Turbulent Flow: Theory & CFD Modeling

A: This choice lands you solidly in the turbulent regime using the standard Re = ρVD/μ basis expected by RANS wall functions. Option B drops density and quietly swaps to ν without stating it, which shifts the order of magnitude. Option C invents a correction that doesn't exist for a round duct. Option D inflates density to a stagnation value that the flow never sees.

A: This selection gives stable pressure-drop predictions without forcing mesh refinement that the schedule won't tolerate. Option B can work but drives y+ and cell count into a corner you didn't budget. Option C is aimed at attached external flows and misses separation physics here. Option D ignores compute reality and adds no safety margin.

A: This step confirms numerical error is under control before you compare physics. Option B assumes the mesh is already adequate, which is backward. Option C tunes numerics to match data and hides discretization error. Option D checks a local metric only after global correlation has already biased you.

A: This outcome directly skews friction and thermal loads, which feeds into unsafe design decisions. Option B is a time-stepping issue, not a wall-resolution one. Option C can happen but isn't the primary consequence of high y+ misuse. Option D is unrelated to wall treatment entirely.