https://ogma.newcastle.edu.au/vital/access/ /manager/Index en-au 5 https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:39519 Wed 27 Jul 2022 13:59:34 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:40503 Wed 13 Jul 2022 15:02:44 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:45594 Wed 02 Nov 2022 13:39:04 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:50689 Wed 02 Aug 2023 11:09:25 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:33015 r = λ, is assessed in various turbulent flows at small to moderate values of the Taylor microscale Reynolds number R_λ. It is found that the contribution of the large-scale terms to the scale by scale energy budget differs from flow to flow. For a fixed R_λ, this contribution is largest on the centreline of a fully developed channel flow but smallest for stationary forced periodic box turbulence. For decaying-type flows, the contribution lies between the previous two cases. Because of the difference in the large-scale term between flows, the third-order longitudinal velocity structure function at r = λ differs from flow to flow at small to moderate R_λ. The effect on the second-order velocity structure functions appears to be negligible. More importantly, the effect of R_λ on the scaling range exponent of the longitudinal velocity structure function is assessed using measurements of the streamwise velocity fluctuation u, with R_λ in the range 500–1100, on the axis of a plane jet. It is found that the magnitude of the exponent increases as R_λ increases and the rate of increase depends on the order n. The trend of published structure function data on the axes of an axisymmetric jet and a two-dimensional wake confirms this dependence. For a fixed R_λ, the exponent can vary from flow to flow and for a given flow, the larger R_λ is, the closer the exponent is to the value predicted by Kolmogorov (Dokl. Akad. Nauk SSSR, vol. 30, 1941a, pp. 299–303) (hereafter K41). The major conclusion is that the finite Reynolds number effect, which depends on the flow, needs to be properly accounted for before determining whether corrections to K41, arising from the intermittency of the energy dissipation rate, are needed. We further point out that it is imprudent, if not incorrect, to associate the finite Reynolds number effect with a consequence of the modified similarity hypothesis introduced by Kolmogorov (J. Fluid Mech., vol. 13, 1962, pp. 82–85) (K62); we contend that this association has misled the vast majority of post K62 investigations of the consequences of K62.]]> Tue 21 Aug 2018 11:32:55 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:38239 3 based on d and the free-stream velocity. A multi-wire probe is deployed to measure simultaneously the fluctuating temperature and its gradient vector, at nominally the same spatial point in the plane of the mean shear. It is found that the coherent streamwise and spanwise temperature derivatives are similarly distributed with respect to the spanwise vortex, exhibiting twin peaks at the temperature fronts, while the coherent lateral component is linked to the rib-like structures. The temperature variance dissipation rate is found to be statistically independent of the temperature fluctuation when the Kármán vortex is so weak (say at x/d = 40) that the large-scale temperature front resulting from the vortex entrainment ceases to be present and the coherent strain rate at the saddle region is relatively small. In addition, the most effective turbulent mixing is found to take place around the temperature front near the wake centerline, which is in contrast to the conjecture by Hussain and Hayakawa (1987).]]> Tue 17 Aug 2021 09:45:53 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:44263 u_i and ω_i (i=1,2,3 ), are simultaneously measured, along with the fluctuating temperature θ and the temperature gradient vector, at nominally the same spatial point in the plane of mean shear at x/d=10 , where x is the streamwise distance from the cylinder axis and d is the cylinder diameter. We believe this is the first time the properties of fluctuating velocity, temperature, vorticity and temperature gradient vectors have been explored simultaneously within the Kármán vortex in detail. The Reynolds number based on d and the free-stream velocity is 2.5 x 10³ . The phase-averaged distributions of θ and ui follow closely the Gaussian distribution for r/d⩽0.2 (r is the distance from the vortex centre), but not for r/d>0.2 . The collapse of the distributions of the mean-square streamwise derivative of the velocity fluctuations within the Kármán vortex implies that the velocity field within the vortex tends to be more locally isotropic than the flow field outside the vortex. A possible physical explanation is that the large and small scales of velocity and temperature fields are statistically independent of each other near the Kármán vortex centre, but interact vigorously outside the vortex, especially in the saddle region, due to the action of coherent strain rate.]]