Generation of Particles and Seeding

One of the most important elements in laser velocimetry yet the most neglected, is the small particle embedded in the flow field that scatters the light necessary to make velocity measurements. The characteristics of this lowly particle are often ignored in the effort to obtain data. This seems strange since it is the primary cause of measurement error. If the particle is too large, it will not follow the flow resulting in an inaccurate representation of the fluid velocity. If the particle is too small, it will not scatter sufficient light to provide the signal-to-noise necessary to minimize measurement uncertainty in the signal processing electronics. When the researcher finally gets around to deciding on the material to be used for the seed particles and the method for their generation, he is faced with a myriad of possibilities. Asking the advice of others leads to frustration since, like politics and religion, everyone has their own belief in the best method---And don''t confuse them with facts stating otherwise. This lecture will attempt to remove the confusion in choosing a seeding method by assessing many of the techniques currently used. It will outline their characteristics and typical limitations imposed by various applications. The lecture will then focus on the ramifications of these methods on measurement accuracy.