Small-scale Surface Roughness Research Articles

Automated Sentinel-1 sea ice type mapping and in-situ validation during the CIRFA-22 cruise

Abstract We present a fully-automated workflow to map sea ice types from Sentinel-1 data and transfer the results in near real-time to the research vessel Kronprins Haakon (KPH) in order to support tactical navigation and decision-making during a research cruise conducted towards Belgica Bank in April and May 2022. We used overlapping SAR and optical imagery to train a pixel-wise classifier for the required season and region, and implemented a processing chain with the Norwegian Ice Service at MET Norway that automatically classifies all Sentinel-1 images covering the area of interest. During the cruise, classification results were available on KPH within hours after image acquisition, which is significantly faster than manually produced ice charts. We evaluate the results both quantitatively, based on manually selected validation regions, and qualitatively in comparison to in-situ observations and photographs. Our findings show that open water, level ice, and deformed ice are classified with high accuracy, while young ice remains challenging due to its variable small-scale surface roughness. This work presents one of the first attempts to transfer automated ice type classification results into the field in near real-time and contributes to bridging the gap between research and operations in automated sea ice mapping.

A semi-analytical thermal model for craters with application to the crater-induced YORP effect

Context. The YORP effect is the thermal torque generated by radiation from the surface of an asteroid. The effect is sensitive to surface topology, including small-scale roughness, boulders, and craters. Aims. The aim of this paper is to develop a computationally efficient semi-analytical model for the crater-induced YORP (CYORP) effect that can be used to investigate the functional dependence of this effect. Methods. This study linearizes the thermal radiation term as a function of the temperature in the boundary condition of the heat conductivity, and obtains the temperature field in a crater over a rotational period in the form of a Fourier series, accounting for the effects of self-sheltering, self-radiation, and self-scattering. By comparison with a numerical model, we find that this semi-analytical model for the CYORP effect works well for K > 0.1 Wm−1 K−1. This semi-analytical model is computationally three-orders-of-magnitude more efficient than the numerical approach. Results. We obtain the temperature field of a crater, accounting for the thermal inertia, crater shape, and crater location. We then find that the CYORP effect is negligible when the depth-to-diameter ratio is smaller than 0.05. In this case, it is reasonable to assume a convex shape for YORP calculations. Varying the thermal conductivity yields a consistent value of approximately 0.01 for the spin component of the CYORP coefficient, while the obliquity component is inversely related to thermal inertia, declining from 0.004 in basalt to 0.001 in metal. The CYORP spin component peaks at an obliquity of 0°, 90°, or 180°, while the obliquity component peaks at an obliquity of around 45° or 135°. For a z-axis symmetric shape, the CYORP spin component vanishes, while the obliquity component persists. Our model confirms that the total YORP torque is damped by a few tens of percent by uniformly distributed small-scale surface roughness. Furthermore, for the first time, we calculate the change in the YORP torque at each impact on the surface of an asteroid explicitly and compute the resulting stochastic spin evolution more precisely. Conclusions. This study shows that the CYORP effect due to small-scale surface roughness and impact craters is significant during the history of asteroids. The semi-analytical method that we developed, which benefits from fast computation, offers new perspectives for future investigations of the YORP modeling of real asteroids and for the complete rotational and orbital evolution of asteroids accounting for collisions. Future research employing our CYORP model may explore the implications of space-varying roughness distribution, roughness in binary systems, and the development of a comprehensive rotational evolution model for asteroid groups.

Squirmer hydrodynamics near a periodic surface topography.

The behaviour of microscopic swimmers has previously been explored near large-scale confining geometries and in the presence of very small-scale surface roughness. Here, we consider an intermediate case of how a simple microswimmer, the tangential spherical squirmer, behaves adjacent to singly and doubly periodic sinusoidal surface topographies that spatially oscillate with an amplitude that is an order of magnitude less than the swimmer size and wavelengths that are also within an order of magnitude of this scale. The nearest neighbour regularised Stokeslet method is used for numerical explorations after validating its accuracy for a spherical tangential squirmer that swims stably near a flat surface. The same squirmer is then introduced to different surface topographies. The key governing factor in the resulting swimming behaviour is the size of the squirmer relative to the surface topography wavelength. For instance, directional guidance is not observed when the squirmer is much larger, or much smaller, than the surface topography wavelength. In contrast, once the squirmer size is on the scale of the topography wavelength, limited guidance is possible, often with local capture in the topography troughs. However, complex dynamics can also emerge, especially when the initial configuration is not close to alignment along topography troughs or above topography crests. In contrast to sensitivity in alignment and topography wavelength, reductions in the amplitude of the surface topography or variations in the shape of the periodic surface topography do not have extensive impacts on the squirmer behaviour. Our findings more generally highlight that the numerical framework provides an essential basis to elucidate how swimmers may be guided by surface topography.