> Tue 11 Oct 2022 13:03:27 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:41375 Tue 02 Aug 2022 15:34:25 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:42750 Thu 01 Sep 2022 14:21:19 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:31548 χ/d = 10, 20 and 40, where χ is the streamwise distance from the cylinder axis and d is the cylinder diameter, with a Reynolds number of 2.5 x 10³ based on the cylinder diameter and the free-stream velocity. A probe consisting of eight hot wires (four X-wires) and four cold wires is used to measure simultaneously the three components of the fluctuating velocity and vorticity vectors, as well as the fluctuating temperature gradient vector at nominally the same point in the plane of the mean shear. It is found that the enstrophy and scalar dissipation spectra collapse approximately at all wavenumbers except around the Kármán vortex street wavenumber for χ/d ≽ 20. The spectral similarity between the streamwise velocity fluctuation u and the passive scalar θ is better than that between the velocity fluctuation vector q and θ. This is closely related to the highly organized lateral velocity fluctuation v in this flow. The present observations are fully consistent with the expectation that small scales of the velocity and temperature fields are more likely to exhibit a close relationship than scales associated with the bulk of the turbulent energy or scalar variance. The variation across the wake of the time scale ratio between scalar and velocity fields is significantly smaller than that of the turbulent Prandtl number.]]> Sat 24 Mar 2018 08:44:26 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:10276 Sat 24 Mar 2018 08:09:11 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:18296 Sat 24 Mar 2018 08:04:25 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:30242 n is a positive integer). It is found that the relation M_2n+1(∂u/∂z)∼R⁻¹_λ is supported reasonably well by hot-wire data up to the seventh order (n=3) on the wake centreline, although it is also dependent on the initial conditions. The present relation N3(∂u/∂y)∼R⁻¹_λ is obtained more rigorously than that proposed by Lumley (Phys Fluids 10:855–858, 1967) via dimensional arguments. The effect of the mean shear at locations away from the wake centreline on M_2n+1(∂u/∂z) and N_2n+1(∂u/∂y) is addressed and reveals that, although the non-dimensional shear parameter is much smaller in wakes than in a homogeneous shear flow, it has a significant effect on the evolution of N_2n+1(∂u/∂y) in the direction of the mean shear; its effect on M_2n+1(∂u/∂z) (in the non-shear direction) is negligible.]]> Sat 24 Mar 2018 07:41:58 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:26852 iso/⋶ is sufficiently close to 1 on the centreline, our main focus is on the isotropic form of the transport equation. It is found that the imbalance between the production of ⋶ due to vortex stretching and the destruction of ⋶ caused by the action of viscosity is governed by the diffusion of ⋶ by the wall-normal velocity fluctuation. This imbalance is intrinsically different from the advection-driven imbalance in decaying-type flows, such as grid turbulence, jets and wakes. In effect, the different types of imbalance represent different constraints on the relation between the skewness of the longitudinal velocity derivative S₁,₁ and the destruction coefficient G of enstrophy in different flows, thus resulting in non-universal approaches of S₁,₁ towards a constant value as the Taylor microscale Reynolds number, R_λ, increases. For example, the approach is slower for the measured values of S₁,₁ along either the channel or pipe centreline than along the axis in the self-preserving region of a round jet. The data for S₁,₁ collected in different flows strongly suggest that, in each flow, the magnitude of S₁,₁ is bounded, the value being slightly larger than 0.5.]]> Sat 24 Mar 2018 07:41:48 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:27364 Sat 24 Mar 2018 07:36:42 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:27921 Sat 24 Mar 2018 07:36:07 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:27919 J. Fluid Mech., vol. 250, 1993, pp. 651-668) or MA, the new model provides a more detailed description of the role the rib-like structures undertake in transporting heat and momentum, and also underlines the importance of the upstream half of the spanwise vortex rollers, instead of only one quadrant of these rollers, as in the MA model, in diffusing heat out of the vortex.]]> Sat 24 Mar 2018 07:36:07 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:30078 Sat 24 Mar 2018 07:31:16 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:30203 q² diffusion term is negligible compared to advection term along the axis and the advection and energy dissipation terms dominate the budget. However, in the CC wake, aside from the advection and energy dissipation terms, the q² diffusion term also contributes significantly to the budget. At the region close to the centreline, the gain of the energy due to the contributions from the advection and diffusion terms is equal to the loss due to the isotropic dissipation, indicating that the isotropic dissipation rate ε iso is a good surrogate of the mean TKE dissipation rate ε.]]