What is the specified pressure of a simple boundary condition with a constant-flow inlet velocity?

Peter Parrish_22382
Peter Parrish_22382 Altair Community Member
edited July 12 in Community Q&A

In the example laid out in ACUSOLVE documentation, image

what would be the pressure specified at this boundary condition?

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Answers

  • acupro
    acupro
    Altair Employee
    edited July 3

    The pressure solution at the inlet would be the result of the calculation - not specified as a boundary condition.  We specify a flow BC (velocity, flow rate, etc) or a pressure BC - but not both.  The pressure is typically specified at the outflow boundary - and there the velocity/flow-rate would be the unknown being solved.

  • Peter Parrish_22382
    Peter Parrish_22382 Altair Community Member
    edited July 11

    Thanks for the information, I would like to try specifying both velocity and pressure at the inlet, with a 0-pres. gradient at the outlet. This way I would hope to remove outlet boundary condition effects on the solution.

  • acupro
    acupro
    Altair Employee
    edited July 11
    Peter Parrish_22382 said:

    Thanks for the information, I would like to try specifying both velocity and pressure at the inlet, with a 0-pres. gradient at the outlet. This way I would hope to remove outlet boundary condition effects on the solution.

    This would overspecify the conditions at the inlet - likely leading to poor convergence (if at all) and spurious results.  The typical outflow boundary condition with a specified pressure applies that pressure as an 'Element Boundary Condition' so the solver attempts to reach that in averaged sense, not applying that pressure at every node.  It's a more natural/conservative boundary condition, as the flow distribution is resolved.

    What is your desired simulation?  Can you add some explanation and images?

  • Peter Parrish_22382
    Peter Parrish_22382 Altair Community Member
    edited July 11

    Thanks, I did not realize that outflow is treated like a natural boundary condition. 

    I am trying to simulate flow past a square cylinder. Here is a link to the Article that I've been following,

    Large-Eddy Simulation of Turbulent Flow Around a Finite-Height Wall-Mounted Square Cylinder Within a Thin Boundary Layer | Request PDF (researchgate.net)

    imageimage                             image       where d = 0.0127m, Uinf = 15 m/s 

     

  • acupro
    acupro
    Altair Employee
    edited July 12
    Peter Parrish_22382 said:

    Thanks, I did not realize that outflow is treated like a natural boundary condition. 

    I am trying to simulate flow past a square cylinder. Here is a link to the Article that I've been following,

    Large-Eddy Simulation of Turbulent Flow Around a Finite-Height Wall-Mounted Square Cylinder Within a Thin Boundary Layer | Request PDF (researchgate.net)

    imageimage                             image       where d = 0.0127m, Uinf = 15 m/s 

     

    I think we already accomplished the task for the inlet velocity profile.

    You'll likely need a very (or very, very, very) refined mesh for LES simulations.  Have you looked at mesh sensitivity, time increment/step sensitivity, inlet turbulence sensitivity, etc?

  • Peter Parrish_22382
    Peter Parrish_22382 Altair Community Member
    edited July 12
    acupro_21778 said:

    I think we already accomplished the task for the inlet velocity profile.

    You'll likely need a very (or very, very, very) refined mesh for LES simulations.  Have you looked at mesh sensitivity, time increment/step sensitivity, inlet turbulence sensitivity, etc?

    I have tried running a simulation with about 15 million volumetric elements (the same as what was applied in the paper), with a refinement so that the surface Y+ at any given time is less than 10. My time stepping is done to achieve a CFL of less than 10, as well as adding second order time integration to help allow a higher CFL. 

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