A. J. Chambers

On the organized motion of a turbulent plane jet

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.

Temperature Ramps in the Atmospheric Surface Layer

Abstract A review of the evidence for the organized temperature structure observed in both the atmospheric surface layer and the laboratory boundary layer reveals similar features between the two turbulent flows. This similarity suggests that the atmospheric temperature ramp may be interpreted as the signature of an organized large-scale motion rather than a necessary consequence of the presence of buoyant plumes. An experiment was conducted in which the translation velocity Ut of the sharp edge of the temperature ramp is determined from the transit time of the ramp between two thermistors placed at the same height in the marine surface layer but separated in a direction parallel to the wind. Ut was found to be in more nearly constant ratio to the local velocity than to the friction velocity. Velocities determined from the phase angle of the temperature cross spectrum and from the optimum temperature cross correlation obtained from the two thermistors are in reasonable agreement with Ut. Cross correlation...

The interaction region of a turbulent plane jet

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.

Organized structures in a turbulent plane jet: topology and contribution to momentum and heat transport

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.

Assessment of local isotropy using measurements in a turbulent plane jet

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 Peclet 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.

Accuracy of moments of velocity and scalar fluctuations in the atmospheric surface layer

A detailed accuracy analysis is presented for moments, up to order four, of both velocity (horizontal u and vertical w) and scalar (temperature θ and humidity q) fluctuations, as well as of the products uw, wθ and wq, in the atmospheric surface layer. The high-order moments and integral time scales required for this analysis are evaluated from data obtained at a height of about 5 m above the ocean surface under stability conditions corresponding to Z/L \- −0.05. Measured moments and probability density functions of some of the individual fluctuations show departures from Gaussianity, but these are sufficiently small to enable good estimates to be obtained using Gaussian instead of measured moments. For the products, the assumption of joint Gaussianity for individual fluctuations provides a reasonable, though somewhat conservative, estimate for the integration times required. The concept of Reynolds number similarity implies that differences in integration time requirements for flows at different Reynolds numbers arise exclusively from differences in integral time scales. A first approximation to the integral time scales relevant to atmospheric flows is presented.

Determination of time constants of cold wires

The time constant of a fine cold wire used as a resistance thermometer can be obtained by a technique which is based on ideas developed for the pulsed‐wire technique for velocity measurement in highly turbulent flows and in regions of flow reversal. The cold wire is placed downstream of, and at right angles to, a wire of larger diameter which is pulsed with a short duration voltage pulse. The response of the cold wire to the change in the temperature of the flow is used to determine the time constant of the wire, once the pulsed wire time constant is known. Frequency responses based on measured time constants for cold wires of 2.5, 1.3, and 0.63 μm diameters are presented as a function of flow velocity.

Budget of the temperature variance in a turbulent plane jet

Abstract Measurements are presented of the advection, production, diffusion and dissipation terms of the mean square temperature fluctuation budget for a two-dimensional jet. Diffusion is more important than advection near the jet centreline. The measured dissipation or destruction, determined using all three components, enables satisfactory closure of the budget in the region of the jet that is free of flow reversal. The difference between the measured dissipation and an estimate, based on local isotropy, is significant across the jet.

Turbulence Reynolds number and the turbulent kinetic energy balance in the atmospheric surface layer

The relation between the turbulence Reynolds numberRλ and a Reynolds numberz* based on the friction velocity and height from the ground is established using direct measurements of the r.m.s. longitudinal velocity and turbulent energy dissipation in the atmospheric surface layer. Measurements of the relative magnitude of components of the turbulent kinetic energy budget in the stability range 0 >z/L ≥ 0.4 indicate that local balance between production and dissipation is maintained. Approximate expressions, in terms of readily measured micrometeorological quantities, are proposed for the Taylor microscale λ and the Kolmogorov length scale η.

Taylor's hypothesis and the probability density functions of temporal velocity and temperature derivatives in a turbulent flow

Equations for the instantaneous velocity and temperature fluctuations in a turbulent flow are used to assess the effect of a fluctuating convection velocity on Taylors 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.