cfd modeling of shallow and small lakes (case study: lake binaba)

Introuduction Lake Binaba Lake Modeling Conclusion

CFD Modeling of Shallow and SmallLakes

(Case Study: Lake Binaba)

Ali AbbasiNick van de Giesen

Department of Water ManagementDelft University of Technology

November 28, 2013

Ali Abbasi — CFD Modeling of Shallow and Small Lakes (Case Study: Lake Binaba) 1/48


1 IntrouductionWhat is CFD?Why CFD?Where is CFD Used?Numerical MethodsModelingFlow Conditions

2 Lake BinabaDescriptionAims of the study

3 Lake ModelingCFD ModelPre-processingSolving the Flow FieldPost-processing

4 ConclusionConclusion



What is CFD?

CFD : Computational Fluid DynamicsThe result of the confluence of fluid dynamics and a rangeof allied subjects:

Numerical MethodsGrid GenerationComputational GeometryComputer-aided Geometric DesignComputer GraphicsParallel Computing

Simulation of fluid engineering systems using modeling andnumerical methodsAnalytical Fluid Dynamics(AFD) and Experimental FluidDynamics(EFD)



Why CFD?

Analysis and design:more cost effective and more rapid than EFDCFD provides high-fidelity database for diagnosting flowfield

Simulation of physical fluid phenomena that are difficultfor experiments:

full scale simulationsenvironmental effects(wind, weather, etc.)hazards (explosions, pollution)physics (plantetary boundary layer)

Knowledge and exploration of flow physics.



Where is CFD Used?

AerospaceAutomotiveBiomedicalHydraulicsMarineOil & GasPower generation...



Where is CFD Used?

Figure 1 : Smoothing the cap over a swimmers head significantly improveshydrodynamic performance



Where is CFD Used?

Figure 2 : Fishway optimization: a numerical modeling study



Where is CFD Used?

Figure 3 : CFD: a valuable design tool in water resources



Where is CFD Used?

Figure 4 : CFD modeling of water flow in a basin



Where is CFD Used?

Figure 5 : Trees will reduce campus storms(Dr. Sasa Kenjere-Delta Magazine)



Numerical Methods

The continuous Initial Boundary Value Problems(IBVPs)are discretized into algebraic equations using numericalmethodsnumerical methods include:

Discretization methodSolvers and numerical parametersGrid generation and transformationHigh performance computation(HPC) and post-processing



Modeling

Modeling is the mathematical physics problem formulationin terms of continuous initial boundary valueproblem(IBVPs)IBVP is in the form of Partial DifferentialEquations(PDEs) with appropriate boundary conditionsand initial conditions.Modeling includes:

Geometry and domain(simple and complex geometry, sizeand shape)CoordinatesGoverning equationsFlow conditionsInitial and boundary conditionsselect on of models for different applications



Flow Conditions

Based on the physics of the fluids phenomena, CFD can bedistinguished into different categories using different criteria:

Viscous vs. Inviscid (Re)External flow or Internal flow(wall bounded or not)Turbelent vs. Laminar(Re)Incompressible vs. Compressible (Ma)Single- vs. Multi-phase flowThermal/density effects(Pr)etc.



Description

Lake Binaba:Location: an artificial lake located in northern GhanaSurface: the average area of the lake surface is 4.5 km2

Average depth: only 3 mMaximum depth: 7 mUsage: a small reservoir, used as a form of infrastructure

for the provision of waterAir temperature: fluctuates between 24 C and 35 CWater surface temperature: varies from 28 C to 33 CClimate: (semi-)arid region



DescriptionLocation

Lake Binaba

Figure 6 : Lake Binaba in Ghana



DescriptionLocation

Lake Binaba

Figure 7 : Location of lake Binaba(Google earth)



Aims of the study

To develop a three-dimensional time-dependenthydrodynamic and heat transfer model(CFD model)Simulating the effects of wind and atmosphere conditionsover a complex bathymetryTo predict the circulation patterns as well as thetemperature distribution in the water bodyTo compute total heat storage of small shalow lakes andreservoirs in order to estimate evaporation from watersurface



CFD Model

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Figure 8 : CFD simulation workflow



Pre-processing

Main steps in preparing the bathymetry of lake to use in CFDmodeling:

Reading initial point cloud (x,y,z coordinates from text file)Adding extra points in point cloud to improve thegenerated STL fileGenerating the STL fileCleaning & reapairing the STL fileGenerating the computational mesh or CFD mesh



Pre-processingReading initial point cloud

Reading the coordinates of points from a text file(points.asc):

Figure 9 : Initial point cloud: 642 points (x,y,z)



Pre-processingAdding extra points in point cloud to improve the generated STL file

