A wake is the turbulent perturbation region that develops behind any body in motion in a fluid. Among the streamlined bodies' wakes, we have to make a difference between the drag wakes, and the propelled body's wakes. For the latter case, the knowledge of the stream characteristics is of a great interest for the detection of vehicles. The purpose of this study is to bring experimental information as complete as possible on the velocity field evolution. In order to do this, we set an experiment in a wind tunnel. The Reynolds number calculated with the upstream velocity and the model diameter is 5.8*104. An automated acquisition device of the three components of the instantaneous velocity vector using a triple hot film anemometric probe was developed especially for this work. It is possible to compare the pure and self-propelled wakes created by the same body. The axial exploration was carried out for a large range extending for the near wake to the far wake (0.2 to 50 diameters downstream from the body) which is a very complete data base. The determination of the time velocity field brings: - three components of mean velocity - six components of Reynolds tensor - third order moments of velocity - power spectral densities in the axial, radial and tangential directions - three components of mean velocity and six components of Reynolds tensor, analyzed in phase with the propeller rotation An original result of this work is the experimental existence of self-preservation for the velocity field in the self-propelled body far wake. Nevertheless, the laws established on an analysis of the motion equations and using a closure based on two opposite rates of flow do not seem to be justified by the experiment. We have determined all the terms of the turbulent kinetic energy equation that can be measured. Thus we proved self-preservation for the turbulent exchanges in the far wake for both configurations. For the self-propelled body, an important result is that production of turbulent kinetic energy is a negligible process in the self-preserved region. If one adds the self-propelled condition to the boundary layer approximation, it is possible to show, by a correct dimensional analysis, that production is smaller than the other terms of the turbulent kinetic energy balance. In the transition zone between near and far wake, the shape of non-dimensional power spectral density is preserved along the radial direction for pure or self-propelled wakes. This means that distribution of energy between the turbulent structures is conserved. In the case of the self-propelled wake, the analysis of power spectral densities points out a wide band signal with spectral lines. The near wake region, up to two diameters downstream from the model, is strongly influenced by excitation phenomena created by the periodic blades passing. This observation behaved us to realize a statistical processing of velocity in phase with the propeller rotation. We obtained a visualization of the organized structures in the flow for a zone that spreads to two diameters downstream from the model.
Experimental study of the turbulent wake of an axisymmetric self-propelled body