Some transient flow calculations are performed for a given material volume, for which the conservation must be guaranteed. Often, the incompressibility equation suffices for this, however, that equation has only a local effect, and does not prevent the numerical loss of creation of material that may originate, for example, from round-off errors or from adaptive meshing. Hence, you can impose a global constraint on the entire fluid domain. For this, at the level of the definition of a sub-task, you should select the item
Volume conservation
When entering the menu, Ansys Polydata informs you about the current known volume of the discretized fluid domain. The discretized attribute is important, and the given value may differ from the expected value. For example, a discretized sphere may have a volume that is slightly lower than the volume of the actual sphere. The current status is also displayed. When the setup involves a flow defined on a moving domain without entry or exit of material, with free surface and Lagrangian remeshing, the volume conservation is enabled by default, and the value is evaluated on the basis of the finite element mesh. For the other cases, the volume conservation is disabled by default. You can activate (or deactivate) it by clicking the option
Enable volume conservation
Subsequently, you can modify the prescribed value of the volume by selecting the option
Modify prescribed volume =
and by introducing a new value. This last option should be used for compensating the possible departure from the expected volume. For obvious reasons, it should not be used for assigning a totally different value to the volume that may eventually be incompatible with the other attributes of the simulation.
Important: When Polydata detects that the volume of fluid is likely not constant, it recommends the creation of a function of time that adapts the prescribed volume in order to be coherent with the inlet(s) and outlets(s). It is your responsibility to define that function and its parameters according to the variations of the volume.
In addition, you can set the prescribed value of the volume by selecting the option
Set prescribed volume = volume of sub-domain(s)
This option makes sure that the prescribed volume is equivalent to the volume of the fluid domain in the mesh, thereby allowing you to use the same setup for different geometries and allowing you to use design points with geometrical parameters in Ansys Workbench.
You can control the volume correction by selecting the option
Correct volume every time step
This option allows you to choose between volume correction at every time step and at every mesh adaption step. By default, this occurs at every mesh adaption step with a correction that is actually spread over n time steps between mesh adaptions. This avoids any large corrections after each mesh adaption. A correction at every time step might be necessary when the volume is not constant and if you want to have the correct volume at each time step.