From the Intro Course, you should already be familiar with the different types of nodes: normal, Ground, and reference. We’ll go into more detail about how they can be used in the fluid aspect.

• This is the name of the normal node shape in the GUNNS_Fluid.xml shape library in GunnsDraw. Most of your nodes will use this shape.
• It has one shape data field you need to set: the node’s initial fluid state.
  • This defines the initial state of its contents.
  • This is a reference to a Fluid State object in the diagram.
  • Syntax is: (Fluid State name). For example: fluidAmbient
  • Note that this defines the properties of an infinitesimal amount of mass.
    • It doesn’t use the Mass shape data from the Fluid State.
    • Mass is assumed zero (non-capacitive node) unless overridden by an attached link.
    • Attached capacitive links, such as GunnsFluidCapacitor, give the node volume, and thus mass and capacitance, and also override this initial Fluid State with their own defined state.
  • Because attached capacitive links override this initial state with their own, this initial state is really only used by non-capacitive nodes.
  • Regardless, a value here is always required.

• In the fluid aspect, Ground represents perfect vacuum with pressure equal to zero.
• The Ground node is just another node object in the network (same GunnsFluidNode class as the regular nodes):
  • Same class so that links can connect to it just like normal nodes
  • Doesn’t have a row in the system of equations — its pressure doesn’t need to be solved for since it’s always zero by definition
  • Has a fluid contents, but most of its properties are zero (like pressure). Zero mass, density, temperature, etc.
• Ground is not always appropriate to use for your “ambient” environment pressure. For instance, if your system is in an atmosphere, then leaks from the system to overboard should go to a node that represents the atmospheric pressure, not Ground. This is shown in the example drawing above.

• This is a normal node that is controlled by attached links to a given “ambient” environment pressure for your system.
  • If environment is pure vacuum, then use Ground node instead.
• Several ways to set this up, depending on how you need to use it:
  • If your system will only ever flow to ambient, and not from ambient, then only the pressure of the ambient node matters, and you can control this with just a GunnsFluidPotential on the ambient node. Set the conductance to a large value to make it an ideal pressure source, and set its pressure to the desire ambient pressure. This can be left as a constant or driven from some dynamic atmosphere model via sim bus, etc.
  • If you need to also be able to flow into your system from ambient, and can assume that the ambient conditions are constant, then use a GunnsFluidTank. Give the tank a very large volume so it has effectively infinite capacitance its contents won’t change significantly due to flows between your system. Initialize its fluid state to represent the ambient pressure, temperature, mixture.
  • If you need to be able to flow both ways and also dynamically change the ambient properties, then it’s best to use a GunnsFluidExternalDemand link (see below).

• The “Fluid Node Frame” link shape in GunnsDraw creates a normal fluid node, just like the Fluid Node shape.
• Same initial fluid state data as a normal Fluid Node.
• Only difference is in the shape graphic itself:
  • More of a square-ish shape, good for sizing to be very large and fitting other shapes inside its area.
  • This can be used to represent a large volume (like a vehicle cabin) in which other systems reside.
  • Objects placed inside its area don’t connect to the node, unless you draw a connection to its edges.

• Works exactly the same as other aspects.
• A convenient alternative to using large Fluid Node Segment shapes.

• The GunnsFluidExternalDemand link makes its attached node act like a boundary condition with the given fluid pressure, temperature, and mixture.
• Normally these are paired with a GunnsFluidExternalSupply in another network via the sim bus to tie the networks together and allow flow between them.
• However, you can also use this without a GunnsFluidExternalSupply and instead drive its inputs from some other model.

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