Orientational properties of fibrous aerosols in atmospheric turbulence

Experimental and computational results of the orientational properties of relatively large fibrous aerosols dispersed in the turbulent atmospheric boundary layer are presented. The mean orientation of the fibers is compared with independent measurements of the variance in the strain rate field, E2ij , associated with the turbulent air motion. Millimeter wave depolarization measurements were used to obtain ensemble average orientation of the dispersed fibers. A spectral analysis of data obtained from nearly collocated and simultaneous high-frequency hot wire probe measurements was employed to estimate E2ij . A distinctive correlation is found between decreasing E2ij and increasing horizontal orientation of the fibers. For a given E2ij significantly different levels of horizontal orientation are observed depending on the fiber length and diameter. A simple model of the motion of a straight rigid fiber in a turbulent viscous medium is presented. This model qualitatively describes the relationship between fiber geometry and relative mean orientation; However, it consistently over estimates the observed mean orientation. Discrepancies may be due to either deficiencies in the model, the quality of the dispersed fibers, or both.