URANS Approach for Open-Channel Bifurcation Flows Modelling

URANS APPROACH FOR OPEN-CHANNEL BIFURCATION FLOWS MODELLING

Adrien Momplot, Gislain Lipeme Kouyi, Emmanuel Mignot, Nicolas Rivière and Jean-Luc Bertrand-

Krajewski

Friday the 30th of August - Sheffield

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OUTLINE Motivations/Background

Objectives

Material and Methods

Results

ConclusionsFriday the 30th of August - 10.40 am - 11.00 am - Sheffield

SPN7 - Session: Monitoring and New Technologies

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MOTIVATIONS Singularities are often encountered in sewers

(junctions, bifurcations, CSOs, etc.) and exhibit: 3D pattern not understood with 1D or 2D

modelling High turbulence Complex mixing processes and complex pollutant

transport

European Water Framework directive imposes monitoring of quality and quantity of conveyed water

What about monitoring of dividing flows ?Friday the 30th of August - 10.40 am - 11.00 am - Sheffield


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Bifurcations instrumentation ? Discharge repartition process in downstream

branches Contaminants transport/monitoring through the

bifurcation

Neary and Sotiroupolous (1996)

MOTIVATIONS

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3D CFD modelling based on RANS equations to understand complex 3D flows

However strong dependence on: Parameters (mesh, boundary conditions) Numerical options (turbulence models, wall

functions, discretization schemes, etc.)

Friday the 30th of August - 10.40 am - 11.00 am - Sheffield


MOTIVATIONS

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Capability of RANS approach to reproduce bifurcation flow - particularly the discharge distribution in downstream branches

Improvements obtained with URANS approach



OBJECTIVES

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MATERIAL AND METHODS Experiments performed in the channel

intersection facility at the LMFA (Laboratoire de Mécanique des Fluides et d’Acoustique, INSA – LYON)



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MATERIAL AND METHODS Different flows investigated

PIV measurements at two elevations (z = 30 mm and z = 90 mm)

Discharge measurements in the lateral downstream branch



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PIV measurement flow Case 1 Case 2 Case 3Inlet discharge Q 4 L/s 4 L/s 4 L/s 4 L/s

Weir height in the main downstream branch 98.13 mm 34.50 mm 25.78 mm 18.41 mm

Weir height in the main lateral branch

98.14 mm 12.16 mm 22.19 mm 32.73 mm

Measured discharge in the lateral branch

Not measured 3.18 L/s 2.05 L/s 0.91 L/s

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MATERIAL AND METHODSMethodology:

RANS approach validation based on PIV measurements

Verification of the discharge distribution using the validated RANS model

Check the URANS improvements



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Optimum mesh is obtained when no significant improvements are observed with further refinements

Cell size Δx*Δy*Δz = 1mm*1mm*5mm in downstream branches



MATERIAL AND METHODS

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RESULTS

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Comparison between experimental data (PIV measurement) and simulations, using different turbulence model (RSM, k-ε RNG and k-ε standard)

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Model setup Turbulence model: RSM Wall function: scalable Free surface representation: VOF model Boundary conditions: mass-flow inlet for

inlet, pressure outlet for outlets Spatial discretisation scheme: Body-Force

Weighted for pressure and Second-Order Upwind for other variables



RESULTS

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RESULTSDischarge distribution:

RANS - steady state simulations give poor quantitative results for Case 1 (most quantitative studies have 10% as a maximum value for relative error)

Case 2 and 3 representations are better



Case 1 Case 2 Case 3Measured Lateral discharge Q (L/s) 3.18 2.05 0.91Simulated Lateral discharge Q (L/s) 2.73 (-16%) 1.87 (-9%) 1.03 (+11%)

Difference between inlet and outlet (L/s) 0.09 (-2,3%) 0.10 (2.5%) 0.07 (1.75%)

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RESULTS URANS calculations are performed for Case 2,

using the steady solution as an initial state



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CONCLUSIONS RANS method is able to reproduce the

horizontal velocity Some trouble to well represent the discharge

distribution in downstream branches URANS approach improves greatly the

results, but with high computational cost ! URANS approach or RANS approach (if

relative error of 16% is acceptable) may help for bifurcation sites monitoring using for e.g only one flow sensor in the

upstream branch?



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THANK YOU FOR YOUR ATTENTION

URANS Approach for Open-Channel Bifurcation Flows Modelling

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