https://novaprd-lb.newcastle.edu.au/vital/access/manager/Index ${session.getAttribute("locale")} 5 Skewness and flatness factors of the longitudinal velocity derivative in wall-bounded flows https://novaprd-lb.newcastle.edu.au/vital/access/manager/Repository/uon:34227 λ). For example, in the region below about 0.2δ (δ is the boundary layer thickness) where Reλ varies significantly, S and F strongly vary with Reλ and can be multivalued at a given Reλ. In the outer region, between 0.3δ and 0.6δ, S, F, and Reλ remain approximately constant. The channel flow direct numerical simulation data for S and F exhibit a similar behavior. These results point to the ambiguity that can arise when assessing the Reλ dependence of S and F in wall shear flows. In particular, the multivaluedness of S and F can lead to erroneous conclusions if y/δ is known only poorly, as is the case for the atmospheric shear layer (ASL). If the laboratory turbulent boundary layer is considered an adequate surrogate to the neutral ASL, then the behavior of S and F in the ASL is expected to be similar to that reported here.]]> Wed 20 Feb 2019 15:55:38 AEDT ]]> Large-scale structures in a turbulent channel flow with a minimal streamwise flow unit https://novaprd-lb.newcastle.edu.au/vital/access/manager/Repository/uon:47127 U ¯ . It is shown that, in the MSU, the large-scale structures become approximately two-dimensional at h+=1020. In this case, the streamwise velocity fluctuation u is energized, whereas the spanwise velocity fluctuation w is weakened significantly. Indeed, there is a reduced energy redistribution arising from the impaired global nature of the pressure, which is linked to the reduced linear–nonlinear interaction in the Poisson equation (i.e. the rapid pressure). The logarithmic dependence of w w ¯ is also more evident due to the reduced large-scale spanwise meandering. On the other hand, the spanwise organization of the large-scale u structures is essentially identical for the MSU and large streamwise domain (LSD). One discernible difference, relative to the LSD, is that the large-scale structures in the MSU are more energized in the outer region due to a reduced turbulent diffusion. In this region, there is a tight coupling between neighbouring structures, which yields antisymmetric pairs (with respect to centreline) of large-scale structures with a spanwise spacing of approximately 3h; this is intrinsically identical with the outer energetic mode in the optimal transient growth of perturbations.]]> Wed 14 Dec 2022 14:17:29 AEDT ]]> Near-wall similarity between velocity and scalar fluctuations in a turbulent channel flow https://novaprd-lb.newcastle.edu.au/vital/access/manager/Repository/uon:7367 Wed 11 Apr 2018 17:20:30 AEST ]]> Correlation between small-scale velocity and scalar fluctuations in a turbulent channel flow https://novaprd-lb.newcastle.edu.au/vital/access/manager/Repository/uon:7366 Wed 11 Apr 2018 12:11:19 AEST ]]> Scaling of normalized mean energy and scalar dissipation rates in a turbulent channel flow https://novaprd-lb.newcastle.edu.au/vital/access/manager/Repository/uon:12259 ε and Cεθ 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 Cε and Cεθ 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 Cε and Cεθ 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 Cε and Cεθ impacts on turbulence modeling. In particular, the similarity between Cε and Cεθ 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. © American Institute of Physics]]> Wed 11 Apr 2018 11:45:27 AEST ]]> Mean temperature calculations in a turbulent channel flow for air and mercury https://novaprd-lb.newcastle.edu.au/vital/access/manager/Repository/uon:47608 Prt for air (Pr = 0.71) and mercury (Pr = 0.025), with a view to calculating the mean temperature. Constant time-averaged (surface) heat flux (CHF) is used as a thermal boundary condition. For each Pr, four values of the Kármán number (h+ = 180, 395, 640, 1020) are used. Datasets for the constant heating source (CHS) are also examined. For Pr = 0.71, Prt is approximately 1.1 at the wall, varies between 0.9 and 1.1 in the region y+ <100, and is approximated by 0.9–0.3(y/h)2 for y/h > 0.2. The latter relation, with a low Re correction term (i.e. 25/h+), yields an excellent prediction for the mean temperature up to h+ = 2000, whereas