Question from tutorial: Buoyant flow in a partitioned cavity

noppawit · October 28, 2010, 06:48

Hello,

I'm trying to simulate a case really similar to the tutorial of Buoyant flow in a Partitioned cavity (Chapter 8).

From the tutorial, the air (Material: Air at 25 C) is initially at 5ºC and one side is heated up with fixed temperature of 75ºC and the opposite side is maintained at 5ºC.

From the result, why the pressure changes not so much? In my opinion refers to ideal gas law, the pressure should increase about 0.25atm. If I want to see 0.25atm increasing, how can I do? And also, what is the different between "Total Pressure" and "Pressure"?

Thank you so much.
Noppawit

ghorrocks · October 28, 2010, 19:14

http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

noppawit · October 31, 2010, 22:00

Does anyone know how to maintain constant volume?

ghorrocks · October 31, 2010, 22:05

? In a closed cavity if you don't move the mesh you keep a constant volume.

noppawit · October 31, 2010, 22:21

Thank you for your replies, I'm afraid that I still don't understand why my case doesn't follow Ideal Gas Law. Since you've mentioned that static mesh, volume is constant.
When I heat the gas, it should follow $\frac{P_{1}}{T_{1}}=\frac{P_{2}}{T_{2}}$ . But from the simulation, it doesn't follow. From my understanding after I switched material to "Air Ideal Gas", CFX calculate the density of air after change in temperature, and it uses calculated density to calculate pressure. I tried to initialize the initial (static) pressure, also vary this initial pressure -->> the result is still the same (very small change in pressure).

Since my gas is a kind of gas expansion, how can I do it?

michael_owen · November 1, 2010, 10:44

Are you running in steady state, or transient?

noppawit · November 1, 2010, 10:54

I have tried both of them. But they are the same, I mean they don't follow ideal gas law. From my result above is transient, at 2s.

michael_owen · November 1, 2010, 10:57

Are you setting the pressure level?

noppawit · November 1, 2010, 11:03

I really don't know what is pressure level? Where should I set it?

If you mean the initialization, I've already tried. But the result is still the same. I tried with 1atm, 2atm,.. in the box of static pressure.

michael_owen · November 1, 2010, 11:39

The model is not conserving mass.

1. Make sure you are using Air Ideal Gas, and NOT Air at 25 C.
2. Run in Transient
3. Make sure that your Heat Transfer option (Domain definition, Fluid Models tab) is set to Total Energy OR
3a. If you use the Thermal Energy option, make sure that you set the minimum number of coefficient loops (Solver Control, Basic Settings tab) to 2
4. On the Solver Control, Basic Settings tab, check on Conservation Target. The default setting may be to loose for a transient simulation with a lot of time steps. Lower it if your mass conservation is poor.
4. On the Advanced Options tab of the Solver Control, check on Pressure Level Information and check on Compressible Transient Option.

noppawit · November 2, 2010, 11:31

Thank you for your reply. I'm trying on michael_owen's method. Roughly, the pressure increases about 2000Pa.

Atze · March 18, 2014, 12:06

Quote:

Originally Posted by michael_owen

The model is not conserving mass.

1. Make sure you are using Air Ideal Gas, and NOT Air at 25 C.
2. Run in Transient
3. Make sure that your Heat Transfer option (Domain definition, Fluid Models tab) is set to Total Energy OR
3a. If you use the Thermal Energy option, make sure that you set the minimum number of coefficient loops (Solver Control, Basic Settings tab) to 2
4. On the Solver Control, Basic Settings tab, check on Conservation Target. The default setting may be to loose for a transient simulation with a lot of time steps. Lower it if your mass conservation is poor.
4. On the Advanced Options tab of the Solver Control, check on Pressure Level Information and check on Compressible Transient Option.

Hi,

I've the same problem in a steady state simulation. How can I change this setting for my case?

thank you

ghorrocks · March 18, 2014, 17:53

It is a transient only option. You should not need to do this sort of thing. Can you explain your problem more fully? I bet there is another more important problem causing it.