Improved Geometric Optics with Topography (IGOT) Model for GNSS-R Delay-Doppler Maps Using Three-Scale Surface Roughness

Although multiple efforts have been made to model global navigation satellite system (GNSS)-reflectometry (GNSS-R) delay-Doppler maps (DDMs) over land, there is still a need for models that better represent the signals over land and can enable reliable retrievals of the geophysical variables. Our paper presents improvements to an existing GNSS-R DDM model by accounting for short-wave diffraction due to small-scale ground surface roughness and signal attenuation due to vegetation. This is a step forward in increasing the model fidelity. Our model, called the improved geometric optics with topography (IGOT), predicts GNSS-R DDM over land for the purpose of retrieving geophysical parameters, including soil moisture. Validation of the model is carried out using DDMs from the Cyclone GNSS (CYGNSS) mission over two validation sites with in situ soil moisture sensors: Walnut Gulch, AZ, USA, and the Jornada Experimental Range, NM, USA. Both the peak reflectivity and the DDM shape are studied. The results of the study show that the IGOT model is able to accurately predict CYGNSS DDMs at these two validation sites.

The relationship between surface roughness and fatigue crack growth rate in AA7050-T7451 subjected to periodic underloads

The accurate prediction of fatigue crack growth rates is necessary to assist with optimising the design, maintenance, and availability of aircraft structures. Roughness induced crack closure is considered by many researchers as an important factor that can impact these growth rates. It has also been suggested that crack closure may be the governing factor behind the accelerative effects of underloads. As such, the focus of this study is on better understanding what correlation there is between the known accelerative effects of underloads, and the local fracture surface roughness of fatigue cracks. Here, specially designed loading spectra are used to quantify the effects of underloads on local fatigue crack growth. Photogrammetry is then used to quantify surface roughness local to the application of the underloads, in order to explore any potential relationship between observed crack growth acceleration and local crack surface roughness. In addition, a general fractographic analysis of the fatigue fracture surface was conducted to understand what correlation there may be between any observed features and the fatigue crack growth rate. It is found that there is little correlation between small-scale surface roughness and the observed acceleration from underloads. However, these is some evidence that large out-of-plane features, such as river patterns, correspond with changes in crack growth rate.

Mitigation of installation-related effects for small-scale borehole-to-surface ERT

Small-scale resistivity inhomogeneities can result from the local distribution of water and the water and nutrient uptake of plants. Measuring small-scale Electrical Resistivity Tomography (ERT) in the field comes with a set of particularities, especially when including borehole electrodes for a better resolution with depth. We apply small-scale borehole-to-surface ERT over a palaeochannel. Combining surface ERT with detailed borehole-to-surface ERT profiles along the measurement line allows a delineation of finer layering within the coarser lithology. Our field setup includes a borehole electrode tool with 20 ring electrodes, electrically coupled to the ground via a conductive mud. Two main points are addressed in this publication: (1) In the field, we electrically coupled the borehole electrodes to the ground by filling the cavities around the tool with a soil mud, i.e., we need to account for the unknown conductive borehole filling in the inversion. If not incorporated, the mud has a considerable influence on the resistivities close to the borehole tool, but also on the region around the surface electrodes. Consequently, alongside with a 3D inversion scheme representing the electrodes with the Complete Electrode Model (CEM), we include the mud as a separate and uncoupled region. We model the geometry of the mud layer around the tool and do not allow an influence of this region on the rest of the model. (2) Due to the small electrode distances and the overall small-scale nature of the array, the depth of installation of the borehole electrode tool must be known accurately in the inversion model. However, it is not easy to measure the tool depth in the field with the required accuracy, due to small-scale surface roughness, e.g., from a weathered loose soil layer at the surface or from vegetation. We also investigate the influence of a tilted tool installation and optimise for the depth and installation angle of the borehole tool before inverting for resistivities. An accurate knowledge of the borehole electrode positions is crucial for a reliable and precise inversion result. The surface electrodes establish a coordinate system around the borehole tool on the surface, with an angle φ describing the direction around the tool in the top view. The sensitive plane (in-plane) is defined as the x-z plane cutting through φ=0° and φ=180°. A tilting of the tool from the vertical direction is described by a tilting angle θ. A tilting of the borehole tool within the sensitive plane manifests in an increased misfit between data points on both sides of the tool, i.e., at φ=0° and φ=180°. We use this difference to optimise on the tool angle. The true depth of the borehole tool is found by searching for a minimum of the objective function, describing the goodness of the found model, while assuming different tool depths in each inversion. We see a minimum of the objective functionwhich can be attributed to the correct depth range, as shown by a synthetic study. Through our optimisations, we can determine a tilting of the tool, i.e., the angle θ, with an accuracy of 2° to 3° and the tool depth with an accuracy of a few centimetres, depending partially on the subsurface resistivities, i.e., our optimisation works mainly in predominantly horizontally layered soils. A tilting in directions out of the sensitive plane (out-of-plane) can be projected onto the sensitive plane, since the out-of-plane tilt has a negligible influence on the data. After this optimisation, we can determine layer resistivities from our field data.

