Mesh independence study for transient simulation

snejball91 · September 21, 2015, 10:06

Hello,
I want to set up a simulation in transient mode and to get good results I want to do a mesh independence study.

Do I have to do the mesh study in transient mode or can I select a value and do the study in steady mode?

Thank you!

ghorrocks · September 21, 2015, 17:32

You have to have a flow with the same characteristics as your intended flow. If you can do this with steady state then do so because it is much quicker.

bestniaz · September 22, 2015, 02:21

Quote:

Originally Posted by ghorrocks

You have to have a flow with the same characteristics as your intended flow. If you can do this with steady state then do so because it is much quicker.

sorry dear, I didnt get the answer,,,
You mean that mesh independence study for transient study can be validated on Steady state problem... As steady state problem is quicker..

ghorrocks · September 22, 2015, 03:11

Yes, that is correct.

If the transient flow is a boundary layer flow with a BL thickness of Xmm then over a bluff body, and if your steady state flow is also a BL with thickness about Xmm then over a bluff body - then a mesh sensitivity study on the steady state flow should be valid for the transient flow.

But if your flow is inherently transient (eg some shock waves, free surface waves, natural convection) then you cannot use the steady state flow.

bestniaz · September 22, 2015, 04:19

Quote:

Originally Posted by ghorrocks

Yes, that is correct.

If the transient flow is a boundary layer flow with a BL thickness of Xmm then over a bluff body, and if your steady state flow is also a BL with thickness about Xmm then over a bluff body - then a mesh sensitivity study on the steady state flow should be valid for the transient flow.

But if your flow is inherently transient (eg some shock waves, free surface waves, natural convection) then you cannot use the steady state flow.

Thanks for your quick help..
Sir , I am a bit new here and also have a lot of searching and browsing..but still confuse in few areas...
How we can find BL thickness?

ghorrocks · September 22, 2015, 06:20

https://en.wikipedia.org/wiki/Boundary_layer_thickness

snejball91 · September 22, 2015, 08:06

First of all: thank you for the information guys.

My setup is actually the air flow through a diffuser with some impacts on velocity and pressure that can only be seen in a transient simulation. So I guess I have to do the mesh study in transient mode.

aiden_1995 · November 17, 2022, 01:04

HI guys
im doing a fsi modelling which i should first do the mesh study and then continue
ive been told i can do it in reynolds max condition,i dont know how?
from which type of solution should i get max Reynolds? at first?
and what does max Reynolds condition mean overally? just to enter max velocity and run the solution? how does it make difference?

ghorrocks · November 17, 2022, 01:09

I do not understand your question.

Are you asking whether you should check the mesh sensitivity at the maximum Reynolds Number you are interested in? If so the answer is yes, you should check that condition if other Reynolds numbers goes into other regimes (eg transitional turbulence) then you should check them as well. But often just checking the highest Reynolds Number is OK, as that is likely to require the finest mesh.

aiden_1995 · November 17, 2022, 01:12

thanks for your quick reply.
sorry i will ask it on fluent forum i made a mistake again.

September 21, 2015, 10:06	Mesh independence study for transient simulation	#1
snejball91 New Member Tom Join Date: Jul 2015 Location: Munich, Germany Posts: 10 Rep Power: 10	Hello, I want to set up a simulation in transient mode and to get good results I want to do a mesh independence study. Do I have to do the mesh study in transient mode or can I select a value and do the study in steady mode? Thank you!

September 22, 2015, 03:11		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,705 Rep Power: 143	Yes, that is correct. If the transient flow is a boundary layer flow with a BL thickness of Xmm then over a bluff body, and if your steady state flow is also a BL with thickness about Xmm then over a bluff body - then a mesh sensitivity study on the steady state flow should be valid for the transient flow. But if your flow is inherently transient (eg some shock waves, free surface waves, natural convection) then you cannot use the steady state flow. bestniaz and 503131132 like this.

September 22, 2015, 06:20		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,705 Rep Power: 143	https://en.wikipedia.org/wiki/Boundary_layer_thickness bestniaz likes this.

November 17, 2022, 01:04	steady mesh study instead of transient for fsi case	#8
aiden_1995 Member naji Join Date: Nov 2022 Posts: 37 Rep Power: 3	HI guys im doing a fsi modelling which i should first do the mesh study and then continue ive been told i can do it in reynolds max condition,i dont know how? from which type of solution should i get max Reynolds? at first? and what does max Reynolds condition mean overally? just to enter max velocity and run the solution? how does it make difference?

November 17, 2022, 01:09		#9
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,705 Rep Power: 143	I do not understand your question. Are you asking whether you should check the mesh sensitivity at the maximum Reynolds Number you are interested in? If so the answer is yes, you should check that condition if other Reynolds numbers goes into other regimes (eg transitional turbulence) then you should check them as well. But often just checking the highest Reynolds Number is OK, as that is likely to require the finest mesh. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

September 21, 2015, 17:32		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,705 Rep Power: 143	You have to have a flow with the same characteristics as your intended flow. If you can do this with steady state then do so because it is much quicker.

September 22, 2015, 08:06		#7
snejball91 New Member Tom Join Date: Jul 2015 Location: Munich, Germany Posts: 10 Rep Power: 10	First of all: thank you for the information guys. My setup is actually the air flow through a diffuser with some impacts on velocity and pressure that can only be seen in a transient simulation. So I guess I have to do the mesh study in transient mode.

November 17, 2022, 01:12		#10
aiden_1995 Member naji Join Date: Nov 2022 Posts: 37 Rep Power: 3	thanks for your quick reply. sorry i will ask it on fluent forum i made a mistake again.

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