An Experimental Investigation of Two-Point Correlations in Two- and Three-Dimensional Turbulent Boundary Layers

Abstract

Two-point correlation measurements of the wall normal fluctuating velocities 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-Doppler-velocimeter fiber-optic probes. The data are relatively free of noise, signal 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 correlations lie along a line inclined away from the wall at 11° and 8°, respectively, which suggests the existence of an outgoing characteristic line affected by only the upstream flow. The decay of the correlation 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 variations, but with longer Δ x ⁺and Δ y ⁺ decay distances, probably because of the 3-D flowacceleration. While the spanwise variation of the correlationcoefficients is symmetric about the fixed point for the 2-D case asdictated by reciprocity, the 3-D case shows a large asymmetry for spanwise variations Δ z ⁺ < 68. The profiles at higher Δ z ⁺ are more symmetric. In general, at a given y the maximum correlation is skewed toa non-zero Δ z. It appears that the skewing of the correlation coefficient in the z direction tracks the sign of $\overline {w3 }$.