CPCE CFD Software & Simulation Techniques 5 — Questions and Answers
Question 1: What is the primary purpose of 'conjugate heat transfer' (CHT) simulations in CFD?
- Model radiation between surfaces only
- Solve coupled fluid flow and solid conduction simultaneously (Correct answer)
- Compute only convective heat transfer coefficients
- Apply isothermal wall boundary conditions
Correct answer: Solve coupled fluid flow and solid conduction simultaneously
CHT simulations solve the energy equations in both fluid and solid domains simultaneously, accounting for conduction within solids coupled to convection in the fluid.
Question 2: In CFD mesh generation, what is 'aspect ratio' and why is a high aspect ratio problematic in non-flow-aligned directions?
- Ratio of cell volume to surface area; high values cause memory issues
- Ratio of the longest to shortest edge of a cell; high values cause numerical errors in cross-flow gradients (Correct answer)
- Ratio of viscous to inertial forces; high values indicate turbulent flow
- Ratio of mesh size to domain size; high values indicate over-refinement
Correct answer: Ratio of the longest to shortest edge of a cell; high values cause numerical errors in cross-flow gradients
Cells with high aspect ratio aligned perpendicular to the flow gradient direction introduce significant truncation error and can cause solver instability.
Question 3: Which feature in STAR-CCM+ allows automatic mesh refinement in regions of high flow gradient during a simulation?
- Mesh morphing
- Adaptive mesh refinement (AMR) (Correct answer)
- Polyhedral conversion
- Surface wrapper
Correct answer: Adaptive mesh refinement (AMR)
STAR-CCM+'s AMR feature automatically refines or coarsens mesh cells based on solution-based criteria such as velocity gradient or pressure gradient magnitude.
Question 4: In the context of CFD solver settings, what is 'false diffusion' and which approach minimizes it?
- Artificial compressibility added to stabilize the solver; minimized by reducing time step
- Numerical diffusion caused by mesh-flow misalignment with first-order schemes; minimized by higher-order schemes and flow-aligned meshes (Correct answer)
- Diffusion introduced by turbulence models; minimized by DNS
- Diffusion from periodic boundary conditions; minimized by increasing domain size
Correct answer: Numerical diffusion caused by mesh-flow misalignment with first-order schemes; minimized by higher-order schemes and flow-aligned meshes
False diffusion arises when flow crosses cell faces obliquely with first-order upwind schemes; using second-order schemes and aligning mesh with the flow reduces it.
Question 5: What role does the 'Courant-Friedrichs-Lewy (CFL) condition' play in transient CFD simulations?
- Limits the maximum mesh size
- Constrains the time step to ensure numerical stability in explicit time-marching schemes (Correct answer)
- Specifies the turbulence intensity at inlets
- Determines the convergence tolerance for iterative solvers
Correct answer: Constrains the time step to ensure numerical stability in explicit time-marching schemes
The CFL condition requires that the time step be small enough that information cannot travel more than one cell per time step, ensuring stability of explicit schemes.
Question 6: Which turbulence model is most commonly used for internal flows in ducts and pipes where accurate prediction of near-wall behavior is critical?
- Spalart-Allmaras (SA)
- Standard k-ε with standard wall functions
- SST k-ω (Correct answer)
- Large Eddy Simulation (LES)
Correct answer: SST k-ω
The SST k-ω model blends k-ω near walls (for accurate boundary layer resolution) with k-ε in the freestream, making it well-suited for adverse pressure gradient and separated flows.
Question 7: In CFD post-processing, what is the difference between 'pathlines' and 'streamlines'?
- Pathlines show pressure contours; streamlines show velocity vectors
- Pathlines trace actual particle trajectories in unsteady flow; streamlines are tangent to velocity field at a single instant (Correct answer)
- Pathlines are used only in laminar flow; streamlines only in turbulent flow
- Pathlines represent averaged flow; streamlines represent instantaneous flow
Correct answer: Pathlines trace actual particle trajectories in unsteady flow; streamlines are tangent to velocity field at a single instant
Streamlines are instantaneous curves tangent to the velocity field, while pathlines trace the actual path of a fluid particle over time and differ from streamlines in unsteady flows.
CPCE CFD Software & Simulation Techniques 5
What is the primary purpose of 'conjugate heat transfer' (CHT) simulations in CFD?