Schlieren
Fluid flow as a complex subject has not yet investigated analytically. Thus, measurement of flow field is complicated and there are not precise analytical relationships. In usual measurement methods, the flow often affected by the probes which are placed in the flow field for experimental measurement of velocity, pressure, temperature, and so on. Optical flow visualization techniques as non-invasive techniques do not interfere with fluid flow, and let scientists to measure the whole flow field non-intrusively in a wide variety of applications of liquid and gaseous media.
In fluid dynamics visualization of flow is applied to observe the flow patterns in order to obtain quantitative or qualitative data of fluid flow. Flow pattern of various phenomena could be visualized by means of optical visualization techniques such as Schlieren and Shadowgraphy techniques. These techniques visualize the flow pattern based on variation of optical refractive index in the flow field.
Although Schlieren and shadowgraphy techniques are similar, but there are several differences between them as follows:
- Schlieren images are formed by lenses optically, while shadowgraphy images are mere shadow, not focused optical ones.
- In Schlieren technique using a knife-edge in order to cutoff the refracted light is necessary, but there is no need to it in shadowgraphy.
- Illuminance level of Schlieren images depends on the first spatial derivative of the refractive index. However shadowgraphy images illuminance level depends on the second spatial derivative of refractive index. Consequently, Schlieren images show the deflection angle while shadowgraphy images show the displacement of light ray due to deflection.
Schlieren:
