CPCE CPCE Mesh Generation & Boundary Conditions 1 — Questions and Answers
Question 1: What is the primary advantage of a structured (hexahedral) mesh over an unstructured (tetrahedral) mesh in CFD?
- It is easier to generate for complex geometries
- It typically yields lower numerical diffusion and higher solver efficiency (Correct answer)
- It supports more turbulence models
- It requires fewer boundary condition definitions
Correct answer: It typically yields lower numerical diffusion and higher solver efficiency
Structured hexahedral meshes align with the flow direction, reducing numerical diffusion and improving solver convergence speed compared to unstructured tetrahedral meshes.
Question 2: What boundary condition is applied at a symmetry plane in a CFD simulation?
- No-slip condition on all velocity components
- Zero normal velocity and zero normal gradients of all other variables (Correct answer)
- Constant pressure specification
- Periodic inlet-outlet linking
Correct answer: Zero normal velocity and zero normal gradients of all other variables
A symmetry boundary sets the normal velocity component to zero and enforces zero normal gradients of scalar variables, effectively mirroring the flow without resolving the full domain.
Question 3: What is the purpose of mesh refinement in the boundary layer region of a CFD simulation?
- To reduce total cell count and computational cost
- To accurately resolve large velocity gradients and capture wall shear stress (Correct answer)
- To apply turbulence model boundary conditions
- To prevent backflow at the outlet
Correct answer: To accurately resolve large velocity gradients and capture wall shear stress
Boundary layer mesh refinement (small cells near walls) is needed to resolve steep velocity gradients in the viscous sublayer and accurately predict wall shear stress and heat transfer.
Question 4: In CFD, what does 'mesh skewness' measure and why is it important?
- The ratio of largest to smallest cell in the domain
- The deviation of a cell's shape from an ideal equilateral form, affecting solution accuracy (Correct answer)
- The number of cells per unit volume
- The alignment of cell faces with the flow direction
Correct answer: The deviation of a cell's shape from an ideal equilateral form, affecting solution accuracy
Mesh skewness quantifies how much a cell deviates from an ideal shape; highly skewed cells degrade interpolation accuracy and can cause solver divergence.
Question 5: What type of boundary condition is a 'velocity inlet' in CFD, and what quantities must typically be specified?
- An outflow condition requiring pressure specification only
- An inlet condition requiring velocity vector, turbulence intensity, and turbulence length scale (Correct answer)
- A wall condition requiring no-slip enforcement
- A periodic condition linking inlet to outlet
Correct answer: An inlet condition requiring velocity vector, turbulence intensity, and turbulence length scale
A velocity inlet requires specifying the velocity magnitude and direction, plus turbulence quantities (intensity and length scale or equivalent) to fully define the incoming flow state.
Question 6: What is a 'conformal mesh' in CFD and why is it preferred over non-conformal interfaces?
- A mesh where all cells are the same size; preferred for uniform accuracy
- A mesh where node points match exactly at shared boundaries, avoiding interpolation errors at interfaces (Correct answer)
- A mesh generated exclusively for structured grids
- A mesh that conforms to the turbulence model requirements
Correct answer: A mesh where node points match exactly at shared boundaries, avoiding interpolation errors at interfaces
A conformal mesh has matching nodes at shared boundaries between mesh regions, eliminating the need for interpolation across the interface and preventing artificial errors at zone connections.
CPCE CPCE Mesh Generation & Boundary Conditions 1
What is the primary advantage of a structured (hexahedral) mesh over an unstructured (tetrahedral) mesh in CFD?