Aggregate formation in 3D turbulent-like flows
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Aggregate formation is an important process in industrial and environmental turbulent flows. Two examples in the environmental area, where turbulent aggregate formation takes place, are raindrop formation in clouds and Marine Snow (aggregate) formation in the upper layer in the oceans.
The dispersion of inertial particles differs from that of (passive) fluid particles and is dominated by particle-turbulence interaction. This is especially important when the particle scales match the small-scale turbulent flow scales. Our motivation to study turbulent aggregate formation comes from the need to describe aggregate formation in small-scale turbulence in the oceans.
For a proper description, the study of aggregate formation in turbulent flows requires a particle-based model, i.e. following single particles. Therefore, three main processes should be modeled: the turbulent flow, the motion of the particles, and the collision between particles and subsequent aggregate formation. In this study we use 3D kinematic simulations to model the turbulent flow. A simplified version of the Maxey-Riley equation is used to describe the motion of the particles. For the collision and aggregate formation a geometrical collision check is used: when the distance between two particles is smaller than the sum of their radii a collision takes place. All the particles that collide stay together to form an aggregate, i.e. 100% coagulation efficiency. To account for the porosity of the aggregates a Fractal Growth Model is used.
In this study the importance of the Stokes number and the fractal dimension of the aggregates on collision rates and aggregate formation has been explored, finding that the preferential concentration plays a very important role in aggregate formation by creating regions of high particle concentration. Other results are: the net effect of fractal growth is to increase the aggregate Stokes number and to decrease the density of the aggregate.
In order to determine the performance and applicability of 3D-KS models on aggregate formation processes, DNS simulations and supplementary laboratory experiments are planned.
KeywordsFractal Dimension Collision Rate Direct Numerical Simulation Aggregate Formation Stokes Number
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