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  • FLUID MECHANICS AND HEAT TRANSFER  (15)
  • data partitioning  (1)
  • 1
    Electronic Resource
    Electronic Resource
    Hybrid spectral finite difference simulations of stratified turbulent flows on distributed memory architectures (1997)
    Chichester : Wiley-Blackwell
    International Journal for Numerical Methods in Fluids 24 (1997), S. 1129-1158 
    Details
    ISSN: 0271-2091
    Keywords: spectral finite difference ; direct numerical simulation ; message-passing computers ; data partitioning ; fractional step methods ; Engineering ; Numerical Methods and Modeling
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: A method for efficient implementation of a combined spectral finite difference algorithm for computation of incompressible stratified turbulent flows on distributed memory computers is presented. The solution technique is the fractional step method with a semi-implicit time advancement scheme. A single-programme multiple-data abstraction is used in conjunction with a static data-partitioning scheme. The distributed FFTs required in the explicit step are based on the transpose method and the large sets of independent tridiagonal systems of equations arising in the implicit steps are solved using the pipelined Thomas algorithm. A speed-up analysis of a model problem is presented for three partitioning schemes, namely unipartition, multipartition and transpose partition. It is shown that the unipartitioning scheme is best suited for this algorithm. Performance measurements of the overall as well as individual stages of the algorithm are presented for several different grids and are discussed in the context of associated dependency and communication overheads. An unscaled speed-up efficiency of up to 91% on doubling the number of processors and up to 60% on an eightfold increase in the number of processors was obtained on the Intel Paragon and iPSC/860 Hypercube. Absolute performance of the code was evaluated by comparisons with performance on the Cray-YMP. On 128 Paragon processors, performance up to five times that of a single-processor Cray-YMP was obtained. The validation of the method and results of grid refinement studies in stably stratified turbulent channel flows are presented. © 1997 John Wiley & Sons, Ltd.
    Additional Material: 20 Ill.
    Type of Medium: Electronic Resource
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  • 2
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    Model consistency in large eddy simulation of turbulent channel flows (1988)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Combinations of filters and subgrid scale stress models for large eddy simulation of the Navier-Stokes equations are examined by a priori tests and numerical simulations. The structure of the subgrid scales is found to depend strongly on the type of filter used, and consistency between model and filter is essential to ensure accurate results. The implementation of consistent combinations of filter and model gives more accurate turbulence statistics than those obtained in previous investigations in which the models were chosen independently from the filter. Results and limitations of the a priori test are discussed. The effect of grid refinement is also examined.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: Physics of Fluids (ISSN 0031-9171); 31; 1884-189
    Format: text
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  • 3
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    Acoustics and dynamics of coaxial interacting vortex rings (1988)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Using a contour dynamics method for inviscid axisymmetric flow we examine the effects of core deformation on the dynamics and acoustic signatures of coaxial interacting vortex rings. Both 'passage' and 'collision' (head-on) interactions are studied for initially identical vortices. Good correspondence with experiments is obtained. A simple model which retains only the elliptic degree of freedom in the core shape is used to explain some of the calculated features.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: Fluid Dynamics Research (ISSN 0169-5983); 3; 1-4,
    Format: text
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  • 4
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    Numerical simulation: Its contributions to turbulence modeling (1992)
    In:  CASI
    Details
    Publication Date: 2013-08-29
    Description: The relative advantages and disadvantages of Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) are discussed. The topics are presented in viewgraph form.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: NASA. Lewis Research Center, Workshop on Engineering Turbulence Modeling; p 487-521
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  • 5
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    Dynamics of a class of vortex rings (1989)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The contour dynamics method is extended to vortex rings with vorticity varying linearly from the symmetry axis. An elliptic core model is also developed to explain some of the basic physics. Passage and collisions of two identical rings are studied focusing on core deformation, sound generation and stirring of fluid elements. With respect to core deformation, not only the strain rate but how rapidly it varies is important and accounts for greater susceptibility to vortex tearing than in two dimensions. For slow strain, as a passage interaction is completed and the strain relaxes, the cores return to their original shape while permanent deformations remain for rapidly varying strain. For collisions, if the strain changes slowly the core shapes migrate through a known family of two-dimensional steady vortex pairs up to the limiting member of the family. Thereafter energy conservation does not allow the cores to maintain a constant shape. For rapidly varying strain, core deformation is severe and a head-tail structure in good agreement with experiments is formed. With respect to sound generation, good agreement with the measured acoustic signal for colliding rings is obtained and a feature previously thought to be due to viscous effects is shown to be an effect of inviscid core deformation alone. For passage interactions, a component of high frequency is present. Evidence for the importance of this noise source in jet noise spectra is provided. Finally, processes of fluid engulfment and rejection for an unsteady vortex ring are studied using the stable and unstable manifolds. The unstable manifold shows excellent agreement with flow visualization experiments for leapfrogging rings suggesting that it may be a good tool for numerical flow visualization in other time periodic flows.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: NASA-TM-102257 , A-90016 , NAS 1.15:102257
