CPCE CFD Software & Simulation Techniques 4 — Questions and Answers
Question 1: In CFD, what is 'operator splitting' used for in time integration?
- Decomposing the domain for parallel processing
- Treating different physics operators with separate integration steps (Correct answer)
- Splitting mesh blocks for AMR
- Separating laminar and turbulent contributions
Correct answer: Treating different physics operators with separate integration steps
Operator splitting (e.g., Strang splitting) integrates different physics terms (advection, diffusion, source) in separate sub-steps, allowing use of optimal schemes for each.
Question 2: What is the key advantage of the Detached Eddy Simulation (DES) approach over pure LES for high-Reynolds-number external flows?
- DES resolves all scales everywhere in the domain
- DES uses RANS near walls to reduce near-wall mesh cost while using LES away from walls (Correct answer)
- DES is always more accurate than LES
- DES eliminates the need for a turbulence model
Correct answer: DES uses RANS near walls to reduce near-wall mesh cost while using LES away from walls
DES is a hybrid method that applies RANS in attached boundary layers (where full LES is prohibitively expensive) and LES in separated regions where large eddies dominate.
Question 3: In ANSYS Fluent, which boundary condition type is most appropriate when the total pressure and total temperature at an inlet are known?
- Velocity inlet
- Mass flow inlet
- Pressure far-field
- Pressure inlet (Correct answer)
Correct answer: Pressure inlet
The pressure inlet boundary condition in Fluent accepts total (stagnation) pressure and temperature, making it appropriate when those quantities are known at the inlet.
Question 4: Which numerical scheme is known for its ability to capture shock waves sharply without spurious oscillations in compressible flow simulations?
- Central difference scheme
- TVD (Total Variation Diminishing) scheme (Correct answer)
- Crank-Nicolson scheme
- Pure Galerkin finite element scheme
Correct answer: TVD (Total Variation Diminishing) scheme
TVD schemes apply flux limiters to suppress oscillations near discontinuities (shocks) while maintaining high-order accuracy in smooth flow regions.
Question 5: What does 'GCI' (Grid Convergence Index) measure in CFD verification?
- Solver residual reduction per iteration
- Discretization error estimate using Richardson extrapolation on multiple mesh levels (Correct answer)
- Turbulence model uncertainty
- Boundary condition sensitivity
Correct answer: Discretization error estimate using Richardson extrapolation on multiple mesh levels
GCI provides a quantitative estimate of discretization error by applying Richardson extrapolation to solutions on systematically refined meshes.
Question 6: In a Volume of Fluid (VOF) simulation for two-phase flow, what does the volume fraction α = 0.5 represent?
- Pure phase 1
- Pure phase 2
- The interface between two phases (Correct answer)
- A phase with 50% density
Correct answer: The interface between two phases
In VOF, α = 0 and α = 1 represent pure phases, while intermediate values (including 0.5) mark the interfacial region where both phases coexist in a cell.
Question 7: Which parallel decomposition method in OpenFOAM divides the mesh by minimizing inter-processor boundary faces?
- Simple decomposition
- Hierarchical decomposition
- Scotch decomposition (Correct answer)
- Manual decomposition
Correct answer: Scotch decomposition
Scotch decomposition uses graph partitioning to minimize the number of inter-processor boundaries, reducing communication overhead in parallel simulations.
CPCE CFD Software & Simulation Techniques 4
In CFD, what is 'operator splitting' used for in time integration?