Slip/No-Slip Flow

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Slip/No-Slip Flow

Posted December 2, 2011 at 23:19 by MatthewSlaby

Hi, I'm Matthew.

You asked what the physical meaning of the slip/no-slip condition means.

You need to look up the term "Knudsen Number".

This is simply a reminder of what CFD is base upon.

This will give insight into your question.

Fluid Dynamics is mostly based on Continuim Mechanics which assumes that the properties of a gas or liquid can be approximated by some very simple rules.

These would be:

What goes into the box, comes out (unless it's stored there) : Continuity

What Momentum goes into the box, comes out (unless it has tangential or normal dissipation and then exerts forces on the walls) : Conservation of Linear Momentum

What Energy goes into the box, comes out (Energy cannot be created or destroyed, so it simply transforms into different forms) : Conservation of Energy

That's the the basics!

Think of a bunch of big balls flowing past a Mountian range. When the ball hits they are deflected and loose Momentum. This slows it down and causes it to drift upward hit anthoner ball slowing it down too. When this keeps occuring, soon the fluid next to the wall becomes almost still.
When this happens the fluid above is retarded be friction and is transmitted to the next layer, and the next, until the retartardation is no longer felt and the fluid velocity is freestream and the "Boundary Layer" is far beneath.

There is no such thing as a "No Slip Fluid Flow". But there are situations where Tangential Forces may be ignored for Engineering Purposes.

If your question was more about how to identify the coding (software) for this, then I'm not your guy. The Physics of it however I can explain.

No-Slip Conditions were used quite a bit in earlier codes. They are based on Euler's Appoximation of the Navier-Stokes Eqns. which had succes with predicting pressure distibutions on bodies exposed to external flows.

What I've described is "Laminar Flow". Check out "Turbulent Flow".

Best Wishes,

Matt
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