CPCE CFD Post-Processing, Validation & Verification 3

Question 1

What is the purpose of computing the 'pressure coefficient' (Cp) distribution in external aerodynamics CFD post-processing?

Accepted Answer

To non-dimensionalize pressure for comparison across different flow speeds and geometries

Answer

The pressure coefficient Cp = (p - p∞) / (0.5ρU²∞) normalizes local pressure against freestream dynamic pressure, enabling comparison of pressure distributions across different scales, speeds, and geometries.

Question 2

In a CFD report for engineering design, which information is essential to include for the simulation to be reproducible?

Accepted Answer

Solver settings, mesh statistics, boundary conditions, turbulence model, and convergence criteria

Answer

Reproducibility requires complete documentation of solver type, discretization schemes, mesh details, all boundary conditions, physics models (e.g., turbulence), and convergence criteria so another engineer can replicate the simulation.

Question 3

Which approach is used to extract a boundary layer velocity profile from a CFD solution for comparison with the Blasius or log-law theoretical profiles?

Accepted Answer

Sampling velocity data along a line normal to the wall at a specific streamwise location

Answer

A line probe (or 'rake') is placed perpendicular to the wall at the desired streamwise station to extract the velocity magnitude versus wall-normal distance, which is then compared with analytical profiles.

Question 4

The 'effective viscosity ratio' (μt/μ) in post-processing of RANS simulations indicates:

Accepted Answer

The local importance of turbulent mixing relative to molecular viscosity

Answer

The turbulent-to-laminar viscosity ratio μt/μ shows where turbulence dominates momentum transport; very high values (>1000) may indicate unrealistic over-prediction of turbulent viscosity in some RANS models.

Question 5

Which numerical artifact can appear in CFD results when using a first-order upwind scheme on a coarse mesh, and is revealed during post-processing?

Accepted Answer

Numerical diffusion — excessive smearing of sharp gradients and shocks

Answer

First-order upwind schemes introduce significant numerical diffusion (false diffusion), which smears sharp features like boundary layers, shocks, and mixing layers, producing overly smooth solutions on coarse meshes.

(CPCE) Certified Professional CFD Engineer Practice Test

(CPCE) Certified Professional CFD Engineer Practice Test

CPCE CFD Post-Processing, Validation & Verification 3