pa-s
So, after the Reynolds number calculation by using the Eqn.1
Re= 161
So, in this case the flow is laminar.
The equation is quite simple to use provided you have taken care in unit selection and conversion. Stick to consistent unit system, i.e, select any one of the unit system and convert the input to that system.

References

### Shibashis Ghosh

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