Roughness measurement of leaf surface based on shape from focus

BackgroundSurface roughness has a significant effect on leaf wettability. Consequently, it influences the efficiency and effectiveness of pesticide application. Therefore, roughness measurement of leaf surface offers support to the relevant research efforts. To characterize surface roughness, the prevailing methods have drawn support from large equipment that often come with high costs and poor portability, which is not suitable for field measurement. Additionally, such equipment may even suffer from inherent drawbacks like the absence of relationship between pixel intensity and corresponding height for scanning electron microscope (SEM).ResultsAn imaging system with variable object distance was created to capture images of plant leaves, and a method based on shape from focus (SFF) was proposed. The given space-variantly blurred images were processed with the proposed algorithm to obtain the surface roughness of plant leaves. The algorithm improves the current SFF method through image alignment, focus distortion correction, and the introduction of NaN values that allows it to be applied for precise 3d-reconstruction and small-scale surface roughness measurement.ConclusionCompared with methods that rely on optical three-dimensional interference microscope, the method proposed in this paper preserves the overall topography of leaf surface, and achieves superior cost performance at the same time. It is clear from experiments on standard gauge blocks that the RMSE of step was approximately 4.44 µm. Furthermore, according to the Friedman/Nemenyi test, the focus measure operator SML was expected to demonstrate the best performance.

Effect of small-scale snow surface roughness on snow albedo and reflectance

Abstract. The primary goal of this paper is to present a model of snow surface albedo accounting for small-scale surface roughness effects. The model is based on photon recollision probability, and it can be combined with existing bulk volume albedo models, such as Two-streAm Radiative TransfEr in Snow (TARTES). The model is fed with in situ measurements of surface roughness from plate profile and laser scanner data, and it is evaluated by comparing the computed albedos with observations. It provides closer results to empirical values than volume-scattering-based albedo simulations alone. The impact of surface roughness on albedo increases with the progress of the melting season and is larger for larger solar zenith angles. In absolute terms, small-scale surface roughness can decrease the total albedo by up to about 0.1. As regards the bidirectional reflectance factor (BRF), it is found that surface roughness increases backward scattering especially for large solar zenith angle values.

Convergence of the iterative T-matrix method.

Among the various methods for computing the T-matrix in electromagnetic and acoustic scattering problems is an iterative approach that has been shown to be particularly suited for particles with small-scale surface roughness. This method is based on an implicit T-matrix equation. However, the convergence properties of this method are not well understood. Here, a sufficient condition for the convergence of the iterative T-matrix algorithm is derived by applying the Banach fixed point theorem. The usefulness of the criterion is illustrated by applying it to predicting, as well as to systematically improving the convergence of the iterative method.

Dune migration and volume change from airborne LiDAR, terrestrial LiDAR and Structure from Motion-Multi View Stereo

