http://nova.newcastle.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:12259 Non-dimensional parameters for the mean energy and scalar dissipation rates C_ε and C_εθ are examined using direct numerical simulation (DNS) data obtained in a fully developed turbulent channel flow with a passive scalar (Pr=0.71) at several values of the Kármán (Reynolds) number h⁺. It is shown that C_ε and C_εθ are approximately equal in the near-equilibrium region (viz., y⁺ = 100 to y/h = 0.7) where the production and dissipation rates of either the turbulent kinetic energy or scalar variance are approximately equal and the magnitudes of the diffusion terms are negligibly small. The magnitudes of C_ε and C_εθ are about 2 and 1 in the logarithmic and outer regions, respectively, when h⁺ is sufficiently large. The former value is about the same for the channel, pipe, and turbulent boundary layer, reflecting the similarity between the mean velocity and temperature distributions among these three canonical flows. The latter value is, on the other hand, about twice as large as in homogeneous isotropic turbulence due to the existence of the large-scale u structures in the channel. The behaviour of C_ε and C_εθ impacts on turbulence modeling. In particular, the similarity between C_ε and C_εθ leads to a simple relation for the scalar variance to turbulent kinetic energy time-scale ratio, an important ingredient in the eddy diffusivity model. This similarity also yields a relation between the Taylor and Corrsin microscales and analogous relations, in terms of h⁺, for the Taylor microscale Reynolds number and Corrsin microscale Peclet number. This dependence is reasonably well supported by both the DNS data at small to moderate h⁺ and the experimental data of Comte-Bellot [Ph. D. thesis (University of Grenoble, 1963)] at larger h⁺. It does not however apply to a turbulent boundary layer where the mean energy dissipation rate, normalized on either wall or outer variables, is about 30% larger than for the channel flow. 2012-12-17T04:40:13.082Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:7366 Direct numerical simulations of a turbulent channel flow with passive scalar transport are used to examine the relationship between small-scale velocity and scalar fields. The Reynolds number based on the friction velocity and the channel half-width is equal to 180, 395 and 640, and the molecular Prandtl number is 0.71. The focus is on the interrelationship between the components of the vorticity vector and those of the scalar derivative vector. Near the wall, there is close similarity between different components of the two vectors due to the almost perfect correspondence between the momentum and thermal streaks. With increasing distance from the wall, the magnitudes of the correlations become smaller but remain non-negligible everywhere in the channel owing to the presence of internal shear and scalar layers in the inner region and the backs of the large-scale motions in the outer region. The topology of the scalar dissipation rate, which is important for small-scale scalar mixing, is shown to be associated with the organized structures. The most preferential orientation of the scalar dissipation rate is the direction of the mean strain rate near the wall and that of the fluctuating compressive strain rate in the outer region. The latter region has many characteristics in common with several turbulent flows; viz. the dominant structures are sheetlike in form and better correlated with the energy dissipation rate than the enstrophy. 2012-01-30T04:59:26.746Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:7367 The degree of near-wall similarity between velocity and scalar fields is examined with the use of direct numerical simulation databases for a smooth wall turbulent channel flow with passive scalar transport. Particular attention is given to correlations between velocity and scalar fluctuations as well as those between their derivatives. In the vicinity of the wall, the largest correlation is between the wall-normal vorticity fluctuation and the spanwise scalar derivative, mainly due to the significantly reduced effect of the fluctuating pressure gradient. Other correlations peak at locations where the effect of the pressure fluctuation is smallest. The near-wall similarity between momentum and thermal streaks is not perfect because momentum streaks are often intensified or weakened by the fluctuating pressure gradient. 2012-01-30T04:59:26.463Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:8900 DNS databases for a turbulent channel flow with a passive scalar at a molecular Prandtl number of 0.71 are used to examine the limiting forms, at zero separation, of the transport equations for the turbulent kinetic energy and scalar variance structure functions. The results support the notion that the limits are identical over a significant portion of the outer region when the Reynolds number is sufficiently large and the normalization is based on Kolmogorov and Batchelor scales. 2011-09-09T05:30:03.659Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:8814 DNS databases in a turbulent channel flow with passive scalar transport and a constant time-averaged heat-flux boundary condition have been used to examine the variation of the turbulent Prandtl number (Prt) across the channel. Two values of the molecular Prandtl number Pr are considered (0.025 and 0.71); in each case, data were obtained for four values of h⁺ (180, 395, 640, 1020). For Pr=0.71, Prt is 1.1 at the wall, varies between 0.9 and 1.1 in the region y⁺ <100, and is represented by 0.9 − 0.3(y/h)² for y/h >0.2. The closeness to unity near the wall is attributed to the excellent similarity between the velocity and scalar fields, whereas the decrease in magnitude in the outer region is most likely associated with the unmixedness of the scalar. A similar description for Prt is not possible for Pr=0.025 due to the strong conductive effects. In this case, the near-wall limiting value is unlikely to approach unity. 2011-09-05T01:50:16.803Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:7852 The relationship between the fluctuating velocity vector and the temperature fluctuation has been examined using direct numerical simulation databases of a turbulent channel flow with passive scalar transport using a constant time-averaged heat flux at each wall for h⁺ = 180, 395, 640 and 1020 (where h is the channel half-width with the superscript denoting normalization by wall variables) at Prandtl number Pr=0.71. The analogy between spectra corresponding to the kinetic energy and scalar variance is reasonable in both inner and outer regions irrespective of whether the spectra are plotted in terms of kx or kz, the wavenumbers in the streamwise and spanwise directions respectively. Whereas all three velocity fluctuations contribute to the energy spectrum when kx is used, the longitudinal velocity fluctuation is the major contributor when kz is used. The quality of the analogy in the spectral domain is confirmed by visualizations in physical space and reflects differences between spatial organizations in the velocity and scalar fields. The similarity between the spectra corresponding to the enstrophy and scalar dissipation rate is not as good as that between the kinetic energy and scalar variance, emphasizing the prominence of the scalar sheets as the centre of the channel is approached. The ratio R between the characteristic time scales of the velocity and scalar fluctuations is approximately constant over a major part of the channel and independent of h⁺, when the latter is sufficiently large. This constancy, which is not observed in quantities such as the turbulent Prandtl number, follows from the spectral similarities discussed in this paper and has implications for turbulent heat transport models. 2011-06-06T06:20:15.389Z ]]>