Roughness Elements Research Articles

Impact of Roughness Spanwise Wavelength on Secondary Flow Rearrangement

Abstract Spanwise heterogeneity in surface roughness generates secondary mean flows in a rough-wall turbulent boundary layer. This study investigates the influence of roughness spanwise wavelength on the \textcolor{blue}{arrangement} of these secondary flows using Direct Numerical Simulation (DNS). We systematically vary the spanwise wavelength, $S/\delta$, from $\pi/16$ to $2\pi$, while maintaining constant roughness height and surface coverage density. Here, $S$ represents the roughness spanwise wavelength, and $\delta$ denotes \textcolor{blue}{the outer length scale, which in this case is the half-channel} height. Secondary flows are observed in all DNS cases, but their configurations depend on the spanwise wavelength. Specifically, small wavelengths result in low-momentum pathways above the roughness elements, whereas large wavelengths lead to high-momentum pathways at these locations. To elucidate the mechanisms behind this rearrangement, we analyze the mean streamwise vorticity transport equation. The findings indicate that shear stress anisotropy induces another pair of secondary vortices above the roughness elements as the spanwise wavelength increases. Given that secondary flows are features of the mean flow, we further examine the budgets of the dispersive kinetic energy (DKE). The analyses reveal that at small spanwise wavelengths, shear production primarily generates DKE, while at large wavelengths, wake production becomes the dominant energy source. Building on these insights, we propose a refined classification for surfaces with spanwise heterogeneity.

Critical Appraisal of Integrated CFD/Surface Roughness Models for Additive Manufactured Swirl Burners

Abstract Additive manufacturing technology creates complex parts that are impossible to be manufactured by traditional methods. However, the process often results in rough and irregular surfaces that can affect performance. Here Computational Fluid Dynamics (CFD) is considered to optimise component design for use in applications such as gas turbine. However, modelling the interactions between turbulent flows and Additive Manufacture (AM)-generated wall roughness affects the predictive capability of numerical models due to difficulty in thoroughly characterising rough wall texture. To address this issue, this study aims to appraise two common wall roughness approaches within the RANS framework: the modified law-of-the-wall and roughness-resolving approaches. The modified law-of-the wall is based on the correlation that converts the measured surface roughness parameters to the equivalent sand-grain roughness height. The latter approach involves the resolution of the roughness elements within the computational grid. The simulations were validated and compared against the velocity data published for the burner with AM swirl nozzle inserts of different surface finishes. The Realizable k-ɛ turbulence model was selected for all the CFD simulations, as it has been already validated for swirling flows. The results showed that the roughness-resolving approach was more accurate than the modified law-of-the wall correlation, with a good match with the experimental velocity data, predicting the velocity shift to the center. The model also revealed the shortened recirculation zone with increasing surface roughness. This is important in predicting flame stability and emissions performance.

Three-Dimensional Particle Tracking Velocimetry Investigation of Flow Dynamics Around Simplified Stones at Low Submergence: Implications for Instream Habitat

Shallow waterways such as rapids, tributaries and smaller streams can have important ecological functions in both free-flowing and regulated rivers. As more intermittent renewable energy is introduced to the energy system to reduce CO2 emissions, the operational conditions of hydropower plants are changing. This implies various flow scenarios that can lead to more locations with shallow depths and larger variations in water levels and velocities, resulting in increased impact on the riverine ecosystem. Accurate predictions of these impacts require an understanding of the flow dynamics near large roughness elements such as boulders or trees in shallow river regions. This study uniquely investigates the effect of relative submergence, i.e., water depth relative to boulder size, on the flow field, turbulence, and potential fish habitats around idealized stone shapes (hemispheres) in shallow open channel flow using time-resolved 3D particle tracking velocimetry. The results indicate that varying relative submergence significantly affects recirculation zones, velocity and vorticity distribution, as well as turbulent kinetic energy. Notably, larger regions of lower velocity downstream of the roughness elements were generated at lower submergences, which might be favorable for fish energy conservation. Valuable insights into ecohydraulic engineering and habitat restoration in shallow waterways can be gained by understanding the fundamental flow mechanisms at low submergence for the flow around large roughness elements.

