Momentum Source, Fan Type Question

Hi all,
I’m running studies on the efficacy of a fan shroud over a heated element - for the most part they’ve been illuminating and the software has been great to use (thanks Simscale for a fantastic product). There is something about the result that I’m having some difficulty interpreting however…

Goal: Assess cooling efficacy of fan cooling shroud over heated element, simulation type is Conjugate Heat Transfer 2.0 and appears to be executing as intended - momentum source is ‘fan’ type and has only one row of airflow and pressure inputs (.0219 cubic meters/sec flow rate, and 7.649 Pa of pressure).

Description of question: Using 5 simulation writes over a 500 ‘s’ study, I notice (what looks like to me) the momentum source (fan) starting at zero, or low airflow, and then beginning to speed up over time. The initial simulation state indicates the heated element getting hot in the absence of airflow - and then cooling down as airflow increases. My question is, since I gave the Momentum Source one power state (.0219 cubic meters/sec flow rate, and 7.649 Pa of pressure), why does it appear to be slowly increasing it’s output from 0 flow to high flow over the duration of the simulation?
Is this simply the air accelerating from a static state to a flow state? If so, why does it take more than 300 simulation seconds to do so?

Project is here: SimScale
The story of increasing airflow is more or less told in the three screenshots that are immediately displayed. If you import the project, observe Test Run 11 - Iterations 1 through 5, with particles, cutting plane, or whatever field you chose to assess airflow state.

Hi Christopher, and thanks for reaching out via the forum.

You can find the details on fan models in this pages:

Anyway, I will ask other members of the community to help you with your particular case, as it is somewhere outside my knowledge zone.

Thanks for replying! I suspect, contrary to appearances this will end up being a question more about how Simscale works than the study in question. I wasn’t able to find information about how the fan curve relates to simulation time in the articles you mentioned, or how having only one line of pressure/flow data will cause the momentum source to behave compared to a constant force.

Hello Christopher,

Given the way the fan curve model is created and meant to be used, I suspect getting valid results with just 1 row of flow rate vs pressure will not be sufficient. The fan curve model is meant to interpolate the fan’s flow rate performance based on the pressure surrounding the fan, so at least 2 rows would need to be defined to linearly interpolate/extrapolate the flow rate it pushes through the cylinder body modeling the fan.

I personally don’t know much about how the fan curve model would behave with just one row of data or if it presents a valid model in the first place just based on how the module was designed. If you can give an additional row of data from the fan you intend to model, then that will internally generate a linear fan curve.

I hope this clarifies things, but feel free to comment again if there’s any other questions/comments regarding the Fan model for momentum sources (or within the Boundary condition setup).