Function Of Roughness Research Articles

Particle Competitive Mechanism Based Multiobjective Rough Clustering Algorithm for Image Segmentation

Rough clustering has attracted increasing attention due to well dealing with the fuzziness and uncertainty of data. It is well known that it needs to manually set the threshold to determine the upper and lower approximations of rough clusters, which may bring a great effect on the clustering performance. When applied to image segmentation, rough clustering is always sensitive to the initialized cluster centers and image noise. Furthermore, only one clustering criterion is considered in rough clustering, which cannot satisfy diverse practical requirements. To handle these issues, a particle competitive mechanism based multiobjective rough clustering algorithm (PCM-MORCA) for image segmentation is proposed. First, a rough intraclass compactness function considering the nonlocal spatial information derived from an image is constructed to overcome the sensitivity to image noise. Next, the constructed rough intraclass compactness function and an interclass separation function are optimized simultaneously to make cluster centers meet diverse segmentation requirements. Then, an adaptive threshold determination mechanism by which the threshold adaptively varies with the clustered data is presented to well determine the upper and lower approximations of rough clusters. After that, to effectively search appropriate cluster centers, a novel pair competition-based particle weight updating strategy is designed for multiobjective particle swarm optimization by improving the elite particle selection and particle update. Finally, a rough clustering index with the nonlocal spatial information is constructed for selecting the optimal solution for PCM-MORCA. Segmentation experiments on Berkeley and magnetic resonance images reveal that PCM-MORCA behaves well on the segmentation accuracy and noise robustness.

An Adaptive Irregular Grid Generation Method for Scattering Simulation of Electrically Large Terrains

This communication provides a novel adaptive irregular grid model (AIGM) to improve the scattering simulation efficiency of electrically large terrains. This model distinguishes the steep and flat regions of the measured topographic point cloud models by a threshold. To determine the optimal threshold, we present the height gradient as the criterion to describe the topographic undulation. The optimal threshold is located at the inflection point of the curve describing topographic similarity in the range of height gradient. This generated AIGM can reduce the grid quantity and maintain consistency with the original point cloud model. It is also valuable in the popular coherent and incoherent (C&I) scattering model of global navigation satellite system reflectometry (GNSS-R). In this communication, the coherent component can be expressed by a simple product of the electric field of the conductor grid, the characteristic function of roughness, and the Fresnel reflection coefficient. The incoherent component comes from the diffuse scattering simulated by the integral equation method (IEM). Compared with the traditional solution to C&I scattering by using the Kirchhoff approximation (KA), the modified C&I model has a more extensive application. The precision of the AIGM-based-C&I model is verified by traditional physical optics (PO). The excellent simulation efficiency of the AIGM-based-C&I model is also demonstrated.

Optimising pool boiling performance of hybrid nanofluids through desirability function analysis

Enhancements in critical heat flux (CHF), heat transfer coefficient (HTC), and other thermal parameters of nanofluids produced during pool boiling of Ag/ZnO hybrid nanoparticles are examined in the current work. We also present a study comparing performance outputs obtained after optimising the conflicting response variables. Copper rods having a cylindrical geometry with 20 mm outer diameter and 35 mm length and their upper face being modified over the die-sink electro-discharge machine (EDM) at various parameter settings were used as a test piece. This process produced test pieces of varying roughness upon which the experimental trials were undertaken. In order to disseminate the hybrid nanoparticles in the base fluid, i.e., DI (de-ionized) water, volume concentrations (ϕ) of 0.02, 0.06, and 0.1% were used to make stable nanofluids utilising a magnetic stirrer and an ultrasonicator. We carried out pool boiling experiments by adapting the Taguchi methodology to design the experiments. Our study demonstrates that the enhancements are a function of surface roughness and nanofluid concentration. The increased concentration of nanoparticles during pool boiling studies raised important metrics like CHF and HTC. Compared to DI water, Ag/ZnO hybrid nanofluids showed excellent gains in HTC (∼250%) and CHF (∼80%) at surface characteristics corresponding to EDM settings of I = 35 A and Ton = 100 µs. The degree of superheat required for boiling incipience contradicted the experimental surface and fluid combination that showed maximum enhancements. As a result, an attempt to optimise the overall boiling performance was undertaken by employing the desirability function analysis (DFA). Considerable improvements were seen in CHF, HTC, and ΔT (Wall superheat) after experimenting with the optimised settings obtained after DFA implementation and determination of the mean of means. No improvements in heat transfer were seen for nanofluid concentrations above ϕ > 0.1% due to the instability of nanofluids above this range.

