Two-color planar Doppler velocimetry

A novel two-color approach to planar Doppler velocimetry (PDV) is demonstrated in a supersonic flow. The technique was implemented using a frequency-doubled Nd:YAG laser and a Nd:YAG-pumped dye laser (λ = 618 nm) for illumination and a color charge-coupled device (CCD) camera for detection. In testing two-color PDV, we obtain good agreement between experimental and theoretical velocities (difference of approximately 11%) in a compressible freejet flow. The two-color approach enjoys an inherent advantage over single-color systems in that both the filtered and reference flow images are captured with a single camera. The need to split the scattering (and direct to two cameras) is eliminated, and the fields of view of the filtered and reference images are naturally aligned. Thus, the difficulty of experimental setup, which is an impediment to the wide application of PDV, is significantly reduced. Furthermore, the needed equipment, at least for one velocity component, was essentially that employed for particle-imaging velocimetry (PIV). However, as a tradeoff one must characterize the irradiance distributions of the two laser sheets, as well as carefully overlap the two sheets within the probe region, and record the beam energies for the two lasers on a shot-by-shot 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, the ease with which the two-color PDV technique can be applied, and its complementary nature with equipment needed for PIV, makes it attractive. The strengths and weaknesses of our particular approach are discussed, namely CCD color bleed effects, as are potential alternatives.