1,721,046 research outputs found
Spectral measurements of reactive and passive scalars in a turbulent reactive-scalar-mixing layer
No full textIntroduction to direct numerical simulation
No full textContents
Introduction B. E. Launder and N. D. Sandham; Part I. Physical and Numerical Techniques: 1. Linear and non-linear eddy viscosity models T. B. Gatski; 2. Second-moment turbulence closure modelling K. Hanjalic and Jakirilic Suad; 3. Closure modelling near the two-component limit T. J. Craft and B. Launder; 4. The elliptic relaxation method P. A. Durbin and B. A. Patterson; 5. Numerical aspects of applying second-moment closure to complex flows M. Leschziner and F.-S. Lien; 6. Modelling heat transfer in near-wall flows Y. Nagano; 7. Introduction to direct numerical simulation N. D. Sandham; 8. Introduction to large-eddy simulation of turbulent flows J. Fröhlich and W. Rodi; 9. Two-point closure strategies C. Cambon; 10. Introduction to pdf approaches in turbulence modelling D. Roekaerts; Part II. Flow Types and Processes and Strategies for Modelling Them: 11. Modelling of separated and impinging flows T. J. Craft; 12. Large eddy simulation of the flow past bluff bodies W. Rodi; 13. LES modelling of industrial flows D. Laurence; 14. Application of TCL modelling to stratified flows T. J. Craft and B. E. Launder; 15. Higher-moment diffusion in stable stratification B. Ilyushin; 16. DNS of by-pass transition P. A. Durbin, R. Jacobs and X. Wu; 17. By-pass transition using conventional closures A. M. Savill; 18. New strategies in modelling by-pass transition A. M. Savill; 19. Compressible, high-speed flows S. Barre, J.-P. Bonnet, T. B. Gatski and N. D. Sandham; 20. Closure strategies for reacting flows W. P. Jones; 21. Pdf strategies for reacting flows D. Roekaerts; 22. TRANS approach to convection in unstably stratified layers K. Hanjalic and S. Kenjeres; 23. Use of higher moments to construct pdfs in stratified flows B. Ilyushin; 24. DNS of separation bubbles G. N. Coleman and N. D. Sandham; 25. Is LES ready for complex flows? B. J. Geurts and A. Leonard; 26. Further developments in two-point closure C. Cambon
Experimental study of low-frequency oscillations and large-scale circulations in turbulent mixed convection
No full textThe formation and dynamics of large-scale circulations in forced and mixed convection has been studied at ambient and elevated fluid pressure by means of particle image velocimetry and temperature measurements.
The study has been conducted in two rectangular containers of the same shape and aspect ratios of Cxz = 1 and Cyz = 5. For the measurements at high fluid pressure the dimensions of the cell have been scaled down by a factor of 5. Air with Pr = 0.7 has been used as fluid in both configurations. Forced
convection has been investigated at Re = 1.01 x 104 and mixed convection has been studied at Ar = 3.3,
Re = 1.01 x 10 up4 and Ra = 2.4 x 10 up8. In this configuration low-frequency oscillations in the heat transfer
between the inlet and outlet have been found for mixed convection. Instantaneous velocity vector fields obtained from particle image velocimetry have been analysed using proper orthogonal decomposition and an algorithm to detect the core and the core centre position of large-scale circulations
Closure strategies for turbulent and transitional flows
No full textTurbulence modelling is a critically important area in any industry dealing with fluid flow, having many implications for computational fluid dynamics (CFD) codes. It also retains a huge interest for applied mathematicians since there are many unsolved problems. This book provides a comprehensive account of the state-of-the-art in predicting turbulent and transitional flows by some of the world’s leaders in these fields. It can serve as a graduate-level textbook and, equally, as a reference book for research workers in industry or academia. It is structured in three parts: Physical and Numerical Techniques; Flow Types and Processes; and Future Directions.
