Schlieren Flow Visualization
Schlieren imaging is a measurement technique that can visualize density gradients via the deflection of light rays due to changes in the refractive index of the gas medium. In the image above, when there is no density gradient, light will arrive at point F. However, if there is a density gradient, light will arrive at point F*, which creates the displacement Δx and Δy in the recording plane. The light source is collimated by the focus lens and passes through the knife edge, finally arriving at the recording plane. The knife edge is used to amplify the difference of the light intensity which is deflected by the density gradient.
This technique was utilized to analyze the shock structure of a supersonic jet . Measuring the flow characteristics in supersonic flows is extremely difficult, and thus Schlieren imaging was applied. The shock structure of the over-expanded supersonic jet is clearly shown above. At the exit, a pair of oblique shocks meet and form a normal shock, which then creates a reflected shock. When the flow passes through a shockwave, density increases, but when the flow passes through an expansion fan, density decreases.
 Park, H. J. (2019). Measuring high frequency density fluctuation induced by supersonic jet with Schlieren photography and Rayleigh scattering. (Master’s thesis, Seoul National University, Seoul, Republic of Korea).