CPCE CPCE Turbulence Modeling & Numerical Methods 1 — Questions and Answers
Question 1: Which turbulence model is known as a two-equation model that solves transport equations for turbulent kinetic energy (k) and its dissipation rate (ε)?
- k-ω SST
- k-ε model (Correct answer)
- Spalart-Allmaras
- Reynolds Stress Model
Correct answer: k-ε model
The k-ε model solves two transport equations for turbulent kinetic energy k and its dissipation rate ε to close the Reynolds-averaged Navier-Stokes equations.
Question 2: What does the acronym 'DNS' stand for in the context of CFD turbulence simulation?
- Dynamic Numerical Simulation
- Direct Numerical Simulation (Correct answer)
- Discrete Navier-Stokes
- Diffusion Noise Scheme
Correct answer: Direct Numerical Simulation
DNS (Direct Numerical Simulation) resolves all turbulent length and time scales without any turbulence modeling, making it highly accurate but computationally expensive.
Question 3: The Courant-Friedrichs-Lewy (CFL) condition is primarily a stability criterion for which class of numerical methods?
- Finite Element Methods
- Boundary Element Methods
- Explicit time-stepping schemes (Correct answer)
- Implicit steady-state solvers
Correct answer: Explicit time-stepping schemes
The CFL condition restricts the time step size in explicit time-stepping schemes to ensure numerical stability by requiring that information does not travel more than one cell per time step.
Question 4: In Large Eddy Simulation (LES), what role does the Sub-Grid Scale (SGS) model play?
- It resolves large eddies directly
- It models the effect of eddies smaller than the grid size (Correct answer)
- It eliminates the need for a computational mesh
- It replaces the Navier-Stokes equations entirely
Correct answer: It models the effect of eddies smaller than the grid size
The SGS model in LES accounts for the influence of small unresolved eddies (smaller than the grid cell size) on the resolved larger-scale flow.
Question 5: Which numerical discretization scheme is second-order accurate in space and is commonly used for convection terms in CFD to balance accuracy and stability?
- First-order upwind
- Central differencing (Correct answer)
- Quick scheme
- Power-law scheme
Correct answer: Central differencing
Central differencing is second-order accurate in space and provides a good balance between accuracy and stability for convection term discretization in CFD.
Question 6: What is the primary advantage of the k-ω SST turbulence model over the standard k-ε model?
- It requires no wall functions
- It performs better in adverse pressure gradient and separated flow regions (Correct answer)
- It is computationally faster
- It eliminates the need for mesh refinement near walls
Correct answer: It performs better in adverse pressure gradient and separated flow regions
The k-ω SST model blends k-ω near walls and k-ε in freestream, giving it superior performance in adverse pressure gradients and separated flows compared to standard k-ε.
CPCE CPCE Turbulence Modeling & Numerical Methods 1
Which turbulence model is known as a two-equation model that solves transport equations for turbulent kinetic energy (k) and its dissipation rate (ε)?