The turbulentDigitalFilterInlet is a digital-filter based boundary condition for vector- and scalar-based quantities (e.g. U or T) to generate synthetic turbulence-alike time-series from input turbulence statistics for LES and DES turbulent flow computations.

In DFM or FSM, a random number set (mostly white noise), and a group of target statistics (mostly mean flow, Reynolds-stress tensor profiles and integral-scale sets) are merged into a new number set (stochastic time-series, yet consisting of the statistics) by a chain of mathematical operations whose characteristics are designated by the target statistics, so that the realised statistics of the new sets could match the target.

Random number sets ---->-| | DFM or FSM ---> New stochastic time-series consisting | turbulence statistics Turbulence statistics ->-|

The main difference between DFM and FSM is that FSM replaces the expensive-to-run streamwise convolution summation in DFM by a simpler and an almost-equivalent-in-effect numerical procedure in order to reduce computational costs. Accordingly, FSM potentially brings computational resource advantages for computations involving relatively large streamwise integral-scale sets and small time-steps.

Synthetic turbulence is generated on a virtual rectangular structured-mesh plane and is mapped onto this patch by the selected mapping method.


The condition requires entries in both the boundary and field files.

Boundary file

    type            patch;

Field file

    // Mandatory entries
    type            turbulentDigitalFilterInlet;
    n               (<label> <label>);
    mean            <PatchFunction1<vector>> or <PatchFunction1<scalar>>;
    R               <PatchFunction1<symmTensor>> or <PatchFunction1<scalar>>;
    L               <tensor> or <vector>;

    // Optional entries
    kernelType      <word>;
    AMIMethod       <word>;
    coordinateSystem  <coordinateSystem>;
    fsm             <bool>;

    // Inherited entries


Property Description Type Required Default
type Type name: turbulentDigitalFilterInlet word yes -
n Number of cells on turbulence generation plane (width height) or (e2 e3) Vector2D<label> yes -
mean Mean quantity PatchFunction1<vector> or PatchFunction1<scalar> yes -
R Reynolds stresses PatchFunction1<symmTensor> or PatchFunction1<scalar> yes -
L Integral scales tensor or vector yes -
kernelType Autocorrelation function form word no Gaussian
AMIMethod Mapping method word no faceAreaWeightAMI
coordinateSystem Coordinate system of turbulence generation plane coordinateSystem no -
fsm Flag to turn on the forward-stepwise method bool no false

The inherited entries are elaborated in:

Options for the kernelType entry:

Property | Description ———————-|——————————————– Gaussian | Standard Gaussian function exponential | Exponential function

  • The order of elements of the Reynolds stresses for the vector-based condition is “(Rxx Rxy Rxz Ryy Ryz Rzz)” and for the scalar-based condition is “R”.
  • The order of elements of the integral scales for the vector-based condition is “(Lxu Lxv Lxw Lyu Lyv Lyw Lzu Lzv Lzw)” and for the scalar-based condition is “(Lxi Lyi Lzi)”, where \c x here indicates the streamwise components and \c y/z lateral components. The streamwise components (i.e. \c x) are Lagrangian time scales in units of seconds and the remaining components are length scales in units of meters.
  • \c adjustTimeStep=true option is only supported by the forward-stepwise method.
  • The default global Cartesian coordinate system “(e1 e2 e3)” corresponds to “(x y z)”.

Further information


Source code:



  • Introduced in version v1906