**hydro#

Description#

This boundary condition is used to impose a surface pressure calculated based on the updated (deformed) geometry, i.e. based on the geometry at the end of each increment (compare this to ***bc **pressure ). Hydro pressure will be defined according to the geometrical normal of a line set in 2D or a face set in 3D.

The pressure imposed on a body is applied in the direction of the surface’s normal 1You may verify a bset’s normals in Zmaster, and change them with the mesher bset_align.. For instance to model a solid submerged in fluid, the pressure’s value should be negative if the mesh uses the convention of outwards normals, and positive if normals point inwards.

See also **hydro_finite_strain.

Syntax#

**hydro bset value [ table ]

bset

This is the character name of a boundary set, i.e. a line set for 2D problems or a face set in 3D problems.

value

Real value for the base or scaling value of the hydrostatic pressure. The table value is scaled by this factor to calculate the pressure at a given time.

table

Character name for a valid loading table or tables which will describe the pressure in time.

Example#

Consider a c2d4 element. The displacement of bottom nodes 1 and 2 in both directions is set to 0. The pressure is applied on the top boundary set, which length is reducing with time due to the displacement U1 applied on nodes 3 and 4.

***bc
 **fix_all_dof 1 2
  **impose_nodal_dof
    3 U1 -0.3 time
    4 U1 0.3 time
 %**pressure top -10. time
   **hydro top -10. time
../../_images/pressure_vs_hydro.png

The value of applied forces depends on the surface as \(force = pressure\times surface\). Since **hydro takes into account the updated position of nodes (3 and 4), the surface on which the pressure is applied diminishes. Hence, the applied force in case of **hydro is lower compared to **pressure.

**hydro_finite_strain#

Description#

This boundary condition is similar to the **hydro boundary condition: it applies a surface pressure calculated on the updated (deformed) geometry. Both conditions apply the same loading forces.

hydro_finite_strain computes the exact (non-symmetric) tangent matrix by perturbation, whereas hydro only computes a (symmetric) approximation of the tangent matrix. Both conditions add non-linearities to the problem. Thus, hydro_finite_strain has a better convergence rate, at the expense of additional computational cost (especially noticeable if this condition is the only one that adds non-symmetric terms to the tangent matrix).

Syntax#

**hydro_finite_strain bset value [ table ]

bset

This is the character name of a boundary set, i.e. a line set for 2D problems or a face set in 3D problems.

value

Basic value for the base or scaling value of the hydrostatic pressure. The table value is scaled by this factor to calculate the pressure at a given time.

table

Character name for a valid loading table or tables which will describe the pressure in time.

Example#

The following example is taken from $Z7TEST/Hyperelastic_test/INP/ring-axi.inp:

***resolution
  % [...]
 **no_symmetrize   % mandatory for hydro_finite_strain

***bc
 **hydro_finite_strain
   upstream   -40.1e5 time
   tip        -40.0e5 time
   downstream -40.0e5 time