Anàlisi de la naturalesa de les estructures coherents en l'estela llunyana d'un cilindre.

Identifier:  TDX:356

Handle: https://hdl.handle.net/20.500.11797/TDX356

Authors:  Vernet Peña, Anton

Abstract:
One of the topics emphasized in the study of turbulent flows in the last few years has been the identification of large scale organized motions called coherent structures. The present work extends the knowledge of the coherent structures in free turbulence by examining the far wake (x/D=150) of a heated cylinder at a Reynolds number of 9700. Multi-probe experiments carried out using hot- and cold-wire anemometry in a low turbulence wind tunnel, measuring temperature and the three velocity components simultaneously in time. The data were analyzed using a conditional pattern recognition technique. Basically, events in the temperature field were used to identify the velocity patterns of the coherent structures. Specifically, the pattern recognition software searched for hot-to-cold transitions in the temperature field. In the horizontal planes, the typical footprint identified with the temperature transitions was a double roller eddy with negative streamwise fluctuations in its centre plane. This eddy is similar to that identified by Grant (1958) with correlation measurements and is the origin for a large number of later investigations (Munford, 1983; Ferré, 1986). The present work shows that, in fact, this double roller structure is part of a larger structure which is shaped like a horseshoe.The use of the conditional pattern recognition technique allows one to obtain the three-dimensional velocity field that characterizes the coherent structure, and its relationship to the temperature field. It was found that the correlation between the temperature fluctuations and the streamwise velocity fluctuations is negative, while the correlation between the same temperature fluctuations and the lateral velocity fluctuations is positive. In addition, it was observed that the individual horseshoe structures occupy at least 40% of the flow and that these structures contain 60% of the turbulent kinetic energy. Thus, the parts of the flow selected by the technique are more energetic that the mean. As well, the role of the coherent structures in the exchange of energy between different turbulent scales, the mean, the coherent field and the incoherent field has been identified. The distributions of the fine scale turbulence and the dissipation were also investigated relative to the horseshoe structure. These results provide information about where and when the structure gains and loses energy. It was found that the majority of the production and dissipation of fine-scale turbulence occur in the central portion of the three-dimensional horseshoe structure.