Special Issue "Under-Expanded Jets"

A special issue of Aerospace (ISSN 2226-4310).

Deadline for manuscript submissions: closed (30 April 2018)

Special Issue Editors

Guest Editor
Dr. Paul Bruce

Department of Aeronautics, Imperial College London, London SW7 2AZ, UK
Website | E-Mail
Interests: high-speed aerodynamics; unsteady compressible flow; experimental methods; transonic/supersonic flow control; experiment/computation comparison
Guest Editor
Dr. Arash Hamzehloo

Faculty of Engineering, Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, UK
Website | E-Mail
Interests: Gas Dynamics; Compressible Flows; Subsonic and Supersonic Jets; Shock Waves and Flow Discontinuities; Aerodynamics; Multiphase Flows; Cavitating and Flashing Flows; Reacting Flows and Combustion; Supercritical Conditions; Numerical Methods for CFD; Advanced Propulsion Systems

Special Issue Information

Dear Colleagues,

Fundamental understanding of the under-expansion process and under-expanded jets is highly beneficial to engineers and scientists of various fields, from aerospace engineering to geophysics. Under-expanded jets are complex high speed flows, which are formed in various engineering applications and devices such as exhaust plumes of aircrafts (rockets and missiles), supersonic combustors, actuators, ejectors and high pressure gaseous injectors. This type of jet can also be observed in geophysical systems (volcanic eruption) and in accidental release of hazardous gases (such as hydrogen) from tiny cracks in high pressure pipelines and reservoirs.

General characteristics of under-expanded jets in the aforementioned examples are similar, however different nozzle diameters, nozzle (hole) shapes, jet/ambient fluids and nozzle pressure ratios would result in significantly different flow behaviours. Although overall structures of under-expanded jets have been comprehensively identified, many specific characteristics and quantitative aspects are still to be determined. Furthermore, there are very limited quantitative data available for under-expanded jets issued form small-size and particularly millimetre-size nozzles. Therefore, this Special Issue aims to provide quantitative insights into the key sonic and mixing characteristics of under-expanded jets particularly those issued from millimetre-size nozzles.

Authors are encouraged to submit high quality manuscripts on analytical, computational (high fidelity modelling) and experimental (advanced quantitative measurement techniques) studies in the field of under-expanded jets. The topics of interest may include, but are not restricted to, free and impinging jets, co and cross-flows, near-nozzle shock structures, Mach disk dimensions and curvature, vortical and coherent structures and shear layers, turbulent mixing characteristics, aeroacoustics and screech tone, shock-shear interactions, viscous effects, farfield characteristics, effects of the ambient medium thermodynamic conditions, effects of the nozzle diameter and topology (lip geometry and exit profile), hydrodynamic instabilities, compressible in-nozzle flows, different jet/ambient fluids, computational modelling with real fluid equation of state and properties, reacting under-expanded jets, numerical methods, such as advanced shock capturing techniques.

Dr. Paul Bruce
Dr. Arash Hamzehloo
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Aerospace is an international peer-reviewed open access quarterly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 550 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Under-Expanded Jet
  • Under-Expansion
  • Nozzle
  • Mach Disk
  • Shock
  • Shock-Shear Interaction
  • Vortical Structures
  • Coherent Structures
  • Shear Layer
  • Impinging Jet
  • Cross Flow
  • Supersonic
  • Turbulent Mixing
  • Screech Tone
  • Aeroacoustics
  • Reacting Flow
  • Actuator
  • Ejector
  • Exhaust Plume
  • Aircraft
  • Rocket
  • Missile
  • Injector
  • Injection
  • Hydrogen

Published Papers (4 papers)

