Journal articles on the topic 'Wall roughness model'
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1
HERWIG, H., D. GLOSS, and T. WENTERODT. "A new approach to understanding and modelling the influence of wall roughness on friction factors for pipe and channel flows." Journal of Fluid Mechanics 613 (October 1, 2008): 35–53. http://dx.doi.org/10.1017/s0022112008003534.
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In this study, it is shown how the equivalent sand roughness required in the Moody chart can be calculated for arbitrarily shaped wall roughnesses. After a discussion of how to define the wall location and roughness height in the most reasonable way, a numerical approach based on the determination of entropy production in rough pipes and channels is presented. As test cases, three different two-dimensional roughness types have been chosen which are representative of regular roughnesses on machined surfaces. In the turbulent range, skin friction results with these test roughnesses can be linked to Nikuradse's sand roughness results by a constant factor. For laminar flows, a significant effect of wall roughness is identified which in most other studies is neglected completely. The dissipation model of this study is validated with experimental data for laminar and turbulent flows.
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Keirsbulck, L., L. Labraga, A. Mazouz, and C. Tournier. "Surface Roughness Effects on Turbulent Boundary Layer Structures." Journal of Fluids Engineering 124, no. 1 (October 15, 2001): 127–35. http://dx.doi.org/10.1115/1.1445141.
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A turbulent boundary layer structure which develop over a k-type rough wall displays several differences with those found on a smooth surface. The magnitude of the wake strength depends on the wall roughness. In the near-wall region, the contribution to the Reynolds shear stress fraction, corresponding to each event, strongly depends on the wall roughness. In the wall region, the diffusion factors are influenced by the wall roughness where the sweep events largely dominate the ejection events. This trend is reversed for the smooth-wall. Particle Image Velocimetry technique (PIV) is used to obtain the fluctuating flow field in the turbulent boundary layer in order to confirm this behavior. The energy budget analysis shows that the main difference between rough- and smooth-walls appears near the wall where the transport terms are larger for smooth-wall. Vertical and longitudinal turbulent flux of the shear stress on both smooth and rough surfaces is compared to those predicted by a turbulence model. The present results confirm that any turbulence model must take into account the effects of the surface roughness.
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Afzal, Noor. "Power Law Velocity Profile in the Turbulent Boundary Layer on Transitional Rough Surfaces." Journal of Fluids Engineering 129, no. 8 (March 4, 2007): 1083–100. http://dx.doi.org/10.1115/1.2746902.
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A new approach to scaling of transitional wall roughness in turbulent flow is introduced by a new nondimensional roughness scale ϕ. This scale gives rise to an inner viscous length scale ϕν∕uτ, inner wall transitional variable, roughness friction Reynolds number, and roughness Reynolds number. The velocity distribution, just above the roughness level, turns out to be a universal relationship for all kinds of roughness (transitional, fully smooth, and fully rough surfaces), but depends implicitly on roughness scale. The open turbulent boundary layer equations, without any closure model, have been analyzed in the inner wall and outer wake layers, and matching by the Izakson-Millikan-Kolmogorov hypothesis leads to an open functional equation. An alternate open functional equation is obtained from the ratio of two successive derivatives of the basic functional equation of Izakson and Millikan, which admits two functional solutions: the power law velocity profile and the log law velocity profile. The envelope of the skin friction power law gives the log law, as well as the power law index and prefactor as the functions of roughness friction Reynolds number or skin friction coefficient as appropriate. All the results for power law and log law velocity and skin friction distributions, as well as power law constants are explicitly independent of the transitional wall roughness. The universality of these relations is supported very well by extensive experimental data from transitional rough walls for various different types of roughnesses. On the other hand, there are no universal scalings in traditional variables, and different expressions are needed for various types of roughness, such as inflectional roughness, monotonic roughness, and others. To the lowest order, the outer layer flow is governed by the nonlinear turbulent wake equations that match with the power law theory as well as log law theory, in the overlap region. These outer equations are in equilibrium for constant value of m, the pressure gradient parameter, and under constant eddy viscosity closure model, the analytical and numerical solutions are presented.
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Gre´goire, G., M. Favre-Marinet, and F. Julien Saint Amand. "Modeling of Turbulent Fluid Flow Over a Rough Wall With or Without Suction." Journal of Fluids Engineering 125, no. 4 (July 1, 2003): 636–42. http://dx.doi.org/10.1115/1.1593705.
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The turbulent flow close to a wall with two-dimensional roughness is computed with a two-layer zonal model. For an impermeable wall, the classical logarithmic law compares well with the numerical results if the location of the fictitious wall modeling the surface is considered at the top of the rough boundary. The model developed by Wilcox for smooth walls is modified to account for the surface roughness and gives satisfactory results, especially for the friction coefficient, for the case of boundary layer suction.
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Aupoix, B. "A General Strategy to Extend Turbulence Models to Rough Surfaces: Application to Smith’s k-L Model." Journal of Fluids Engineering 129, no. 10 (April 27, 2007): 1245–54. http://dx.doi.org/10.1115/1.2776960.
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A general procedure to extend turbulence models to account for wall roughness, in the framework of the equivalent sand grain approach, is proposed. It is based on the prescription of the turbulent quantities at the wall to reproduce the shift of the logarithmic profile and hence provide the right increase in wall friction. This approach was previously applied to Spalart and Allmaras one equation (1992, “A One-Equation Turbulence Model for Aerodynamic. Flows,” 30th Aerospace Sciences Meeting and Exhibit, Reno, NV, AIAA paper No. 92-0439;1994, ibid, Rech. Aerosp. 1, pp. 5–21). Here, the strategy is detailed and applied to Smith’s two-equation k-L model (1995, “Prediction of Hypersonic Shock Wave Turbulent Boundary Layer Interactions With The k-l Two Equaton Turbulence Model,” 33rd Aerospace Sciences Meeting and Exhibit, Reno, NV, Paper No. 95-0232). The final model form is given. The so-modified Spalart and Allmaras and Smith models were tested on a large variety of test cases, covering a wide range of roughness and boundary layer Reynolds numbers and compared with other models. These tests confirm the validity of the approach to extend any turbulence model to account for wall roughness. They also point out the deficiency of some models to cope with small roughness levels as well as the drawbacks of present correlations to estimate the equivalent sand grain roughness.
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Ivanov, Martin, and Sergey Mijorski. "Development of thermal bridge numerical model, based on conjugate heat transfer and indoor and outdoor environment parameters." E3S Web of Conferences 180 (2020): 04011. http://dx.doi.org/10.1051/e3sconf/202018004011.
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The presented study reveals the development of a 3D numerical model for thermal bridge assessment, based on conjugate heat transfer and CFD methods. With the developed model, thermal simulations are performed, in order to analyse the interaction between different ambient conditions and material properties. The results show that the wall boundary layer profiles are depended on the attached air flow velocity magnitude and implemented wall roughness. The parametric analysis, of the varying ambient air temperatures, confirm the linear dependence to the internal wall surface temperatures. The demonstrated correlations, in regard of the attached air flow velocity magnitude and wall roughness heights, are non-linear. The most characteristic result, achieved in the simulation study, is the impact of the wall roughness, over the internal wall temperature. The increase of the roughness leads to significant increase of the internal wall temperature. Explanation may be found in the boundary layer flow velocity magnitude near the external wall, which decreases the heat energy transfer between the solid and cold fluid medias.
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Anderson, William. "Amplitude modulation of streamwise velocity fluctuations in the roughness sublayer: evidence from large-eddy simulations." Journal of Fluid Mechanics 789 (January 26, 2016): 567–88. http://dx.doi.org/10.1017/jfm.2015.744.
