Dissipation element analysis of scalar field in turbulent jet flow

For better understanding of turbulence, the geometry of turbulent structures in turbulent jet flow should be analyzed. The aim of the present work was to experimentally verify the dissipation element theory on highly resolved two-dimensional measurements turbulent jets using Rayleigh scattering technique. The statistical analysis of the characteristic parameters of dissipation elements; namely the linear length connecting the extremal points and the absolute value of the scalar difference at these points, respectively was also investigated. Rayleigh scattering was used to topographically produce 2D images of turbulent mixing to obtain the concentration distribution of two gases in a turbulent shear flow. The scalar field obtained was subdivided into numerous finite size regions. In each of these regions local extremal points of the fluctuating scalar are determined via gradient trajectory method. Gradient trajectories starting from any point in the scalar field phi(x, y) in the directions of ascending and descending scalar gradients will always reach a minimum and a maximum point where del phi = 0. The dissipation element has two extremal points (one maximal and one minimal) and two saddle points at the boundaries. (C) 2011 Elsevier Inc. All rights reserved.