CPCE CFD Post-Processing, Validation & Verification 1 — Questions and Answers
Question 1: In CFD post-processing, what does a streamline represent?
- A line tangent to the velocity vector field at every point at a given instant (Correct answer)
- The path traced by a fluid particle over time
- A line connecting points of equal pressure in the flow domain
- A contour of constant vorticity magnitude
Correct answer: A line tangent to the velocity vector field at every point at a given instant
A streamline is a curve that is instantaneously tangent to the velocity vector at every point, representing the direction of flow at a specific time instant.
Question 2: Which metric is used to assess spatial discretization error in CFD by comparing solutions on systematically refined grids?
- Courant-Friedrichs-Lewy (CFL) number
- Grid Convergence Index (GCI) (Correct answer)
- Reynolds Averaging Error (RAE)
- Kolmogorov length scale ratio
Correct answer: Grid Convergence Index (GCI)
The Grid Convergence Index (GCI) is the standard metric recommended by ASME and AIAA to quantify spatial discretization uncertainty using Richardson extrapolation across refined grids.
Question 3: What is the primary purpose of a Q-criterion isosurface in CFD post-processing?
- To visualize regions of high pressure gradient
- To identify and visualize vortical structures in the flow (Correct answer)
- To show areas where the mesh quality is poor
- To mark zones of flow separation from walls
Correct answer: To identify and visualize vortical structures in the flow
The Q-criterion identifies vortices by locating regions where the rotation rate tensor dominates the strain rate tensor (Q > 0), making it useful for visualizing coherent vortical structures.
Question 4: In verification and validation (V&V) of CFD, 'verification' specifically refers to:
- Comparing CFD results against experimental data to assess physical accuracy
- Ensuring the numerical algorithm correctly solves the governing equations (Correct answer)
- Checking that the mesh meets quality thresholds such as skewness and aspect ratio
- Validating that the turbulence model is appropriate for the flow regime
Correct answer: Ensuring the numerical algorithm correctly solves the governing equations
Verification confirms that the mathematical model is solved correctly — i.e., the numerical implementation is free of coding errors and discretization converges — without regard to whether the model represents reality.
Question 5: When monitoring residuals during a steady-state CFD simulation, which condition best indicates convergence?
- Residuals exceed 1e-2 for all transport equations
- Residuals plateau at a constant non-zero value and key monitor quantities stabilize (Correct answer)
- Residuals oscillate periodically with decreasing amplitude
- Residuals drop to exactly zero for the continuity equation
Correct answer: Residuals plateau at a constant non-zero value and key monitor quantities stabilize
True convergence in steady-state CFD is indicated when residuals have leveled off (not necessarily at machine precision) and engineering quantities of interest such as drag, lift, or mass flow rate have stabilized.
Question 6: In CFD results, a non-dimensionalized wall distance y+ ≈ 1 is required when using:
- Wall functions in the k-ε model
- Low-Reynolds-number turbulence models that resolve the viscous sublayer (Correct answer)
- Inviscid Euler solvers near solid walls
- Large Eddy Simulation with a coarse near-wall mesh
Correct answer: Low-Reynolds-number turbulence models that resolve the viscous sublayer
Low-Reynolds-number turbulence models and wall-resolved LES require y+ ≈ 1 to accurately capture the viscous sublayer without relying on wall-function approximations.
Question 7: Which technique is used to extract a force coefficient (e.g., drag coefficient) from a CFD solution?
- Integrating velocity magnitude over the inlet face
- Integrating pressure and viscous stress over the body surface and normalizing by dynamic pressure and reference area (Correct answer)
- Computing the gradient of the streamfunction along the wake centerline
- Averaging the total pressure drop across the domain
Correct answer: Integrating pressure and viscous stress over the body surface and normalizing by dynamic pressure and reference area
Drag coefficient is obtained by integrating both pressure forces and viscous (shear) forces over the body surface, then dividing by the product of the reference dynamic pressure and reference area.
CPCE CFD Post-Processing, Validation & Verification 1
In CFD post-processing, what does a streamline represent?