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Recent studies have demonstrated that large- and very-large-scale motions in the logarithmic region of turbulent boundary layers ‘amplitude modulate’ dynamics of the near-wall region (Marusicet al.,Science, vol. 329, 2010, pp. 193–196; Mathiset al.,J. Fluid Mech., vol. 628, 2009a, pp. 311–337). These contributions prompted development of a predictive model for near-wall dynamics (Mathiset al.,J. Fluid Mech., vol. 681, 2011, pp. 537–566) that has promising implications for large-eddy simulations of wall turbulence at high Reynolds numbers (owing to the presence of smaller scales as the wall is approached). Existing studies on the existence of amplitude modulation in wall-bounded turbulence have addressed smooth-wall flows, though high Reynolds number rough-wall flows are ubiquitous. Under such conditions, the production of element-scale vortices ablates the viscous wall region and a new near-wall layer emerges: the roughness sublayer. The roughness sublayer depth scales with aggregate roughness element height,$h$, and is typically$2h\sim 3h$. Above the roughness sublayer, Townsend’s hypothesis dictates that turbulence in the logarithmic layer is unaffected by the roughness sublayer (beyond its role in setting the friction velocity and thus inducing a deficit in the mean streamwise velocity known as the roughness function). Here, we present large-eddy simulation results of turbulent channel flow over rough walls. We follow the decoupling procedure outlined in Mathiset al.(J. Fluid Mech., vol. 628, 2009a, 311–337) and present evidence that outer-layer dynamics amplitude modulate the roughness sublayer. Below the roughness element height, we report enormous sensitivity to the streamwise–spanwise position at which flow statistics are measured, owing to spatial heterogeneities in the roughness sublayer imparted by roughness elements. For$y/h\gtrsim 1.5$(i.e. above the cubes, but within the roughness sublayer), topography dependence rapidly declines.
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Lin, Xiaohui, Fu-bing Bao, Xiaoyan Gao, and Jiemin Chen. "Molecular Dynamics Simulation of Nanoscale Channel Flows with Rough Wall Using the Virtual-Wall Model." Journal of Nanotechnology 2018 (June 24, 2018): 1–7. http://dx.doi.org/10.1155/2018/4631253.
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Molecular dynamics simulation is adopted in the present study to investigate the nanoscale gas flow characteristics in rough channels. The virtual-wall model for the rough wall is proposed and validated. The computational efficiency can be improved greatly by using this model, especially for the low-density gas flow in nanoscale channels. The effect of roughness element geometry on flow behaviors is then studied in detail. The fluid velocity decreases with the increase of roughness element height, while it increases with the increases of element width and spacing.
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Durbin, P. A., G. Medic, J. M. Seo, J. K. Eaton, and S. Song. "Rough Wall Modification of Two-Layer k−ε." Journal of Fluids Engineering 123, no. 1 (November 17, 2000): 16–21. http://dx.doi.org/10.1115/1.1343086.
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A formulation is developed to apply the two-layer k−ε model to rough surfaces. The approach involves modifying the lν formula and the boundary condition on k. A hydrodynamic roughness length is introduced and related to the geometrical roughness through a calibration procedure. An experiment has been conducted to test the model. It provides data on flow over a ramp with and without surface roughness.
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Богомолов, Дмитрий, Dmitriy Bogomolov, Валерий Порошин, Valeriy Poroshin, Валентин Нижник, and Valentin Nizhnik. "Mathematical model of heat flux in continuous media in thin 2d channel with moving rough wall." Bulletin of Bryansk state technical university 2014, no. 4 (December 5, 2014): 100–108. http://dx.doi.org/10.12737/23096.
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The mathematical model of heat flux in continuous media in thin channel with moving rough wall in 2D approach is described.. The results of the comparisons of flow factors and Mussel numbers in channels with smooth walls and channels with real stochastic wall roughness are shown. Both static and dynamic cases were investigated.
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Zhang, Xia, and Lixing Zhou. "A second-order moment particle–wall collision model accounting for the wall roughness." Powder Technology 159, no. 2 (November 2005): 111–20. http://dx.doi.org/10.1016/j.powtec.2005.07.005.
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Naimi, M., and F. B. Gessner. "Calculation of Fully-Developed Turbulent Flow in Rectangular Ducts With Nonuniform Wall Roughness." Journal of Fluids Engineering 119, no. 3 (September 1, 1997): 550–58. http://dx.doi.org/10.1115/1.2819279.
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The predictive capabilities of four transport-type turbulence models are analyzed by comparing predictions with experimental data for fully-developed flow in (1) a rectangular duct with a step change in roughness on one wall (Case 1), and (2) a square duct with one rib-roughened wall (Case 2). The models include the Demuren-Rodi (DR) k-ε model, the Sugiyama et al. (S) k-ε model, the Launder-Li (LL) Reynolds stress transport equation model, and the differential stress (DS) model proposed recently by the authors. For the first flow situation (Case 1), the results show that the DS model yields improved agreement between predicted and measured primary and secondary mean velocity distributions in comparison to the DR and LL models. For the second flow situation (Case 2), the DS model is superior to the DR and S models for predicting experimentally observed mean velocity, turbulence kinetic energy, and Reynolds stress anisotropy behavior, especially in the vicinity of a corner formed by the juncture of adjacent smooth and rough walls. The results are analyzed in order to explain why the DR model leads to the formation of a spurious secondary flow cell near this corner that is not present in the experimental flow.
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Abderrahaman-Elena, Nabil, Chris T. Fairhall, and Ricardo García-Mayoral. "Modulation of near-wall turbulence in the transitionally rough regime." Journal of Fluid Mechanics 865 (March 1, 2019): 1042–71. http://dx.doi.org/10.1017/jfm.2019.41.
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Direct numerical simulations of turbulent channels with rough walls are conducted in the transitionally rough regime. The effect that roughness produces on the overlying turbulence is studied using a modified triple decomposition of the flow. This decomposition separates the roughness-induced contribution from the background turbulence, with the latter essentially free of any texture footprint. For small roughness, the background turbulence is not significantly altered, but merely displaced closer to the roughness crests, with the change in drag being proportional to this displacement. As the roughness size increases, the background turbulence begins to be modified, notably by the increase of energy for short, wide wavelengths, which is consistent with the appearance of a shear-flow instability of the mean flow. A laminar model is presented to estimate the roughness-coherent contribution, as well as the displacement height and the velocity at the roughness crests. Based on the effects observed in the background turbulence, the roughness function is decomposed into different terms to analyse different contributions to the change in drag, laying the foundations for a predictive model.
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GAMRAT, G., M. FAVRE-MARINET, S. LE PERSON, R. BAVIÈRE, and F. AYELA. "An experimental study and modelling of roughness effects on laminar flow in microchannels." Journal of Fluid Mechanics 594 (December 14, 2007): 399–423. http://dx.doi.org/10.1017/s0022112007009111.
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Three different approaches were used in the present study to predict the influence of roughness on laminar flow in microchannels. Experimental investigations were conducted with rough microchannels 100 to 300μm in height (H). The pressure drop was measured in test-sections prepared with well-controlled wall roughness (periodically distributed blocks, relative roughness k* =k/0.5H≈0.15) and in test-sections with randomly distributed particles anchored on the channel walls (k* ≈0.04–0.13). Three-dimensional numerical simulations were conducted with the same geometry as in the test-section with periodical roughness (wavelength L). A one-dimensional model (RLM model) was also developed on the basis of a discrete-element approach and the volume-averaging technique. The numerical simulations, the rough layer model and the experiments agree to show that the Poiseuille number Po increases with the relative roughness and is independent of Re in the laminar regime (Re<2000). The increase in Po observed during the experiments is predicted well both by the three-dimensional simulations and the rough layer model. The RLM model shows that the roughness effect may be interpreted by using an effective roughness height keff. keff/k depends on two dimensionless local parameters: the porosity at the bottom wall; and the roughness height normalized with the distance between the rough elements. The RLM model shows that keff/k is independent of the relative roughness k* at given k/L and may be simply approximated by the law: keff/k = 1 − (c(ϵ)/2π)(L/k) for keff/k>0.2, where c decreases with the porosity ϵ.