Instability characteristics induced by roughness elements in the rotating-disk boundary layer of a rotor-stator cavity

Instability characteristics induced by roughness elements in the rotating-disk boundary layer of a rotor-stator cavity

Roughness and Energy Losses Induced by Mussel Growth on the Walls of Hydraulic Structures and Application to a Water Transfer Project

AbstractMussel biofouling increases energy losses in hydraulic structures. The first contribution of this paper is the quantification of the mussel‐induced equivalent sand roughness ks as function of the mussel attachment density N and the shell length L. Laboratory experiments reveal that ks/L ≈ 1.5 for a continuous regular layer of mussels, which is found for N L2 > 1.2. For 0.5 < N L2 < 1.2, the mussels form a continuous irregular roughness layer with increased values of ks/L of up to 2.4. These geometrical irregularities are interpreted as macro‐roughness elements, that is, roughness elements with a spatial scale larger than that of an individual mussel. For N L2 < 0.5, the density of the irregularities is too low to act as macro‐roughness elements leading to ks/L < 1.5. The second contribution is the establishment of a threshold criterion for the importance of filtering activity on ks based on data from the here reported experiments and data reported in literature in other configurations and/or with other mussel species. It is found that laboratory conditions are often close to the threshold value but that mussel filtering is always negligible in large hydraulic structures. The third contribution is the development of a method based on 3‐D numerical simulations for estimating a Darcy‐Weisbach friction factor f for walls that are only partially covered with patches of mussels. An application example illustrates how the thus obtained f can be used in a 1‐D model for quantifying the additional energy losses in large water transfer projects.

Numerical investigation on comparative performance assessment of solar air heater with different artificial roughness elements in a triangular duct

Numerical investigation on comparative performance assessment of solar air heater with different artificial roughness elements in a triangular duct

Turbulence scale and strength analysis in the roughness and inertial sublayers over urban areas: A wind tunnel study

The dissimilarity of dynamics and turbulence structures between the roughness and inertial sublayers (RSL, ISL) over roughness elements show that ISL turbulence is much more homogeneous. Conventional logarithmic law of the wall is merely applicable for the RSL mean-wind-speed profiles. To characterize the turbulence scales and elucidate the mechanism, the flows in the atmospheric surface layer (ASL) over real urban morphology are measured in wind tunnel experiments fabricated by 3D-printing, reduced-scale model of downtown Kowloon Peninsula, Hong Kong. Elevated dispersive stress at urban canopy signifies the influence from individual buildings and inhomogeneous RSL flows. A positive momentum contribution appears in the low-frequency regime. The large-scale turbulence in RSL thus enhances the mixing and transport, resulting in inhomogeneous flows. The contribution from ejection Q2 and sweep Q4 increases and decreases, respectively, with increasing RSL motion strength. The conventional −5/3 law is observed by empirical mode decomposition (EMD) and the largest amplitude fluctuation occurs more often when the turbulence length scale is comparable to the turbulent boundary layer (TBL) thickness. The single-point amplitude modulation (AM) shows that the large- and small-scale turbulence correlates tightly at the bottom of RSL. Besides, the joint probability density function (JPDF) illustrates that accelerating large scales often occur with decelerating small scales, and they are intensified with increasing wall-normal distance. As a result, large-scale turbulence influences substantially the flow structures over urban areas and the small-scale turbulence (even) in RSLs in the vicinity of building obstacles.

Changes in the Transition Process of Roughness-Induced Crossflow Vortices due to Freestream Turbulence

AbstractLaminar-turbulent transitions in boundary layers are one of the major research topics in fluid dynamics. In this study, we focused on a three-dimensional boundary layer formed on a swept flat plate. In this boundary layer, the crossflow instability is dominant, and the instability induces crossflow vortices (CFVs). Many studies have focused on the dependency of the transition process on the intensities of steady or unsteady disturbances, which correspond to a roughness element and freestream turbulence (FST), respectively. On the other hand, the effects of the FST wavelength are still unclear. Moreover, there is a lack of knowledge about the transition processes caused by both steady and unsteady disturbances. We investigated how the transition process of a stationary structure caused by cylindrical roughness changes depending on the FST wavelength using direct numerical simulations. We classified transition processes into two types: processes in which stationary structures grow into CFVs and processes in which hairpin vortices are generated on the stationary structures. The former is further classified into four types depending on the presence or absence of FST and on the FST wavelength. We revealed the contributions of different FST wavelengths to the transition process changes. The short-wavelength FST provides hairpin vortices to the stationary structure at low roughness height conditions because of its high-frequency components. The long-wavelength FST changes the process due to unsteady fluctuations influencing the stationary structure. In summary, the transition processes undergo different mechanisms between short- and long-wavelength FST.

