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E. Mignot , N. Rivière, D. Doppler, I. Vinkovic, P.-H. Bazin

E. Mignot , N. Rivière, D. Doppler, I. Vinkovic, P.-H. Bazin Laboratoire de Mécanique des Fluides et d’Acoustique Université de Lyon ( France). Hydrodynamique environnementale expérimentale au LMFA. Elargissement Thèse Han Lei. Ecoulement torrentiel autour d’un obstacle. Cavité.

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E. Mignot , N. Rivière, D. Doppler, I. Vinkovic, P.-H. Bazin

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

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

  3. 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 on the discharge distribution

  4. Introduction • Island Natural bifurcations • Delta • Cut-off

  5. Introduction Artificial bifurcations • Severe floods in dense urbanized areas Flow takes place in streets and crossroads Some crossroads are 3-branch bifurcations (Bonneaud, 2002)

  6. Introduction Subcritical, 3 branch, open-channel, bifurcation Qb • General flow pattern: • Dividing interface • Recirculation zone • Secondary flows Qu Qd (Neary et al., 1999) Main concern : Prediction of discharge distribution 6

  7. Introduction Rq (correl.) Discharge distribution AvailableEquations to describe the flow • Momentum /x : Ramamurthy et al. (1990) • Energy : head loss coefficient unknown Qd Qu Empirical relationship (Rivière et al., 2007) Qb Ramamurthy et al. (1990) Rivière et al. (2007) Valid if no obstacle 7

  8. Topic Present experiment Quantify impact of obstacles near the bifurcation? (Bonneaud, 2002) Qb Qu • Modification of discharge distribution • depends on • Flow characteristics (h, U, b …) • Obstacle shape and size • Obstacle location Qd 8

  9. Experimental set-up (LMFA – INSA Lyon –Université de Lyon) • - 3 or 4 open-channels • Central intersection • Glass walls (optical access) Velocity measurement PIV technics White light sheet Add particles = PSP 50 m High-frequency camera (30Hz) Discharge measurement Water depth measurement 9

  10. Channel section Boundary conditions: Qu: upstream flow-rate Cd: downstream weir crest Cb: branch weir crest 20 cm b=30 cm Measurements: Qb: branch flow-rate hu, hb, hd depths Experimental approach (LMFA – INSA Lyon –Université de Lyon) Experimental scheme Qu Ld=2.6m Lu=2m Qu Cd Upstream Tank Qd PIV area Downstream Tank Pump Qb Lb=2.6m Qb Cb Lateral Tank

  11. Obstacle configurations 5cm • Methodology • Fixed boundary conditions (Qu, Cb, Cd) • Measure discharge distribution • Introduce 9 obstacle one after the other • Measure the modification of outlet discharges 4cm 5cm 4cm 1 2 3 4 5 6 7 8 9

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

  13. Influence of the Froude number Rq0 0.39 ; hu0 /b0.14 7 obstacle  Qb 3 obstacle  Qb Previously described 2 Fru0 (without obstacle) If Fru0 , impact of obstacles  Stagnation point depth  and so horizontal pressure gradients  13

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

  15. Influence of the other flow parameters • hu0 /b  0.15 ; Fru0 0.445 7 3 2 Influence of the initial discharge distribution 3 2 Rq0  Rq : moves the separating streamline compared with the obstacle Rq : modifies recirculation width Rq0  15

  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 … ? 16

  17. Current works Streamlines Fieldlines Separating streamline Applications * Turbulent modeling * Pollutant dispersion • Separating streamline • Rapid main flow • Slow recirculation zone Reynolds shear stress 17

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