SCALAR IMAGING VELOCIMETRY MEASUREMENTS OF THE VELOCITY-GRADIENT TENSOR FIELD IN TURBULENT FLOWS .2. EXPERIMENTAL RESULTS

Scalar imaging velocimetry is here applied to experimental turbulent f low scalar field data to yield the first fully resolved, non-intrusive laboratory measurements of the spatio-temporal structure and dynamics of the full nine-component velocity gradient tensor field del u(x,t), as well as the pressure gradient field del p(x,t), in a turbulent flo w. Results are from turbulent flows at outer scale Reynolds numbers in the range 3,000 less than or equal to Re(delta)less than or equal to 4,200, with Taylor scale Reynolds numbers Re-lambda approximate to 45. These give a previously inaccessible level of detailed experimental a ccess to the spatial structure ill the velocity gradient tensor field at the small scales of turbulent flows, and through the much longer te mporal dimension of these four-dimensional data spaces allow access to the inertial range of scales as well. Sample spatio-temporal data pla nes and probability distributions spanning more than 75 advection time scales (lambda(nu)/U) are presented for various dynamical fields of i nterest, including the three components of the velocity field u(x,t), the nine components of the velocity gradient tensor field del u(x,t) t hrough the full vector vorticity field pi(i)(x,t) and tensor strain ra te field epsilon(ij) (x,t), the kinetic energy dissipation rate field Phi(x,t) = 2 nu epsilon:epsilon(x,t), the enstrophy field 1/2 pi .pi(x ,t), the enstrophy production rate field <pi .>epsilon .pi(x,t), and t he pressure gradient field del p(x,t). Continuity tests show agreement with the zero divergence requirement that exceeds the highest values reported from single-point, invasive, multi-probe measurements. Distri butions of strain rate eigenvalues as well as alignments of the strain rate eigenvectors with both the vorticity and scalar gradient vectors are in agreement with DNS results, as are distributions of the measur ed helicity density fields u .pi(x,t). Results obtained for the true k inetic energy dissipation rate field show good agreement, up to 14th-o rder, with previous inertial range structure function exponents measur ed by Anselmet et al. [J. Fluid Mech. 140, 63 (1984)] at much higher R eynolds numbers. In addition, probability distributions scaled on inne r variables show good agreement among buoyant and non-buoyant turbulen t flow cases, further suggesting that these results are largely indica tive of the high Reynolds number state of the inner scales of fully de veloped turbulent flows. (C) 1996 American Institute of Physics.