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  • 1
    Electronic Resource
    Electronic Resource
    FLOW CONTROL WITH NONCIRCULAR JETS1 (1999)
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Fluid Mechanics 31 (1999), S. 239-272 
    Details
    ISSN: 0066-4189
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Noncircular jets have been the topic of extensive research in the last fifteen years. These jets were identified as an efficient technique of passive flow control that allows significant improvements of performance in various practical systems at a relatively low cost because noncircular jets rely solely on changes in the geometry of the nozzle. The applications of noncircular jets discussed in this review include improved large- and small-scale mixing in low- and high-speed flows, and enhanced combustor performance, by improving combustion efficiency, reducing combustion instabilities and undesired emissions. Additional applications include noise suppression, heat transfer, and thrust vector control (TVC). The flow patterns associated with noncircular jets involve mechanisms of vortex evolution and interaction, flow instabilities, and fine-scale turbulence augmentation. Stability theory identified the effects of initial momentum thickness distribution, aspect ratio, and radius of curvature on the initial flow evolution. Experiments revealed complex vortex evolution and interaction related to self-induction and interaction between azimuthal and axial vortices, which lead to axis switching in the mean flow field. Numerical simulations described the details and clarified mechanisms of vorticity dynamics and effects of heat release and reaction on noncircular jet behavior. The research on noncircular jets has also led to technology transfer. A topic that started as an academic curiosity-an interesting flow phenomenon-subsequently has had various industrial applications. The investigations reviewed include experimental, theoretical, numerical, and technological aspects of the subject.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1146/annurev.fluid.31.1.239
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  • 2
    Electronic Resource
    Electronic Resource
    Mixing Enhancement in Supersonic Free Shear Flows (1995)
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Fluid Mechanics 27 (1995), S. 375-417 
    Details
    ISSN: 0066-4189
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1146/annurev.fl.27.010195.002111
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  • 3
    Electronic Resource
    Electronic Resource
    Mode switching in supersonic circular jets (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 868-873 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An underexpanded supersonic circular jet was studied experimentally in a pressure ratio range of 2–15, yielding an equivalent Mach number range of 1.1–2.4 downstream of the nozzle. Spark Schlieren photography and near-field pressure measurements showed large changes in the jet structure for different Mach numbers. For nearly sonic exit velocity, the jet had a dominant symmetric structure and spreading rate similar to subsonic conditions. For an exit velocity equivalent to M〉1.2, the jet switched mode to a helical structure with an accompanying substantial increase in the spreading rate. A transitional region was determined for 1.12〈M〈1.18, in which the jet switched modes intermittently from axisymmetric to helical. For Mach numbers higher than 1.6, another structural change was observed, related to the emergence of a normal shock in the first shock cell. The structural changes affected the near-field pressure fluctuations of the jet. The jet with the helical structure had a strong pressure field component in the upstream direction that could be related to the higher spreading rate of the jet.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857384
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  • 4
    Electronic Resource
    Electronic Resource
    Streamwise and spanwise vortex interaction in an axisymmetric jet. A computational and experimental study (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 8 (1996), S. 1515-1524 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The near-field of an azimuthally excited round jet was investigated in a combined computational/experimental study. The reaction zones in the jet were visualized using OH Planar-Laser- Induced-Fluorescence (PLIF) diagnostics. Both axisymmetric and azimuthal modes of the jet were excited to stabilize its spatial structure. Three-dimensional flame visualization of the laboratory jet reconstructed from multiple two-dimensional images acquired at constant phase angle, reveal a complex structure of the reaction zone. Time-dependent numerical simulations provided insight into the underlying fluid-dynamical processes leading to this flame structure. Simulations of reactive and non-reactive free jets used a Monotonically Integrated Large-Eddy-Simulation (MILES) approach, multi-species diffusive transport, global finite-rate chemistry and appropriate inflow/outflow boundary conditions. The flow visualizations of the experimental and computational jets strongly resemble each other, revealing tight coupling between axisymmetric vortex rings and braid (rib) vortices. The jet vorticity evolution is dominated by the dynamics of vortex-ring self-deformation induced by the azimuthal excitation imposed at the jet exit, the dynamics of rib vortices forming in the braid regions between undulating vortex rings, and strong interactions between rings and ribs. The observed topological features of the flow are directly related to the nearly-inviscid jet vorticity dynamics. These processes affect the mixing pattern of the jet, resulting in localized regions of high fuel concentration leading to combustion inactive regions in the flame, and other regions with enhanced mixing and a proper air-to-fuel ratio in the flame where the combustion process is intense. The vorticity dynamics and ensuing mixing processes determine the regions of combustion within the flame and thus the overall heat release pattern. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868927
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  • 5
    Electronic Resource
    Electronic Resource
    Effect of convective Mach number on mixing of coaxial circular and rectangular jets (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 29-36 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Rectangular supersonic free and coaxial jets were used to enhance mixing relative to a circular jet in a convective Mach number range of 0.5 to 2.2. The different convective Mach numbers were obtained by changing the central jet gas composition, the temperatures of the inner and outer flows, and the velocity of the coaxial flow. The experimental techniques used were schlieren photography, total pressure, and gas-sampling measurements. For all test conditions the rectangular jets showed substantial improved mixing relative to a circular jet. The free jets showed high mixing in the circumferential region of the jet while the coaxial jet had a high mixing rate inside the central jet.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857860
