Abstract: The laminar flow and instability mechanism of a round jet are investigated by visualization methods of dye and particle tracers. The toroidal backflow in a low speed round jet issuing from a nozzle was observed successfully for the first time. The flow visualization photographs demonstrated both the evolution of jet flow and effect on it of bottom boundary of the test vessel. It was found that the axisymmetric disturbance occurring in the real jet and its instability process are corresponding just right to ...