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paul175
Posts: 14
Registered: ‎08-15-2013

trouble changing scenario to compressable flow

125 Views, 11 Replies
08-15-2013 12:04 PM

as an incompressible ... things worked and seemed to make sense,

but the air was moving at mach 2.6 ... hence the switch to compressible

 

fluid is air ... 1 inlet 9 outlets. 

boundary conditions were:

inlet: piecewise linear temperature and pressure over 2 milliseconds

outlet: 0 pressure

 

changes for compressibility:

set air to 'variable'

select 'compressible' in solver dialog

boundary conditions are:

inlet: total temperature and total pressure and mass flow

outlet: pressure 0, and 'unknown'

 

simulation gets to the second time step and crashes ...

 

I read the paper on interior compresibles and didn't catch my mistake.

What did miss ?

 

/Steven

 

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Product Support
Royce_adsk
Posts: 645
Registered: ‎08-24-2011

Re: trouble changing scenario to compressable flow

08-15-2013 12:22 PM in reply to: paul175

Hi Paul,

 

It seems to me that you have overconstrained your model a bit with the compressible setup.

 

Review the suggest model setup guidelines here:

http://wikihelp.autodesk.com/Simulation_CFD/enu/2014/Help/0360-Learning360/0400-Simulati400/0435-Com...

 

My suggestion based on what you wrote would be to remove the mass flow rate and the unknown boundary conditions and give it another shot.  That would make the model setup jive with the 1st bullet point model setup in the above link.



Royce Abel
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Contributor
paul175
Posts: 14
Registered: ‎08-15-2013

Re: trouble changing scenario to compressable flow

08-15-2013 01:26 PM in reply to: Royce_adsk

if I remove the mass fow rate, the summary begins with "0 inlets" and the rest is crap ...

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Royce_adsk
Posts: 645
Registered: ‎08-24-2011

Re: trouble changing scenario to compressable flow

08-15-2013 02:00 PM in reply to: paul175

Hi Paul,

 

The summary files labels ALL pressure boundary conditions are 'Outlets' even though they very well could be inlets.  It is just book post-processing keeping and not important.

 

What does 'rest is crap' in regards to?

 

-Royce



Royce Abel
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Contributor
paul175
Posts: 14
Registered: ‎08-15-2013

Re: trouble changing scenario to compressable flow

08-15-2013 04:22 PM in reply to: Royce_adsk

I am referring to the 'Messages' tab in the simulator, where it does differentiate between inputs and outputs.

Despite inititial conditions for the air volume of pressure=0, velocity=(0,0,0) and temp=72 ... I am seeing pressures (psi) in the millions, velocities (in/sec) in the bilions, and temperatures of absolute zero -- -459F .

the simulation seems very confused ... or is it me ?


Since the first message I have replaced the total pressure constant with a piecewise linear from 13601 to 0(psi); total temperature constant with a piecewise linear from 2092 to 22 (degrees C); and the mass flow constant with a piecewise linear from 1.75 to 0 (kg/s). The simulator still crashes in the first few iterations ... and reports bizarre values.

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Contributor
paul175
Posts: 14
Registered: ‎08-15-2013

Re: trouble changing scenario to compressable flow

08-15-2013 04:34 PM in reply to: paul175

maybe its my computations of the new boundary conditions...

 

input temp = 3361F, pressure = 8000psi, velocity = 2970 ft/s
so speed of sound = 3190 ft/s ... and mach number = 0.9311
from nasa I got a gamma of 1.3028622041 for 3361F and 8000psi
I get a total temp of 2092.295673C and a total pressure of 13600.77257psi
mass flow is 1.75 grams in 2 milliseconds -- or .875 kg/s

by 2 milliseconds temp and pressure are back to ambient and flow==0

 

I'm setting these as piecewise linear boundary conditions. And yet ...

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Contributor
paul175
Posts: 14
Registered: ‎08-15-2013

Re: trouble changing scenario to compressable flow

08-15-2013 05:02 PM in reply to: paul175

this help ?

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Product Support
Royce_adsk
Posts: 645
Registered: ‎08-24-2011

Re: trouble changing scenario to compressable flow

08-16-2013 06:36 AM in reply to: paul175
I was just going to ask for that. I'll check it out later today.

Thanks!


Royce Abel
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Product Support
Royce_adsk
Posts: 645
Registered: ‎08-24-2011

Re: trouble changing scenario to compressable flow

08-16-2013 02:17 PM in reply to: Royce_adsk

Can we make this 1/4 symmetric?

Also, could we move the projectile to be placed outside of the fluid domain in CAD.  We will use the modtion initial position settings to place it in the barrel.  this will allow us to turn on mesh enhancement inside of the barrel.

 



Royce Abel
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Product Support
Royce_adsk
Posts: 645
Registered: ‎08-24-2011

Re: trouble changing scenario to compressable flow

08-16-2013 03:07 PM in reply to: Royce_adsk

Just to give you an idea of how I am troubleshooting this problem. I am first turning OFF the transient motion portion of the setup and will work on getting the problem solving the compressible flow transiently to determine the needed mesh and solver configurations for this to work.

 

First Stab.png

 



Royce Abel
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