Product: ABAQUS/Explicit
A fluid exchange definition:
can be used to model flow between a single fluid cavity and its environment or flow between two fluid cavities;
can be used to prescribe mass- or volume-based flux into or out of a cavity;
can model the venting of a cavity through an exhaust orifice;
can model flow through cavity walls such as leakage through a porous fabric; and
has a name that can be used to identify history output of mass flow rates out of a cavity.
The fluid exchange capability in ABAQUS/Explicit is very general and can be used to define flow in and out of a cavity either as a prescribed function or based on the pressure difference arising from analysis conditions. You must associate the fluid exchange definition with a name.
| Input File Usage: | *FLUID EXCHANGE, NAME=name |
To define fluid flow between a fluid cavity and its environment, specify the single reference node associated with the fluid cavity. In the discussion that follows this fluid cavity is referred to as the primary cavity. When the flow is defined as a prescribed function, the flow can either be into or out of the primary cavity. If the flow is into the cavity, the properties of the material flowing in are assumed to be the instantaneous properties of the material in the cavity itself. When the flow behavior is based on analysis conditions, flow can occur only out of the primary cavity. For this case ABAQUS/Explicit will use the fluid cavity pressure and the specified constant ambient pressure to calculate the pressure difference used to determine the flow rate.
| Input File Usage: | Use the following options: |
*FLUID CAVITY, NAME=primary_cavity_name, REF NODE=primary_cavity_reference_node *FLUID EXCHANGE, NAME=fluid_exchange_name primary_cavity_reference_node |
To define fluid flow between two fluid cavities, specify the reference nodes associated with the primary and secondary fluid cavities. When the flow is based on analysis conditions, the fluid will flow from the high pressure or upstream cavity to the low pressure or downstream cavity.
| Input File Usage: | Use the following options: |
*FLUID CAVITY, NAME=primary_cavity_name, REF NODE=primary_cavity_reference_node *FLUID CAVITY, NAME=secondary_cavity_name, REF NODE=secondary_cavity_reference_node *FLUID EXCHANGE, NAME=fluid_exchange_name primary_cavity_reference_node, secondary_cavity_reference_node |
The flow rate from the primary cavity for any fluid exchange property is proportional to the effective leakage area. The leakage area may represent the size of an exhaust orifice, the area of a porous fabric enclosing the cavity, or the size of a pipe between cavities. You can specify the effective leakage area.
You can also define a surface that represents the leakage area by specifying the name of the surface on the boundary enclosing the primary fluid cavity. The effective area for fluid exchange is based on the area of the surface unless you specify the area directly. If both the effective area and a surface are specified, the area of the surface will be ignored. If neither is specified, the effective area defaults to 1.0.
| Input File Usage: | *FLUID EXCHANGE, EFFECTIVE AREA=effective_area, SURFACE=surface_name |
There are several different types of fluid exchange properties available in ABAQUS/Explicit to define the rate of fluid flow from a fluid cavity to the environment or between two cavities. The fluid exchange property can be as simple as prescribing the mass or volume flow rate directly. More complex leakage mechanisms such as those found on automotive airbags can be modeled by defining the mass or volume leakage rate as a function of the pressure difference, 
; the absolute pressure, 
; and the temperature, 
. For the purposes of evaluating the flow rate between two cavities, the absolute pressure and temperature are taken from the high pressure or upstream cavity. The flow is always in the direction from the high pressure cavity to the low pressure or downstream cavity. The cavity absolute pressure and temperature are always used to calculate the flow between a cavity and the environment.
You must associate the fluid exchange property with a name. This name can then used to associate a certain property with a fluid exchange definition.
| Input File Usage: | Use the following options: |
*FLUID EXCHANGE, NAME=fluid_exchange_name, PROPERTY=property_name *FLUID EXCHANGE PROPERTY, NAME=property_name |
Fluid flux into or out of the primary fluid cavity can be defined directly by prescribing the mass flow rate per unit area, 
. The mass flow rate is
is the effective area.Fluid flux can also be defined by prescribing a volumetric flow rate per unit area, 
. The mass flow rate is
is the density.A negative value for or will generate flux into the primary fluid cavity. When a second fluid cavity is not defined, the state of the fluid flowing into the primary cavity is assumed to be that of the fluid already present in the primary cavity.