Adding extra points to define the water surfaceAdding extra points to improve the bathymetryUsing ArcMap to interpolate between points

Figure 10 : Improved point cloud: 68802 points (x,y,z)(V.S:100)



Pre-processingGenerating the STL file

STL (STereoLithography) format is similar to a TIN,except it defines the shell of a volume with a set ofinterlocking triangular facests:facet normal ni nj nk

outer loopvertex v1x v1y v1zvertex v2x v2y v2zvertex v3x v3y v3z

endloopendfacet

UsingMeshLab(free and open-source STL generator)



Pre-processingGenerating the STL file

Figure 11 : STL file from meshLab(V.S:100)



Pre-processingCleaning the STL file

The generated STL file should be clean to can be used inCFD modelingUsing addmesh (free and open-source STL cleaner)

Figure 12 : Final STL file(V.S:100)



Pre-processingSeperating Boundaries

Water surfaceBottom and sides of lake

Figure 13 : Final geometry used in meshing(V.S:100)



Pre-processingGenerating the computational mesh or CFD mesh

Refinig the mesh near the boundariesnPoints:9,241,729; nCells:7,913,145∆x = 1.5m; ∆y = 1.6m; ∆z = 0.25(0.125)m

Figure 14 : CFD mesh(Vertical Scale:100)



Pre-processingChecking the mesh

Generated mesh should be satisfy the criteria

Figure 15 : z component of normal vector of cells(V.S:100)



Solving the Flow FieldEquations

Continuity equation:

∂uj∂xj

= 0, (1)

Momentum equations using Boussinesq approach

(2)

∂ui∂t + ∂

∂xj(ujui)

− ∂

∂xj

{νeff

[(∂ui∂xj

+ ∂uj∂xi

)− 2

3

(∂uk∂xk

)δij

]}=

− ∂p∂xi

+ gi [1 − β(T − Tref )]

Temperature in the water body

∂T∂t + ∂

∂xj(Tuj) − κeff

∂

∂xk(∂T∂xk

) = ST (3)



Solving the Flow FieldInitail Conditions

ICs should not affect final resultsOnly affect the number of iterations to reach convergedsolution

Figure 16 : Initial condition for T



Solving the Flow FieldBoundary Conditions

Temperature

ρ0Cp

(κeff

∂T∂z

)surf

= Hnet (4)

Velocity

τsurf ,u = ρ0

((νt + ν)∂u

∂z

)(5)

τsurf ,v = ρ0

((νt + ν)∂v

∂z

)(6)

τsurf ,u = CDρair(v2

10 + u210

)12 .u10 (7)

τsurf ,v = CDρair(v2

10 + u210

)12 .v10 (8)



Solving the Flow FieldMeteorogical measurements

0 10 20 30 40 5015

20

25

30

35

40

T[C]

Tair

Twater−surface

0 10 20 30 40Time[hr]

−400

−200

0

200

400

600

800

1000

HeatF

luxes[W

/m

2]

HLA

HLW

Hns

HS

HE

Hnet

Rs

Figure 17 : Time-dependent parameters using as B.C



Solving the Flow FieldMeteorogical measurements

Figure 18 : Time-dependent parameters using as B.C



Solving the Flow FieldSolver

Open Source Field Operation and Manipulation(OpenFOAM)

Open-Source LibraryFree of Chargein LINUXC++ LibraryLinking with PYTHON

Special Issuenew SOLVERS and UTILITIES Can be Created byUSERS



Solving the Flow FieldRunning in parallel

OF runs in parallelWe need to run the model in parallelDecomposing the domain accoeding to the availabe sourcesUsing MPIOF was tested at least for 1000 cores!Using GPU

Lake BinabaWe are running the model on 64 and 36 nodes



Post-processing

Figure 19 : Bathymetry of lake Binaba



Post-processingVelocity

Figure 20 : Velocity in t=3930 s (V.S:10)



Post-processingTemperature

Figure 22 : Temperature in t=3930 s (V.S:10)



Post-processingSource term in T

Figure 23 : Temperature source term in t=3930 s (V.S:10)



Post-processingTemperature

Figure 25 : Temperature in t=5640 s (V.S:10)



Conclusion

CFD modeling and graphical output provides a detailedvisual representation of the modeled systemModel results provides a high degree of confidence forproject owners, designers, and other stakeholdersComputational fluid dynamics (CFD) analysis has provento be a valuable design tool in the water resourcesModelling is one of the best means to gain understandingof complex flow fieldsWind over water surface affects lake currents, sensible andlatent heat fluxesBuoyancy effect due to density gradiant in water bodyshould be considerd in temperature profile



Thanks For your Attention


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