A modified k-ω turbulence model for improved predictions of neutral atmospheric boundary layer flows

A modified k-ω turbulence model is developed for neutral atmospheric boundary layer flows by introducing additional source terms to the equations of the standard model to remove its inconsistency with the inlet conditions. Rough wall functions based on aerodynamic roughness are adopted to overcome the limitations of the standard wall functions that are based on sand grain roughness. Moreover, the diagnostic and prognostic approaches are considered when simulating the flow. The diagnostic approach is approximate, relying on satisfying mass conservation by solving an optimization problem, whereas the prognostic approach resolves the Navier-Stokes equations with a turbulence model. The performances of the developed modified k-ω model, an existing modified k-ε model, and the standard SST k-ω model are compared. To calculate the concentration field, the Eulerian approach which solves a concentration transport equation, and the Lagrangian approach which adopts the particle method with core radius spreading for computing diffusion are used. Results generated with the diagnostic approach demonstrate its unsuitability to calculate wind flows over complex geometries. On the other hand, the prognostic approach, with the three mentioned turbulence models, was successful in reproducing the flow data of CEDVAL A1 and B1 experiments in domains composed of one building and an array of buildings. In addition, a good agreement with the concentration measurements is obtained when using the newly modified k-ω and the SST k-ω models to account for turbulence, with the Eulerian method outperforming the Lagrangian dispersion approach.

Direct numerical simulations of turbulent channel flow over ratchet roughness

The influence of the orientation of ratchet-type rough surfaces on their fluid dynamic roughness effect is investigated using direct numerical simulations of turbulent channel flow at Re_{tau }=395. The ratchet length-to-height ratio is varied from ell /k=2 to 16 for a fixed ratchet height of k/delta =0.1 where delta is the mean channel half-height. The results show that both roughness function and mean flow and turbulence statistics strongly depend on the ratchet orientation. Existing empirical formulae, which estimate the roughness function Delta U^+ or the equivalent sand-grain roughness k_s based on surface-slope related parameters such as the effective slope or the Sigal-Danberg parameter, fail to accurately predict the differences between ratchet surfaces with high windward slopes and ratchet surfaces with high leeward slopes.

Simulation of turbulent flow over roughness strips

Heterogeneous roughness in the form of streamwise aligned strips is known to generate large scale secondary motions under turbulent flow conditions that can induce the intriguing feature of larger flow rates above rough than smooth surface parts. The hydrodynamical definition of a surface roughness includes a large scale separation between the roughness height and the boundary layer thickness which is directly related to the fact that the drag of a laminar flow is not altered by the presence of roughness. Existing simplified approaches for direct numerical simulation of roughness strips do not fulfil this requirement of an unmodified laminar base flow compared with a smooth wall reference. It is shown that disturbances induced in a modified laminar base flow can trigger large-scale motions with resemblance to turbulent secondary flow. We propose a simple roughness model that allows us to capture the particular features of turbulent secondary flow without impacting the laminar base flow. The roughness model is based on the prescription of a spanwise slip length, a quantity that can directly be translated into the Hama roughness function for a homogeneous rough surface. The heterogeneous application of the slip-length boundary condition results in very good agreement with existing experimental data in terms of the secondary flow topology. In addition, the proposed modelling approach allows us to quantitatively evaluate the drag increasing contribution of the secondary flow. Both the secondary flow itself and the related drag increase reveal a very small dependence on the gradient of the transition between rough and smooth surface parts only. Interestingly, the observed drag increase due to secondary flows above the modelled roughness is significantly smaller than the one previously reported for roughness resolving simulations. We hypothesise that this difference arises from the fact that roughness resolving simulations cannot truly fulfil the requirement of large scale separation.

