CPCE CPCE Mesh Generation & Boundary Conditions 2

Question 1

What is the 'pressure outlet' boundary condition used for in CFD, and what is commonly specified?

Accepted Answer

It specifies a gauge static pressure at the domain exit, allowing flow to leave the domain naturally

Answer

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?

Accepted Answer

Ratio of longest to shortest cell dimension; very high ratios can cause solver ill-conditioning and inaccuracy

Answer

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?

Accepted Answer

When the geometry and flow repeat spatially, allowing a section of the domain to represent the full geometry

Answer

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?

Accepted Answer

The state where refining the mesh no longer significantly changes the solution; verified by comparing key results on successively finer meshes

Answer

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?

Accepted Answer

The fluid velocity at the wall equals the wall velocity (zero for stationary walls), enforcing viscous adherence

Answer

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.

(CPCE) Certified Professional CFD Engineer Practice Test

(CPCE) Certified Professional CFD Engineer Practice Test

CPCE CPCE Mesh Generation & Boundary Conditions 2