Investigation of the Near Wake Steady Flow development of a Fractal Square Grid using Particle Image Velocimetry

Turbulent flow generated by a square fractal grid is studied by 2-dimensional 2-component particle image velocimetry. These type of grids are designed to generate multi-scale turbulence by directly exciting a wide range of fluctuation length scales in the flow. The purpose of this study is to get the whole flow field pattern downstream of a 4-iteration square fractal grid. The grid is placed inside a test section of a suction type wind tunnel. With the help of transparent walls, it is possible to take shots exactly at the vicinity of the grid. Attaching a wide angle lens to the camera turned out a relatively large field of view in which the overall flow pattern could be observed. This gives enough information of how wakes of each individual bar are formed and how they mutually interact, although the vector spacing is not enough to resolve the wakes produced by the small bars. Among the quantities which are analyzed from the obtained instantaneous velocity fields are the mean vorticity, Reynolds shear stress and turbulent kinetic energy. Flow structures generated throughout the grid close-up region are realized with the help of zvorticity information. This will help for understanding the origins of the unusual flow properties downstream of the fractal square grids. There should be kept in mind that the region of interest is not limited to the close-up area of the grid but far downstream of the grid, where flow obtains its homogeneity and isotropy is also important and will be left for future studies.

Keywords:

Multi-scale turbulence , Fractal turbulence grid , Particle Image Velocimetry