1,721,024 research outputs found
Are urban-canopy velocity profiles exponential?
No full textUsing analyses of data from extant direct numerical simulations and large-eddy simulations of boundary-layer and channel flows over and within urban-type canopies, sectional drag forces, Reynolds and dispersive shear stresses are examined for a range of roughness densities. Using the spatially-averaged mean velocity profiles these quantities allow deduction of the canopy mixing length and sectional drag coefficient. It is shown that the common assumptions about the behaviour of these quantities, needed to produce an analytical model for the canopy velocity profile, are usually invalid, in contrast to what is found in typical vegetative (e.g. forest) canopies. The consequence is that an exponential shape of the spatially-averaged mean velocity profile within the canopy cannot normally be expected, as indeed the data demonstrate. Nonetheless, recent canopy models that allow prediction of the roughness length appropriate for the inertial layer’s logarithmic profile above the canopy do not seem to depend crucially on their (invalid) assumption of an exponential profile within the canopy
Vortex flows in open cylindrical-section cavities
No full textFlow within a large-aspect-ratio cylindrical vortex cell has been explored experimentally. The flow was driven by a shear layer above an opening in the cylinder circumference. Reynolds numbers, based on the length of the opening and the velocity just outside it, exceed 50,000. It is shown that the expected solid body rotation within the cell, with a constant velocity gradient across most of the core, is qualitatively present, but is significantly distorted by three-dimensional effects. Nonetheless, turbulence levels within the core are very low, only rising to levels similar to those in regular turbulent shear flows within the driving mixing layer itself and the cell-wall boundary layers
The stability of laminar symmetric separated wakes
Full text linkTime-dependent computations of the two-dimensional incompressible uniform-velocity laminar flow past a normal flat plate (of unit half-width) in a channel are presented. Attention is restricted to cases in which the well-known anti-symmetric (von Kármán-type) vortex shedding is suppressed by the imposition of a symmetry plane on the downstream plate centreline. With a further symmetry plane at the channel's upper boundary, the only two governing parameters in the problem are the channel half-width, H, and the Reynolds number, Re (based on the body half-width and the upstream velocity, U). The former is restricted to the range 3?H?30 and the interest lies in determining the nature of the initial instability which occurs in the separated wake as Re is gradually increased. It is found that for sufficiently large H and at a critical Re, a long-time-scale global (supercritical) instability is initiated, which in its saturated (limit) state takes the form of ‘lumps’ of vorticity being periodically shed from the tail end of the separated bubble. Stability calculations of corresponding mean flow profiles (typical of those found in the separated wake) are undertaken by examining the impulse response of particular profiles via appropriate solution of the Orr–Sommerfeld equation. The results of this analysis extend those available from related published work and are consistent with the behaviour found from the numerical computations. Taken together, all the results suggest that this type of global instability may be generic to many kinds of separated wakes and, indeed, may provide the fundamental explanation for the very low-frequency oscillations often noticed in fully turbulent wake bubbles
Dataset for LES and RANS for turbulent flow over arrays of wall-mounted obstacles, Flow Turbulence and Combustion
No full textDataset supporting paper Xie, Z-T, Castro, IP (2006) LES and RANS for turbulent flow over arrays of wall-mounted obstacles, Flow Turbulence and Combustion, 76(3), 291-312, DOI: 10.1007/s10494-006-9018-6</span
Turbulent flows: an Introduction
No full textThis book presents an introduction to the fundamentals of turbulent flow. Its focus is on understanding and simplifying the equations of motion for various classes of flow, so as to elucidate the most crucial and practically important aspects of the physics. Adopting a classical approach concentrated on canonical flows of various kinds, the book includes wisdom from the last few decades of research, supplementing this with biographical accounts of the 'subject giants' who have shaped the field. Practical exercises are also included, making use of online data sets that can be directly accessed while reading, allowing teachers to construct a wide range of further exercises for students, as well as facilitating independent study and analysis.Key features• Aimed as a supplement to final year engineering or physical science undergraduate and/or first year graduate courses in turbulence, or as a basis for those entering turbulence research.• Authored by two experts in the field from different generations, ensuring a broad perspective.• Contains example questions.• Provides programmes for the analysis of turbulence data, including recent data from leading research laboratories
Turbulence intensity in wall-bounded and wall-free flows