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MacDonald, M., L. Chan, D. Chung, N. Hutchins, and A. Ooi. "Turbulent flow over transitionally rough surfaces with varying roughness densities." Journal of Fluid Mechanics 804 (September 8, 2016): 130–61. http://dx.doi.org/10.1017/jfm.2016.459.
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We investigate rough-wall turbulent flows through direct numerical simulations of flow over three-dimensional transitionally rough sinusoidal surfaces. The roughness Reynolds number is fixed at $k^{+}=10$, where $k$ is the sinusoidal semi-amplitude, and the sinusoidal wavelength is varied, resulting in the roughness solidity $\unicode[STIX]{x1D6EC}$ (frontal area divided by plan area) ranging from 0.05 to 0.54. The high cost of resolving both the flow around the dense roughness elements and the bulk flow is circumvented by the use of the minimal-span channel technique, recently demonstrated by Chung et al. (J. Fluid Mech., vol. 773, 2015, pp. 418–431) to accurately determine the Hama roughness function, $\unicode[STIX]{x0394}U^{+}$. Good agreement of the second-order statistics in the near-wall roughness-affected region between minimal- and full-span rough-wall channels is observed. In the sparse regime of roughness ($\unicode[STIX]{x1D6EC}\lesssim 0.15$) the roughness function increases with increasing solidity, while in the dense regime the roughness function decreases with increasing solidity. It was found that the dense regime begins when $\unicode[STIX]{x1D6EC}\gtrsim 0.15{-}0.18$, in agreement with the literature. A model is proposed for the limit of $\unicode[STIX]{x1D6EC}\rightarrow \infty$, which is a smooth wall located at the crest of the roughness elements. This model assists with interpreting the asymptotic behaviour of the roughness, and the rough-wall data presented in this paper show that the near-wall flow is tending towards this modelled limit. The peak streamwise turbulence intensity, which is associated with the turbulent near-wall cycle, is seen to move further away from the wall with increasing solidity. In the sparse regime, increasing $\unicode[STIX]{x1D6EC}$ reduces the streamwise turbulent energy associated with the near-wall cycle, while increasing $\unicode[STIX]{x1D6EC}$ in the dense regime increases turbulent energy. An analysis of the difference of the integrated mean momentum balance between smooth- and rough-wall flows reveals that the roughness function decreases in the dense regime due to a reduction in the Reynolds shear stress. This is predominantly due to the near-wall cycle being pushed away from the roughness elements, which leads to a reduction in turbulent energy in the region previously occupied by these events.
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Taylor, R. P., H. W. Coleman, and B. K. Hodge. "Prediction of Turbulent Rough-Wall Skin Friction Using a Discrete Element Approach." Journal of Fluids Engineering 107, no. 2 (June 1, 1985): 251–57. http://dx.doi.org/10.1115/1.3242469.
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A discrete element model for turbulent flow over rough surfaces has been derived from basic principles. This formulation includes surface roughness form drag and blockage effects as a constituent part of the partial differential equations and does not rely on a single-length-scale concept such as equivalent sandgrain roughness. The roughness model includes the necessary empirical information on the interaction between three-dimensional roughness elements and the flow in a general way which does not require experimental data on each specific surface. This empirical input was determined using data from well-accepted experiments. Predictions using the model are compared with additional data for fully-developed and boundary layer flows. The predictions are shown to compare equally well with both transitionally rough and fully rough turbulent flows without modification of the roughness model.
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Annus, Ivar, Anatoli Vassiljev, Nils Kändler, and Katrin Kaur. "Determination of the corresponding roughness height in a WDS model containing old rough pipes." Journal of Water Supply: Research and Technology-Aqua 69, no. 3 (October 1, 2019): 201–9. http://dx.doi.org/10.2166/aqua.2019.080.
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Abstract The aim of the paper was to determine the influence of irregular pipe wall roughness on the flow velocity in a water distribution system (WDS) containing old pipes. Field studies have shown that due to pipe wall build-up, the shape of the inner pipe surface can vary temporally and spatially. This will lead to unrealistic pipe roughness values when calibrating the WDS model using nominal pipe diameters. Therefore, in this study, three types of pipe wall build-up were investigated using EPANET2 and computational fluid dynamics (CFD) to estimate the velocity correction coefficients for EPANET2 calculations. It was shown that in old rough pipes, the mean velocities are higher than expected, indicating that in water quality estimation in a WDS, actual pipe diameters with reasonable roughness need to be defined.
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Yang, Xiang I. A., Jasim Sadique, Rajat Mittal, and Charles Meneveau. "Exponential roughness layer and analytical model for turbulent boundary layer flow over rectangular-prism roughness elements." Journal of Fluid Mechanics 789 (January 18, 2016): 127–65. http://dx.doi.org/10.1017/jfm.2015.687.
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We conduct a series of large-eddy simulations (LES) to examine the mean flow behaviour within the roughness layer of turbulent boundary layer flow over rough surfaces. We consider several configurations consisting of arrays of rectangular-prism roughness elements with various spacings, aspect ratios and height distributions. The results provide clear evidence for exponential behaviour of the mean flow with respect to the wall normal distance. Such behaviour has been proposed before (see, e.g., Cionco, 1966 Tech. Rep. DTIC document), and is represented as $U(z)/U_{h}=\exp [a(z/h-1)]$, where $U(z)$ is the spatially/temporally averaged fluid velocity, $z$ is the wall normal distance, $h$ represents the height of the roughness elements and $U_{h}$ is the velocity at $z=h$. The attenuation factor $a$ depends on the density of the roughness element distribution and details of the roughness distribution on the wall. Once established, the generic velocity profile shape is used to formulate a fully analytical model for the effective drag exerted by turbulent flow on a surface covered with arrays of rectangular-prism roughness elements. The approach is based on the von Karman–Pohlhausen integral method, in which a shape function is assumed for the mean velocity profile and its parameters are determined based on momentum conservation and other fundamental constraints. In order to determine the attenuation parameter $a$, wake interactions among surface roughness elements are accounted for by using the concept of flow sheltering. The model transitions smoothly between ‘$k$’ and ‘$d$’ type roughness conditions depending on the surface coverage density and the detailed geometry of roughness elements. Comparisons between model predictions and experimental/numerical data from the existing literature as well as LES data from this study are presented. It is shown that the analytical model provides good predictions of mean velocity and drag forces for the cases considered, thus raising the hope that analytical roughness modelling based on surface geometry is possible, at least for cases when the location of flow separation over surface elements can be easily predicted, as in the case of wall-attached rectangular-prism roughness elements.
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Wu, Sicong, Kenneth T. Christensen, and Carlos Pantano. "Modelling smooth- and transitionally rough-wall turbulent channel flow by leveraging inner–outer interactions and principal component analysis." Journal of Fluid Mechanics 863 (January 29, 2019): 407–53. http://dx.doi.org/10.1017/jfm.2018.899.
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Direct numerical simulations (DNS) of turbulent channel flow over rough surfaces, formed from hexagonally packed arrays of hemispheres on both walls, were performed at friction Reynolds numbers $Re_{\unicode[STIX]{x1D70F}}=200$, $400$ and $600$. The inner normalized roughness height $k^{+}=20$ was maintained for all Reynolds numbers, meaning all flows were classified as transitionally rough. The spacing between hemispheres was varied within $d/k=2$–$4$. The statistical properties of the rough-wall flows were contrasted against a complementary smooth-wall DNS at $Re_{\unicode[STIX]{x1D70F}}=400$ and literature data at $Re_{\unicode[STIX]{x1D70F}}=2003$ revealing strong modifications of the near-wall turbulence, although the outer-layer structure was found to be qualitatively consistent with smooth-wall flow. Amplitude modulation (AM) analysis was used to explore the degree of interaction between the flow in the roughness sublayer and that of the outer layer utilizing all velocity components. This analysis revealed stronger modulation effects, compared to smooth-wall flow, on the near-wall small-scale fluctuations by the larger-scale structures residing in the outer layer irrespective of roughness arrangement and Reynolds number. A predictive inner–outer model based on these interactions, and exploiting principal component analysis (PCA), was developed to predict the statistics of higher-order moments of all velocity fluctuations, thus addressing modelling of anisotropic effects introduced by roughness. The results show excellent agreement between the predicted near-wall statistics up to fourth-order moments compared to the original statistics from the DNS, which highlights the utility of the PCA-enhanced AM model in generating physics-based predictions in both smooth- and rough-wall turbulence.
