|M.Sc Student||Sabban Lilach|
|Subject||Measurements of Pollen Settling in Quiescent Air and in Near|
Homogeneous Isotropic Turbulence
|Department||Department of Mechanical Engineering||Supervisor||Dr. Rene Van Hout|
|Full Thesis text|
The pollen settling velocity in a turbulent flow field is an essential input parameter to any pollen dispersal model. An important parameter in the particle-turbulence interaction is the Stokes number, defined as the ratio between the particle response time and a characteristic flow time scale e.g. the Kolmogorov time scale. For pollen, typical Stokes numbers are of order one indicating that pollen response to turbulence can be significant.
In this research settling velocity of pollen were measured in quiescent air as well as in near homogeneous, isotropic turbulence generated in a 40 cm3 transparent turbulence chamber. Settling velocities measured in quiescent air corresponded well to values reported in the literature. Turbulent flow inside the chamber was generated by 8 woofers mounted on the chamber's corners. Each woofer was driven independently at a randomly changing frequency. The turbulent flow field was validated using stereoscopic particle image velocimetry at two woofer amplification settings. Near homogeneous, isotropic turbulence was obtained with small mean velocities (in most cases less than 10% of rms values). The dissipation rate necessary to determine the Kolmogorov flow scales was estimated using three different methods. The direct calculation corrected for insufficient spatial resolution was chosen as the most reliable one. Dissipation rates were 2.93 m2/s3 and 5.02 m2/s3 for the two woofer amplifications and Reynolds numbers based on the Taylor length scale were 135 and 150.
Three different pollen types, ragweed (~20mm), pine (~60 mm), corn (~80 mm) as well as polystyrene spheres (~80 mm) were released into the chamber. Settling trajectories were measured using high-speed, inline digital holographic cinematography. The holographic measurements enabled 3D tracking of particles in the center of the chamber. The frame rate was approximately twice the Kolomogorov time scale. In contrast to straight settling trajectories in quiescent air, results show a large effect of the turbulent eddies on the settling trajectories that become highly meandering. For all cases, increased mean settling velocities are obtained in turbulent conditions in particular for pollen with Stokes numbers ranging between 1 and 2.