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Research

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Open AccessArticle Energy-Dynamics Resulting in Turbulent and Acoustic Phenomena in an Underexpanded Jet
Received: 11 April 2018 / Revised: 23 April 2018 / Accepted: 25 April 2018 / Published: 1 May 2018
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Abstract
Underexpanded jets exhibit interactions between turbulent shear layers and shock-cell trains that yield complex phenomena that are absent in the more commonly studied perfectly expanded jets. We quantitatively analyze these mechanisms by considering the interplay between hydrodynamic (turbulence) and acoustic modes, using a
[...] Read more.
Underexpanded jets exhibit interactions between turbulent shear layers and shock-cell trains that yield complex phenomena that are absent in the more commonly studied perfectly expanded jets. We quantitatively analyze these mechanisms by considering the interplay between hydrodynamic (turbulence) and acoustic modes, using a validated large-eddy simulation. Using momentum potential theory (MPT) to achieve energy segregation, the following observations are made. The sharp gradients in fluctuations introduced by the shock-cell structure are captured mostly in the hydrodynamic mode, whose amplitude is an order of magnitude larger than the acoustic mode. The acoustic mode has a relatively smoother distribution, exhibiting a compact wavepacket form. Proper orthogonal decomposition (POD) identifies the third-to-sixth cells as the most dynamic structures. The imprint of shock cells is discernible in the nearfield of the acoustic mode, primarily along the sideline direction. Energy interactions that feed the acoustic mode remain compact in nature, facilitating a simple propagation technique for farfield noise prediction. The farfield sound spectra show peak directivity at 30 to the downstream axis. The POD modes of the acoustic component also identify two main energetic components in the wavepacket: one representative of the periodic internal structure and the other of intermittent downstream lobes. The latter component occurs at exactly the same frequency as, and displays high correlation with, the farfield peak noise spectra, making the acoustic mode a better predictor of the dynamics than velocity fluctuations. Full article
(This article belongs to the Special Issue Under-Expanded Jets)
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Open AccessArticle Influence of Nozzle Exit Conditions on the Near-Field Development of High Subsonic and Underexpanded Axisymmetric Jets
Received: 19 February 2018 / Revised: 17 March 2018 / Accepted: 19 March 2018 / Published: 29 March 2018
Cited by 1 | PDF Full-text (59539 KB) | HTML Full-text | XML Full-text
Abstract
Detailed knowledge of jet plume development in the near-field (the first 10–15 nozzle exit diameters for a round jet) is important in aero-engine propulsion system design, e.g., for jet noise and plume infrared (IR) signature assessment. Nozzle exit Mach numbers are often high
[...] Read more.
Detailed knowledge of jet plume development in the near-field (the first 10–15 nozzle exit diameters for a round jet) is important in aero-engine propulsion system design, e.g., for jet noise and plume infrared (IR) signature assessment. Nozzle exit Mach numbers are often high subsonic but improperly expanded (e.g., shock-containing) plumes also occur; high Reynolds numbers (O (106)) are typical. The near-field is obviously influenced by nozzle exit conditions (velocity/turbulence profiles) so knowledge of exit boundary layer characteristics is desirable. Therefore, an experimental study was carried out to provide detailed data on nozzle inlet and exit conditions and near-field development for convergent round nozzles operated at Nozzle Pressure Ratios (NPRs) corresponding to high subsonic and supersonic (underexpanded) jet plumes. Both pneumatic probe and Laser Doppler Anemometry (LDA) measurements were made. The data revealed that internal nozzle acceleration led to a dramatic reduction in wall boundary layer thickness and a more laminar-like profile shape. The addition of a parallel wall extension to the end of the nozzle allowed the boundary layer to return to a turbulent state, increasing its thickness, and removing vena contracta effects. Differences in nozzle exit boundary layers exerted a noticeable influence but only in the first few diameters of plume development. The addition of the exit extension removed the vena contracta effects of the convergence only design. At underexpanded NPRs, this change to nozzle geometry modified the shock cell pattern and shortened the potential core length of the jet. Full article
(This article belongs to the Special Issue Under-Expanded Jets)
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Graphical abstract

Open AccessArticle Design of a Facility for Studying Shock-Cell Noise on Single and Coaxial Jets
Received: 12 January 2018 / Revised: 24 February 2018 / Accepted: 25 February 2018 / Published: 1 March 2018
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Abstract
Shock-cell noise occurs in aero-engines when the nozzle exhaust is supersonic and shock-cells are present in the jet. In commercial turbofan engines, at cruise, the secondary flow is often supersonic underexpanded, with the formation of annular shock-cells in the jet and consequent onset
[...] Read more.
Shock-cell noise occurs in aero-engines when the nozzle exhaust is supersonic and shock-cells are present in the jet. In commercial turbofan engines, at cruise, the secondary flow is often supersonic underexpanded, with the formation of annular shock-cells in the jet and consequent onset of shock-cell noise. This paper aims at describing the design process of the new facility FAST (Free jet AeroacouSTic laboratory) at the von Karman Institute, aimed at the investigation of the shock-cell noise phenomenon on a dual stream jet. The rig consists of a coaxial open jet, with supersonic capability for both the primary and secondary flow. A coaxial silencer was designed to suppress the spurious noise coming from the feeding lines. Computational fluid dynamics (CFD) simulations of the coaxial jet and acoustic simulations of the silencer have been carried out to support the design choices. Finally, the rig has been validated by performing experimental measurements on a supersonic single stream jet and comparing the results with the literature. Fine-scale PIV (Particle Image Velocimetry) coupled with a microphone array in the far field have been used in this scope. Preliminary results of the dual stream jet are also shown. Full article
(This article belongs to the Special Issue Under-Expanded Jets)
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Review

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Open AccessReview Shock-Wave Structure of Supersonic Jet Flows
Received: 28 April 2018 / Revised: 31 May 2018 / Accepted: 5 June 2018 / Published: 7 June 2018
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Abstract
In the present paper, we give a brief overview of the studies of supersonic jet flows which were performed recently with the aim of gaining experimental data on the formation of the shock-wave structure and jet mixing layer in such flows. Considerable attention
[...] Read more.
In the present paper, we give a brief overview of the studies of supersonic jet flows which were performed recently with the aim of gaining experimental data on the formation of the shock-wave structure and jet mixing layer in such flows. Considerable attention is paid to a detailed description of discharge conditions for supersonic jets, to enable the use of measured data for making a comparison with numerical calculations. Data on the 3D flow structure in the mixing layer of the initial length of a supersonic jet are reported. Scientific interest in this phenomenon is due to its practical significance in studying the possibility of intensifying the mixing process as well as in studying the sound-generation process. Full article
(This article belongs to the Special Issue Under-Expanded Jets)
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