Seeding arrangement for PIV

One of the most important steps in PIV measurements is seeding of the flow. In order to consider PIV as a non-intrusive technique, it is necessary that the addition of tracer particle does not alter the flow properties. Proper seeding is essential to capture complicated flow details, for example, the recirculation zone. Seeding should be homogeneous (spatially uniform) and sufficient (of high enough density). The injection of tracer particle has to be done without significantly disturbing the flow, but in a way and at a location that ensures homogeneous distribution of the tracers. Particles should be of small diameter so that they follow the original local air velocity without causing any disturbance. The particle density should ideally match that of the fluid to eliminate velocity lag. This issue is adequately taken care of by micron-sized particles for which surface forces are in excess of body forces.

For the present investigation, tracer particles (namely, droplets of corn oil) were added to the main air flow by a number of copper tubes upstream of the honeycomb section. A large number of tiny holes, 0.1 mm diameter were drilled along the length of the copper tubes to make the seeding uniform over the entire test section. The seeding density was adjusted through an air pressure control valve. Laskin nozzles were used to produce oil droplets as tracers. For the range of frequencies in the wake, an expected slip velocity error of 0:3% to 0:5% relative to the instantaneous local velocity is expected in the present study (Adrian, 1991).

Laskin nozzles are widely used as atomizers of non-volatile liquids due to simplicity of design and the resulting uniform particle size distribution. The picture of the Laskin nozzle seed generator has been shown in Figure 3.12. A detailed schematic drawing of the Laskin nozzle seed generator is shown in Figure 3.13. The particles should be small in size, spherical in shape, of appropriate density and refractive index, and non-volatile. Above all, the liquid should be non-toxic and of low cost. The particles should be efficient scatterer of the illuminating laser light. This largely decides the illuminating laser type and the recording hardware i.e. camera. For example, if a given particle scatters weakly, then one would have to employ more powerful lasers or a more sensitive camera, both of which can drive up costs, as well as the associated safety issues. Corn oil was used for the present work, in view of its high surface tension required for producing small particles along with favorable light scattering properties.