| Input File Usage: | To prescribe a flux based on mass flow rate: |
*FLUID EXCHANGE PROPERTY, TYPE=MASS FLUX To prescribe a flux based on volumetric flow rate: *FLUID EXCHANGE PROPERTY, TYPE=VOLUME FLUX |
The mass flow rate, 
, can be related to pressure difference by both viscous and hydrodynamic resistance coefficients such as
is the pressure difference, is the effective area, 
is the viscous resistance coefficient, and 
is the hydrodynamic resistance coefficient. The resistance coefficients can be functions of the average absolute pressure, average temperature, and average of any user-defined field variables. A positive value of 
corresponds to flow out of the first cavity.| Input File Usage: | *FLUID EXCHANGE PROPERTY, TYPE=BULK VISCOSITY, DEPENDENCIES=n viscous resistance coefficient, hydrodynamic resistance coefficient |
The mass flow rate through a vent or exhaust orifice that can be approximated by one-dimensional, quasi-steady, and isentropic flow is given (Bird, Stewart and Lightfoot, 2002) by
is the dimensionless discharge coefficient, is the vent or exhaust orifice area, is the temperature in the upstream fluid cavity, 
is the absolute zero on the temperature scale being used, and 
is the absolute pressure in the upstream fluid cavity. The pressure ratio, 
, is defined as 
is the absolute pressure in the orifice. The critical pressure, 
, at which chocked or sonic flow occurs is defined as
is the ratio of the constant pressure heat capacity, 
, and the constant volume heat capacity, 
:
, is then given by
is equal to the ambient pressure for flow out of a single fluid cavity or the downstream cavity pressure for flow between two fluid cavities.The value of the discharge coefficient can be a function of the absolute upstream pressure, upstream temperature, and any user-defined field variables. Fluid exchange through a vent or exhaust orifice is valid only for pneumatic fluids.
| Input File Usage: | *FLUID EXCHANGE PROPERTY, TYPE=ORIFICE, DEPENDENCIES=n discharge coefficient |
The mass flow rate due to leakage through fabric can be expressed as
is the dimensionless fabric leakage or discharge coefficient and is the effective fabric leakage area.The value of the discharge coefficient can be a function of absolute upstream pressure, upstream temperature, and any user-defined field variables.
| Input File Usage: | *FLUID EXCHANGE PROPERTY, TYPE=FABRIC LEAKAGE, DEPENDENCIES=n discharge coefficient |
The overall mass flow rate can be calculated from a specified mass flow rate per unit area, , by
is the effective area.In this case you can define the mass flow rate per unit area in a table depending on the absolute value of pressure difference and, optionally, on the average absolute pressure, average temperature, and average value of any user-defined field variables. Values for and 
must be positive and start from zero.
| Input File Usage: | *FLUID EXCHANGE PROPERTY, TYPE=MASS RATE LEAKAGE, DEPENDENCIES=n 0, 0 |
The overall mass flow rate can alternatively be calculated from a specified volumetric flow rate per unit area, , by
is the effective area and is the density.In this case you can define the volumetric flow rate per unit area in a table depending on the absolute value of pressure difference and, optionally, on the average absolute pressure, average temperature, and average value of any user-defined field variables. Values for and 
must be positive and start from zero.
| Input File Usage: | *FLUID EXCHANGE PROPERTY, TYPE=VOLUME RATE LEAKAGE, DEPENDENCIES=n 0, 0 |
Fluid exchange will not occur unless the fluid exchange definition is activated in an analysis step.
| Input File Usage: | Use the following options to activate a fluid exchange for a given analysis step: |
*FLUID EXCHANGE, NAME=fluid_exchange_name *FLUID EXCHANGE INTERACTION fluid_exchange_name |
By default, the magnitude of the flow is based on the specified flow behavior. A time variation of flow magnitude during a step can be introduced by an amplitude curve. The magnitude based on the specified flow behavior is multiplied by the amplitude value to obtain the actual mass flow rate. For example, a time variation of prescribed mass or volumetric flux can be defined.
An amplitude curve may be used to trigger an event for fluid exchange in the middle of a step. For example, an airbag may deploy at some predetermined time during a step, and it may be desirable to close off all exhaust orifices until the actual deployment. A step amplitude curve that starts at zero and steps up at deployment time could be used for this purpose.
| Input File Usage: | Use the following options: |
*AMPLITUDE, NAME=amplitude_name *FLUID EXCHANGE INTERACTION, AMPLITUDE=amplitude_name |
By default, flow can occur both in and out of the primary fluid cavity when a second node is included in the fluid exchange definition. You can limit the flow direction such that fluid only flows out of the primary fluid cavity. This method is relevant only for a fluid exchange definition based on analysis conditions and not prescribed mass or volume flux.
| Input File Usage: | *FLUID EXCHANGE INTERACTION, OUTFLOW ONLY |
By default, when you modify the activation of a fluid exchange definition or activate a new fluid exchange definition, all existing fluid exchange activations in the step remain. When modifying an existing activation, all applicable data must be respecified.
Activated fluid exchange definitions remain active in subsequent steps unless deactivated. You can choose to deactivate all fluid exchange definitions in the model and optionally reactivate new ones. If you deactivate any fluid exchange definition in a step, all fluid exchange definitions must be respecified.
| Input File Usage: | Use the following option to modify an existing fluid exchange activation or to specify an additional fluid exchange activation (default): |
*FLUID EXCHANGE INTERACTION, OP=MOD Use the following option to deactivate all fluid exchange definitions in the model and optionally reactivate new ones: *FLUID EXCHANGE INTERACTION, OP=NEW |
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