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Aerospace
Special Issues
Under-Expanded Jets
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Under-Expanded Jets

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

Deadline for manuscript submissions: closed (30 April 2018) | Viewed by 48602

Special Issue Editors

Dr. Paul Bruce

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

E-Mail Website
Guest Editor
Faculty of Engineering, Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, UK
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 submissions that pass pre-check are 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 monthly 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 2400 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

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Published Papers (6 papers)

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Research

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28 pages, 16984 KiB  
Article
Effects of Nozzle Pressure Ratio and Nozzle-to-Plate Distance to Flowfield Characteristics of an Under-Expanded Jet Impinging on a Flat Surface
by Duy Thien Nguyen, Blake Maher and Yassin Hassan
Aerospace 2019, 6(1), 4; https://doi.org/10.3390/aerospace6010004 - 6 Jan 2019
Cited by 27 | Viewed by 9155
Abstract
The current work experimentally investigates the flowfield characteristics of an under-expanded turbulent jet impinging on a solid surface for various nozzle-to-plate distances 2.46 D j , 1.64 D j , and 0.82 D j ( D j is the jet hydraulic diameter), and [...] Read more.
The current work experimentally investigates the flowfield characteristics of an under-expanded turbulent jet impinging on a solid surface for various nozzle-to-plate distances 2.46 D j , 1.64 D j , and 0.82 D j ( D j is the jet hydraulic diameter), and nozzle pressure ratios (NPRs) ranging from 2 to 2.77 . Planar particle image velocimetry (PIV) measurements were performed in the central plane of the test nozzle and near the impingement surface. From the obtained PIV velocity vector fields, flow characteristics of under-expanded impinging jets, such as mean velocity, root-mean-square fluctuating velocity, and Reynolds stress profiles, were computed. Comparisons of statistical profiles obtained from PIV velocity measurements were performed to study the effects of the impingement surface, nozzle-to-plate distances, and NPRs to the flow patterns. Finally, proper orthogonal decomposition (POD) analysis was applied to the velocity snapshots to reveal the statistically dominant flow structures in the impinging jet regions. Full article
(This article belongs to the Special Issue Under-Expanded Jets)
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16 pages, 6238 KiB  
Article
Analysis of Coherent Structures in an Under-Expanded Supersonic Impinging Jet Using Spectral Proper Orthogonal Decomposition (SPOD)
by Shahram Karami and Julio Soria
Aerospace 2018, 5(3), 73; https://doi.org/10.3390/aerospace5030073 - 6 Jul 2018
Cited by 34 | Viewed by 6599
Abstract
The spatiotemporal dynamics of the coherent structures in an under-expanded supersonic impinging jet are studied using a spectral proper orthogonal decomposition technique. For this analysis, a large eddy simulation of an under-expanded supersonic impinging jet at a pressure ratio of 3.4 and a [...] Read more.
The spatiotemporal dynamics of the coherent structures in an under-expanded supersonic impinging jet are studied using a spectral proper orthogonal decomposition technique. For this analysis, a large eddy simulation of an under-expanded supersonic impinging jet at a pressure ratio of 3.4 and a stand-off distance of 2 jet diameters at a Reynolds number of 50,000 is performed. The mean flow fields illustrate some striking features of this flow, such as an oblique shock, a stand-off shock, a Mach disk, and a recirculation bubble. The spectral proper orthogonal decomposition method is applied to time-resolved three-dimensional flow fields. The accumulative energy of modes within each azimuthal mode number reveals that the first three azimuthal modes contain most of the energy of the flow. The spectra of these azimuthal modes show that the flow exhibits a low-ranked behaviour with discrete frequencies at the optimal symmetric azimuthal mode while other two azimuthal modes have negligible contributions in this behaviour. Three peaks are observed in the spectra of the optimal symmetric azimuthal mode. The spatial fields of the streamwise velocity and pressure of these peaks show that the complex structures are consequences of the under-expansion, Mach disk, and the impingement. Strong hydrodynamic instabilities exist in the shear layer of the jet in the optimal azimuthal mode at each of these dominant frequencies. High-amplitude acoustic waves are also present in the near-field of the jet. These acoustic waves are strong at the nozzle lip, suggesting that a feedback loop linking these two processes exists for dominant frequencies in the optimal mode. High cross-spectrum density of near-field pressure fluctuations and streamwise velocity fluctuations near the nozzle lip at these frequencies confirms the hydro-acoustic coupling, which is necessary to close the feedback loop. Full article
(This article belongs to the Special Issue Under-Expanded Jets)
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26 pages, 4793 KiB  
Article
Energy-Dynamics Resulting in Turbulent and Acoustic Phenomena in an Underexpanded Jet
by Unnikrishnan Sasidharan Nair, Kalyan Goparaju and Datta Gaitonde
Aerospace 2018, 5(2), 49; https://doi.org/10.3390/aerospace5020049 - 1 May 2018
Cited by 11 | Viewed by 6231
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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26 pages, 59539 KiB  
Article
Influence of Nozzle Exit Conditions on the Near-Field Development of High Subsonic and Underexpanded Axisymmetric Jets
by Miles T. Trumper, Parviz Behrouzi and James J. McGuirk
Aerospace 2018, 5(2), 35; https://doi.org/10.3390/aerospace5020035 - 29 Mar 2018
Cited by 12 | Viewed by 7751
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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29 pages, 21169 KiB  
Article
Design of a Facility for Studying Shock-Cell Noise on Single and Coaxial Jets
by Daniel Guariglia, Alejandro Rubio Carpio and Christophe Schram
Aerospace 2018, 5(1), 25; https://doi.org/10.3390/aerospace5010025 - 1 Mar 2018
Cited by 6 | Viewed by 6787
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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18 pages, 6963 KiB  
Review
Shock-Wave Structure of Supersonic Jet Flows
by Valery Zapryagaev, Nikolay Kiselev and Dmitry Gubanov
Aerospace 2018, 5(2), 60; https://doi.org/10.3390/aerospace5020060 - 7 Jun 2018
Cited by 25 | Viewed by 9892
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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