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Dawid Tale - One of the best experts on this subject based on the ideXlab platform.
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determining velocity and friction factor for turbulent flow in smooth tubes
International Journal of Thermal Sciences, 2016Co-Authors: Dawid TaleAbstract:Abstract The most popular explicit correlations for the friction factor in smooth tubes are reviewed in this paper. The friction factor for the turbulent flow in smooth tubes is required in some correlations when calculating the Nusselt number. To calculate the friction factor, the velocity profile in a turbulent smooth wall-tube must be estimated at first. The radial velocity distribution was determined using either universal velocity profile found experimentally by Reichardt or by integration the momentum equation using the eddy diffusivity model of Reichardt. The friction factor obtained by using the universal velocity profile gives better results than that obtained from the momentum equation when compared with the Prandtl–von Karman–Nikuradse equation. Based on the velocity profiles proposed by Reichardt the friction factor was calculated as a function of the Reynolds number and subsequently two formulas for the friction were proposed. They have satisfactory accuracy when comparing with the implicit Prandtl–von Karman–Nikuradse equation. Thus, it was concluded that the universal velocity profile proposed by Reichardt will provide good results when it is taken into account while integrating the energy conservation equation. There is also a considerable number of experimental correlations for the friction factor in smooth tubes. All these relationships were compared with the experimental data and with the implicit Prandtl–von Karman–Nikuradse equation that is considered as a standard to test the explicit approximations. The Colebrook and Filonienko explicit correlations are widely used when calculating the Nusselt number for the turbulent flow but they have noticeable errors for small Reynolds number ranged from 3000 to 7000 for the Colebrook relation and from 3000 to 30,000 for the Filonienko relation. For this reason, a new simple and accurate correlation for the friction factor for Reynolds numbers between 3000 and 107 is proposed in the paper.
Peter Nielsen - One of the best experts on this subject based on the ideXlab platform.
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VERTICAL SCALES AND SHEAR STRESSES IN WAVE BOUNDARY LAYERS OVER MOVABLE BEDS
Coastal Engineering Proceedings, 2011Co-Authors: Peter Nielsen, Paul A. GuardAbstract:Unified scaling rules are provided for smooth and rough wave boundary layers. It is shown that the rough equivalent of the smooth, or viscous, vertical scale , the Stokes’ length, is a function of r, the Nikuradse roughness and A, the near-bed semi excursion of the wave motion. Realizing this equivalence of viscous and rough scales a unified description in the style of Colebrook’s (1939) formulae for steady flow friction can be devised based on the unified vertical scale. That is, unified smooth and rough wave friction factor formulae can be used with adequate accuracy. A general procedure is given for deriving the unified vertical scale from velocity data including data from mobile bed experiments, which enable determination of the equivalent Nikuradse roughness from these experiments. Presently available sheet flow data show a velocity structure, which corresponds to a Nikuradse roughness r of the order 50 to 100 grain diameters. Instantaneous shear stresses derived through the usual momentum integral from sheet flow experiments show that the shear stress varies strongly through the sheet flow layer with the value at the lowest level of sediment motion being 2 to 3 times the value at the undisturbed bed level. The corresponding Nikuradse roughnesses are about 2.5d50 corresponding to the undisturbed bed level and 100d50 for the stress at the lowest level of sediment motion. With this strong variation of the shear stress through the layer of moving sediment, it is not at all obvious what should be understood by THE BED SHEAR STRESS in the context of wave sediment transport.
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Unsteady flow effects on bed shear stress and sheet flow sediment transport
Coastal Engineering 2008, 2009Co-Authors: Peter A. Guard, Peter NielsenAbstract:Recent measurements of velocity and concentration profiles in oscillatory boundary layers over mobile beds of cohesionless sediment have revealed important details of the physical processes involved. New estimates of the hydraulic roughness of mobile beds indicate that the equivalent Nikuradse roughness is much larger than the 2.5d(50) commonly assumed. The vertical gradient of the total shear stress is often large, making it unclear at what elevation we should evaluate the so-called "bed shear stress". Since this is the input for most simple sediment transport formulae, it is important to establish what is meant by this term.
