Learning Forum

[douniaextra1]

Hello everyone!

I was simulating a square box with trays as a porous media, where the air is flowing from the inlet passing by the porous media and then go out from the outlet. I only entered the power-law coefficients C0 C1 and the porosity. but then I realised that there is something wrong with it, because I found that in both cases with or without porous media it shows the same results. so I was wondering if someone can help me with this and show me how to simulate a porous media using the power law coefficients?  I'll be so grateful for any comments or answers. Thanks.

[Amine Ben Hadj Ali]

In the User's Guide you will find tips how to calculate the inertial and permeability coefficients based on your measured / know pressure drop or Ergun's like approximation.

This can be helpful 2:

https://youtu.be/T-2wfMQwYms

[douniaextra1]

Thank you so much for answering me. But I have no measurements to calculate the inertia and permeability coefficients. all I have is C0 and C1 the power law model coefficients.

Is it wrong to use just the power law model without the inertia and permeability coefficients ?

Best regards,

Dounia

[Amine Ben Hadj Ali]

No is not wrong.

[douniaextra1]

so why it doesn't work? there is absolutely no difference between adding porous media and take it off, it's like FLUENT does not take in count the power-law coefficients. 

Best regards,

Dounia

[Amine Ben Hadj Ali]

You will get the same velocity etc.. Only the contour of pressure is different across the porous zone if you are not using the physical formulation.

[douniaextra1]

the contour of pressure does not change if I take off the porous media,

in your opinion, where is the problem?

Best regards,

Dounia

 

[douniaextra1]

the picture in the right is the result I got ( velocity distribution), the left one is how it should be. where do you think I have a problem?

by the way, thank you so much for answering me I really appreciate it

Best regards,

Dounia

[Amine Ben Hadj Ali]

Add more details about case settings, cell zone and boundary conditions. What is the aim of this run? Are you comparing to a Fluent run?

[douniaextra1]

can I have your email so I can send to you a report  PDF that contains the details about my simulation step by step, or you can send me a message in my Gmail: douniaextra1@gmail.com

Best regards,

Dounia

[Amine Ben Hadj Ali]

No I'm sorry. Stick to the community please.

[douniaextra1]

it's ok, I understand. well, I'm just gonna send you some pictures so you can understand my geometry and what I'm trying to do.

first of all, here is my geometry: 

this is a drying chamber made of wood with 2 trays as a porous media. air is heated at the bottom by

the mean of a horizontal absorber which is considered as a source of energy

The air flows into the dryer through an inlet, absorbs energy (heat) from the absorber (Aluminum), flows

through the drying chamber and outflows from the outlet.

trays were assumed as porous media for airflow. It is modeled by the power law model using C0 and C1.

boundary conditions in the inlet: P= 100000 Pa  and  T=304 K

boundary conditions in the outlet: P= 100000 Pa

Wooden wall boundary conditions: u=v=0

there is convective loss in all the walls and in the absorber,

the standard k-? model is used to describe the flow.

I know it's a lot but please be patient with me, and again I'm so grateful for keeping answering me

Best regards,

Dounia

 

[Amine Ben Hadj Ali]

I guess they took buoyancy effects into account.

[douniaextra1]

I entered the gravity value... Please! can you check for me those pictures and see if there is something wrong.

[Rob]

What density model did you use? 

[douniaextra1]

for air, I'm using Boussinesq density

[douniaextra1]

in solution initailization, I realized that the turbulence coefficients equals Zero, is that something wrong? 

[Amine Ben Hadj Ali]

As the communication is not really optimal can you use ideal gas density and provide as operating density the density corresponding to the surrounding temperature (if the same at inlet and outlet).

[douniaextra1]

It didn't work! 

in solution initialization, I realized that the turbulence coefficients equals Zero, is that something wrong? 

[Amine Ben Hadj Ali]

Hey what did not work: If you want us  to help: be patient and provide details! 

Regarding the values of TKE and EPSILON: Yes that might lead to problem. Make ini from Inlets or do a hybrid ini as you have pressure boundaries!

[douniaextra1]

I'm sorry if it seems that I'm not patient. But I'm truly patient and trying to provide you with all the details.

what I was trying to say, is that I used ideal gas density but it still the same problem.

I tried to enter the turbulence coefficients randomly( i tried different values) in the inlet and outlet. Yes, there is a change but it has nothing to do with the results that I want to achieve.

I tried also the Hybrid initialization, it gave me the same result.

Best regards,

Dounia

[Amine Ben Hadj Ali]

Is the reference case transient? Do you have information on that?

