Need help in boundary conditions

sampathevs · July 3, 2013, 01:20

Hello all.

I am working on a case study related problem. static pressure for outlet condition is given in the paper. But, in FLUENT, for outlet boundary condition, it is showing as gauge pressure.

Should i give the the static pressure in the given Gauge pressure box ? or should i perform any calculations to find the gauge pressure ?

If any calculations needed, Please help me with the calculations.

michujo · July 3, 2013, 03:27

Hi, if you are working with incompressible flow I think you just need the gauge pressure. In that case the flow is not sensitive to the mean pressure level, but rather to the pressure gradients. You can set the gauge pressure to zero and let the code calculate the pressure at the inlet that matches the pressure drop along the duct.
Quoting Fluent's User's Guide:

8.14.2 Operating Pressure, Gauge Pressure, and Absolute Pressure

FLUENT avoids the problem of roundoff error (discussed in Section 8.14.1) by subtracting the operating pressure (generally a large pressure roughly equal to the average absolute pressure in the flow) from the absolute pressure, and using the result (termed the gauge pressure). The relationship between the operating pressure, gauge pressure, and absolute pressure is shown below. The absolute pressure is simply the sum of the operating pressure and the gauge pressure:

(8.14-1) $p_{abs}=p_{operating}+p_{gauge}$

All pressures that you specify and all pressures computed or reported by FLUENT are gauge pressures.

Cheers,
Michujo.

sampathevs · July 4, 2013, 03:15

Thanks a lot Michujo. Your information is very helpful.

July 3, 2013, 01:20	Need help in boundary conditions	#1
sampathevs Member EMAVI V SURYA SAMPATH Join Date: Aug 2011 Location: Malaysia Posts: 85 Rep Power: 15	Hello all. I am working on a case study related problem. static pressure for outlet condition is given in the paper. But, in FLUENT, for outlet boundary condition, it is showing as gauge pressure. Should i give the the static pressure in the given Gauge pressure box ? or should i perform any calculations to find the gauge pressure ? If any calculations needed, Please help me with the calculations.

July 3, 2013, 03:27		#2
michujo Senior Member Join Date: Dec 2011 Location: Madrid, Spain Posts: 134 Rep Power: 15	Hi, if you are working with incompressible flow I think you just need the gauge pressure. In that case the flow is not sensitive to the mean pressure level, but rather to the pressure gradients. You can set the gauge pressure to zero and let the code calculate the pressure at the inlet that matches the pressure drop along the duct. Quoting Fluent's User's Guide: 8.14.2 Operating Pressure, Gauge Pressure, and Absolute Pressure FLUENT avoids the problem of roundoff error (discussed in Section 8.14.1) by subtracting the operating pressure (generally a large pressure roughly equal to the average absolute pressure in the flow) from the absolute pressure, and using the result (termed the gauge pressure). The relationship between the operating pressure, gauge pressure, and absolute pressure is shown below. The absolute pressure is simply the sum of the operating pressure and the gauge pressure: (8.14-1) $p_{abs}=p_{operating}+p_{gauge}$ All pressures that you specify and all pressures computed or reported by FLUENT are gauge pressures. Cheers, Michujo. Last edited by michujo; July 3, 2013 at 05:35.

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July 4, 2013, 03:15		#3
sampathevs Member EMAVI V SURYA SAMPATH Join Date: Aug 2011 Location: Malaysia Posts: 85 Rep Power: 15	Thanks a lot Michujo. Your information is very helpful.