Measurements of the structure of the Reynolds stress in a turbulent boundary layer

Abstract

Additional experimental studies of the structure of Reynolds stress which supplement our previous work (Willmarth & Lu 1971) are reported. The velocity at the edge of the viscous sublayer is again used as a detector signal for bursts and sweeps. The signal uv obtained from an X-wire probe at various locations is conditionally sampled and sorted into four quadrants of the u, v plane. Using this method it is found that, when the velocity u w at the edge of the viscous sublayer becomes low and decreasing, a burst occurs. On the other hand, a sweep occurs when u w becomes large and increasing. The convection speeds of the bursts and the sweeps are found to be equal and are about 0·8 times the local mean velocity and 0·425 times the free-stream velocity at a distance y ≈ 0·15δ* from the wall (δ* is the displacement thickness). Throughout the turbulent boundary layer, the bursts are the largest contributors to $\overline{uv}$ with the sweeps the second largest. On average, the bursts account for 77% of $\overline{uv} $ , while the sweeps provide 55%; the excess percentage over 100% is due to the other small negative contributions. Characteristic mean time intervals are obtained for both bursts and sweeps from certain unique features of the measurements of fractional contributions to $\overline{uv}$ from different events. Both mean time intervals are approximately equal and constant for most of the turbulent boundary layer. The scaling of the mean time interval between bursts with outer flow variables is confirmed. It is suggested that many of the features of the fluctuating flow revealed by the measurements may be explained by convection past the measuring station of an evolving deterministic flow pattern such as the hairpin vorticity model of Willmarth & Tu (1967).