Characteristics of droplet dispersion and interpersonal exposure in indoor and outdoor dining areas of urban street: Elementary numerical investigations

Abstract

Indoor and outdoor dining areas are special spaces where people generally sit close and talk face-to-face for a long time (∼hour). Semi-open dining area is of popularity among them, one of the reason is the thoughts that outdoor dining area have experienced small infectious risks, even though this opinion has not been thoroughly validated yet. As a novelty, this study conducts CFD simulations to investigate droplet dispersion and face-to-face exposure risk of customers in the indoor and outdoor dining areas of typical street canyons (H/W=1) by coupling indoor/outdoor airflows. Different scenarios are considered including index patient locations, the setting of the desk isolation board (DIB, HDIB=0.6m or 1.2m), and the outdoor semi-open street roof (SSR).Results show that indoor dining spaces with single-sided ventilation experience one-order poorer ventilation (net escape velocity NEV∼0.036m/s) and subsequently 10-200 times higher face-to-face droplet trap fraction than those in outdoor dining areas (NEV∼0.407m/s, TF∼3-102ppm). Indoor desk isolation board can decrease face-to-face TF by 50% but in outdoors, TF without desk-isolation board is 6.5ppm for 50μm droplet and this value changes little or slightly increases as HDIB is 0.6m (∼8.9ppm) or 1.2m (∼7.7ppm), indicating DIB seems not helpful to reduce airborne exposure risk in outdoors. Moreover, the setting of an outdoor semi-open street roof (SSR) may reduce NEV in the indoor or outdoor dining area by half or 90% respectively. Therefore, SSR may raise indoor and outdoor customers' face-to-face exposure risk (e.g. TF) by hundreds of times and seventy times respectively.