238
11 Non-intrusive Measurement Techniques
– spectroscopic techniques based on the physical process during the intermolecular
interaction between the illumination source and the fluid or the material used for
the surface of the model; this will be covered in Chap. 12.
11.2 Interferometry
11.2.1 Light Interference and Refractive Index
The first category of techniques listed above is based on the deviation of the path
of light due to the change in refractive index in certain region of a flow where the
velocity changes rapidly. In a gas or air as in most aerodynamics applications the
refractive index, n, and the density, ρ, are related by Gladstone and Dale’s relation
given as:
n = Bρ + A
where A and B are constants. Therefore, the variation in the optical property of the
flow could be detected using a technique capable of sensing the change in density
or vice versa. Based on this relation the region with variation in density, for instance
in the presence of a shock or expansion wave, mixing zones and boundary layers,
can be visualised and captured. This technique is an extension to the visualisation
technique presented in Sect. 7.5.
Interferometry has historically been used to visualise compressible flows where
spectacular images of the density field can be captured (see Sect. 7.5.3) and provide
quantitative information which is very useful to study transonic and supersonic flows.
The basic principle of interferometry is to capture the interference of two waves
emitted from the same monochromatic, coherent source on a sensitive screen or
nowadays straight onto a CCD sensor:
– the wave passing the working section, through the flow in concern,
– the wave passing outside the working section, through the unperturbed flow.
Both waves interacts on the optical sensor to produce an interference fringes
pattern which are lines of equal phase along the optical path length, nl, the product
of the refractive index, n, and the geometrical length of light beam, l. If the flow is
uniform, with the interferometry system set-up properly, the optical sensor will be
covered by a white fringe which corresponds to the constant phase. But once nonuniformity is introduced in the flow the density variation will result in a change in
refractive and hence a deviation in the optical path and phase shift of the light wave.
The shifted and unshifted beams interact to add or subtract energy to each other and
this produces interference fringe patterns or the interferogram of the flow.
For a two-dimensional planar flow, in a working section with a width, b, the change
in the length of the optical path introduced by the flow at all points is given as:
11 Non-intrusive Measurement Techniques
– spectroscopic techniques based on the physical process during the intermolecular
interaction between the illumination source and the fluid or the material used for
the surface of the model; this will be covered in Chap. 12.
11.2 Interferometry
11.2.1 Light Interference and Refractive Index
The first category of techniques listed above is based on the deviation of the path
of light due to the change in refractive index in certain region of a flow where the
velocity changes rapidly. In a gas or air as in most aerodynamics applications the
refractive index, n, and the density, ρ, are related by Gladstone and Dale’s relation
given as:
n = Bρ + A
where A and B are constants. Therefore, the variation in the optical property of the
flow could be detected using a technique capable of sensing the change in density
or vice versa. Based on this relation the region with variation in density, for instance
in the presence of a shock or expansion wave, mixing zones and boundary layers,
can be visualised and captured. This technique is an extension to the visualisation
technique presented in Sect. 7.5.
Interferometry has historically been used to visualise compressible flows where
spectacular images of the density field can be captured (see Sect. 7.5.3) and provide
quantitative information which is very useful to study transonic and supersonic flows.
The basic principle of interferometry is to capture the interference of two waves
emitted from the same monochromatic, coherent source on a sensitive screen or
nowadays straight onto a CCD sensor:
– the wave passing the working section, through the flow in concern,
– the wave passing outside the working section, through the unperturbed flow.
Both waves interacts on the optical sensor to produce an interference fringes
pattern which are lines of equal phase along the optical path length, nl, the product
of the refractive index, n, and the geometrical length of light beam, l. If the flow is
uniform, with the interferometry system set-up properly, the optical sensor will be
covered by a white fringe which corresponds to the constant phase. But once nonuniformity is introduced in the flow the density variation will result in a change in
refractive and hence a deviation in the optical path and phase shift of the light wave.
The shifted and unshifted beams interact to add or subtract energy to each other and
this produces interference fringe patterns or the interferogram of the flow.
For a two-dimensional planar flow, in a working section with a width, b, the change
in the length of the optical path introduced by the flow at all points is given as: