Question 1

Which turbulence model should I use for aerodynamic simulations?

Accepted Answer

k-ω SST (Menter) is the industry standard for external aerodynamics. It blends k-ω near walls with k-ε in the freestream, giving excellent accuracy for adverse pressure gradients and separation. Use y+≈1 with enhanced wall treatment. For simple internal flows, k-ε Realizable with y+=30-300 is sufficient and cheaper.

Question 2

What is y+ and how do I calculate the first-cell height?

Accepted Answer

y+ is the dimensionless wall distance defined as y+ = u*·Δy/ν, where u* = √(τ_w/ρ) is the friction velocity and ν is kinematic viscosity. For standard wall functions use y+=30–300; for low-Re treatment (SST) use y+≈1. The calculator estimates u* from the flat-plate Cf correlation and gives the required Δy.

Question 3

How do I set inlet turbulence boundary conditions?

Accepted Answer

Given turbulence intensity Tu (%) and length scale L: k = 1.5(Tu·U)², ε = Cμ^(3/4)·k^(3/2)/L, ω = k^(1/2)/(Cμ^(1/4)·L), with Cμ=0.09. For pipe inlet use Tu≈3-5%; for atmospheric boundary layer use Tu≈10-20% with L equal to the turbulent integral length scale.

Question 4

When should I use LES instead of RANS?

Accepted Answer

Use LES when accurate unsteady flow structures matter: aeroacoustics, combustion, massive separation, and turbulent mixing. LES directly resolves large eddies and only models sub-grid scales, giving far superior accuracy but at 100–10000× the cost of RANS. Detached Eddy Simulation (DES) offers a practical compromise for high-Re wall-bounded flows.

Model	Free Shear	Adverse ∂P/∂x	Separation	Heat Transfer	Rotation	Cost
k-ε Std	⭐⭐⭐	⭐⭐	⭐⭐	⭐⭐⭐	⭐	Low
k-ε RNG	⭐⭐⭐	⭐⭐⭐	⭐⭐	⭐⭐⭐	⭐⭐	Low
k-ε Real.	⭐⭐⭐	⭐⭐⭐	⭐⭐⭐	⭐⭐⭐	⭐⭐	Low
k-ω Std	⭐⭐	⭐⭐⭐	⭐⭐⭐	⭐⭐	⭐⭐	Low
k-ω SST	⭐⭐⭐	⭐⭐⭐⭐	⭐⭐⭐⭐	⭐⭐⭐	⭐⭐	Low–Med
SA	⭐⭐	⭐⭐⭐	⭐⭐⭐	⭐⭐	⭐⭐	Lowest
LES	⭐⭐⭐⭐⭐	⭐⭐⭐⭐⭐	⭐⭐⭐⭐⭐	⭐⭐⭐⭐⭐	⭐⭐⭐⭐⭐	Very High

Turbulence Model Selection Guide & y⁺ Calculator

Law of the Wall & RANS Closure