The role of screech tones in mixing of an underexpanded rectangular jet
TL;DR: In this article, Schlieren photographs of the flow field along with hot-wire measurements in the jet were obtained at different pressure ratios and it was shown that the self-excitation helps to induce large scale vortical motions in a converging rectangular nozzle.
About: This article is published in Journal of Sound and Vibration.The article was published on 1986-04-08. It has received 164 citations till now. The article focuses on the topics: Jet (fluid) & Nozzle.
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TL;DR: Noncircular jets have been 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 jet rely solely on changes in the geometry of the nozzle as discussed by the authors.
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 selfinduction 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.
537 citations
TL;DR: In this article, the mixing augmentation methods employed efficiently in sub- sonic flows failed to work at elevated Mach numbers, and some were inefficient because they were utilized outside their effective range.
Abstract: Recent interest in supersonic combustion (scramjets) and noise reduction for the high speed civil transport (HSCT) plane prompted renewed research in supersonic mixing processes and means to control them. The scramjet propulsion concept requires rapid mixing between fuel and air in order to minimize the size of the combustor and affect the performance of the entire vehicle system. Also, accelerated mixing of exhaust plumes with coflowing air has been shown to lead to jet noise reduction. Other examples of technological applications requiring control of mixing in compressible flows include thrust augmenting ejectors, thrust vector control, metal deposition, and gas dynamic lasers. The technological challenge of mixing enhancement in compressible flows stems from the inherently low growth rates of supersonic shear layers. Many mixing augmentation methods employed efficiently in sub sonic flows failed to work at elevated Mach numbers, and some were inefficient because they were utilized outside their effective range. Never-
316 citations
TL;DR: In this paper, the spreading characteristics of several asymmetric nozzles, and a set of rectangular orifices are compared, covering a jet Mach number range of 0.3-2.0.
Abstract: The spreading characteristics of jets from several asymmetric nozzles, and a set of rectangular orifices are compared, covering a jet Mach number range of 0.3-2.0. The effect of 'tabs' for a rectangular and a round nozzle is also included in the comparison. Compared to a round jet, the jets from the asymmetric nozzles spread only slightly more at subsonic conditions whereas at supersonic conditions, when 'screech' occurs, they spread much more. The dynamics of the azimuthal vortical structures of the jet, organized and intensified under the screeching condition, are thought to be responsible for the observed effect at supersonic conditions. Curiously, the jet from a 'lobed' nozzle spreads much less at supersonic condition compared to all other cases; this is due to the absence of screech with this nozzle. Screech stages inducing flapping, rather than varicose or helical, flow oscillation cause a more pronounced jet spreading. At subsonic conditions, only a slight increase in jet spreading with the asymmetric nozzles contrasts previous observations by others. The present results show that the spreading of most asymmetric jets is not much different from that of a round jet. This inference is further supported by data from the rectangular orifices. In fact, jets from the orifices with small aspect ratio (AR) exhibit virtually no increase in the spreading. A noticeable increase commences only when AR is larger than about 10. Thus, 'shear layer perimeter stretching', achieved with a larger AR for a given cross-sectional area of the orifice, by itself, proves to be a relatively inefficient mechanism for increasing jet spreading. In contrast, the presence of streamwise vortices or 'natural excitation' can cause a significant increase - effects that might explain the observations in the previous investigations. Thus far, the biggest increase in jet spreading is observed with the tabs. This is true in the subsonic regime, as well as in the supersonic regime, in spite of the fact that screech is eliminated by the tabs. The characteristic spreading of the tabbed jets is explained by the induced motion of the tab-generated streamwise vortex pairs. The tabs, however, incur thrust loss; the flow blockage and loss in thrust coefficient, vis-a-vis the spreading increase, are evaluated for various configurations.
293 citations
Cites background from "The role of screech tones in mixing..."
...This was further invesigated by Sherman, Glass & Duleep (1976), and, for a rectangular jet, by Krothapalli et al. (1986), leading to similar conclusions that jet spreading increased in the presence of screech....
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TL;DR: A detailed overview of the state of the art in the field of supersonic jet screech can be found in this paper, including a historical perspective, a summary of recent developments and a critical assessment of the current state-of-the-art.
263 citations
TL;DR: In this article, the authors developed a linear shock cell model for non-axisymmetric supersonic jets from convergent-divergent nozzles operating at off-design conditions, where the mixing layer of the jet is approximated by a vortex sheet.
166 citations
References
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TL;DR: In this article, Spark shadow pictures and measurements of density fluctuations suggest that turbulent mixing and entrainment is a process of entanglement on the scale of the large structures; some statistical properties of the latter are used to obtain an estimate of entrainedment rates, and large changes of the density ratio across the mixing layer were found to have a relatively small effect on the spreading angle.
