Flow field features of fractal impinging jets at short nozzle to plate distances

An experimental analysis of the flow field generated by an impinging jet equipped with a fractal grid insert located at the nozzle exit is carried out by means of Tomographic Particle Image Velocimetry. The Reynolds number based on the nozzle exit section diameter d is set to 15,000. The presence of the grid leads to a non uniform curvature of the jet shear layer. As a consequence, azimuthal instabilities are triggered in proximity of the nozzle exit section, causing the production of streamwise vortices. Furthermore, the organization of the coherent structures in proximity of the impinged wall is discussed and related to the convective heat transfer distribution. Owing to the presence of counter rotating wall vortices along the diagonals of the on-plate imprint of the fractal grid, a region of minimum in the scalar transfer map can be detected. In addition to that, similarly to the well known vortex rings that characterize the case of round jet without turbulence promoter, the presence of azimuthally coherent structures that might be generated by Kelvin–Helmholtz instability of the jet shear layer is presented and discussed.