Boundary Condition Pitfalls and Mitigation in CFD

The accuracy of Computational Fluid Dynamics (CFD) solutions relies heavily on the quality of boundary conditions. Boundary conditions define the physics at the boundaries of the solution domain and drive the flow behavior. Well-posed, realistic boundary conditions are crucial for obtaining accurate solutions. Improper or unrealistic boundary conditions can lead to inaccurate or non-physical results, making it challenging to obtain meaningful solutions.

Outlet Boundary:

• Pitfall: Outlet placed too close to obstacles, leading to inaccurate flow development and violation of zero-gradient assumption.
• Mitigation: Place the outlet sufficiently far downstream (beyond 10 heights of the last obstacle) to ensure fully developed flow.

Wall Boundary (Turbulent Flow):

• Pitfall: Insufficient near-wall mesh resolution $(y^+ < 11.63)$ for accurate wall function application.
• Mitigation: Refine the mesh near walls to maintain $y^+ \ > 11.63 $ preferably between (30-500). This helps to improve accuracy.

Symmetry Boundary:

• Pitfall: Misapplication in symmetric flows, leading to inaccurate solutions. Geometric symmetry does not guarantee that flow possess the same symmetry.
• Mitigation: Verify true flow symmetry before applying the symmetry condition.

General Considerations:

• Inappropriate combinations: Combining certain boundary conditions can lead to under-specified problems.
• Mitigation: Follow recommended combinations based on flow type (e.g., walls with inlet/outlet, walls with constant pressure).