Issues with measurements of the convective velocity of large-scale structures in the compressible shear layer of a free jet

Experiments were conducted to measure convective velocity of large-scale structures in the shear layer of a Mach 2.0 free jet with a convective Mach number of 0.87 and Reynolds number based on a nozzle diameter of 2.6x10(6). Real-time flow visualization and planar Doppler velocimetry (PDV) measurements along with space-time correlation were used. The results reveal that regardless of the measurement technique employed, whether qualitative flow visualization or quantitative PDV, the method of seeding can have a significant effect on the ability to accurately identify, track, and measure convective velocity of large-scale structures. This is apparent by the relatively large spread in convective velocity results obtained in the same jet by using various methods of seeding and qualitative and quantitative visualization. Convective velocity measurements made under the best seeding conditions available with PDV indicate a convective velocity of structures that is close to the value predicted by theory up to the convective Mach number of 0.87 used in the current work. This is in contrast to previously reported convective velocity measurements (including those of the present authors) that indicated a deviation from the theoretically predicted value but were based on measurements made under less than ideal seeding conditions. (C) 2008 American Institute of Physics.