Fluids

Fluids

Stokes/Creeping Flow

    • dray19
      Subscriber

      Hello,


      I am looking at the fluid dynamics in a square microchannel for 0.1<=Re<=1000. This involves determining the hydrodynamic entrance length and the friction factor for both the developing and developed regions.


      I have conducted a mesh independent study for several Re>100 and validated the results with previous researchers experimental and analytical work


      When I did the mesh independent study (7 meshes) for Re=0.1,1,10, inlet pressure increases by 2% (~0.01, 0.1, 1 Pa respectively) with each mesh refinement. The other parameters (velocity, entrance length) seem to constant expect entrance length changed in last few meshes. I increased the number of elements in each direction by 25% to give approximately 2x the number of elements. 


      Should I keep increasing the mesh? Am I missing something else? I have read (forums) that fluent should be able to handle creep flow but maybe it can't.


      Any help is greatly appreciated.


      Attached are the results for Re=0.1 mesh study


      Model Information:


      Microchannel: 100um x 100um x 210um 


      Model Domain: 50um x 50um x 210um (symmetry applied)


      CFD: Ansys Fluent 19.1


      Double Precision


      Solver - Pressure-Based, Steady State


      Model – Laminar


      Residual – 10E-6: Continuity, Velocity in X,Y,Z Direction


      Methods:


      Scheme: SIMPLE


      Spatial Discretization


      Gradient: Least Squares Cell Based


      Pressure: Second Order


      Momentum: Second Order Upwind


       

    • Raef.Kobeissi
      Subscriber

      hi,


      how did the pressure inlet increase by 2% ? This is a boundary condition and should remain constant regardless of the mesh, can you send a picture that shows this increase.


      Regards

    • Karthik R
      Administrator

      Hello,


      Could you please elaborate your meshing strategy near the wall? How is your boundary layer mesh? How does it change with each successive refinement?


      Thank you.


      Best Regards,


      Karthik 

    • dray19
      Subscriber

      Raef,


      I am using a Velocity-Inlet and Pressure-Outlet boundary conditions. If I apply a Pressure-Inlet B.C, the velocity profile is fully developed at the inlet and I can't determine entrance length or developing friction factor.

    • dray19
      Subscriber

      Kremella,


      I set the number of elements for each direction (ex: 15 in x, 15 in y, 65 in z-axial direction) and apply a bias factor to have smaller elements at the walls and in the developing flow regime. For refining, I increased the number of elements in each direction by 25%


       


       

    • Karthik R
      Administrator

      Hello,


      Please take a look at this discussion on the student community. This might help you understand mesh independence better.


      https://forum.ansys.com/forums/topic/convergence-of-results-in-the-study-of-mesh-independence/


      Thank you.


      Best Regards,


      Karthik

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