Effect of the surface finish on the mechanical properties and cellular adhesion in (Ce,Y)-TZP/Al2O3 ceramic composites for denture implants

Ceramic composites based on (Ce, Y)-TZP/Al2O3 system have great potential for applications as dental implants due to their unusually great balance between good mechanical properties and resistance to hydrothermal degradation. Surface roughness plays an important role in controlling these properties, but few studies have investigated the relationship between cytocompatibility and surface roughness, at levels considered moderate and low, comparable to titanium implants. In this work, bending strength, hydrothermal degradation and biological evaluation of a ceramic composite based on (Ce,Y)-TZP/Al2O3 system were investigated as a function of surface roughness. Compacted samples were sintered at 1500 °C - 2h and then submitted to different surface treatments: Group 1 composed of samples with smooth surfaces, Group 2 and Group 3 composed of rough surfaces (grinded with 15 μm or 45 μm diamond sandpaper, respectively. Samples were characterized by X-ray diffraction, scanning electron microscopy, contact angle and optical profilometry and then subjected to hydrothermal degradation tests in autoclave (134 °C - 2 bar) using artificial saliva. The Piston-on-three-balls (P–3B) testing was used to determine flexural strength. To assess indirect cytotoxicity, samples were immersed in the culture medium for NIH-3T3 cells for 72 h. Furthermore, cell adhesion and proliferation were investigated using MG63 cells (human osteosarcoma) after 3, 7, 14, and 21 days of culture. Cytotoxicity, adhesion, and cell proliferation were examined by the Methyl Tetrazolium salt (MTS) and Alizarin Red, using a confocal laser microscope. The results indicated that the materials have high resistance to degradation. Furthermore, the (Ce,Y)-TZP/Al2O3 composites are not cytotoxic. The flexural strength of the composites was 913 ± 103 MPa in samples presenting original (smooth) surface, however, a reduction in the order of 17% was observed in samples containing rough surfaces. The rougher samples show the best cellular adhesion and proliferation, leading to the formation of a mineralized matrix after 21 days. These results clearly suggest that the new (Ce,Y)-TZP/Al2O3 brand is strong and highly biocompatible and warrants further study.

A Novel Approach for Computational Fluid Dynamics Analysis of Mean Wind Loads on Heliostats

Abstract A computational fluid dynamics study about the aerodynamic loads over a heliostat due to an atmospheric boundary layer flow using a modified k–ɛ turbulence model is presented. A new formulation is used, in which the model quantities vary with the velocity field. Modified wall functions for roughness were used at the bottom of the computational domain to achieve horizontal homogeneity of the airflow. Good horizontal homogeneity for the streamwise and spanwise velocity and turbulence intensity profiles were found. The incident profiles were compared with the inlet ones. The average percentage differences were 0.22% for velocity and 0.43% for turbulent intensity. Good agreement was found between the numerical data and theoretical values of the streamwise and spanwise shear stress at the bottom of the domain. The aerodynamic coefficients of the heliostat at different elevation angles were obtained, and a good agreement was found between the numerical data concerning the wind tunnel experimental values. An average percentage difference of 3.1% was found for drag, 6.5% for lift, and 6.0% for overturning. A significant improvement was obtained by using this new formulation with respect to a non-modified k–ɛ turbulence model. The average differences of the aerodynamic coefficients were 6.6% for drag, 12.4% for lift, and 10.1% for overturning. The velocity, turbulent kinetic energy, and pressure fields at different elevation angles were analyzed. It was found that at an elevation of 60 deg, the stagnation point of the flow occurs at the superior edge of the heliostat, causing the maximum lift force over the structure.

Investigation of rough-wall turbulence over barnacle roughness with increasing solidity using direct numerical simulations

Surface fouling by marine organisms such as barnacles has a major impact on the shipping sector. We use direct numerical simulations to investigate how the fluid dynamical properties of a surface change as it is gradually covered by barnacle colonies. The roughness effect peaks at intermediate coverage when approximately half of the surface is covered by barnacles. We further investigate how the roughness is perceived by the outer part of the flow using the blanketing layer concept and find a linear relationship between the roughness function and the effective slope of the blanketing layer.

