CPCE Fluid Flow & Heat Transfer Analysis 3 — Questions and Answers
Question 1: In CFD mesh generation, what is the primary advantage of a structured hexahedral mesh over an unstructured tetrahedral mesh?
- Easier generation for complex geometries
- Lower numerical diffusion and better convergence (Correct answer)
- Automatic local refinement capability
- Better representation of curved surfaces
Correct answer: Lower numerical diffusion and better convergence
Hexahedral structured meshes produce lower numerical diffusion due to their alignment with flow direction and better convergence behavior in CFD solvers.
Question 2: What does the Courant-Friedrichs-Lewy (CFL) condition limit in transient CFD simulations?
- Maximum mesh size relative to geometry
- Time step size relative to convective transport (Correct answer)
- Turbulence intensity at inlet boundaries
- Pressure residual convergence criterion
Correct answer: Time step size relative to convective transport
The CFL condition requires that the time step be small enough that information does not travel more than one cell width per time step, ensuring numerical stability.
Question 3: In natural convection problems, which dimensionless number determines the flow regime and heat transfer rate?
- Reynolds number
- Rayleigh number (Correct answer)
- Eckert number
- Biot number
Correct answer: Rayleigh number
The Rayleigh number (Ra = Gr × Pr) characterizes natural convection and determines whether flow is laminar or turbulent in buoyancy-driven problems.
Question 4: What type of CFD simulation approach resolves all turbulent length and time scales without any turbulence modeling?
- Reynolds-Averaged Navier-Stokes (RANS)
- Large Eddy Simulation (LES)
- Direct Numerical Simulation (DNS) (Correct answer)
- Detached Eddy Simulation (DES)
Correct answer: Direct Numerical Simulation (DNS)
DNS resolves the complete range of turbulent scales from the Kolmogorov microscales to the integral length scales, requiring no turbulence model but enormous computational resources.
Question 5: In a CFD simulation of conjugate heat transfer, what is solved simultaneously?
- Laminar and turbulent flow regions
- Compressible and incompressible flow zones
- Fluid flow and solid conduction heat transfer (Correct answer)
- Forced and natural convection regimes
Correct answer: Fluid flow and solid conduction heat transfer
Conjugate heat transfer CFD simulations simultaneously solve the fluid energy equation in flow regions and the heat conduction equation in solid regions.
Question 6: Which finite volume discretization scheme is second-order accurate in space and centered in nature?
- First-order upwind scheme
- Power law scheme
- Central differencing scheme (Correct answer)
- QUICK scheme
Correct answer: Central differencing scheme
The central differencing scheme evaluates face values as arithmetic averages of neighboring cell values, yielding second-order spatial accuracy but prone to instability at high Peclet numbers.
Question 7: For flow over a cylinder, at what approximate Reynolds number does the von Kármán vortex street first appear?
- Re ≈ 5
- Re ≈ 40 (Correct answer)
- Re ≈ 150
- Re ≈ 1000
Correct answer: Re ≈ 40
Von Kármán vortex shedding begins at approximately Re ≈ 40-50 when the symmetric steady wake becomes unstable and alternating vortices are shed.
CPCE Fluid Flow & Heat Transfer Analysis 3
In CFD mesh generation, what is the primary advantage of a structured hexahedral mesh over an unstructured tetrahedral mesh?