Entrance Conditions (Velocity Boundary Layer)

Velocity Boundary Layers! Yaaaaaay!

But Sir What is a Boundary Layer?

• • When a fluid moves along a surface, the fluid that is in contact with the surface is halted to zero velocity due to the ever present shear stress.

  • The zone or “layer” where the fluid is accelerating from zero velocity to its main stream maximum flow rate is known as the “Boundary Layer” 

• You can probably tell by this detailed diagram that a strong assumption is made…That the flow created is axisymmetric. Making it a whole lot easier for us.

• Luckily in our module we don’t have to calculate the velocity of set points along this boundary layer, but only the mean velocity.

How do we calculate this mean velocity?

• • Well as you may well have noticed….there is not reference velocity temperature!!!

  • So instead we use the cross sectional means  used for mass calculations.

  • Thus bringing us this equations for average velocity for incompressible flow in a circular pipe of radius R:

• • Unfortunately from my brief flick through. I cannot find these equations in the “Little book of Thermofluids” so you’d better remember these.

  • If you would like a handy .pdf of all you need to know and more. Click on a meme.