Synthetic schlieren is a process that is used to visualize the

flow Flow may refer to: Science and technology * Fluid flow, the motion of a gas or liquid * Flow (geomorphology), a type of mass wasting or slope movement in geomorphology * Flow (mathematics), a group action of the real numbers on a set * Flow (psyc ...

of a

fluid In physics, a fluid is a liquid, gas, or other material that may continuously motion, move and Deformation (physics), deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are M ...

of variable

refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

. 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 inhomogeneit ...

method of visualization, it consists of a

digital Digital usually refers to something using discrete digits, often binary digits. Businesses *Digital bank, a form of financial institution *Digital Equipment Corporation (DEC) or Digital, a computer company *Digital Research (DR or DRI), a software ...

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 , s2cid=123499058 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 phenome ...

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 ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

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 defined by IUPAC as "A liquid or solid phase containing more than one substance, when for convenience one (or more) substance, which is called the solvent, is treated differently from the other substances, which are ...

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, also known as sodium chloride (NaCl), in water ...

, 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 compressibility, compressible, flows are usually treated as being incompressible flow, incom ...

, caused by

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

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 eve ...

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