FLUID138 Element Description

FLUID138 models the viscous fluid flow behavior through short channels (that is, holes) in microstructures moving perpendicular to fixed surfaces. FLUID138 can be used in conjunction with FLUID136 elements to determine the stiffening and damping effects that the fluid exerts on the moving perforated microstructure.

FLUID138 assumes isothermal flow at low Reynolds numbers. The channel length must be small relative to the acoustic wave length, and the pressure change must be small relative to the ambient pressure. FLUID138 accounts for gas rarefaction effects and fringe effects due to the short channel length.

As with FLUID136, FLUID138 is applicable to static, harmonic, and transient analyses. FLUID138 can be used to model two different flow regimes: continuum theory and high Knudsen number.

In contrast to FLUID116, this element is more accurate for channels of rectangular cross section, allows channel dimensions to be small compared to the mean free path, allows modeling of evacuated systems, and considers fringe effects at the inlet and outlet. These effects can considerably increase the damping force in the case of short channel length. See FLUID138 - 3D Viscous Fluid Link Element in the Mechanical APDL Theory Reference for more details about this element.

FLUID138 Input Data

The element is defined by two nodes. The I node is located at the center of the cross-section of the hole region on the same plane as the nodes used to model the squeeze film fluid region (FLUID136 elements). The J node is located at the opposite face of the structure through the channel depth.

Figure 138.1: FLUID138 Geometry

KEYOPT(1) specifies the flow regime. The Knudsen number can be calculated from the mean free fluid path at a reference pressure, the operating pressure, and the lateral dimensions.

For rectangular channels, DIM is the smallest lateral dimension. For circular channels, DIM is the radius.

For continuum theory to be valid (KEYOPT(1) = 0), the Knudsen number should be less than 0.01. If the Knudsen number is greater than 0.01 (KEYOPT(1) = 1 or 2), the dynamic viscosity is adjusted to account for the slip flow boundary.

The fluid environment is defined by a set of real constants:

For rectangular channels, DIM1 and DIM2 specify the lateral dimensions of the channel. For circular channels, DIM1 specifies the radius of the channel and DIM2 is not used, PAMB specifies the ambient (that is, surrounding) pressure, PREF specifies the reference pressure for the mean free fluid path, and MFP specifies the mean free fluid path at reference pressure PREF.

For continuum theory (KEYOPT(1) = 1), DIM1, DIM2 (if rectangular channel), and PAMB must be specified.

For high Knudsen numbers (KEYOPT(1) = 1), DIM1, DIM2 (if rectangular channel), PAMB, PREF and MFP must be specified. PREF and MFP are used to adjust the dynamic viscosity.

FLUID138 does not support any loadings. To preserve the pressure drop through the hole, the PRES degree of freedom for the nodes of the FLUID136 elements at the periphery of the hole must be coupled to the PRES degree of freedom for node I of the FLUID138 element representing the hole, and the pressure degree of freedom for node J must be set to the surrounding ambient pressure.

FLUID138 Output Data

The solution output associated with the element is in two forms:

A general description of solution output is given in Table 136.2: FLUID136 Element Output Definitions. See the Basic Analysis Guide for ways to view results.

The Element Output Definitions table uses the following notation:

A colon (:) in the Name column indicates that the item can be accessed by the Component Name method (ETABLE, ESOL). The O column indicates the availability of the items in the file jobname.out. The R column indicates the availability of the items in the results file.

In either the O or R columns, “Y” indicates that the item is always available, a letter or number refers to a table footnote that describes when the item is conditionally available, and “-” indicates that the item is not available.

Table 138.2: FLUID138 Item and Sequence Numbers lists output available through the ETABLE command using the Sequence Number method. See The General Postprocessor (POST1) in the Basic Analysis Guide and The Item and Sequence Number Table in this reference for more information. The following notation is used in Table 129.2: FLUID129 Item and Sequence Numbers:

FLUID138 Assumptions and Restrictions

FLUID138 Product Restrictions

There are no product-specific restrictions for this element.