4 Equations and 13 Unknowns

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  • AbubakarAbubakar Posts: 84Member
    edited January 29

    Hi Karthik,

    In one of the basic turbulent flow video the instructor is saying that in Incompressible Navier stroke equation we have 4 equations and 4unknowns(u,v,w,p) which is correct but the instructor is saying that after averaging we are getting 13 unknowns how? We are getting only three additional unknowns so, total it becomes 7unknowns not 13 right? And the additional unknowns which we are getting after averaging is nothing but the fluctuations of velocity in each direction.

    Can you please help me on this? For your reference I have attached the SS where the instructor is saying that we have 13 unknowns after averaging.

    Thank you,

    Abubakar khan

  • Kalyan GoparajuKalyan Goparaju Posts: 243Member

    Hello Abubakar,

    For RANS equations, the unknowns are not the velocity fluctuations, they are infact the product of velocity fluctuations. Therefore, the total unknowns are u,v,w,p,u'u',u'v',u'w',v'u',v'v',v'w',w'u',w'v',w'w' = 13. Making use of the symmetric nature of the stress tensor, we can safely say u'v'=v'u', u'w'=w'u', v'w'=w'v'. Therefore, the actual number of unknowns are 10!

    I hope this answers your question.

    Regards,

    Kalyan

    PS - I dropped the bar from the terms for simplicity.

  • AbubakarAbubakar Posts: 84Member
    edited January 30

    Hi Kalyan Sir,

    I got my answer, thanks for clearing my doubt.

    I have a request, can you please make a video on a SIMPLE and PISO algorithm.


    @Kalyan Goparaju

    Thank you,

    Abubakar khan

  • AbubakarAbubakar Posts: 84Member

    Hi Kalyan Sir,

    I got my answer, thanks for clearing my doubt. I have a request, can you please make a video on SIMPLE and PISO algorithm.

    @Kalyan GoparajuThank you,

    Abubakar khan

  • KremellaKremella Posts: 2,664Admin

    Hello,

    Thank you for your suggestion. If you are looking for additional details about SIMPLE and PISO coupling schemes, I'd recommend that you look into Section 28.4.3 Pressure-Velocity Coupling from our Fluent Theory Guide. This should provide you with some mathematical background behind these schemes.

    Here is the link.

    If you are looking for help on how to access this link, I suggest that you read the instructions specified in the following post.

    Karthik

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