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Mitchell, R. J. "Model studies on the stability of confined fills." Canadian Geotechnical Journal 26, no. 2 (May 1, 1989): 210–16. http://dx.doi.org/10.1139/t89-030.
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Stabilized backfills are used extensively for ground control in most bulk underground mining operations. The stability of the fill face during pillar ore removal is of prime concern and has been the subject of considerable research because of many factors, including the effects of wall interaction and the high costs of stabilization. Centrifuge modelling data presented in this paper clearly show that fill confined between sloped walls, which is the most common prototype condition, is much more stable than fill between vertical rock walls, a condition previously studied. This study also shows that wall roughness contributes substantially to the fill stability and that current design criteria are quite conservative. Data from stress transducers, mounted on centrifuge strongboxes to monitor stress changes on the fill boundaries during gravitational stress simulations, show that fill arching and wall shear effects are responsible for the improved stability of confined fills. Key words: mine backfill, stability, lateral stresses, arching, centrifuge model, fill geometry.
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García-López, Erika, Juansethi Ibarra-Medina, Hector Siller, Jan Lammel-Lindemann, and Ciro Rodriguez. "Surface Finish and Back-Wall Dross Behavior during the Fiber Laser Cutting of AZ31 Magnesium Alloy." Micromachines 9, no. 10 (September 24, 2018): 485. http://dx.doi.org/10.3390/mi9100485.
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Magnesium alloys are of increasing interest in the medical industry due to their biodegradability properties and better mechanical properties as compared to biodegradable polymers. Fiber laser cutting of AZ31 magnesium alloy tubes was carried out to study the effect of cutting conditions on wall surface roughness and back-wall dross. During the experiments, an argon gas chamber was adapted in order to avoid material reactivity with oxygen and thus better control the part quality. A surface response methodology was applied to identify the significance of pulse overlapping and pulse energy. Our results indicate minimum values of surface roughness (Ra < 0.7 μm) when the spot overlapping is higher than 50%. A back-wall dross range of 0.24% to 0.94% was established. In addition, a reduction in back-wall dross accumulations was obtained after blowing away the dross particles from inside the tube using an argon gas jet, reaching values of 0.21%. Laser cutting experimental models show a quadratic model for back-wall dross related with the interaction of the pulse energy, and a linear model dependent on pulse overlapping factor for surface roughness.
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Scaggs, W. F., R. P. Taylor, and H. W. Coleman. "Measurement and Prediction of Rough Wall Effects on Friction Factor—Uniform Roughness Results." Journal of Fluids Engineering 110, no. 4 (December 1, 1988): 385–91. http://dx.doi.org/10.1115/1.3243568.
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The results of an experimental investigation of the effects of surface roughness on turbulent pipe flow friction factors are presented and compared with predictions from a previously published discrete element roughness model. Friction factor data were acquired over a pipe Reynolds number range from 10,000 to 600,000 for nine different uniformly rough surfaces. These surfaces covered a range of roughness element sizes, spacings and shapes. Predictions from the discrete element roughness model were in very good agreement with the data.
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Straka, Petr, and Jaromír Příhoda. "Extension of the algebraic transition model for the wall roughness effect." EPJ Web of Conferences 114 (2016): 02114. http://dx.doi.org/10.1051/epjconf/201611402114.
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Wang, Haoli, Yuan Wang, and Jiazhong Zhang. "Influence of Ribbon Structure Rough Wall on the Microscale Poiseuille Flow." Journal of Fluids Engineering 127, no. 6 (June 25, 2005): 1140–45. http://dx.doi.org/10.1115/1.2060733.
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The regular perturbation method is introduced to investigate the influence of two-dimensional roughness on laminar flow in microchannels between two parallel plates. By superimposing a series of harmonic functions with identical dimensional amplitude as well as the same fundamental wave number, the wall roughness functions are obtained and the relative roughness can be determined as the maximal value of the product between the normalized roughness functions and a small parameter. Through modifying the fundamental wave number, the dimensionless roughness spacing is changed. Under this roughness model, the equations with respect to the disturbance stream function are obtained and analyzed numerically. The numerical results show that flowing in microchannels are more complex than that in macrochannels; there exist apparent fluctuations with streamlines and clear vortex structures in microchannels; the flow resistances are about 5–80% higher than the theoretical value under different wall-roughness parameters. Furthermore, analysis shows that the effect of roughness on the flow pattern is distinct from that on the friction factor.
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Tang, Feng, Yue Zhong Li, and Xiao Ming Guan. "A Study of Velocity Distribution Impact of Wall Roughness on Ultrasonic Gas Flowmeter." Advanced Materials Research 433-440 (January 2012): 349–52. http://dx.doi.org/10.4028/www.scientific.net/amr.433-440.349.
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In order to study the effects of velocity distribution of ultrasonic gas flow meter based on wall roughness, , a full developed turbulence model is established based on the theory of prandtl mixing length and flow loss coefficent λ has been calculated by using Colebrook friction correlation formula after analyzing time interval difference measuring method of ultrasonic flow meter. Through Matlab calculating and simulating velocity distribution in different conditions of wall roughness about the model shows and through Fluent simulating, velocity vector and pressure distribution in smooth and rough tube have been compared and analyzed. The experimental results shows that quantification of velocity distribution based on wall roughness can been solved with the model, and the analysis results have certain instructive significance to the development of ultrasonic gas flow meter.
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Gu¨lich, J. F. "Effect of Reynolds Number and Surface Roughness on the Efficiency of Centrifugal Pumps." Journal of Fluids Engineering 125, no. 4 (July 1, 2003): 670–79. http://dx.doi.org/10.1115/1.1593711.
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A procedure has been developed to predict the effects of roughness and Reynolds number on the change in efficiency from a model or baseline to a prototype pump (“efficiency scaling”). The analysis of individual losses takes into account different roughnesses of impeller, diffuser/volute, impeller side disks, and casing walls in the impeller side rooms. The method also allows to predict the effect of roughness and Reynolds number on the hydraulic efficiency. The calculations are based on physical models but the weighting of impeller versus diffuser/volute roughness and the fraction of scalable losses within impeller and diffuser/volute are determined empirically from the analysis of tests with industrial pumps. The fraction of scalable impeller/diffuser/volute losses is found to decrease with growing specific speed. Roughness effects in the diffuser/volute are stronger than in the impeller, but the dominance of the stator over the rotor decreases with increasing specific speed. The procedure includes all flow regimes from laminar to turbulent and from hydraulically smooth to fully rough. It is validated by tests with viscosities between 0.2 to 3000 cSt and Reynolds numbers between 1500 and 108. The hydraulic losses depend on the patterns of roughness, near-wall turbulence, and the actual velocity distribution in the hydraulic passages. These effects—which are as yet not amenable to analysis—limit the accuracy of any efficiency prediction procedure for decelerated flows.
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Shishkina, Olga, and Claus Wagner. "Modelling the influence of wall roughness on heat transfer in thermal convection." Journal of Fluid Mechanics 686 (September 27, 2011): 568–82. http://dx.doi.org/10.1017/jfm.2011.348.
