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Backward-facing step flow is a benchmark problem that has been studied in various fields, such as airfoils, diffusers, boilers, nuclear reactors, electronic devices, and air-conditioning ducts. In this study, a rigid rectangular flapping longitudinal vortex generator was mounted at the step of the channel to investigate the fluid flow and heat transfer characteristics at three flapping frequencies (0.167, 0.25, and 0.5 Hz) in the Reynolds number range of 3000 to 8000, while maintaining at a constant heat flux. When the fluid flowed over the backward-facing step with flapping longitudinal vortex generator, a train of longitudinal vortices developed simultaneously. At a flapping frequency of 0.167 Hz, the developed high-intensity longitudinal vortices were stable and augmented the heat transfer by 38.54 % more than the smooth channel. The friction factor at 0.167 Hz was found to be 19.47 % and 25.33 % greater than at the higher frequencies of 0.25 and 0.5 Hz, respectively.