248
11 Non-intrusive Measurement Techniques
Fig. 11.9 Two pairs of laser beams in an LDV set-up for two components measurements
(© ONERA)
produced by passage of different particles. This ensures that all 3-components are
related to the same particle and resolved simultaneously in time which is imperative
for determining velocity fluctuations and the turbulence quantities.
11.4.5 Flow Seeding
The confidence level in the LDV measurement is usually set by the validation routine
which relies on the quality and quantity of particles seeded or entrained in the flow.
The choice of seeding particles is dictated by several factors:
– Size: Smaller particles tend to follow the path of the streamline more precisely
however the intensity of light scattered is lower, hence weaker signal.
– Appropriately sized particles can bond to each other to form larger particles which
affect the accuracy of measurement as they deviate from the actual streamline and
are more problematic in rapidly compressing or expanding flows.
– For propulsion studies which involve heated jets, the particles need to withstand
high temperatures without auto-igniting or vaporising.
– Some products used for seeding particles are not compatible with PSP and TSP
– Concerning, health and environmental impact, some particles can have harmful
health and environmental effects. Therefore, one must ensure that the tunnel is
11 Non-intrusive Measurement Techniques
Fig. 11.9 Two pairs of laser beams in an LDV set-up for two components measurements
(© ONERA)
produced by passage of different particles. This ensures that all 3-components are
related to the same particle and resolved simultaneously in time which is imperative
for determining velocity fluctuations and the turbulence quantities.
11.4.5 Flow Seeding
The confidence level in the LDV measurement is usually set by the validation routine
which relies on the quality and quantity of particles seeded or entrained in the flow.
The choice of seeding particles is dictated by several factors:
– Size: Smaller particles tend to follow the path of the streamline more precisely
however the intensity of light scattered is lower, hence weaker signal.
– Appropriately sized particles can bond to each other to form larger particles which
affect the accuracy of measurement as they deviate from the actual streamline and
are more problematic in rapidly compressing or expanding flows.
– For propulsion studies which involve heated jets, the particles need to withstand
high temperatures without auto-igniting or vaporising.
– Some products used for seeding particles are not compatible with PSP and TSP
– Concerning, health and environmental impact, some particles can have harmful
health and environmental effects. Therefore, one must ensure that the tunnel is