Persistence of local anisotropy of passive scalars in wall-bounded flows

Local isotropy of passive scalars in fully developed turbulent channel flow is studied by way of direct numerical simulations. We observe a persistent small-scale anisotropy that (i) persists after the flow has undergone substantial mixing and (ii) is independent of the original large-scale anisotropic initial conditions of the scalar field. This latter observation is in sharp contrast with the persistent local anisotropy observed in homogeneous, isotropic turbulence with imposed mean scalar gradients, for which the small-scale anisotropy is directly correlated to the imposed large-scale anisotropy by way of ramp-cliff structures. The anisotropy observed in the present work is linked to the production of θ due to the mean velocity gradient. A major implication of the work is that local isotropy of passive scalar fields may never hold in flows in which mean velocity gradients exist, even after mean scalar gradients have been eliminated by the turbulence

3DFLUX: A high-order fully three-dimensional flux integral solver for the scalar transport equation

We present a detailed derivation of a high-order, fully three-dimensional, conservative, monotonicity preserving, flux integral method for the solution of the scalar transport equation. This algorithm, named 3DFLUX, produces highly accurate solutions that are nearly unaffected by numerical dissipation, at a realistic computational cost. The performance of 3DFLUX is characterized by means of several challenging multidimensional tests. 3DFLUX is nominally third-order in space and second-order in time, however, at low Courant numbers, it appears to be superconvergent and, depending on the problem solved, is fourth-order or higher in space. Finally, 3DFLUX is used to simulate advection-diffusion of a complex temperature field in an incompressible turbulent flow of practical relevance, and its results are in excellent agreement with experimental measurements

The Use of Active Grids in Experimental Facilities

Active grids allow for the turbulence in experimental facilities to be tailored through a broad range of turbulence intensities and Reynolds numbers. This work provides an overview of the active grids that presently exist around the globe as well as advances in turbulence research that are a result of their use. Focus is placed on homogeneous turbulent flows, turbulent boundary layers, and model testing