Antisymmetric Oscillation Modes in Rectangular Screeching Jets

Abstract

In this paper, the origin and the properties of the oscillation modes in screeching nonideally expanded rectangular jets are investigated using compressible implicit LES of rectangular supersonic jets. At the exit of a converging diverging rectangular nozzle of aspect ratio 2 and of design Mach number 1.5, the jets are under-and over-expanded. Seven simulations with four different temperature ratios ranging from 1 to 3 and two different nozzle pressure ratios are performed. The geometry of the nozzle and the exit conditions are chosen such that to match the experimental study conducted at the University of Cincinnati. First, the over-expanded jets are studied. It is shown that the total number of shock cells decreases with increased temperature ratio. However, the temperature does not influence the size of the first shock cell and the linear decrease of the shock cell size in the downstream direction. The spreading of the jet is observed to be higher along the minor axis plane than along the major axis plane. The intensity of the screech noise increases with the temperature ratio in the present study although the opposite is observed in the experiments. Moreover, for jet temperature ratios of 2.5 and 3, the strong flapping motion of the jet along the minor axis plane due to the screech feedback mechanism yields to an antisymmetric organization of the Mach wave radiation. Thereafter, the near-and far-field acoustic are studied. In the near-field, screech tones are captured, whose frequencies are consistent with both experimental data and theoretical models. In the far-field, four acoustic components typical of non-ideally expanded supersonic jets are observed, namely the screech noise, the broadband shock-associated noise, the mixing noise and the Mach wave noise. Their directivities and frequencies are in agreement with experimental results and models. The mechanism of the screech noise generation is studied by using a Fourier decomposition of the pressure field. For the four over-expanded jets, a flapping motion along the diagonal or along the minor axis plane of the jet is noted. Finally, the hypothesis that the acoustic waves completing the feedback loop in these jets are linked to the upstreampropagating acoustic wave modes of the equivalent ideally expanded jets is tested. Using a jet vortex sheet model to describe the dispersion relations of these modes, it is found that this hypothesis allows us to explain the antisymmetric jet oscillation observed at the screech frequencies. Based on frequency-wavenumber decomposition of the pressure fluctuations in the jets, it is shown that at the screech frequencies, acoustic waves propagating in the upstream direction at the ambient speed of sound exist also in the jet flow, additionally

The generation mechanism of higher screech tone harmonics in supersonic jets

“…(2003) and Gojon et al (2019)) follow a well-defined trend, although the nozzle contours and aspect ratios are different. However, the screech frequencies exceed the analytically derived frequency ranges for neutral wave feedback.…”

“…The wavenumber, k, is normalised by the nozzle exit height, h e . p 0,n /p ∞ = 2.5, AR = 9 p 0,n /p ∞ = 2.75, AR = 9 p 0,n /p ∞ = 3.67, AR = 9 p 0,n /p ∞ = 4.5, AR = 9 p 0,n /p ∞ = 2.75, AR = 7 p 0,n /p ∞ = 3.67, AR = 7 p 0,n /p ∞ = 4.5, AR = 7 p 0,n /p ∞ = 2.5, AR = 7 Panda et al (1997) Alkiskar et al (2003) Gojon et al (2019 FIGURE 8. Measured frequencies of screech and its harmonics plotted on top of the permitted frequency bands for neutral waves (as defined by Tam & Norum (1992)) as function of the ideal-expanded Mach number.…”

“…Tam & Hu (1989) found a special family of upstream propagating waves which follow the jet column and close the screech feedback loop. The existence of these neutral waves was confirmed numerically by Shen & Tam (2002), Gojon, Bogey & Mihaescu (2018), Gojon, Gutmark & Mihaescu (2019) and experimentally by Edgington-Mitchell et al (2018). However, the mechanism generating the screech tones is debated.…”

Intermittent modal coupling in screeching underexpanded circular twin jets

“…The effect of jet temperature ratio on twin-jet coupling remains an open topic for investigation. The addition of heat is generally associated with a suppression of screech in single jets (Shen & Tam 2000), although under certain conditions it has also been seen to amplify the acoustic tones (Gojon et al 2017;Gojon, Gutmark & Mihaescu 2019). Strong coupling associated with the production of screech tones was observed in the two primary full-scale aircraft studies (Berndt 1984;Seiner et al 1986), indicating that in full-scale systems, screech is still present in heated twin-jet configurations.…”

“…The addition of heat is generally associated with a suppression of screech in single jets (Shen & Tam 2000), although under certain conditions it has also been seen to amplify the acoustic tones (Gojon et al. 2017; Gojon, Gutmark & Mihaescu 2019). Strong coupling associated with the production of screech tones was observed in the two primary full-scale aircraft studies (Berndt 1984; Seiner et al.…”