Time-developing combined-convection boundary layers induced by imposing aiding and opposing flows to the natural-convection boundary layer in air along a hot vertical flat plate have been examined with a direct numerical simulation. As the freestream velocity increases, the transition from laminar to turbulence delays for aiding flow and quickens for opposing now, corresponding well to actual observations in space-developing flows The calculated profiles of mean streamwise velocity, mean temperature, intensities of streamwise velocity fluctuation and temperature fluctuation in the laminarization process oft he boundary layer for aiding flow agree relatively well with the existing data Also, the distributions Of turbulent statistics and instantaneous fluid motions in the combined-convection boundary layers with adding and opposing flows are displayed. and the regimes of the boundary layer flows obtained from the calculations are compared with those observed in the experiment. (C) 2009 Elsevier Ltd. All rights reserved.