Mixing length

The length scale of the turbulent eddies entering through the inlet. With the intensity it sets the specific dissipation rate $\omega$ at the inlet. Blank uses 0.07 times the hydraulic diameter of the inlet.

Last updated 2026-11-03

Theory

OpenFOAM's turbulentMixingLengthFrequencyInlet computes $\omega$ from the inlet $k$ and the mixing length, equation (1) (OpenFOAM user guide, 2026). A larger length means larger, slower eddies and a smaller $\omega$ , i.e. turbulence that decays more slowly downstream of the inlet. The hydraulic diameter is that of the exported inlet surface, $D_h = 4A/P$ .

With "Fully developed" (Inlet turbulence) the typed value is not used and $\ell = 0.07\,D_h$ .

Equations

\omega = \frac{\sqrt{k}}{C_\mu^{1/4}\,\ell}, \qquad C_\mu = 0.09, \qquad \ell_{blank} = 0.07\,D_h, \qquad D_h = \frac{4A}{P}

(1)

where	meaning	unit
$\omega$	Specific dissipation rate at the inlet	1/s
$k$	Turbulent kinetic energy at the inlet	m²/s²
$\ell$	Mixing length	m
$D_h$	Hydraulic diameter of the inlet patch	m
$A, P$	Area and perimeter of the inlet patch	m², m

Default and rule

Default: blank = 0.07 D_h of the inlet
Rule: Blank: $0.07\,D_h$ .
Source: Inlet condition as documented for OpenFOAM (OpenFOAM user guide, 2026). The estimate $0.07\,D_h$ for fully developed pipe flow is a widely used engineering value.

Valid range > 0; used only with 'Intensity and length'

Values

Positive, and smaller than the inlet diameter; $0.07\,D_h$ is the usual choice for pipe flow.

References 1

OpenCFD Ltd. (2026). OpenFOAM user guide. openfoam.com. Web page

Theory

Equations

Default and rule

Valid range > 0; used only with 'Intensity and length'

Related 3

References 1