Near-wall turbulence alteration with the transpiration-resistance model

A set of boundary conditions called the transpiration-resistance model (TRM) is investigated in altering near-wall turbulence. The TRM was proposed by Lacis et al. (J. Fluid Mech., vol. 884, 2020, p. A21) as a means of representing the net effect of surface micro-textures on their overlying bulk flows. It encompasses conventional Navier-slip boundary conditions relating the streamwise and spanwise velocities to their respective shears through the slip lengths $\ell _x$ and $\ell _z$ . In addition, it features a transpiration condition accounting for the changes induced in the wall-normal velocity by expressing it in terms of variations of the wall-parallel velocity shears through the transpiration lengths $m_x$ and $m_z$ . Greater levels of drag increase occur when more transpiration takes place at the boundary plane, with turbulent transpiration being predominately coupled to the spanwise shear component for canonical near-wall turbulence. The TRM reproduces the effect of a homogeneous and structured roughness up to $k^{+}\approx 18$ , encompassing the regime of smooth-wall-like turbulence described using virtual origins (Luchini, 1996 Reducing the turbulent skin friction. In Computational Methods in Applied Sciences’ 96 (Paris, 9–13 Sept. 1996), pp. 465–470. Wiley; Ibrahim et al., J. Fluid Mech., vol. 915, 2021, p. A56) and slightly beyond it. The transpiration factor is defined as the product of the slip and transpiration lengths, i.e. $(m\ell )_{x,z}$ . This factor contains the compound effect of the wall-parallel velocity occurring at the boundary plane and increased permeability, both of which lead to the transport of momentum in the wall-normal direction. A linear relation between the transpiration factor and the roughness function is observed for regularly textured surfaces in the transitionally rough regime of turbulence. This shows that such effective flow quantities can be suitable measures for characterizing rough surfaces in this flow regime.

Error modelling method of extended Kalman filter‐based terrain referenced navigation system for integrity assurance under nominal conditions

Navigation safety is further emphasised for terrain referenced navigation (TRN) systems used in middle-range unmanned aerial vehicles (UAVs) than those in cruise missiles flying through mountainous areas, because middle-range UAVs may fly to regions and cities containing both combatants and civilians. For safety assurance of TRN systems, this paper proposes a method to compute protection levels (PLs) to assure the required level of integrity under nominal conditions. For obtaining PLs of TRN systems, all the components of the TRN measurement errors should be identified and overbounded. This study models every error component: the radar altimeter error, the vertical error in the terrain elevation database, the lateral offset induced error, and the interpolation error. Based on the characteristics of TRN systems using terrain information for obtaining navigation solutions, this study developed new models for the lateral offset induced error and the interpolation error as a function of terrain roughness by examining the effect of terrain roughness on the error sources. The simulation results demonstrate that the latitude and longitude PLs obtained by applying the proposed models can sufficiently overbound the navigation solution error for TRN systems under nominal conditions; especially the proposed method shows remarkable performance in regions with low terrain roughness.

The Graphene Structure's Effects on the Current-Voltage and Photovoltaic Characteristics of Directly Synthesized Graphene/n-Si(100) Diodes.

Graphene was synthesized directly on Si(100) substrates by microwave plasma-enhanced chemical vapor deposition (MW-PECVD). The effects of the graphene structure on the electrical and photovoltaic properties of graphene/n-Si(100) were studied. The samples were investigated using Raman spectroscopy, atomic force microscopy, and by measuring current–voltage (I-V) graphs. The temperature of the hydrogen plasma annealing prior to graphene synthesis was an essential parameter regarding the graphene/Si contact I-V characteristics and photovoltaic parameters. Graphene n-type self-doping was found to occur due to the native SiO2 interlayer at the graphene/Si junction. It was the prevalent cause of the significant decrease in the reverse current and short-circuit current. No photovoltaic effect dependence on the graphene roughness and work function could be observed.