> Sat 24 Mar 2018 07:31:04 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:25900 isoalong the centreline in the far-wake of a circular cylinder is derived by applying the limit at small separations to the two-point energy budget equation. It is found that the imbalance between the production and the destruction of ⋶_iso, respectively due to vortex stretching and viscosity, is governed by both the streamwise advection and the lateral turbulent diffusion (the former contributes more to the budget than the latter). This imbalance differs intrinsically from that in other flows, e.g. grid turbulence and the flow along the centreline of a fully developed channel, where either the streamwise advection or the lateral turbulent diffusion of ⋶_iso governs the imbalance. More importantly, the different types of imbalance represent different constraints on the relation between the skewness of the longitudinal velocity derivative S and the destruction coefficient of enstrophy G. This results in a non-universal approach of S towards a constant value as the Taylor microscale Reynolds number R_λ increases. For the present flow, the magnitude of S decreases initially (R_λ≤40) before increasing (R_λ>40) towards this constant value. The constancy of S at large Rλ violates the modified similarity hypothesis introduced by Kolmogorov (J. Fluid Mech., vol. 13, 1962, pp. 82-85) but is consistent with the original similarity hypotheses (Kolmogorov, Dokl. Akad. Nauk SSSR, vol. 30, 1941b, pp. 299-303 (see also 1991 Proc. R. Soc. Lond. A, vol. 434, pp. 9-13)) , and, more importantly, with the almost completely self-preserving nature of the plane far-wake.]]> Sat 24 Mar 2018 07:28:16 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:24300 Sat 24 Mar 2018 07:14:38 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:45528 Mon 31 Oct 2022 14:45:03 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:32998 Exp. Fluids, vol. 53, 2012, pp. 1005–1013) based on the universality of the dissipation range of the longitudinal velocity spectrum normalized by the Kolmogorov scales also applies in the present flow despite the strong perturbation from the Kármán vortex street and violation of local isotropy at small x/d. The appropriateness of the spectral chart method is consistent with Antonia et al.’s (Phys. Fluids, vol. 26, 2014, 45105) observation that the two major assumptions in Kolmogorov’s first similarity hypothesis, i.e. very large Taylor microscale Reynolds number and local isotropy, can be significantly relaxed. The data also indicate that vorticity spectra are more sensitive, when testing the first similarity hypothesis, than velocity spectra. They also reveal that the velocity derivatives δu/δy and δv/δx play an important role in the interaction between large and small scales in the present flow. The phase-averaged data indicate that the energy dissipation is concentrated mostly within the coherent spanwise vortex rollers, in contrast with the model of Hussain (J. Fluid Mech., vol. 173, 1986, pp. 303–356) and Hussain & Hayakawa (J. Fluid Mech., vol. 180, 1987, p. 193), who conjectured that it resides mainly in regions of strong turbulent mixing.]]> Mon 20 Aug 2018 15:42:53 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:41306 Mon 01 Aug 2022 12:23:31 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:41272 Mon 01 Aug 2022 09:56:30 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:27672 θ is derived by applying the limit at small separations to the generalized form of Yaglom's equation in two types of flows, those dominated mainly by a decay of energy in the streamwise direction and those which are forced, through a continuous injection of energy at large scales. In grid turbulence, the imbalance between the production of ∈_θ due to stretching of the temperature field and the destruction of ∈_θ by the thermal diffusivity is governed by the streamwise advection of ∈_θ by the mean velocity. This imbalance is intrinsically different from that in stationary forced periodic box turbulence (or SFPBT), which is virtually negligible. In essence, the different types of imbalance represent different constraints imposed by the large-scale motion on the relation between the so-called mixed velocity-temperature derivative skewness S_T and the scalar enstrophy destruction coefficient G_θ in different flows, thus resulting in non-universal approaches of S_T towards a constant value as Re_λ increases. The data for S_T collected in grid turbulence and in SFPBT indicate that the magnitude of S,sub>T is bounded, this limit being close to 0.5.]]> Fri 10 Nov 2023 15:44:15 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:52774 Fri 10 Nov 2023 15:39:09 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:45709 Fri 04 Nov 2022 09:03:17 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:49836 Fri 02 Jun 2023 15:57:06 AEST ]]>