Accurate simulations of the flow in the human airway are essential for advancing diagnostic methods. Many existing computational studies rely on simplified geometries or turbulence models, limiting their simulation's ability to resolve flow features such shear-layer instabilities or secondary vortices. In this study, direct numerical simulations were performed for inspiratory flow through a detailed airway model which covers the nasal mask region to the 6th bronchial bifurcation. Simulations were conducted at two physiologically relevant \textsc{Reynolds} numbers with respect to the pharyngeal diameter, i.e., at Re_p=400 (resting) and Re_p=1200 (elevated breathing). These values characterize resting and moderately elevated breathing conditions. A lattice-Boltzmann method was employed to directly simulate the flow, i.e., no turbulence model was used. The flow field was examined across four anatomical regions: 1) the nasal cavity, 2) the naso- and oropharynx, 3) the laryngopharynx and larynx, and 4) the trachea and carinal bifurcation. The total pressure loss increased from 9.76 Pa at Re_p=400 to 41.93 Pa at Re_p=1200. The nasal cavity accounted for the majority of this loss for both Reynolds numbers, though its relative contribution decreased from 81.3% at Re_p=400 to 73.4% at Re_p=1200. At Re_p=1200, secondary vortices in the nasopharyngeal bend and turbulent shear-layers in the glottis jet enhanced the local pressure losses. In contrast, the carinal bifurcation mitigated upstream unsteadiness and stabilized the flow. A key outcome is the spatial correlation between the pressure loss and the onset of flow instabilities across the four regions. This yields a novel perspective on how the flow resistance and vortex dynamics vary with geometric changes and flow rate.

翻译：对人体气道内流动进行精确模拟对于改进诊断方法至关重要。现有许多计算研究依赖于简化几何或湍流模型，限制了其模拟解析剪切层不稳定性或二次涡等流动特征的能力。本研究对通过一个覆盖鼻罩区域至第六级支气管分叉的详细气道模型的吸气流动进行了直接数值模拟。模拟基于咽部直径在两个生理相关的雷诺数下进行，即Re_p=400（静息状态）和Re_p=1200（增强呼吸状态）。这些数值表征了静息和中等增强呼吸条件。研究采用格子玻尔兹曼方法直接模拟流动，即未使用任何湍流模型。流动场在四个解剖区域进行了考察：1）鼻腔，2）鼻咽与口咽部，3）喉咽部与喉部，4）气管与隆突分叉。总压损从Re_p=400时的9.76 Pa增加到Re_p=1200时的41.93 Pa。鼻腔在两个雷诺数下均贡献了压损的主要部分，但其相对贡献从Re_p=400时的81.3%下降到Re_p=1200时的73.4%。在Re_p=1200时，鼻咽弯曲处的二次涡和声门射流中的湍流剪切层增强了局部压力损失。相比之下，隆突分叉缓解了上游的非定常性并使流动趋于稳定。一个关键结果是四个区域中压力损失与流动不稳定性起始之间的空间相关性。这为流动阻力和涡动力学如何随几何变化与流量改变提供了新的视角。