WALL-PRESSURE AND COHERENT STRUCTURES IN A TURBULENT BOUNDARY-LAYER ON A CYLINDER IN AXIAL-FLOW

Measurements of wall pressure and streamwise velocity fluctuations in a turbulent boundary layer on a cylinder in an axial air flow (delta/a = 5.04, Re-theta = 2870) have been used to investigate the turbulent flow structures in the cylindrical boundary layer that contribute to t he fluctuating pressure at the wall in an effort to deduce the effect of transverse curvature on the structure of boundary layer turbulence. Wall pressure was measured at a single location with a subminiature e lectret condenser microphone, and the velocity was measured throughout a large volume of the boundary layer with a hot-wire probe. Auto- and cross-spectral densities, cross-correlations, and conditional samplin g of the pressure and streamwise velocity indicate that two primary gr oups of flow disturbances contribute to the fluctuating pressure at th e wall: (i) low-frequency large-scale structures with dynamical signif icance across the entire boundary layer that are consistent with a pai r of large-scale spanwise-oriented counter-rotating vortices and (ii) higher frequency small-scale disturbances concentrated close to the wa ll that are associated with the burst-sweep cycle and are responsible for the short-duration large-amplitude wall pressure fluctuations. A b idirectional relationship was found to exist between both positive and negative pressure peaks and the temporal derivative of u near the wal l. Because the frequency of the large-scale disturbance observed acros s the boundary layer is consistent with the bursting frequency deduced from the average time between bursts, the burst-sweep cycle appears t o be linked to the outer motion. A stretching of the large-scale struc tures very near the wall, as suggested by space-time correlation conve ction velocity results, may provide the coupling mechanism. Since the high-frequency disturbance observed near the wall is consistent with t he characteristic frequency deduced from the average duration of burst ing events, the bursting process provides the two characteristic time scales responsible for the bimodal distribution of energy near the wal l. Because many of the observed structural features of the cylindrical boundary layer are similar to those observed in flat-plate turbulent boundary layers, transverse curvature appears to have little effect on the fundamental turbulent structure of the boundary layer for the mod erate transverse curvature ratio used in this investigation. From diff erences that exist between the turbulence intensity, skewness, and spe ctra of the streamwise velocity, however, it appears that transverse c urvature may enhance (i.e. energize) the large-scale motion owing to t he reduced constraint imposed on the flow by the smaller cylindrical w all.