Neil D. Sandham - One of the best experts on this subject based on the ideXlab platform.
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Direct numerical simulation of turbulent channel flow over a surrogate for Nikuradse-type roughness
Journal of Fluid Mechanics, 2017Co-Authors: Manan Thakkar, Angela Busse, Neil D. SandhamAbstract:A tiled approach to rough surface simulation is used to explore the full range of roughness Reynolds numbers, from the limiting case of hydrodynamic smoothness up to fully rough conditions. The surface is based on a scan of a standard grit-blasted comparator, subsequently low-pass filtered and made spatially periodic. High roughness Reynolds numbers are obtained by increasing the friction Reynolds number of the direct numerical simulations, whereas low roughness Reynolds numbers are obtained by scaling the surface down and tiling to maintain a constant domain size. In both cases, computational requirements on box size, resolution in wall units and resolution per minimum wavelength of the rough surface are maintained. The resulting roughness function behaviour replicates to good accuracy the experiments of Nikuradse (1933 VDI-Forschungsheft , vol. 361), suggesting that the processed grit-blasted surface can serve as a surrogate for his sand-grain roughness, the precise structure of which is undocumented. The present simulations also document a monotonic departure from hydrodynamic smooth-wall results, which is fitted with a geometric relation, the exponent of which is found to be inconsistent with both the Colebrook formula and an earlier theoretical argument based on low-Reynolds-number drag relations.
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Dataset for 'DNS of turbulent channel flow over a surrogate for Nikuradse-type roughness'
2017Co-Authors: Manan Thakkar, Angela Busse, Neil D. SandhamAbstract:Data related to the publication: Thakkar, M., Busse, A. & Sandham, N.D. (2017) DNS of turbulent channel flow over a surrogate for Nikuradse-type roughness.Table1.csv contains data from Table 1 in the paper. Additionally, it contains two more columns, showing the values of ks+ (equivalent sand-grain roughness height in wall-units) and Nikuradse's A parameter, for all cases considered.
Ahmed Al-salaymeh - One of the best experts on this subject based on the ideXlab platform.
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Investigations of Tripping Effect on the Friction Factor in Turbulent Pipe Flows
Journal of Fluids Engineering, 2009Co-Authors: Ahmed Al-salaymeh, O. A. BayoumiAbstract:Tripping devices are usually installed at the entrance of laboratory-scale pipe test sections to obtain a fully developed turbulent flow sooner. The tripping of laminar flow to induce turbulence can be carried out in different ways, such as using cylindrical wires, sand papers, well-organized tape elements, fences, etc. Claims of tripping effects have been made since the classical experiments of Nikuradse (1932, Gesetzmassigkeit der turbulenten Stromung in glatten Rohren, Forschungsheft 356, Ausgabe B, Vol. 3, VDI-Verlag, Berlin), which covered a significant range of Reynolds numbers. Nikuradse's data have become the metric by which theories are established and have also been the subject of intense scrutiny. Several subsequent experiments reported friction factors as much as 5% lower than those measured by Nikuradse, and the authors of those reports attributed the difference to tripping effects, e.g., work of Durst et al. (2003, "Investigation of the Mean-Flow Scaling and Tripping Effect on Fully Developed Turbulent Pipe Flow," J. Hydrodynam., 15(1), pp. 14―22). In the present study, measurements with and without ring tripping devices of different blocking areas of 10%, 20%, 30%, and 40% have been carried out to determine the effect of entrance condition on the developing flow field in pipes. Along with pressure drop measurements to compute the skin friction, both the Pitot tube and hot-wire anemometry measurements have been used to accurately determine the mean velocity profile over the working test section at different Reynolds numbers based on the mean velocity and pipe diameter in the range of 1.0 × 10 5 ―4.5 × 10 5 . The results we obtained suggest that the tripping technique has an insignificant effect on the wall friction factor, in agreement with Nikuradse's original data.
