Large-Eddy Simulation of Time Evolution and Instability of Highly Underexpanded Sonic Jets

High-pressure jet injection into quiescent air is a challenging fluid dynamics problem in the field of aerospace engineering. Although plenty of experimental, theoretical, and numerical studies have been conducted to explore this flow, there is a dearth of literature detailing the flow evolution and instability characteristics, which is vital to the mixing enhancement design and jet noise reduction. In this paper, a density-based solver for compressible supersonic flow, astroFoam, is developed based on the OpenFOAM library. Large-eddy simulations of highly underexpanded jets with nozzle pressure ratios from 5.60 to 11.21 at a Reynolds number around 10(5) are carried out with a highresolution grid. A grid-convergence study has been conducted to confirm the fidelity of the large-eddy simulation results. The large-eddy simulation results have also been validated against available literature data in terms of the time-averaged near-field properties of underexpanded jets. The turbulent transition processes are revealed based on the instantaneous flow features and are quantitatively resolved according to the jet penetration and maximum width. The vorticity analysis is conducted to understand the turbulent transition mechanism, and it is found that the vortex stretching term plays a leading role on the distortion of the vortex rings in the near field of the jets. The dominant instability modes of jets, visualized by helicity, are quantitatively revealed based on the spectrum and relative phase of pressure fluctuation. The single helical modes corresponding to a phase angle close to +/- 180 deg with the 1 + 1 helices are dominant for nozzle pressure ratios of 5.60 and 7.47, whereas the complex and multiple helices for the other two higher nozzle pressure ratios are due to the superposition of the single and double helical modes. In addition, the performance of the coarse mesh and different subgrid-scale models on capturing the dominant instability characteristics in large-eddy simulation of underexpanded jets is investigated

Flow characteristic of highly underexpanded jets from various nozzle geometries

Flow characteristics of highly underexpanded jets at the same nozzle pressure ratio of 5.60 but issuing from four different nozzles, i.e., the circular, elliptic, square, and rectangular nozzles, are studied using large eddy simulations. The results show that the square jet penetrates fastest, although the turbulence transition is similar for different jets. The penetration rates of different jets show the similar linear dependency on the square root of time, but the penetration constant Gamma for the noncircular jets deviates more than 5% from the theoretical value of 3.0. The circular and square jets both correspond to a three-dimensional helical instability mode, while the elliptic and rectangular jets haveatwo-dimensional flapping instability in their minor axis planes. All the jets undergo a Mach reflection forming the Mach disk, but the Mach disk in the elliptic and rectangular jets is not easily visible. The intercepting shocks in the square jet originate at the four corners of the nozzle exit at first, while the formation of the intercepting shocks is only observed in the major axis planes for the elliptic and rectangular jets. In addition, great differences are observed on the mixing characteristics between different jets. In particular, the elliptic jet penetrates slowest, has the shortest length of jet potential core, and takes the largest mixing area. (C) 2017 Elsevier Ltd. All rights reserved.</p

Numerical investigation of characteristic frequency excited highly underexpanded jets

A highly underexpanded jet with a nozzle pressure ratio of 5.60 is excited simultaneously by the inherent characteristic frequencies in the steady jet of 14.569 kHz 37.086 kHz and 45.695 kHz as well as other two reference frequencies of 1.0 kHz and 40.0 kHz. The flow characteristic of the excited jets is revealed by comparing to the steady jet using large eddy simulation technique. For low-frequency excitation (fe = 1.0 kHz) the flow and acoustic fields of the forcing jet are similar with the steady jet. However when the jet is excited by high frequencies the acoustic source moves to the nozzle exit and the jet potential core together with the near-field shocks oscillate periodically at the excitation frequency. The excitation at fe = 1.0 kHz increases the mixing area since y/D = 24 from the nozzle exit which is contrary to the effect of other high frequencies that enhances the mixing in the near-field region but decreases the mixing area since y/D = 18. The peak frequency of the excited jets generally becomes identical to the excitation frequency once being excited except the fe = 1.0 kHz and h = 40.0 kHz jets. High-order harmonics of the dominant frequency are observed in the pressure spectrum of jets excited by high frequencies and the dominant mode turns into the axisymmetric mode from the original helical one accordingly. In particular forcing the jet with the axisymmetric mode of h = 14.569 kHz provides the fewest shock cells but the largest amplitude in shock oscillation the most harmonics in the spectrum and the largest mixing area within 8 &lt; y/D &lt; 12. 2017 Elsevier Masson SAS. All rights reserved.</p

A comparative study of highly underexpanded nitrogen and hydrogen jets using large eddy simulation

Three-dimensional large eddy simulations (LES) of highly underexpanded hydrogen and nitrogen jets at the same nozzle pressure ratio (NPR) of 5.60 and at a Reynolds number around 105 are performed. The classical near-field structures of highly underexpanded jets are well captured by LES, especially the shape and size of Mach barrel for both jets are very similar and agree well with the available literature data. However, the flow field and the shock structures after the Mach disk differ significantly. The density in the annular shear layer of H-2 jet is much lower because of its smaller molecular weight. Meanwhile, the H-2 jet has a much longer jet core and more shock cells. The dominant instability mode is helical for the N-2 jet, but is axisymmetric for the H-2 jet. There are two discrete peaks of f(s) = 37.086 kHz and f(2s) = 45.695 kHz in the spectrum of the N-2 jet, while the spectrum of the H-2 jet is characterized by a fundamental screech frequency of f(s) = 47.020 kHz and its high-order harmonics. The H-2 jet mixes more rapidly with the ambient air but has a much smaller mixing area on cross-section planes. Mixing between the ambient air and fuel still takes places at the jet boundary defined according to the mixture fraction of Z = 0.02, and the area of fully turbulent region of the highly underexpanded jets seems to be less predicted based on the traditional vorticity T/NT (turbulent/non-turbulent) interface for both jets. Copyright (C) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved