No-slip condition

From Wikipedia, the free encyclopedia

In fluid dynamics, the no-slip condition states that at a boundary, fluids have zero velocity relative to the boundary.

The fluid velocity at all liquid–solid boundaries is equal to that of the solid boundary. Conceptually, one can think of the outermost molecule of a fluid sticking to the surfaces past which it flows.

1 Physical justification
2 Exceptions
3 See also
4 External links

[edit] Physical justification

This section is a stub. You can help by expanding it.

[edit] Exceptions

As with most engineering approximations, the no-slip condition does not always hold in reality. For example, at very low pressure (e.g., at high altitude), even when the continuum approximation still holds there may be so few molecules near the surface that they "bounce along" down the surface.

While the no-slip condition is used almost universally in modelling of viscous flows, it is sometimes neglected in favour of the 'no-penetration condition' (where the fluid velocity normal to the wall is set to the wall velocity in this direction, but the fluid velocity parallel to the wall is unrestricted) in elementary analyses of inviscid flow, where the affect of boundary layers is neglected.

The no-slip condition poses a problem in viscous flow theory at contact lines: places where a boundary between two liquids meets a solid boundary. Here, the no-slip boundary condition implies that the position of the contact line does not move, which is not observed in reality. The rate of movement of the contact line is known to be dependent on the angle the contact line makes with the solid boundary, but the mechanism behind this is not yet fully understood.