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.