Sand dunes are commonly regarded as a challenge to traditional photogrammetry due their homogeneous texture and spectral response. In this work we present an evaluation of Structure from Motion-Multi View Stereo (SfM-MVS) to obtain high-resolution elevation data of coastal sand dunes based on images acquired by Remotely Piloted Aircraft (RPA). A Digital Elevation Model (DEM) of a dunefield in Southern Brazil was generated from 810 photos captured by an RPA at 100 m above the takeoff point in February 2019. Image matching was successful in all areas of the survey due the presence of superficial features (footprints and sandboard tracks) and visibility of the sedimentary stratification, highlighted by heavy minerals. Altimetric accuracy of the SfM-MVS DEM was validated by comparison with Terrestrial LiDAR (TLS) data collected during the same fieldwork campaign of the RPA flights. The SfM-MVS DEM was then compared to an Airborne LiDAR (ALS) DEM from October 2010. While the SfM-MVS and TLS DEMs are very similar, without any major difference in elevation or in the reconstruction of topographic features, the SfM-MVS DEM presents a small scale surface roughness not visible in the TLS DEM. The Feature Preserving DEM Smoothing (FPD) algorithm was applied to the SfM-MVS DEM with good results in terms of surface smoothing, but without any significant changes in descriptive statistics and error metrics, with an RMSE of 0.08 m and MAE of 0.06 m for both the original and the FPD-filtered DEM. Displacement of dune crest lines from the ALS and SfM-MVS DEMs resulted in a migration rate of ≈5 m/year between 2010 and 2019, in good agreement with rates derived from satellite images and historical aerial photographs of the same area. Sand volume change in the same period showed a decrease of only 0.2%, which can be related to the installation of sand fences to promote dune stabilization and sand removal from the front of the dune field to keep a road open to vehicles. ALS can cover large areas in little time but its high cost still remains a barrier to wider usage, especially by researchers in developing countries. TLS has an intermediate cost but demands more fieldwork and more processing time. In our case we needed three days for the TLS survey and around three weeks to produce a DEM of ≈80 400m2. On the other hand, we were able to cover ≈740 900m2 with six flight missions in under three hours, with ≈13 h processing time in a medium-range workstation. This makes SfM-MVS a low-cost solution with fast and reliable results for 3D modelling and continuous monitoring of coastal dunes.

Effects of partial slip on the local-global linear stability of the infinite rotating disk boundary layer

A numerical investigation is undertaken on the effect of small-scale surface roughness on the local absolute and global stability of the flow due to a rotating disk. Surface roughness is modeled via the imposition of the partial-slip wall boundary condition, with radial and concentric anisotropic roughnesses and isotropic roughness considered. The effect of the partial-slip parameters on the neutral characteristics for absolute instability is presented, while the azimuthal mode numbers required for global linear instability to occur are determined for the genuine inhomogeneous base flow. Predictions for the threshold values for the azimuthal mode numbers needed for globally unstable behavior are also computed by coupling solutions of the Ginzburg–Landau equation with the local linear stability properties obtained using the homogeneous flow approximation. These are found to be in excellent agreement with the exact values realized from the numerical simulations. In general, surface roughness is demonstrated to stabilize the absolute instability and the global linear instabilities.

The significant impact of ribs and small-scale roughness on cylinder drag crisis

The impact of flow-normal ribs and small-scale surface roughness on the drag and vortex shedding of a circular cylinder was investigated. Three rib heights, four relative rib spacings and three different forms of micro-roughness were combined to produce 28 unique surface coatings for the cylinder. The drag was measured in a wind tunnel for Reynolds numbers in the range 20,000<Re<160,000, representing nearly a decade change centred about the drag crisis. The drag measurements were complemented by hot-wire measurements in the wake to investigate the vortex shedding frequency. The results show significant average drag reduction, up to 23%, for most of the ribbed geometries compared to a smooth cylinder for Re<160,000. Increasing the rib height was found to reduce the critical Reynolds number and increase the minimum drag coefficient. Varying the rib spacing resulted in an ‘‘optimal” spacing, approximately five times the rib height, that caused the lowest critical Reynolds number. Increasing the micro-roughness resulted in a reduction in the critical Reynolds number and an increase in the minimum drag coefficient.

The emergence of small-scale self-affine surface roughness from deformation.

Most natural and man-made surfaces appear to be rough on many length scales. There is presently no unifying theory of the origin of roughness or the self-affine nature of surface topography. One likely contributor to the formation of roughness is deformation, which underlies many processes that shape surfaces such as machining, fracture, and wear. Using molecular dynamics, we simulate the biaxial compression of single-crystal Au, the high-entropy alloy Ni36.67Co30Fe16.67Ti16.67, and amorphous Cu50Zr50 and show that even surfaces of homogeneous materials develop a self-affine structure. By characterizing subsurface deformation, we connect the self-affinity of the surface to the spatial correlation of deformation events occurring within the bulk and present scaling relations for the evolution of roughness with strain. These results open routes toward interpreting and engineering roughness profiles.