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AbstractThe objective of this study is to approximate heat transport in thermal convection enhanced by the roughness of heated/cooled horizontal plates. The roughness is introduced by a set of rectangular heated/cooled obstacles located at the corresponding plates. An analytical model to estimate the Nusselt number deviations caused by the wall roughness is developed. It is based on the two-dimensional Prandtl–Blasius boundary layer equations and therefore is valid for moderate Rayleigh numbers and regular wall roughness, for which the height of the obstacles and the distances between them are significantly larger than the thickness of the thermal boundary layers. To validate this model, the transport of heat and momentum in rectangular convection cells is studied in two-dimensional Navier–Stokes simulations, for different aspect ratios of the obstacles. It is found that the model predicts the heat transport with errors ${\leq }6\hspace{0.167em} \% $ for all investigated cases.
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Bo, Zheng, Qi Zhao, Xiaorui Shuai, Jianhua Yan, and Kefa Cen. "Numerical study on the pressure drop of fluid flow in rough microchannels via the lattice Boltzmann method." International Journal of Numerical Methods for Heat & Fluid Flow 25, no. 8 (November 2, 2015): 2022–31. http://dx.doi.org/10.1108/hff-12-2014-0379.
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Purpose – The purpose of this paper is to provide a quantitative assessment on the effect of wall roughness on the pressure drop of fluid flow in microchannels. Design/methodology/approach – The wall roughness is generated by the method of random midpoint displacement (RMD) and the lattice Boltzmann BGK model is applied. The influences of Reynolds number, relative roughness and the Hurst exponent of roughness profile on the Poiseuille number are investigated. Findings – Unlike the smooth channel flow, Reynolds number, relative roughness and the Hurst exponent of roughness profiles play critical roles on the Poiseuille number Po in rough microchannels. Modeling results indicate that, in rough microchannels, the rough surface configuration intensifies the flow-surface interactions and the wall conditions turn to dominate the flow characteristics. The perturbance of the local flows near the channel wall and the formation of recirculation regions are two main features of the flow-surface interactions. Research limitations/implications – The fluid flow in parallel planes with surface roughness is considered in the current study. In other words, only two-dimensional fluid flow is investigated. Practical implications – The LBM is a very useful tool to investigate the microscale flows. Originality/value – A new method (RMD) is applied to generate the wall roughness in parallel plane and LBM is conducted to investigate the pressure drop characteristics in rough microchannels.
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Patel, V. C., and J. Y. Yoon. "Application of Turbulence Models to Separated Flow Over Rough Surfaces." Journal of Fluids Engineering 117, no. 2 (June 1, 1995): 234–41. http://dx.doi.org/10.1115/1.2817135.
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Principal results of classical experiments on the effects of sandgrain roughness are briefly reviewed, along with various models that have been proposed to account for these effects in numerical solutions of the fluid-flow equations. Two models that resolve the near-wall flow are applied to the flow in a two-dimensional, rough-wall channel. Comparisons with analytical results embodied in the well-known Moody diagram show that the k–ω model of Wilcox performs remarkably well over a wide range of roughness values, while a modified two-layer k–ε based model requires further refinement. The k–ω model is applied to water flow over a fixed sand dune for which extensive experimental data are available. The solutions are found to be in agreement with data, including the flow in the separation eddy and its recovery after reattachment. The results suggest that this modeling approach may be extended to other types of surface roughness, and to more complex flows.
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Jun, Li, Chunyuan Ma, Wang Tao, Jingcai Chang, and Xiqiang Zhao. "Effects of roughness on the performance of axial flow cyclone separators using numerical simulation method." Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 233, no. 7 (February 26, 2019): 914–27. http://dx.doi.org/10.1177/0957650919831892.
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An axial flow cyclone is a separator with high efficiency and low resistance. Researchers have extensively studied the structure and parameters that have the greatest influence on its performance. However, the influence of wall roughness on the performance of axial flow cyclones has been neglected for a long time. The wall roughness height can be changed by the manufacturing process and the effect of particles on the wall. Thus, in this study, the effects of roughness on an axial flow cyclone are investigated using a numerical simulation method. The Reynolds stress model and discrete phase model are used for gas and particle prediction and the simulation result were verified through experimentation. The results of the numerical simulation show that the roughness height has big influence on axial flow cyclones. The separation efficiency decreases and static pressure drop increases with increasing roughness height. This happens especially at high inlet velocity. The tangential velocity decreases, particularly near the inner surface of the cyclone, and axial velocity increases in the center of the pipe. The trends show that the degree of change reduced for all parameters with increasing roughness height.
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Li, Lei, Yuliang Su, Han Wang, Guanglong Sheng, and Wendong Wang. "A New Slip Length Model for Enhanced Water Flow Coupling Molecular Interaction, Pore Dimension, Wall Roughness, and Temperature." Advances in Polymer Technology 2019 (December 17, 2019): 1–12. http://dx.doi.org/10.1155/2019/6424012.
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In this paper, a slip length model is proposed to analyze the enhanced flow based on the Hagen–Poiseuille equation. The model considers the multimechanisms including wall-water molecular interactions, pore dimensions, fractal roughness, and temperature. The increasing wall-water interactions result in the greater slip length and flow enhancement factor. The increased temperature enhances the kinetic energy of water molecules that leads to great surface diffusion coefficient and small work of adhesion. The wall roughness can decrease the slip length and flow enhancement factor in hydrophilic nanopores. This work studies the effects of multimechanisms on slip length and flow enhancement factor theoretically, which can accurately describe the liquid flow in nanopores.
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Lambeth, Christopher, Ziyu Wang, Kristina Kairaitis, Abouzar Moshfegh, Ahmad Jabbarzadeh, and Terence Amis. "Modelling mucosal surface roughness in the human velopharynx: a computational fluid dynamics study of healthy and obstructive sleep apnea airways." Journal of Applied Physiology 125, no. 6 (December 1, 2018): 1821–31. http://dx.doi.org/10.1152/japplphysiol.00233.2018.
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We previously published a unique methodology for quantifying human velopharyngeal mucosal surface topography and found increased mucosal surface roughness in patients with obstructive sleep apnea (OSA). In fluid mechanics, surface roughness is associated with increased frictional pressure losses and resistance. This study used computational fluid dynamics (CFD) to analyze the mechanistic effect of different levels of mucosal surface roughness on velopharyngeal airflow. Reconstructed velopharyngeal models from OSA and control subjects were modified, giving each model three levels of roughness, quantified by the curvature-based surface roughness index (CBSRI0.6) (range 24.8–68.6 mm−1). CFD using the k-ω shear stress transport turbulence model was performed (unidirectional, inspiratory, steady-state, 15l/min volumetric flow rate), and the effects of roughness on flow velocity, intraluminal pressure, wall shear stress, and velopharyngeal resistance ( Rv) were examined. Across all models, increasing roughness increased maximum flow velocity, wall shear stress, and flow disruption while decreasing intraluminal pressures. Linear mixed effects modeling demonstrated a log-linear relationship between CBSRI0.6 and Rv, with a common slope (log( Rv)/CBSRI0.6) of 0.0079 [95% confidence interval (CI) 0.0015–0.0143; P = 0.019] for all subjects, equating to a 1.9-fold increase in Rv when roughness increased from control to OSA levels. At any fixed CBSRI0.6, the estimated difference in log( Rv) between OSA and control models was 0.9382 (95% CI 0.0032–1.8732; P = 0.049), equating to an 8.7-fold increase in Rv. This study supports the hypothesis that increasing mucosal surface roughness increases velopharyngeal airway resistance, particularly for anatomically narrower OSA airways, and may thus contribute to increased vulnerability to upper airway collapse in patients with OSA. NEW & NOTEWORTHY Increased mucosal surface roughness in the velopharynx of patients with obstructive sleep apnea (OSA) has recently been identified, but its role in OSA pathogenesis is unknown. This is the first study to model the impact of increased roughness on airflow mechanics in the velopharynx. We report that increasing roughness significantly affects airflow, increasing velopharyngeal resistance and potentially increasing the vulnerability to upper airway collapse, particularly in those patients with an already compromised anatomy.
