A schlieren photo shows the density gradients in a two-dimensional flow field. The schlieren technique utilizes the feature that the density of a gas is intrinsically tied to the refractive index of light. The schlieren technique transforms density gradients of the gas into changes of brightness or colour in a photo or video (see explanation of method below). Effects of compressible flow as shocks, wakes, vortices and flow separation are revealed, but also thermal effects can be visualized.
Supersonic turbine flow: flow acceleration, shocks and wakes
Flow separation at profile front
Detail: separation bubble by shock boundary layer interaction
Supersonic exit flow in an HP turbine cascade: on the left with separation on the suction side, on the right with rear total separation at still higher exit Mach numbers
Flow separation in steam turbine cascade at Mach numbers higher than design
Turbine cascade flow with Flat Plate profiles: Mach number increasing from top left to bottom right ; visualization of density gradients, especially zones of flow acceleration, compression shocks, wakes and von Karman vortex streets; light source: flash of 20 ns duration