Influence of coherent structures on the near- and far-field evolution of an axisymmetric turbulent jet.

Abstract

The role of the initial conditions in affecting the near field and the evolution of an axisymmetric turbulent jet towards the far field is examined. The jet's near-field coherent structures are manipulated with the aid of noncircular geometries of identical open area $D^2_e$ , including a square and fractal exit, for comparison with a classical round orifice. The flow is studied between 0 and 26$D_e$ with the aid of planar particle image velocimetry (PIV), hot-wire anemometry and tomographic PIV. This study shows that the fractal orifice significantly changes the near-field properties of the jet, breaking-up the coherent structures and affects the entrainment of background (quiescent) fluid into the turbulent stream. Despite the different state of coherence, it is found that many of the jet's turbulent characteristics scale with the number of eddy turnover times rather than simply the streamwise coordinates. Investigating the jet's evolution towards the self-similar state, the presence of non-equilibrium dissipation was found for all jets studied, concurrent to self-similar scalings for mean velocity and Reynolds stresses. The thesis is concluded by analysing the turbulent/non-turbulent interface (TNTI) and the fine scales of round and fractal jets at various streamwise locations. In the near field, the exit geometry appears to primarily impact on the inner interface, separating the laminar core from the turbulent stream, rather than the outer interface between the jet and the background fluid. Moreover, analysing the fine scales of the flow, the presence of two interfaces in the near field constrains the development of the turbulent properties and the jets appear as if only "local" strain was present. Finally, the evolution of the jet properties and of the small scales of the flow from the TNTI towards the turbulent bulk is examined, which suggests that the TNTI has a thickness of one longitudinal Taylor lenghtscale $\lambda_f $.