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  • 1
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    Coherent structures - Reality and myth (1983)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: Large-scale coherent structures (CS) in turbulent shear flows are characterized, reviewing recent theoretical and experimental investigations. The use of computers as a research tool and the flow-visualization experimental technique are introduced, CS are defined, the history of their discovery is traced, and their main characteristics are listed. Topics discussed and illustrated include the initial condition of the free shear layer, triple and double decomposition, topological features of CS, detection and eduction of CS, phase alignment via cross correlation, induced versus natural structures, the bursting phenomenon, turbulent spot, streaks, bursting frequency, the axisymmetric mixing layer, vortex pairing in an axisymmetric jet, CS and jet noise, broadband noise amplification via pure-tone excitation, CS interaction in a plane-jet near field, the Taylor hypothesis applied to CS, negative production, and the validity of the Reynolds-number similarity hypothesis. It is found that the coherent Reynolds stress, vorticity, and production are not much greater than the time-averaged values for fully developed flows with significant incoherent turbulence, suggesting that the importance of CS may have been exaggerated in some recent studies.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: Physics of Fluids (ISSN 0031-9171); 26; 2816-285
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  • 2
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    Role of coherent structures in turbulent shear flows (1981)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: A definition for the large-scale coherent structure is presented, and the nature and role of coherent structures in turbulent shear flows are examined. The equations governing the coherent motions and the experimental considerations as well as constraints in the investigations of coherent structures in wall-bounded and free turbulent shear flows are discussed. Results from a few of our recent and on-going studies of coherent structures in excited and unexcited free turbulent shear flows are reviewed. These results show that coherent structures are dominant in transport in the early stages of their formation, but not in the self-preserving regions of turbulent shear flows.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: vol. 4; Aug. 198
    Format: text
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  • 3
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    Evaluation of the SPAR thermal analyzer on the CYBER-203 computer (1982)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: The use of the CYBER 203 vector computer for thermal analysis is investigated. Strengths of the CYBER 203 include the ability to perform, in vector mode using a 64 bit word, 50 million floating point operations per second (MFLOPS) for addition and subtraction, 25 MFLOPS for multiplication and 12.5 MFLOPS for division. The speed of scalar operation is comparable to that of a CDC 7600 and is some 2 to 3 times faster than Langley's CYBER 175s. The CYBER 203 has 1,048,576 64-bit words of real memory with an 80 nanosecond (nsec) access time. Memory is bit addressable and provides single error correction, double error detection (SECDED) capability. The virtual memory capability handles data in either 512 or 65,536 word pages. The machine has 256 registers with a 40 nsec access time. The weaknesses of the CYBER 203 include the amount of vector operation overhead and some data storage limitations. In vector operations there is a considerable amount of time before a single result is produced so that vector calculation speed is slower than scalar operation for short vectors.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: Computational Aspects of Heat Transfer in Struct.; p 405-424
    Format: application/pdf
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  • 4
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    The computation of complex jet and nozzle flows (1980)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: APL JANNAF 12th Plume Technol. Meeting, Vol. 1; p 57-84
    Format: text
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  • 5
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    The 'preferred mode' of the axisymmetric jet (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Detailed distributions of different time-average and phase-average flow properties for an axisymmetric free jet under controlled perturbation at the jet preferred mode are explored. The data are compared with the corresponding unexcited jet data. It is found that the effect of the perturbation is to increase jet spread and mean velocity decay, as well as to increase the peak values of the time-average fluctuation intensities and Reynolds stress in the axisymmetric mixing layer.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: Journal of Fluid Mechanics; 110; Sept
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  • 6
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    Turbulence suppression in free shear flows by controlled excitation (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: 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 0.017 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.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: Journal of Fluid Mechanics; 103; Feb. 198
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  • 7
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    Vortex pairing in a circular jet under controlled excitation. I - General jet response. II - Coherent structure dynamics (1980)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: 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 0.012 and 0.85, 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.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: Journal of Fluid Mechanics; 101; Dec. 11
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  • 8
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    The dominant coherent structure of the circular jet organized by controlled perturbation (1982)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The dominant large-scale coherent structure of a circular jet has been organized through controlled perturbation at the jet Strouhal number of 0.3, induced acoustically in two air jets through cavity resonance. Selected phases of the coherent structure were studied for three Reynolds numbers (25,000; 55,000; and 110,000), and two limiting initial conditions; laminar and fully turbulent initial boundary layers. Distributions of phase-average structure properties, i.e., longitudinal and lateral velocities, coherent vorticity and Reynolds stress, phase-random turbulence intensities and Reynolds stress, and streamline and pseudo-streamfunction patterns are obtained. The phase-random turbulence intensity is greatest at the structure center where the coherent vorticity is also the maximum. Some properties of the structure show mild but systematic dependence on Reynolds number.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
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  • 9
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    The 'whistler-nozzle' phenomenon (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The whistler nozzle is a simple device which can induce jet self-excitations of controllable amplitudes and frequencies and appears highly promising for many applications involving turbulent transport, combustion and aerodynamic noise. The characteristics of this curious phenomenon are documented for different values of the controlling parameters and attempts to explain the phenomenon. It is shown that the whistler excitation results from the coupling of two independent resonance mechanisms: shear-layer tone resulting from the impingement of the pipe-exit shear layer on the collar lip, and organ-pipe resonance of the pipe-nozzle. The crucial role of the shear-layer tone in driving the organ-pipe resonance is proven by reproducing the event in pipe-ring and pipe-hole configurations in the absence of the collar. It is also shown that this phenomenon is the strongest when the self-excitation frequency matches the preferred mode of the jet. Previously announced in STAR as N83-20706
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: Journal of Fluid Mechanics (ISSN 0022-1120); 134; Sept
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  • 10
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    The coupling between scales in shear flows (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The relationship between large and small-scale motions in turbulent shear flows is studied using short-time correlation measurements. Boundary layers, plane and axisymmetric mixing layers, plane wakes, and the far fields of plane and circular jets are examined. The coupling is found to be significant in all flows. Phase reversal across the shear region is found to occur in the boundary layers and the mixing layers only. This is interpreted to mean that in these layers the streamwise extent of the large structure over which the small-scale activity reaches a peak moves from one transverse end of the large structure over which its ensemble-averaged u-fluctuation is positive to another where it is negative. In the far field of jets and wakes, the small-scale peak activity remains confined to that half of the streamwise extent of the large structure where its ensemble-averaged u-fluctuation is positive.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: Physics of Fluids (ISSN 0031-9171); 27; 2221-222
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