Fluid (Enclosed Cavity)
Add a Fluid subnode under Enclosed Cavity to model the pressure load acting on the cavity walls due to changes in fluid volume. The fluid can be modeled as a compressible or incompressible fluid by defining an equation of state.
The Fluid subnode is contributing with the Prescribed Pressure subnode.
See also Enclosed Fluids in the Structural Mechanics Theory chapter.
Fluid Properties
Select the fluid Compressibility — Compressible gas, adiabatic (default), Compressible gas, isothermal, Compressible fluid, Incompressible, or User defined.
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When Compressible gas, adiabatic is selected, enter a Reference pressure pref (default 1 atm), the Reference temperature Tref (default 293.15 K), and the Ratio of specific heats γ (default 1.4). The gas temperature is computed and stored in a variable called <item>.T, where <item> corresponds to the tag of the Fluid node, for example solid.enc1.fl1.
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When Compressible gas, isothermal is selected, enter the Reference pressure pref (default 1 atm).
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When Compressible fluid is selected, enter the Reference pressure pref (default 1 atm) and the Compressibility of fluid χf (default 4e-10 Pa-1). By entering a compressibility which is much smaller than the surrounding solid (for example, 1/(1e3*K) where K a representative bulk modulus of the solid), the cavity will become nearly incompressible. Extremely low compressibility may lead to numerical instabilities.
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When User defined is selected, enter an expression for the Pressure p. The default is an expression similar to (-1[GPa])*solid.enc1.DeltaV/solid.enc1.V_ref. The pressure is typically a function of the volume change. To apply a constant pressure load, use the Prescribed Pressure node instead.
For Compressible gas, adiabatic, Compressible gas, isothermal, and Compressible fluid, the reference pressure can be specified in two ways.
Specify the Reference pressure — From prescribed pressure or Used defined.
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When From prescribed pressure is selected, the reference pressure is equal to the pressure given in one or more Prescribed Pressure nodes. See Prescribed Pressure (Enclosed Cavity).
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When User defined is selected, enter an expression for pref.
Advanced
From the Reference volume list, select Automatic, From undeformed configuration, From deformed configuration, or User defined.
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When Automatic is selected, the reference volume is equal to the volume during the initialization of the study step (spatial frame configuration). In particular, this is useful when the cavity is already deformed due to an initial overpressure applied in a previous study step.
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When From undeformed configuration is selected, the reference volume is equal to the volume of the undeformed configuration (material frame configuration).
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When From deformed configuration is selected, the reference volume is equal to the volume during the initialization of the study step (spatial frame configuration). In particular, this is useful when the cavity is already deformed due to an initial overpressure applied in a previous study step.
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When User defined is selected, enter an expression for the reference volume, Vref. The default is an expression similar to withsol('sol1',solid.enc1.V).
From the Formulation list, select Automatic, Pressure formulation, or Strain formulation.
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When Automatic is selected, the method to compute the fluid pressure is automatically selected. This setting should be appropriate for most cases.
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When Pressure formulation is selected, the fluid pressure is computed with a global auxiliary DOF, which represents the fluid pressure.
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When Strain formulation is selected, the fluid pressure is computed with a global auxiliary variable that represents the volume change.
Location in User Interface
Context Menus
Solid Mechanics > More > Enclosed Cavity > Fluid
Membrane > More > Enclosed Cavity > Fluid
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Physics tab with Enclosed Cavity selected in the model tree:
Attributes > Fluid