Gas transport in double bifurcation subjected to asymmetric reciprocating flow

High-frequency oscillatory ventilation is a medical ventilation technique that aims to be lung-protective by only moving small volumes of air in and out of the airway per cycle; these small tidal volumes mean that gas transport must be achieved by mechanisms other than bulk advection. Nonlinear mean streaming is one such mechanism. This paper uses direct numerical simulations to study the generation of nonlinear mean streaming in a double bifurcation, a geometry representative of the bifurcating structure of the airway. Specifically, the impact of an asymmetric waveform (where the inflow time is shorter than the outflow time) and the base frequency of reciprocation is measured. We show that the streaming flow strength is nearly independent of the waveform asymmetry at high Reynolds numbers representative of the large upper airways. However, the streaming flow strength can be significantly enhanced by asymmetry at low Reynolds numbers representative of the smaller lower airways. We show that there is an optimal outflow to inflow time ratio in the vicinity of 1.4, and confirm that the impact of asymmetry on the streaming strength occurs for a range of base frequencies spanning an order of magnitude.