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Wall skin friction and mean velocity profiles of fully developed turbulent pipe flows
Experimental Thermal and Fluid Science, 2007Co-Authors: E.-s. Zanoun, F. Durst, O. Bayoumy, Ahmed Al-salaymehAbstract:Abstract The friction factor λ ( Re ) and the mean velocity U + = f ( y + ) measurements of Nikuradse [J. Nikuradse, Gesetzmassigkeite der turbulenten Stromung in glatten Rohren, Forschg. Arb. Ing.-Wes. No. 356 (1932); J. Nikuradse, Stromungsgesetze in rauhen Rohren, Forschg. Arb. Ing.-Wes. No. 361 (1933)] of fully developed turbulent flows in smooth and rough pipes are of vital consideration since they provided the data by which established theories have been developed in the last few decades. The pressure gradient and the resultant friction factor of Nikuradse’ smooth pipe agree well with the authors’ own results. On the other hand, the Nikuradse’s corresponding mean velocity profile measurements show differences from measurements presented in this. The differences might be attributed to the state of Nikuradse’s flow at the location of velocity profile measurements in addition to differences in the applied measuring techniques. It is concluded that pitot tubes do usually not have the needed spatial resolution in the near-wall region and produces therefore velocity overshoots under y + = 300 when used in turbulent pipe shear flows. Hence, when the lower limit for the log-range starts at y + ⩽ 50, which was common for almost all previous work up to the late 1990s, a higher value for the so-called von Karman constant ( κ ) of the logarithmic velocity profile resulted. In addition to pitot tube velocity measurements, hot-wire measurements are provided, showing that the slope (i.e., 1/ κ ) of the logarithmic velocity profile is inconsistent with value deduced from the λ ( Re ) measurements performed by the authors. Nikuradse’s pitot tube velocity data also yield log law constants that are not reflected by their corresponding pipe friction measurements. However, the authors observed that the hot wire and pitot tube results are about the same if the inner limit of the log range of the logarithmic velocity profile is y + ⩾ 300 and the effect of the mean shear gradient is minimal under the same condition.
Costantino Manes - One of the best experts on this subject based on the ideXlab platform.
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A phenomenological model to describe turbulent friction in permeable-wall flows
Geophysical Research Letters, 2012Co-Authors: Costantino Manes, Luca Ridolfi, Gabriel G. KatulAbstract:[1] Describing the canonical properties of turbulent flows over rough-permeable walls such as gravel beds, vegetated- or snow-covered surfaces have, to date, resisted complete theoretical treatment. The major complication in describing such geophysical flows is that the friction factor - Reynolds number relationships significantly deviate from their conventional Nikuradse curves or Moody diagrams derived over impermeable rough boundaries. A novel phenomenological model that describes such anomalous behavior is proposed. It expands the approach in Gioia and Chakraborty (2006) developed for rough-impermeable pipes to include finite velocity effects within the porous wall and canonical length scales governing the momentum exchanges between interstitial and superficial flows.
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Turbulent friction in flows over permeable walls
Geophysical Research Letters, 2011Co-Authors: Costantino Manes, Luca Ridolfi, Dubravka Pokrajac, Vi Nikora, Davide PoggiAbstract:[1] The experimental results of Nikuradse and the concept of hydraulically smooth, transitional, and rough flow regimes are commonly used as a benchmark for data interpretation and modeling of hydraulic resistance. However, Nikuradse's experiments were carried out in pipes with impermeable rough-walls whereas many geophysical flows occur over permeable walls and thus the permeability effects need to be quantified and accounted for. On the basis of our own experimental results, it is shown that wall permeability influences flow resistance dramatically and that the conventional ‘hydraulically-rough regime’, for which the friction factor depends only on the ratio of the roughness size to the flow thickness, does not apply to flows over permeable walls. Indeed, even at high Reynolds number (Re), the friction factor progressively increases with increasing Re. Possible mechanisms that explain this behavior, as well as the implications of these results for modeling of the friction factors and hyporheic exchange in porous-bed rivers are discussed.