Enhancing Fish Passage Efficiency: Lessons from UHE Porto Primavera’s Fish Ladder

Dams severely affect aquatic environments and block the longitudinal migration of fish. In order to mitigate the negative effects generated by these developments, fish passes, or fishways, are implemented in dams with the purpose of restoring river connectivity and allowing the movement of migrants. Nevertheless, fishways in neotropical areas often face design and construction issues that can reduce their efficiency and selectively disadvantage species with limited swimming capabilities. This study analyzes how a fish ladder on the Paraná River influences the black armored catfish (Rhinelepis aspera), a benthic, long-distance migratory species important to commercial fisheries. A total of 200 individuals were PIT-tagged and monitored for four months. The results showed that although many fish successfully located the fishway, only a small portion (3.5%) managed to complete the ascent. The interaction between the hydraulic characteristics of the fishway and the fish condition factor played a significant role in ascent performance. Our findings underscore the importance of assessing fishway suitability for benthic neotropical species to support conservation efforts in the Upper Paraná River Basin. To improve passage rates for R. aspera, we recommend optimizing flow conditions by adjusting orifice and notch configurations, incorporating roughness elements, and modifying resting pool designs. These adaptations would reduce energy expenditure for ascending fish, enhancing fishway performance and contributing to the sustainability of migratory species in this region.

Characteristics of hydraulic jump in asymmetrical trapezoidal channel with rough beds: experimental investigations

Understanding the dynamics of hydraulic jumps is crucial for optimizing the design of stilling basins in dams, enhancing energy dissipation efficiency, and reducing corrosion risks in hydraulic structures. This work aims to investigate the effect of bed geometry and roughness on the properties of hydraulic jump in an asymmetric trapezoidal channel, including parameters such as sequential depths, roller length and energy loss. Experiments were carried out under open channel flow conditions using three different bottom roughness element heights and mm. The channel's bottom is inclined transversely with a slope of covering a wide range of inflow Froude Number . Results indicate that the increase in bottom roughness leads to a decrease in the subsequent depth ratio by 28.91% compared to a hydraulic jump in a smooth bed. It was also found that the average reduction in roller length on the shallow and deep sides is 21.62% and 20.4%, respectively. Increasing the height of the roughness element enhances the relative energy dissipation by 8.53%. Finally, empirical equations were developed to describe hydraulic jump characteristics based on the Froude number and roughness element height, aiding in the optimal design of stilling basins.

Direct numerical simulation of boundary layer transition induced by roughness element in a low Reynolds number compressor blade

The surface roughness of blades in low Reynolds number compressors significantly impacts the transition of the boundary layer and the profile loss. This paper employs direct numerical simulation (DNS) to investigate the boundary layer flow within a low Reynolds number compressor cascade, where an isolated roughness element is situated on the suction surface. A cylinder and a ramp are crafted and simulated by the embedded boundary method (EBM). The research centers on assessing the influence of varied roughness element shapes on the downstream velocity field, laminar separation bubble (LSB), and transition. DNS findings indicate that the cylinder's downstream shear and disturbances are relatively weaker, prompting separation-induced transition within its boundary layer, while the ramp's intense disturbances lead to bypass transition. The lift-up mechanism and the self-induced amplification of streamwise gradient of spanwise velocity are intrinsic to the flow instability during the bypass transition. Both cases can diminish total pressure loss, with the ramp yielding a relatively higher loss due to the contribution of a large-scale turbulent boundary layer to the total pressure loss. In terms of profile loss, the terms of mean kinetic energy dissipation and production associated with the suction-surface-normal gradient of mean streamwise velocity are predominant.

Effect of roughness elements on receptivity of hypersonic blunt cone boundary layers

Effect of roughness elements on receptivity of hypersonic blunt cone boundary layers

