Resuspension of spherical particles in the oscillating grid turbulence

Report on the first results of the research project of the Turbulence Structure Laboratory at the Tel Aviv University

Transcript

1. Particle resuspension in an oscillating grid turbulent flow Hadar Traugott and Alex Liberzon Turbulence Structure Laboratory, Tel Aviv University SAFL, Sep 26, 2012 . . . . . .
2. Outline • Background and motivation • Goals and objectives • Experimental set up • Preliminary results • Discussion . . . . . .
3. Tel Aviv, Israel • top 10 Urban Beaches list • top 10 travel destinations on airbnb • Tel Aviv’s White City is UNESCO World Heritage Site, contains 4,000 International Style buildings, many of which were built in the 1930s and 1940s • top 10 most vibrant cities in nightlife category, “Miami of Middle East” • one of the best cities to open a start-up . . . . . .
4. Tel Aviv University - No. 1 in research output among Israeli universities - No. 1 in citation impact among Israeli universities - No. 1 choice among Israeli students - 15 international study programs - Over 1,200 international students - 2,000,000 people served in TAU’s 17 affiliated hospitals - 130 research centers and institutes - 2 Oscar-nominated films . . . . . .
5. Turbulence Structure Laboratory - Not on the scale of SAFL :) - Established in 70’s by Prof. A. Tsinober - 2 Faculty members - 2 Senior Researchers, 1 Engineer - 3 Ph.Ds, 12 M.Sc., 6 undergraduate students 2XU´SKLORVRSK\µ² learn from the change 7 Turbulence Polymers Particles Forcing Lagrangian Eulerian . . . . . .
6. Major developments Miniature hot-film probe - sub-Kolmogorov scales turbulence - Borisenkov et al. (2011) tip size is 20 - 60 µm sensor thickness is 20 nm 3D Particle Tracking Velocimetry - originally from ETH Zurich - FPGA based real-time image processing Kreizer (2010,2011) - open source software: http://3dptv.github.com . . . . . .
7. Motivation is both basic and practical • Sediment transport: “Incipient motion condition constitutes one of the classical and central problems of sediment transport In rivers, coastal areas and atmospheric flows” • Industry: • Silicon wafer cleaning • Pneumatic conveying • Fluidized beds • Biomedical applications (pulsating flows) . . . . . .
8. Incipient motion • Mean flow properties: parameterization of the problem • Forces acting on the particle: entrainment forces / resisting forces • Turbulence: fluctua- tions/instantaneous, drag/lift, impulse/drag, local/non-local Flow Fluid force Weight Flow shear distribution + + + + + - High Pressure - - - Low Pressure Friction Drag Lift . . . . . .
9. Relevant terminology • Incipient motion of spherical bed particles from a smooth wall under turbulent flow conditions without mean shear • Incipient motion due to fluid motion refers to the beginning of movement (not only disturbed) of bed particles that stayed at rest. • We focus on the lift-off events at the moment (though we have the full history). Lift-off occurs only when the instantaneous lift force acting on the particle exceeds its effective weight force. The lift-off (pick-up) probability is a fraction of time when the lift force is greater than the effective weight for a given time interval, or the percentage of the number of particles in motion on a fixed area of bed surface (Einstein 1942, 1950) . . . . . .
10. Two general “views” on the problem Deterministic view: mean flow τw = µ ∂U ∂y y=0 Shields (1936) Stochastic view: turbulence, probability −ρ⟨u′v′⟩ + ρ⟨u′2⟩ + ρ⟨v′2⟩ + . . . Einstein (1949) * However: particles are observed to move also under very weak turbulent flow conditions (e.g. Paintal, 1971) * In all models, turbulence is assumed to have a major influence on the phenomenon . . . . . .
11. Unsteadiness Literature review on particle removal by Smedley et al. (1999): . . 1 unsteady flow effects play a vital role in particle removal even in the steady methods due to the start-up transient as the flow is initiated. In most steady-flow experiments, a high entrainment rate is observed when the flow is first turned on; The transients during flow start-up produce higher forces on the bound particles than the subsequent steady flows. . . 2 In periodic unsteady flows, such as a pulsed jet, the transients are generated repeatedly. Nelson (2001), Smart and Habersack (2007), Schmeeckle et al. (2007): • entrainment due to deceleration, high pressure fluctuations, rare episodes of specially high lift force • two important quantities: the instantaneous fluid acceleration/deceleration and vertical normal stress component . . . . . .
12. Acceleration vs shear USGS Sediment Transport group: “Sediment flux changing greatly for different values of the bed stress. The velocity field (which drives lift and drag on the particles) is not tied directly to the stress because of the acceleration of the flow.” . . . . . .
13. Open channel “external” turbulence From: “Effect of external turbulence on sediment pickup rate” by Okayasu, Fujii and Isobe, Proc. Intl. Conf. Coastal Eng., 2010 The pickup rates could be up to three times . . . . . .
14. Breaking waves “external” turbulence From: Sediment inception under breaking tidal bores by Khezri and Chanson, Mech. Res. Comm. 41 (2012) . . . . . .
