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Three-component vorticity measurements in a turbulent grid flow

Published online by Cambridge University Press:  10 November 1998

R. A. ANTONIA
Affiliation:
Department of Mechanical Engineering, The University of Newcastle, N.S.W. 2308, Australia
T. ZHOU
Affiliation:
Department of Mechanical Engineering, The University of Newcastle, N.S.W. 2308, Australia
Y. ZHU
Affiliation:
Department of Mechanical Engineering, The University of Newcastle, N.S.W. 2308, Australia Current Address: Advanced Fluid Dynamics Laboratory, CSIRO Division of Building, Construction and Engineering, PO Box 56, Graham Road, Highett, Vic. 3190, Australia.

Abstract

All components of the fluctuating vorticity vector have been measured in decaying grid turbulence using a vorticity probe of relatively simple geometry (four X-probes, i.e. a total of eight hot wires). The data indicate that local isotropy is more closely satisfied than global isotropy, the r.m.s. vorticities being more nearly equal than the r.m.s. velocities. Two checks indicate that the performance of the probe is satisfactory. Firstly, the fully measured mean energy dissipation rate 〈ε〉 is in good agreement with the value inferred from the rate of decay of the mean turbulent energy 〈q2〉 in the quasi-homogeneous region; the isotropic mean energy dissipation rate 〈εiso〉 agrees closely with this value even though individual elements of 〈ε〉 indicate departures from isotropy. Secondly, the measured decay rate of the mean-square vorticity 〈ω2〉 is consistent with that of 〈q2〉 and in reasonable agreement with the isotropic form of the transport equation for 〈ω2〉. Although 〈ε〉≃〈εiso〉, there are discernible differences between the statistics of ε and εiso; in particular, εiso is poorly correlated with either ε or ω2. The behaviour of velocity increments has been examined over a narrow range of separations for which the third-order longitudinal velocity structure function is approximately linear. In this range, transverse velocity increments show larger departures than longitudinal increments from predictions of Kolmogorov (1941). The data indicate that this discrepancy is only partly associated with differences between statistics of locally averaged ε and ω2, the latter remaining more intermittent than the former across this range. It is more likely caused by a departure from isotropy due to the small value of Rλ, the Taylor microscale Reynolds number, in this experiment.

Type
Research Article
Copyright
© 1998 Cambridge University Press

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