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OmkarJ
Posts: 406
Registered: ‎10-02-2012

Advection schemes

547 Views, 15 Replies
12-04-2012 01:37 AM

I pick this thread from one of my recent experiences. Typically, first order advection schemes (ADV1) and second order accurate advection schemes (ADV5) would produce different results because of artificial viscosity in ADV1. But would these results be very much affected ONLY while simulating distributed resistance, which models a negative pressure gradient zone? Or do these schemes always produce significantly different results for ALL types of physics, viz. incompressible turbulent flow, heat transfer, compressible flow etc.?

 

Regards

OJ

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*Expert Elite*
OmkarJ
Posts: 406
Registered: ‎10-02-2012

Re: Advection schemes

12-05-2012 03:56 AM in reply to: OmkarJ

Anyone, please?

 

Regards

OJ

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*Expert Elite*
OmkarJ
Posts: 406
Registered: ‎10-02-2012

Re: Advection schemes

01-14-2013 06:36 AM in reply to: OmkarJ

Someone?

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wildej
Posts: 726
Registered: ‎08-25-2011

Re: Advection schemes

01-15-2013 01:29 AM in reply to: OmkarJ

We would recommend switching away from ADV1 and to ADV5 for many purposes now. Standard incompressible flow should be OK but ADV5 will be better for compressible and pressure driven flows, those with resistances as you rightly say and also for many heat transfer calculations now too.



Jon Wilde
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Active Contributor
ssenbore
Posts: 34
Registered: ‎11-25-2013

Re: Advection schemes

05-13-2014 10:47 AM in reply to: wildej
I was about to open a new forum poost but I guess it can go here.

I am simulating a Venturri nozzle as a flow measuring device, and when I use ADV5, the pressure difference comes out ~5 psi less than if I use ADV1. This difference is significant (~ 25% of the entire pressure drop across the nozzle).

Which one is more accurate than the other? When would it be more advisable to switch to AdV5 vs ADV1? Or would ADV4 be more appropriate?

If this helps, my setup is:

1018 psi absolute at the inlet

9 million lbm/hr of water at the outlet

Mesh adaptation is used to solve to 99% mesh independence in both cases.

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srhusain
Posts: 55
Registered: ‎04-02-2014

Re: Advection schemes

05-13-2014 11:00 AM in reply to: ssenbore

Some points to consider here:

  1. You should be applying a flow rate at the inlet and pressure at the outlet as a a standard setup. Any particular reason why you have it the other way around?
  2. Based on a pressure of 1000 psi, 5 psi is a small discrepancy
  3. Beyond that, I would recommend Adv5 over Adv1as it is more stable
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Active Contributor
ssenbore
Posts: 34
Registered: ‎11-25-2013

Re: Advection schemes

05-13-2014 11:14 AM in reply to: srhusain

Some points to consider here:

 
You should be applying a flow rate at the inlet and pressure at the outlet as a a standard setup. Any particular reason why you have it the other way around?
I know the inlet pressure and mass flow rate.  There is a head loss across the venturri nozzle that I dont know.  Hence I thought it was wiser to put the pressure at the inlet.

Based on a pressure of 1000 psi, 5 psi is a small discrepancy
True.  But for the Pressure Drop across the big and small diameter sections of the venturri, the lab data is 16.7 psi, so a variatioin of 5 psi in pressure drop is HUGE.
 
Beyond that, I would recommend Adv5 over Adv1as it is more stable
Thanks.  Would ADV4 be a consideration? I know the help says that its specially tuned for flows in long narrow ducts.  This nozzle is 5 ft long, with a large dia of 1.5 ft and small dia of 1 ft.  Does that translate to 'narrow' in CFD terms?
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Employee
srhusain
Posts: 55
Registered: ‎04-02-2014

Re: Advection schemes

05-13-2014 11:24 AM in reply to: ssenbore

You are correct about the pressure drop issue.

Based on the dimensions you mentioned, the model should be amenable to analysis with Adv5

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Valued Contributor
nhahn
Posts: 87
Registered: ‎01-18-2006

Re: Advection schemes

05-13-2014 11:35 AM in reply to: ssenbore
In this case then, would it make more sense to set the scenario environment to 1018 psi, the inlet to specified mass flow, and the outlet to zero psi gage? Or does this end up the same (I wonder). Make sure your material properties are appropriate for the elevated pressure, too.

The bigger issue here is that based on your scale, and the fact that you are interested in the losses due to the nozzle (not in the pressure distribution in the flowfield), what you really need to be interested in is getting the viscous losses right. So focus on good meshing in the boundary layer, make sure y+ is low (enable y+ adaptation) and choose advection scheme to minimize artificial dissipation -- ADV 1 is out, ADV 5 better. Maybe even ADV3 ?

Finally, make sure you know where/how the lab data was gathered - it can't be an average across the exit plane like you can do in CFD - so if it was a pitot tube try to use a results point the same location, if not on centerline.
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Employee
srhusain
Posts: 55
Registered: ‎04-02-2014

Re: Advection schemes

05-13-2014 11:46 AM in reply to: nhahn

Hi:

 

These are good points.

 

One thing about incompressible flow (unless you are concerned or on the lookout for cavitation), is that the pressure is not a thermodynamic quantity and is purely mechanical in nature. So, when density is fixed (frequently a good idealization), the boundary condition for pressure is largely a relative value and the interesting result is the variation of pressure itself (such as a drop or a rise) within the model.

 

In the end, it depends on your judgement and the assumptions you want to make regarding the material properties when setting up the problem.

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