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A purely Lagrangian vortex method is employed to investigate turbulent flows past a rough circular cylinder under moving wall effect at large-gap regime, namely h*/d* = 0.45 and 0.80 (h* establishes the gap height between the moving wall base and the cylinder underside, and d* defines the outer cylinder diameter), yielding data in a transition regime, between the supercritical and transcritical regimes. In the large-gap regime, strong vortical structures are cyclically generated at the rear part of the cylinder. The numerical simulations utilize a two-dimensional model to simulate surface roughness effect. That model is inspired in a point set strategically located close to a body surface to inject momentum in its boundary layer and thus to capture a delay in the separation point of flow. Special attention is directed toward the relative roughness size variation, being that each test case always starts from an upper-subcritical Reynolds number of Re = 1.0 × 105. The present work contributes in the literature: (i) providing a detailed study of temporal evolution of pressure distribution, simultaneously computed with the integrated aerodynamic loads (drag and lift forces), Strouhal number and angle of boundary layer separation and (ii) examining the possibility of predicting suppression of vortex shedding. Such approach has been successfully applied within the study of bluff body aerodynamics at small-gap regime (i.e., h*/d* < 0.25), where the vortex shedding frequency completely disappears. Overall, the results reveal the sensitivity of the numerical technique to capture changes on the aerodynamic characteristics of a cylinder. For test with the rougher cylinder at h*/d* = 0.45, the big and small peaks on the drag curve are more influenced by increasing of noises because of surface roughness effect; consequently, the drag curve oscillates around small values reducing the mean drag coefficient. A drag reduction around 13% is identified as compared to a smooth cylinder. The lift force also reduces and still remains positive. The position of the separation points of the flow also changes, which characterizes boundary layer detachment with delay. The results are substantiated by a greater base pressure of the rougher cylinder; furthermore, only small differences are identified in the base pressure between both smooth and less rough cylinders, which explains the nearly constant level of drag force on them.

This paper investigates a long-standing question about the effect of surface roughness on turbulent flow: What is the equivalent roughness sand-grain height for a given roughness topography? Deep neural network (DNN) and Gaussian process regression (GPR) machine learning approaches are used to develop a high-fidelity prediction approach of the Nikuradse equivalent sand-grain height $k_s$ for turbulent flows over a wide variety of different rough surfaces. To this end, 45 surface geometries were generated and the flow over them simulated at ${Re}_\\tau =1000$ using direct numerical simulations. These surface geometries differed significantly in moments of surface height fluctuations, effective slope, average inclination, porosity and degree of randomness. Thirty of these surfaces were considered fully rough, and they were supplemented with experimental data for fully rough flows over 15 more surfaces available from previous studies. The DNN and GPR methods predicted $k_s$ with an average error of less than 10 % and a maximum error of less than 30 %, which appears to be significantly more accurate than existing prediction formulae. They also identified the surface porosity and the effective slope of roughness in the spanwise direction as important factors in drag prediction.

Sequel to the fact that hybrid nanofluidic systems (e.g. scalable micro-/nanofluidic device) exhibit greater thermal resistance with increasing nanoparticle concentration, little is known on the significance of thermal radiation, surface roughness and linear stability of water conveying alumina and copper oxide nanoparticles. This study presents the effects of thermal radiation and surface roughness on the complex dynamics of water conveying alumina and copper oxide nanoparticles, in the case where the thermophysical properties of the resulting mixture vary meaningfully with the volume fraction of solid nanomaterials, as well as with the Brownian motion and thermophoresis microscopic phenomena. Based on the linear stability theory and normal mode analysis method, the basic partial differential equations governing the transport phenomenon were non-dimensionalized to obtain the simplified stability equations. The optimum values of the critical thermal Rayleigh number depicting the onset of thermo-magneto-hydrodynamic instabilities were obtained using the power series method and the Chock–Schechter numerical integration. The increase in the strength of Lorentz forces, thermal radiation and surface roughness has a stronger stabilizing impact on the appearance of convection cells. On the contrary, the stability diminishes with the increasing values of the volumetric fraction and diameter of nanomaterials. The partial substitution of the alumina nanoparticles by the copper oxide nanomaterials in the mixture stabilizes importantly the hybrid nanofluidic medium.

This study is part of an ongoing project in which we experimentally examine the effect of composition (refractive index) and surface roughness on the scattering matrix elements of a set of well-characterized rough regolith simulants. A set of four cylindrical Mars regolith simulants with controlled degrees of porosity and surface roughness is studied. Photopolarimetric measurements, spanning scattering angles from 94° to 177°, were conducted at a wavelength of 488 nm at the Instituto de Astrofísica de Andalucía Cosmic Dust Laboratory. These results are compared with those previously obtained at 640 nm. The higher absorption of the Mars simulants at 488 nm compared to 640 nm produces a significant effect on all elements of the scattering matrix. The trends previously observed in the diagonal elements of the scattering matrix and in the albedo as a function of surface roughness remain unaffected by the differences in refractive indices at both wavelengths. The comparison with the previous study enables the investigation of sample reddening across the scattering-angle range, with a trend also observed between surface roughness and reddening. Moreover, within our measured angular range, the effect of absorption is consistent with Umov’s law, whereas the effects of surface roughness do not appear to support its validity. Nearly all samples exhibit a shallow negative polarization branch.

