Direct numerical simulations of lung models: do we need them?

Abstract: We recently carried out the first direct numerical simulation of a rigid lung model including an idealized single branch with 23 successive bifurcations, at a steady inhalation rate of 60 L/min. Our findings suggest that intense and fast flow rate fluctuations can occur deeper in the bronchial tree than previously thought. In a rigid bronchial tree, these fluctuations appear to be the inevitable consequence of unsteady flow splitting happening in higher bifurcations, where the flow is chaotic; they cannot be damped by viscosity because of mass conservation and propagate down to the terminal portion of the model. What happens in the physiological case? We do not know, because there is a lack of data on the degree of anisotropy deformation of the alveolar ducts. However, assuming perfectly synchronous expansion and contraction – which is the only condition compatible with the suppression of all time scales different than the breathing cycle – is quite counterintuitive.