Synthetic schlieren is a process that is used to visualize the flow of a

fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear ...

of variable

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

. Named after the

schlieren Schlieren ( ; , ) are optical inhomogeneities in transparent media that are not necessarily visible to the human eye. Schlieren physics developed out of the need to produce high-quality lenses devoid of such inhomogeneities. These inhomogeneitie ...

method of visualization, it consists of a

digital Digital usually refers to something using discrete digits, often binary digits. Technology and computing Hardware *Digital electronics, electronic circuits which operate using digital signals **Digital camera, which captures and stores digital i ...

camera or video camera pointing at the flow in question, with an illuminated target pattern behind. The method was first proposed in 1999.{{citation , doi=10.1017/S0022112099005017 , first1=B. R. , last1=Sutherland , first2=S. B. , last2=Dalziel , first3=G. O. , last3=Hughes , first4=P. F. , last4=Linden , title=Visualization and measurement of internal waves by "synthetic schlieren". Part 1: Vertically oscillating cylinder , journal=Journal of Fluid Mechanics , volume=390 , issue=1 , pages=93–126 , year=1999 , bibcode=1999JFM...390...93S Variations in refractive index cause the light from the target to

refract In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...

as it passes through the fluid, which causes a distortion of the pattern in the image seen by the camera. Pattern matching algorithms can measure this distortion and calculate a qualitative

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

field of the flow. The method of synthetic schlieren can be used to observe any flow which has variations in refractive index. Commonly these are caused by variations in concentration of a

solute In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solvent ...

in an

aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be re ...

, or variations in the density of a

compressible flow Compressible flow (or gas dynamics) is the branch of fluid mechanics that deals with flows having significant changes in fluid density. While all flows are compressible, flows are usually treated as being incompressible when the Mach number (the r ...

, caused by

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

variations. As with the optical schlieren method, the clearest results are obtained from flows which are largely two-dimensional.

References

Fluid dynamics

See also

References