Experimental investigation of multilayer particle deposition and resuspension between periodic steps in turbulent flows

- Formation and removal of a particle multilayer between periodic steps was studied.
- Particle multilayer build up shows linear tendency against time.
- Particle resuspension shows strong dependency on fluid friction velocity.
- Similarities found between single and multilayer particle deposition/resuspension.

Multilayer particle deposition and resuspension between periodic steps in a turbulent channel flow was experimentally studied. A particle laden turbulent flow in a horizontal channel was generated by means of a small-scale wind tunnel. Periodic steps were placed on the channel floor to induce a complex flow field characterised by flow separations, recirculation and reattachment zones.The turbulent flow field was recorded by means of a stereoscopic Particle Image Velocimetry system. The dimensions of the flow field structures agree well with the specifications from comparable studies. Polydisperse graphite particles were injected into the flow field during a pure deposition regime. The multilayer build up between the periodic steps was measured using a laser distance sensor coupled with a linear stage. The particle layer build up shows a linear tendency against time and the deposition velocities are in good agreement with similar investigations. After the deposition scenario the particle dispersion was switched off and particle resuspension was induced by the stepwise increase of the fluid velocity. The particle resuspension was found to begin in clusters close to the recirculation zones and was accomplished at lower friction velocities than necessary for single particle resuspension. Surface roughness analysis of the multilayer interface characterises the growth of the loose particle agglomerates during particle deposition and the instant removal of these structures during particle resuspension. Furthermore, particle bed porosity measurements were performed by gravimetrical and x-ray methods to characterise the bed porosity. Spatial correlations between the turbulent flow field and the multilayer thickness distribution indicate the influence of inertia impaction, turbulent dispersion and gravitational settling on the particle deposition process. The multilayer particle resuspension seems to be influenced by the mean and fluctuating flow field structures. The results indicate similarities between single particle and multilayer particle formation during particle deposition and resuspension processes.