LES Predictions of Self-Sustained Oscillations in Homogeneous Density Round Free Jet

The paper presents a detailed LES analysis of turbulent round jets dominated by the mechanism of Kelvin-Helmholtz (K-H) instability and the so-called self-sustained regime, which is characterised by large velocity fluctuations, reminiscent of the behaviour of excited jets. It is shown that the occurrence of this regime is largely conditioned by the type and parameters of the inlet jet velocity profile, i.e., the shear layer momentum thickness 𝜃, turbulence intensity T i. A high order numerical code based on the combined pseudo-spectral / compact difference methods is used in the simulations. Analysis is performed for the Reynolds number R e = 1 × 104 with 𝜃 characterised by R/𝜃 = 16, 20, 24, 28 and 32 (with R - jet radius) and for T i = 10−2, 10−3, 10−4. Two inlet velocity profiles are used in the simulations: hyperbolic tangent and Blasius. Comparisons focus on the axial velocity profiles and the spectra of the velocity signals. It is shown that in the self-sustained regime the results obtained with the Blasius profile are significantly closer to the experimental data. Sensitivity tests of the self-sustained regime on the sub-grid modelling are performed based on four well known models: classical and dynamic Smagorinsky, the filtered structure function model of Ducros et al. (JFM, 1996) and the relatively new model proposed by Vreman (PoF, 2004). It is shown that in the case of the classical Smagorinsky model an excess of sub-grid dissipation prevents the appearance of self-sustained velocity oscillations and in effect gives results significantly different from the remaining models. On the other hand, when the jets are dominated by K-H instability all the models lead to very similar solutions.