4.38 FLUID38 Dynamic Fluid Coupling

4.38 FLUID38 Dynamic Fluid Coupling (UP19980821 ) 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. For certain cases the axisymmetric harmonic fluid element, FLUID81 (with MODE = 1), can also be used. See Section 14.38 of the ANSYS Theory Reference for more details about this element.

Figure 4.38-1 FLUID38 Dynamic Fluid Coupling



4.38.1 Input Data

The node locations and the coordinate system for this element are shown in Figure 4.38-1. The element is defined by two nodes and several real constants. The real constants are defined as follows:

R1

Radius of inner cylinder (Length). Node I refers to inner boundary.

R2

Radius of outer cylinder (Length). Node J refers to outer boundary.

L

Length of cylinders (Length).

F

Darcy friction factor for turbulent flow.

DX,DZ

Estimate of peak relative amplitudes between inner and outer boundaries for the x and z motions, respectively (Length).

WX,WZ

Estimate of resonant frequencies in the x and z response directions (Rad/Time).

CX,CZ

Flow and geometry constants for the x and z motions, respectively (Mass/Length).

M1

Mass of fluid displaced by the inner boundary (Boundary 1) (Mass).

M2

Mass of fluid that could be contained within the outer boundary (Boundary 2) in absence of the inner boundary (Mass).

MHX, MHZ

Hydrodynamic mass for motion in the x and z directions, respectively (Mass).

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 Table 4.38-1. A general description of element input is given in Section 2.1.

Table 4.38-1 FLUID38 Input Summary

Element Name

FLUID38

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

R2, R1, L, F, DX, DZ,WX, WZ if KEYOPT(3) = 0
M2, M1, MHX, MHZ, DX, DZ,WX, WZ, CX, CZ if KEYOPT ( 3 ) = 2

Material Properties

DENS if KEYOPT ( 3 ) = 0
None if KEYOPT ( 3 ) = 2

Surface Loads

None

Body Loads

Temperature: T ( I ),T ( J )

KEYOPT(3)

0 - Concentric circular cylinders
2 - Concentric arbitrary cylinders

KEYOPT(6)

0, 2 - Flow axis parallel to nodal Y axis (UX, UZ degrees of freedom)
1 - Flow axis parallel to nodal X axis (UX, UZ degrees of freedom) 3 - Flow axis parallel to nodal Z axis (UX, UY degrees of freedom)

4.38.2 Output Data

There is no element solution output associated with the element.

4.38.3 Assumptions and Restrictions

The element operates in the nodal coordinate system (see Section 2.3.2). 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.

4.38.4 Product Restrictions

There are no product-specific restrictions for this element.