BSA Flow Software Installation and User's Guide - CSI

7.1.8 Seeding
Seeding as flow field tracers
In LDA it is not actually the velocity of the flow that is measured, but the
velocity of particles suspended in the flow. In this respect these seeding
particles can be considered to be the actual velocity probes, and seeding
considerations are thus important in LDA.
The particles must be small enough to track the flow accurately, yet large
enough to scatter sufficient light for the photodetector to be able to detect the
Doppler frequency. Ideally, the particles should also be neutrally buoyant in
the fluid, that is they should have approximately the same density as the
fluid itself, but in many experiments this is a secondary consideration.
Durst, Melling & Whitelaw (1981) state the following desired properties of
seeding particles:
Particles whose motion is used to represent that of a fluid should be:
• Able to follow the flow.
• Good light scatterers.
• Conveniently generated.
• Cheap.
• Non-toxic, non-corrosive, non-abrasive.
• Non-volatile, or slow to evaporate.
• Chemically inactive.
• Clean.
In general the motion of particles suspended in a fluid is affected by
• Particle shape.
• Particle size.
• Relative density of particle and fluid.
• Concentration of particles in the fluid.
• Body forces.
The shape of the seeding particles affect the drag exerted on the particle by
the surrounding fluid, and the size of the particles along with their relative
density influence their response to velocity changes of the surrounding fluid.
The concentration of particles affect particle motion through interaction
between different particles. In practice the concentrations used are normally
so low, that particle interaction can be neglected.
Also body forces, such as gravity, can normally be ignored, except in very
slow flows, where buoyancy of the seeding particles may be an issue. Also
in experiments including for example electrostatic fields, body forces may be
of importance, but in such cases, they will probably be part of the
experiment, and as such they can not really be considered a disturbance.
Particle motion Since the analysis of particle motion is rather complicated even for spherical
particles, and “real” particles can’t be modelled properly anyway, only
spherical particles in an infinite fluid have been analysed. It is assumed, that
the results apply qualitatively also for particles of more irregular shape. This
assumption is good for liquid particles and fair for monodisperse solid
particles, but poor for other solid particles, such as agglomerates.
Basset derived the equation of motion for a sphere relative to an infinite,
stagnant fluid in 1888, and in 1959 Hinze expanded this to a moving fluid,
BSA Flow Software:Reference guide 7-17