An experimental study of a turbulent jet impinging on a flat surface

The flow characteristics of an isothermal turbulent jet impinging normally on a flat plate was studied experimentally using hot-wire anemometry. The air jet discharged from a round pipe with an inner diameter D and the distance between the pipe exit and the flat impingement plate was 9D. The Reynolds number, based on the jet centerline velocity at pipe exit and the pipe inner diameter, was 10,338. Measurements were performed in the free- and wall-jets, mean velocity, RMS, Reynolds shear stress, higher-order moments of velocity fluctuations, dissipation spectrum as well as spatial characteristic scale results being presented. Moreover, a multiscale analysis method based on wavelet transform was applied to gain deeper insight into the multiscale turbulent structures and to extract the most essential scales governing the development of the turbulent flow structures of the impinging jet. The experimental results show that the normal velocity fluctuations have the greatest skewness and the highest intermittency in the mixing region and this characteristic is more prominent downstream the pipe exit. The radial velocity fluctuations at a small distance from the flat surface trend towards Gaussian values far away the jet centerline. The strongest dissipation structures grows with larger scales and lower frequencies far away from the pipe exit, and the frequency range of the dissipation structures is reduced far away from the jet centerline together with the spatial distribution of the dissipation structures expanding with larger scales and lower frequencies. The scales in the center of the mixing region have the most energy, from which the scales belonging to the inertial range have a trend to shift to the larger scales with the increasing radial traverse, thus the scales of the coherent structures belonging to the dissipation region get more as well as larger. The scales belonging to the inertial range in the wall-jet region tend to the larger scales with the increasing normal traverse.