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The first experimental measurements are presented for the full three-dimensional spatial scalar energy spectrum Er(k) resulting from the molecular mixing of a Sc n I conserved scalar quantity in a turbulent flow. These results are based on threeand four-dimensional laser induced fluorescence imaging measurements of a dye concentration field ((x,t) in the self-similar far-field of an axisymmetric turbulent jet at local outer-scale Reynolds numbers Res between 3,000 and 6,000. Each of the resulting data sets consist of up to 3 billion individual data points, arranged into a temporal sequence of individual three-dimensional spatial data volumes, each of which consists of a series of parallel two-dimensional data planes. The resolution within each spatial data volume is finer than the local strain-limited molecular diffusion scale hD in all three dimensions. In addition, the temporal resolution between successive data volumes is made finer than the local diffusion scale advection time hD/u when time differentiation of the scalar field is of interest. This resolution, together with <(x,f), and in particular the scalar dissipation rate field (ReSc)-' Vc.Vc(x,f), throughout the data space. Results are presented for various statistics of the scalar and scalar gradient fields. The three-dimensional character of these measurements allows direct evaluation of the three-dimensional spatial scalar spectrum Ey(k) associated with turbulent mixing. The resulting isotropic spectrum function Ec(k), integrated from this full spectrum, is compared with its individual one-dimensional counterparts E&,) and E&) and E<@,) as a test of the smallscale isotropy hypothesis. These one-dimensional spatial spectra are compared with previous temporal specmm measurements. and with Batchelor's classical theory of the high wavenumber spectrum for Sc B I scalar mixing i n turbulent flows the signal quality achieved, permits differentiation of the data to determine the scalar gradient vector field V d'

Four-dimensional laser induced fluorescence study of the structure of molecular mixing in turbulent flows