Horizontal rolls over localized heat source in a cylindrical layer

•Convection over a localized heat source was studied experimentally and numerically.•Complex system of secondary flows over the heater was observed.•Transitions from transverse rolls to radial rolls were found.•Transverse rolls formation is a periodic process within a wide range of parameters.

Convection over a localized heat source in a cylindrical layer was studied experimentally and numerically for fluids with different values of Prandtl number. A basic flow produced by a horizontal temperature gradient occupies the whole layer and leads to unstable temperature stratification over the heating area and the formation of a complex system of secondary flows. The main focus of our study was the spatial and temporal evolution of small-scale convective structures in the boundary layer of a basic flow. Transitions from transverse rolls to radial rolls and further to their superposition were found in experiment and numerical simulation. Various types of visualization revealed co-existence of different kinds of secondary flows. Complex convective patterns over the heating area are temporally periodic. The characteristic frequency of transverse rolls depends on Rayleigh number for a wide range of governing parameters.