The control of horizontal square cylinder wake using thermal buoyancy has been experimentally investigated at low Reynolds numbers. The cylinder with an aspect ratio of 60 is mounted in a vertical test cell. The cylinder is electrically heated such that the buoyancy aids to the inertia of the mean flow. The operating parameters i.e. Reynolds number (87–118) and Richardson number (0.065–0.171) are varied to examine the flow behaviour over a range of experimental conditions. Laser schlieren-interferometry has been used for visualization and analysis of flow structures. The complete vortex shedding sequence has been recorded using a highspeed camera. The suppression of vortex shedding by heat input has been demonstrated by schlieren image visualization, time traces of light intensity, corresponding power spectra and Strouhal number. The study provides new experimental information on processes and mechanisms involved in the heat-induced changes of the vortex structures under the influence of buoyancy. The formation length of the vortex structures increases with increase in Richardson number i.e. heating level. The sequence of instantaneous schlieren images show that shape of vortex structures becomes slender at a sufficiently high Richardson number and the vortices from opposite shear layers rub with each other without increasing the circulation level and the two shear layers combine to form a single plume. The plume becomes steady at critical value of heat input leading to suppression of vortex shedding. The corresponding spectra evolve from having a clear peak at the vortex shedding frequency to broadband spectra when vortex shedding is suppressed.

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