E. Mignot, N. Rivière, D. Doppler, I. Vinkovic, P.-H. BazinLaboratoire de Mécanique des Fluides et d’Acoustique

Université de Lyon (France)

2

y

x

Qxi

Qxo Qyi

Qyo L=2m

b=0.3my

Channel intersection : 0.3x0.3m,Slopes 0-5%, 0<Fr<5

ElargissementThèse Han Lei

Hydrodynamique environnementale expérimentale au LMFA

Thèse Cai WeiCavité

IntersectionsThèse PH Bazin

d

h,U

L

g

B

gTv

Tc

Ecoulement torrentielautour d’un obstacle

QMaj QminQMaj Qmin

Lit composé

Jet torrentiel

* Torrentiel - Fluvial* 3-4 branches* Distribution Q - PIV* Simple - obstacles

E. Mignot, N. Rivière, P.-H. BazinLaboratoire de Mécanique des Fluides et d’Acoustique

Université de Lyon (France)

Open-channel bifurcations:Open-channel bifurcations: Impact of singularitiesImpact of singularities

on the discharge distributionon the discharge distribution

Introduction

• Delta

• Cut-off

• IslandNatural bifurcationsNatural bifurcations

Introduction

• Severe floods in dense urbanized areas

Flow takes place in streets and crossroads

(Bonneaud, 2002)

Artificial bifurcationsArtificial bifurcations

Some crossroads are3-branch bifurcations

Subcritical, 3 branch, open-channel, bifurcation

6

Introduction

(Neary et al., 1999)

General flow pattern:• Dividing interface• Recirculation zone• Secondary flows

Main concern : Prediction of discharge distribution

Qu Qd

Qb

7

Introduction

Ramamurthy et al. (1990)

• Momentum /x : Ramamurthy et al. (1990)• Energy : head loss coefficient unknown

Empirical relationship (Rivière et al., 2007)

Available Equations to describe the flowDischarge distribution

Valid if no obstacle

Qu

Qd

Qb

);(d

b

dd

u

u

bq C

C

gCbC

Qf

Q

QR

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 0,2 0,4 0,6 0,8 1

Rq ex p.

Rq

co

rr.

+5%

-5%

Rq-sub (eq.4)

Rq (exp.)

Rq (correl.)

Rivière et al. (2007)

8

Topic

Quantify impact of obstacles near the bifurcation?

Present experiment

Modification of discharge distribution depends on

• Flow characteristics (h, U, b …)• Obstacle shape and size

• Obstacle location

(Bonneaud, 2002)

Qu

Qd

Qb

9

Experimental set-up

White light sheet

PIV technics

(LMFA – INSA Lyon – Université de Lyon)

- 3 or 4 open-channels- Central intersection- Glass walls (optical access)

Add particles = PSP 50 mHigh-frequency camera (30Hz)

Velocity measurement

Discharge measurement

Water depth measurement

UpstreamTank

DownstreamTank

LateralTank

PumpQu

Qb

Cd

Cb

Boundary conditions:

Qu: upstream flow-rate

Cd: downstream weir crest

Cb: branch weir crest

10

Qu

Qb

Qd

Lu=2m Ld=2.6m

Lb=2.6m

PIV area

Experimental approach(LMFA – INSA Lyon – Université de Lyon)

Experimental scheme

Channel section

20 c

m

b=30 cm

Measurements:

Qb: branch flow-rate

hu, hb, hd depths

1 2 3 4 5

6 7 8 9 11

4cm

4cm

5cm

5cm

Methodology• Fixed boundary conditions (Qu, Cb, Cd)• Measure discharge distribution• Introduce 9 obstacle one after the other• Measure the modification of outlet discharges

Obstacle configurations

12

Results : Discharge distribution

Qu=2L/s

Qb=0.75L/s Qb=0.74L/s

Qb=0.77L/s

Qb=0.70L/s

Qb=0.73L/s

No obstacle O-1 O-2

O-4 O-7O-3

Qb=0.75L/s

Streamwise acceleration

Lateral branch blockage

Downstream blockageO-5

Qb=0.76L/s

Side deflection

Ux

13

Influence of the Froude number

-15

-10

-5

0

5

10

0.2 0.3 0.4 0.5 0.6 0.7 0.8

∆R

q

1 2 3 4 5 6 7 8 9

Previously described

Fru0 (without obstacle)

If Fru0 , impact of obstacles Stagnation point depth and so horizontal pressure gradients

3

2

7

Rq0 0.39 ; hu0 /b 0.14

u

bb

Q

QQ 0

obstacle Qb

obstacle Qb

Influence of the other flow parameters

incoming Froude nb.

discharge distribution

inlet water depth

Dimensional analysis

14

b

hR u

h0

0 0

00

u

bq Q

QR 2/3

02/1

00

u

uu

hbg

QFr

As Fr , impact of obstacle More complex

Influence of the other flow parameters

Rq : moves the separating streamline compared with the obstacle

Rq : modifies recirculation width

15

Influence of the initial discharge distribution

-8

-6

-4

-2

0

2

4

6

0.2 0.3 0.4 0.5 0.6 0.7 0.8Rq0

∆R

q

1 2 3 4 5 6 7 8 9u

bb

Q

QQ 0

u

bq Q

QR 0

0

hu0 /b 0.15 ; Fru0 0.445

3

2

7

O0-C O2-C O3-C O7-C 1

2

3

O0-C O2-C O3-C O7-C 1

2

3

2 3

Rq0

Rq0

16

Conclusions

•Magnitude of discharge modification depends on• Location of obstacle• Froude number of inflow / reference distribution• shape / size not studied here

• Obstacles modify the discharge distribution by about [-15 ; +10 %]• Non negligible modifications when compared to other errors:

Sidewalks – Roughness – Shape of crossroads – Exchange with buildings - sewer networks … ?

Current works

• Separating streamline• Rapid main flow• Slow recirculation zone

Applications* Turbulent modeling* Pollutant dispersion

Reynolds shear stress

Streamlines Fieldlines Separating streamline

17