The growth, breakdown, and transition to turbulence of counter-rotating streamwise vortices, generated via a Görtler instability mechanism, was used to experimentally model the eddy structures found in transitional and turbulent flat-plate boundary layers. The naturally occurring vortices have been studied using smoke-wire visualization and multiple-probe hot-wire rakes. Results show that low-speed regions are formed between the vortices as low-momentum fluid is removed away from the wall. The low-speed regions grow in the normal direction faster than a nominally Blasius boundary layer and create strongly inflexional normal and spanwise profiles of the streamwise velocity component. Instability oscillations develop on these unstable profiles that scale with the local shear-layer thickness and velocity difference. Contrary to expectations however, the spatial scales of the temporal velocity fluctuations correlate better with the velocity gradient in the spanwise direction than with the normal velocity gradient. The nonlinear growth of the oscillations is quite rapid and breakdown into turbulence occurs within a short timescale.

The outer intermittent region of a fully developed turbulent boundary layer with zero pressure gradient was extensively explored in the hope of shedding some light on the shape and motion of the interface separating the turbulent and non-turbulent regions as well as on the nature of the related large-scale eddies within the turbulent regime. Novel measuring techniques were devised, such as conditional sampling and conditional averaging, and others were turned to new uses, such as reorganizing in map form the space-time auto- and cross-correlation data involving both the U and V velocity components as well as I, the intermittency function. On the basis of the new experimental results, a conceptual model for the development of the interface and for the entrainment of new fluid is proposed.

The wall structure of the turbulent boundary layer was examined using hot-wire rakes and conditional sampling techniques. Instantaneous velocity measurements indicate a high degree of coherence over a considerable area in the direction normal to the wall. Aty+= 15, there is some evidence of large-scale correlation in the spanwise direction, but almost no indication of the streamwise streaks that exist in the lower regions of the boundary layer. Conditional sampling showed that the normal velocity is directed outwards in regions of strong stream-wise-momentum deficit, and inwards when the streamwise velocity exceeds its mean value. The conditionally averaged Reynolds shear stress was approximately an order of magnitude greater than its conventionally averaged value and decayed slowly downstream.

Measurements in turbulent channel flow with forced oscillations covering a wide range of frequencies (ω+ = 0.03–0.0005) and amplitudes (10–70% of centreline velocity) are presented and discussed. Phase averages of the velocity <u> across the flow, and of the wall shear stress <τ>, as well as the turbulent fluctuations <u′u′> and <t′t′> are obtained with LDA and hot-film techniques. The time-mean quantities, except u’2, are only slightly affected by the imposed oscillations whatever their frequency and amplitude. It is shown that the appropriate similarity parameter for the oscillating quantities ũ and ĩ is the non-dimensional Stokes length l+s (or the frequency ω+ = 2/l+2s). In the regime of high-frequency forcing (l+s < 10) the oscillating flow ũ and ĩ are governed by purely viscous shear forces although the time-mean flow is fully turbulent. This behaviour may be explained by the physical significance of l+s. At lower frequency l+s 10, the oscillating flow is influenced by the turbulence, in particular the amplitude of ĩ increases with respect to the Stokes amplitude and becomes proportional to l+s. The relative amplitude of <u′u′> and <t′t′> decreases sharply with increasing forcing frequency once l+s < 25. This decay of the turbulence response is faster for the wall shear stress. For forcing frequencies such that l+s > 12, <u′u′> and <t′t′> lag behind <u> and <τ> by respectively about 75 and 130 viscous time units. These lags decrease by a factor 2 at higher forcing frequencies. It is shown that in the log layer, the turbulence modulation diffuses away from the wall with a diffusivity equal to that of the time-mean turbulence. The imposed oscillations are felt down to the small scales of the turbulence as may be evidenced from the cyclic modulation of the Taylor microscale, the skewness and the flatness factors of δu′/δt. The modulations of the skewness and the flatness go through a maximum around l+s = 12.

A fully developed turbulent boundary layer with a zero pressure gradient was explored by using temperature as a passive contaminant in order to study the large-scale structure. The temperature tracer was introduced into the flow field by heating the entire wall to approximately 12°C above the free-stream temperature. The most interesting observation was the existence of a sharp internal temperature front, characterized by a rapid decrease in temperature, that extended throughout the entire boundary layer. In the outer, intermittent region, the internal temperature front was always associated with the upstream side of the turbulent bulges, i.e. the ‘backs’. It extended across the entire logarithmic region and was related to the sharp acceleration associated with the bursting phenomenon near the wall. Conditional averages of the velocities measured with the temperature front revealed that it was associated with an internal shear layer. The results suggest that this shear layer provides a dynamical relationship between the large structures in the outer, intermittent region and the bursting phenomenon near the wall.