On the Stokes number and characterization of aerosol deposition in the respiratory airways

Aerosol deposition in the respiratory airways has traditionally been examined in terms of the Stokes number based on the reference flow timescale. This choice leads to large scatter in deposition efficiency when plotted against the reference Stokes number because the velocity and length scales experienced by advected particles deviate considerably from the reference values. A time-average of the particle local Stokes number should be adopted instead. Our results demonstrate that this average, or effective, Stokes number can deviate significantly from the reference value, in particular in the intermediate Stokes number range where variation across subjects is largest

The Effect of wake Turbulence Intensity on Transition in a Compressor Cascade

Direct numerical simulations of separating flow along a section at midspan of a low-pressure V103 compressor cascade with periodically incoming wakes were performed. By varying the strength of the wake, its influence on both boundary layer separation and bypass transition were examined. Due to the presence of small-scale three-dimensional fluctuations in the wakes, the flow along the pressure surface undergoes bypass transition. Only in the weak-wake case, the boundary layer reaches a nearly-separated state between impinging wakes. In all simulations, the flow along the suction surface was found to separate. In the simulation with the strong wakes, separation is intermittently suppressed as the periodically passing wakes managed to trigger turbulent spots upstream of the location of separation. As these turbulent spots convect downstream, they locally suppress separation. © 2014 Springer Science+Business Media Dordrecht

Wavy Taylor vortices in molecular dynamics simulation of cylindrical Couette flow

Molecular dynamics simulations of flow between concentric rotating cylinders are performed. As the relative speed between the two cylinders is increased, a spontaneous flow bifurcation occurs and vortices form in a stationary-vortex or traveling-wavy-vortex configuration. The former emerges when the axial boundary conditions constrain the flow by reflection, and the traveling-wavy-vortex flow develops when the axial boundaries are relaxed to periodic conditions. The flow bifurcation is triggered by the thermal fluctuations in the system, and the resulting flow field is in agreement with previous experimental observations. In addition, the temporal growth of the Fourier mode that characterizes the wavy-vortex motion is well described by Landau's theory for Hopf bifurcations. The spatiotemporal energy spectrum is evaluated in order to characterize the instability in terms of its azimuthal wave number and wave speed

Prediction of flow and aerosol deposition in the extrathoracic airways using an implicit immersed boundary method

The effect of intrasubject variation on the turbulent flow and aerosol deposition in the extrathoracic airways is studied in two realistic mouth-throat geometries from the same subject. An immersed boundary method is applied which simplifies the task of grid generation for the complex extrathoracic geometries and allows the use of a structured grid solver. Curvilinear grids that roughly follow the shape of the geometries are adopted, allowing for much higher resolution within the geometries than Cartesian grids commonly used in IB methods. An added advantage is that the grid lines are approximately aligned with the streamlines, which reduces numerical diffusive errors. The numerical simulations allow us to explain in vitro aerosol deposition data in the literature for the same mouth-throat models. The position of the tongue during inhalation is shown to have a significant impact on both the mean flow patterns and the turbulence intensities, which in turn affects extrathoracic deposition