Stochastic tracking and deposition of particles inside a complex geometry

The development of new medicines for inhalation requires a series of procedures including in-vitro tests using special testing devices, which simulate the human airways to assess the local deposition of drugs. Such tests require samples of the new drugs and the series of tests are costly and time-consuming. A numerical model for the two-phase flow inside the first chamber of one of these special devices, the Twin Impinger (TI), has been developed. Due to the effects of drag, gravity and virtual mass forces, computer-generated particles travel through the virtual TI, which was discretised using a structured mesh. An Eulerian model has been used to simulate the continuous phase and a Lagrangian stochastic eddy-interaction model (EIM) has been chosen to simulate the behaviour of the discrete phase. Aiming to reduce the uncertainty associated to turbulence modelling of the continuous phase in a complex flow, detailed experimental data has been taken for the mean and turbulent air velocity fields. These data have been obtained by the use of a 2D Laser Doppler Anemometer, employing the continuity concept to generate the third velocity component. The interaction between particles was not considered because it was supposed that the particles were already dispersed enough inside the TI. Finding the Eulerian control volume where the particle was after each Lagrangian time-step was attained in a circular search around the previous particle position, in preferential directions, for the closest mesh node. The Eulerian cell values for the mean and turbulent velocity fields were assumed everywhere except near the wall, where the gradients are larger, and corrections in both the flow model and the EIM were required. The interaction between the particles and the walls of the TI considered a coefficient of restitution depending on the angle of impact of the particles, which were considered adhered if their kinetic energy after impact was not sufficient to compensate the van der Waals forces between the wall and the particle. A simple procedure to compute the distance between the particle and the near-wall velocity led to preferential deposition in lines orientated along the mesh, and the formation of unphysical particle clusters. Major improvements to the global model performance were obtained by the use of near-wall corrections in the EIM and employing a more elaborate algorithm for the wall distance. However, these changes increased the computational cost of the numerical simulations