Abstract
The disturbances added to trigger turbulence in the simulation of jet flows are mostly implemented with multiple azimuthal modes. The validity and rationale of using such multi-modes are still not very clear. In this paper, large eddy simulations are performed to study a compressible isothermal jet. Flow forcing methods for promoting transition are considered. Two multi-mode disturbances and a single-mode excitation are shown to trigger rapid flow transition, but the development of mixing layers in downstream varies. The most organized vortical structures produced by the vortex rolling/pairing process are observed when the jet is excited by the single-mode forcing, while the shear layers are closer to turbulence when the two multi-mode disturbances are employed. In order to unveil the mechanism behind such differences, dynamic mode decompositions on the axial velocity are conducted to analyze the spatiotemporal coherent structures. The results show that near the nozzle exit, the multi-mode disturbances have triggered more high frequency coherent structures, while the single-mode forcing has produced both low and high frequencies. The low frequency components are transmitted downstream of the jet, leading to poor prediction of the turbulent flow properties.
Highlights
In recent years, due to the question of Viswanathan (2004) of possible contamination by spurious facility in the jet far-field noise measurements of Tanna (1977), the effect of jet initial conditions on jet flow has regained attention in aerodynamics
Different nozzle exit conditions may initially lead to a laminar or turbulent shear layer, which affects the flow characteristics of the mixing layer and the noise level in the far field
The coherent structures appeared to be persistent in the initial laminar jet, but they were less obvious in the initial turbulent jet
Summary
Due to the question of Viswanathan (2004) of possible contamination by spurious facility in the jet far-field noise measurements of Tanna (1977), the effect of jet initial conditions on jet flow has regained attention in aerodynamics. Different nozzle exit conditions may initially lead to a laminar or turbulent shear layer, which affects the flow characteristics of the mixing layer and the noise level in the far field. As revealed by Bridges and Hussain (1987), the additional noise component of the laminar jet was generated by vortex pairing during the transition of the shear layer. Bogey and Sabatini (2019) reviewed these studies and confirmed the presence of persistent large-scale coherent structures in jets with a laminar exit velocity profile. Since the instability waves grew predominantly at higher Strouhal numbers in scitation.org/journal/adv the initial turbulent jet than those of the laminar jet, instability waves in the former disappeared quickly during the boundary layer/shear layer transition but continued to grow over a long distance from the nozzle in the latter case
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