CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > General Forums > Main CFD Forum

Compare two simulations with same friction Reynolds and different bulk Reynolds

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree6Likes
  • 3 Post By sbaffini
  • 1 Post By FMDenaro
  • 2 Post By LuckyTran

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 3, 2020, 11:15
Default Compare two simulations with same friction Reynolds and different bulk Reynolds
  #1
Senior Member
 
Join Date: Jan 2018
Posts: 123
Rep Power: 6
Moreza7 is on a distinguished road
Hi,
I have simulated a duct flow using LES with Bulk Reynolds number = 2800 and friction Reynolds number=160.
There's a DNS data with bulk Reynolds number = 2500 and friction Reynolds number = 160.

Can I compare these two results or they should have the same Bulk Reynolds number? Can I scale the results?
Moreza7 is offline   Reply With Quote

Old   March 3, 2020, 12:04
Default
  #2
Senior Member
 
sbaffini's Avatar
 
Paolo Lampitella
Join Date: Mar 2009
Location: Italy
Posts: 1,948
Blog Entries: 29
Rep Power: 36
sbaffini will become famous soon enoughsbaffini will become famous soon enough
Send a message via Skype™ to sbaffini
Well, you can compare them, but only one of them is probably correct, if any. Guess which one I'm referring to?

However, it is typical for LES simulations of wall bounded turbulent flows to produce higher mean velocities for given pressure drops, when low order methods (<3) are used. In contrast, higher order methods tend to produce lower mean velocities.

The culprit here is that, ideally LES is a model and DNS is correct. Your model is failing here. Is it important? It depends from what you actually want to achieve, what is the scope of your analysis. If the result is too off you probably have to refine your grid at the wall (probably spanwise).

If you scale the results you simply obtain the correct Re number but now with the wrong friction Re number. There is no escape from this.
FMDenaro, pcosta and Moreza7 like this.
sbaffini is offline   Reply With Quote

Old   March 3, 2020, 12:11
Default
  #3
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,198
Rep Power: 66
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
As addressed by Paolo, fixing the pressure driving force for a prescribed Re_tau number can produce quite different results in terms of the bulk Re number.

For this reason I suggest to get a solution without any SGS model to assess the effect of this latter on the averaged velocity.
Moreza7 likes this.
FMDenaro is offline   Reply With Quote

Old   March 3, 2020, 12:21
Default
  #4
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,071
Rep Power: 60
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
I just want to add that (assuming constant viscosity), for fully developed flow in a duct, the bulk Reynolds number and friction Reynolds number are proportional to one another via the friction factor.

One way to interpret this is, if you target one (either the bulk or friction Reynolds number) and you also get the right friction factor, then you will match the other one. That is, if you match one of the Reynolds numbers with the right pressure drop, you also match the other Reynolds number.

So if you try to target the same friction Reynolds number, the reason you don't also have the same bulk Reynolds number is because your solution is slightly different for whatever reason. It's not an error per se, just that your model produces a slightly different result.
sbaffini and Moreza7 like this.
LuckyTran is offline   Reply With Quote

Reply

Tags
large eddy simulation

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On



All times are GMT -4. The time now is 19:01.