Abstract: Plane turbulent mixing between two streams of different gases (especially nitrogen and helium) was studied in a novel apparatus Spark shadow pictures showed that, for all ratios of densities in the two streams, the mixing layer is dominated by large coherent structures High-speed movies showed that these convect at nearly constant speed, and increase their size and spacing discontinuously by amalgamation with neighbouring ones The pictures and measurements of density fluctuations suggest that turbulent mixing and entrainment is a process of entanglement on the scale of the large structures; some statistical properties of the latter are used to obtain an estimate of entrainment rates Large changes of the density ratio across the mixing layer were found to have a relatively small effect on the spreading angle; it is concluded that the strong effects, which are observed when one stream is supersonic, are due to compressibility effects, not density effects, as has been generally supposed
3,339 citations
TL;DR: In this article, hot-wire measurements in a plane incompressible jet are reported, where the flow was found to be self-preserving beyond x/d > 40 and measurements were made up to x/D = 120.
Abstract: Results of hot-wire measurements in a plane incompressible jet are reported. The flow was found to be self-preserving beyond x/d > 40 and measurements were made up to x/d = 120. The quantities measured include mean velocities, turbulence intensities and third- and fourth-order terms, as well as two-point correlations and the intermittency factor. Conditional sampling techniques were used to obtain exclusively data within the turbulent zone of the jet. The results are compared with previous investigations.This is the third paper in a sequence providing data on turbulent free shear flows.
515 citations
TL;DR: In this article, the conditions most favorable to vortex parting were determined as a function of the excitation Strouhal number, the Reynolds number, and the initial shear-layer state.
Abstract: Hot-wire and flow visualization studies were performed in three air jets subjected to pure-tone excitation The instability, vortex roll-up, and transition to the controlled excitation were investigated The conditions most favorable to vortex parting were determined as a function of the excitation Strouhal number, the Reynolds number, and the initial shear-layer state; it was shown that the rolled-up vortex rings undergo pairing under 'the shear layer mode', and the 'jet-column mode' when the Strouhal numbers based on the initial shear-layer momentum thickness are 0012 and 085, respectively Coherent ring-like vortical structures could be educed to the end of the potential core; however, the paired vortex becomes weaker with increasing downstream distances The transverse transport of 'u' momentum by the coherent structures was much larger during the pairing process than in regions where a single vortex is studied
469 citations
TL;DR: In this article, a blowdown-type air supply system was used to provide the airflow to a cylindrical settling chamber 1.75 m in length and 0.6 m in diameter.
Abstract: Hot-wire measurements in an incompressible rectangular jet, issuing into a quiet environment at ambient conditions, are presented. A blow-down-type air supply system was used to provide the airflow to a cylindrical settling chamber 1.75 m in length and 0.6 m in diameter. The measurements were made with constant-temperature anemometers in conjunction with linearizers. The two signals from the linearizers were sent through a sum and difference unit which was calibrated from dc to 100 kHz. The distributions of mean velocity and the turbulence shear stresses were measured in the two central planes of the jet stations up to 115 widths downstream of the nozzle exit. Three distinct regions characterized the jet flow field: a potential core origin, a two-dimensional-type region, and an axisymmetric type region. The onset of the second region appeared to be at a location where the shear layers separated by the short dimension of the nozzle meet; and the third region occurred at a downstream location where the two shear layers from the short edges of the nozzle meet. In the central plane, similarity was found both in the mean velocity and shear stress profiles beyond 30 widths downstream of the nozzle exit; profiles of rms velocity showed similarity in the second, but not the third region.
348 citations
TL;DR: In this paper, Turulence suppression in the near field of a free shear flow under controlled excitation is investigated in four circular jets, a plane jet, and a plane mixing layer, and the most pronounced suppression occurs when the shear layer is excited at a frequency 40% higher than the natural roll-up frequency.
Abstract: Turbulence suppression in the near field of a free shear flow under controlled excitation is investigated in four circular jets, a plane jet, and a plane mixing layer The suppression is a consequence of an excitation-induced modification of the shear layer structure and occurs at the excitation frequency corresponding to the maximally unstable disturbance frequency of the initial free shear layer The most pronounced suppression occurs when the shear layer is excited at a frequency 40% higher than the natural roll-up frequency Excitation at a Strouhal number of about 0017 produces a rapid roll-up and early breakdown of the shear layer, and thus inhibits the formation of the energetic large-scale vortices which otherwise survive farther downstream, grow to larger sizes, and undergo successive pairings in the corresponding unexcited flow
276 citations