## Simulation CFD

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# Advection schemes

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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

# Re: Advection schemes

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Anyone, please?

Regards

OJ

# Re: Advection schemes

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Someone?

# Re: Advection schemes

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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**

# Re: Advection schemes

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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.

# Re: Advection schemes

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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?
- Based on a pressure of 1000 psi, 5 psi is a small discrepancy
- Beyond that, I would recommend Adv5 over Adv1as it is more stable

# Re: Advection schemes

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Some points to consider here:

Based on a pressure of 1000 psi, 5 psi is a small discrepancy

# Re: Advection schemes

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You are correct about the pressure drop issue.

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

# Re: Advection schemes

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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.

# Re: Advection schemes

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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.