[dsofi32]

When doing a volumetric rendering of Pressure, both the Global and Local minimum value are -14.78 Pa. Yet, when I create the expression "minVal(Pressure)@part_1", the result is -40.4 Pa.


What is CFD-Post's expression used to calculate each value? I would like an expression to calculate, for each time-step of my simulation, the minimum pressure value of the volumetric rendering.


Thanks in advance for the help.

[ghorrocks]

You have to be careful when you look at minimum values (or maximums). They are likely to be highly mesh sensitive. Also be aware of the general numerical modelling advice that taking the max or min of a function amplifies noise (whereas means and medians reduces it). So it is always going to be hard to get a reliable number on the maximum or minimum of a simulation variable.

The difference between the global/local minimum and the CEL expression is likely to be because it is evaluated on different locations in the control volume. CFD-Post just uses the nodal values but the solver has the full resolution results with integration point values as well - so CEL done in the solver is evaluated on a finer and more accurate variable field than CFD-Post. The difference should reduce with mesh refinement (unless the minimum is associated with a step change effect like a shock wave).

[Opaque]

In addition, what is "part_1"?

Does include any boundaries? If so, are you using hybrid or conservative values when visualizing, or computing the minVal() function?

If part_1 is defined as mesh location, or physics location in CFX-Pre, you may also want to evaluate minVal(Pressure)@part_1 as a monitor point and compare vs CFD-Post.

You also mentioned transient, so be careful with Global. Is Global on the locator, or Global for the simulation (transient minimum included)?

[dsofi32]

Thank you very much for your reply @Opaque. To answer your questions:
- "part_1" is the fluid domain and I think it includes everything, including the boundaries. I am not familiar with "mesh location", the geometry was imported and selected as a Fluid in DesignModeler, if that helps.

- I am not sure what "hybrid or conservative values" means. I did a volume rendering of pressure and looked at the minimum value of the scale, and the minVal was used in an expression.
- Regarding the Global aspect, I have results from Fluent for several time steps, and I tested this with a different time step and the minimum values were different from these (and from each other, once again). So I don't think Global values are related to time, but instead related to the geometry.


Is there a way to find the minimum pressure through another method in CFD-Post?

[dsofi32]

Thanks for the explanation @ghorrocks.


If I were to plot the pressure rendering and picked the CEL expression (which I am assuming is the minVal), then wouldn't CFD-Post return an error, since the minimum I picked is lower than the global/local minimum?

[ghorrocks]

If I were to plot the pressure rendering and picked the CEL expression (which I am assuming is the minVal), then wouldn't CFD-Post return an error, since the minimum I picked is lower than the global/local minimum?

I do not understand what you are saying - but why not just try it and find out?

[dsofi32]

Hello again @ghorrocks


I think I wasn't clear enough in my reply. Since the minimum Global value measured is -14.78 Pa, if I specified a different minimum (in this case, using minVal which returns -40.4 Pa), CFD-Post could return an error. But you are right, I could test this out and confirm, but I do not think it is necessary.


Do you know any other ways of returning the minimum of the pressure for the chosen time step? This way I could potentially validate one of the two values.

[ghorrocks]

The best, and most accurate way of getting the minimum pressure in each time step is to define a monitor point with the CEL expression minVal(p)@region name. Then rerun the simulation and the solver will output the minimum value with the full integration point accuracy. You can get the data in the solver manager.