Border
Use the Border node to set a pressure condition at the border and the border flow type.
The Border flow condition is used to account for the pressure drop caused by the flow converging into the gap, outside the thin layer. The acoustic elongation option does this by adding additional thickness to the thin layer beyond the edge of the geometry (assuming a pressure gradient in this layer equal to that at the boundary). The out-of-plane motion calculates the pressure gradient due to circulation of the fluid caused by motion of the journal or bearing surfaces using a predefined model. Alternatively, the pressure gradient at the boundary can be specified using an arbitrary expression, which allows for more sophisticated, user-defined models.
Border Settings
Select a Border condition — Zero pressure, Pressure, or Border flow.
When Liquid with cavitation is selected in the physics interface, Starvation is also available.
For Pressure, enter a Pressure pf0 to define pf = pf0 on the edge. If the reference pressure pref, defined at the physics interface level, is 0, pf0 is the absolute pressure. Otherwise, pf0 is the relative pressure.
For Border flow, select a Border flow type — Acoustic boundary condition-absolute elongation, Acoustic boundary condition-relative elongation, Out-of-plane motion, or User defined.
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For Acoustic Boundary condition-absolute elongation, enter a Border elongation ΔL.
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For Acoustic boundary condition-relative elongation, enter a Relative border elongation ΔLr.
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For Out-of-plane motion, enter the Model coefficients η, ζ, and χ with defaults 0, 0, and 1, respectively. Also enter the Slip length, L. The default slip length uses an if condition to set the slip length equal to that defined in the Hydrodynamic Journal Bearing node if a suitable Film flow model is selected. If the Slip length is not defined in the Hydrodynamic Journal Bearing node, it defaults to 0.1 μm.
Note that the coefficients can be specified according to the results given in the paper by Gallis and Torczynski (Ref. 8). The following values are recommended:
where h is the gap height. The above equations are valid in the range 0 ≤ Λ/h ≤ 1.
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For User defined enter a Normal pressure gradient. For Starvation enter a value for the Fluid fraction θ0.