No-slip model: Qch is a constant and the continuum assumption holds (Ref. 4):

Slip model: Qch is a function of Ks. This model assumes minor rarefaction so that the no-slip condition in the channel boundaries is not valid (Ref. 4, Ref. 5):

Qch is a function of Ks. For this model, the modified Reynolds equation is valid with 5% accuracy for 0 Kn 880 (Ref. 7):

Qch is a function of D and . For this model, the modified Reynolds equation is valid with 1% accuracy for D 0.01 (Kn 88.6) and 0,7 1 (Ref. 7):

Geometry Orientations

The Film Damping physics interface calculates the film-thickness variation from the normal displacements of the moving solid and the channel base.

The normal displacement, on the other hand, is a product of the boundary normal and the boundary deformation of the solid wall and the channel base. The implementation of the Film-Damping Shell and Lubrication Shell physics interfaces assumes that the film boundaries are parallel so that the two normals are parallel but have opposite signs. The orientation of the normal vector n has also an effect on how to view the relative locations of the structures in Figure 5-1.

If simple blocks are used, the normal n always points out of the structure and it correctly points toward the fluid film outside of the solid domain. However, depending on how the geometry is built, the geometric normal does not necessarily correspond

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to the from-solid-to-fluid vector, in which case the wall can be set to normal in the Gap Properties section in the Fluid-Film Properties feature.

For the Thin-Film Flow and Thin-Film Gas Flow physics interfaces, the wall normal always points into the screen, which essentially means that the geometry is viewed in Figure 5-1 from above.

References for the Thin-Film Flow Interfaces

1.K.S. Breuer, “Lubrication in MEMS,” The MEMS Handbook, M. Gad-el-Hak, editor, CRC Press, 2002.

2.M. Bao, “Air Damping,” Analysis and Design Principles of MEMS Devices, Elsevier, 2005.

3.A. Burgdorfer, “The Influence of the Molecular Mean Free Path on the Performance of Hydrodynamic Gas Lubricated Bearings,” Transducers of the ASME, March 1959.

4.S.D. Senturia, Microsystem Design, Kluwer Academic Publishers, 2003.

5.T. Veijola, A. Pursula, and P. Råback, “Extending the Validity of Squeezed-film Damper Models with Elongations of Surface Dimensions,” J. Micromech. Microeng., vol. 15, pp. 1624–1636, 2005.

6.M. Bao, H. Yang, Y. Sun, and P.J. French, “Modified Reynolds Equation and Analytical Analysis of Squeeze-film Air Damping of Perforated Structures,” J. Micromech. Microeng., vol. 13, pp. 795–800, 2003.

7.T. Veijola, H. Kuisma, and J. Lahdenperä, “The Influence of Gas-surface Interaction on Gas Film Damping in a Silicon Accelerometer,” Sensors and Actuators, vol. A 66, pp. 83–92, 1998.

8.T. Veijola and P. Råback, “A Method for Solving Arbitrary MEMS Perforation Problems with Rare Gas Effects,” NSTI-Nanotech, vol. 3, 2005.

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6

M u l t i p h a s e F l o w B r a n c h

The fluid flow interfaces are grouped by type under the Fluid Flow main branch in the Model Wizard. This chapter discusses applications involving the Multiphase Flow branch () in the Model Wizard. The Mechanisms for Modeling Multiphase Flow helps you choose the best one to start with. Also see Bubbly Flow and Mixture Model Branches and Euler-Euler Model Branch for details about those interfaces.

In this chapter:

•The Laminar Flow, Two-Phase, Level Set and Phase Field Interfaces

•The Turbulent Flow, Two-Phase, Level Set and Phase Field Interfaces

•Theory for the Two-Phase Flow Interfaces

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