Estimating the 3D shape of volcanic ash to better understand sedimentation processes and improve atmospheric dispersion modelling

The sedimentation rate of volcanic ash through the atmosphere influences its travel distance, with important implications for aviation and health. The fall velocity of a particle depends on its size and density, but also shape, and volcanic ash is not spherical. To capture the sedimentation of ash, atmospheric dispersion models use empirical drag equations calibrated using geometric shape descriptors. However, particle shape data are scarce and there is no standard method of shape measurement. In addition, shape measurements are not always available during an eruption, when dispersion models are used operationally to forecast ash hazard. We assess the variability in the shape of volcanic ash from Icelandic eruptions using X-ray computed tomography. To consider how good different drag equations and shape descriptors are at representing the sedimentation of volcanic ash we compare calculated fall velocities to measured fall velocities of volcanic ash in air in a settling column. We then suggest the best drag equations and shape descriptors for use in atmospheric dispersion models. We find that shape-dependent drag equations produce more accurate results than a spherical approximation. However, accurate drag calculations based on the shape descriptor sphericity, which is a function of surface area, require the imaging resolution to be within the range of 102 - 105 voxels per particle (where a voxel is a volumetric pixel) as surface area is sensitive to imaging resolution. We suggest that the large-scale form of the particle impacts sedimentation more than small-scale surface roughness. Shape descriptors based on ratios between principal axis lengths are more practical as they are less variable among particle size classes and much less sensitive to imaging resolution. Finally, we use particle shape data from this study and literature sources to make recommendations on default values for use with atmospheric dispersion models where no shape data are available.

First Evaluation of Topography on GNSS-R: An Empirical Study Based on a Digital Elevation Model

Understanding the effects of Earth’s surface topography on Global Navigation Satellite Systems Reflectometry (GNSS-R) space-borne data is important to calibrate experimental measurements, so as to provide accurate soil moisture content (SMC) retrievals. In this study, several scientific observables obtained from delay-Doppler maps (DDMs) ⟨ | Y r , t o p o ( τ , f ) | 2 ⟩ generated on board the Cyclone Global Navigation Satellite System (CyGNSS) mission were evaluated as a function of several topographic parameters derived from a digital elevation model (DEM). This assessment was performed as a function of Soil Moisture Active Passive (SMAP)-derived SMC at grazing angles θ e ~ [20,30] ° and in a nadir-looking configuration θ e ~ [80,90] °. Global scale results showed that the width of the trailing edge (TE) was small T E ~ [100, 250] m and the reflectivity was high Γ ~ [–10, –3] dB over flat areas with low topographic heterogeneity, because of an increasing coherence of Earth-reflected Global Positioning System (GPS) signals. However, the strong impact of several topographic features over areas with rough topography provided motivation to perform a parametric analysis. A specific target area with little vegetation, low small-scale surface roughness, and a wide variety of terrains in South Asia was selected. A significant influence of several topographic parameters i.e., surface slopes and curvatures was observed. This triggered our study of the sensitivity of T E and Γ to SMC and topographic wetness index ( T W I ). Regional scale results showed that T E and Γ are strongly correlated with the T W I , while the sensitivity to SMC was almost negligible. The Pearson correlation coefficients of T E and Γ with T W I are r Γ ~ 0.59 and r T E ~−0.63 at θ e ~ [20, 30] ° and r Γ ~ 0.48 and r T E ~ −0.50 at θ e ~ [80, 90] °, respectively.

Spectrophotometric modeling and mapping of Ceres

We report a comprehensive analysis of the global spectrophotometric properties of Ceres using the images collected by the Dawn Framing Camera through seven color filters from April to June 2015 during the RC3 (rotational characterization 3) and Survey mission phases. We derived the Hapke model parameters for all color filters. The single-scattering albedo of Ceres at 555 nm wavelength is 0.14 ± 0.04, the geometric albedo is 0.096 ± 0.006, and the bolometric Bond albedo is 0.037 ± 0.002. The asymmetry factors calculated from the best-fit two-term Henyey-Greenstein (HG) single-particle phase functions (SPPFs) show a weak wavelength dependence from −0.04 at 438 nm increasing to 0.002 at >900 nm, suggesting that the phase reddening of Ceres is dominated by single-particle scattering rather than multiple scattering or small-scale surface roughness. The Hapke roughness parameter of Ceres is derived to be 20° ± 6°, with no wavelength dependence. The phase function of Ceres presents appreciably strong scattering around 90° phase angle that cannot be fitted with a single-term HG SPPF, suggesting possible stronger forward scattering component than other asteroids previously analyzed with spacecraft data. We speculate that such a scattering characteristic of Ceres might be related to its ubiquitous distribution of phyllosilicates and high abundance of carbonates on the surface. We further grouped the reflectance data into a 1° latitude-longitude grid over the surface of Ceres, and fitted each grid independently with both empirical models and the Hapke model to study the spatial variations of photometric properties. Our derived albedo maps and color maps are consistent with previous studies [Nathues, A., et al., 2016, Planet. Space Sci. 134, 122–127; Schröder, S.E., et al., 2017, Icarus 288, 201–225]. The SPPF over the surface of Ceres shows an overall correlation with albedo distribution, where lower albedo is mostly associated with stronger backscattering and vice versa, consistent with the general trend among asteroids. On the other hand, the Hapke roughness parameter does not vary much across the surface of Ceres, except for the ancient Vendimia Planitia region that is associated with a slightly higher roughness. Furthermore, the spatial distributions of the SPPF and the Hapke roughness do not depend on wavelength. Based on the wavelength dependence of the SPPF of Ceres, we hypothesize that the regolith grains on Ceres either contain a considerable fraction of μm-sized or smaller particles, or are strongly affected by internal scatterers of this size.