Superhydrophobicity is closely linked to the chemical composition and geometric characteristics of surface roughness. Building on our structural studies on water and air–water interfaces, this work aims to elucidate the mechanism underlying the wetting transition from the Wenzel to the Cassie state on a hydrophobic surface. In the Wenzel state, the grooves are filled with water, meaning that the surface roughness becomes embedded in the liquid. To evaluate the effects of surface roughness on water structure, a wetting parameter (WRoughness) is proposed, which is closely related to the geometric characteristics of roughness, such as pillar size, width, and height. During the wetting transition from Wenzel to Cassie states, the critical wetting parameter (WRoughness,c) may be expected, which corresponds to the critical pillar size (ac), width (wc), and height (hc). The Cassie state is expected when the WRoughness is less than WRoughness,c (ac), decreasing width (hc). Additionally, molecular dynamic (MD) simulations are conducted to demonstrate the effects of surface roughness on superhydrophobicity.

Most effective method to find the roughness parameters in rarefied gas flow is to calculate them from aerodynamic measurements, solving the inverse problem. The value of the main roughness parameter obtained from the solution of inverse problem is substantially higher (at least 1,25–1,5 times) than similar value of the same parameter measured from the profile diagrams. Thus, the effect of surface roughness in aerodynamic values of rough surface in rarefied gas flow is always significantly underestimated. First main reason of it is the low precision of roughness parameter measurements from the profile diagrams, and the second is based on usual lack of taking into account aerodynamic shadowing effect.

Time crystals refer to a state realized in an open system that spontaneously breaks time translation symmetry. There are ongoing discussions, both theoretical and experimental, regarding their realization and potential extensions, which highlights the need to explore new demonstrations in non-equilibrium systems. In this study, we demonstrate that the dripping of a superfluid can exhibit time crystallinity. We visualized pendant droplets of superfluid ⁴He dripping from various shaped surfaces and observed that the dripping period became consistently discretized when the edge of the droplet was free to move along the surface. This edge motion resulted in oscillation periods that were independent of the droplet’s volume, effectively eliminating the influence of variations in volume and flow rates on the timing of the dripping. Consequently, the dripping process became regular. The unimpeded movement of the droplet’s edge is a characteristic of superfluids and is facilitated by a preexisting superfluid thin film on the surface, which minimizes the effects of surface roughness and enhances the mobility of the edge. We examined the stability of the discretized periods as a function of input flow rate and concluded that the superfluid dripping system spontaneously broke continuous time translation symmetry, making it analogous to a continuous time crystal, but with multiple stable phases. In contrast, when the edge of the droplet was pinned, the dripping period displayed a wide distribution, resembling the irregular dripping behavior seen in classical fluids.

To assess the biogeophysical impacts of land cover land use change (LCLUC) on surface temperature, two observation-based metrics and their applicability in climate modeling were explored in this study. Both metrics were developed based on the surface energy balance, and provided insight into the contribution of different aspects of land surface change (such as albedo, surface roughness, net radiation and surface heat fluxes) to changing climate. A revision of the first metric, the intrinsic biophysical mechanism, can be used to distinguish the direct and indirect effects of LCLUC on surface temperature. The other, a decomposed temperature metric, gives a straightforward depiction of separate contributions of all components of the surface energy balance. These two metrics well capture observed and model simulated surface temperature changes in response to LCLUC. Results from paired FLUXNET sites and land surface model sensitivity experiments indicate that surface roughness effects usually dominate the direct biogeophysical feedback of LCLUC, while other effects play a secondary role. However, coupled climate model experiments show that these direct effects can be attenuated by large scale atmospheric changes (indirect feedbacks). When applied to real-time transient LCLUC experiments, the metrics also demonstrate usefulness for assessing the performance of climate models and quantifying land-atmosphere interactions in response to LCLUC.

In this study, a polymer-based surface plasmon resonance (SPR) sensor for refractive index measurements was designed and manufactured via inkjet 3D printing; then, it was optically characterized. Next, it was investigated how the surface finish of the 3D printed optical waveguide affects the sensor performance, i.e., its sensitivity. More in detail, it was studied how the surface roughness changes with the placement of the 3D printed items on the building platform. To achieve this purpose, a Phase I distribution-free quality monitoring analysis of the selected manufacturing process was implemented for a small pilot production run. The aim was to check the stability of surface roughness versus the placement of the 3D printed parts on the building platform. The 3D printed sensor’s surface roughness was assessed through a profilometry study. In particular, the surface roughness was determined for the core of the optical waveguide used to excite the SPR phenomena. Furthermore, the SPR sensors were optically characterized to find the existing relationship between their sensitivity and the considered quality of surface finish. In particular, by varying the surface roughness of the used waveguide, the light scattering in the waveguide changes, and the SPR sensitivity changes too, similarly to the light-diffusing fibers covered by gold nanofilms where the guided light is scattered through a plurality of voids distributed in the core. The procedure followed to investigate the sensor roughness, and establishing their performance enabled the optimal operative range for their application in practice to be identified. Finally, a better knowledge of the 3D printing manufacturing process has been achieved to improve quality of surface finish.

This paper presents a numerical study of the effects of blade roughness on wind turbine performance and annual energy production and how these effects may be partially mitigated through improved control. Three rotors are designed using the NACA4415, S801 and S810 airfoils, and blade element momentum theory is used to model wind turbine behaviour. The aerodynamic lift and drag data for the clean and roughened airfoils are taken from previous experimental work. These show that surface roughness leads to a decreased airfoil lift coefficient and an increased drag coefficient across an angle of attack range typical for large wind turbines, which will clearly lead to decreased turbine performance. Three separate control methods are considered for wind turbines with roughened blades operating at each of four candidate wind sites with different wind speed distributions. Results show that, compared to clean rotor blades, the roughened blades lead to a performance drop in the range of 2.9–8.6% for a torque based control strategy. A control‐based performance recovery strategy, in which the controller gain coefficient is re‐optimised, increased roughened rotor annual energy production by 0.1–1.0%.