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Is Cfd Turbulence Modeling free to use?

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Who published Cfd Turbulence Modeling?

ashfordeOU (@ashfordeou) published this skill. Their other Agent Skills are listed on their SkillMD profile.

Cfd Turbulence Modeling by ashfordeou

Name: Cfd Turbulence Modeling
Author: ashfordeOU

Cfd Turbulence Modeling

Use when you must estimate the wall-normal first cell height for a CFD mesh: compute the y plus value from the friction velocity and kinematic viscosity, derive the friction velocity from the wall shear stress or the skin friction coefficient, and recommend the turbulence model and wall treatment for the boundary layer. Produces the y plus value, the friction velocity, and the turbulence model recommendation that size the near-wall mesh. Trigger: turbulence model, y plus, friction velocity, boundary layer, wall treatment, cfd.

ashfordeOU Updated Sep 17, 2026

File contents

CFD Turbulence Modeling (aerodynamics/cfd/cfd-turbulence-modeling)

Use when the task is CFD turbulence modeling for a wall-bounded flow: sizing the first cell height with y plus, deriving the friction velocity, and selecting the turbulence model and wall treatment.

Domain quick reference

The dimensionless wall distance is y+ = y * u_tau / nu, with y in meters, u_tau in m/s, and nu in m^2/s; it measures how far the first cell center sits from the wall in wall units.
The friction velocity u_tau = sqrt(tau_w / rho), with tau_w in Pa and rho in kg/m^3, scales the near-wall velocity profile.
From the skin friction coefficient, u_tau = v_inf * sqrt(cf / 2), with v_inf in m/s.
The target y+ drives the model choice: resolve the viscous sublayer at y+ about 1 (k-omega SST), blend at y+ about 30 (realizable k-epsilon), or use wall functions up to y+ about 300 (SA with wall functions).
NACA Report 824 anchors classic airfoil boundary-layer data used to sanity-check skin friction and transition behavior.

Workflow

Collect y, u_tau, and nu (or tau_w and rho, or cf and v_inf).
Compute the y plus value with y_plus.
Derive the friction velocity with friction_velocity or friction_velocity_from_cf.
Choose the turbulence model with turbulence_model_recommendation.
Size the first cell height from the target y+.

Pitfalls

Targeting y+ about 30 while running a low-Reynolds-number model that needs the sublayer resolved; the model and the y+ target must agree.
Mixing units in u_tau: seconds for the friction velocity with centimeters for y gives a y plus off by orders of magnitude.
Ignoring separation when choosing the model; wall functions degrade in separated regions, so flag separated flow even when the y+ band suggests a wall-function model.

Behavior contract (gate 3)

The y plus, friction velocity, and recommendation logic is exercised by the gate 3 contract test: scripts/test_cfd_turbulence.py against scripts/cfd_turbulence_logic.py (stdlib unittest, offline). Run: python3 scripts/test_cfd_turbulence.py

Compliance

NACA Report 824 is US government work (public domain); summary and reference data only, per standards-map.yaml. Turbulence modeling guidance is common CFD methodology, not reproduced text.
compliance: STANDARDS-REF, gated: false.

---name: cfd-turbulence-modelingdescription: Use when you must estimate the wall-normal first cell height for a CFD mesh: compute the y plus value from the friction velocity and kinematic viscosity, derive the friction velocity from the wall shear stress or the skin friction coefficient, and recommend the turbulence model and wall treatment for the boundary layer. Produces the y plus value, the friction velocity, and the turbulence model recommendation that size the near-wall mesh. Trigger: turbulence model, y plus, friction velocity, boundary layer, wall treatment, cfd.license: Apache-2.0---# CFD Turbulence Modeling (aerodynamics/cfd/cfd-turbulence-modeling)Use when the task is CFD turbulence modeling for a wall-boundedflow: sizing the first cell height with y plus, deriving thefriction velocity, and selecting the turbulence model and walltreatment.## Domain quick reference- The dimensionless wall distance is y+ = y * u_tau / nu, with y in meters, u_tau in m/s, and nu in m^2/s; it measures how far the first cell center sits from the wall in wall units.- The friction velocity u_tau = sqrt(tau_w / rho), with tau_w in Pa and rho in kg/m^3, scales the near-wall velocity profile.- From the skin friction coefficient, u_tau = v_inf * sqrt(cf / 2), with v_inf in m/s.- The target y+ drives the model choice: resolve the viscous sublayer at y+ about 1 (k-omega SST), blend at y+ about 30 (realizable k-epsilon), or use wall functions up to y+ about 300 (SA with wall functions).- NACA Report 824 anchors classic airfoil boundary-layer data used to sanity-check skin friction and transition behavior.## Workflow1. Collect y, u_tau, and nu (or tau_w and rho, or cf and v_inf).2. Compute the y plus value with y_plus.3. Derive the friction velocity with friction_velocity or friction_velocity_from_cf.4. Choose the turbulence model with turbulence_model_recommendation.5. Size the first cell height from the target y+.## Pitfalls- Targeting y+ about 30 while running a low-Reynolds-number model that needs the sublayer resolved; the model and the y+ target must agree.- Mixing units in u_tau: seconds for the friction velocity with centimeters for y gives a y plus off by orders of magnitude.- Ignoring separation when choosing the model; wall functions degrade in separated regions, so flag separated flow even when the y+ band suggests a wall-function model.## Behavior contract (gate 3)The y plus, friction velocity, and recommendation logic isexercised by the gate 3 contract test: scripts/test_cfd_turbulence.pyagainst scripts/cfd_turbulence_logic.py (stdlib unittest, offline).Run:python3 scripts/test_cfd_turbulence.py## Compliance- NACA Report 824 is US government work (public domain); summary and reference data only, per standards-map.yaml. Turbulence modeling guidance is common CFD methodology, not reproduced text.- compliance: STANDARDS-REF, gated: false.

Frequently asked questions