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  • 6
    Electronic Resource
    Electronic Resource
    Near field dynamics of subsonic free square jets. A computational and experimental study (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 7 (1995), S. 1483-1497 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Results of a combined numerical and experimental investigation of the near field of low-subsonic air square jets are presented. The study focuses on examining the role of initial conditions and other features of the jet dynamics in determining the nature and frequency of occurrence of axis switching and the related mechanisms which enhance entrainment, mixing, and turbulence production. Three different experimental square jet facilities were utilized, including orifice jets with low and high initial turbulence level, and pipe jets. Unsteady, spatially developing jets were investigated computationally using direct and monotonically-integrated large-eddy simulation approaches, and appropriate inflow/outflow boundary conditions. Insight on the axis-switching process was obtained using the detailed database from the simulations to investigate how the unsteady vorticity dynamics reflects on the time-averaged properties of the jet cross sections. The different experimental jets were chosen such that important parameters affecting the initial conditions could be tested. Depending on the particular initial conditions of the subsonic jets studied, several or no axis switchings were observed in the first few diameters of jet development. Observed trends reported, include the effects of initial conditions such as, ratio of equivalent diameter to characteristic-momentum-thickness, turbulence level, nonuniform azimuthal momentum-thickness distributions, and Reynolds number. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868534
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  • 7
    Electronic Resource
    Electronic Resource
    Near-field pressure radiation and flow characteristics in low supersonic circular and elliptic jets (1988)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 31 (1988), S. 2524-2532 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The near-field pressure fluctuations of circular and elliptic underexpanded supersonic jets were studied experimentally. Unlike the case of low subsonic jets, the pressure fluctuation characteristics at the minor axis plane of the elliptic jet were very different from those of the major axis plane. The amplitude of the pressure fluctuations at the minor axis was more than an order of magnitude higher than at the other plane. This section of the jet was also characterized by a larger spreading rate and higher amplification rate of the velocity fluctuations. The circular jet was similar to the major axis plane of the elliptic jet. The spectra of the near-field pressure fluctuations of both jets exhibited the highest peak at a frequency corresponding to the jets' preferred mode frequency. The spectral peak related to the screech tone was much stronger at the minor axis plane and had the same frequency as at the other plane. The amplitude of the dominant pressure fluctuation frequencies was mapped in the entire near field, and each one was found to be dominant in a different region.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.867011
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  • 8
    Electronic Resource
    Electronic Resource
    Flow and acoustic features of a supersonic tapered nozzle (1992)
    Springer
    Experiments in fluids 13 (1992), S. 49-55 
    Details
    ISSN: 1432-1114
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The acoustic and flow characteristics of a supersonic tapered jet were measured for free and shrouded flow configurations. Measurements were performed for a full range of pressure ratios including over- and underexpanded and design conditions. The supersonic tapered jet is issued from a converging-diverging nozzle with a 3∶1 rectangular slotted throat and a conical diverging section leading to a circular exit. The jet was compared to circular and rectangular supersonic jets operating at identical conditions. The distinct feature of the jet is the absence of screech tones in the entire range of operation. Its near-field pressure fluctuations have a wide band spectrum in the entire range of measurements, for Mach numbers of 1 to 2.5, for over- and underexpanded conditions. The free jet's spreading rate is nearly constant and similar to the rectangular jet, and in a shroud, the pressure drop it is inducing is linearly proportional to the primary jet Mach number. This behavior persisted in high adverse pressure gradients at overexpanded conditions, and with nozzle divergence angles of up to 35°, no inside flow separation was observed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00208075
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  • 9
    Electronic Resource
    Electronic Resource
    Selective control of flow coherence in triangular jets (2004)
    Springer
    Experiments in fluids 6 (2004), S. 129-135 
    Details
    ISSN: 1432-1114
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The triangular jet was investigated for use as a passive device to enhance fine-scale mixing and to reduce the coherence of large-scale structures in the flow. The suppression of the structures is vital to the enhancement of molecular mixing, which is important for efficient chemical reactions including combustion. The sharp corners in the jet injector introduced high instability modes into the flow via the non-symmetric mean velocity and pressure distribution around the nozzle. Both aerodynamic and hydrodynamic flows showed the difference between the flow at the corner (vertex) and at the flat side. While highly coherent structures could be generated at the flat side, the corner flow was dominated by highly turbulent small-scale eddies. The flow characteristics were tested using hotwire anemometry for mean flow and turbulence analysis, and flow visualization in air and water.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00196464
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  • 10
    Electronic Resource
    Electronic Resource
    Coherent and random structure in reacting jets (1990)
    Springer
    Experiments in fluids 10 (1990), S. 147-156 
    Details
    ISSN: 1432-1114
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Combustion rate and stability are strongly related to the interaction of fluid dynamics with the combustion process in reactive flow systems. The ability to control the combustion depends on the understanding of this interaction. This paper describes the use of the Planar Laser Induced Fluorescence (PLIF) technique to get insight into these processes, by looking at the coherent and random structures of a combusting jet. The coherent structures educed by phase-locked averaging are significantly different from instantaneous pictures, which are highly three dimensional. The PLIF images clarify the relationship between the large-scale structures and the generation of small-scale turbulence.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00215023
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