Nasal cavity turbulence

An additional form of air-conditioning is concerned with the removal of particulates, such as dust, microorganisms and allergens, from the inspired air. The large cross-sectional area of the nasal cavity and relatively low air velocities are ideal for particle deposition, as is the turbulence caused beyond constrictions where changes in air flow direction occur. The efficiency of particle removal from the air-stream is dependent on a number of factors including the aerodynamic diameter of the inhaled particles ... 

Turbulence is enhanced by a passage with nonuniform cross section and by twists and turns and protrusions. Notice that the nasal cavity has a large surface area, it is of variable cross section, and the path of the air makes a large turn before entering the throat. The large cross-sectional area means that the air velocity slows from its value at the nostrils, increasing the residence time of the air in the cavity and giving it the time to be heated and humidified. 

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