Compressible flow or incompressible flow equation?

  • Thread starter ulfaazmi
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  • #1
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Hello everyone..
I am using 2-dimension Navier Stokes equation, but I confused that my problem is compressible or incompressible flow form, because if I have initial pressure and temperature and velocity for x-axis only in one grid that they are so very high but the others grid are zero. Can I consider the equation is compressible flow??


Thank you
 

Answers and Replies

  • #5
bigfooted
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Then the flow is compressible.
But the effects of compressibility might be negligible. You should compute the highest local Mach number $$Ma=U/c$$ in your flow. If it is smaller than say 0.1, the effects of compressibility can be ignored and you can use the incompressible Navier Stokes equations. If it is higher than 0.3, then you should definitely use the compressible form.
 
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  • #6
cjl
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Then the flow is compressible. Can you provide more details on your difficulty?
Not necessarily. Gas flows can be simplified as incompressible depending on the details - you should look at pressure variation thoughout your flowfield, as well as local mach numbers. If your pressure variations are small relative to the overall static pressure and the local mach number never exceeds 0.3 or so, you can reasonably treat the problem as incompressible.
 
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  • #7
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But the effects of compressibility might be negligible. You should compute the highest local Mach number $$Ma=U/c$$ in your flow. If it is smaller than say 0.1, the effects of compressibility can be ignored and you can use the incompressible Navier Stokes equations. If it is higher than 0.3, then you should definitely use the compressible form.
Thank you for your help, actually the driving force of flow is caused by pressure. So, initially the flow is in a steady state in a closed horizontal pipe until pressure is introduced into that flow caused by a high speed projectile hits the wall of pipe. It also affecting temperature becomes higher at the area exactly in the center of the left side of pipe. Can I guess the velocity can be obtained from the equation :
P = ρ/U ?
 
  • #8
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Not necessarily. Gas flows can be simplified as incompressible depending on the details - you should look at pressure variation thoughout your flowfield, as well as local mach numbers. If your pressure variations are small relative to the overall static pressure and the local mach number never exceeds 0.3 or so, you can reasonably treat the problem as incompressible.
Thank you for your help
Then the flow is compressible. Can you provide more details on your difficulty?
Thank you for your help, actually I have problem to get the correct equation of my study. I am difficult to converting the equation into the non dimensional system because the condition is not clear yet. If it doesn't bother you, would you like to check my equation, I will send it in a picture because it is difficult to write here, also include with the illustration of my problem??
 
  • #9
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Thank you for your help

Thank you for your help, actually I have problem to get the correct equation of my study. I am difficult to converting the equation into the non dimensional system because the condition is not clear yet. If it doesn't bother you, would you like to check my equation, I will send it in a picture because it is difficult to write here, also include with the illustration of my problem??
Sure. Let's see what you got. Just upload it.
 
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  • #10
cjl
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An illustration of your problem would be very helpful for us to determine the best approach to take.
 
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  • #12
cjl
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That link doesn't work - it says access denied. You could try uploading to a site like imgur or something, or adding an attachment here...
 
  • #13
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That link doesn't work - it says access denied. You could try uploading to a site like imgur or something, or adding an attachment here...
I am so sorry about that. Actually, I did not find any tool to send an attachment directly from here. So, as you mentioned like "imgur", I tried to upload my images there, and this is the link "https://imgur.com/a/zFtoldc". I hope it can be loaded.
Thank you very much.
 
  • #14
cjl
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So what exactly are you trying to calculate? The particle is labeled as traveling at 6.5km/s (!), which puts it solidly in the hypersonic flow regime, and compressibility will be necessary.
 
  • #15
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So what exactly are you trying to calculate? The particle is labeled as traveling at 6.5km/s (!), which puts it solidly in the hypersonic flow regime, and compressibility will be necessary.
I want to get the result of velocity, pressure, and temperature after iteration until reaching steady state and make a simulation using Fortran. But, I don't know whether the illustration of horizontal pipe is correct or not, because it is nonsymmetric. If I illustrate in a half of pipe like the third picture, is it right?
 

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