Following a review of the difficulties associated with the measurement and interpretation of statistics of the small-scale motion, the evidence for and against local isotropy is assessed in the light of measurements in a turbulent plane jet at moderate values of the Reynolds and Péclet numbers. These measurements include spatial derivatives with respect to different spatial directions of the longitudinal velocity fluctuation and of the temperature fluctuation. Relations between mean-square values of these derivatives suggest strong departures from local isotropy for both velocity and temperature. In contrast, the locally isotropic forms of the vorticity and temperature dissipation budgets are approximately satisfied. Possible contamination of the fine-scale measurements by the anisotropic large-scale motion is assessed in the context of the measured structure functions of temperature and of the measured skewness of the streamwise derivative of temperature. Structure functions are, within the framework of local isotropy, consistent with the average frequency and amplitude of temperature signatures that characterize the quasi-organized large-scale motion. Conditional averages associated with this motion account, in an approximate way, for the skewness of the temperature derivative but make negligible contributions to the skewness of velocity derivatives. The degree of spatial organization of the fine structure is inferred from conditional statistics of temperature derivatives.

In the self-preserving region of a slightly heated turbulent plane jet, conventional isocorrelation contours of velocity and temperature fluctuations support the existence of organized large-scale structures. Temperature fronts associated with these structures were visually detected using a spanwise rake of cold wires. This method of detection was then used to condition velocity and temperature fluctuations and products of these fluctuations. Ensemble-averaged velocity vectors, constructed in the plane of main shear, suggest a topology for the organized motion in which the temperature front is identified with the diverging separatrix connecting adjacent structures on the same side of the centreline. Coherent stresses and heat fluxes are particularly significant near the diverging separatrix. Contributions by the coherent and random motions to the averaged momentum and heat transports are generally of the same order of magnitude.

A turbulent spot is induced by a spark triggered in one of the laminar boundary layers in the entrance region of a two-dimensional duct flow. The development of the spot is studied using ensemble-averaged velocity and wall shear stress in the plane of symmetry of the spot. Following an initial growth of the spot, the potential-flow field associated with this spot triggers a second spot on the opposite wall of the duct. This new spot propagates at the same convection velocity as the original spot and grows until the turbulent regions occupied by the two spots completely fill the width of the duct. This transition mechanism differs significantly from that observed for a plane Poiseuille flow, where the spot fills the duct almost immediately after it is generated.

All three velocity fluctuations and the temperature fluctuation have been measured in a slightly heated turbulent plane jet. Attention is focused on the interaction region of the flow, which is situated between the location where the two mixing layers nominally merge and that which corresponds to approximate self-preservation.

For the jet considered here the mixing-layer structures are symmetrical with respect to the centreline, and when they meet in the interaction region the redistribution of turbulence quantities is dramatic. This redistribution is examined in detail. Also examined is the effect of the generation, in the interaction region, of new structures, asymmetric with respect to the centreline, which evolve into the self-preserving flow region downstream.

Turbulence parameters, such as the turbulent Prandtl number, probability density functions, skewness and flatness factors, are also presented, primarily to guide computer simulations of this flow. The superposition procedure of Weir, Wood & Bradshaw (1981), which assumes that the turbulence structure of each mixing layer is not significantly altered by the interaction, is not appropriate to the present flow.

Measurements of space–time correlations of longitudinal and normal velocity fluctuations and of temperature fluctuations support the existence of counter-rotating spanwise structures appearing alternately on opposite sides of the jet centreline in the self-preserving region of the flow. The frequency of these structures closely satisfies self-preservation. The asymmetric arrangement of the structures is first observed downstream of the position where the jet mixing layers nominally merge but upstream of the onset of self-preservation. Closer to the jet exit, the space–time correlations indicate the existence of spanwise structures that are symmetrical about the centreline.

