The structure and behavior of the bimodal flow of the horseshoe vortex at the nose of a wing-body junction flow was studied. The wing consists of a 3:2 elliptic nose and a NACA 0020 tail joined at the maximum thickness (t). Measurements were performed with an approach flow conditions of U_ref = 27.5 m/s, Re_θ = 6700 at x/t=-2.15, and δ/t=0.5.

A rapidly-scanning two-velocity-component laser-Doppler anemometer system was developed for use in investigating this flow. U and V velocity components were measured simultaneously with surface pressure measurements at the location of the most bimodal pressure histogram (x/t=-0.26).

Mean (U, V) and rms (u’, v’) velocity components were obtained at four x locations, x/t= -0.15, -0.20, -0.25, -0.30, and show the same flow features measured in previous studies at this facility.

Cross-correlations between the velocity and the surface pressure fluctuations were obtained. Large correlations were found between the u fluctuations (x/t= -0.15, -0.25, and -0.30) near the wall, y/t < 0.05, and the surface pressure fluctuations. The z fluctuations for y/t > 0.1 at all four x-locations lead the surface pressure fluctuations.

Space-time correlations between the velocity fluctuations near the wall with the velocity fluctuations along the scan were also obtained. The correlations at x/t=-0.25 and x/t=-0.30 show that the fluctuations in the outer region, y/t > 0.1, are significantly correlated with and lead the velocity fluctuations near the wall.

These measurements support a model of a single primary junction vortex that changes size and location in front of the wing. The strength or circulation of this vortex varies by only 20%. Event-threshold conditional-averages of velocity were obtained based on the surface pressure signal, which is sensitive to the movement of the junction vortex. These show that the junction vortex is concentrated near the nose, with large backflow, when the surface pressure signal is above the mean. The junction vortex is larger, with smaller backflow near the nose, when the surface pressure signal is below the mean.

The velocity-pressure cross-correlations and space-time correlations indicate that the behavior of the junction vortex is influenced by fluctuations originating upstream and propagating inward and downward toward the wing.