Inlet BC defenition

assafwei · April 27, 2014, 11:06

Hi All,

I am new to CDF and now trying to learn the basic theory.
I stumbled upon a problem and couldnt understand the logic behind inlet definition.
If we are talking about compressible fluids, why cant I specify an inlet with only volume flow rate? whay do I have to specify either mass flow rate, or specify pressure or density in the inlet? If I only specify volume flow rate, cant the software calculate density on its own? I know I am missing something, but cant figure out what is it that I am missing...

Thanks.

mojijoon · April 29, 2014, 04:25

hi
Density depends on pressure and temperature. It means if you change the pressure in piston like cylinder, because of changing in volume, density will change.So in order to define a really known BC you have to specify either volume flow rate with density (pressure ) or multiply them to have the mass flow rate.

assafwei · April 29, 2014, 06:53

Hi, Thanks for the answer.

That makes sense, so once I specify volume flow and pressure at the inlet I am set. Does this mean static pressure or total pressure at the inlet (static pressure is a bit more difficult for me to measure with my lab test setup)?
I gues total pressure will not cut it?
I do have a static pressure sensor in another location in the test chamber (not at the inlet). Is volume flow rate at the inlet and static pressure at an arbitrary location in the fluid volume enough to fully define the problem?

Thanks.

DaveyBaby · April 29, 2014, 10:21

Hi!
In order to solve the N.S equations over a domain you need initial conditions within the entire domain and also boundary conditions over the whole domain surface including the inlet.

You could do a hand calculation based on Bernouilli's P* formula (assuming you know the velocity at the point within the domain where you measure the pressure) and use it along with your inlet velocity to estimate and set the inlet pressure.

You also need to make sure that you are using the correct fluid and have the correct temperature set. Star CCM+ has defaults of Air and Water at 20C I think.

April 27, 2014, 11:06	Inlet BC defenition	#1
assafwei New Member Join Date: Mar 2014 Posts: 20 Rep Power: 12	Hi All, I am new to CDF and now trying to learn the basic theory. I stumbled upon a problem and couldnt understand the logic behind inlet definition. If we are talking about compressible fluids, why cant I specify an inlet with only volume flow rate? whay do I have to specify either mass flow rate, or specify pressure or density in the inlet? If I only specify volume flow rate, cant the software calculate density on its own? I know I am missing something, but cant figure out what is it that I am missing... Thanks.

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April 29, 2014, 04:25		#2
mojijoon New Member Join Date: Jan 2014 Posts: 4 Rep Power: 12	hi Density depends on pressure and temperature. It means if you change the pressure in piston like cylinder, because of changing in volume, density will change.So in order to define a really known BC you have to specify either volume flow rate with density (pressure ) or multiply them to have the mass flow rate.

April 29, 2014, 06:53		#3
assafwei New Member Join Date: Mar 2014 Posts: 20 Rep Power: 12	Hi, Thanks for the answer. That makes sense, so once I specify volume flow and pressure at the inlet I am set. Does this mean static pressure or total pressure at the inlet (static pressure is a bit more difficult for me to measure with my lab test setup)? I gues total pressure will not cut it? I do have a static pressure sensor in another location in the test chamber (not at the inlet). Is volume flow rate at the inlet and static pressure at an arbitrary location in the fluid volume enough to fully define the problem? Thanks.

April 29, 2014, 10:21		#4
DaveyBaby Member DaveyBaby Join Date: May 2013 Posts: 46 Rep Power: 12	Hi! In order to solve the N.S equations over a domain you need initial conditions within the entire domain and also boundary conditions over the whole domain surface including the inlet. You could do a hand calculation based on Bernouilli's P* formula (assuming you know the velocity at the point within the domain where you measure the pressure) and use it along with your inlet velocity to estimate and set the inlet pressure. You also need to make sure that you are using the correct fluid and have the correct temperature set. Star CCM+ has defaults of Air and Water at 20C I think.