Particle resuspension from moving flexible surfaces

Abstract

This thesis strives to achieve a truly unified picture of particle resuspension by focusing on moving flexible surfaces, inspired by human activities and natural phenomena. Three specific studies were conducted and are summarized as follows: Study 1 parametrically investigated dust resuspension from contaminated fabrics (with four levels of initial dust load: 1, 10, 20, and 30 g/m2) subjected to force-induced vibrations (with low frequencies ranging from 0 to 6 Hz), inspired by human activities, e.g., shaking clothes. It was found that different settings of vibration duration, vibration frequency, and initial dust load can lead to significant differences in the resuspension results. Flexible fabric motion and multilayer dust motion were demonstrated as major contributors through visualization experiments. The observed phenomena of acceleration amplification effect along the fabric and various particle-particle interactions provided a crucial basis for our reasonable assumptions in the mathematical description. A set of semi-empirical correlations was thus developed. Study 1 laid a solid foundation for the subsequent studies. Study 2 parametrically investigated dust resuspension from four typical fabrics (cotton, linen, silk, and polyester) exposed to airflow with ordinary velocities within 0 ~ 10 m/s, including a moving fabric case and a fixed fabric case, inspired by wind-induced natural phenomena, e.g., flags and leaves fluttering in the wind. Following the modeling configuration adopted in Study 1, a set of semi-empirical correlations, involving air velocity, fabric motion mode, fabric type, and airflow duration was developed for the moving fabric case. It was found that a stronger-than-expected resuspension was triggered by short-term accelerating airflow. The resuspension enhancement of over 90% was reported for the moving fabrics compared with the fixed ones. Fabric motion induced by airflow was proposed to account for these resuspension findings. Fabric acceleration was then demonstrated to be a key factor in evaluating the resuspension for such a scenario. Compared with Study 1, the case of flow-induced fluttering flexible surfaces was recognized as the one with greater resuspension potential, which deserved further theoretical development. Study 3 therefore performed a single-particle dynamics analysis for spherical particles on a flow-induced fluttering flexible substrate. The effects of the flow-induced flutter on the resuspension, and the underlying mechanisms initiating the resuspension in the fluttering substrate case, were quantitatively illustrated by comparing with the fixed substrate case. A proportional relationship (i.e., fluttering frequency ∝ air velocity) connected the physical meanings of the semi-empirical models in Study 1 and Study 2, implying that the form is proposed to be applicable in a wide range of scenarios involving moving surfaces. A force caused by the fluttering acceleration was identified as an important factor enhancing the resuspension. The existence of a significant flutter-induced aerodynamic force/moment enhancement acting on the particles was demonstrated as another necessary factor, charting a new course for future studies. The three studies are interconnected and progressive. This thesis not only systematically studies particle resuspension from moving flexible surfaces, but also helps enrich the understanding of particle dynamics.