Force Calculation FLUENT

sanket2309 · March 2, 2020, 04:16

1)I want to calculate net force due to pressure across body. I am using force variable in x direction for that. From basic understanding, F = P1A1 - P2A2 , my question is, In FLUENT , in above equation, whether it will consider P1 and P2 as gauge or absolute values ?

Absolute Pressure = Operating + Gauge pressure

If my P1 and P2 are negative, then there are chances of blunder because force is vector quantity (direction also matters).

2)In CFD post, pressure contours we are getting are gauge or they are absolute (operating +gauge)?

Thank you.

vinerm · March 2, 2020, 04:28

Though force is a vector, pressure is not. The direction comes from the face, which is a vector. The pressure used in the Fluent is always gauge. The operating pressure cannot exert any force since it is on both sides of the surface, until it is assumed that the other side is vacuum. Negative pressure means suction, so, if you have negative pressure on a surface, it is being pulled instead of being pushed.

CFD-post also works with gauge pressure.

In Fluent as well as CFD-Post, absolute pressure values are mentioned as Absolute.

Ryuzaki · December 23, 2023, 15:07

I have the same issue.

I am trying to simulate transonic cavity flow (Ma 0.85) and i need to calculate the aerodynamic force on the rear wall of the cavity.

My initial conditions are Tstatic=250K, Pstatic=50000 Pa and U=250 m/s. When

When I define the operating value as 50000 Pascal and enter the gauge pressure value in the pressure far-field boundary condition as 0, and when I enter the operating pressure value as 0 and enter the gauge pressure value in the pressure far-field boundary condition as 50000 Pascal, I get very different results and I cannot decide which one is correct. .

I know that Fluent calculate p_abs = p_gauge+p_op. Here, the results I get when I enter P_op=0 Pa and p_gauge= 50000 Pa and P_op=50000 Pa and p_gauge= 0 Pa are very different from each other. I can't understand why and which one is correct.

March 2, 2020, 04:16	Force Calculation FLUENT	#1
sanket2309 New Member SANKET BHATT Join Date: Nov 2019 Posts: 10 Rep Power: 6	1)I want to calculate net force due to pressure across body. I am using force variable in x direction for that. From basic understanding, F = P1A1 - P2A2 , my question is, In FLUENT , in above equation, whether it will consider P1 and P2 as gauge or absolute values ? Absolute Pressure = Operating + Gauge pressure If my P1 and P2 are negative, then there are chances of blunder because force is vector quantity (direction also matters). 2)In CFD post, pressure contours we are getting are gauge or they are absolute (operating +gauge)? Thank you.

March 2, 2020, 04:28	Force	#2
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 35	Though force is a vector, pressure is not. The direction comes from the face, which is a vector. The pressure used in the Fluent is always gauge. The operating pressure cannot exert any force since it is on both sides of the surface, until it is assumed that the other side is vacuum. Negative pressure means suction, so, if you have negative pressure on a surface, it is being pulled instead of being pushed. CFD-post also works with gauge pressure. In Fluent as well as CFD-Post, absolute pressure values are mentioned as Absolute. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

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December 23, 2023, 15:07		#3
Ryuzaki New Member Ali Can Join Date: Apr 2021 Posts: 28 Rep Power: 5	I have the same issue. I am trying to simulate transonic cavity flow (Ma 0.85) and i need to calculate the aerodynamic force on the rear wall of the cavity. My initial conditions are Tstatic=250K, Pstatic=50000 Pa and U=250 m/s. When When I define the operating value as 50000 Pascal and enter the gauge pressure value in the pressure far-field boundary condition as 0, and when I enter the operating pressure value as 0 and enter the gauge pressure value in the pressure far-field boundary condition as 50000 Pascal, I get very different results and I cannot decide which one is correct. . I know that Fluent calculate p_abs = p_gauge+p_op. Here, the results I get when I enter P_op=0 Pa and p_gauge= 50000 Pa and P_op=50000 Pa and p_gauge= 0 Pa are very different from each other. I can't understand why and which one is correct.