3-COMPONENT VORTICITY MEASUREMENTS IN A TURBULENT GRID FLOW

All components of the fluctuating vorticity vector have been measured in decaying grid turbulence using a vorticity probe of relatively simp le geometry (four X-probes, i.e. a total of eight hot wires). The data indicate that local isotropy is more closely satisfied than global is otropy, 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 [epsilon] is in good agreement with the value inferred from the rate of decay of the mean turbulent energy [q(2)] in the quasi-homogeneous region; the isotropic mean energy dissipation rate [epsilon(iso)] agre es closely with this value even though individual elements of [epsilon ] indicate departures from isotropy. Secondly, the measured decay rate of the mean-square vorticity [omega(2)] is consistent with that of [q (2)] and in reasonable agreement with the isotropic form of the transp ort equation for [omega(2)]. Although [epsilon] similar or equal to [e psilon(iso)], there are discernible differences between the statistics of epsilon and epsilon(iso); in particular, epsilon(iso) is poorly co rrelated with either epsilon or omega(2). The behaviour of velocity in crements has been examined over a narrow range of separations for whic h the third-order longitudinal velocity structure function is approxim ately linear. In this range, transverse velocity increments show large r departures than longitudinal increments from predictions of Kolmogor ov (1941). The data indicate that this discrepancy is only partly asso ciated with differences between statistics of locally averaged epsilon and omega(2), the latter remaining more intermittent than the former across this range. It is more likely caused by a departure from isotro py due to the small value of R-lambda, the Taylor microscale Reynolds number, in this experiment.