This chapter provides an overview how the mass balance in V-HAB is maintained and explains some underlying principles that explain why certain limitations in V-HAB exist. For example why branch and P2P flowrates cannot be changed directly but require a specific function and update order to work correctly.
In order to make this easier to understand we will use a consistent example similar to the simple flow tutorial. Two phases with a mixture of hydrogen and oxygen exchange mass. Phase A has a constant time step of 1 second while phase B uses a constant timestep of 2 seconds. Each block of H2/O2 represents one gram of this substance. Blocks outside of any of the phases were removed from phase A already but have not yet been added to phase B yet. The arrow above the pictures represents the time, the red dot the current simulation time.
First we look at the procedure of mass transfer if the flowrate does not change (since gifs are used for the examples they are expandable so you are not distracted by the other examples):
This is a very simple case, where in fact not much can go wrong. But it is helpfull to understand the example itself as the following examples will be more complicated. In this case two seconds were simulated and a total mass of 4 g was transfered between the two phases.
The next example shows what happens if the update order is not maintained and can be used to explain why you are not simply allowed to change a flowrate directly in V-HAB. In this case the initial flowrate of 2 g/s is changed to 4 g/s during the execution:
here phase B did not call a massupdate before the flowrate was changed. Therefore, phase B assumes the flowrate of 4 g/s when it does call its massupdate, which results in 2 g of matter beeing generated as they are added to phase B but never taken from phase A.
In order to prevent such issues V-HAB has a framework which ensures that both mass updates are executed before a flowrate can change. This prevents mass balance issues from occuring, but makes it a bit complicated to change the flowrate.
In this case the massupdate of phase B is executed before the new flowrate is set, which ensures that both phases use the correct mass flow rate for all transfers.