FLUID139 Element Description

FLUID139 is a uniaxial element which models the fluid behavior between a sliding surface and a fixed wall. The viscous flow between surfaces is represented by a series connection of mass-damper elements whereby each node corresponds to a local fluid layer. The element has applications for modeling the fluid damping effects in microsystems such as comb drive fingers, large horizontally moving plates in seismic devices, etc. The element can be used in conjunction with other elements to model complete structural-fluid damping interaction, or stand-alone to add damping effects in a lumped sense to a structure. For low frequency applications, Couette flow assumptions is used. At higher frequencies where inertial effects become important, Stokes flow theory is used. First and second order slip flow models can be activated for systems which operate at high Knudsen numbers. The element is applicable to large deflection cases where the surface area exposed to a fixed wall changes with displacement (such as in comb fingers). See FLUID139 in the Mechanical APDL Theory Reference for more details about this element.

FLUID139 Input Data

The element is defined by two nodes. The I node is connected to the first "wall" and the J (or I+32) node is attached to the second "wall". Either wall may be constrained from moving, or both walls may move with respect to one another.

Figure 139.1:  FLUID139 Geometry

The 2-node option (KEYOPT(2) = 0) is recommended for systems which operate at frequencies below the cut-off frequency.

The 32-node option (KEYOPT(2) = 1) is necessary for Stokes flow models where only a small fluid layer at the wall is accelerated due to fluid inertia. For the 32 node option (KEYOPT(2) = 1), the first node is node I, and the 32nd node is node J. The intermediate node numbers (2-31) must be defined, but their location may be arbitrary. The geometric location of node I and J is not important as their separation distance is computed from the real constant gap separation GAP.

The fluid environment is defined by the real constants.

Gap is the local gap separation (i.e., width of fluid domain). AREA is the surface area. DADU is the change in the overlap area with respect to the surface displacement. That is, DADU is the first derivative of AREA with respect to displacement. If the surface area is constant, then DADU is the width of the overlap surface. PAMB is the ambient (i.e., surrounding) pressure. PREF is the reference pressure for which the mean free path of the fluid is defined. MFP is the mean free path of the fluid at PREF.

FLUID139 can be used to model continuous flow or slip flow boundary conditions. If the Knudsen number is less than 0.01, then continuous flow boundary conditions are valid. If the Knudsen number is greater than 0.01, but not near 1, then first order slip flow boundary conditions are valid. If the Knudsen number is near 1, then extended slip flow boundary conditions are valid. KEYOPT(3) is used to specify fluid blow boundary conditions. KEYOPT(3) = 0 specifies continuous flow. KEYOPT(3) = 1 specifies first order slip flow boundary conditions. KEYOPT(3) = 2 specifies extended slip flow boundary conditions. See Flow Regime Considerations in the Fluids Analysis Guide for a complete discussion of flow regimes and calculation of the Knudsen number.

FLUID139 can be loaded by nodal displacements at the interface nodes using the D command or by nodal forces using the F command. A combination of FLUID139 and structural elements allows a simultaneous fluid-structure domain simulation.

FLUID139 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 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:

FLUID139 Assumptions and Restrictions

The element assumes isothermal viscous flow. All the fluid properties are at a constant temperature (TUNIF) within a load step, even if you specify material properties with temperature dependencies (using MP). See Slide Film in the Mechanical APDL Theory Reference for more information on the governing equations.

This element cannot be used in a distributed solution.

FLUID139 Product Restrictions

There are no product-specific restrictions for this element.