Ultraviolet germicidal irradiation (UVGI) inactivates viral aerosols in indoor environments. Upper room UVGI systems use wall or ceiling mounted fixtures to create a disinfection zone above the occupied zone. The performance of upper room UVGI systems varies with indoor airflow induced by mechanical ventilation and thermal plumes from occupants, which carries contaminated air into the disinfection zone where viral aerosols are partially inactivated before circulating back into the breathing zone. This study used computational fluid dynamics (CFD) modeling to investigate the effect of an upper room UVGI system on spatial distributions of viral aerosols with UV-C susceptibility representative of coronaviruses as a function of ventilation system characteristics. Upper-room UVGI confined elevated viral aerosol concentrations to the vicinity of an infector, while the room average viral aerosol concentration was reduced by two orders of magnitude relative to a case without UVGI. Return air recirculation rates and ventilation strategy (i.e. mixing vs. displacement) had notable effects on the disinfection effectiveness of the UVGI system. For mixing ventilation, as the recirculation air flow rate increased from 0 to 5.3 h-1 with a fixed outdoor air flow of 0.7 h-1, UVGI inactivation escalated by 62%. Mixing ventilation at 0.7 h-1 without recirculation in the room with a volume of 108 m3 was 30% more effective in inactivating airborne viruses than displacement ventilation, due to the higher air mixing.