26.8.3 Fluid link elements

**Products: **ABAQUS/Standard ABAQUS/Explicit

Fluid link elements:

are provided to simulate transfer of fluid between two cavities that are modeled with hydrostatic fluid elements or between a single cavity and the environment; and

must be connected to a cavity reference node on at least one end.

Defining a fluid link

At least one node of the element must be the cavity reference node of a fluid-filled cavity. The other node of the element can be unconnected and the pressure fixed with a boundary condition (“Boundary conditions,” Section 27.3.1), or it can be the cavity reference node of a second fluid-filled cavity.

Fluid properties of the nodes of the link are assumed to be the same as in their respective cavities. ABAQUS will not check whether a fluid link element has been defined between two cavities that are filled with dissimilar fluids; e.g., a fluid link element between a liquid-filled cavity and a gas-filled cavity. If this situation exists, the mass transferred from one cavity is converted to the fluid of the second cavity. Therefore, you must verify that the results obtained are meaningful. If fluid is transferred between a cavity and the environment, the fluid properties of the environment are assumed to be the same as those of the fluid in the cavity.

If one node of a fluid link element is unconnected and no boundary condition is applied, no fluid flow will occur through the link. This feature can be used to “seal” the cavity from the environment by removing the boundary condition during the analysis.

Defining the mass flow rate through the link

The fluid link properties determine the mass flow rate through the link. The mass flow rate is specified as a function of the pressure differential and may also depend on the average pressure and temperature as well as external field variables. The fluid link will account for differences in fluid density between the cavities. The density in each cavity is a function of the temperature and, for compressible materials, of the pressure in the cavity. The conversion of the mass flow rate into volume flow rates at the ends is based on the fluid properties given for the cavity or cavities. Hence, the volume flow rates at the ends of the link may not balance.

Two methods are available for specifying the mass flow rate. In either case you must associate the mass flow rate with a set of fluid link elements.

Input File Usage: | *FLUID LINK, ELSET= |

You can define an implicit functional relationship between the mass flow rate, *q*, and the pressure difference, , between the two nodes:

where and the subscripts 1 and 2 refer to nodes 1 and 2 of the element. This method is the default for specifying the mass flow rate. The viscous resistance coefficient, , and the hydrodynamic resistance coefficient, , can be functions of the average pressure, , and average temperature, , in the link, as well as the average of any user-defined field variables in the link. A positive value of

Input File Usage: | *FLUID LINK, TYPE=FUNCTION |

You can input a table of *q* versus , , , and field variables. ABAQUS will interpolate linearly between values specified in the table. If one of the independent variables is outside the range of specified values, ABAQUS will use the value that is closest in the table.

Input File Usage: | *FLUID LINK, TYPE=TABULAR |

Including viscous effects

Viscous effects in fluid link elements can be included in steady-state harmonic response analysis by using the direct-solution steady-state dynamic procedure (“Direct-solution steady-state dynamic analysis,” Section 6.3.4). In this case the linearized response is considered to be a perturbation about a nonlinear prepressurized fluid state, which implies that the vibration amplitude is sufficiently small that the fluid link response in the dynamic phase of the problem can be treated as a linear perturbation about the prepressurized state.