| Undesirable airborne particulate matter is present in the indoor environment largely because of the humans that inhabit it, and the disturbances that cause the matter to become airborne are also largely generated by the inhabitants. Understanding the process by which particles are deposited and then suspended in the air (resuspension), is an important part of understanding how to maintain indoor environments at appropriate levels of quality. The focus of the present work is that the whole history of a resuspended particle is important. The process by which the particle is deposited upon a surface determines the condition of the particle on a surface. The particle could be isolated, in contact with other particles, or part of an agglomerate (a set of particles that adhere to each other and operate as a single unit). The particle then resides upon the surface for a time, during which it may be subject to variations in temperature, humidity, and other environmental parameters that may change the degree to which the particle is attached to the surface or other particles. The particle may also be subject to mechanical operations that may change the physical dimensions of the particle. In order to be resuspended, the particle must then be detached from the surface. This could be caused by mechanical disturbances, such as vibration, or by aerodynamic disturbances, such as the impact of a turbulent flow structure upon a surface. Once a particle is detached, it then must be transported away from the surface by the fluid before the particle can be considered resuspended. In the present work, we concentrate upon the deposition and transport phases of a resuspended particle's life cycle. To study the deposition process, we formulate a Monte Carlo deposition model. This model tells us that the condition of aerodynamic isolation, or separation from other particles by a significant number of particle diameters, is very unlikely for most of the particulate matter that is of practical interest. Examination of the forces upon particles leads us to conclude that for small particles of unit density, the velocity gradient lift forces that are sometimes blamed for resuspension are not significant when compared to the weight of the particle. The relative importance of the particle weight is borne out in particle transport simulations in an axisymmetric footstep flow simulation, in which most of the unit density particles are not lifted away from the surface. We simulate this flow problem, which has been studied experimentally and computationally, using an immersed boundary technique. We find relatively poor comparison with the experiments and investigate these differences using the simpler, related problem of the impingement of a vortex upon a plane wall. From these results, we conclude that turbulence in the footstep case is likely the cause of the differences between our calculations and the experiments. Our simulations and models indicate that much of the resuspension observed in experiments is not the result of single, isolated particles being forcibly detached from a surface, but must come from other on-surface conditions and disturbances. |
