Fluids

Fluids

Extreme pressure in boundary layer

    • Frank96CZ
      Subscriber

      Hello,

      I was wondering if you could help me with this problem. I got nearly straight pipe in 2D and i am using axisymmetric option. when i try to use these discretization schemes (PRESTO, QUICK, QUICK, QUICK) and SIMPLE p-v-coupling the pressure in boundary layer at the inlet skyrockets to extreme values. I attached a pictures to better ilustrate.

    • Karthik R
      Administrator
      Hello,nIn any scenario, the velocity of the fluid adjacent to the stationary wall must be 0. This is true for any location irrespective of whether you are looking at the inlet or center of the pipe or outlet. In your case, you are imposing a constant velocity at the inlet of 5 m/s. You might want to change your approach slightly here. Instead of using a constant velocity, could you please try this?nTake a small section of the pipe, run a streamwise periodic simulation. It means that your inlet and outlet boundaries are periodic. This will give you a fully developed solution after you complete your steady-state run. Write out the velocity profile at the outlet boundary.nRead this profile file into your Fluent simulation and impose this as your new inlet velocity condition for your simulation.nAlso, make sure you are using second-order schemes. And for this problem, you don't need to use PRESTO! or QUICK. Second-order upwind should be sufficient. PRESTO! is generally used when you have strong body forces in your simulation.nLet me know your findings.nThank you.nKarthikn
    • Frank96CZ
      Subscriber
      Hello,nthanks for your answer, During this time i figured out, that the main problem is in inflation itself, anytime I suppress this option the results look fine. So my solution was to delete the inflation and use more precise decomposition. Picture below.n Then i used edge sizing to define boundary layer meshing. picture below.nand then more edge sizings on the interior fluid itself. picture below.nplus Face Meshing and multizone quad/tri method set to all quad and the result is in the picture. the picture shows the middle part where the pipe is narrowing. it is Venturi´s nozzle.nI really do not know why the inflation was making such a mess but with this mesh the result looks good. picture.nI thank you very much for your proposal @Kremella , but i am not sure what you mean. I never used streamwise periodic simulation and i am not really sure how to do it. I am sorry but since the problem is resolved, I am not sure i have the energy to go through the necessary tutorials to use streamwise p. sim. I hope you wont be angry if I won´t try your solution. Also, the PRESTO and QUICK will be necessary in the later stages where i will use RSM optimization to maximize cavitation and minimize pressure losses. the geometry can get a pretty wild when spline is applied on the narrowing part instead of two lines. Since i am using Latin hypercube to create my DoE in some design points the diameter in the most narrow part can get to 1mm.nnThanks a lot again and if the problem returns i know what should i try next.n
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