Changes in density
The density variations in a fluid can be visualized using the optical methods (shadowgraph, Schlieren and interferometry), which highlight the changes in the refractive index of the fluid. These methods furthermore allow measurements of density (especially interferometry). These optical methods, besides being non-intrusive and giving a global view of the flow field, as do all methods of visualization, have peculiar advantages and disadvantages:
■ The inertia of the system is practically zero, the study of unsteady motions is possible with the temporal discrimination allowed by the speed of the shut-off system (camera, video camera). This feature is particularly useful in supersonic wind tunnels and even more in hypersonic ones in which the duration of the tests is very short.
■ Optical methods are able not only to visualize streams at Mach numbers lower than those considered typical of compressible aerodynamics, but can also visualize changes in fluid density due to heat transfer or mass diffusion.
■ All optical methods give the value of the density variations integrated along the optical path; increasing difficulties are encountered in
interpreting the images going from two-dimensional plane fields to axially symmetric fields and to three-dimensional fields.
■ Limits on these methods are low density hypersonic motions and low density gradients.