FLUID38


Dynamic Fluid Coupling

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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.

Figure 38.1: FLUID38 Geometry

FLUID38 Geometry

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.NameDescription
Concentric Circular Cylinders: KEYOPT(3) = 0
1R2Radius of outer cylinder (length); node J refers to outer boundary
2R1Radius of inner cylinder (length); node I refers to inner boundary
3LLength of cylinders
4FDarcy friction factor for turbulent flow
5DXEstimate of maximum relative amplitude DX
6DZEstimate of maximum relative amplitude DZ
7WXEstimate of resonant X frequency (Rad/Time)
8WZEstimate of resonant Z frequency (Rad/Time)
Concentric Arbitrary Cylinders: KEYOPT(3) = 2
1M2Mass of fluid that could be contained within the outer boundary (Boundary 2) in absence of inner boundary.
2M1Mass of fluid displaced by the inner boundary (Boundary 1)
3MHXHydrodynamic mass in X direction
4MHZHydrodynamic mass in Z direction
5DXEstimate of maximum relative amplitude DX
6DZEstimate of maximum relative amplitude DZ
7WXEstimate of resonant X frequency (Rad/Time)
8WZEstimate of resonant Z frequency (Rad/Time)
9CXFlow and geometry constant for X motion (mass/lenght)
10CZFlow 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.

FLUID38 Product Restrictions

There are no product-specific restrictions for this element.