Direct numerical simulation-based characterization of pseudo-random roughness in minimal channels

Direct numerical simulations (DNS) are used to systematically investigate the applicability of the minimal-channel approach (Chung et al., J. Fluid Mech., vol. 773, 2015, pp. 418–431) for the characterization of roughness-induced drag on irregular rough surfaces. Roughness is generated mathematically using a random algorithm, in which the power spectrum (PS) and probability density function (p.d.f.) of the surface height can be prescribed. Twelve different combinations of PS and p.d.f. are examined, and both transitionally and fully rough regimes are investigated (roughness height varies in the range $k^+ = 25$ –100). It is demonstrated that both the roughness function ( ${\rm \Delta} U^+$ ) and the zero-plane displacement can be predicted with ${\pm }5\,\%$ accuracy using DNS in properly sized minimal channels. Notably, when reducing the domain size, the predictions remain accurate as long as 90 % of the roughness height variance is retained. Additionally, examining the results obtained from different random realizations of roughness shows that a fixed combination of p.d.f. and PS leads to a nearly unique ${\rm \Delta} U^+$ for deterministically different surface topographies. In addition to the global flow properties, the distribution of time-averaged surface force exerted by the roughness onto the fluid is calculated. It is shown that patterns of surface force distribution over irregular roughness can be well captured when the sheltering effect is taken into account. This is made possible by applying the sheltering model of Yang et al. (J. Fluid Mech., vol. 789, 2016, pp. 127–165) to each specific roughness topography. Furthermore, an analysis of the coherence function between the roughness height and the surface force distributions reveals that the coherence drops at larger streamwise wavelengths, which can be an indication that very large horizontal scales contribute less to the skin-friction drag.

Phenofit: An R package for extracting vegetation phenology from time series remote sensing

Abstract Satellite‐derived vegetation indices (VIs) provide a way to analyse vegetation phenology over decades globally. However, these data are often contaminated by different kinds of optical noise (e.g. cloud, cloud shadow, snow, aerosol), making accurate phenology extraction challenging. We present an open‐source state‐of‐the‐art R package called to extract vegetation phenological information from satellite‐derived VIs. adopts state‐of‐the‐art phenology extraction methods, such as a weight updating function for reducing optical noise contamination, a growing season division function for separating the VI time series into different vegetation cycles, and rough and fine fitting functions for reconstructing VI time series. They work together to make phenology extraction from frequently contaminated VIs easier and more accurate. Compared against other widely used phenology extraction tools, for example, and , provides flexible input and output options, a practical growing season division function, rich curve fitting and phenology extraction functions, and robust performance under different kinds of optical noise. In addition to working with VIs from mesoscale satellites (e.g. MODIS and AVHRR), can also reconstruct vegetation time series and extract phenology using other sources, such as VIs from high‐resolution optical satellites (e.g. Sentinel‐2 and Landsat) and radar satellites (e.g. Sentinel‐1), vegetation greenness indices from digital cameras and gross primary production estimations from eddy‐covariance sites. As such, can contribute to the study of ecological process dynamics and assist in effective modelling of global change impacts on vegetation, as caused by climate variability and human intervention. Code and data of case studies are available at https://zenodo.org/record/6425745 (Kong, 2022a).

Heat transfer analysis of double discrete arc roughness with different relative rib altitudes and relate to a single discrete arc in the solar air heater

An artificially roughened solar air heater (SAH) with double discrete arc roughness is experimentally explored in this paper. Relative rib altitude (e/Dh ) varies from 0.018 to 0.034, while constant relative rib pitch (P/e), staggered pitch ratio (P′/P) and segment ratio (S′/S ) of 10, 0.5 and 1.6, respectively, are considered. The Reynolds number (Re) varies from 3000 to 14,000. Augmented Stanton number (St) and thermo-hydraulic performance (THPP) with a moderate friction factor (f) are reported for e/Dh of 0.034. The maximum St and f of 2.32 and 3.10 times, respectively, are the result for the rough plate related to basic configuration, while THPP of roughened SAH is augmented up to 1.60. The outcomes of the present roughness are compared with single discrete and continuous arc geometry under identical flow states and confirm the superior performance under the parameters tested. The correlations for heat transfer function (G) and roughness function (R) are also developed.

