Two-Color Planar Doppler Velocimetry

Abstract

Anovel two-colorapproachto planarDopplervelocimetry (PDV)isdemonstratedin a supersonice ow. Thetechnique was implemented using a frequency-doubled Nd:YAG laser and a Nd:YAG-pumped dye laser (¸ =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 » 11%) in a compressible freejet e ow. The two-color approach enjoys an inherent advantage over single-color systems in that both the e ltered and reference e ow images are captured with a single camera. The need to split the scattering (and direct to two cameras ) is eliminated, and the e elds of view of the e ltered and reference images are naturally aligned. Thus, the dife culty of experimental setup, which is an impediment to the wide application of PDV, is signie cantly 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 thetwo lasers on a shot-by-shot basis. Furthermore, asseed particlesizeincreasesbeyond theRayleigh limit(where dRayleigh <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.