Time-developing thermally-driven boundary layers created by imposing aiding and opposing freestreams on the natural-convection boundary layer in water along a heated vertical flat plate have been examined with a direct numerical simulation. The numerical results reveal that, with a slight increase in freestream velocity, the transition to turbulence delays for aiding flow and quickens for opposing flow. This fact is linked to the experimental results indicating that heat transfer rates of the turbulent combined-convection boundary layer decrease for aiding flow and increase for opposing flow with increasing freestream velocity. In response, turbulence statistics obtained for aiding flow such as the intensities of velocity and temperature fluctuations, Reynolds shear stress and turbulent heat fluxes become smaller than those for the pure natural-convection boundary layer suggesting the way to laminarization of the boundary layer, while those for opposing flow are not much different from the observations for the pure natural-convection boundary layer. To improve the significance of the present numerical results, the association of turbulence statistics between time- and space-developing flows has been also investigated. Consequently, the numerical results for time-developing flow are converted to those for space-developing flow through the integral thickness of the velocity boundary layer for pure natural convection, and thus the regimes of boundary layer flows in water can be quantitatively assessed. Moreover, by visualizing the flow field of the combined-convection boundary layers, it is found that the long-drawn high- and low-speed fluid motions with weak fluctuations for aiding flow and the large scale fluid motions for opposing flow are more apparent than those for the pure natural-convection boundary layer.

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