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CFD Calculators

Practical tools for computational fluid dynamics engineers and researchers — built for real-world workflows. Fast, accurate, and no fluff.

Pure client-side JavaScript
Peer-validated formulas
Internal & external flows
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Engineering Tools

Everything you need before running your CFD case

Estimate flow regimes, set turbulence boundary conditions, and determine wall spacing — all before opening your solver.

U_max Hydraulic Diameter D_h D_h Re = ρUDh ─── μ
// 01 — FLOW CHARACTERIZATION

Hydraulic Diameter & Reynolds Number Calculator

Determine the Reynolds number and flow regime for internal and external flows. Supports arbitrary duct cross-sections via hydraulic diameter, circular pipes, flat plates, and more.

Inputs
Flow type (internal / external) Velocity (m/s) Density (kg/m³) Dynamic viscosity (Pa·s) Length / diameter (m)
Outputs
Reynolds number Re Hydraulic diameter Dh Flow regime Laminar / transitional / turbulent
-5/3 Energy input Dissipation κ (1/m) E(κ) k ε ω μ_t Computed outputs →
// 02 — BOUNDARY CONDITIONS

Turbulence Properties Calculator

Set inlet turbulence boundary conditions for RANS solvers. Converts intuitive parameters — turbulence intensity and length scale — into the solver-required variables k, ε, ω, and eddy viscosity ratio.

Inputs
Turbulence intensity I (%) Reference velocity U (m/s) Turbulent length scale l (m)
Outputs
Turbulent kinetic energy k Dissipation rate ε Specific dissipation ω Eddy viscosity ratio μ_t/μ
Δy₁ y⁺ ≤ 1 y⁺ ~ 30 y⁺ ~ 100 Boundary layer δ δ(x) Wall (no-slip)
// 03 — NEAR-WALL RESOLUTION

Y⁺ Calculator

Calculate y⁺ values and determine the required first cell height to achieve your target y⁺ for wall-bounded flows. Works for both internal duct flows and external boundary layers.

Inputs
Flow type (internal / external) Freestream velocity (m/s) Density & viscosity Target y⁺ value
Outputs
Wall shear stress τ_w Friction velocity u_τ First cell height Δy₁ y⁺ for given Δy₁
Π similarity Re inertia/viscous Nu heat transfer Pr momentum/thermal Ma flow / speed of sound St Strouhal Fr inertia/gravity
// 04 — FLOW SIMILARITY

Non-Dimensional Numbers Calculator

Compute the full suite of non-dimensional numbers used in fluid mechanics and heat transfer. Scale experiments, generalize results across fluids and geometries, and establish dynamic similarity between model and prototype.

Numbers covered
Reynolds (Re) Mach (Ma) Prandtl (Pr) Nusselt (Nu) Strouhal (St)
Also includes
Froude (Fr) Weber (We) Grashof (Gr) Rayleigh (Ra) Euler (Eu)
Key Formulas

Physics behind the calculators

All formulas are documented with references to standard CFD textbooks and NASA technical reports.

Reynolds Number
D_h = 4A/P for non-circular ducts. Laminar: Re < 2300, Turbulent: Re > 4000.
Turbulent Kinetic Energy
I = turbulence intensity. ε = Cμ^(3/4)·k^(3/2)/l. ω = k^(1/2)/(Cμ^(1/4)·l).
First Cell Height (y⁺)
u_τ = √(τw/ρ). For internal flows: τw via Blasius friction factor. External: Cf via Schlichting correlation.
Friction Velocity
τw = Cf·(1/2)·ρ·U². Cf from Schlichting: 0.0592·Re^(-1/5) for turbulent flat plate.
Heat Transfer — Dittus-Boelter Correlation
n = 0.4 for heating, 0.3 for cooling. Valid for 0.6 < Pr < 160, Re > 10,000 — the standard forced-convection correlation referenced across the calculators above.
Who actually uses this

Same calculators, different reasons to open them

🎓
Students
Check a homework Reynolds number or y⁺ target in seconds instead of hunting through a textbook appendix for the right correlation.
🔬
Researchers
Sanity-check mesh sizing and boundary-layer resolution before launching a multi-hour OpenFOAM or CONVERGE run.
⚙️
Industry Engineers
Get a quick first-cell-height or heat-transfer estimate to scope a simulation contract without opening a full CFD license.
More tools coming

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