Definition of cp and cp-total

Altair Forum User
Altair Forum User
Altair Employee
edited October 2020 in Community Q&A

Hi,

 

i am looking for the definitions of pressure and total pressure coefficient (cp and cp-tot) for AcuFieldview.

I would be glad for your help.

 

Thanks in advance

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Answers

  • acupro
    acupro
    Altair Employee
    edited March 2018

    I'm not sure I completely understand the question.  The quantity written as 'pressure' by AcuSolve is static pressure (P).  Total Pressure would be static pressure plus dynamic pressure - or P + 1/2 * Rho * Vel^2.  You can create variables using math functions in AcuFieldView by f_x icon towards the top of the AcuFieldView main window.

  • Altair Forum User
    Altair Forum User
    Altair Employee
    edited March 2018

    Hi acupro,

     

    sorry for the misunderstanding of my question. I will try to explain better:

    For VWT Simulations i need to define the pressure coefficient to show the pressure distributions on a car. I think, the formula for that is following from ydigits description:

     

    ('pressure'-0)/(0.5*'vwtRho'*('velocity_magnitude'+'vwtVel')^2)

     

    At this point, there are some questions: where is the variable 'velocity_magnitude' from? Theoretically it should be without velocity magnitude.

    Why is the reference free stream pressure 0?

    My expectation is to see the cp value 1 at stagnation point of the car. But it is around 2 or 3 according to VWT analysis report. How could it be like that?

    I am a bit confused because of this. Maybe i miss an important point by the way.

     

    Thanks for your help.

  • acupro
    acupro
    Altair Employee
    edited March 2018

    It seems there may be a mistake in that formula.  You wouldn't sum velocity_magnitude and vwtVel - the 'velocity' term squared should be the overall freestream velocity, which is likely vwtVwl.  The pressure difference would be the local static pressure P minus some reference pressure.  You could use the integrated static pressure at the inlet for the reference pressure.  I suppose you could also use the integrated pressure at the outlet, which may be 0 if that is the boundary condition used.  So long as you always use the same definition, then your comparisons would be valid.

  • Altair Forum User
    Altair Forum User
    Altair Employee
    edited March 2018

    Thanks acupro. I will try with the inlet static pressure.

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