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Flow Anomaly Near Boundaries

Question asked by Joel Turco on Apr 10, 2011
Latest reply on Apr 16, 2011 by Rich Bayless

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G'day,

We are running a number of external flow scenarios and continue having issues with the flow results near the boundaries of the computational domain.

Our system is fairly simple, we are trying to analyse subsea flow over a simplified seabed and around block shaped objects.

We can't understand why the velocity is speeding up near the domain edges (see attached picture).

Our input velocity is in the X-Direction and no matter what velocity we have the same problem.

Any help on this matter would be much appreciated.

Thank you!

Joe Galliera

Apr 16, 2011 12:32 AM

Correct Answer

We solved their problem offline... here is the resolution.

Joel,

I think that the manual pressure dependency (151 kPa to 101 kPa from 0 m at sea floor to 4.5 m at top of sea) for ambient that you defined makes the difference. I set it as a pressure potential and put the pressure that is at the origin (see picture); it figures out the pressure at the top for you after running a few iterations. Now it only gets to 110 kPa (instead of 101 kPa) but that could possibly be caused by your own calcs for pressure at the sea floor accounts for sea water with salinity, thus here you might need to define a new water fluid with different density to account for the difference.

See the reply in context

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Bill McEachern Apr 11, 2011 8:51 AM

What exactly are you doing? You are obviously running a transient analysis. at the time given on the plot is 13.5 seconds. what is the time varying parameter (BC?) and what is the objective of the analysis? I take it, it is an external analysis. If so what is the free stream velocity set as the BC?

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Joel Turco @ Bill McEachern on Apr 11, 2011 9:02 PM

I am trying to analyse the flow profile in and around this block-like arrangement underwater. Also, I will look to determined the pressures that the block faces are exposed to under various conditions. I have modelled the seabed as a planar surface with a specified roughness and the blocks sit on top of it. The botom of the computational domain is flush against the bottom of the planar surface, such that the entire upper face of this planar surface (the 'seabed') is contained within the computational domain.

Initially, I was running steady state analyses and encountered the same issue with flow near the walls of the computational domain. However, I thought I would try a transient analysis to a) see if it made any difference, and b) allow for fluctuating inlet velocities down the track. In this scenario, the free stream velocity is constant at approx 5.3 m/s in the x-direction.

Thanks again for your help.

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Joe Galliera

Apr 11, 2011 5:26 PM

Could it be perhaps that you are modeling as a solid the outer boundary, like a wind tunnel? For an external flow, you do not model the boundaries but the external envelope is determined by the computational domain. By simply sharing your files with you VAR, they would be able to help you to clear up this issue.

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Joel Turco @ Joe Galliera on Apr 11, 2011 9:09 PM

I thought I may have inadvertently done that at first but I don't think that is the case. I used the external flow option and simply extended the computational domain to an appropriate size.

I will contact my VAR, as you said. Thanks!

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Joel Turco @ Joe Galliera on Apr 14, 2011 10:59 PM

Thanks for your help so far. We have experimented with increasing and decreasing the size of the computational domain. The best results we have obtained so far are achieved when the bottom of the computational domain is set below the seabed surface. Therefore, the surface of the seabed is within the computational domain.

The unusual results that appeared at the long sides of the domain are resolved by making them symmetry boundaries. However, making the downstream boundary a symmetry condition does not fix the problem and gives very unrealistic results.

We are still experiencing stagnation points at the bottom of the inlet boundary and a lot of stagnation at the downstream boundary, as shown in the images attached.

Once again, any advice would be much appreciated.

We have contacted Solidworks and they are having similar issues.

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Joe Galliera

@ Joel Turco on Apr 14, 2011 11:15 PM

Weird. I could possibly remodel what I see in your pics, but I'm sure I'll miss some detail from your model. Can you share with me? My email address is in my user profile.

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Joe Galliera

@ Joe Galliera on Apr 16, 2011 12:32 AM

We solved their problem offline... here is the resolution.

Joel,

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Image1.jpg30.4 KB

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Rich Bayless @ Joe Galliera on Apr 16, 2011 8:11 AM

Hello Joe,

Can you post a simple model (or three) to show what you did?

How to model hydrostatic pressure and gravity flow has come up before, here:

https://forum.solidworks.com/message/168367#168367

Clicking on help or searching the tutorials at the time didn't shed much light on what was happening. Seems to me that this is one area where three or four examples would really help.

Thanks, Rich.