A Novel Approach for Characterizing Solutions of Rough Optimization Problems Based on Boundary Region.

Rough set theory, presented by Pawlak in 1981, is one of the most well-known methods for communicating ambiguity by estimating an item based on some knowledge rather than membership. The concept of a rough function and its convexity and differentiability in regard to its boundary region are discussed in this work. The boundary notion is also used to present a new form of rough programming issue and its solutions. Finally, numerical examples are provided to demonstrate the proposed method and emphasize its advantages over other approaches.

Justifying network reconstruction when switching to low temperature district heating

The aim of the paper is to deduce a way to make low temperature district heating (LTDH) more feasible by involving operational data. Using design data only, the total network cost compared to traditional system increases by 28.4%. New assessment gives only 18.6 million EUR of costs to modernize DH network, which provide 2.4 million EUR (12.9%) as savings. The lower total network cost is achieved when the usage of existing pipes is higher. Only one section of the research brings negative trend to differential pressure head. The reason is a higher pressure drop that derives from the fact that the Colebrook-White formulae expresses the Darcy friction factor as a function of Reynolds number and pipe relative roughness. However, it does not supersede the positive effect provided by adjusting heat demands and supply/return temperatures closer to operational values. To sum up, operational data including network and outdoor temperatures, flow rate and pressure, results into more accurate evaluation. Moreover, costs for network reconstruction are sensitive to errors, when considering the overall costs to switch to low temperature technologies. This method is fast and scalable, able to include any type of a district heating system for which basic demand data is available.

Scaling of rough-wall turbulence in a transitionally rough regime

In this Letter, we propose a roughness scaling method based on the direct numerical simulation of turbulent channel flow over three-dimensional sinusoidal rough walls in a transitionally rough regime. A new coupling scale Reτ/(k¯+S)n is defined by combining the Reynolds number Reτ and the roughness parameter k¯+S, where k¯+ is the mean roughness height, S is the roughness steepness, and n is the scaling exponent—which depends on the roughness type. The relationships between Reτ, k¯+S, the roughness function, the friction factor, and the peak of the streamwise turbulent velocity fluctuation are explored. This approach provides an effective way for collapsing rough-wall turbulence data with different Reτ and k¯+S.

Определение оптимальных режимов резания аналитико-графическим методом для обеспечения заданной микронеровности фрезерованной поверхности

For determination of the surface roughness function after face milling a complete factorial experiment has been fea-tured. An adequate mathematical model of roughness and significant variables are calculated. Hypersurfaces and function level lines are projected, optimal and maximum cutting modes are found in a graphical-analytic way.

Amplitude and wavelength scaling of sinusoidal roughness effects in turbulent channel flow at fixed

Direct numerical simulations are performed for incompressible, turbulent channel flow over a smooth wall and different sinusoidal wall roughness configurations at a constant$Re_\tau = 720$. Sinusoidal walls are used to study the effects of well-defined geometric features of roughness-amplitude,$a$, and wavelength,$\lambda$, on the flow. The flow in the near-wall region is strongly influenced by both$a$and$\lambda$. Establishing appropriate scaling laws will aid in understanding the effects of roughness and identifying the relevant physical mechanisms. Using inner variables and the roughness function to scale the flow quantities provides support for Townsend's hypothesis, but inner scaling is unable to capture the flow physics in the near-wall region. We provide modified scaling relations considering the dynamics of the shear layer and its interaction with the roughness. Although not a particularly surprising observation, this study provides clear evidence of the dependence of flow features on both$a$and$\lambda$. With these relations, we are able to collapse and/or align peaks for some flow quantities and, thus, capture the effects of surface roughness on turbulent flows even in the near-wall region. The shear-layer scaling supports the hypothesis that the physical mechanisms responsible for turbulent kinetic energy production in turbulent flows over rough walls are greatly influenced by the shear layer and its interaction with the roughness elements. Finally, a semiempirical model is developed to predict the contribution of pressure and skin friction drag on the roughness element based purely on its geometric parameters and the corresponding shear-layer velocity scale.