Atze · March 19, 2014, 03:18

Hi Glenn,

I answered you in another post. By the way my problem is the same of noppawit but in steady state. I tried this tutorial but internal Absolute Pressure doesn't change with temperature

October 28, 2010, 06:48	Question from tutorial: Buoyant flow in a partitioned cavity	#1
noppawit New Member Noppawit Sippawit Join Date: Oct 2010 Posts: 14 Rep Power: 16	Hello, I'm trying to simulate a case really similar to the tutorial of Buoyant flow in a Partitioned cavity (Chapter 8). From the tutorial, the air (Material: Air at 25 C) is initially at 5ºC and one side is heated up with fixed temperature of 75ºC and the opposite side is maintained at 5ºC. From the result, why the pressure changes not so much? In my opinion refers to ideal gas law, the pressure should increase about 0.25atm. If I want to see 0.25atm increasing, how can I do? And also, what is the different between "Total Pressure" and "Pressure"? Thank you so much. Noppawit

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October 28, 2010, 19:14		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

October 31, 2010, 22:00		#3
noppawit New Member Noppawit Sippawit Join Date: Oct 2010 Posts: 14 Rep Power: 16	Does anyone know how to maintain constant volume?

October 31, 2010, 22:05		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	? In a closed cavity if you don't move the mesh you keep a constant volume.

October 31, 2010, 22:21		#5
noppawit New Member Noppawit Sippawit Join Date: Oct 2010 Posts: 14 Rep Power: 16	Thank you for your replies, I'm afraid that I still don't understand why my case doesn't follow Ideal Gas Law. Since you've mentioned that static mesh, volume is constant. When I heat the gas, it should follow $\frac{P_{1}}{T_{1}}=\frac{P_{2}}{T_{2}}$ . But from the simulation, it doesn't follow. From my understanding after I switched material to "Air Ideal Gas", CFX calculate the density of air after change in temperature, and it uses calculated density to calculate pressure. I tried to initialize the initial (static) pressure, also vary this initial pressure -->> the result is still the same (very small change in pressure). Since my gas is a kind of gas expansion, how can I do it?

November 1, 2010, 10:44		#6
michael_owen Senior Member Michael P. Owen Join Date: Mar 2009 Posts: 196 Rep Power: 17	Are you running in steady state, or transient?

November 1, 2010, 10:54		#7
noppawit New Member Noppawit Sippawit Join Date: Oct 2010 Posts: 14 Rep Power: 16	I have tried both of them. But they are the same, I mean they don't follow ideal gas law. From my result above is transient, at 2s.

November 1, 2010, 10:57		#8
michael_owen Senior Member Michael P. Owen Join Date: Mar 2009 Posts: 196 Rep Power: 17	Are you setting the pressure level?

November 1, 2010, 11:03		#9
noppawit New Member Noppawit Sippawit Join Date: Oct 2010 Posts: 14 Rep Power: 16	I really don't know what is pressure level? Where should I set it? If you mean the initialization, I've already tried. But the result is still the same. I tried with 1atm, 2atm,.. in the box of static pressure.

November 1, 2010, 11:39		#10
michael_owen Senior Member Michael P. Owen Join Date: Mar 2009 Posts: 196 Rep Power: 17	The model is not conserving mass. 1. Make sure you are using Air Ideal Gas, and NOT Air at 25 C. 2. Run in Transient 3. Make sure that your Heat Transfer option (Domain definition, Fluid Models tab) is set to Total Energy OR 3a. If you use the Thermal Energy option, make sure that you set the minimum number of coefficient loops (Solver Control, Basic Settings tab) to 2 4. On the Solver Control, Basic Settings tab, check on Conservation Target. The default setting may be to loose for a transient simulation with a lot of time steps. Lower it if your mass conservation is poor. 4. On the Advanced Options tab of the Solver Control, check on Pressure Level Information and check on Compressible Transient Option.

November 2, 2010, 11:31		#11
noppawit New Member Noppawit Sippawit Join Date: Oct 2010 Posts: 14 Rep Power: 16	Thank you for your reply. I'm trying on michael_owen's method. Roughly, the pressure increases about 2000Pa.

March 18, 2014, 17:53		#13
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	It is a transient only option. You should not need to do this sort of thing. Can you explain your problem more fully? I bet there is another more important problem causing it.

March 19, 2014, 03:18		#14
Atze Member Join Date: Apr 2010 Location: Pisa / Italy Posts: 62 Rep Power: 16	Hi Glenn, I answered you in another post. By the way my problem is the same of noppawit but in steady state. I tried this tutorial but internal Absolute Pressure doesn't change with temperature