Dynamic
Fluid Coupling
FLUID38 Element Description
FLUID38 is used to represent a dynamic coupling between two points of a structure. The coupling is based on the dynamic response of two points connected by a constrained mass of fluid. The points represent the centerlines of concentric cylinders. The fluid is contained in the annular space between the cylinders. The cylinders may be circular or have an arbitrary cross-section. The element has two degrees of freedom per node: for example, translations in the nodal x and z directions. The axes of the cylinders are then assumed to be in the nodal y directions. The element may be used in any structural dynamic analysis. See FLUID38 in the Mechanical APDL Theory Reference for more details about this element.
FLUID38 Input Data
The node locations and the coordinate system for this element are shown in Figure 38.1: FLUID38 Geometry. The element is defined by two nodes and several real constants. The real constants are defined in Table 38.1: FLUID38 Real Constants.
KEYOPT(3) is used to select the form of the fluid coupling element. The form of the element determines the real constants required, the material properties (if any), and the matrices calculated. The density is input as material property DENS and is evaluated at the average of the two node temperatures. The damping matrix is calculated only if F is nonzero. KEYOPT(6) is used to select the direction of operation for the element. If KEYOPT(6) = 1, the X and Y labels used in this description should be interchanged. Similarly, if KEYOPT(6) = 3, interchange the Z and Y labels.
A summary of the element input is given in "FLUID38 Input Summary". A general description of element input is given in Element Input.
FLUID38 Input Summary
- Nodes
I, J
- Degrees of Freedom
UX, UZ if KEYOPT(6) = 0 or 2, or UY, UZ if KEYOPT(6) = 1, or UX, UY if KEYOPT(6) = 3 - Real Constants
If KEYOPT(3) = 0:
R2, R1, L, F, DX, DZ, WX, WZ If KEYOPT(3) = 2:
M2, M1, MHX, MHZ, DX, DZ, WX, WZ, CX, CZ See Table 38.1: FLUID38 Real Constants for a description of the real constants - Material Properties
MP command: DENS if KEYOPT (3) = 0 None if KEYOPT (3) = 2 - Surface Loads
None
- Body Loads
- Temperature --
T(I), T(J)
- Special Features
None
- KEYOPT(3)
Cross-section of cylinders:
- 0 --
Concentric circular cylinders
- 2 --
Concentric arbitrary cylinders
- KEYOPT(6)
Flow axis parallel to:
- 0, 2 --
Nodal Y axis (UX, UZ degrees of freedom)
- 1 --
Nodal X axis (UY, UZ degrees of freedom)
- 3 --
Flow axis parallel to nodal Z axis (UX, UY degrees of freedom)
Table 38.1: FLUID38 Real Constants
| No. | Name | Description |
|---|---|---|
| Concentric Circular Cylinders: KEYOPT(3) = 0 | ||
| 1 | R2 | Radius of outer cylinder (length); node J refers to outer boundary |
| 2 | R1 | Radius of inner cylinder (length); node I refers to inner boundary |
| 3 | L | Length of cylinders |
| 4 | F | Darcy friction factor for turbulent flow |
| 5 | DX | Estimate of maximum relative amplitude DX |
| 6 | DZ | Estimate of maximum relative amplitude DZ |
| 7 | WX | Estimate of resonant X frequency (Rad/Time) |
| 8 | WZ | Estimate of resonant Z frequency (Rad/Time) |
| Concentric Arbitrary Cylinders: KEYOPT(3) = 2 | ||
| 1 | M2 | Mass of fluid that could be contained within the outer boundary (Boundary 2) in absence of inner boundary. |
| 2 | M1 | Mass of fluid displaced by the inner boundary (Boundary 1) |
| 3 | MHX | Hydrodynamic mass in X direction |
| 4 | MHZ | Hydrodynamic mass in Z direction |
| 5 | DX | Estimate of maximum relative amplitude DX |
| 6 | DZ | Estimate of maximum relative amplitude DZ |
| 7 | WX | Estimate of resonant X frequency (Rad/Time) |
| 8 | WZ | Estimate of resonant Z frequency (Rad/Time) |
| 9 | CX | Flow and geometry constant for X motion (mass/lenght) |
| 10 | CZ | Flow and geometry constant for Z motion (mass/length) |
FLUID38 Output Data
There is no element solution output associated with the element unless element reaction forces and/or energies are requested.
FLUID38 Assumptions and Restrictions
The element operates in the nodal coordinate system (see Elements That Operate in the Nodal Coordinate System).
No fluid coupling exists in the flow axis direction.
The element has no nodal coordinate system transformation to account for nonparallel nodal coordinate systems.
Nodes I and J may be located anywhere in space (preferably coincident).
The lumped mass option [LUMPM] is not available with this element.