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 options 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 wall or base 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 (the default), Pressure, or Border flow. The option Starvation is also available when Liquid with cavitation is selected in the Physical Model settings.
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For Pressure enter a Pressure pfilm0 (SI unit: Pa) to define pfilm = pfilm0 on the boundary, edge, or point. The default is 0 Pa.
If the reference pressure pref = 0 in the Reference Pressure section, then pfilm0 represents the absolute pressure; otherwise, pfilm0 sets the relative pressure with respect to pref.
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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. The default is 0.1 μm.
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For Acoustic boundary condition-relative elongation enter a Relative border elongation ΔLr. The default is 0.7 (dimensionless).
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For Out-of-plane motion enter the Model coefficients η, ζ, χ with defaults 0, 0, 1, respectively. Also enter the Slip length, Λ (SI unit: m). The default slip length uses an if condition to set the slip length equal to that defined in the Fluid-Film Properties node when a suitable Film flow model is selected. If the slip length is not defined in the Fluid-Film Properties node, it defaults to 0.1μm.
Note that the coefficients can be specified according to the results given in Ref. 9. The following values are recommended:
where h is the gap height. The above expressions are valid in the range 0 ≤ Λ/h ≤ 1.
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For User defined enter a Normal pressure gradient. The default is pf/(0.1 μm) N/m3.
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For Starvation enter a value for the Fluid fraction θ0. The default is 0.6 (dimensionless).