    Format: application/pdf
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  • 6
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    Large-eddy simulation of turbulent flow with a surface-mounted two-dimensional obstacle (1993)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: In this paper, we perform a large eddy simulation (LES) of turbulent flow in a channel containing a two-dimensional obstacle on one wall using a dynamic subgrid-scale model (DSGSM) at Re = 3210, based on bulk velocity above the obstacle and obstacle height; the wall layers are fully resolved. The low Re enables us to perform a DNS (Case 1) against which to validate the LES results. The LES with the DSGSM is designated Case 2. In addition, an LES with the conventional fixed model constant (Case 3) is conducted to allow identification of improvements due to the DSGSM. We also include LES at Re = 82,000 (Case 4) using conventional Smagorinsky subgrid-scale model and a wall-layer model. The results will be compared with the experiment of Dimaczek et al.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: Annual Research Briefs, 1992; p 97-111
    Format: application/pdf
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  • 7
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    Large eddy simulation of the flow in a transpired channel (1989)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The flow in a transpired channel has been computed by large eddy simulation. The numerical results compare very well with experimental data. Blowing decreases the wall shear stress and enhances turbulent fluctuations, while suction has the opposite effect. The wall layer thickness normalized by the local wall shear velocity and kinematic viscosity increases on the blowing side of the channel and decreases on the suction side. Suction causes more rapid decay of the spectra, larger mean streak spacing and higher two-point correlations. On the blowing side, the wall layer structures lie at a steeper angle to the wall, whereas on the suction side this angle is shallower.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: AIAA PAPER 89-0375
    Format: text
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  • 8
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    Model consistency in the large eddy simulation of turbulent channel flows (1987)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Various combinations of filters and subgrid scale stress models for large eddy simulation of the Navier-Stokes equations are studied by a priori tests and numerical simulations. Consistency between model and filter is found to be essential to ensure accurate results. Results and limitations of the a priori test are discussed. The effect of grid refinement is also examined.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: AIAA PAPER 87-1446
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  • 9
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    Local intermodal energy transfer of the secondary instability in a plane channel (1987)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: A mathematical technique for analyzing local energy-transfer rates among wave-vector triads is developed and applied to the data generated by direct numerical simulations of waves with various types of initial conditions. Starting the simulation with a primary two-dimensional wave and random noise produced structures very similar to those which evolved from three-dimensional center modes. The local transfer rates determined in this case help to explain the eventual deformation of the primary two-dimensional wave which was observed in wind-tunnel experiments. Including weak streamwise vortices in the initial flow field results in large amounts of energy being transferred to the K-type modes early in the simulation. The later development of the waves (and hence the energy-transfer rates) is similar to the previous case.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: AIAA PAPER 87-1202
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  • 10
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    Numerical studies of natural transition in a decelerating boundary layer (1992)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: The laminar-turbulent transition in a decelerating flat-plate boundary layer is investigated by solving numerically the incompressible time-dependent three-dimensional Navier-Stokes equations. In the early stages of the transition, two-dimensional and slightly oblique waves are found to grow because of an inflexional instability. Their subsequent nonlinear interactions trigger the breakdown and determine the pattern of the Lambda vortices. The tips of the Lambda vortices are rarely aligned in the flow direction and appear in local patches, often near the dislocation in the initial wave pattern. The observed features are accounted for by a simple wave-interference model.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: Journal of Fluid Mechanics (ISSN 0022-1120); 240; 433-468
    Format: text
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