The technique of simultaneous hot-wire anemometry and flow visualization has been used to determine the average characteristics of two important scales of motion in the outer region of turbulent boundary layers: large scale motions (average length 1.6δ), and ’’typical eddy’’ motions (average streamwise length approximately 200 ν/uτ). Results showed that the Reynolds number dependent ’’typical eddies’’ produced most of the Reynolds stress in the outer half of the layer at Rϑ≈1200, and that they are formed on the upstream side of large scale motions at all Reynolds numbers investigated. This phase relationship explains the scaling of the frequency of occurrence of outer layer bursts (which are identified with ’’typical eddies’’) on the free stream velocity and overall boundary layer thickness, although it is found that the lengths of the ’’typical eddies’’ scale on inner layer variables. In the log region, roughly one-half of the large scale motions sampled had zone-averaged streamwise velocity defects. Ensemble averaged results showed that they were associated with significant Reynolds stress contributions. A structural model showing the phase relationship of ’’typical eddies’’ and large scale motions is presented.

Spanwise vorticity measurements have been performed in zero-pressure-gradient boundary layers over the range 1010 < Rθ < 4850 (Rθ ≡ U∞ θ/ν, where U∞ is the free-stream velocity and θ is the momentum deficit thickness) using a four-wire probe. In addition, experiments quantifying the spatial and temporal resolution required to obtain an accurate statistical representation of the small-scale structure of wall-bounded turbulence were performed. Furthermore, a thorough investigation of statistical convergence for a variety of fluctuating quantities was performed. Comparisons with earlier high-resolution studies indicate that the maximum value of u′/uτ increases with increasing Reynolds number over the given Rθ range (u′ ≡ r.m.s. u, and uτ is the friction velocity). It is suggested that detecting this dependence provides a good measure of probe resolution. In general it was found that statistics of velocity gradients were distinctly more sensitive to finite probe size than velocity statistics. Wire spacing experiments suggest that Wyngaard's (1969) criterion is to a good approximation valid even under anisotropic conditions. Furthermore, it was found that instantaneously spatial averaging of ∂u/∂t caused significant attenuation in the resulting r.m.s., and that this averaging procedure is sensitive to the level of mean shear. A simple method of estimating how noise in the u-velocity signals enters into the ∂u/∂y signals is presented. The convergence study shows that statistical convergence criteria developed from free-shear flows severely underestimates the averaging times required in boundary layers. A table of general convergence criteria is provided.

Four-wire hot-wire probe measurements are used to examine the relationships between the motions bearing spanwise vorticity, ωz, and the mechanisms responsible for turbulent stress transport. The measurements were carried out in thick zero pressure gradient turbulent boundary layers over the Reynolds number range, 1010&lt;Rθ&lt;4850. Because of the scale of the flow, the spatial resolution of the probe was, in general, very good. The capabilities of the probe allowed all of the terms in the balance equation for 〈u2〉, except the pressure strain correlation, to be directly measured. Regarding wall region vorticity dynamics, velocity–vorticity correlation measurements are used to investigate ωz motion contributions to the gradients of the turbulent stresses. In particular, the measured positive correlation between v and ωz for y+&lt;15 is shown to be consistent with the large ∂〈−uv〉/∂y gradient in this region, and physically, with the outward motion of sublayer streaks. Relationships between the motions bearing ωz and the sweep and ejection bursting events are examined, and the results are discussed in connection with the turbulent diffusion term in the budget for 〈u2〉. The present results indicate that in the near-wall region (but outside the sublayer) approximately two-thirds of the contributions to the turbulent diffusion term come from the correlation between the uv shear product and ωz fluctuations. The overall results are discussed relative to coherent motion interactions and the known properties of the inner region vorticity field.

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