The Wall node includes a set of boundary conditions describing fluid-flow conditions at stationary, moving, and leaking walls.
Select a Boundary condition for the wall.
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1 The default
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No slip is the default boundary condition for a stationary solid wall for laminar flow. The condition prescribes u = 0; that is, the fluid at the wall is not moving.
The Slip option prescribes a no-penetration condition, u·n=0. It is implicitly assumed that there are no viscous effects at the slip wall and hence, no boundary layer develops. From a modeling point of view, this can be a reasonable approximation if the main effect of the wall is to prevent fluid from leaving the domain.
The Sliding wall boundary condition is appropriate if the wall behaves like a conveyor belt; that is, the surface is sliding in its tangential direction. A velocity is prescribed at the wall and the boundary itself does not have to actually move relative to the reference frame.
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For 3D components, values or expressions for the Velocity of sliding wall uw should be specified. If the velocity vector entered is not in the plane of the wall, COMSOL Multiphysics projects it onto the tangential direction. Its magnitude is adjusted to be the same as the magnitude of the vector entered.
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For 2D components, the tangential direction is unambiguously defined by the direction of the boundary. For this reason, the sliding wall boundary condition has different definitions in different space dimensions. A single entry for the Velocity of the tangentially moving wall Uw should be specified in 2D.
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For an arbitrary wall movement, the condition u = uw may be prescribed. In this case, the components of the Velocity of moving wall uw should be specified.
This boundary condition may be used to simulate a wall where fluid is leaking into or leaving the domain with the velocity u = ul through a perforated wall. The components of the Fluid velocity ul on the leaking wall should be specified.
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