A first step towards the implementation of a p ppartial slip boundary condition inthe free-surface CFD code ComFLOW

Hugo Hartmann, Mart Borsboom, Ivo Wenneker

6/22/20096/22/2009

Outline

1. Deltares applications1. Deltares applications

2. Introduction and motivation

3. Development and implementation partial-slip boundary condition

4. Test-cases

5. Pump sump application

6. Conclusions and discussion

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Deltares applications as a starting point (1)

Waves and currents around coastal defense systems (breakwaters)

• forces, wave impact, run-up, overtopping• scour protection• unsteadiness, turbulence, breaking waves, air entrainment

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Deltares applications as a starting point (2)

Free surface currents in pump sumps

• Efficiency, damage prevention• Unsteadiness, turbulence, free surface vortices,

tornadoes, air entrainment, forces

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Introduction – research question

• A significant part of our consultancy projects g p y p jconsists of hydraulic investigations of pumping station design

• Focus on optimal pump performance (no cavitation, vibrations, pre-rotation) by optimizing the approach flow

• Today, we see a growing number of requests for numerical CFD studies instead of physical investigationsinvestigations

Is there potential for ComFLOW as an alternative to the commercial CFD code CFX?

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Introduction – ComFLOW

What is ComFLOW?• 3D free- surface Navier-Stokes solver, only no-slip BC• developed by RuG (prof. Veldman) in collaboration with MARIN,

TU-Delft, Force Technologies (Norway), supported by shipyards and g ( y) pp y pyengineering/oil companies (JIP)

Numerical methods in ComFLOW1

• non-uniform Cartesian grid• finite volume discretization, staggered grid• pressure-correction methodp essu e co ec o e od• VOF• mixed central-upwind convection scheme• arbitrary geometry (cut-cell method)arbitrary geometry (cut cell method)

Since 2008: Deltares is part of the modeling team

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1 For more information visit www.math.rug.nl/~veldman/comflo/comflo.html

Motivation

What is needed for applications including free-surface and turbulence?pp g• More accurate schemes (in progress)• Addition of turbulence models (to be done by a PhD student in

ComFLOW-3 JIP)ComFLOW-3 JIP)• Partial-slip boundary conditions to model turbulent boundary layers

This work: development of partial-slip boundary condition (first step towards modeling of turbulent flows)• Formulation• Discretization• Implementation

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Partial-slip boundary condition

With implementation of the partial-slip boundary condition we intend to better represent the wall boundary layersy y

ui u uut

uiΔn/2

ui ui

ut

ut

No-sliput = 0 (L = 0)

Partial sliput = α ui (0 < L < ∞)

Free sliput = ui (L = ∞)

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From no-slip to partial-slip

• Impermeability + No-slipp y p

un = 0ut = 0

u = 0v = 0

• Impermeability + Partial-slip

ut 0 v 0

y

un = 0cos(θ)u + sin(θ)v = 0

Too complex!...Too complex!...

Except for:pv v?

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?

Feasible implementation including wall orientation

1. In Cartesian coordinates (2D)1. In Cartesian coordinates (2D)

2. Approximation 1: Flow parallel to boundary

3. Approximation 2: Flow uniform along boundary

4. 4 Equations, 2 variables

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Implementation

multiple values of virtual velocities

• Straightforward implementation

Partial slip information in central coefficient• Partial-slip information in central coefficient

• Similar treatment for v-velocity

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• Note: un = 0 is accurately covered in discretized fluxes!

Test-case 1

• Laminar flow (Re = 1000) in straight channel aligned with gridlinesLaminar flow (Re 1000) in straight channel aligned with gridlines• 20 grid cells in cross-flow direction (resolution 0.5 mm)

city

(m/s

)U

-vel

oc

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Y (m)

Test-case 2

• Laminar flow (Re = 1000) in 45° inclined channelLaminar flow (Re 1000) in 45 inclined channel• Grid resolution 0.5 mm

city

(m/s

)U

-vel

oc

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Y (m)

Pump sump application

• Geometry: pump compartment (l x w x h = 11 x 3.2 x 4.35 m3)Geometry: pump compartment (l x w x h 11 x 3.2 x 4.35 m )

• 1 Pump: square suction line (l x w = 0.73 x 0.73 m2)

• Pump capacity: 15,530 m3/h

• Asymmetric inflow (due to e.g. upstream structures)(due to e.g. upstream structures)

• Compare profiles 1.6 m upstream of suction line

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Pump sump application – preliminary results

X = 8 m ComFLOW(1.6 m upstream center line suction pipe)

ComFLOW• 60180 cells (structured)• 1st order upwind scheme

Z = -2 mZ = -1 m

• No turbulence• Partial-slip

Z = -2 mZ = -1 m

CFX• 93728 cells (unstructured)93728 cells (unstructured)• CDS (2nd order) scheme• k-ε model + wall functions

Z = -3 m Z = -4 m

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Conclusions

• Simplified partial-slip boundary condition has been implemented inSimplified partial slip boundary condition has been implemented in the ComFLOW code

• Correct trends are predicted (no-slip vs. free-slip)

• But: verification of the software implementation and investigation of its• But: verification of the software implementation and investigation of its accuracy needs to be completed!

• Coupling of partial-slip length to wall-roughness height using boundary layer theory

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Outlook

This work was a ‘first step towards’….This work was a first step towards ….

Accurate modeling of turbulent flows in ComFLOW lies far ahead with challenging ‘hurdles’ in between:

• Development of full partial-slip model• Development of full partial-slip model• Inclusion of turbulence model(s) • Implementation of stable higher-order advection schemes

This will be covered in the STW project (accepted!)

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Result of inaccurate flow modeling around the pillars??

Thank you for your attention!

The authors acknowledge prof. Arthur Veldman and dr. Roel Luppes (RuG) f th i t t thi k

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for their support to this work