The effects of the air variable properties (density, viscosity and thermal conductivity) on the transient buoyancy-driven flows established in open square cavities are investigated. Two-dimensional, laminar, transitional and turbulent simulations are obtained, mainly considering uniform heat flux heating conditions. For the fully understanding of the transient flow, different configurations are considered, including those chosen for comparison purposes. The low-Reynolds k-omega turbulence model is employed. The average Nusselt number and the dimensionless mass-flow rate have been obtained for a wide range of the Rayleigh number varying from 10(5) to 10(12). The changes produced along the time in the flow patterns inside the cavity when the effects of variation of properties are relevant, are also shown. The transient evolution of the flow is strongly affected when the heating parameter is high enough. (C) 2016 Elsevier Ltd. All rights reserved.