This experimental study aims to better the knowledge of the flow-mixing phenomena involved in the first period of a backdraft, before the potential reignition of fuel gases in the enclosure (step not studied in the study). The authors describe the aerodynamics mechanisms of the evolution of thermal instabilities leading to the formation and propagation of a gravity wave appearing when dense fresh air enters an enclosure rich in hot gases (mixture of combustion products and fuel gases). A specific device and an experimental procedure were developed with flow conditions representative of a backdraft, but the tests were performed with inert gases in which fresh air entering in a enclosure containing heated air. Time-resolved laser tomography and particle image velocimetry measurements were performed to describe the wave displacement. The results show a strongly unsteady flow with formation in a pulsative mode of large scale Kelvin-Helmholtz structures. These obtained experimental results are essential for the calibration and validation of the subgrid turbulence model used by the numerical model.