Forebody and leading edge vortex measurements using planar doppler velocimetry

The planar Doppler velocimetry (PDV) technique has been demonstrated by employing it in a large-scale wind tunnel to record velocity fields surrounding a model of a generic fighter plane. The PDV instrument employed here included the following: (i) a frequency monitoring system for measuring the laser frequency corresponding to each set of scattering images; (ii) two detector systems (each composed of two 16-bit CCD cameras), one viewing the model from the top of the wind tunnel and the second from the side; (iii) iodine vapour cells based on the straved-cell design, which eliminated the need for separate temperature control of the iodine reservoir; iv) a vibration-isolated, injection-seeded, Q-switched Nd: YAG laser and (v) custom data acquisition software for linking the four cameras, the laser and the frequency monitor. The PDV instrument was validated by comparing the PDV-derived velocity to the known value in the empty wind tunnel. An error of about 1 m s^-1 out of an 18.9 m s^-1 velocity component was found; the image noise component (resulting primarily from the speckle effect) was found to be about 1 m s^-1. In addition, as a result of laser-sheet impingement on the model surface, velocities near the model surfaces are biased by background scattering effects. Nonetheless, it has been shown that PDV can be used effectively to map velocity fields with high spatial resolution over complex model geometries. Frame-averaged velocity images recorded at four axial stations along the model have shown the formation of forebody and leading-edge vortices and their complex interaction in the presence of the wing flow field.