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Effendy, Marwan, Yu Feng Yao, and Jun Yao. "Effect of Mesh Topologies on Wall Heat Transfer and Pressure Loss Prediction of a Blade Coolant Passage." Applied Mechanics and Materials 315 (April 2013): 216–20. http://dx.doi.org/10.4028/www.scientific.net/amm.315.216.
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This paper studies the effect of mesh topologies such as hybrid and structured meshes on the evaluation of wall heat transfer coefficient (HTC) and pressure loss of a blade cooling passage. An experimental model is chosen; it has five-row of stream wise staggered elliptical pin-fin fitted inside a 10owedge-shape duct and one-row of fillet circular pin-fin in the exit region. Simulations consider two types; i.e. warm test with isothermal wall condition and cold test with adiabatic wall condition respectively, in order to evaluate flow and thermal characteristics such as HTC and pressure loss. Further simulations are carried out by varying Re number, wall surfaces roughness, inlet turbulence intensity and turbulence models. It was found that for unstructured or structured mesh with proper near wall and middle passage grid resolutions, CFD predicted HTC and pressure loss are in good agreement with available experimental data. The wall surface roughness is found to have significant impact on HTC, simulations produce results in better agreement with experimental measurements. Simulation results also confirm that inlet turbulence intensity and turbulence model have insignificant effect of predicting the pin-fin wall and end wall heat transfer coefficient.
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Boulle, A., I. C. Infante, and N. Lemée. "Diffuse X-ray scattering from 180° ferroelectric stripe domains: polarization-induced strain, period disorder and wall roughness." Journal of Applied Crystallography 49, no. 3 (May 4, 2016): 845–55. http://dx.doi.org/10.1107/s1600576716005331.
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A key element in ferroic materials is the presence of walls separating domains with different orientations of the order parameter. It is demonstrated that 180° stripe domains in ferroelectric films give rise to very distinct features in their diffuse X-ray scattering (DXS) intensity distributions. A model is developed that allows the determination of not only the domain period but also the period disorder, the thickness and roughness of the domain walls, and the strain induced by the rotation of the polarization. As an example, the model is applied to ferroelectric/paraelectric superlattices. Temperature-dependent DXS measurements reveal that the polarization-induced strain decreases dramatically with increasing temperature and vanishes at the Curie temperature. The motion of ferroelectric domain walls appears to be a collective process that does not create any disorder in the domain period, whereas pinning by structural defects increases the wall roughness. This work will facilitatein situquantitative studies of ferroic domains and domain wall dynamics under the application of external stimuli, including electric fields and temperature.
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Lin, Jian-Hung, and Keh-Chin Chang. "A Modeling Study on Particle Dispersion in Wall-Bounded Turbulent Flows." Advances in Applied Mathematics and Mechanics 6, no. 06 (December 2014): 764–82. http://dx.doi.org/10.4208/aamm.2014.m533.
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AbstractThree physical mechanisms which may affect dispersion of particle’s motion in wall-bounded turbulent flows, including the effects of turbulence, wall roughness in particle-wall collisions, and inter-particle collisions, are numerically investigated in this study. Parametric studies with different wall roughness extents and with different mass loading ratios of particles are performed in fully developed channel flows with the Eulerian-Lagrangian approach. A low-Reynolds-numberk–εturbulence model is applied for the solution of the carrier-flow field, while the deterministic Lagrangian method together with binary-collision hard-sphere model is applied for the solution of particle motion. It is shown that the mechanism of inter-particle collisions should be taken into account in the modeling except for the flows laden with sufficiently low mass loading ratios of particles. Influences of wall roughness on particle dispersion due to particle-wall collisions are found to be considerable in the bounded particle–laden flow. Since the investigated particles are associated with large Stokes numbers, i.e., larger thanO(1), in the test problem, the effects of turbulence on particle dispersion are much less considerable, as expected, in comparison with another two physical mechanisms investigated in the study.
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Aupoix, B., and P. R. Spalart. "Extensions of the Spalart–Allmaras turbulence model to account for wall roughness." International Journal of Heat and Fluid Flow 24, no. 4 (August 2003): 454–62. http://dx.doi.org/10.1016/s0142-727x(03)00043-2.
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Hu, Ya Lun, Zheng Li, and Zhong Xu. "Simulation of Microbubbles Drag Reduction on Nonsmooth Surface with Hydrophobic Property." Applied Mechanics and Materials 300-301 (February 2013): 3–9. http://dx.doi.org/10.4028/www.scientific.net/amm.300-301.3.
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The friction resistance accounts for a large proportion of the total resistance, during the navigation of ships and underwater vehicles. Drag reduction techniques can significantly reduce the friction resistance of the wall and improve the speed of navigation. This paper combine microbubbles drag reduction technology and nonsmooth and hydrophobic surface technology. Building different analysis models, considering the dimples size of nonsmooth surface , the contact angle of surface and wall roughness, study the law between drag reduction and parameters of the wall by gas-liquid two-phase flow model. Under the same conditions, analysis results show that the performance of drag reduction is mainly determined by the dimple size of nonsmooth surface. The lager dimples cause stronger turbulence and loss more energy. The drag reduction effect is declined. There is a linear relationship between the drag reduction and the contact angle of hydrophobic surface. The drag reduction is enhanced by increasing the contact angle. But the principle is complicated between drag reduction and the roughness of the wall. There are different roughness to achieve the best effect under different flow velocities.
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Kleinstreuer, C., and J. Koo. "Computational Analysis of Wall Roughness Effects for Liquid Flow in Micro-Conduits." Journal of Fluids Engineering 126, no. 1 (January 1, 2004): 1–9. http://dx.doi.org/10.1115/1.1637633.
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Fluid flow in microchannels or microtubes may differ in terms of wall frictional effects, and hence flow rates, when compared to macrochannels. Focusing on steady laminar fully developed flow of a liquid in different micro-conduits, relative surface roughness is captured in terms of a porous medium layer (PML) model. The new approach allows the evaluation of microfluidics variables as a function of PML characteristics, i.e., layer thickness and porosity, uncertainties in measuring hydraulic diameters as well as the inlet Reynolds number. Specifically, realistic values for the PML Darcy number, relative surface roughness, and actual flow area are taken into account to match observed friction factors in micro-conduits. The model predictions compared well with measured data sets for systems with significant relative roughness values. Although other surface effects may have influenced the experimental results as well, surface roughness is found to affect the friction factor and hence the flow parameters in relatively rough channels, e.g., those which are made of aluminum or stainless steel by way of micro-cutting processes.
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Li, Mingzhong, Chunting Liu, and Guodong Zhang. "Calibration of the Interaction Parameters between the Proppant and Fracture Wall and the Effects of These Parameters on Proppant Distribution." Energies 13, no. 8 (April 23, 2020): 2099. http://dx.doi.org/10.3390/en13082099.
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Saltation and reputation (creep) dominate proppant transport rather than suspension during slickwater fracturing, due to the low sand-carrying capacity of the slickwater. Thus, the interaction parameters between proppants and fracture walls, which affect saltation and reputation, play a more critical role in proppant transport. In this paper, a calibration method for the interaction parameters between proppants and walls is built. A three-dimensional coupled computational fluid dynamics–discrete element method (CFD–DEM) model is established to study the effects of the interaction parameters on proppant migration, considering the wall roughness and unevenly distributed diameters of proppants. The simulation results show that a lower static friction coefficient and rolling friction coefficient can result in a smaller equilibrium height of the sand bank and a smaller build angle and drawdown angle, which is beneficial for carrying the proppant to the distal end of the fracture. The wall roughness and the unevenly distributed diameter of the proppants increase the collision between proppant and proppant or the wall, whereas the interactions have little impact on the sandbank morphology, slightly increasing the equilibrium height of the sandbank.
