Question

The Reynolds number of a flow is the ratio of
A. Gravity to viscous force
B. Gravity force to pressure force
C. Inertia force to viscous force
D. Viscous force to pressure forces

Answer 1

Recall the basics of the concept viscosity. The terms Reynolds number is related to the viscosity of the fluid as the different fluids have different viscosities and hence have different velocities. Remind the definition of the Reynolds number in terms of the forces acting on the flowing fluid and choose the correct option.

Complete step by step solution:
The Reynolds number is the number which determines whether the flow of the fluid is laminar or turbulent.
The ratio of gravity to viscous force, gravity force to pressure force or viscous force to pressure force does not give the information about the type of the liquid flow.
Hence, options A, B, and D are incorrect.
The different fluids have different flow velocities due to their different viscosities.
Hence, the Reynolds number is the ratio of the inertia forces for the viscous forces inside the fluid as the different layers of the fluid have different velocities because of their relative motion.
Therefore, the Reynolds number of a flow is the ratio of inertia force to pressure force.

So, the correct answer is “Option C”.

Additional information:
For the laminar flow, the Reynolds number is less than 2300 and for turbulent flow, the Reynolds number is more than 4000.
The flow of the fluid having Reynolds number between 2300 and 4000 is called transitional flow.

Note:
We have given that the different layers of the fluid have different velocities. The students may think that as there are different velocities of the fluid in different layers but there is only one fluid flowing having different layers. The different velocities of the fluid occur due to the positions of the layers of the same fluid from the outermost boundary.