Time resolved analysis of steady and oscillating flow in the upper human airways

In this experimental study a thorough analysis of the steady and unsteady flow field in a realistic transparent silicone lung model of the first bifurcation of the upper human airways will be presented. To determine the temporal evolution of the flow time-resolved particle-image velocimetry recordings were performed for a Womersley number range 3.3 ≤ α ≤ 5.8 and Reynolds numbers of Re _D  = 1,050, 1,400, and 2,100. The results evidence a highly three-dimensional and asymmetric character of the velocity field in the upper human airways, in which the influence of the asymmetric geometry of the realistic lung model plays a significant role for the development of the flow field in the respiratory system. At steady inspiration, the flow shows independent of the Reynolds number a large zone with embedded counter-rotating vortices in the left bronchia ensuring a continuous streamwise transport into the lung. At unsteady flow the critical Reynolds number, which describes the onset of vortices in the first bifurcation, is increased at higher Womersley number and decreased at higher Reynolds number. At expiration the unsteady and steady flows are almost alike.