242
11 Non-intrusive Measurement Techniques
Holographic interferometry is easier to set-up than the classical interferometry,
due to the extended distance over which a laser source preserves its coherence. Thus,
the optical path length of the reference and the transmitted beam does not have to
be adjusted very accurately as opposed to a white light source. Besides, a good
quality glass will be sufficient for the test section windows and the lens and other
instrumentations are more affordable.
Interferometry is a powerful tool for studying very complex compressible flow
fields which are easily perturbed by solid probes. It’s an accurate technique which
can provide a considerable amount of information over a large experimental domain
or field of view. However, its extent of application is reduced in the case of separated
flow where the density is almost constant and it is impossible to make any measurements of turbulence. In practice, interferometry is limited to two-dimensional planar
or axisymmetric flow. The extension to three-dimensional flow is possible through
tomographic imaging, but the set-up, operational mode and process of image reconstruction, is so challenging that the technique has been applied only to very few rare
cases.
11.3 Mechanism of Light Scattering
Light scattering is the property of particles that allows the reflection of a light wave
in all directions. The scattering takes place without the loss of energy therefore the
wavelength remains constant. The type of scattering depends on the form of the
particles and their size, to distinguish between:
– Rayleigh scattering is the elastic scattering which occurs when the diameter of
an assumed spherical particle is smaller than the wavelength of the incident wave,
typically a tenth of a nanometre smaller. This scattering or reflection takes place
without losses and isotropically in all directions.
– Mie scattering is the elastic scattering occurring when the diameter of the particle
is larger than the wavelength of the incident wave, of the orders of micrometre. The
reflection of the light is not isotropic and this depends on the form of the particles
being spherical or cylindrical and their size. The intensity of the scattered waves
depends a lot on the angle of scatter as shown in the Mie-scattering plot in Fig. 11.4.
– Raman scattering is the inelastic counterpart of Rayleigh scattering, where the
energy is not constant. The difference in energy between a photon absorbing energy
and emitting energy is equal to the difference between two different energy states
of scattering object. The use of Raman scattering in aerodynamics is presented
Chap. 12.
11 Non-intrusive Measurement Techniques
Holographic interferometry is easier to set-up than the classical interferometry,
due to the extended distance over which a laser source preserves its coherence. Thus,
the optical path length of the reference and the transmitted beam does not have to
be adjusted very accurately as opposed to a white light source. Besides, a good
quality glass will be sufficient for the test section windows and the lens and other
instrumentations are more affordable.
Interferometry is a powerful tool for studying very complex compressible flow
fields which are easily perturbed by solid probes. It’s an accurate technique which
can provide a considerable amount of information over a large experimental domain
or field of view. However, its extent of application is reduced in the case of separated
flow where the density is almost constant and it is impossible to make any measurements of turbulence. In practice, interferometry is limited to two-dimensional planar
or axisymmetric flow. The extension to three-dimensional flow is possible through
tomographic imaging, but the set-up, operational mode and process of image reconstruction, is so challenging that the technique has been applied only to very few rare
cases.
11.3 Mechanism of Light Scattering
Light scattering is the property of particles that allows the reflection of a light wave
in all directions. The scattering takes place without the loss of energy therefore the
wavelength remains constant. The type of scattering depends on the form of the
particles and their size, to distinguish between:
– Rayleigh scattering is the elastic scattering which occurs when the diameter of
an assumed spherical particle is smaller than the wavelength of the incident wave,
typically a tenth of a nanometre smaller. This scattering or reflection takes place
without losses and isotropically in all directions.
– Mie scattering is the elastic scattering occurring when the diameter of the particle
is larger than the wavelength of the incident wave, of the orders of micrometre. The
reflection of the light is not isotropic and this depends on the form of the particles
being spherical or cylindrical and their size. The intensity of the scattered waves
depends a lot on the angle of scatter as shown in the Mie-scattering plot in Fig. 11.4.
– Raman scattering is the inelastic counterpart of Rayleigh scattering, where the
energy is not constant. The difference in energy between a photon absorbing energy
and emitting energy is equal to the difference between two different energy states
of scattering object. The use of Raman scattering in aerodynamics is presented
Chap. 12.