a calculation based on Prt = 0.85 underestimates the mean temperature. The calculated maximum wall-normal turbulent heat flux and Nusselt number also agree well with the empirical relations over a wide range of h+. For Pr = 0.025, Prt departs significantly from unity inside the inner region (y/h < 0.2) owing to the strong conductive effect, whilst the magnitude in the outer region (y/h > 0.2) tends to approach that corresponding to Pr = 0.71 as h+ increases due to the increase in the Peclet number. The h+ dependence of Prt in the logarithmic and outer regions is represented adequately by the turbulent Peclet number, i.e.Pet≡Pr(vt/V. The resulting Prt relation, which is an extension of the expression established by Kays (1994), leads to a correct calculation of the mean temperature not only for mercury (Pr = 0.025) but also for liquid sodium (Pr = 0.01). The mean temperature defect profile exhibits an outer-layer similarity when Pe(≡RebPr) ≥ 2000; the Nusselt number is represented by reasonably well.]]> Tue 24 Jan 2023 11:14:51 AEDT ]]> Analogy between velocity and scalar fields in a turbulent channel flow https://novaprd-lb.newcastle.edu.au/vital/access/manager/Repository/uon:7852 Sat 24 Mar 2018 08:42:25 AEDT ]]> Turbulent Prandtl number in a channel flow for Pr=0.025 and 0.71 https://novaprd-lb.newcastle.edu.au/vital/access/manager/Repository/uon:8814 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.]]> Sat 24 Mar 2018 08:38:24 AEDT ]]> Analogy between small-scale velocity and passive scalar fields in a turbulent channel flow https://novaprd-lb.newcastle.edu.au/vital/access/manager/Repository/uon:8900 Sat 24 Mar 2018 08:36:42 AEDT ]]> Relationship between the heat transfer law and the scalar dissipation function in a turbulent channel flow https://novaprd-lb.newcastle.edu.au/vital/access/manager/Repository/uon:30615 Pr of 0.71. A logarithmic dependence on the Kármán number h+ is established for the integrated mean scalar in the range h+ ≽400 where the mean part of the total scalar dissipation exhibits near constancy, whilst the integral of the turbulent scalar dissipation rate ̅ϵθ increases logarithmically with h+. This logarithmic dependence is similar to that established in a previous paper (Abe & Antonia, J. Fluid Mech., vol. 798, 2016, pp. 140–164) for the bulk mean velocity. However, the slope (2.18) for the integrated mean scalar is smaller than that (2.54) for the bulk mean velocity. The ratio of these two slopes is 0.85, which can be identified with the value of the turbulent Prandtl number in the overlap region. It is shown that the logarithmic h+ increase of the integrated mean scalar is intrinsically associated with the overlap region of ̅ϵθ, established for h+ (≽400). The resulting heat transfer law also holds at a smaller h+ (≽200) than that derived by assuming a log law for the mean temperature.]]> Sat 24 Mar 2018 07:39:00 AEDT ]]> Relationship between the energy dissipation function and the skin friction law in a turbulent channel flow https://novaprd-lb.newcastle.edu.au/vital/access/manager/Repository/uon:29610 b = 2:54 ln(h⁺)+ 2:41 (Ub is the bulk mean velocity). This latter relationship is established on the basis of energy balances for both the mean and turbulent kinetic energy. When h⁺ is smaller than 300, viscosity affects the integrals of both the mean and turbulent energy dissipation rates significantly due to the lack of distinct separation between inner and outer regions. The logarithmic h⁺ dependence of U⁺b is clarified through the scaling behaviour of the turbulent energy dissipation rate ε̅ in different parts of the flow. The overlap between inner and outer regions is readily established in the region 30/h⁺ ≼ y/h ≼ 0:2 for h⁺ ≽ 300. At large h⁺ (≽5000) when the finite Reynolds number effect disappears, the magnitude of [could not be replicated] approaches 2.54 near the lower bound of the overlap region. This value is identical between the channel, pipe and boundary layer as a result of similarity in the constant stress region. As h⁺ becomes large, the overlap region tends to contribute exclusively to the 2.54 ln(h⁺) dependence of the integrated turbulent energy dissipation rate. The present logarithmic h⁺ dependence of U⁺b is essentially linked to the overlap region, even at small h⁺.]]> Sat 24 Mar 2018 07:32:05 AEDT ]]>