**** Entered By: jcooper @ 10/02/2007 05:14 PM ****
Q1: Why do the results of the massFlow function differ from an area integration of Density*Velocity?
A1: The difference between the results of massFlow and a summation of Density*Velocity at an interface have to do with the orientation of the interface. If the interface orientation is either curved or not aligned with X, Y, or Z, these evaluations will not match up.
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Q2: How can I duplicate the results of the massFlow function so that I can understand what it is doing?
A2: The massFlow function provides the correct evaluation because it takes the projected area normal to the surface at each point to do an integration of Density*Velocity, as follows:
areaInt_x(Velocity u * Density)@ Interface + areaInt_y(Velocity v * Density)@ Interface + areaInt_z(Velocity w * Density)@ Interface 3
This can be matched on an interface or boundary with a known orientation by selecting the orientation before doing the integral. In an expression, use the appropriate velocity component as follows:
Mdot =areaInt(Velocity w*Density )@Out (Surface normal in Z direction)
The massFlow function uses integration point (IP) data on boundaries and interfaces when these are available. IP data is not available in expressions (unless you use the Mass Flow variable, which is again using IP data), so in general, it is not possible to match the accuracy of the built-in CFX massFlow function for surfaces that are curved or whose normals do not align with X, Y or Z.
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**** Entered By: jcooper @ 10/02/2007 05:14 PM **** Q1: Why do the results of the massFlow function differ from an area integration of Density*Velocity? A1: The difference between the results of massFlow and a summation of Density*Velocity at an interface have to do with the orientation of the interface. If the interface orientation is either curved or not aligned with X, Y, or Z, these evaluations will not match up. ************************************************** Q2: How can I duplicate the results of the massFlow function so that I can understand what it is doing? A2: The massFlow function provides the correct evaluation because it takes the projected area normal to the surface at each point to do an integration of Density*Velocity, as follows: areaInt_x(Velocity u * Density)@ Interface + areaInt_y(Velocity v * Density)@ Interface + areaInt_z(Velocity w * Density)@ Interface 3 This can be matched on an interface or boundary with a known orientation by selecting the orientation before doing the integral. In an expression, use the appropriate velocity component as follows: Mdot =areaInt(Velocity w*Density )@Out (Surface normal in Z direction) The massFlow function uses integration point (IP) data on boundaries and interfaces when these are available. IP data is not available in expressions (unless you use the Mass Flow variable, which is again using IP data), so in general, it is not possible to match the accuracy of the built-in CFX massFlow function for surfaces that are curved or whose normals do not align with X, Y or Z. |
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