Correlation measurements and structure angles in a turbulent boundary layer recovering from convex curvature

Simultaneous, multiple point turbulence measurements were made in a boundary layer upstream and downstream of a 90° convex bend. Two-point space-time correlation coefficients R_uu(O,ξ,O,τ) were computed for several separation distances ξ. The time delay τ_max corresponding to the maximum value of R_uu was used to define a structure angle θ, where θ = tan^-1 [ξ/(U_cτ_max)], and U_c is the best estimate for the convection velocity. For the undisturbed boundary layer, θ takes low values near the wall, increases to 30°-40° in the central portion of the layer, and increases further near the boundary layer edge. At the exit from the convex curvature, θ is largely unchanged. In the relaxing boundary layer θ appears to increase over most of the boundary layer thickness. The sensitivity of the structure angle to the probe separation distance, and the significance of the results in terms of large-scale organized motions are discussed. The instantaneous structure angle can be defined in terms of short-time correlations, and some of the difficulties associated with this concept are also discussed.