Modeling transient negative pressure (suction pressure) in outlet

N_mrtz · July 18, 2013, 15:18

Hello,

I am new in fluent. I a modeling the two branches system of pipes with two inlets and one outlet. The sucking pressure(negative pressure) is applied in outlet to force the fluid to flow from inlet. This pressure is transient and varies by time. Also, it is important to have a constant mass flow rate which is known form outside. Both inside have the same condition. The system is incompressible.

I have some question related to boundary condition:

I applied mass flow rate for inlet (we have to get constant mass flow rate from outlet, so I divided this constant number by two, because we have two inlets and put it as mass flow inlet for each inlets)
Also, I assigned the pressure outlet which is the profile(user defined profile) as a function of time.

As I read in fluent articles, the mass flow inlet is more for compressible flow, so should I change it to any other boundary? I do not have any other information for inlet.
Also, for pressure outlet, I have profile of pressure, (not equation), I mean that I have the values of pressure versus time. I got these values from experimental results. These values are negative, should I apply the negative value on the profile or I have to change it to the positive value?

I appreciate if you can help me.

blackmask · July 18, 2013, 21:59

For incompressible flow, the pressure difference matters so that you do not have to change the negative values into positive ones. You could use the velocity inlet b.c. instead of mass flow because the former is more robust. May I ask what is the purpose of your simulation? To find the pressure at the inlet?

July 18, 2013, 15:18	Modeling transient negative pressure (suction pressure) in outlet	#1
N_mrtz New Member Join Date: Jul 2013 Posts: 1 Rep Power: 0	Hello, I am new in fluent. I a modeling the two branches system of pipes with two inlets and one outlet. The sucking pressure(negative pressure) is applied in outlet to force the fluid to flow from inlet. This pressure is transient and varies by time. Also, it is important to have a constant mass flow rate which is known form outside. Both inside have the same condition. The system is incompressible. I have some question related to boundary condition: I applied mass flow rate for inlet (we have to get constant mass flow rate from outlet, so I divided this constant number by two, because we have two inlets and put it as mass flow inlet for each inlets) Also, I assigned the pressure outlet which is the profile(user defined profile) as a function of time. As I read in fluent articles, the mass flow inlet is more for compressible flow, so should I change it to any other boundary? I do not have any other information for inlet. Also, for pressure outlet, I have profile of pressure, (not equation), I mean that I have the values of pressure versus time. I got these values from experimental results. These values are negative, should I apply the negative value on the profile or I have to change it to the positive value? I appreciate if you can help me.

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July 18, 2013, 21:59		#2
blackmask Senior Member Join Date: Aug 2011 Posts: 421 Blog Entries: 1 Rep Power: 22	For incompressible flow, the pressure difference matters so that you do not have to change the negative values into positive ones. You could use the velocity inlet b.c. instead of mass flow because the former is more robust. May I ask what is the purpose of your simulation? To find the pressure at the inlet?