Natural convection in a side-heated cavity: visualization of the initial flow features

The shadowgraph technique is used for visualizing the early stages of the flow in a water-filled square cavity which is suddenly heated and cooled on the opposing sidewalls. These sidewalls are perfect heat conductors, while the top and bottom boundaries of the cavity are thermally insulating. For the first time, a clear visualization is given of the first group of waves travelling up the hot boundary layer immediately after start-up, of the initial horizontal intrusions and of the second group of waves resulting from the interaction of the incoming intrusion with the boundary layer. The properties of the waves are examined and shown to be in agreement with previous numerical predictions. Special attention is paid to the initial horizontal intrusions, and their connection to gravity currents is discussed. One feature in particular which makes these intrusions unique is the interaction with the first group of waves coming from the vertical boundary layers. This interaction together with what appears to be a thermal instability of the intrusion nose are discussed in detail.