## Question

Which of the following statements about the Reynolds Number Are FALSE?

I Is a dimensionless number

II If the Reynolds Number is great than 2000, the flow is laminar

III As the hydraulic diameter of the pipe increases so does the Reynolds Number

IV The greater the viscous forces, the greater the Reynolds Number

A) All of the statements are false

B) II & IV

C) I & IV

D) I, II, III

## Answer

Today’s engineering exam question looks at what the Reynolds Number is and what factors can and can not change the Reynolds Number.

### Equation

The general equations are:

### Variables

ν = kinematic viscosity

μ = dynamic viscosity

D_{h} = Hydraulic Diameter (also known as equivalent diameter)

*v* = velocity

*A* = Area

ρ = density

### Solution

The **Reynolds Number** (*Re*) is a dimensionless number that is the ratio of inertia forces to viscous forces. When this number is less than 2,000, the flow is laminar. When the number is greater than 4,000 the flow is considered turbulent.

I – The Reynolds number is a dimensionless number. This statement is true.

II – When the Reynolds number is less that 2000 the flow is laminar. This statement is false

III – Since the hydraulic diameter is in the numerator. An increase will result in a larger Reynolds Number. This statement is true.

IV – As the viscous forces increase the Reynolds number decreases. This statement is false.

Since II and IV are false, the correct answer is B

Click here to view the previous PE/FE engineering sample question