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Singh, Kalyan Kumar, and Dhiraj Kumar. "Experimental investigation and modelling of drilling on multi-wall carbon nanotube–embedded epoxy/glass fabric polymeric nanocomposites." Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture 232, no. 11 (December 20, 2016): 1943–59. http://dx.doi.org/10.1177/0954405416682277.
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The primary objective of this research is to investigate the effect of multi-wall carbon nanotubes on drilling of multi-wall carbon nanotube–embedded epoxy/glass fabric polymeric nanocomposites. The experiments were conducted on composites with varying the weight percentage of multi-wall carbon nanotubes content to analyse drilling-induced delamination and surface roughness, which affect the quality and property of the drilled holes. The drilling parameters considered are spindle speed, feed rate and drill diameter. The microstructure of the holes was characterized using field emission scanning electron microscopy methods. For correlating the effect of the weight percentage of carbon nanotubes with the referred drilling parameters, a mathematical model was used, based on response surface methodology. For development of the mathematical model, four factors, namely, spindle speed, feed rate, diameter of drill and weight percentage of carbon nanotubes, were taken into account. The result established that delamination and surface roughness are reduced as multi-wall carbon nanotubes’ content increases. Maximum improvement in delamination factor was observed in the case of 1.0 wt% multi-wall carbon nanotube–embedded epoxy/glass fabric polymeric nanocomposite, which is 25% and 31.09% at the entrance and exit sides of the hole, respectively. With an increase in the feed rate and the drill diameter, delamination factor increases; however, with an increase in spindle speed, delamination factor decreases. Lower value of surface roughness (1.113 µm) was observed in 1.5 wt% of multi-wall carbon nanotube–embedded epoxy/glass fabric polymeric nanocomposite. However, surface roughness increases with an increase in feed rate and drill diameter.
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Cui, Jie, Virendra C. Patel, and Ching-Long Lin. "Prediction of Turbulent Flow Over Rough Surfaces Using a Force Field in Large Eddy Simulation." Journal of Fluids Engineering 125, no. 1 (January 1, 2003): 2–9. http://dx.doi.org/10.1115/1.1524587.
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A force field model to simulate turbulent flow over a surface with arbitrary roughness is described. A given roughness is decomposed into resolved and subgrid-scale roughness, conceptually similar to the flow decomposition in large eddy simulation (LES). For a given flow and Reynolds number, a Cartesian grid is selected to satisfy LES requirements. This grid determines the geometric features of the roughness that are formally resolved. The force field used to represent this resolved roughness is determined during the LES solution process, without any empirical input. The subgrid roughness that is not resolved is modeled by a random force distribution in which a drag coefficient is specified. Use of this approach to model surface roughness is demonstrated by calculations of the flow in a duct with a wavy wall with superimposed fine-grain roughness.
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Cavar, Dalibor, Pierre-Elouan Réthoré, Andreas Bechmann, Niels N. Sørensen, Benjamin Martinez, Frederik Zahle, Jacob Berg, and Mark C. Kelly. "Comparison of OpenFOAM and EllipSys3D for neutral atmospheric flow over complex terrain." Wind Energy Science 1, no. 1 (May 20, 2016): 55–70. http://dx.doi.org/10.5194/wes-1-55-2016.
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Abstract. The flow solvers OpenFOAM and EllipSys3D are compared in the case of neutral atmospheric flow over terrain using the test cases of Askervein and Bolund hills. Both solvers are run using the steady-state Reynolds-averaged Navier–Stokes k–ϵ turbulence model. One of the main modeling differences between the two solvers is the wall-function approach. The OpenFOAM v.1.7.1 uses a Nikuradse's sand roughness model, while EllipSys3D uses a model based on the atmospheric roughness length. It is found that Nikuradse's model introduces an error dependent on the near-wall cell height. To mitigate this error the near-wall cells should be at least 10 times larger than the surface roughness. It is nonetheless possible to obtain very similar results between EllipSys3D and OpenFOAM v.1.7.1. The more recent OpenFOAM v.2.2.1, which includes the atmospheric roughness length wall-function approach, has also been tested and compared to the results of OpenFOAM v.1.7.1 and EllipSys3D. The numerical results obtained using the same wall-modeling approach in both EllipSys3D and OpenFOAM v.2.1.1 proved to be almost identical. Two meshing strategies are investigated using HypGrid and SnappyHexMesh. The performance of OpenFOAM on SnappyHexMesh-based low-aspect-ratio unstructured meshes is found to be almost an order of magnitude faster than on HypGrid-based structured and high-aspect-ratio meshes. However, proper control of boundary layer resolution is found to be very difficult when the SnappyHexMesh tool is utilized for grid generation purposes. The OpenFOAM is generally found to be 2–6 times slower than EllipSys3D in achieving numerical results of the same order of accuracy on similar or identical computational meshes, when utilization of EllipSys3D default grid sequencing procedures is included.
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Robertson, Iain, Adrien Becot, Adrian Gaylard, and Ben Thornber. "Automotive Drag Reduction through Distributed Base Roughness Elements." Applied Mechanics and Materials 553 (May 2014): 267–72. http://dx.doi.org/10.4028/www.scientific.net/amm.553.267.
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This paper focuses on the effect of base roughness added to the rear of an automotive reference model, the Windsor model. This roughness addition was found to reduce both the drag and the lift of the model. RANS CFD simulations presented here replicate the experimentally observed drag reduction and enable a detailed examination of the mechanisms behind this effect. Investigations into the wake structure of the configurations with base roughness and the baseline case without base roughness showed the main changes to the wake to include a reduction in the overall size of the wake with base roughness present. Furthermore a reduction in the near wall velocities at the rear of the model caused stretching of the upper and lower vortices, a more turbulent near wake and pressure recovery over much of the rear face. This leads to reduce levels of pressure drag on the model.
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Bavière, R., G. Gamrat, M. Favre-Marinet, and S. Le Person. "Modeling of Laminar Flows in Rough-Wall Microchannels." Journal of Fluids Engineering 128, no. 4 (November 22, 2005): 734–41. http://dx.doi.org/10.1115/1.2201635.
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Numerical modeling and analytical approach were used to compute laminar flows in rough-wall microchannels. Both models considered the same arrangements of rectangular prism rough elements in periodical arrays. The numerical results confirmed that the flow is independent of the Reynolds number in the range 1–200. The analytical model needs only one constant for most geometrical arrangements. It compares well with the numerical results. Moreover, both models are consistent with experimental data. They show that the rough elements drag is mainly responsible for the pressure drop across the channel in the upper part of the relative roughness range.
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Bou-Zeid, Elie, Marc B. Parlange, and Charles Meneveau. "On the Parameterization of Surface Roughness at Regional Scales." Journal of the Atmospheric Sciences 64, no. 1 (January 1, 2007): 216–27. http://dx.doi.org/10.1175/jas3826.1.
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Abstract A parameterization for surface roughness and blending height at regional scales, under neutral atmospheric stability, is studied and tested. The analysis is based on a suite of large-eddy simulations (LES) over surfaces with varying roughness height and multiple variability scales. The LES are based on the scale-dependent Lagrangian dynamic subgrid-scale model, and the surface roughnesses at the ground are imposed using the rough-wall logarithmic law. Several patterns of roughness distribution are considered, including random tiling of patches with a wide distribution of length scales. An integral length scale, based on the one-dimensional structure function of the spatially variable roughness height, is used to define the characteristic surface variability scale, which is a critical input in many regional parameterization schemes. Properties of the simulated flow are discussed with special emphasis on the turbulence properties over patches of unequal roughness. The simulations are then used to assess a generalized form of the parameterization for the blending height and the equivalent surface roughness at regional scales that has been developed earlier for regular patterns of surface roughness (regular stripes). The results are also compared with other parameterizations proposed in the literature. Good agreement is found between the simulations and the regional-scale parameterization for the surface roughness and the blending height when this parameterization is combined with the characteristic surface variability scale proposed in this paper.
