Incorrect pressure calculation at inlet

Floszwa · October 9, 2014, 11:01

Hello,

I have encountered a problem during my simulation of a particle in a laminar tube flow.

I try to simulate the equilibrium position of a particle for Re = 500 (pipe Reynolds number). To simulate the movement of the particle, the flow at the wall has the negative value of the particle velocity. At the inlet is a parabolic flow profile minus the particle velocity. The velocity along the pipe is shown in the following picture:
flow profile.JPG

First I did the same simulation with a smaller particle and the results were as expected. The following picture shows the pressure development in the pipe. The inlet is in the upper right corner and the pressure does not change there.
smaller particle.JPG

After I raised the diameter of the particle, the results are different as shown in the following picture:
larger particle.JPG
As you can see, the pressure for the inlet changes along the x axis although only the diameter of the particle was changed.

I moved the inlet away from the particle (as shown in the picture for the larger particle), but nothing changed. So does anyone have an idea why the pressure at the inlet changes and how I can solve this problem?

Thank you in advance.

Best regards!
Florian

ghorrocks · October 9, 2014, 19:04

Have you checked your outlet boundary is far enough away from the sphere? Is the simulation fully converged?

Floszwa · October 16, 2014, 11:35

The simulation was fully converged.

I moved the outlet farther away from the particle (I doubled the distance between the particle and the outlet) and repeated the simulations, but the results stayed the same. So the outlet boundary should be far enough away from the particle.

ghorrocks · October 16, 2014, 19:21

What do the streamlines look like? Is there any sign of a recirculation?

Floszwa · October 22, 2014, 08:24

Hey,

I made a picture of the streamlines.
Stream lines.JPG
There seems to be a recirculation.

ghorrocks · October 22, 2014, 18:43

Then that is the problem. Your boundary condition is too close. You need to move your boundary further away so the flow is entirely in one direction.

(There are other solutions but this is the easiest and often the best)

Floszwa · October 30, 2014, 09:38

In this case the particle Reynolds number is clearly below 1. For such a low particle Reynolds number the particle won't effect the flow this way, if the boundarys are already that far away from the particle.

ghorrocks · October 30, 2014, 17:56

But your image clearly shows lots of recirculations. Are you sure it is converged? Some of the recirculations appear to be a distance away from the particle - what is generating those?

murx · November 6, 2014, 12:19

Hey Glenn,

Thanks for your effort! I work with Floszwa on this problem and just want to point out some things that were not made really clear so far.

The simulation is carried out in the frame of reference of the sphere/particle. Thus, the velocity profile is shifted and there is inflow and outflow at both boundaries, see picture 1.

Even for simulations that show physical results (those carried out for lower pipe Reynolds numbers ~ 100), some streamlines turn at the particle and exit through the same boundary as they entered. However... this seems physically meaningful to me as the inflow velocity of these streamlines is close to zero. I attached screenshots of the streamlines and corresponding pressure profiles for the working simulation (bottom, Re_pipe = 100) and the simulation not giving physical results (top, Re_pipe = 500), see picture 2.

The flow can be regarded as almost creeping (Re_particle < 1). To my understanding there should not be a recirculation (in terms of vortices forming behind the prticle). Even if there is, for those low particle reynolds numbers, I would expect the recirculations to fade in a distance in the order of magnitude of one particle diameter. We moved the outlet as far as ~40 particle diameters away. The simulations are converged. Do still you think we should go even further?

ghorrocks · November 6, 2014, 17:05

Quote:

The simulations are converged.

I doubt that. What you are seeing looks just like an inadequately converged simulation. Why do you say it is converged?

murx · November 7, 2014, 05:11

What I meant is that the residuals decreased by several orders of magnitude and they do not decrease further when the claculation is carried on. You're right, that does not necessarily mean it is converged to the real solution or rather to the real solution of the problem we try to model.

But why is that?

The RMS-residuals are in the order of 10^-7. For lower Reynolds numbers, we could get physical results even at significantly higher residuals.

October 9, 2014, 11:01	Incorrect pressure calculation at inlet	#1
Floszwa New Member Florian Join Date: May 2013 Posts: 12 Rep Power: 13	Hello, I have encountered a problem during my simulation of a particle in a laminar tube flow. I try to simulate the equilibrium position of a particle for Re = 500 (pipe Reynolds number). To simulate the movement of the particle, the flow at the wall has the negative value of the particle velocity. At the inlet is a parabolic flow profile minus the particle velocity. The velocity along the pipe is shown in the following picture: flow profile.JPG First I did the same simulation with a smaller particle and the results were as expected. The following picture shows the pressure development in the pipe. The inlet is in the upper right corner and the pressure does not change there. smaller particle.JPG After I raised the diameter of the particle, the results are different as shown in the following picture: larger particle.JPG As you can see, the pressure for the inlet changes along the x axis although only the diameter of the particle was changed. I moved the inlet away from the particle (as shown in the picture for the larger particle), but nothing changed. So does anyone have an idea why the pressure at the inlet changes and how I can solve this problem? Thank you in advance. Best regards! Florian

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October 9, 2014, 19:04		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Have you checked your outlet boundary is far enough away from the sphere? Is the simulation fully converged?

October 16, 2014, 11:35		#3
Floszwa New Member Florian Join Date: May 2013 Posts: 12 Rep Power: 13	The simulation was fully converged. I moved the outlet farther away from the particle (I doubled the distance between the particle and the outlet) and repeated the simulations, but the results stayed the same. So the outlet boundary should be far enough away from the particle.

October 16, 2014, 19:21		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	What do the streamlines look like? Is there any sign of a recirculation?

October 22, 2014, 08:24		#5
Floszwa New Member Florian Join Date: May 2013 Posts: 12 Rep Power: 13	Hey, I made a picture of the streamlines. Stream lines.JPG There seems to be a recirculation.

October 22, 2014, 18:43		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Then that is the problem. Your boundary condition is too close. You need to move your boundary further away so the flow is entirely in one direction. (There are other solutions but this is the easiest and often the best)

October 30, 2014, 09:38		#7
Floszwa New Member Florian Join Date: May 2013 Posts: 12 Rep Power: 13	In this case the particle Reynolds number is clearly below 1. For such a low particle Reynolds number the particle won't effect the flow this way, if the boundarys are already that far away from the particle.

October 30, 2014, 17:56		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	But your image clearly shows lots of recirculations. Are you sure it is converged? Some of the recirculations appear to be a distance away from the particle - what is generating those?

November 7, 2014, 05:11		#11
murx Member Max Join Date: May 2011 Location: old europe Posts: 88 Rep Power: 15	What I meant is that the residuals decreased by several orders of magnitude and they do not decrease further when the claculation is carried on. You're right, that does not necessarily mean it is converged to the real solution or rather to the real solution of the problem we try to model. But why is that? The RMS-residuals are in the order of 10^-7. For lower Reynolds numbers, we could get physical results even at significantly higher residuals.