Modeling of bed roughness using a geometry function and forcing - - PowerPoint PPT Presentation

Introduction Solver set-up Case set-up Post-Processing

Modeling of bed roughness using a geometry function and forcing terms in the momentum equations

Jonatan Margalit

A project work in the course CFD with OpenSource software, taught by H˚ akan Nilsson at Chalmers University of Technology, Gothenburg, Sweden

01-12-2014

Jonatan Margalit Modeling of bed roughness in OpenFOAM 01-12-2014 1 / 28

Introduction Solver set-up Case set-up Post-Processing

Background

A tutorial of how to set up an OpenFOAM case with a rough bed using porosity to define the bed elements. Wall functions have quite limited success with LES. Good for simulating near bed dynamics. Use of structured mesh with porous bed elements Bed elements - normal distribution. Method based on Stoesser, T. (2010). ”Physically Realistic Roughness Closure Scheme to Simulate Turbulent Channel Flow over Rough Beds within the Framework of LES.” J. Hydraul. Eng., 136(10), 812-819.

Jonatan Margalit Modeling of bed roughness in OpenFOAM 01-12-2014 2 / 28

Introduction Solver set-up Case set-up Post-Processing

Shear stress in open-channel flow

Force balance in time averaged pressure driven channel flow: ∂ ∂y

• µ∂u

∂y − ρu′v′

• = ∂p

∂x, Denoting the shear with τ: ∂τ ∂y = ∂p ∂x, Solution for the mean shear stress: τ = −∂p ∂xh

• 1 − y

h

• .

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Introduction Solver set-up Case set-up Post-Processing

Velocity profiles

Log law over a smooth bed: u Uf = 1 κ ln

• y+

+ 5.5, Log law over a rough wall: u Uf = 1 κ ln 30y ks

• ,

with friction velocity Uf =

• τb/ρ.

From wikipedia

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Introduction Solver set-up Case set-up Post-Processing

Getting started

Start by sourcing your OpenFOAM-2.2.x:

source $HOME/OpenFOAM/OpenFOAM-2.2.x/etc/bashrc

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Introduction Solver set-up Case set-up Post-Processing

Configuring the solver

Make a copy of the pimpleFoam solver in the user directory:

cd $FOAM_APP cp -r --parents solvers/incompressible/pimpleFoam $WM_PROJECT_USER_DIR cd $WM_PROJECT_USER_DIR/solvers/incompressible

Rename the copied pimpleFoam solver:

mv pimpleFoam pimpleFoam_mod cd pimpleFoam_mod mv pimpleFoam.C pimpleFoam_mod.C

Cleanup the directory and delete the unnecessary folders and files:

wclean rm -rf SRFPimpleFoam/ pimpleDyMFoam/ Allwmake Make/linux*

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Introduction Solver set-up Case set-up Post-Processing

Write out sgs and viscous stresses

Add the following before the runtime.write() in pimpleFoam_mod.C.

nuGradU = laminarTransport.nu()*-2*dev(symm(fvc::grad(U))); B = turbulence->R();

Add to createFields.H

volSymmTensorField nuGradU ( IOobject ( "nuGradU", runTime.timeName(), mesh, IOobject::NO_READ, IOobject::AUTO_WRITE ), laminarTransport.nu()*-2*dev(symm(fvc::grad(U))) );

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Introduction Solver set-up Case set-up Post-Processing

Write out sgs and viscous stresses

Add to createFields.H

volSymmTensorField B ( IOobject ( "B", runTime.timeName(), mesh, IOobject::NO_READ, IOobject::AUTO_WRITE ), laminarTransport.nu()*-2*dev(symm(fvc::grad(U))) );

Modify Make/files and compile with wmake

Jonatan Margalit Modeling of bed roughness in OpenFOAM 01-12-2014 8 / 28

Introduction Solver set-up Case set-up Post-Processing

Copying a case from the tutorials

Copy the tutorial case channel395:

cp -r $FOAM_TUTORIALS/incompressible/pimpleFoam/channel395 $FOAM_RUN run

In the run folder make another copy of channel395 and call it roughChannel:

cp -r channel395 roughChannel

Now run the channel395 case while you configure the roughChannel:

blockMesh -case channel395 pimpleFoam -case channel395 > channel395/log.run &

We will use the results for comparison.

