Pressure Boundary Condition Problem

rbjet · July 8, 2013, 12:41

Hi,

I have a problem setting the boundary condition for my desired problem. I am setting a free-surface 2D problem with a tank. At the bottom of the tank I need to apply pressure to the fluid and check the disturbance of the water-air surface, it is a transient problem (the pressure data is the amplitude of different frequencies of a sound changing over time and space(x axis)). I found a tutorial about how do set a transient profile with a .csv file, and everything seems to be working. However, I have to chose inlet, outlet, opening, symmetry or wall as boundary condition... I've tried all of them, and neither one fit properly for my purpose because I don't want any flow in my tank.
So, how could I set a pressure transient profile boundary condition without mass flow?
I have almost no experience in CFD, it's a summer project. I would be very grateful if someone could help me with this.

ghorrocks · July 13, 2013, 08:51

There is no such thing as a pressure boundary with no mass flow. The mass flow is required to obtian the pressure you request - you cannot have one without the other.

Can you explain what you are modelling better? Please show an image and describe what you know about this bottom boundary - where does the pressure come from and why do you say there is no mass flow?

rbjet · July 15, 2013, 14:02

Hi ghorrocks,

Thanks for your reply. Well, I want to apply pressure to a static fluid on a tank (maybe you could imagine like several pistons moving at the bottom of the tank), I have a pressure profile at the bottom and it changes over time. The pressure profile represents the visualization of a song. It would be like if the bars of the "music visualizer" hit the static fluid on the bottom of the tank, without flow. I not sure if its correct, but when I changed the boundary of the top of the tank from "opening" to "wall" it stopped the flow and I can see the surface disturbance. Do you think it is correct? I saved a video.

https://www.dropbox.com/s/sr6ot0jf9f...isturbance.wmv

If it is everything right, now I have to take data from the surface, I could do this using a chart, but I need to export the data for 1024 points over the bottom and I didn't find where could I set exporting options.

Thank you again.

ghorrocks · July 15, 2013, 22:45

Are you trying to see the free surface motion resulting from acoustic vibrations at the bottom of the tank?

If so the easiest way of generating these vibrations is probably using a normal pressure opening. The frequency of this vibration will be very fast and averaged over time there will be no mass flow, but at any point in time the flow is travelling in or out to generate the pressure waves.

July 8, 2013, 12:41	Pressure Boundary Condition Problem	#1
rbjet New Member Robert Bento Florentino Join Date: Jun 2013 Posts: 2 Rep Power: 0	Hi, I have a problem setting the boundary condition for my desired problem. I am setting a free-surface 2D problem with a tank. At the bottom of the tank I need to apply pressure to the fluid and check the disturbance of the water-air surface, it is a transient problem (the pressure data is the amplitude of different frequencies of a sound changing over time and space(x axis)). I found a tutorial about how do set a transient profile with a .csv file, and everything seems to be working. However, I have to chose inlet, outlet, opening, symmetry or wall as boundary condition... I've tried all of them, and neither one fit properly for my purpose because I don't want any flow in my tank. So, how could I set a pressure transient profile boundary condition without mass flow? I have almost no experience in CFD, it's a summer project. I would be very grateful if someone could help me with this.

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July 13, 2013, 08:51		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	There is no such thing as a pressure boundary with no mass flow. The mass flow is required to obtian the pressure you request - you cannot have one without the other. Can you explain what you are modelling better? Please show an image and describe what you know about this bottom boundary - where does the pressure come from and why do you say there is no mass flow?

July 15, 2013, 14:02		#3
rbjet New Member Robert Bento Florentino Join Date: Jun 2013 Posts: 2 Rep Power: 0	Hi ghorrocks, Thanks for your reply. Well, I want to apply pressure to a static fluid on a tank (maybe you could imagine like several pistons moving at the bottom of the tank), I have a pressure profile at the bottom and it changes over time. The pressure profile represents the visualization of a song. It would be like if the bars of the "music visualizer" hit the static fluid on the bottom of the tank, without flow. I not sure if its correct, but when I changed the boundary of the top of the tank from "opening" to "wall" it stopped the flow and I can see the surface disturbance. Do you think it is correct? I saved a video. https://www.dropbox.com/s/sr6ot0jf9f...isturbance.wmv If it is everything right, now I have to take data from the surface, I could do this using a chart, but I need to export the data for 1024 points over the bottom and I didn't find where could I set exporting options. Thank you again.

July 15, 2013, 22:45		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Are you trying to see the free surface motion resulting from acoustic vibrations at the bottom of the tank? If so the easiest way of generating these vibrations is probably using a normal pressure opening. The frequency of this vibration will be very fast and averaged over time there will be no mass flow, but at any point in time the flow is travelling in or out to generate the pressure waves.