Re = Reynolds Number

The Reynolds number, Re, is used to predict whether the flow of fluid through a tube will be laminar or turbulent.

When Re < 2000, flow tends to be mostly laminar in quality.

A Re > 2000 suggests turbulent flow.

Re is given by the equation:

Re = (P * V * D)/(n)
            
where 

      Re = Reynold's number, dimensionless

      P  = density of fluid, kg/m3

      V = fluid velocity, m/s

      D = diameter of tubing or pipe, m
	
      n = dynamic viscosity of fluid, Pascal*s

A sample problem:

TRUE or FLASE:

All other factors being equal, a lower viscosity would yield less turbulent flow.

Answers:

FALSE.

If you are comparing the flow characteristics of two fluids, say, water and molten lava (which is several thousand times more viscous than water), you would note that the two fluids would flow very differently through the same tubes.

You could predict this from the Reynolds number, which is inversely proportional to viscoisty. A fluid with low viscosity (water) would give you a higher Reynolds number than a fluid with high viscosity (lava). As the Reynolds number gets higher, the likelihood for turbulent flow increases.

So, all other factors kept constant, lava would flow with less turbulence than would water.

For more information on this topic, please refer to West , page 105.

Also, check out the following links that may be helpful:

• Blood Velocity and Turbulence
• TK!Solver Fluid Flow Problem
• And, as always, if you have questions, or if you can suggest a helpful link, please e-mail me.

No!
Please don't make me go to any of those pages!
I want to go somewhere completely different!