Particle Mixing and Diffusion in the Turbulent Wake of a Single Cylinder

Abstract

Particle mixing and turbulent diffusion from a dilute monodisperse aerosol stream into the turbulent wake of a single cylinder was studied over a velocity range of 6.1, 12.2, and 18.3 m/sec. Dioctyl phthalate droplets with diameters of 4, 8, and 15 μm were used resulting in effective Stokes numbers ranging from 0.18 to 5.11. Particle velocity and concentration data in the aerosol stream around the cylinder was taken with a laser doppler anemometer. The velocity and concentration data were analyzed empirically, examining diffusivities via the particles' rms velocity, and also using a model from a similarity solution to the equation of motion, assuming a constant diffusivity in the wake.

The experimental data showed good agreement between the models and the air and particle momentum data. The concentration data contained more scatter, but was consistent with the air and particle momentum data. The air momentum Peclet numbers agreed with literature values for cylinder wakes. Diffusivity ratios, ε_m/ε_mp, and ε_m/ε_p, were found to be functions of the Stokes number, being less than one for low Stokes numbers, greater than one for Stk > 0.33. Particle momentum diffusivity, ε_mp, was lower than particle concentration diffusivity, ε_p, for Stk > 1.0. The diffusivity ratio, ε_m/ε_mp, was found to be well described by a function including the particle Stokes number and the Strouhal number of the cylinder.