Numerical simulation of cohesive sediment transport in the open annular flume

Transcription

1 Numerical simulation of cohesive sediment transport in the open annular flume Irina Klassen Prof. Dr.-Ing. Nestmann Gesinus, 30. July 2010 KIT The cooperation of Forschungszentrum Karlsruhe GmbH and Universität Karlsruhe (TH)

2 Outline Motivation Open annular flume Experimental and numerical results of the hydraulics Experimental results of measuring the settling of cohesive sediments Numerical results of the settling of cohesive sediments Measured floc size versus modelled particle size Influence factors on the settling in model and laboratory experiments Conclusions 2

3 Motivation Problem: Is it possible to simulate the measured settling behaviour of cohesive sediments with the numercial program SSIIM? Simplification of suspended load calculation in SSIIM: Suspended load is calculated by size fractions (diameter and fall velocity have to be given) No flocculation processes for cohesive sediments can be simulated Method: Testing of both different sediment size fractions and formulas for calculating the settling velocity in the numerical model Comparison of calibrated particle size versus measured floc size Analysis of hydraulic and sediment parameters on the settling behaviour 3

4 Open annular flume small flume large flume Measurements and numerical simulation of the hydraulics Experimental analysis of the settling behaviour 4

5 Open annular flume V x Small small flume, Flume V x < large flume, V x Measurements and numerical simulation of the hydraulics Experimental analysis of the settling behaviour 5

6 Open annular flume TKE small flume, TKE > large flume, TKE Measurements and numerical simulation of the hydraulics Experimental analysis of the settling behaviour 6

7 Open annular flume Measured data of the settling Measurements and numerical simulation of the hydraulics Experimental analysis of the settling behaviour Does the numerical model calculate the same settling behaviour? 7

8 Simulation results variation of sediment size fractions large flume Stokes (1850): w s 2 d = ρ g 18ν d_1 (μm) fraction (%) d_2 (μm) fraction (%) d_3 (μm) fraction (%) d_50 (μm) Original grading curve 0, ,5 Representative fractions ,4 8

9 Simulation results different formulas of the settling velocity w S Settling velocity formulas w S (d): Stokes: Zanke: Cheng: w s w S w s 2 d = ρ g 18ν ν ρ' g 3 = ,01 d 1 2 d ν 1/ 3 2 ν 25 1,2 ρ g = + 2 d d ν 1,5 5 Different approaches for the calculation of w S yield similar results 9

10 Measured floc size vs. modelled particle size d 50,floc > d 50,particle Measured floc size: d μm Possible reasons for different sediment sizes: modelled TKE measured TKE different densities: ρ floc ρ particle Modelled particle size: d 50 7,4 μm 10

11 Influence of the density on the settling velocity w S Flocs (d μm) Winterwerp (1999) Particles (fractional, d 50 7,4 μm) Stokes (1850) w S d F 1 α ( ρ S ρw ) g 3 d D F = DP 18β µ 1+ 0,15 Re P 0,687 w S 2 d = ρ' g 18ν d (μm) fraction (%) 17 4 d F = 3 d F = < 3 d F = 2, d F = 2,09 d F = 1,95 d F = 1,8 w S, floc = 0, mm/s < w S, particle = 0,0488 mm/s Why is the settling bahviour the same, although the settling velocities are different? 12

12 Possible reasons: Overpredicted modelled TKE values near the bed particles are more swirled up Assumed fractal dimension d F 2, but more compact flocs (e.g. d F 2,3) Flow through fractal permeable flocs can increase the settling velocity (Johnson et al., 1996) 12

13 Influence factors on the settling large flume small flume measured floc size 15 μm 8 μm modelled particle size Velocity V x measured + modelled TKE measured +modelled > > 7,4 μm 3 μm > < General characteristics of the influence factors are the same in SSIIM and experiment Opposing interaction of the hydraulic and sediment size factors is observed in SSIIM and experiment 13

14 Conclusions The measured concentration courses could be calibrated in both flumes by testing different sediment size fractions and settling velocities formulas Different settling velocity formulas exert similar results The calibrated particle sizes are about 50 % smaller than the measured floc sizes possible reasons: - ρ floc < ρ particle - overpredicted modelled TKE values near the bed - increased settling velocites of permeable flocs Both models show an opposing interaction of the influencing factors on the settling Next steps Flocculation model in SSIIM LDA measurements to compare TKE values 14

15 Thank you for your attention! 15

16 Influence of the hydraulic conditions on the settling large flume small flume particle size 7,4 μm 7,4 μm Velocity V X = > TKE < 16