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7 Flow Visualisation Techniques
7.6 Short Exposure Time Visualisation
The transient and unsteady nature of many aerodynamic phenomena (inlet starting of
a supersonic nozzle, flying projectile, explosion, turbulence, propagation of waves,
etc.) has led to the development of visualisation methods in which the phenomenon is
frozen. From the beginning of their development, shadowgraph, Schlieren technique
and differential interferometry were used in the case of high-speed flows with the
main difference being the technique of capturing the image on a sensor. In those
days, the tests were performed in total darkness using spark system to generate a
bright spark with a very short exposure time (a few ns), which freezes the flow
on a photographic plate with almost no blurring. Then, spark systems were used in
series and a high-speed drum camera with the photographic film attached to the drum
which allowed capturing several images during a test. Typically, in hypersonic flow,
8 images were recorded for 3 ms of flow. This recording technique was abandoned in
the 2000s with the advent of high-speed digital cameras. The most efficient models
allow an aperture opening time of less than 300 ns, which is comparable to the
exposure time of a spark gap. The acquisition rate can be up to 25,000 frames per
second for a resolution of 1280 by 800 pixels.
Methods such as PIV, based on the particle displacement, also require an almost
instantaneous image obtained by illuminating the field with a pulsed laser (see
Sect. 11.6).
7.7 Visualisation by Induced Light Emission
These techniques, mainly used in low-density hypersonic flows, are based on
the emission of light by the atoms of the gas under the effect of ionisation (see
Chap. 12).
7.7.1 Glow Discharge Method
The gas is ionised by an intense electric field, which produces a plasma under the
effect of the excitation of the nitrogen molecules. This results in a light emission
whose intensity is a function of the local density. The process, which only works
in very low density flows, highlights shock waves, boundary layers, mixing zones,
7 Flow Visualisation Techniques
7.6 Short Exposure Time Visualisation
The transient and unsteady nature of many aerodynamic phenomena (inlet starting of
a supersonic nozzle, flying projectile, explosion, turbulence, propagation of waves,
etc.) has led to the development of visualisation methods in which the phenomenon is
frozen. From the beginning of their development, shadowgraph, Schlieren technique
and differential interferometry were used in the case of high-speed flows with the
main difference being the technique of capturing the image on a sensor. In those
days, the tests were performed in total darkness using spark system to generate a
bright spark with a very short exposure time (a few ns), which freezes the flow
on a photographic plate with almost no blurring. Then, spark systems were used in
series and a high-speed drum camera with the photographic film attached to the drum
which allowed capturing several images during a test. Typically, in hypersonic flow,
8 images were recorded for 3 ms of flow. This recording technique was abandoned in
the 2000s with the advent of high-speed digital cameras. The most efficient models
allow an aperture opening time of less than 300 ns, which is comparable to the
exposure time of a spark gap. The acquisition rate can be up to 25,000 frames per
second for a resolution of 1280 by 800 pixels.
Methods such as PIV, based on the particle displacement, also require an almost
instantaneous image obtained by illuminating the field with a pulsed laser (see
Sect. 11.6).
7.7 Visualisation by Induced Light Emission
These techniques, mainly used in low-density hypersonic flows, are based on
the emission of light by the atoms of the gas under the effect of ionisation (see
Chap. 12).
7.7.1 Glow Discharge Method
The gas is ionised by an intense electric field, which produces a plasma under the
effect of the excitation of the nitrogen molecules. This results in a light emission
whose intensity is a function of the local density. The process, which only works
in very low density flows, highlights shock waves, boundary layers, mixing zones,