Effects of ventilation on the indoor spread of COVID-19

Abstract

Keywords: drops; plumes/thermals; turbulent mixing.

Graphical abstract

Figure 1.

Schematic illustrating ventilation flows with the various flow elements such as the body plume, inlet flows, stratification and arrows indicating entrainment and mixing. (a) Mixing ventilation, the hot air rises to the ceiling and, except near the inlets and vents, the indoor conditions remain approximately uniform. (b) Displacement ventilation when the occupant does not wear a mask. The secondary breathing plume stratifies below the hot upper layer, and the fluid in the secondary layer gets entrained into the body plume and exhausted out of the indoor space. (c) Displacement ventilation when the occupant wears a mask. In this scenario, near its origin, the breathing plume gets caught into the body plume and exhausted out from the upper layer. (d) When we turn off the mechanical ventilation input and instead open the doors and windows of space with a top-level opening, ignoring the effect of the wind, it effectively creates a displacement ventilation scenario (here shown when no mask is worn).

Figure 2.

The concentration measured at different heights in a naturally ventilated office in London. (a) Time variations over five weekdays and (b) the mean stratification and mean temperature profile during working hours (9:00 to 17:00).

Figure 3.

Differential synthetic schlieren images of the thermal plumes produced by a person in a quiescent environment. In panels (a–c) no mask is worn, while in panels (d–f) a non-surgical three-ply disposable mask is worn. The subject is (a,d) sitting quietly breathing through their nose, (b,e) saying ‘also’ when speaking at a conversational volume and (c,f) laughing. Video sequences of each of these can be found in the supplementary material.

Figure 4.

Synthetic schlieren images of the forced convection and thermal wake produced by someone walking slowly through a quiescent room. (a) Thermal convection is not visible in front of the person but a thermal signal is clearly visible in the high-Reynolds-number wake of a person walking without breathing. (b) The thermal signature of the wake one second after the passage of the person. Significant mixing has occurred without obvious effects of buoyancy. (c) Air exhaled by nasal breathing is swept around the head by the motion and entrained into the wake. (d) Although laughing still produces a jet that reaches in front of the subject, it is soon overtaken and the associated breath is incorporated into the wake.