The flow source effect forces flow into or out of a node at a given rate. GunnsFluidSource is the simplest version. Some derived types that add features are:

• GunnsFluidExternalSupply — the counterpart to GunnsFluidExternalDemand in creating a network-to-network flow interface, takes the flow demand from the demand link and adds/removes it to the supply node.
• GunnsFluidMetabolic and GunnsFluidMetabolic2 — models human metabolics consumption of O2 and production of CO2, H2O & heat by removing/adding these directly to the node contents.
• GunnsFluidSourceBoundary — a more capable version of GunnsFluidSource for creating flow source boundary conditions.
• GunnsGasDisplacementPump and GunnsLiquidDisplacementPump — model positive-displacement pumps like pistons, etc.

The most important thing to remember about the flow source effect is that it forces flow regardless of the network’s capacity to receive or supply that flow. It’s common sense that if you were to cram a finite number of molecules into an infinitesimally small volume, then the pressure and temperature in that volume would go to infinity. The same thing happens in GUNNS – this causes a network “blow-up” which is usually not recoverable. So you must be careful when using these links.

Some links have built-in checks that prevent these kinds of conditions, but most don’t. Some have tips in their GunnsDraw link help pages for how to avoid such conditions.

When flowing into a node, always make sure there is room for that flow. Likewise when flowing out of a node, always make sure there is enough mass in the local network to supply that flow. Otherwise you can cause:

Negative absolute pressure or temperature is not possible in the real world, but it is in the GUNNS network solution. The generic network solver, which is used in all 3 aspects, doesn’t care about the sign of the potential vector solution nor does it constrain the solution to be positive, because the electrical aspect in particular needs the option of having negative voltages in a solution. Contrary to pressure & temperature, there is no absolute voltage scale. Many links, including most flow source effect links, can “pull a vacuum” and cause the solution to go negative when used improperly. The fluid properties does not allow negative pressures, so at the end of the frame when the fluid properties are balanced, the fluid pressure will be constrained to zero while the node potential remains negative, and this causes large state error. It’s a bad place to be, so let’s not go there.

The source effect can flow individual fluid constituents in a fluid mixture as well as the bulk fluid. The human metabolics links are an example — they add & remove specific constituents like O2 & CO2 from the bulk air mixture which will also contain N2.

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