No full textTurbulence intensity variations in the outer region of turbulent shear flows are considered, in the context of the diagnostic plot first introduced by Alfredsson et al. (Phys. Fluids, vol. 23, 2011, 041702) and for both (smooth and rough) wall-bounded flows and classical free shear flows. With U U
defined as the mean velocity within the flow, U e Ue
as a suitable reference velocity and u ′ u′
as the root mean square of the fluctuating velocity, it is demonstrated that, for wall flows, the attached eddy hypothesis yields a closely linear diagnostic plot ( u ′ /U u′/U
versus U/U e U/Ue
) over a certain Reynolds number range, explaining why the relation seems to work well for both boundary layers and channels despite its lack of any physical basis (Castro et al., J. Fluid Mech., vol. 727, 2013, pp. 119–131). It is shown that mixing layers, jets and wakes also exhibit linear variations of u ′ /U u′/U
versus U/U e U/Ue
over much of the flows (starting roughly from where the turbulence production is a maximum), with slopes of these variations determined by the total mean strain rate, characterised by Townsend’s flow constant R s Rs
. The diagnostic plot thus has a wider range of applicability than might have been anticipated.</p
Near wall flow over urban-like roughness
No full textIn this study, comprehensive measurements over a number of urban-type surfaces with the same area density of 25% have been performed in a wind tunnel. The experiments were conducted at a free stream velocity of 10 m s-1 and the main instrumentation was 120 ° x-wire anemometry, but measurement accuracy was checked using laser Doppler anemometry. The results have confirmed the strong three-dimensionality of the turbulent flow in the roughness sublayer and the depths of the inertial sublayer (log-law region) and roughness sublayer for each surface have been determined. Spatial averaging has been used to remove the variability of the flow in the roughness sublayer due to individual obstacles and it is shown that the spatially averaged mean velocity in the inertial sublayer and roughness sublayer can, together, be described by a single log-law with a mean zero-plane displacement and roughness length for the surface, provided that the proper surface stress is known. The spatially averaged shear stresses in the inertial sublayer and roughness sublayer are compared with the surface stress deduced from form drag measurements on the roughness elements themselves. The dispersive stress arising from the spatial inhomogeneity in the mean flow profiles was deduced from the data and is shown to be negligible compared with the usual Reynolds stresses in the roughness sublayer. Comparisons have been made between a homogeneous (regular element array) surface and one consisting of random height elements of the same total volume. Although the upper limits of the inertial sublayer for both surfaces were almost identical at equivalent fetch, the roughness sublayer was much thicker for the random surface than for the uniform surface, the friction velocity and the roughness length were significantly larger and the 'roughness efficiency' was greater. It is argued that the inertial sublayer may not exist at all in some of the more extreme rough urban areas. These results will provide fundamental information for modelling urban air quality and forecasting urban wind climates
Dataset for: Secondary motions in turbulent ribbed channel flows
No full textThis is a dataset linked with an article "Secondary motions in turbulent ribbed channel flows" published in Journal of Fluid Mechanics. The dataset includes various .xlsx and .plt files that are accessed by using Microsoft Excel and Tecplot 360.</span
Axisymmetric jets impinging on porous walls
No full textThe flow of axisymmetric turbulent jets impinging on porous walls has been studied experimentally. It is shown how the overall flow structure depends on the porosity of the surface. For high porosities (open area ratios, β, in excess of around 40% say) the porous wall, or screen, leads to a sudden increase in jet width and decrease in mean and fluctuating velocities, a direct consequence of the momentum flux extracted because of the screen drag. Lower porosities can lead to the appearance of radial wall jets on the upstream side of the screen but, in contrast to the corresponding case of planar jet impingement (Cant et al. in Exp Fluids 32:16–26, 2002), such wall jets never occur on the downstream side. The axial downstream velocities thus remain positive for all porosities. Jet growth rates for β ≥ 0.45 are initially increased by the screen, but once β ≤ 0.4 momentum extraction by the screen is virtually complete, so that velocities become very small. Again, unlike in the corresponding planar case (for β ~ 0.4), recirculating regions upstream of the screen never occur. A simple argument is suggested to explain the fundamental differences in flow behaviour between planar and axisymmetric jet impingement onto porous screens and it is concluded that in the latter case the effects of the screen are generally more benign and unsurprising. Nonetheless, these axisymmetric flows, like the corresponding planar ones, provide a serious challenge for computational modelling
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