Calculation of the coefficients of forces and moment

ThomasYepes · July 7, 2020, 14:02

Hello, I am replicating a simulation of a fluent turbine in CFX. My idea is to check the drag, moment and lift coefficients. I know that the ones I got in Fluent are close to what I want to do but when I make the same simulation using the sliding mesh method in CFX, I get completely different data. The simulation is running fine but I don't know what may be affecting the data so dramatically. I wanted to see in the Post processing that it also allows calculating the expression in a certain Time Step and comparing the values with what the CFX monitors show are different, something that seems strange to me if the two are calculating according to the solution made by CFX . Any recommendation?

PS: I am using exactly the same turbulence model and solution method. It also exports the mesh that I used in fluent, as in its 2D modeling, the program added a thickness and added the condition of symmetry between faces.

CFX.zip

Opaque · July 7, 2020, 15:25

Are you using a non-zero Reference Pressure (CFX), or Operating Pressure (Fluent) in the model?

If you read the output file in CFX, it states the reported forces/moments to do not include the contribution of the Reference Pressure.

ThomasYepes · July 7, 2020, 17:46

Thanks for answering. I let you know that I have been learning simulation for a few months. Yes, I am using a CFX reference pressure other than 0. Exactly 1 atm. In that case where I failed and how could I solve it, Thank you very much for your help.

REFERENCE STATE:
Option = Specified Point
Reference Pressure = 1 [atm]
Reference Specific Enthalpy = 0. [J/kg]
Reference Specific Entropy = 0. [J/kg/K]
Reference Temperature = 25 [C

I am trying to use reference pressure on the material at 0 Atm but I don't see a favorable change. I have no idea what a mistake I am making.

Opaque · July 8, 2020, 11:44

Is that the source of your differences with Fluent? If so, just set the Domain/Reference Pressure value to 0, and adjust your boundary condition accordingly. Nothing else.

The value of the Reference Pressure should not change the final result of the solution of the flow field in any way of form. It is a numerical detail to improve round off errors. If you run in double precision, it is not needed.

Unfortunately, the Reference Pressure value is not included in the post-processing quantities and it may cause confusion.

ThomasYepes · July 8, 2020, 12:29

Hi, I just checked that the reference pressure is not the source of the results I am getting in CFX. As you say it does not noticeably affect the final results. For now I have no idea what the source of the difference I get is.

July 7, 2020, 14:02	Calculation of the coefficients of forces and moment	#1
ThomasYepes New Member Outside US and Canada Join Date: Jun 2020 Posts: 14 Rep Power: 5	Hello, I am replicating a simulation of a fluent turbine in CFX. My idea is to check the drag, moment and lift coefficients. I know that the ones I got in Fluent are close to what I want to do but when I make the same simulation using the sliding mesh method in CFX, I get completely different data. The simulation is running fine but I don't know what may be affecting the data so dramatically. I wanted to see in the Post processing that it also allows calculating the expression in a certain Time Step and comparing the values with what the CFX monitors show are different, something that seems strange to me if the two are calculating according to the solution made by CFX . Any recommendation? PS: I am using exactly the same turbulence model and solution method. It also exports the mesh that I used in fluent, as in its 2D modeling, the program added a thickness and added the condition of symmetry between faces. CFX.zip

July 7, 2020, 15:25		#2
Opaque Senior Member Join Date: Jun 2009 Posts: 1,815 Rep Power: 32	Are you using a non-zero Reference Pressure (CFX), or Operating Pressure (Fluent) in the model? If you read the output file in CFX, it states the reported forces/moments to do not include the contribution of the Reference Pressure. ThomasYepes likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

July 7, 2020, 17:46		#3
ThomasYepes New Member Outside US and Canada Join Date: Jun 2020 Posts: 14 Rep Power: 5	Thanks for answering. I let you know that I have been learning simulation for a few months. Yes, I am using a CFX reference pressure other than 0. Exactly 1 atm. In that case where I failed and how could I solve it, Thank you very much for your help. REFERENCE STATE: Option = Specified Point Reference Pressure = 1 [atm] Reference Specific Enthalpy = 0. [J/kg] Reference Specific Entropy = 0. [J/kg/K] Reference Temperature = 25 [C I am trying to use reference pressure on the material at 0 Atm but I don't see a favorable change. I have no idea what a mistake I am making. Last edited by ThomasYepes; July 8, 2020 at 00:09.

July 8, 2020, 11:44		#4
Opaque Senior Member Join Date: Jun 2009 Posts: 1,815 Rep Power: 32	Is that the source of your differences with Fluent? If so, just set the Domain/Reference Pressure value to 0, and adjust your boundary condition accordingly. Nothing else. The value of the Reference Pressure should not change the final result of the solution of the flow field in any way of form. It is a numerical detail to improve round off errors. If you run in double precision, it is not needed. Unfortunately, the Reference Pressure value is not included in the post-processing quantities and it may cause confusion. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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July 8, 2020, 12:29		#5
ThomasYepes New Member Outside US and Canada Join Date: Jun 2020 Posts: 14 Rep Power: 5	Hi, I just checked that the reference pressure is not the source of the results I am getting in CFX. As you say it does not noticeably affect the final results. For now I have no idea what the source of the difference I get is.