Jonatan Margalit Modeling of bed roughness in OpenFOAM 01-12-2014 9 / 28

Introduction Solver set-up Case set-up Post-Processing

Edit the mesh

Open constant/polyMesh/blockMeshDict and edit the vertices section to the following:

vertices ( (0 0 0) (4 0 0) (0 0.2 0) (4 0.2 0) (0 1 0) (4 1 0) (0 0 2) (4 0 2) (0 0.2 2) (4 0.2 2) (0 1 2) (4 1 2) );

Jonatan Margalit Modeling of bed roughness in OpenFOAM 01-12-2014 10 / 28

Introduction Solver set-up Case set-up Post-Processing

Edit the mesh

Then edit the blocks section in the following way:

blocks ( hex (0 1 3 2 6 7 9 8) (60 25 40) simpleGrading (1 1 1) hex (2 3 5 4 8 9 11 10) (60 35 40) simpleGrading (1 6 1) );

Finally, change the boundary type of the topWall into a patch:

topWall { type patch; faces ((4 10 11 5)); }

Run blockMesh

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Introduction Solver set-up Case set-up Post-Processing

Boundary and internal conditions of the fields

Set all internalFields to 0 in the 0 folder files. For the velocity set the boundary condition at the topWall to be of the type slip, in order to have a rigid lid surface:

topWall { type slip; }

In the turbulence energy file k set a Neumann boundary condition at the top wall by modifying it into:

topWall { type zeroGradient; }

Jonatan Margalit Modeling of bed roughness in OpenFOAM 01-12-2014 12 / 28

Introduction Solver set-up Case set-up Post-Processing

Mapping fields

Map the fields of the lower half of the channel in channel395 into your rough channel. This is done by first copying the mapFieldsDict into the system folder:

cp -r $FOAM_APP/utilities/preProcessing/mapFields/mapFieldsDict system/

Modify mapFieldsDict:

patchMap ( bottomWall bottomWall sides1_half0 sides1_half0 sides2_half0 sides2_half0 inout1_half0 inout1_half0 inout2_half0 inout2_half0 ); cuttingPatches ( topWall );

Map the fields: mapFields ../channel395/

Jonatan Margalit Modeling of bed roughness in OpenFOAM 01-12-2014 13 / 28

Introduction Solver set-up Case set-up Post-Processing

Generating the rough bed zone

Download the file boxes.C from pingpong and place it in the system folder. Compile the file with

g++ -std=c++11 boxes.C -o boxes

Run the file boxes:

./boxes > log.boxes

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Introduction Solver set-up Case set-up Post-Processing

Generating the rough bed zone

Copy the topoSetDict file into the system folder:

cp -r $FOAM_APP/utilities/mesh/manipulation/topoSet/topoSetDict .

Edit topoSetDict to the following:

actions ( { name bed; type cellSet; action new; source boxToCell; sourceInfo { boxes ( #include "boxes.txt" ); } } );

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Introduction Solver set-up Case set-up Post-Processing

Adding sources with fvOptions

Run topoSet followed by setsToZones Now open up system/fvOptions

momentumSource { type pressureGradientExplicitSource; active

• ff;

//on/off switch selectionMode all; //cellSet // points //cellZone pressureGradientExplicitSourceCoeffs { fieldNames (U); Ubar ( 0.1335 0 0 ); } }

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Introduction Solver set-up Case set-up Post-Processing

Adding sources with fvOptions

porosity { type explicitPorositySource; active true; selectionMode cellZone; cellZone bed; explicitPorositySourceCoeffs { type DarcyForchheimer; DarcyForchheimerCoeffs { d d [0 -2 0 0 0 0 0] (1e12 1e12 1e12); f f [0 -1 0 0 0 0 0] (1e12 1e12 1e12); coordinateSystem { e1 (1 0 0); e2 (0 1 0); } } } } Jonatan Margalit Modeling of bed roughness in OpenFOAM 01-12-2014 17 / 28

