1. Formation of Near-Wall Particle-Streaks in Particle-Laden Wall-Bounded Turbulent Flows Luís M. Portela and Valérie Ferrand Kramers Laboratory Delft University of Technology, The Netherlands Lorentz Center Turbulence Workshop August 21-30, 2006, Leiden, The Netherlands

2. Small Heavy Particles Fluid Particles Motivation Particle Behavior Strong accumulation very near the walls Spatially organized in « streaks » // to the walls fluid streaks (Kline et al. 1967) Turbulence ModulationParticles Fluid Strong and non-isotropic attenuation of the turbulence intensity Dynamics of interaction mechanisms still poorly understood (two-way coupling) (Rogers and Eaton 1991, Kulick et al. 1994, Paris and Eaton 1999, Li et al. 2001, Portela and Oliemans 2002) ( Young and Hanratty 1991, Pedinotti et al. 1992, Wang and Squires 1996, Rouson and Eaton 2000, Susuki et al. 2000)

3. Objectives of the Study Gain a better understanding of their formation process Particle streaks very near the wall Particle-fluid dynamics Fluid Particles Turbulence structure

4. L X =5H, L Y =2H, L Z =H 64 grid points, periodic BC 48 grid points, refined near the walls elastic bouncing walls Channel Geometry and Flow Parameters high-resolution LES Eulerian-Lagrangian Simulations N P  400 000 point particles approach Stokes drag no gravity no collisions

5. Particle Accumulation Very Near the Wall C t + 60 channel lengths (30H) with bulk velocity time evolution of C very near the wallmean concentration profile Z + Z + Y + t1t1 + Z + =2

6. Spatial Distribution of Particles Very Near the Wall Ejection Sweep u’ Z + =2 one-way coupling Particles move up and down the wall responding to the ejection and sweep events ( Rashidi et al. 1990) By extension, both « streaky patterns » are also generally attributed to the streamwise vortices ( Pedinotti et al. 1992, Wang and Squires 1996, Rouson and Eaton 2000, Susuki et al. 2000, Marchioli and Soldati 2002)

10. streamwise vortices length : “residence time” : Spatial Distribution of the Particles Very Near the Wall very elongated : associated with a long life time : particle streaks are

11. Formation Process of the Particle Streaks

12. ….. Determination of the “fully converged” fluid streamlines fluid-streaks “baselines” 20 L X Y + X + X + Very good correlation with the particle-streaks

14. Formation Process of the Particle Streaks Y + X + X + Y + X + X + Time t +  t + ;  t +  1000 streamwise vortices cover 6 channel lengths (= 30 H)

15. Formation Process of the Particle Streaks Fixed fluid field + W P =0 Slow process of collection due to a local interaction with the fluid streaks Particle streaks formation = Y + X + X + Particles initially uniformly distributed  t +  1000

16. Particle-Streaks Effect on the Near-Wall Turbulence + Relative destruction of the streamwise fluctuations Turbulence attenuation very near the wall not directly affected by the particle streaks PDF u’/u rms no particular modulation of u’<0 % near-wall particles are clearly associated with low u  u’<0 Z + =2 / Z + Z

17. Particle-Streaks Effect on the Near-Wall Structure

18. Two-Way Coupling Effect on the Particle Streaks One-way coupling Two-way coupling Y + X + X + qualitatively equivalent for one-way and two- way coupling cases slightly stronger and more defined for the two- way coupling case Particles streaks are: Z + / more elongated and less wiggly fluid streaks fluid-streaks « baselines » not modified

19. Conclusions and Perspective Particle streaks formation Particle streaks don’t play a significant role in turbulence modification slow process induced by the local fluid streaks qualitatively equivalent in one-way and two-way coupling Even though the concentration is strong very near the wall, the preferential concentration there is not a key point in determining turbulence level Modeling should be focused on particle effect in the buffer layer and log-law layer.