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MacDonald, M., D. Chung, N. Hutchins, L. Chan, A. Ooi, and R. García-Mayoral. "The minimal-span channel for rough-wall turbulent flows." Journal of Fluid Mechanics 816 (February 28, 2017): 5–42. http://dx.doi.org/10.1017/jfm.2017.69.
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Roughness predominantly alters the near-wall region of turbulent flow while the outer layer remains similar with respect to the wall shear stress. This makes it a prime candidate for the minimal-span channel, which only captures the near-wall flow by restricting the spanwise channel width to be of the order of a few hundred viscous units. Recently, Chung et al. (J. Fluid Mech., vol. 773, 2015, pp. 418–431) showed that a minimal-span channel can accurately characterise the hydraulic behaviour of roughness. Following this, we aim to investigate the fundamental dynamics of the minimal-span channel framework with an eye towards further improving performance. The streamwise domain length of the channel is investigated with the minimum length found to be three times the spanwise width or 1000 viscous units, whichever is longer. The outer layer of the minimal channel is inherently unphysical and as such alterations to it can be performed so long as the near-wall flow, which is the same as in a full-span channel, remains unchanged. Firstly, a half-height (open) channel with slip wall is shown to reproduce the near-wall behaviour seen in a standard channel, but with half the number of grid points. Next, a forcing model is introduced into the outer layer of a half-height channel. This reduces the high streamwise velocity associated with the minimal channel and allows for a larger computational time step. Finally, an investigation is conducted to see if varying the roughness Reynolds number with time is a feasible method for obtaining the full hydraulic behaviour of a rough surface. Currently, multiple steady simulations at fixed roughness Reynolds numbers are needed to obtain this behaviour. The results indicate that the non-dimensional pressure gradient parameter must be kept below 0.03–0.07 to ensure that pressure gradient effects do not lead to an inaccurate roughness function. An empirical costing argument is developed to determine the cost in terms of CPU hours of minimal-span channel simulations a priori. This argument involves counting the number of eddy lifespans in the channel, which is then related to the statistical uncertainty of the streamwise velocity. For a given statistical uncertainty in the roughness function, this can then be used to determine the simulation run time. Following this, a finite-volume code with a body-fitted grid is used to determine the roughness function for square-based pyramids using the above insights. Comparisons to experimental studies for the same roughness geometry are made and good agreement is observed.
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Lin, J. H., and K. C. Chang. "Particle Dispersion Simulation in Turbulent Flow Due to Particle-Particle and Particle-Wall Collisions." Journal of Mechanics 32, no. 2 (August 19, 2015): 237–44. http://dx.doi.org/10.1017/jmech.2015.63.
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AbstractSimulation of the 3-D, fully developed turbulent channel flows laden with various mass loading ratios of particles is made using an Eulerian-Lagrangian approach in which the carrier-fluid flow field is solved with a low-Reynolds-number k-ε turbulence model while the deterministic Lagrangian method together with binary-collision hard-sphere model is applied for the solution of particle motion. Effects of inter-particle collisions and particle-wall collisions under different extents of wall roughness on particle dispersion are addressed in the study. A cost-effective searching algorithm of collision pair among particles is developed. It is found that the effects of inter-particle collisions on particle dispersion cannot be negligible when the ratio of the mean free time of particle to the mean particle relaxation time of particle is less or equal to O(10). In addition, the wall roughness extent plays an important role in the simulation of particle-wall collisions particularly for cases with small mass loading ratios.
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Shukla, R., S. S. Bhatt, A. Medhavi, and R. Kumar. "Effect of Surface Roughness during Peristaltic Movement in a Nonuniform Channel." Mathematical Problems in Engineering 2020 (July 15, 2020): 1–8. http://dx.doi.org/10.1155/2020/9643425.
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In this study, the effect of the roughness parameter during the peristaltic transport of a Newtonian fluid in a nonuniform channel has been explored. The motivation of this study comes from various research studies in the area of life sciences and engineering, which reveal that the wall of living beings’ arteries and all other surfaces have roughness to some extent. As peristalsis is a major mode of transporting biological fluids in various organs, the effect of surface roughness during peristaltic flow becomes very significant. The problem of peristaltic motion of a Newtonian fluid through a rough nonuniform channel having sinusoidal-shaped roughness has been investigated in the current work. To analyze the flow, analytic formulation of pressure rise, friction force, velocity, and pressure gradient has been carried out under the low Reynolds number and long-wavelength approximation. Results obtained for zero surface roughness from the current model are in complete agreement with previous studies available in the literature that have been carried out without considering the surface roughness of the wall. Numerical outcomes for the properties mentioned above have been plotted for analyzing the impact of roughness on the physical and flow parameters.
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He, Ning, and Bin Qin. "Influence Analysis of Roadway Friction on Shock Wave Attenuation." Applied Mechanics and Materials 178-181 (May 2012): 1619–22. http://dx.doi.org/10.4028/www.scientific.net/amm.178-181.1619.
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Friction is the extremely important factor when considering the interaction between the shock wave and the tunnel wall. But so far, the impact of wall friction on the shock wave is mainly measured by experimental methods. This paper mainly discusses the effect of wall friction on the shock wave attenuation, without considering roughness, roughness elements, the viscosity of air, and the complex relationship between them; the numerical simulation calculation model is established with DYNA calculation software; the influence law of friction coefficient on tunnel shock wave propagation and attenuation is given based on friction coefficient between air medium and tunnel wall, so as to provide guidance to mining, reduce the impact of tunnel explosion shock wave on personnel and equipment and provide a design basis for shaft safety works.
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Bons, Jeffrey P., and Stephen T. McClain. "The Effect of Real Turbine Roughness With Pressure Gradient on Heat Transfer." Journal of Turbomachinery 126, no. 3 (July 1, 2004): 385–94. http://dx.doi.org/10.1115/1.1738120.
Full textAbstract:
Experimental measurements of heat transfer (St) are reported for low speed flow over scaled turbine roughness models at three different freestream pressure gradients: adverse, zero (nominally), and favorable. The roughness models were scaled from surface measurements taken on actual, in-service land-based turbine hardware and include samples of fuel deposits, TBC spallation, erosion, and pitting as well as a smooth control surface. All St measurements were made in a developing turbulent boundary layer at the same value of Reynolds number Rex≅900,000. An integral boundary layer method used to estimate cf for the smooth wall cases allowed the calculation of the Reynolds analogy 2St/cf. Results indicate that for a smooth wall, Reynolds analogy varies appreciably with pressure gradient. Smooth surface heat transfer is considerably less sensitive to pressure gradients than skin friction. For the rough surfaces with adverse pressure gradient, St is less sensitive to roughness than with zero or favorable pressure gradient. Roughness-induced Stanton number increases at zero pressure gradient range from 16–44% (depending on roughness type), while increases with adverse pressure gradient are 7% less on average for the same roughness type. Hot-wire measurements show a corresponding drop in roughness-induced momentum deficit and streamwise turbulent kinetic energy generation in the adverse pressure gradient boundary layer compared with the other pressure gradient conditions. The combined effects of roughness and pressure gradient are different than their individual effects added together. Specifically, for adverse pressure gradient the combined effect on heat transfer is 9% less than that estimated by adding their separate effects. For favorable pressure gradient, the additive estimate is 6% lower than the result with combined effects. Identical measurements on a “simulated” roughness surface composed of cones in an ordered array show a behavior unlike that of the scaled “real” roughness models. St calculations made using a discrete-element roughness model show promising agreement with the experimental data. Predictions and data combine to underline the importance of accounting for pressure gradient and surface roughness effects simultaneously rather than independently for accurate performance calculations in turbines.