Zero-Volume Node
AVL in North America Simulation
Pushing the boundaries of mobility technology with the speed of simulation.
Every Saturday, Thomas Glatz and Michael Bambula are excited to bring the developers on stage and provide you an extended read into various simulation topics.
by Michael Kolmbauer and Robert Fairbrother
Thermodynamic gas simulations are well known for their special requirements with respect to the simulation step sizes in explicit time integration procedures. Beside the well-known CFL (Courant–Friedrichs–Lewy) criteria also the size of the volumes play an important role for simulation step size restrictions. Small volumes in Plenums?typically require small simulation step sizes for a stable simulation and therefore cause slow simulations. If you have models, e.g. in a gas paths of cooling circuits, where you are not interested in a detailed time resolution of the thermodynamic behavior, but in the heat distribution over the systems, and if the model should run stable and fast with large simulation time step sizes, then the Gas node?component can give you an adequate tool for overcoming numerical instabilities and/or slow simulations due to small volumes in?Plenums.
With the component?Gas Node?AVL CRUISE? M offers a quasi-steady solution procedure for gas circuits. Using this feature simulation step size restrictions due to small volumes in Plenums can easily be overcome.
What is supported?
In a gas circuit, components of type?Plenum?can be topologically exchanged with a?Gas Node.?Gas Nodes?and?Plenums?can be arbitrarily combined in a model, allowing any feasible constellation. In contrast to the?Plenum, no additional component input is required in the?Gas Node, see Figure 1.
What is special about the Gas Node?
The component?Gas Node?represents the numerical limiting case of a?Plenum?with zero volume. Hence, the corresponding conservation equations obtain an algebraic structure.
Due to the algebraic part, the ordinary differential equations (ODEs) describing the gas circuit evolve to differential?algebraic?equations (DAEs), which requires an appropriate solver choice. Automatically configured and topologically partitioned linear and nonlinear solvers in combination with the established explicit time integration methods provide an efficient solution procedure. Therefore, no additional solver configurations are required in the GUI, everything is configured automatically in the background. A Gas Node should be used instead of a Plenum in cases where physically there is only a very small volume between the components. For example, the valve arrangement attached to a hydrogen tank (see example below). It should also be used when a small Plenum size causes excessive run times. The actual Plenum size limit when to switch using a Gas Node is case dependent with a strong link to the mass flow.
What are the known limitations?
The usage of the Gas Node imposes several limitations on the modeling of the gas circuit, which are all automatically checked within CRUISE M.
What are the known limitations?
The usage of the Gas Node imposes several limitations on the modeling of the gas circuit, which are all automatically checked within CRUISE M.
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Examples
?Bus cabin cooling model
This example demonstrates a bus cabin cooling model and features two modeling approaches: gas circuit using transient Plenums and quasi-stationary using Gas Nodes. The boundary conditions are taken from a transient vehicle simulation and are fed to the model via a Multicolumn map component. The inputs consist of the air flap opening splitting the flow between the evaporator and the heater, the recirculation flaps opening, the vehicle velocity used for the calculation of the outer convective heat transfer and the heat exchanged between the ventilation air and the refrigerant in the evaporator. In the Model parameters component, the user can modify the number of passengers currently on board and the contribution of the solar radiation.
The use of Gas Nodes in this case shows comparable results to a plenum model and allows the model to run up to 30 times faster and significantly faster than real time (factor 0.005). The applied solver is Heun, an explicit Runge-Kutta integration scheme, where internally Newton’s method is used in order to solve the algebraic equations related to the Gas Nodes. The use of an explicit solver with a fixed time step means the model can also be used on real time systems.
Hydrogen Tank
Hydrogen fuel tanks are used in all kinds of applications involving the use or generation of hydrogen such as fuel cell systems. The Gas Node can be used to model the multiple valve connections to such a tank and avoid the need to use a small Plenum if you need to connect all the valves before connecting to the Plenum itself. This provides a more realistic model, for example, when the tank is full then flow through the charging valve goes directly out through the relief valve rather than through the tank Plenum when all restrictions are connected directly to the tank Plenum. The use of a small Plenum instead of the tank node would cause excessive run times.
As much as we love to nerd out about simulation and read lengthy articles about it, we have to cut it short at this point.
We want to thank Michael Kolmbauer and Robert Fairbrother for the insights and the impressive work that is performed day to day behind the scenes.
Real-world activities and their real-time limitations bring this Simulation Saturday to an end, but stay tuned for another one soon!
Cheers, Thomas and Michael
What’s next:
You tell us!
We love to hear all of it in the comments and encourage you to: - learn more on our product sites:?AVL Advanced Simulation Technologies Tools - try it:?Rescale - simulate for free via?AVL University Partnership Program as a student and academic researcher. - get in contact via [email protected] and [email protected]
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