Abstract: This article presents an investigation on a correlation relevant to flow separation during shock wave–boundary layer interaction in a hypersonic flow. The study includes experimental as well as numerical components. The hypersonic shock–boundary layer interactions were visualized to gauge the extent of flow separation on a flat plate on external shock impingement. The pressure in the vicinity of shock impingement was measured. Through visualized images, the applicability of the correlation to flow separation due to externally impinged shock waves was analyzed. Qualitative numerical simulations were carried out to have an insight into the hypersonic shock–boundary layer interactions. Graphical abstract: [Figure not available: see fulltext.] © 2015, The Visualization Society of Japan.