Introduction Solver set-up Case set-up Post-Processing

Configuring controlDict

Run with adjusting the time step to a Courant number:

writeControl adjustableRunTime; adjustTimeStep yes; maxCo 0.5;

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Introduction Solver set-up Case set-up Post-Processing

Average the stresses in controlDict

Add to fieldAverage:

nuGradU { mean

• n;

prime2Mean

• n;

base time; window 125; } B { mean

• n;

prime2Mean

• n;

base time; window 125; }

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Introduction Solver set-up Case set-up Post-Processing

Set PIMPLE parameters

Set the pressure reference in fvSolution:

PIMPLE { nOuterCorrectors 1; nCorrectors 2; nNonOrthogonalCorrectors 0; pRefPoint (0 1 0); pRefValue 0; }

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Introduction Solver set-up Case set-up Post-Processing

Define LES properties

In constant/LESProperties use delta cubeRootVol Van Driest delta not suitable as it is intended for wall patches.

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Introduction Solver set-up Case set-up Post-Processing

Adjusting the Reynolds number

The last thing to do before the simulation starts is to reduce the viscosity due to the reduction of the channel depth with the creation of the bed inside the domain. Open constant/transportProperties and reduce the viscosity of the fluid to say:

nu nu [ 0 2 -1 0 0 0 0 ] 1.9e-05;

The case is now fully set-up. Run it with your modified solver:

pimpleFoam_mod > log.run &

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Introduction Solver set-up Case set-up Post-Processing

Post-processing with postChannel

Open constant/postChannelDict and uncheck symmetry:

symmetric false;

Copy the postChannel utility into the utility directory in the user directory:

cp -r $FOAM_UTILITIES/postProcessing/miscellaneous/postChannel \ $WM_PROJECT_USER_DIR/utilities cd $WM_PROJECT_USER_DIR/utilities

Now rename the folder and files:

mv postChannel postChannelRough cd postChannelRough wclean mv postChannel.C postChannelRough.C mv channelIndex.C channelIndexRough.C

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Introduction Solver set-up Case set-up Post-Processing

Adding fields to postChannel

Add the stress fields to be read in readFields.H:

volSymmTensorField BMean ( IOobject ( "BMean", runTime.timeName(), mesh, IOobject::MUST_READ ), mesh ); volScalarField Bxy(BMean.component(symmTensor::XY)); volSymmTensorField nuGradUMean ( IOobject ( "nuGradUMean", runTime.timeName(), mesh, IOobject::MUST_READ ), mesh ); volScalarField nuGradUxy(nuGradUMean.component(symmTensor::XY)); Jonatan Margalit Modeling of bed roughness in OpenFOAM 01-12-2014 24 / 28

Introduction Solver set-up Case set-up Post-Processing

Collapsing fields into spatially averaged profiles

Generate depth profiles of the stresses in collapse.H:

scalarField BxyValues(channelIndexing.collapse(Bxy)); scalarField nuGradUxyValues(channelIndexing.collapse(nuGradUxy)); makeGraph(y, BxyValues, "tauSgs", path, gFormat); makeGraph(y, nuGradUxyValues, "tauVisc", path, gFormat);

Edit Make/files and compile the utility:

postChannelRough.C channelIndexRough.C EXE = $(FOAM_APPBIN)/postChannelRough

Compile with wmake

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Introduction Solver set-up Case set-up Post-Processing

Post-processing the cases

Check the progress of the simulations. If time steps have been written, run:

postChannel -case channel395 postChannelRough -case roughChannel

Each case should now have a folder named graphs with the profiles in various time steps.

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Introduction Solver set-up Case set-up Post-Processing

Plotting with gnuplot

Obtain the mean pressure gradient in the rough channel from the log file:

cd roughChannel cat log.run | grep ’pressure gradient’ | cut -d’ ’ -f11 | tr -d ’,’ > gradP.txt awk ’{total += $1} END { print total/NR }’ gradP.txt

Download the file plotProfiles.gplt from pingpong, place it in the case folder and execute it with gnuplot to visualize the profiles:

gnuplot plotProfiles.gplt

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Introduction Solver set-up Case set-up Post-Processing

Profiles

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