Properties

Digital-filter based inlet boundary condition for velocity, i.e. U, to generate synthetic turbulence-alike time-series for LES and turbulent DES flow computations from input turbulence statistics.
Theoretical expressions of the implementation are based on
- the digital-filter method (DFM) [32]
- the forward-stepwise method (FSM) [96]
- the mass-inflow rate correction [31]
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 length-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 length-scale sets and small time-steps.
Synthetic turbulence is generated on a virtual rectangular structured-mesh plane, which is parallel to the chosen patch, and is mapped onto this patch by the selected mapping method.

Required fields:

U    | Velocity    [m/s]

Usage

Example of the boundary condition specification:

<patchName>
{
    // Mandatory entries (unmodifiable)
    type                turbulentDigitalFilterInlet;
    n                   (<nHeight> <nWidth>);
    L                   (<L1> <L2> ... <L9>);
    R                   uniform (<Rxx> <Rxy> <Rxz> <Ryy> <Ryz> <Rzz>);
    UMean               uniform (1 0 0);
    Ubulk               10.0;

    // Optional entries (unmodifiable)
    fsm                 false;
    Gaussian            true;     // always false for FSM
    fixSeed             true;
    continuous          false;
    correctFlowRate     true;
    mapMethod           nearestCell;
    perturb             1e-5;
    C1                  -1.5707;  //-0.5*PI;
    C1FSM               -0.7854   //-0.25*PI;
    C2FSM               -1.5707;  //-0.5*PI;

    // Optional (inherited) entries
    ...

where the entries mean:

Property	Description	Type	Required	Default
type	Type name: turbulentDigitalFilterInlet	word	yes	-
n	Number of cells on turbulence generation plane	tuple of labels	yes	-
L	Integral length-scale set (Lxu Lxv Lxw Lyu Lyv Lyw Lzu Lzv Lzw) [m]	tensor	yes	-
R	Reynolds stress tensor set (xx xy xz yy yz zz) [m2/s2]	symmTensorField	yes	-
UMean	Mean velocity profile [m/s]	vectorField	yes	-
Ubulk	Characteristic patch-normal bulk flow speed [m/s]	scalar	yes	-
fsm	Flag to turn on the forward-stepwise method	bool	no	false
Gaussian	Autocorrelation function form	bool	no	true
fixSeed	Flag to fix random-number generator seed to 1234 or generate a new seed based on clock-time per simulation	bool	no	true
continuous	Flag to write random-number sets at output time, and to read them on restart. Otherwise, generate new random-number sets of restart	bool	no	false
correctFlowRate	Flag to correct mass-inflow rate on turbulence plane in (only) streamwise direction	bool	no	true
mapMethod	Interpolation-to-patch method	word	no	nearestCell
perturb	Point perturbation for planarInterpolation mapMethod	scalar	no	1e-5
C1	Model constant shaping autocorrelation function (KSJ:Eq. 14)	scalar	no	-0.5*PI
C1FSM	Model coefficient in FSM (XC:Eq. 14)	scalar	no	-0.25*PI
C2FSM	Model coefficient in FSM (XC:Eq. 14)	scalar	no	-0.5*PI

The inherited entries are elaborated in:

fixedValue

Options for the fsm entry:

false | Method due to (Klein et al., 2003)
true  | Method due to (Xie-Castro, 2008)

Options for the Gaussian entry:

true  | Gaussian function
false | Exponential function (only option for FSM)

Options for the mapMethod entry:

nearestCell         | One-to-one direct map, no interpolation
planarInterpolation | Bilinear interpolation

Patch-profile input is available for two entries:

R     | Reynolds stress tensor
UMean | Mean velocity

where the input profiles and profile coordinates are located in:

Coordinates | $FOAM_CASE/constant/boundaryData/<patchName>/points
R/UMean     | $FOAM_CASE/constant/boundaryData/<patchName>/0/{R, UMean}

points file contains a list of three-dimensional coordinates, and profile data files provide a value corresponding to each coordinate.

Notes on entries

mapMethod=planarInterpolation option needs point coordinates that can form a plane.
adjustTimeStep=true option is currently not fully supported.
In order to obtain Reynolds stress tensor information, experiments, RANS simulations or engineering relations can be used.
continuous=true means deterministic-statistical consistent restart (relatively more expensive), and continuous=false means deterministic discontinuity in synthetic turbulence time-series by keeping statistical consistency (relatively cheaper).
For L, the first three entries should always correspond to the length scales in association with the convective (streamwise) mean flow direction.
Streamwise integral length scales are converted to integral time scales by using Taylor's frozen turbulence hypothesis, and Ubulk.

Further information

Tutorial:

$FOAM_TUTORIALS/verificationAndValidation/turbulentInflow/PCF

Source code:

Foam::turbulentDigitalFilterInletFvPatchVectorField

History

Introduced in version v1906

Table of Contents

Properties

Usage

Notes on entries

Further information