Learning Forum

[kartikeya parmar]

i want to design a porous medium in lid driven cavity

The porosity i am using is 0.9.

The Darcy number is 0.1

The Grasoff number =10^4

I am trying to simulate but not getting the result the nusselt number i am getting is around 66 but the actual nusselt number is 12. the reynolds number is 100.

can anyone please help related to viscous resistance and inertial resistance values

[Rob]

Porosity is how open the zone is, permeability is how well flow can pass through that zone. The latter is what you need data to calculate for the resistance coefficients in Fluent. 

[kartikeya parmar]

Thank you for your kind reply

If my darcy number is 0.1 and dimension of block is 1mx1m so permeability is 0.1 and the viscous resistance then becomes 10. but providing this value gives the heat trasnfer which is not appropriate. do i need to enter any momentum source term separately and any term in energy equation to get desired nusselt number.

[kartikeya parmar]

Hello sir i have looked so much but still i am not able to learn how to model fluid flow in ansys fluent. The darcy number i am taking is 0.1 and The particle diameter is 10^-3 t viscous resitance as 10^7 and not considering any inertial resistance but even though i change richardson number from 0.01 to 5 by keeping Gr=10^4

i did not see any signifacent change in total surface heat flux and so in Nusselt number.The solid material is copper and fluid is water.

kindly help if you have any course related to porous media flow kindly let me know i can purchase as well.

 

[Rob]

You might want to review how Fluent calculates the Nusselt Number. 

[kartikeya parmar]

Hello sir i have looked so much but still i am not able to learn how to model fluid flow in porous media in ansys fluent. The darcy number i am taking is 0.1 and The particle diameter is 10^-3 t viscous resitance as 10^7 and not considering any inertial resistance but even though i change richardson number from 0.01 to 5 by keeping Gr=10^4

i did not see any signifacent change in total surface heat flux and so in Nusselt number.The solid material is copper and fluid is water.

kindly help if you have any course related to porous media flow kindly let me know i can purchase as well.

[Rob]

Take a step back from all of the non dimensional numbers. Break the model into parts on paper, leave the solver alone for now. 

You have a moving porous zone, and a particle diameter. Are the spheres packed? If so, check the manual for the equation to give the resistance coefficients. 

Motion can be done in different ways. How is the lid moving in reality? If you look at the options in Fluent, what best suits this? 

What other boundary or physics effects do you need to consider? Do you have turbulence, heat transfer, reactions or the like? 

Put that all down on paper, and add in the models you're going to use in Fluent. You may need to talk to your supervisor to agree on some of the assumptions. Finally, return to the Ansys software, correct anything that's wrong and alter settings to suit. 

There isn't anything extra on porous media as the manual is generally sufficient: it's evolved over the last 20+ years as porous media is one of the older models. The only extra content may be in the Knowledge System on here (box at the top to the left of Search) but that's probably only the experimental data section of the manual converted into a spreadsheet. 

[kartikeya parmar]

Just i have one doubt 

What is darcy number in porous media it is K/D^2 where D=characterisc length of the block or it is D=particle diameter.

I am using a square cavity of dimension 1mx1m so if paper says a darcy number of 0.1 what should i select K=0.1 and viscous resistance as 10

 

[Rob]

The porous media is set based on resistance coefficients. For a packed bed of uniform diameter spheres there's an equation in the manual to calculate these values. 

The Darcy Number is a nondimensional value, you decide which dimension you take. If it doesn't say in the paper you may have a problem. 

 

[kartikeya parmar]

 

I have calculated resistance coefficients based on the manual formula and it comes out to be K=4.87×10^(-7) and C2=480.10. But the only thing i am not able to understand that in paper says calculation for different darcy number like 0.1,0.01,0.001. how i change the K value as K value i have calculated using K=(d^2xphi^3)/(150x(1-0.9)^2).

 

Also if i take K=4.87×10^(-7) viscous resistance is very high and then if i change Richarson number i see no difference in plot.

please help

Just tell me how to change Darcy number as paper says for different darcy number 

[Rob]

We don't tend to worry about the Darcy Number, the resistance coefficients are what Fluent needs, which come from the equations. From your last comments the authors have used the Darcy Number to correlate something, that doesn't mean they set it in the model to get the data.  I can do an experiment get velocity v pressure in a system and then report Reynolds Number v Pressure in a paper; if I don't define diameter in the paper it's impossible to work out what I did. 

[kartikeya parmar]

 

just i want to ask suppose Reynolds number is equal to (rhoxVxD)/mu if anyone wants to change reynolds number for a particular fluid he can do so by changing velocity 

similarly darcy number is Da=K/D^2 so if anyone want s to change Darcy number can he do so by changing permiability K 

That is my first question and my second question is does This K is equal to The inverse of viscous ressitance as mentioned in Ansys 

I just need the answer of these two questions

By the way thank you so much for your time and effort actually i am new to this heat trnasfer through porous media . i am following the paper "

Analysis of momentum and energy transfer in a lid-driven

cavity filled with a porous medium"

 

 

 

[Rob]

In reality we'd alter the speed or resistance of the media. Then calculate the nondimensional numbers. We may also calculate the number and then alter the experimental whatever to give a nice plot. 

In CFD we set stuff (speed, resistance coefficients etc) and can then find everything else. Depending on what is set we can also calculate the nondimensional numbers. But.... You are using a porous media approximation, so you use the permeability & other data to get the resistance coefficients in Fluent: they're based on resistance to flow. I can't say if you can directly equate the numbers as there are two components, C2 and 1/alpha which then use v^2 and v to determine pressure loss. 

The issue with the above is I can do the pressure v flow experiment for a packed bed of spheres and a sponge and get the same resistance coefficients (if the two media are suitable). But, does that return the same Darcy Number? That I can't say as I don't know: as an industrial user of the code I'm interested in pressure loss, flow rate, heat flux etc and try to avoid the nondimensional comparisons. 

[Shanmugam Dhinakaran]

i want to ask is it like for defining darcy number we need to divide our plate for different sections and apply porous medium only to a particular section. like if my plate is 1m in length and i need to analyze flow for darcy number 0.1 so do i need to divide my plate in two parts and apply porous medium only to 0.1m length section. i am very confussed with applying darcy number in porous medium

[kartikeya parmar]

does switching to dynamic meshing creates any change in the result 

my flow is steady does any one suggest

how to put dynamic meshing in a lidd riven cavity

[Rob]

Please re-read my previous comment. 

If it's steady state you can't use dynamic mesh as the remeshing is inherently time dependent. 

[kartikeya parmar]

is there a way to vary packing height in ansys and does this effect darcy number. my point is if permeability is fixed how we can very darcy number. or simply how we can vary darcy number in ansys

”

Effect of packing height and location of porous media on heat transfer in a cubical cavity

Are extended Darcy simulations sufficient?”

i am attaching a paper clip in which they kept permeability constant but vary the darcy number by varying the packing height  please help me understand

 

[Rob]

We vary the resistance coefficients and region those coefficients are applied to. So for one layer or two we'd simply make the porous zone a different size.