Convection over localized heat source in a cylindrical layer was studied experimentally for fluids with different values of Prandtl number. Basic flow produced by horizontal temperature gradient occupies the whole layer and leads to the unstable temperature stratification over the heating area and formation of complex system of secondary flows. The main focus of our study was spatial and temporal evolution of small-scale convective structures. Transition from transverse rolls to radial rolls and further to their superposition was observed and studied in details. It was found that transverse rolls appears periodically for a wide range of governing parameters. Physical interpretation of secondary flows formation is proposed.