Fluids

Fluids

mismatched force values from Fluent force report and shear stress integral

    • Edmond Lam
      Subscriber

      Hi all, I am simulating a laminar multiphase flow in Fluent using the VOF model. However, when I am trying to extract forces acting on a no-slip wall parallel to the xz plane, the value from Fluent force report using the direction vector (1,0,0) is different from the one reported by a surface integral of X-wall shear stress on the same surface. In fact, in the Fluent force report, the viscous force component along the direction vector (0,1,0) is non-zero, which does not make sense for a wall parallel to the xz plane. In comparison, the surface integral of Y-wall shear stress on the same surface results in a value almost equal to zero (E-27). Are there any ideas? Thanks.

    • SRP
      Subscriber

      Hi,

      Will you kindly send a screenshot of the geometry, force report results, and surface integral of the shear stress in the relevant direction?

      Thank you

      Saurabh

    • Edmond Lam
      Subscriber

      Hi Saurabh,

      Thank you for your reply.

      The geometry is very simple, only a box. The surface I am interested in is the bottom surface, which is parallel to the xz plane.

      Here are the results from the force reports and surface integrals:

       
      As you can see, the forces are very different.
      This mismatch seems to occur when on this surface there are 2 phases.
    • DrAmine
      Ansys Employee

      Can you please include the report of the force vector and not the dot product of the force vector with the direction.

    • Edmond Lam
      Subscriber

      Here it is, thanks:

       

      As mentioned, this mismatch seems to occur only when there are 2 phases in contact with the wall. (I have 2 phases in total for the case.)

      I have wall adhesion turned on for the wall and a specfied contact angle. The surface tension is modelled using CSF. I am wondering if using these models adds additional terms to the force calculation on the wall in Fluent.

       

      Thank you very much.

    • DrAmine
      Ansys Employee

      Surface tension force should is not part of the force report.

      For a VOF case with two phases I am not able to reproduce your issue. Is your problem reproducable on a simple dummy case?

    • Edmond Lam
      Subscriber

      Thank you for your reply.

      I ran two simple dummy cases for 10 time steps after patching a liquid droplet with a velocity in -x.

      The boundary conditions for the 2 cases are exactly the same.

      There is no gravity, and the wall that I am measuring forces is at the bottom with wall adhesion enabled, surface tension modelled with CSF.

      1) a liquid droplet moving in the -x direction in its vapour:

      Here, with only one phase (vapour) in contact with the bottom wall, integral of x-wall shear stress is exactly the same as the force report in (1 0 0). The integral of static pressure is also exactly the same as the pressure component in force report in (0 1 0). However, although the integral of y-wall shear stress is almost zero, the viscous component in force report in (0 1 0) is non-zero, or at least much larger.

      2) a liquid droplet moving in the -x direction in its vapour, but also touching the bottom wall:

      Here, with both phases in contact with the bottom wall, the integral of x-wall shear stress no longer matches the force report in (1 0 0). The integral of static pressure also no longer matches the pressure component in force report in (0 1 0). And similarly, although the integral of y-wall shear stress is still almost zero, the viscous component in force report in (0 1 0) is still much larger.

    • DrAmine
      Ansys Employee

      Can you check if there is a mismatch if you disable the surface tension force?

       

    • Edmond Lam
      Subscriber

      The mismatch is gone when wall adhesion or surface tension modelling is disabled. But the integral of y-wall shear stress (E-27, almost zero) is still different from the viscous component of the force report in the (0 1 0) direction (non-zero).

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