Scalar Imaging Velocimetry Studies of Turbulent Flow Structure and Dynamics

摘要

The first fully-resolved, non-intrusive, experimental measurements of the spatio-temporal structure and dynamics of the full nine-component velocity gradient tensor field Vu(x,t) in a turbulent flow are here obtained by applying the scalar imaging velocimetry technique (Phys. Fluids A 4, 2191-2206) to laboratory turbulent flow scalar field data. A variational method implementing this concept is described in which weighted residuals of the conserved scalar transport equation, the continuity condition, and a derivative smoothness condition are minimized over the space of velocity fields. The technique is applied to direct numerical simulation (DNS) data for the limiting case of turbulent mixing of a Sc = 1 passive scalar field. The spatial velocity fields u(x,t) obtained correlate well with the exact DNS results, as do statistics of the velocity and velocity gradient fields. The method is then applied to fully resolved four-dimensional Sc >> 1 scalar field imaging measurements from a laboratory turbulent flow. Results give the first fully resolved data for the time-varying (u, v, w) vector velocity component fields simultaneously everywhere on a regular three-dimensional spatial grid in a turbulent flow. Direct differentiation of these fields yields the spatial structure in the full velocity gradient tensor field components. From these, the vector vorticity field wi(x,t) and tensor strain rate field epsilon-ij(x,t) are extracted, as are the kinetic energy density field k(x;t), the kinetic energy dissipation rate field phi(x;t), and the enstrophy field W(x;t). Finally, extraction of the time evolution in these fields is demonstrated by applying this scalar imaging velocimetry method to perform the inversion for the velocity field at several sequential time steps.