Two-color planar Doppler velocimetry

A novel two-color approach to planar Doppler velocimetry (PDV) is demonstra ted in a supersonic how The technique was implemented using a frequency-dou bled Nd:YAG laser and a Nd:YAG-pumped dye laser (lambda = 618 mn) for illum ination and a color charge-coupled device (CCD) camera for detection. In te sting two-color PDV, we obtain good agreement between experimental and theo retical velocities (difference of similar to 11%) in a compressible freejet how The two-color approach enjoys an inherent advantage over single-color systems in that both the filtered and reference dow images are captured wit h a single camera The need to split the scattering (and direct to two camer as) is eliminated, and the fields of view of the filtered and reference ima ges are naturally aligned. Thus, the difficulty of experimental setup, whic h is an impediment to the wide application of PDV, is significantly reduced . Furthermore, the needed equipment, at least for one velocity component, w as essentially that employed for particle-imaging velocimetry (PIV). Howeve r, as a tradeoff one must characterize the irradiance distributions of the two laser sheets, as well as carefully overlap the tao sheets within the pr obe region, and record the beam energies for the two lasers on a shot-by sh ot basis. Furthermore, as seed particle size increases beyond the Rayleigh limit (where d(Rayleigh) < 100 nm for visible wavelengths) the ratio of red and green scattering signals will depend on particle size. Nonetheless, th e ease with which the two-color PDV technique can be applied, and its compl ementary nature with equipment needed for Pn! makes it attractive. The stre ngths and weaknesses of our particular approach are discussed, namely CCD c olor bleed effects, as are potential alternatives.