CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

Basic Understanding

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 5, 2009, 02:55
Default Basic Understanding
  #1
Abduri
Guest
 
Posts: n/a
Hi Guys,

I want to model a supersonic flight at altitude of 82km. Free stream conditions up there are:

- static pressure 4.6Pa - static temperature 217K - density 7.31e-5 kg m^-3

I take these values as inlet condition in pre (static temperature and static pressure). I found out to set a domain reference pressure of 1 bar is good. With 0 bar reference pressure, the calculation crashes directly.

When I see the post results, freestream temperature and pressure values are as set as in inlet.

But my freestream density is being calculated according to the reference pressure with the ideal gas equation, so not static pressure at inlet.

Is my model still correct? Is my flight going at low density (10Pa) or atmospheric density (1bar)?

Thanks in advance!
  Reply With Quote

Old   March 5, 2009, 06:25
Default Re: Basic Understanding
  #2
johnny
Guest
 
Posts: n/a
Note that the absolute pressure at the inlet = reference pressure + pressure specified for inlet. So if you are trying to set 4.6 Pa at the inlet, when you specify a reference pressure of 1 bar you should set the inlet pressure to -999 995.4 Pa.

The 0 bar reference pressure case should be the correct setup. However, I'm not sure that at such low densities that the assumption that the fluid is a continuum is true. You may want to check a fluid text to see if your free stream conditions are considered rarefied flows.
  Reply With Quote

Old   March 5, 2009, 08:25
Default Re: Basic Understanding
  #3
Abduri
Guest
 
Posts: n/a
Are you sure about that? Aren't the results expect for the values of the density still correct?

Because: Values for the freestream pressure and temperature still seem to make sense. Except for the value of freestream density. Additionally, at every point of the freestream the ideal gas theorem is violated because the density is much too high. How is this possible?
  Reply With Quote

Old   March 5, 2009, 22:20
Default Re: Basic Understanding
  #4
Glenn Horrocks
Guest
 
Posts: n/a
Hi,

Why don't you set the ambient pressure (4.6 Pa) as the reference pressure and specify pressure relative to this. This is how the reference pressure is meant to work.

Regards, Glenn Horrocks
  Reply With Quote

Old   March 5, 2009, 22:58
Default Re: Basic Understanding
  #5
Abduri
Guest
 
Posts: n/a
Hi Johnny,

I have tried what you said. So I kept reference pressure at 1 bar and inlet static pressure at -99.995Pa to have 5Pa at freestream.

What I got, are same values as when I set inlet static pressure at 5Pa without reducing it by 1 bar. Fresstream pressure, temperature and density did not change. And still: ideal gas theorem is still violated in both cases.

What's wrong?
  Reply With Quote

Old   March 5, 2009, 23:06
Default Re: Basic Understanding
  #6
Abduri
Guest
 
Posts: n/a
Hi Glenn,

so you mean:

1. Domain Reference Pressure at 4.6Pa. 2. Inlet Relative Static Pressure at 0Pa.

Is that right?

Cheers
  Reply With Quote

Old   March 6, 2009, 05:19
Default Re: Basic Understanding
  #7
Bart Prast
Guest
 
Posts: n/a
Abduri,

What's the Knudsen number in your case? If it is unity or larger than your simulation doesn't make sense.
  Reply With Quote

Old   March 6, 2009, 07:54
Default Re: Basic Understanding
  #8
Bart Prast
Guest
 
Posts: n/a
I checked some numbers for you: http://www.imamod.ru/~serge/arc/conf...gs/pdf/542.pdf

Your Knudsen number should be fine. But a reference pressure of 0 or your starting pressure should work, like Glenn said.

Bart
  Reply With Quote

Old   March 7, 2009, 07:12
Default Re: Basic Understanding
  #9
Abduri
Guest
 
Posts: n/a
Alright, thank you guys. I understand now how I have to setup the model. Values for freestream pressure, temperature and especially density make sense.

Anyhow, I don't understand how given values at a certain point in freestream flow can violate ideal gas theorem.
  Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
FSI problem , Help needed for my basic questions kmgraju CFX 1 July 29, 2011 01:04
Basic Concept of Secondary flow in Compressor Blade Saima Main CFD Forum 1 November 23, 2010 08:10
[snappyHexMesh] basic understanding of sHM colinB OpenFOAM Meshing & Mesh Conversion 1 October 6, 2010 09:17
Basic Blade Design Code apoorv Main CFD Forum 0 April 5, 2010 07:37
Basic Poiseuille flow simulation question Ashish Main CFD Forum 0 October 2, 2007 14:05


All times are GMT -4. The time now is 02:15.