Chapter 11
Non-intrusive Measurement Techniques
11.1 Basic Principle of Non-intrusive Techniques
Non-intrusive measurement techniques are based on optical techniques and have
tremendous advantages, but as all measurement techniques they have their limitations
and drawbacks. Aside the arduous technical issues associated with data processing
and analysis, their main physical limitation is simply based on visual access of the
experimental domain of interest and also the ability to guide the light/laser source
for sufficient illumination. This depends a lot on the experimental facility and sometimes the working section has to be redesigned and equipped with sufficiently large
windows with high optical quality glass or Perspex which is a strict requirement for
classical interferometry techniques. For applications involving the use of invisible
lighting, infra-red for instance, the windows have to be made of special material
such quartz and zinc selenide (revisited in Sect. 9.5.3). This issue can be addressed
by placing the instrumentations within the working section itself, if possible or it is
easier done in experiment in open atmosphere.
The focal length of lenses needed for some optical techniques could restrict their
use in some facilities with large working sections. Also, in high speed testing facilities
mechanical vibrations and high intensity sound could damage the fragile optical
components which tend to be quite expensive. During optical diagnosis, even if
this sounds trivial, a high-quality window is critical to compensate for losses in
illumination due to the development of wall boundary layer over the window, tracer
particles seeded in the flow but more importantly optical aberration from the window
itself.
Non-intrusive methods for investigation could be classified in three categories:
– techniques based on the change in the optical property of the fluid such as the
refractive index due to a density or temperature of the flow, see Sect. 11.2;
– techniques based on tracking particles, either those present naturally in the fluid or
seeded artificially, assuming that these particles are being convected at the same
speed of the flow, these techniques will be presented in Sects. 11.3 and 11.4;
© Springer Nature Switzerland AG 2020
B. Chanetz et al., Experimental Aerodynamics,
Springer Tracts in Mechanical Engineering,
https://doi.org/10.1007/978-3-030-35562-3_11
237
Non-intrusive Measurement Techniques
11.1 Basic Principle of Non-intrusive Techniques
Non-intrusive measurement techniques are based on optical techniques and have
tremendous advantages, but as all measurement techniques they have their limitations
and drawbacks. Aside the arduous technical issues associated with data processing
and analysis, their main physical limitation is simply based on visual access of the
experimental domain of interest and also the ability to guide the light/laser source
for sufficient illumination. This depends a lot on the experimental facility and sometimes the working section has to be redesigned and equipped with sufficiently large
windows with high optical quality glass or Perspex which is a strict requirement for
classical interferometry techniques. For applications involving the use of invisible
lighting, infra-red for instance, the windows have to be made of special material
such quartz and zinc selenide (revisited in Sect. 9.5.3). This issue can be addressed
by placing the instrumentations within the working section itself, if possible or it is
easier done in experiment in open atmosphere.
The focal length of lenses needed for some optical techniques could restrict their
use in some facilities with large working sections. Also, in high speed testing facilities
mechanical vibrations and high intensity sound could damage the fragile optical
components which tend to be quite expensive. During optical diagnosis, even if
this sounds trivial, a high-quality window is critical to compensate for losses in
illumination due to the development of wall boundary layer over the window, tracer
particles seeded in the flow but more importantly optical aberration from the window
itself.
Non-intrusive methods for investigation could be classified in three categories:
– techniques based on the change in the optical property of the fluid such as the
refractive index due to a density or temperature of the flow, see Sect. 11.2;
– techniques based on tracking particles, either those present naturally in the fluid or
seeded artificially, assuming that these particles are being convected at the same
speed of the flow, these techniques will be presented in Sects. 11.3 and 11.4;
© Springer Nature Switzerland AG 2020
B. Chanetz et al., Experimental Aerodynamics,
Springer Tracts in Mechanical Engineering,
https://doi.org/10.1007/978-3-030-35562-3_11
237