Light reflection as a simple experimental method in compressible boundary-layer studies

Light reflection is employed as an experimental method to measure the local refractive index of a gas at the interface with a wall, and is valid even if rather strong gradients in temperature, density, or chemical composition of the gas prevail, as is the case in a compressible boundary layer adjacent to a wall. The wall is assumed optically transparent, with a plane light wave reflecting from the wall/gas interface and from the boundary layer. The reflections from the boundary layer vanish if the ratio of boundary layer thickness to wavelength is not too small, and reflectivity measurements yield the refractive index of the gas at the wall, while the refractive index of the gas depends on the density and chemical composition of the gas. Applications to a viscous sidewall and thermal end-wall boundary layer in shock tubes are discussed.