The Regimes of Twin-jet Flow

A​uthors: Iván Padilla-Montero, Daniel Rodríguez
(​School of Aeronautics (ETSIAE), Universidad Politécnica de Madrid, Spain)

Vincent Jaunet, Stève Girard, Damien Eysseric and Peter Jordan
(​Institut Pprime, Université de Poitiers – CNRS – ISAE/ENSMA, France)

Animated schlieren visualization of the twin-jet flow field generated by convergent-divergent nozzles in the T200 laboratory (Institut Pprime, Poiters, France). The nozzle pressure ratio is continuously increased to yield jet Mach numbers ranging from 𝑀! = 0 to 𝑀j = 1.9, enabling the system to go through the overexpanded, perfectly-expanded and underexpanded flow regimes. The different components of supersonic jet noise (Mach-wave radiation, BBSAN and screech resonance) can be identified in the schlieren visualizations, together with the shock-cell structure associated with each flow regime. The snapshot included in this document corresponds to the underexpanded regime at 𝑀j = 1.9.

Jet-noise pollution is currently recognized as one of the major environmental challenges of the modern aerospace industry. The status of current technology calls for the development of novel noise mitigation and control strategies that can meet the envisioned long-term sustainable development goals. Supersonic twin-jet engines, often employed in launchers and high-speed aircraft, constitute a complex flow system whose aeroacoustic characterization and control is a state-of-the-art challenge. Large-scale coherent structures developing in the core and mixing layers of each jet are known to play a fundamental role in the generation of noise. Past studies in single jets have shown that such structures, also known as wavepackets, can be successfully modelled via linear stability theory. The existence of wavepackets in twin jets is expected by extension of the evidence gathered for single round jets, but experimental works aimed at their identification and characterization in twin-jet systems are scarce and mostly focused on the screech resonance phenomenon. For this reason, new experimental investigations are necessary to assess the validity of current wavepacket models for twin-jet configurations and to gain additional insight on twin-jet mixing noise and broadband shock-associated noise (BBSAN).
The schlieren visualization presented here was obtained in 2022 as part of an experimental campaign dedicated to study the mechanisms of noise generation in supersonic twin jets and validate state-of-the-art theoretical models. In particular, the data comes from measurements taken in the T200 compressible wind tunnel at the PROMETEE platform of Institut Pprime (France). The experimental setup consists of two identical round convergent-divergent nozzles with a nozzle exit diameter of 2.5 cm. The jets are unheated and subject to a fixed total temperature of 𝑇! = 300 K. Different schlieren recordings were taken at different operating conditions with a high-speed camera featuring a sampling frequency of 68 kHz. The animation presented here was obtained from a recording of the transient flow field occurring between the opening of the reservoir valve (no flow, 𝑀! = 0) up to the establishment of a 𝑀! = 1.9 flow condition, therefore permitting the visualization of the different flow regimes experienced by the system in a short period of time. These high-speed schlieren measurements have enabled the eduction of the large-scale coherent structures present in the twin-jet flow system by means of data driven techniques, and, for the case of perfectly-expanded conditions, their quantitative comparison against linear stability theory. In particular, it has allowed the extraction of the structure and properties of Kelvin-Helmholtz instabilities in the twin-jet flow, which manifest as coupled toroidal and flapping oscillations between each jet. For overexpanded conditions, the associated measurements have also allowed the characterization of the structure of the different wave components involved in the twin-jet screech resonance.

Related recent publications:

1. Padilla-Montero, I., Rodríguez, D., Jaunet, V., and Jordan, P. (2024). “Eduction of coherent structures from schlieren images of twin jets using SPOD informed with momentum potential theory in the spectral domain”. Theoretical and Computational Fluid Dynamics, 38, 375-401. DOI: 10.1007/s00162-024-00699-w
2. Padilla-Montero, I., Rodríguez, D., Jaunet, V., and Jordan, P. (2024). “Experimental validation of PSE wavepacket models for supersonic twin jets using schlieren imaging and potential-momentum-theory-based SPOD”. In: 30th AIAA/CEAS Aeroacoustics Conference, Rome, Italy, 4-7 June 2024 (AIAA 2024-3146). DOI: 10.2514/6.2024-3146
3. Padilla-Montero, I., Rodríguez, D., Jaunet, V., Girard, S., Eysseric, D., and Jordan, P. (2023). “Investigation of coherent motions and noise radiation in twin supersonic jets using high-speed schlieren images”. In: Astolfi, A., Asdrubali, F., Shtrepi, L. (eds.) 10th Convention of the European Acoustics Association. European Acoustics Association. DOI: 10.61782/fa.2023.0103

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