Heating-induced drag reduction in relative movement of parallel plates

An external force is required to maintain the relative movement of horizontal plates. It is shown that this force is reduced when the plates are subject to a spatially distributed heating. The largest reduction occurs for heating wavelengths of the order of distance between the plates with its magnitude increasing proportionally to the second power of the relevant Rayleigh number. It is shown that a sufficiently strong heating eliminates the need for the driving force altogether. The drag-reducing effect is active only in small Reynolds number flows and is stronger in fluids with smaller Prandtl numbers.