The Nature of Boundary-Layer Turbulence at a High Subsonic Speed

Simultaneous measurements of velocity, wall-pressure, and wall-shear fluctuations in a turbulent boundary layer at A/oo =0.64 and Re0 108,000 have been analyzed to obtain a description of coherent (or quasiordered) structures present in the flow. A conditional sampling analysis has been applied to the digitized fluctuations to identify the occurence of such events in the data, and to establish the relationship between the fluctuating quantities during individual and ensemble averaged events. It has been determined that the mean period between readily identifiable coherent structures, when scaled by flow velocity and boundary-layer thickness, falls within the range of values obtained by other investigators for the mean period between bursts in the wall region of low-speed flows. The development of the velocity fluctuation profile during passage of these structures also seems to show definite similarities to the development of burst profiles as depicted by other investigators. The vertical scale of a typical structure is on the order of d* and its duration is on the order of 10 d'/U^. Since our measurements are made outside the wall region of the boundary layer, this flow phenomenon is obviously some aspect of the large-scale outer structure. The correspondence in frequency of occurence and profile development to the wall region bursts, together with the high degree of correlation between the wall shear and the outer structure, may be an indication of a possible link or interaction between the two flow processes. This is being examined further by repeating the measurements at several lower velocities so that the wall region occupies an increasingly larger fraction of the boundary layer.