Showing papers by "R. A. Antonia published in 1989"

Scaling of the ‘‘bursting’’ period in turbulent boundary layer and duct flows

Abstract: Measurements have been made in fully developed turbulent duct and boundary layer flows of the average bursting period over a one decade variation in Reynolds number. The bursting period was determined by two methods: the variable interval time averaging scheme in the form used by Blackwelder and Haritonidis [J. Fluid Mech. 132, 87 (1983)] and the modified u‐level scheme of Luchik and Tiederman [J. Fluid Mech. 174, 529 (1987)] in which ejections from the same burst are grouped. All these authors concluded that the bursting period scaled on inner variables. The present results indicate that this conclusion is valid only over a relatively small Reynolds number range, in which low Reynolds number effects are known to be important. At higher Reynolds numbers, scaling on mixed variables provides the best agreement with the data although scaling on outer variables cannot be ruled out.

Topology of a turbulent boundary layer with and without wall suction

Abstract: Measurements of velocity and temperature fluctuations are made in a turbulent boundary layer with nominally zero pressure gradient for two different slightly heated wall conditions: impermeable and porous surfaces. The temperature fluctuations are measured at three points in the flow to permit the identification of two spatially coherent events: coolings and heatings. Conditional velocity vectors in the plane of mean shear are viewed in a reference frame which translates at a constant velocity. Conditioning is on coolings, heatings or a combination of these events. Sectional streamlines, derived from the velocity vector data, show a succession of critical points: saddles and unstable foci. Coolings are aligned with diverging separatrices through the saddles whereas heatings are identified with the foci. Coolings are associated with a large strain rate and also a large spanwise vorticity: this result seems consistent with the presence of hairpin vortices which extend to different distances from the wall. In contrast, the strain rate and spanwise vorticity are small at the foci. The stabilising influence of suction is observed in the topology of the organised motion and in the contribution from this motion to the conventional stresses, temperature variance and heat fluxes.

On the origin of the organised motion in the turbulent far-wake of a cylinder

Abstract: Spectra and spectral coherences of mainly the lateral velocity fluctuations, measured in the wake of a circular cylinder, strongly suggest that the origin of the far-wake organised motion can be traced back to the cylinder. A possible mechanism for the development of the organised motion is suggested.

Velocity vector cone angle in turbulent flows

Abstract: Use is made of computer simulated turbulent signals to calculate the rms of the velocity cone angle. The calculation compares favourably with X-probe data close to the axis of a circular jet. In this flow, the vector cone angle can exceed 90°, even on the axis. As a consequence, the rms values of the cone angle and of the lateral velocity fluctuation can be seriously underestimated with a 90° X-probe. Support for this is provided by measurements with a 120° X-probe.

Phase-averaging in the turbulent far-wake

Abstract: The dominant peak in the spectrum of the lateral velocity fluctuation in the turbulent far-wake of a circular cylinder can be used to describe the organised motion in this region, in much the same way as phase-averaging has been used to study the organised motion in the near-wake

Flow reversal and intermittency of a turbulent jet

Abstract: An attempt is made to quantify the importance of flow reversal in the intermittent region of a turbulent circular jet discharged into still air and its influence on both conventional and conditional velocity fluctuations. It is found that flow reversal coincides with high probability with nonturbulent periods but occurs for only a small fraction of the time for which the flow is nonturbulent. This result is consistent with the observation that conventional and zone-averaged Reynolds stresses and heat fluxes are essentially unaffected by flow reversal

Instantaneous dissipations of turbulent energy and temperature in a wake

Abstract: Approximations to the instantaneous dissipation of temperature variance and to an instantaneous pseudodissipation of kinetic energy in the turbulent far wake of a cylinder have been measured, using either four hot wires or four cold wires arranged in the form of a square. Results, including probability density functions and spectra as well as joint probability density functions and cospectra between individual components of the dissipations, suggest a close similarity between the two dissipations. The influence of the organized motion on the flow structure is briefly discussed in the context of cospectra and correlation coefficients for the components of the two dissipations. One implication of the similarity between the spectra of the two dissipations is the constancy of the time scale ratio, defined on a spectral basis, of the velocity and temperature fields.

On the Internal Order and Structures in a Transitional Spot

Abstract: A transitional (laminar to turbulent) spot which develops on a slightly heated laminar boundary layer is investigated. Flow structures of nearly the same scale as the spot have been identified within the spot. The number of structures in the spot increases with streamwise distance, however all structures are convected with the same velocity. This convection velocity depends on the normalized distance from the wall and is almost identical to that of the leading edge of the spot. The increase in the number of structures accounts for the streamwise growth of the spot. There is sufficient evidence to suggest that, the new structures are formed near the trailing edge of the spot.