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Abduri March 5, 2009 02:55

Basic Understanding
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!

johnny March 5, 2009 06:25

Re: Basic Understanding
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.

Abduri March 5, 2009 08:25

Re: Basic Understanding
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?

Glenn Horrocks March 5, 2009 22:20

Re: Basic Understanding

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

Abduri March 5, 2009 22:58

Re: Basic Understanding
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?

Abduri March 5, 2009 23:06

Re: Basic Understanding
Hi Glenn,

so you mean:

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

Is that right?


Bart Prast March 6, 2009 05:19

Re: Basic Understanding

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

Bart Prast March 6, 2009 07:54

Re: Basic Understanding
I checked some numbers for you:

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


Abduri March 7, 2009 07:12

Re: Basic Understanding
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.

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