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8 Replies

Bill McEachern Apr 11, 2011 8:51 AM
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Correct Answer
What exactly are you doing? You are obviously running a transient analysis. at the time given on the plot is 13.5 seconds. what is the time varying parameter (BC?) and what is the objective of the analysis? I take it, it is an external analysis. If so what is the free stream velocity set as the BC?
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- Joel Turco @ Bill McEachern on Apr 11, 2011 9:02 PM
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  Correct Answer
  I am trying to analyse the flow profile in and around this block-like arrangement underwater. Also, I will look to determined the pressures that the block faces are exposed to under various conditions. I have modelled the seabed as a planar surface with a specified roughness and the blocks sit on top of it. The botom of the computational domain is flush against the bottom of the planar surface, such that the entire upper face of this planar surface (the 'seabed') is contained within the computational domain.
  
  Initially, I was running steady state analyses and encountered the same issue with flow near the walls of the computational domain. However, I thought I would try a transient analysis to a) see if it made any difference, and b) allow for fluctuating inlet velocities down the track. In this scenario, the free stream velocity is constant at approx 5.3 m/s in the x-direction.
  
  Thanks again for your help.
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Joe Galliera Apr 11, 2011 5:26 PM
Mark Correct
Correct Answer
Could it be perhaps that you are modeling as a solid the outer boundary, like a wind tunnel? For an external flow, you do not model the boundaries but the external envelope is determined by the computational domain. By simply sharing your files with you VAR, they would be able to help you to clear up this issue.
Like • Show 0 Likes0
Actions
- Joel Turco @ Joe Galliera on Apr 11, 2011 9:09 PM
  Mark Correct
  Correct Answer
  I thought I may have inadvertently done that at first but I don't think that is the case. I used the external flow option and simply extended the computational domain to an appropriate size.
  
  I will contact my VAR, as you said. Thanks!
  Like • Show 0 Likes0
  Actions
- Joel Turco @ Joe Galliera on Apr 14, 2011 10:59 PM
  Mark Correct
  Correct Answer
  Thanks for your help so far. We have experimented with increasing and decreasing the size of the computational domain. The best results we have obtained so far are achieved when the bottom of the computational domain is set below the seabed surface. Therefore, the surface of the seabed is within the computational domain.
  
  The unusual results that appeared at the long sides of the domain are resolved by making them symmetry boundaries. However, making the downstream boundary a symmetry condition does not fix the problem and gives very unrealistic results.
  
  We are still experiencing stagnation points at the bottom of the inlet boundary and a lot of stagnation at the downstream boundary, as shown in the images attached.
  
  Once again, any advice would be much appreciated.
  We have contacted Solidworks and they are having similar issues.
  Attachments
  - Foundation Permeability_4.jpg219.2 KB
  - Foundation Permeability_3.jpg142.1 KB
  - Foundation Permeability.jpg112.4 KB
  - Foundation Permeability_1.jpg163.5 KB
  - Foundation Permeability_2.jpg100.8 KB
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  - Joe Galliera @ Joel Turco on Apr 14, 2011 11:15 PM
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    Weird. I could possibly remodel what I see in your pics, but I'm sure I'll miss some detail from your model. Can you share with me? My email address is in my user profile.
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    - Joe Galliera @ Joe Galliera on Apr 16, 2011 12:32 AM
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      Correct Answer
      We solved their problem offline... here is the resolution.
      
      Joel,
      I think that the manual pressure dependency (151 kPa to 101 kPa from 0 m at sea floor to 4.5 m at top of sea) for ambient that you defined makes the difference. I set it as a pressure potential and put the pressure that is at the origin (see picture); it figures out the pressure at the top for you after running a few iterations. Now it only gets to 110 kPa (instead of 101 kPa) but that could possibly be caused by your own calcs for pressure at the sea floor accounts for sea water with salinity, thus here you might need to define a new water fluid with different density to account for the difference.
      Attachments
      Image1.jpg30.4 KB
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      - Rich Bayless @ Joe Galliera on Apr 16, 2011 8:11 AM
        Mark Correct
        Correct Answer
        Hello Joe,
        
        Can you post a simple model (or three) to show what you did?
        
        How to model hydrostatic pressure and gravity flow has come up before, here:
        
        https://forum.solidworks.com/message/168367#168367
        
        Clicking on help or searching the tutorials at the time didn't shed much light on what was happening. Seems to me that this is one area where three or four examples would really help.
        
        Thanks, Rich.
        Like • Show 0 Likes0
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Flow Anomaly Near Boundaries

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