IDEAL GAS: not fulfilled even though set up?

Fettmon · November 2, 2015, 08:31

Hi everybody!

My aim is to simulate pressure equalization in inviscid air (density: ideal gas; Cp = 1004.9 J*K^-1*kg^-1; M = 28.966 g*mol^-1).

Therefor I patch 2 different regions with 2 different initial pressures. I have obtained converging results only with a density-based solver and I had chosen an explicit transient formulation with CFL=0.75 for starting.

My continuity, velocity and energy residuals are nicely declining during calculation, BUT when I calculate my ideal gas resiudal with

Absolute Pressure-R/28.966[g mol^-1]*Total Temperature*Density

I obtain residuals up to 30% of the local absolute pressure. This is a problem, because I'd like to comprare the results with ideal analytic solutions.

My questions are therefore:
1) Is my way of calculating the ideal gas residual (see expression above) correct?
2) If 1) is yes, what can I do to get results which are more consistent with the ideal gas law?

Thank you and kind regards!
Gergö

raghulvr · November 2, 2015, 09:26

Firstly,why are you choosing transient simulation for solving the problem?
There is no need for including time-based simulation here.Just try with steady stste simulations.That might help you out!

Fettmon · November 2, 2015, 10:02

Thank you for replying, Rahul!

The pressure equalization I want to simulate shall take place in a shock tube and I am interested mainly in the properties (velocity, pressure difference, shape of face etc.) of the generated shock wave. Rarefraction waves and their reflections are of interest, too. I'd not be able to examine the waves, if I wouldn't use transient formulation.

Fettmon · November 3, 2015, 07:03

Problem solved. Question 1) has to be answered with 'No', because the variable Temperature (static temperature) is needed instead of Total Temperature (stagnation temperature), ergo the correct expression is

Absolute Pressure-R/28.966[g mol^-1]*Temperature*Density

which is almost zero everywhere in my simulation. Which is super

raghulvr · November 3, 2015, 07:32

Thank you mate! Am also new to CFD.I am trying to gain as much knowledge as i could in the area of CFD.Also thanks for sharing your experience with me

November 2, 2015, 08:31	IDEAL GAS: not fulfilled even though set up?	#1
Fettmon New Member Gergö Schmidt Join Date: Oct 2015 Posts: 5 Rep Power: 10	Hi everybody! My aim is to simulate pressure equalization in inviscid air (density: ideal gas; Cp = 1004.9 JK^-1kg^-1; M = 28.966 gmol^-1). Therefor I patch 2 different regions with 2 different initial pressures. I have obtained converging results only with a density-based solver and I had chosen an explicit transient formulation with CFL=0.75 for starting. My continuity, velocity and energy residuals are nicely declining during calculation, BUT when I calculate my ideal gas resiudal with Absolute Pressure-R/28.966[g mol^-1]Total TemperatureDensity* I obtain residuals up to 30% of the local absolute pressure. This is a problem, because I'd like to comprare the results with ideal analytic solutions. My questions are therefore: 1) Is my way of calculating the ideal gas residual (see expression above) correct? 2) If 1) is yes, what can I do to get results which are more consistent with the ideal gas law? Thank you and kind regards! Gergö

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November 2, 2015, 09:26		#2
raghulvr Member Rahul Join Date: Sep 2015 Posts: 39 Rep Power: 10	Firstly,why are you choosing transient simulation for solving the problem? There is no need for including time-based simulation here.Just try with steady stste simulations.That might help you out!

November 2, 2015, 10:02		#3
Fettmon New Member Gergö Schmidt Join Date: Oct 2015 Posts: 5 Rep Power: 10	Thank you for replying, Rahul! The pressure equalization I want to simulate shall take place in a shock tube and I am interested mainly in the properties (velocity, pressure difference, shape of face etc.) of the generated shock wave. Rarefraction waves and their reflections are of interest, too. I'd not be able to examine the waves, if I wouldn't use transient formulation.

November 3, 2015, 07:03		#4
Fettmon New Member Gergö Schmidt Join Date: Oct 2015 Posts: 5 Rep Power: 10	Problem solved. Question 1) has to be answered with 'No', because the variable Temperature (static temperature) is needed instead of Total Temperature (stagnation temperature), ergo the correct expression is Absolute Pressure-R/28.966[g mol^-1]TemperatureDensity which is almost zero everywhere in my simulation. Which is super

November 3, 2015, 07:32		#5
raghulvr Member Rahul Join Date: Sep 2015 Posts: 39 Rep Power: 10	Thank you mate! Am also new to CFD.I am trying to gain as much knowledge as i could in the area of CFD.Also thanks for sharing your experience with me