Materials: Fluid

Description

The fluid material model is used to model fluid-structure interaction such as a fluid-filled tanks and water-filled dams. The element automatically accounts for the energy loss at free surfaces (surface tension) by the inclusion of a free face term (integral over the surface of fluid density times acceleration due to gravity).

In addition, to reduce (and in most cases eliminate) the many circulation modes usually found in natural frequency analysis, the element matrices are augmented by the addition of a circulation matrix that inhibits these modes.

The circulation matrix is expressed as:

where is a penalty coefficient and is a general strain-displacement matrix:

where the curl of the displacement field can be expressed as

The penalty parameter is used to add some stiffness to the shear terms of the element matrices. The amount of stiffness added equals the penalty parameter times the bulk modulus. In the absence of these terms, the element material matrix is singular and in most cases will not allow the model to solve. For a fluid with zero shear stiffness the shear terms should be zero, but in practice fluids such as water will be represented reasonably well with the default penalty parameter.

Note that the mass matrix is always assembled using the Consistent method (see SOLVERS Parameters: ELEMENTS) when fluid elements are used, irrespective of the element mass matrix selection in the solver defaults.