I have not asked to modify the values at inlet/outlet but to initialize as Hybrid or initialize from Inlet.

Using Ideal gas density: have you set the operating density to the surrounding density?

[douniaextra1]

Yes, I tried the Hybrid initialization and initialization from the inlet and it gave me the same result.

Yes I set the operating density and there is no difference in the result

Yes, the reference case is transient.

Best regards,

Dounia

[Amine Ben Hadj Ali]

And are you sure that the picture from their runs is an instantaneous contour plots or rather an average one?

That density corresponds to which temperature? Is that the same temperature you used at inlet and outlets? I hope 304 K.

What about now you mesh resolution: you require a good near wall mesh with at least 10 cells in the boundary layers. Yplus of one is a must here. I will then use SST model as turbulence model.

[Amine Ben Hadj Ali]

304 K> 1.161191 kg/m³

[douniaextra1]

the picture from their run is an instantaneous contour plot.

the density that I used correspond to 300k, but in the inlet I have 304k. should I change the temperature?

the mesh is good here is the picture:

Best regards,

Dounia

 

 

[douniaextra1]

thank you for the density, I didn't know it before

[Amine Ben Hadj Ali]

You need to use the density corresponding to the surrounding and the surrounding is at 304 K. 

[douniaextra1]

Hello!

I just wanna ask you why K and epsilon value equal to Zero in my situation here?

PS: the inlet and outlet conditions are pressure type, not Velocity type.

Best regards,

Dounia

[Amine Ben Hadj Ali]

That is only for initialization (steady-state not really heartbreaking) but is not okay if it shows zero values. Can you reset and then repeat "Compute from" Inlet".

[Amine Ben Hadj Ali]

Please remove the mark as "Is Solved" as it is irritating a bit. (if you can remove that).

[douniaextra1]

I tried to reset but it still giving me Zero,

and I tried also to remove the mark "is solved" but I couldn't, it was just accidentally.

Best regards,

Dounia

[Amine Ben Hadj Ali]

Provide a value and then try to compute from all zones and then back from inlet.

[douniaextra1]

I have no measurements to calculate the K and epsilon value, and I have no idea how these values should be, can you give me certain values? 

Best regards,

Dounia

[Amine Ben Hadj Ali]

That are initial values: Start with moderate values. Have a look into Inlet turbulence quantities in the User's Guide and use that for domain initialization

[douniaextra1]

Hi!

in this flow, I used the inertial and viscous resistance to model the porous media, But again I have a problem, the air doesn't cross the porous media as you can see, it work as an obstacle, even if I changed the porosity nothing changes!! 

please, can you tell me why the air does not cross the porous media?

what should I do? so the air can cross the porous media.

Why when I changed the porosity the flow does not change? 

Best regards and best wishes,

Dounia

 

[Lucy]

Hi! Did you solve this problem? I'm struggling with a similar issue. 

[Amine Ben Hadj Ali]

Post your porous media settings here.

[douniaextra1]

Best regards,

Dounia

[Amine Ben Hadj Ali]

Are you sure about the inverse permeability coefficients and inertia? How did you calculate that? Post the velocity vectors please.

[douniaextra1]

I don't have a velocity vector, I used different values of inertial and viscous resistance that I take from articles or youtube videos, but it still the same problem.

[Amine Ben Hadj Ali]

Show the velocity vectors Graphics>Vectors.

 

So do not know the values which is required for you case right?

[douniaextra1]

No, I don't know the values which are required for my case! 

[Amine Ben Hadj Ali]

Where is the porous zone located in your screenshots? Please help us to understand so that we can help you.

Please share the residual plot of the run.

[Salman ayyoob]

Hi sir /madam

I want to know , if we use non darcy equation  ( for turbulent flow in porous media) and we have provided viscous resistance only, do we get the correct result? or we also have to calculate the inertial resistance and apply interial resistance as well ?

Would we need to provide power law also or not?

[Salman ayyoob]

I have calculated 1/a after modification in hydraulic gradient (i) and provied the value of 1/a only for non darcy range(turbulent flow condition), and I am getting results close to the experimental result.

So, do you think it ould be correct?

And also tell me when will we use power law?

[Rob]

If you don't provide all of the data then you may find the resistance isn't as expected, but as you don't have all the data you may not know the result isn't as you'd expect. 

[Rob]

It could be correct, that's something you need to check for yourself. 

I don't think I've ever used the Power Law option. Looking at the maths it's for when the more usual form doesn't fit the pressure loss data.