How to solve Non-Dimensional model equations in Fluent!!

Member Posts: 8

The non-dimensional (ND) equations (continuity, r-momentum, z-momentum and energy equations) are to be solved with fluent. Now, how can one implement this in Fluent?

Approach 1: The comparison of coefficients from ND model equations and Fluent's governing equation could be used to modify the thermophysical properties (temperature-dependent) of solids/fluids. Is it the correct methodology?

Approach 2: Is there a way to approach the problem using UDS or UDF? If so, for which equations one should write UDS/UDF. The thermophysical properties are temperature-dependent.

• UKPosts: 11,730Forum Coordinator

We usually alter the material properties to suit. Maybe if you explain why you want to do this someone can offer advice.

• Posts: 30Member

@Rob Thanks for the quick response!

I am simulating the behaviour of the porous medium under various conditions. As the material properties are highly temperature-dependent (for solid as well as fluid), I wish to know if UDS/UDF are a better choice or altering material properties, for solving the model equations (in ND form). Moreover, I want to implement specific correlations for wall heat transfer coefficient (HTC) and effective thermal conductivity, which in turn will be updated as per the temperature.

I hope it helps to gain clarity now.

• UKPosts: 11,730Forum Coordinator

I'd model everything with the correct properties: the whole point of simulation is that we can avoid a lot of the assumptions made when we mess with non-dimensional terms.

If you want a specific wall HTC on the inside you'll need to rewrite the wall functions, that will require UDF.

• Posts: 30Member

@Rob Thanks for the update!!

Wall HTC is surely user dependent in case of a porous media, i. e. Constant or udf etc. How about effective thermal conductivity (or Peclet number in N-D form) for LTE approach. In that case, if one wants to implement the effective thermal conductivity then, do altering the material properties is a good choice or some UDF?

• Posts: 30Member

@Rob Kindly see to the following query also, which I faced now.

The second question in this regard is about the dimensionless temperature which ranges from 0 to 1. The (hot) fluid is coming to the bed and the non-dimensional temperature (theta) is 1, and the porous bed (LTE model) is at theta=0. But while Initialization, I am getting the following message in the console.

"temperature limited to 1.000000e+00 in 60400 cells on zone 2".

And an initial condition of theta=0 is not displayed in the contours.

• UKPosts: 11,730Forum Coordinator
• GermanyPosts: 7,890Forum Coordinator
• Posts: 30Member

@Rob Thanks for the reply, and apologies for the 2 similar posts. I was not able to see the post from my account, so had to post it separately. Continued at https://forum.ansys.com/discussion/33812/issue-in-initialization-of-temperature-field-in-porous-medium#latest

• Posts: 30Member

Thanks for the update.

I would like to share an observation here, and kindly let me know your opinion.

The model is developed in non-dimensional form, so the inputs to Fluent are made in that manner. Now, what I observed was that, the limits of (static) temperature were set from 1 K - 5000 K by default. Upon reducing it to 0 K- 1 K, the simulation ran fine. So the solution remains the same, provided the model inputs are consistent with the mathematical model developed.

• UKPosts: 11,730Forum Coordinator

Good workaround. Except that if you use any temperature dependent properties and get a temperature of 0K you may get a divide by zero issue. The whole point of CFD is that we don't need to non-dimensionalise or scale to be able to model something.

• Posts: 30Member

@Rob All the help is greatly appreciated!!

I agree with your view; however, the packed bed case which I am tackling involves a lot of parameters, therefore non-dimensionalization was deemed necessary. Further, in my case, I have thermophysical properties that are temperature-dependent only (100 K-- 300 K) and I have made the input accordingly in form of UDF/polynomials which depend upon theta (ND temperature). So far, I have not encountered any issues.