| Summary: | The effects of thermal radiation and viscous dissipation on the flow and heat transfer of a hybrid nanofluid (Cu–Al2O3/water) past a circular cylinder are investigated for both assisting and opposing flows. The numerical results reveal that the flow and energy fields adjacent to the rear stagnation point are strongly affected by the opposing flow rather than the assisting flow. For the assisting flow, the size of the vortex increases for the higher volume fraction of Cu nanoparticles and Reynolds number, but it decreases with an increase in the Eckert number, conduction–radiation parameter, surface temperature parameter, and Grashof number. In the case of the opposing flow, the converse scenario is observed for all parameters except the Eckert number. For a certain set of parameters, the size of the vortex for the assisting flow is always smaller than that for the opposing flow. In general, the heat transfer for the assisting flow is stronger than that for the opposing flow. For both types of flows, the Nusselt number significantly increases owing to the increase in the volume fraction of Cu nanoparticles and Reynolds number; however, it diminishes for the higher values of the Eckert number, conduction–radiation parameter, and surface temperature parameter. It is remarkable that the Nusselt number for the Cu–Al2O3/water hybrid nanofluid is found to be higher than that for the Al2O3/water nanofluid.
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