CFD Software & Simulation Techniques Flashcards

What is the primary purpose of 'conjugate heat transfer' (CHT) simulations in CFD?

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.

In CFD mesh generation, what is 'aspect ratio' and why is a high aspect ratio problematic in non-flow-aligned directions?

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.

Which feature in STAR-CCM+ allows automatic mesh refinement in regions of high flow gradient during a simulation?

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.

In the context of CFD solver settings, what is 'false diffusion' and which approach minimizes it?

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.

What role does the 'Courant-Friedrichs-Lewy (CFL) condition' play in transient CFD simulations?

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.

Which turbulence model is most commonly used for internal flows in ducts and pipes where accurate prediction of near-wall behavior is critical?

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.

In CFD post-processing, what is the difference between 'pathlines' and 'streamlines'?

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.