PRACTICAL CFD Q&A
Short answers for the questions that interrupt a simulation.
A growing Q&A layer for students and engineers. Repeated questions can graduate into full Technical Cases, calculators and eventually the Abecator AI CFD Engineer.
Start with recurring CFD decisions.
These answers are deliberately practical. For solver-specific details, always confirm the implementation in the documentation for the version you are using.
What y+ should I target?
Choose the near-wall modelling strategy first. A wall-resolved RANS approach commonly aims around y+ ≈ 1, while conventional high-Re wall functions place the first point in the log layer. Do not pick a target independently of the turbulence model and wall treatment. Use the y+ guide and calculator.
Is first prism-layer thickness the same as the y+ wall distance?
Not automatically. In the y+ definition, y is the wall-normal distance to the near-wall solution location. A mesher may ask for geometric first-layer thickness. For a simple orthogonal cell the centroid is roughly halfway through the layer, but software conventions and real cell shapes differ.
Can I decide laminar or turbulent flow from Reynolds number alone?
Reynolds number is a key indicator, but transition depends on geometry, disturbances, roughness and pressure gradients. Pipe-flow thresholds are not universal thresholds for every external or internal flow. Use the Reynolds calculator as an engineering classification aid, not a universal transition switch.
Does Courant number have to be below 1?
No universal rule applies to all CFD solvers. Explicit methods often have strict CFL-type stability limits; implicit methods may remain stable at larger values. Even when stable, a large time step can under-resolve transient physics. Use local cell size and relevant transport or signal speed, then perform time-step sensitivity checks.
Why can a CFD case converge in residuals but still be wrong?
Residual reduction only shows part of the numerical story. Check mass/energy conservation, engineering monitors, mesh independence, boundary-condition realism, timestep independence for transient cases, and whether the chosen physical models represent the actual problem.
What should I check first when a simulation diverges?
Start with geometry/mesh quality, boundary conditions, material properties and units, initialization, timestep or relaxation, and whether the physics model is appropriate. Then localize where the instability begins instead of changing many solver settings at once.
Should I import CAD and then define physics, or define physics before final geometry preparation?
Finish the geometry topology and the region/boundary/interface structure as early as practical before investing heavily in boundary assignments and reporting. Geometry changes can invalidate face splits, contacts or boundary mappings, so a stable geometry/mesh workflow reduces rework.
Can I import SolidWorks geometry into STAR-CCM+?
Yes. A neutral CAD format such as Parasolid or STEP is commonly used. The exact best choice depends on geometry quality and your CAD workflow. After import, check part surfaces, topology and contacts before meshing.
How Q&A becomes a useful knowledge system.
Abecator is designed so a useful answer does not disappear inside a chat thread.
Question
A student, engineer or startup brings a practical CFD problem.
Short answer
The core engineering direction is captured clearly and safely.
Technical case
Repeated or high-value questions become searchable problem→diagnosis→fix pages.
Tool / AI
Structured knowledge can later support calculators, decision advisors and the AI CFD assistant.
Q&A is also the foundation for the future assistant.
The planned AI assistant will be grounded in curated CFD references, Abecator technical material and structured workflows rather than acting as an unbounded generic chatbot.