15. Resuspension under oscillating grids Lyn (1995): modified Shields parameter for the zero-mean shear flows under oscillating grids: θ∗ = TKE g(s − 1)d Redondo and co-workers (e.g. Cont. Shelf. Res. 2001) focused on the lift off of sediments in ocean benthic layers: • A turbulent r.m.s velocity u′ lower than the u∗ Shields critical velocity is required to start the sediment motion. • One is able to lift off sediments much more efficiently with the grid stirring experiments (zero-mean flow) as compared to the shear-induced turbulence experiment. . . . . . .
16. Resuspension under oscillating grids (contd.) Belinksy et al. (2005): resuspension, settling and diffusion of particles in a water column (lakes, reservoirs) using oscillating grids: • measurements are 1D, forcing is assumed to be according to the grid function: u′ = CfS1.5M0.5z−1 with empirical coefficients • the modified Shields is found to be close to the original Shields for open channel flows “only if the TKE exceeds the critical value, will resuspension occur” . . . . . .
17. Research goals . . 1 To develop improved experimental methods for measuring turbulent flows, w/o additives such as particles, colloids, dilute polymers . . 2 To develop better understanding of the mechanics of turbulence-particle interactions, specifically for the particle-detachment from the surfaces • detailed measurements of particle and fluid motion under well controlled (or well characterized) flows: oscillating grids, driven shear flows, separation points, stratification . . . . . .
18. Research objectives . . 1 Direct 3D position/velocity/acceleration data to simultaneously quantify flow properties and particle motion through the various phases of re-suspension. . . 2 Identification of fluid motion patterns during particle resuspension, linking them to turbulent flow properties * The focus is on the pick-up or lift-off phase. . . . . . .
19. 3D Particle Tracking Velocimetry . . . . . .
20. Basics of the 3D-PTV . . . . . .
21. 3D-PTV algorithm 9  2-5 ± PT V processing scheme (Willneff, 2003).  2-6 ± Stereo-matching -matching is based on epipolar geometry (see  2.6 2.6 below 2.6). We measure directly the full gradient tensor along the particle trajectories: ∂ui/∂xj(x, t) 5.4. Object space based tracking techniques Est con nat tim Fig. 15: Main processing steps . . . . . .
22. 3D-PTV is now fully operational at SAFL . . . . . .
23. How our data looks like . . . . . .
24. Particle motion 1.5 2 2.5 3 3.5 4 4.5 −40 −20 0 20 t [sec] x [mm] x y z 1.5 2 2.5 3 3.5 4 4.5 −0.5 0 0.5 t [sec] u [m/s] u v w 1.5 2 2.5 3 3.5 4 4.5 −0.02 0 0.04 0.08 t [sec] a [m/s2] a x a y a z . . . . . .
25. The detachment moment . . . . . .
26. Fluid-particle correlations: is it local? ρii(r) = ⟨U(k) i (x)Vi(x + r)⟩ ⟨U2 i ⟩1/2⟨V2 i ⟩1/2 . . . . . .
27. Fluid-particle correlations 0 10 20 30 40 50 60 70 80 90 r [mm] −0 . 4 −0 . 2 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 1 . 2 ρii ρ11 (r) ρ22 (r) * Transverse (vertical) components are apparently locally correlated, not so the longitudinal ones. . . . . . .
28. Integral view: pick-up rates 300 350 400 450 Re 4 6 8 10 12 14 Initial pick-up time t0 300 350 400 450 Re 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 Pickup rate * Reynolds number based on the grid velocity, mesh size . . . . . .
29. Full flow field - one needs PIV u [m/s] v [m/s] . . . . . .
30. Intrinsic inhomogeneity −0.05 0 0.05 0.1 0.15 −0.06 −0.04 −0.02 0 0.02 0.04 0.06 X /L Ref=300 Ref=340 Ref=380 Ref=420 −0.05 0 0.05 0.1 0.15 −0.06 −0.04 −0.02 0 0.02 0.04 0.06 0.08 0.1 x/L . . . . . .
31. Strongly inhomogeneous flow field U [m/s] V [m/s] . . . . . .
32. urms [m/s] vrms [m/s] . . . . . .
33. Sharp changes in Reynolds stresses . . . . . .
34. Detachment events at specific locations U [m/s] V [m/s] . . . . . .
35. urms [m/s] vrms [m/s] . . . . . .
36. Reynolds stresses . . . . . .
37. Back to the particle-flow analysis . . . . . .
38. Directly estimated drag force . . . . . .
39. Zoom out: transients 0.1 0.2 0.3 0.4 0.5 0.6 0.5 1 1.5 2 2.5 3 3.5 4 4.5 x 10−9 Nominal drag force [N] Time [s] . . . . . .
40. Local TKE ? 0 10 20 30 40 50 60 70 80 90 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 Time [frame] TKE [m2/s2] . . . . . .
41. Impulse? . . . . . .
42. Impulse vs drag . . . . . .
43. Particle acceleration vs forces ∑ ⃗ F = m⃗ a . . . . . .
44. Ongoing research . . 1 Probe the question of the sweeps/ejections vs acceleration/deceleration events: • Some works report that local velocity of the fluid is higher than the velocity of the particles that leave the bed - it is explained as u′ < 0, v′ > 0 ejection events. Others (see above) referred to the sweep events. We will address this question directly. • It can be also due to large acceleration/deceleration, and not of the relative velocities, i.e. not related to the sweeps/ejections. . . 2 Probe directly the balance of forces on the particle at the initial motion on the bed: relative velocity, pressure gradients, added mass . . 3 Repeat the analysis on a rough bed, adhesive particles, coagulation of particles . . 4 Shear flow cases, e.g. lid driven cavity . . . . . .
45. Acknowledgments . . 1 Tracey Hayse, MIT . . 2 Turbulence Structure Laboratory team . . 3 Funding agencies: ISF, Wolfson Charitable Trust . . . . . .