3. Operands

3.1. Operand MODELE_FLUIDE

♦ MODELE_FLUIDE = fluid

thermal model that is assigned to the part of the mesh that corresponds to the fluid. The edge elements at the fluid/structure interface must be present in the fluid model. It is on this model that we solve the Laplace equation with a « fluid flow » boundary condition, to have the pressure field throughout the fluid and*fortiori* the pressure field at the fluid/structure interface.

The nodes of the elements of the fluid mesh must all be numbered so that the Jacobian of the element is positive throughout the fluid mesh.

3.2. Operand MODELE_INTERFACE

♦ MODELE_INTERFACE = interf

thermal interface model bringing together all the edge elements defining the fluid/structure interface. It is on this model that we calculate the terms of the matrices added.

The calculation depends on the orientation of the normal of these interface elements. Care must be taken to ensure that this standard is oriented on all these elements, from the structure to the fluid (convention adopted).

If the fluids that surround the structure have different densities, the interface model must be defined by as many groups of cells that are not geometrically merged as there are fluids in contact with the structure. For example, a pipe may have its interior surface in contact with a fluid of density 1, and its exterior surface in contact with a fluid of density 2. The interface model is therefore built on two groups of cells that are not geometrically merged, one in contact with the fluid of density 1 (interfa1) and the other in contact with the fluid of density 2 (interfa2).

3.3. Operands CHAM_MATER/CHARGE

♦ CHAM_MATER = matflui

Field of material defining the fluid immersing the structure. This material is defined by equivalent thermal characteristics in operator DEFI_MATERIAU [U4.43.01]. Thermal conductivity (keyword LAMBDA) is always taken to be equal to1. Specific heat (keyword RHO_CP) plays the role of fluid density.

♦ CHARGE = load

[char_ther] thermal load required to solve the linear system arising from the Laplace equation in the fluid. One must impose a any temperature (which in fact plays the role of pressure) on an any node of the fluid mesh, in order to make the system not singular. This operation is done with the operator AFFE_CHAR_THER [U4.44.02].

3.4. Operands MODE_MECA/CHAM_NO/MODELE_GENE

♦/MODE_MECA = fashion

Dynamic modes calculated on the structure model. If there are several non-related structures immersed in the same fluid, for which it is desired to determine the added matrices comprising the terms of coupling by the fluid, the structure model that is defined brings together all the immersed structures. The modes used by the operator are the modes calculated for the overall structure.

/CHAM_NO = field

Field with travel nodes assigned to groups of nodes defining the fluid/structure interface. By assigning such a field of movement to the group (s) of interface nodes using the operator CREA_CHAMP, for example, it is possible to easily simulate rigid body modes of structures. This operand is to be used if one wants to estimate the terms of the added and coupling matrices for a plane geometry without first performing a modal calculation of the structure in a vacuum, and if one wants to assign these terms in discrete elements (modeling DIS_T). The INFO operand must be active to print the calculated terms.

Attention:

The use of this operand **excludes the use of the operand* NUME_DDL_GENE. Therefore, one cannot do a modal, transitory or harmonic calculation using the operand CHAM_NO.

In fact, to calculate water modes, it is necessary to have a generalized mass matrix and a generalized structure stiffness matrix. This assumes having done a modal calculation on the structure in a vacuum (which is precisely what we want to avoid) .

This operand is ultimately only used to **evaluate the matrix of added mass, stiffness, and damping, and use them to couple beam-like structures by discrete mass elements, for example.*

/MODELE_GENE = modgen

Generalized model built by operator DEFI_MODELE_GENE [U4.65.02]. This keyword is to be used when doing a calculation by dynamic substructuring, and when one wants to calculate the added mass matrix coupling all the substructures. In this case, the substructures may be in different mesh files, and these files may themselves be distinct from the fluid mesh file. Substructures that are repeatable within the fluid need only be meshed once, but care will be taken at the level of the fluid mesh to mesh all the fluid/structure interfaces. In addition, care must be taken to ensure that the fluid interface nodes best coincide with the structure interface nodes, in order to be able to copy, on the basis of a geometric proximity criterion, the values of the structure displacement fields onto the fluid interface nodes (see operand DIST_REFE).

Attention:

The use of this operand **makes it mandatory to use the operand* NUME_DDL_GENE.

3.5. Operand OPTION

This keyword specifies the calculation of mass, stiffness or added damping that you want to do, using “MASS_AJOU”, “RIGI_AJOU”, or “AMOR_AJOU” respectively. The last two calculation options require the presence of a stationary (thermal) potential (see operand POTENTIEL).

3.6. Operand DIST_REFE

Reference distance to be entered when calculating added mass on a generalized model. This distance is an absolute geometric criterion intended to copy structural displacement values into a thermal fluid model, in order to solve the Laplace equation of the unsteady pressure field. By default, it is equal to 10—2 m.

3.7. Operand NOEUD_DOUBLE

◊ NOEUD_DOUBLE = 'OUI'

This operand is to be used when calculating added mass from a generalized model that includes a substructure meshed by a wire or surface mesh (i.e. without thickness such as a beam or shell) and surrounded by two fluids. In this case, at the level of the mesh, it is necessary to split the nodes of the fluid interfaces from those of the structure, in order to be able to calculate the hydrodynamic pressure jump on both sides of the structure (cf. figure below).

3.8. Operand POTENTIEL

◊ POTENTIEL = phi

Stationary (thermal) potential required to calculate the added damping and stiffness of the structure subjected to potential flow. This potential is produced by the operator THER_LINEAIRE [U4.54.01].

3.9. Operand NUME_DDL_GENE

◊ NUME_DDL_GENE = number

Generalized numbering based on the mechanical modes of the overall structure. The presence of this operand makes it possible to calculate an added mass matrix of the Matr_asse_Gene_R type. It must necessarily be present if one subsequently wants to perform modal, harmonic or transitory calculations.

3.10. Operand INFO

◊ INFO =

Indicates the level of printing of operator results on file MESSAGE.

1:	no impression,
2:	impression of the upper triangular part of the matrices with added mass, added damping or stiffness.

Column layout.

3.11. Keyword factor SOLVEUR

See [U4.50.01].

3.12. Operand AVEC_MODE_STAT

Allows you to unplug the calculation of the terms of mass added to the static modes contained in the modal base of the substructures in the case of a calculation with a generalized model (cf. [§3.4]).

3.13. Operand NUME_MODE_MECA

Allows you to specify a list of mode numbers used for the calculation.