E. Mignot, N. Rivière, D. Doppler, I. Vinkovic, P.-H. Bazin Laboratoire de Mécanique des Fluides et d’Acoustique Université de Lyon (France)

2 Elargissement Thèse Han Lei Hydrodynamique environnementale expérimentale au LMFA Thèse Cai Wei Cavité Intersections Thèse PH Bazin Ecoulement torrentiel autour d’un obstacle Lit composé Jet torrentiel * Torrentiel - Fluvial * 3-4 branches * Distribution Q - PIV * Simple - obstacles

E. Mignot, N. Rivière, P.-H. Bazin Laboratoire de Mécanique des Fluides et d’Acoustique Université de Lyon (France) Open-channel bifurcations: Impact of singularities Impact of singularities on the discharge distribution on the discharge distribution

Introduction Delta Cut-off Island Natural bifurcations

Introduction Severe floods in dense urbanized areas Flow takes place in streets and crossroads (Bonneaud, 2002) Artificial bifurcations Some crossroads are 3-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 QuQu QdQd QbQb

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 flow Discharge distribution Valid if no obstacle QuQu QdQd QbQb R q (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) QuQu QdQd QbQb

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  m High-frequency camera (30Hz) Velocity measurement Discharge measurement Water depth measurement

Upstream Tank Downstream Tank Lateral Tank Pump QuQu QbQb CdCd CbCb Boundary conditions: Q u : upstream flow-rate C d : downstream weir crest C b : branch weir crest 10 QuQu QbQb QdQd L u =2m L d =2.6m L b =2.6m PIV area Experimental approach (LMFA – INSA Lyon – Université de Lyon) Experimental scheme Channel section 20 cm b=30 cm Measurements: Q b : branch flow-rate h u, h b, h d depths

cm 5cm Methodology Fixed boundary conditions (Q u, C b, C d ) Measure discharge distribution Introduce 9 obstacle one after the other Measure the modification of outlet discharges Obstacle configurations

12 Results : Discharge distribution Q u =2L/s Q b =0.75L/s Q b =0.74L/s Q b =0.77L/s Q b =0.70L/s Q b =0.73L/s No obstacle O-1 O-2 O-4 O-7 O-3 Q b =0.75L/s  Streamwise acceleration Lateral branch blockage Downstream blockage O-5 Q b =0.76L/s Side deflection UxUx

13 Influence of the Froude number Previously described Fr u0 (without obstacle) If Fr u0 , impact of obstacles  Stagnation point depth  and so horizontal pressure gradients  R q0  0.39 ; h u0 /b  0.14 obstacle  Q b obstacle  Q b

Influence of the other flow parameters incoming Froude nb. discharge distribution inlet water depth Dimensional analysis 14 As Fr , impact of obstacle  More complex

Influence of the other flow parameters  R q : moves the separating streamline compared with the obstacle  R q : modifies recirculation width 15 Influence of the initial discharge distribution h u0 /b  0.15 ; Fr u0  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 StreamlinesFieldlines Separating streamline 17