CPCE CPCE Mesh Generation & Boundary Conditions 2 — Questions and Answers
Question 1: What is the 'pressure outlet' boundary condition used for in CFD, and what is commonly specified?
- It specifies velocity at the exit plane
- It specifies a gauge static pressure at the domain exit, allowing flow to leave the domain naturally (Correct answer)
- It enforces zero velocity gradient at the outlet
- It applies a mass flow rate constraint at the exit
Correct answer: It specifies a gauge static pressure at the domain exit, allowing flow to leave the domain naturally
A pressure outlet boundary specifies the static gauge pressure at the exit, allowing the solver to compute the outflow velocity profile naturally while maintaining the specified back pressure.
Question 2: What is 'mesh aspect ratio' and what problem arises when it is excessively large?
- Ratio of total cells to boundary cells; causes memory overflow
- Ratio of longest to shortest cell dimension; very high ratios can cause solver ill-conditioning and inaccuracy (Correct answer)
- Ratio of mesh density near walls to the freestream; causes incorrect y+ values
- Ratio of hexahedral to tetrahedral cells; causes mesh incompatibility
Correct answer: Ratio of longest to shortest cell dimension; very high ratios can cause solver ill-conditioning and inaccuracy
Aspect ratio is the ratio of a cell's longest edge to its shortest; extremely large aspect ratios can make the discretized equations ill-conditioned and reduce solution accuracy.
Question 3: In what situation is a 'periodic' boundary condition used in CFD?
- When the inlet and outlet pressures are equal
- When the geometry and flow repeat spatially, allowing a section of the domain to represent the full geometry (Correct answer)
- When the simulation is transient and repeats over time
- When turbulence intensity is uniform throughout the domain
Correct answer: When the geometry and flow repeat spatially, allowing a section of the domain to represent the full geometry
Periodic boundary conditions are applied when the geometry repeats in space (e.g., a turbine blade passage or a pipe with repeating fins), so only one repeating unit needs to be simulated.
Question 4: What is 'mesh independence' (or grid independence) in CFD and how is it verified?
- A mesh that does not deform during simulation; verified by checking cell quality metrics
- The state where refining the mesh no longer significantly changes the solution; verified by comparing key results on successively finer meshes (Correct answer)
- A mesh that works with any turbulence model; verified by running multiple models
- A mesh generated without user intervention; verified by automated tools
Correct answer: The state where refining the mesh no longer significantly changes the solution; verified by comparing key results on successively finer meshes
Mesh independence means the solution has converged with respect to spatial discretization; it is verified by running simulations on progressively finer meshes until key quantities (drag, pressure drop, etc.) change by less than an acceptable tolerance.
Question 5: What is the 'no-slip' wall boundary condition in CFD?
- The wall allows fluid to slip freely with zero shear stress
- The fluid velocity at the wall equals the wall velocity (zero for stationary walls), enforcing viscous adherence (Correct answer)
- The wall has a specified mass flux through its surface
- The wall maintains a constant pressure equal to ambient
Correct answer: The fluid velocity at the wall equals the wall velocity (zero for stationary walls), enforcing viscous adherence
The no-slip condition sets the fluid velocity at a solid wall equal to the wall's own velocity (zero for a stationary wall), physically representing viscous adhesion of real fluids to surfaces.
Question 6: What is a 'polyhedral mesh' and what is its advantage in CFD over tetrahedral meshes?
- A mesh made of only hexahedral cells; advantage is perfect flow alignment
- A mesh with cells having many faces; advantage is fewer cells needed for same accuracy, better gradient calculation (Correct answer)
- A mesh for 2D axisymmetric simulations only; advantage is faster generation
- A mesh with refinement only at walls; advantage is reduced y+ values
Correct answer: A mesh with cells having many faces; advantage is fewer cells needed for same accuracy, better gradient calculation
Polyhedral cells have many neighbors, providing better gradient estimates and typically requiring fewer cells than tetrahedral meshes for equivalent accuracy, reducing overall cell count and computational cost.
CPCE CPCE Mesh Generation & Boundary Conditions 2
What is the 'pressure outlet' boundary condition used for in CFD, and what is commonly specified?