Visual examination of simultaneous temperature traces from a rake of cold wires placed across a turbulent boundary layer had enabled the identification of coherent temperature fronts. An X-wirelcold-wire arrangement was used simultaneously with the rake to provide measurements of the velocity fluctuations u (longitudinal) and v (normal) and the temperature fluctuation θ. Conditional averages of u, v, θ and products uv, uθ, vθ were obtained by application of conditional techniques based on the detection of the temperature fronts using information obtained at only one point in space. These averages, obtained at various positions across the layer, have been compared with those obtained when the rake was used to detect the fronts. The comparison has indicated that none of the one-point detection techniques is in good quantitative agreement with the rake detection technique, the largest correspondence between the rake technique and any ofthe other one-point techniques being only 51 yo. With the exception of the hole technique used in conjunction with the quadrant decomposition analysis, conditional averages obtained from one-point techniques are in reasonable qualitative agreement with those deduced using the rake.

Simultaneous measurements of the longitudinal velocity U, normal velocity V, and temperature T have been made in turbulent spots generated in a slightly heated laminar boundary layer. The measurements were made in the plane of symmetry of the spot at several streamwise stations downstream of the spot-producing spark. Ensemble averages of U, V and T, obtained relative to the leading edge of the spot, are presented in terms of disturbances relative to the unperturbed values in the laminar flow. Ensemble-averaged disturbances of U, V and T in the outer part of the spot are consistent with a picture of a relatively large vortical structure with a spanwise vorticity in the same sense as that of the laminar flow. A rather dominant feature of these distributions is the large negative disturbance in V and U near the leading edge in the outer part of the spot; associated with this disturbance is a positive perturbation in T. The terms contributing to the ensemble-averaged values of UV, UT and VT are obtained and discussed. In the case of UV, contributions by the disturbance are found to be of the same order as those by the random turbulence superposed on the disturbance. For VT, the disturbance appears to play a more dominant role in the transfer of heat. The conical property of the spot is tested for disturbance temperature and velocity fields by comparing contours, at three streamwise stations, of the velocity and temperature disturbances using the co-ordinates introduced by Cantwell, Coles & Dimotakis (1978). The results show that the conical transformation, which was found to be a good approximation for the full velocity field, is not an accurate representation for the disturbance fields of velocity and temperature measured relative to laminar values. An alternative viscous transformation represents more accurately the disturbance fields. Features of the turbulence within the spot are compared with those obtained in a fully turbulent boundary layer.

Conditional averages of longitudinal, normal velocity and temperature fluctuations and of their products have been obtained in a slightly heated boundary layer with zero pressure gradient over a momentum-thickness Reynolds-number range 990 ≤ Rm ≤ 7100. These averages are based on the identification of coherent temperature fronts that extend across most of the layer. The average period between fronts is approximately independent of Rm when Rm is greater than about 1500. The streamwise length scale of the fronts and the magnitude of velocity and temperature derivatives associated with the fronts scale on the thickness of the layer except for Rm, less than about 3000. This scaling is consistent with the Reynolds-number independence, for Rm greater than about 3000, of both mean and turbulent velocity and temperature fields. Conditional averages are discussed in the context of Head & Bandyopadhyay's (1978) suggestion, based on smoke-flow visualization, that the boundary layer consists almost exclusively of hairpin eddies.

Equations for the instantaneous velocity and temperature fluctuations in a turbulent flow are used to assess the effect of a fluctuating convection velocity on Taylor's hypothesis when certain simplifying assumptions are made. The probability density function of the velocity or temperature derivative is calculated, with an assumed Gaussian probability density function of the spatial derivative, for two cases of the fluctuating convection velocity. In the first case, the convection velocity is the instantaneous longitudinal velocity, assumed to be Gaussian. In the second, the magnitude of the convection velocity is equal to that of the total velocity vector whose components are Gaussian. The calculated probability density function shows a significant departure, in both cases, from the Gaussian distribution for relatively large amplitudes of the derivative, at only moderate values of the turbulence intensity level. The fluctuating convection velocity affects normalized moments of measured velocity and temperature derivatives in the atmospheric surface layer. The effect increases with increasing order of the moment and is more significant for odd-order moments than even-order moments.