Surface Layer Formation in the Earliest Stages of Corrosion of Steel in CO2-Saturated Brine at 80°C: Studied by In Situ Synchrotron X-ray Methods

Grazing-incidence small-angle X-ray scattering (GISAXS) from polycrystalline steel shows features associated with the underlying microstructure, and, in the initial stages of corrosion, with the development of very small-scale surface roughness, on the nanometer height scale. A 1Cr0.25Mo pipeline steel in hot, CO2-saturated brine develops the very small-scale surface roughness significantly faster than a simple carbon steel, although the overall dissolution current density for the two steels is almost the same. We speculate that it is due to the presence of a layer comprising ‘blobs’ of amorphous FeCO3, which grow to spread over the surface and eventually cover it, because it is significantly larger in height scale than the roughness developed during the initial stages of anodic dissolution in acidified NaCl solution, where no surface film is expected. The greater roughness on the 1Cr0.25Mo steel can be interpreted as due to small pre-crystalline nuclei, that form at much lower supersaturation, and grow faster, than on mild steel. Grazing-incidence X-ray diffraction studies show at later stages the apparent preferential dissolution of smaller crystallites of iron, with spatial size scale 0.1–1 μm. This develops significantly more slowly on the 1Cr0.25Mo than on the simple carbon steel.

Contact angle variation on single floating spheres and its impact on the stability analysis of floating particles

The contact angle (CA) as a measure of surface hydrophobicity is of scientific and technological importance for many wetting and dewetting processes. CA hysteresis (i.e., the difference between the advancing and receding CAs) due to roughness on a flat surface has been recognized but is not considered in the stability analysis of floating particles. In this study, the CA of single floating spheres at the air-water interface was determined directly from the deformed meniscus by applying a new method recently developed by our team (Feng, D.-x. and Nguyen, A. V., 2016. A novel quantitative analysis of the local deformation of the air-water surface by a floating sphere. Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol. 504, pp. 407–413). The experiments for each of the single spheres were repeated to estimate the variation in the CA. The experimental results show a normal distribution of the CA. The CA hysteresis occurs randomly because of the dispersion of small-scale surface roughness. This random hysteresis substantially affects the stability analysis of floating spheres. The analysis also shows that the CA determined by the force balance method (valid for ideally smooth surfaces) yields higher values than those determined on the basis of the deformed meniscus around the particle. This difference is attributed to the effect of surface roughness on the CA hysteresis, which should be considered in analyses of the stability and detachment of floating particles.

小尺度表面粗糙度与介质吸收对介质折射率测量的影响

小尺度表面粗糙度与介质吸收对介质折射率测量的影响

INVESTIGATION OF SURFACE ROUGHNESS IMPACT ON MEAN WIND FLOW USING RNG k-ε MODEL

Wind flow in the urban boundary layer is influenced by both large- and small-scale surface roughness. In this study, Reynolds-averaged Navier-Stokes simulations using the renormalisation group (RNG) k-ε model were performed to study the wind flow in square arrays with small-scale roughness elements at the front and back of cubical obstacles at packing densities of 25.0% and 30.9%. The presence of small-scale roughness reduces streamwise velocity but increases turbulent kinetic energy. Moreover, small vortices are formed within the canopy because of small-scale roughness. The generated streamwise velocity profiles are similar at packing densities of 25.0% and 30.9%, but the drag coefficient is higher in the latter case. In brief, the impact of small-scale roughness on urban wind flow is considerable. The results of this study can contribute to future research on wind flow, particularly in the urban environment.