An experimental investigation of two-point correlations in two- and three-dimensional turbulent boundary layers

Two-point correlation measurements of the wall normal fluctuating velocitie s were made in two-dimensional (2-D) and pressure-driven three-dimensional (3-D) turbulent boundary layers. These data are needed for characterization and modeling of active-motion length scales, especially for 3-D flows. The fine-probe-volume data were measured using two custom-designed laser-Doppl er-velocimeter fiber-optic probes. The data are relatively free of noise, s ignal broadening, and bias effects. Favorable comparisons with direct-numerical-simulation (DNS) results in the near-wall region of the 2-D flow validate the experimental techniques used here. For a given fixed probe location, non-dimensional correlation values scale best on the probe separation. For both the 2-D and 3-D cases, peak c orrelations lie along a line inclined away from the wall at 11 degrees and 8 degrees, respectively, which suggests the existence of an outgoing charac teristic line affected by only the upstream flow. The decay of the correlat ion coefficient occurs nearer the wall than away from the wall relative to the fixed probe location. The variations for the 3-D flow correlations are similar to the 2-D variati ons, but with longer Delta x(+) and Delta y(+) decay distances, probably be cause of the 3-D flow acceleration. While the spanwise variation of the cor relation coefficients is symmetric about the fixed point for the 2-D case a s dictated by reciprocity, the 3-D case shows a large asymmetry for spanwis e variations |Delta z(+)| < 68. The profiles at higher |Delta z(+)| are mor e symmetric. In general, at a given y the maximum correlation is skewed to a non-zero Delta z. It appears that the skewing of the correlation coeffici ent in the z direction tracks the sign of w ((3)) over bar.