As the only broad account of the subject, it will prove indispensable for all working in CFD, whether academics interested in turbulent flows, industrial researchers in CFD interested in understanding the models embedded in their software (or seeking more powerful models) or graduate students needing an introduction to this vital area.<br/
Developments in turbulence research: a review based on the 1999 Programme of the Isaac Newton Institute, Cambridge
Full text linkRecent research is making progress in framing more precisely the basic dynamical and statistical questions about turbulence and in answering them. It is helping both to define the likely limits to current methods for modelling industrial and environmental turbulent flows, and to suggest new approaches to overcome these limitations. Our selective review is based on the themes and new results that emerged from more than 300 presentations during the Programme held in 1999 at the Isaac Newton Institute, Cambridge, UK, and on research reported elsewhere. A general conclusion is that, although turbulence is not a universal state of nature, there are certain statistical measures and kinematic features of the small-scale flow field that occur in most turbulent flows, while the large-scale eddy motions have qualitative similarities within particular types of turbulence defined by the mean flow, initial or boundary conditions, and in some cases, the range of Reynolds numbers involved. The forced transition to turbulence of laminar flows caused by strong external disturbances was shown to be highly dependent on their amplitude, location, and the type of flow. Global and elliptical instabilities explain much of the three-dimensional and sudden nature of the transition phenomena. A review of experimental results shows how the structure of turbulence, especially in shear flows, continues to change as the Reynolds number of the turbulence increases well above about 104 in ways that current numerical simulations cannot reproduce. Studies of the dynamics of small eddy structures and their mutual interactions indicate that there is a set of characteristic mechanisms in which vortices develop (vortex stretching, roll-up of instability sheets, formation of vortex tubes) and another set in which they break up (through instabilities and self- destructive interactions). Numerical simulations and theoretical arguments suggest that these often occur sequentially in randomly occurring cycles. The factors that determine the overall spectrum of turbulence were reviewed. For a narrow distribution of eddy scales, the form of the spectrum can be defined by characteristic forms of individual eddies. However, if the distribution covers a wide range of scales (as in elongated eddies in the ‘wall’ layer of turbulent boundary layers), they collectively determine the spectra (as assumed in classical theory). Mathematical analyses of the Navier–Stokes and Euler equations applied to eddy structures lead to certain limits being defined regarding the tendencies of the vorticity field to become infinitely large locally. Approximate solutions for eigen modes and Fourier components reveal striking features of the temporal, near-wall structure such as bursting, and of the very elongated, spatial spectra of sheared inhomogeneous turbulence; but other kinds of eddy concepts are needed in less structured parts of the turbulence. Renormalized perturbation methods can now calculate consistently, and in good agreement with experiment, the evolution of second- and third-order spectra of homogeneous and isotropic turbulence. The fact that these calculations do not explicitly include high-order moments and extreme events, suggests that they may play a minor role in the basic dynamics. New methods of approximate numerical simulations of the larger scales of turbulence or ‘very large eddy simulation’ (VLES) based on using statistical models for the smaller scales (as is common in meteorological modelling) enable some turbulent flows with a non-local and non-equilibrium structure, such as impinging or convective flows, to be calculated more efficiently than by using large eddy simulation (LES), and more accurately than by using ‘engineering’ models for statistics at a single point. Generally it is shown that where the turbulence in a fluid volume is changing rapidly and is very inhomogeneous there are flows where even the most complex ‘engineering’ Reynolds stress transport models are only satisfactory with some special adaptation; this may entail the use of transport equations for the third moments or non-universal modelling methods designed explicitly for particular types of flow. LES methods may also need flow-specific corrections for accurate modelling of different types of very high Reynolds number turbulent flow including those near rigid surfaces.This paper is dedicated to the memory of George Batchelor who was the inspiration of so much research in turbulence and who died on 30th March 2000. These results were presented at the last fluid mechanics seminar in DAMTP Cambridge that he attended in November 1999
Large-Scale Structures and Heat Transport of Turbulent Forced and Mixed Convection in a Closed Cavity
No full textAn experimental investigation of flow structure formation in turbulent forced and mixed convection in a closed rectangular cavity with an aspect ratio of 1:1:5 and air as working fluid is presented. Forced
convection at 1.07x10^4 < Re < 3.5x10^4 and mixed convection at 1.01x10^4 < Re < 3.4x10^4 and Ra=2.4x10^8 are studied under well-defined conditions by means of Particle Image Velocimetry and
local temperature measurements. For purely forced convection a 2D mean wind, which can be approximated by a solid body rotation (Rankine vortex), is found. With increasing Archimedes number
(Ar), realised by a temperature gradient between bottom and ceiling of the convection cell, this structure becomes instable, leading to four convection
rolls for Ar>1.7. These rolls are oriented in longitudinal direction of the cell and their rotation direction alternates for different Archimedes numbers, realising an updraught for Ar=2.2 and a downdraught for
Ar=1.7 and Ar=3.3 in the central cross section. Temperature measurements of the outflowing air reveal a temporally and spatially constant fingerprint with two maxima over the length of the outlet.
Investigations of the enthalpy transport of the fluid indicate the existence of a maximal enthalpy transport by the fluid for approximately Ar=0.6
Flow field characterization of a rotating cylinder
No full textDirect numerical simulation is used to study the flow field around an infinitely long circular cylinder rotating in fluid with no outer boundary. Wall shear stresses and normal pressure fluctuations are considered with reference to flat, non-rotating geometries to help identify any flow field differences introduced by Coriolis forces. In the present case, Coriolis forces are experienced only by the turbulence field. The dominant effect is to decrease the streamwise turbulent velocity level relative to the other two components. A consequential effect is that the two components of wall shear stress fluctuations become almost equal and spectra for streamwise and spanwise wall shear stress fluctuations become almost identical. This is a distinctly different behaviour from that of non-rotating flat plate and straight pipe flows. Instantaneous wall shear stress fluctuations indicate a near wall flow structure similar to that of other boundary layers with sweeps and ejections. No flow reversals of wall shear stress are indicated. A good correlation of the wall shear stresses and the turbulent kinetic energy exists for y(+) < 10. Budgets of Reynolds normal stress components illustrate the role played by Coriolis forces in the production and redistribution of turbulence energies. Wall pressure fluctuations are found to be of much larger spatial extent than velocity fluctuation scales while the probability density distribution of pressure fluctuations is almost Gaussian but does display a Reynolds number effect for skewness and Kurtosis. The ratio of rms pressure fluctuations to mean streamwise wall shear stress follows closely that for flat plate boundary layer and channel flows. (C) 2008 Elsevier Inc. All rights reserved
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