Boundary Conditions
The boundary conditions constrain the flow into the system by either prescribing a fluid velocity or constrain the pressure on the boundary. The Border flow option allows to specify the value of the pressure gradient in the normal direction such that:
The Border flow type selection list allows four ways to specify the value of Γ:
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The two Acoustic elongation options assume a linear gradient in the pressure outside the domain away from the boundary. The pressure gradient is calculated by dividing the pressure at the boundary, pfilm, by an elongation length, ΔL, such that:
The elongation length ΔL can either be specified as an absolute length or as a relative elongation of the width.
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The Out-of-plane motion option calculates the pressure gradient at the boundary using a model described in Ref. 9. This model is suitable for including the effects of gas damping when the thin-film is subjected to out-of-plane motion of the walls or base (for example, when a microbeam is oscillating above an extended surface). When this option is selected, the pressure gradient is defined according to the coefficients ζ, η, and χ
where U is the out-of-plane velocity, Λ is the slip length at the walls, and p∞ is the ambient pressure far away from the domain.
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The User defined option allows to enter values or expressions for the normal pressure gradient.