Effect of Mesh Density on 1 2D flow profile

hinterland1 · August 17, 2017, 05:58

Hi all, I have just created a simple 2-d case.

Inlet: 0.01m/s velocity inlet
Outlet: 0Pa pressure outlet
Walls: No Slip, Non Moving

Convergence criteria 1e-4

Now the thing is, I am using the same conditions, and varying the mesh density. I realised that the higher the mesh, the higher the deviation from a parabolic flow profile.

This is the compiled graph of the mesh density vs outlet flow profile.

What is happening here? Shouldn't a higher mesh density result in a higher accuracy (parabolic outlet flow) instead of a flat one?

hinterland1 · August 18, 2017, 01:46

Bump.

Does anyone actually know why this phenomenon is so???

CeesH · August 18, 2017, 05:05

Is the velocity 0.1 (as mentioned in picture) or 0.01 m/s?

If it is 0.1, the entrance length of the flow is approx. 0.05*Re*velocity = 50 cm (Re = 1000), so your flow is not fully developed - hence the flattened profile at the exit.

juliom · August 18, 2017, 12:08

What makes you think it is wrong? What is the Reynolds number? What location are you using to take the profile? These should be the questions you might be asking yourself.
It seems that you are taking the profile at a location where the conditions are turbulent. Look at the wall the gradient looks steeper there.

LuckyTran · August 18, 2017, 21:22

What makes you think the parabolic profile is the answer? You have developing flow in a pipe. It's only parabolic once it is fully developed.

With increasing mesh density, you better resolve real gradients and have less numerical diffusion. Hence, with a better mesh you correctly get the non-parabolic profile. With a coarse mesh, the numerical diffusion increases causes the flow to false approach the parabolic profile faster.

August 17, 2017, 05:58	Effect of Mesh Density on 1 2D flow profile	#1
hinterland1 New Member HINTERLAND Join Date: Aug 2017 Posts: 3 Rep Power: 9	Hi all, I have just created a simple 2-d case. Inlet: 0.01m/s velocity inlet Outlet: 0Pa pressure outlet Walls: No Slip, Non Moving Convergence criteria 1e-4 Now the thing is, I am using the same conditions, and varying the mesh density. I realised that the higher the mesh, the higher the deviation from a parabolic flow profile. This is the compiled graph of the mesh density vs outlet flow profile. What is happening here? Shouldn't a higher mesh density result in a higher accuracy (parabolic outlet flow) instead of a flat one?

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August 18, 2017, 01:46		#2
hinterland1 New Member HINTERLAND Join Date: Aug 2017 Posts: 3 Rep Power: 9	Bump. Does anyone actually know why this phenomenon is so???

August 18, 2017, 05:05		#3
CeesH Senior Member Cees Haringa Join Date: May 2013 Location: Delft Posts: 607 Rep Power: 0	Is the velocity 0.1 (as mentioned in picture) or 0.01 m/s? If it is 0.1, the entrance length of the flow is approx. 0.05Revelocity = 50 cm (Re = 1000), so your flow is not fully developed - hence the flattened profile at the exit.

August 18, 2017, 12:08		#4
juliom Senior Member Julio Mendez Join Date: Apr 2009 Location: Fairburn, GA. USA Posts: 290 Rep Power: 18	What makes you think it is wrong? What is the Reynolds number? What location are you using to take the profile? These should be the questions you might be asking yourself. It seems that you are taking the profile at a location where the conditions are turbulent. Look at the wall the gradient looks steeper there.

August 18, 2017, 21:22		#5
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,764 Rep Power: 66	What makes you think the parabolic profile is the answer? You have developing flow in a pipe. It's only parabolic once it is fully developed. With increasing mesh density, you better resolve real gradients and have less numerical diffusion. Hence, with a better mesh you correctly get the non-parabolic profile. With a coarse mesh, the numerical diffusion increases causes the flow to false approach the parabolic profile faster.