Effect of Initial Velocity Profile on the Development of Round Jets

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Theeffectsofnonuniforminitialvelocityproe lesonthedownstreamevolutionofroundturbulentincompressible jets have been investigated experimentally. Jets evolving from two nonuniform initial velocity proe les, one with an axisymmetric fully developed proe le and the other with an asymmetric initial proe le, were compared to jets with top-hat initial velocity distributions. The Reynolds number of the present jets was 2 :4 £ 10 4 , based on the exit bulk velocity and the source diameter. The jets exited from pipes of circular cross section. For the jet with the axisymmetric initial velocity, the pipe was straight and produced a fully developed proe le at the exit. For the jet with the initially asymmetric velocity distribution, the e ow passed through a 90-deg bend in the pipe before exiting. Velocity measurements were carried out using hot-wire anemometry extending from the exit plane up to 80 jet diameters downstream. The evolution of both jets towards a self-preserving state is rapid. The initial asymmetry of the second jet vanishes by 9 pipe diameters. By 15 pipe diameters, the mean velocity proe les of both jets are self-preserving and follow very well the Gaussian distribution. Although the far-e eld mean and turbulent trends of the present jets are qualitatively similar to those jets with uniform initial-velocity distributions, there are some quantitative differences between them. The present jets develop into a self-preserving state faster than those jets with a uniform initial-velocity proe le. On the other hand, the initial growth of turbulence intensities and the far-e eld decay rates of the present jets are smaller than the jets with uniform initial-velocity proe les.