Detection of standing internal gravity waves in experiments with convection over a wavy heated wall

Convection over a wavy heated bottom wall in the air flow has been studied in experiments with the Rayleigh number of ∼10⁸. It is shown that the mean temperature gradient in the flow core inside a large-scale circulation is directed upward, which corresponds to the stably stratified flow. In the experiments with a wavy heated bottom wall, we detect large-scale standing internal gravity waves (IGWs) excited in the regions with the stably stratified flow. The wavelength and the period of these waves are much larger than the turbulent spatial and time scales, respectively. In particular, the frequencies of the observed large-scale waves vary from 0.006 Hz to 0.07 Hz, while the turbulent time in the integral scale is about 0.5 s. The measured spectra of these waves contain several localized maxima that imply an existence of waveguide resonators for large-scale standing IGWs. For comparisons, experiments with convection over a smooth plane bottom wall at the same mean temperature difference between the bottom and upper walls have also been conducted. In these experiments, various locations with a stably stratified flow are also found, and large-scale standing IGWs are observed in these regions.