Transition to turbulence in rough plane Couette flow

This DNS study considers transition to turbulence in plane Couette flow (pCf) with a rough stationary wall and a smooth moving wall. The roughness elements are square ribs of height $k=0.2h$ (where $h$ is the half-channel height). Two different pitch separations, $\lambda =2k$ and $10k$ , are considered, i.e. d-type and k-type roughness, respectively. The transition in both rough pCf cases takes place through a stage of alternate laminar–turbulent bands aligned in an oblique fashion. However, roughness causes a shift in the transitional Reynolds number ( $Re$ ) range. In the k-type roughness, stable bands are observed in the range $Re \in [300, 325]$ , which is a downwards shift from the transitional $Re$ range for the smooth pCf ( $Re \in [325,400]$ ). The d-type roughness, on the other hand, surprisingly shifts the transitional $Re$ range upwards to $Re \in [350,425]$ . This peculiar behaviour is associated with the ability of the ribs to shed and regenerate vorticity. Large-scale components extracted using a filtering process relate to the transition bands and flow parallel to the oblique laminar–turbulent boundaries. Counter-rotating vortices are present in the turbulent regions of the flow field, which exist in tandem with the high- and low-velocity streaks. Another interesting observation is the secondary Reynolds shear stresses, $-\overline {v^{\prime }u^{\prime }}$ and $-\overline {w^{\prime }v^{\prime }}$ , which are non-zero in the transitional regime, in contrast to the turbulent regime where they are negligible.

Predicting flow resistance in rough‐bed rivers from topographic roughness: Review and open questions

AbstractMost ways of predicting flow resistance in shallow rivers with a partial or complete cover of coarse sediment use a bed‐sediment grain diameter as a roughness length scale. However, beds with the same grain size distribution differ in roughness and flow resistance depending on how the larger grains are arranged, the nature of any bedforms and the possible complications of bedrock or rough banks. This has led to interest in predicting flow resistance using metrics of the topographic roughness of the bed. Some researchers have used the standard deviation of bed elevation as a roughness length scale. An alternative for channels containing boulders is to regard the bed as an array of large roughness elements. Fluvial research to date using these two approaches is limited and inconclusive. We review potentially relevant findings from the much more extensive literature in boundary‐layer meteorology and various branches of engineering and note links between the distribution‐statistics and element‐array approaches. The skewness of the elevation distribution is widely seen as important but it is unclear how best to use it for flow prediction. Other open questions include the scale dependence of topographic metrics, and what type of flow resistance equation to use them in. Calibration and testing of new prediction methods require flow data from reaches with known roughness statistics. This need should be met partly by measurements at field sites or in flume models of them, but also by flume experiments and numerical simulations using synthetic roughness.

Rough surfaces in underexplored surface morphology space and their implications on roughness modelling

We report direct numerical simulations results of the rough-wall channel, focusing on roughness with high $k_{rms}/k_a$ statistics but small to negative $Sk$ statistics, and we study the implications of this new dataset on rough-wall modelling. Here, $k_{rms}$ is the root mean square, $k_a$ is the first-order moment of roughness height, and $Sk$ is the skewness. The effects of packing density, skewness and arrangement of roughness elements on mean streamwise velocity, equivalent roughness height ( $z_0$ ) and Reynolds and dispersive stresses have been studied. We demonstrate that two-point correlation lengths of roughness height statistics play an important role in characterizing rough surfaces with identical moments of roughness height but different arrangements of roughness elements. Analysis of the present as well as historical data suggests that the task of rough-wall modelling is to identify geometric parameters that distinguish the rough surfaces within the calibration dataset. We demonstrate a novel feature selection procedure to determine these parameters. Further, since there is no finite set of roughness statistics that distinguish between all rough surfaces, we argue that obtaining a universal rough-wall model for making equivalent sand-grain roughness ( $k_s$ ) predictions would be challenging, and that each rough-wall model would have its applicable range. This motivates the development of group-based rough-wall models. The applicability of multi-variate polynomial regression and feedforward neural networks for building such group-based rough-wall models using the selected features has been shown.

Towards a new roughness parametrization through the effective distribution function

Turbulent flows over rough surfaces can be encountered in a wide range of engineering applications. Despite the progress made after several decades of studies, the prediction of drag and roughness function from the surface geometrical parameters remains an open question. Several methods have shown encouraging results. However, they lack generality and present some scatter in the data. In this paper we propose a new parameter, the effective distribution ( $ED$ ), which lays foundation on the effective slope with some changes to take into account the sheltering effect of large roughness elements and the drag induced by pinnacles higher than the average roughness elements. To develop this new correlation between geometrical features of the wall and the drag, we performed a set of simulations of the turbulent flow over a rough surface made of triangular elements varying their height and spatial distribution. The $ED$ correlates quite well both with the drag and the roughness function for a wide range of cases having different mean roughness height, skewness and kurtosis. To further validate the $ED$ , and assessing how it can be generalized to real rough wall, an irregular wall made from the superposition of random sinusoidal function was considered. Results were consistent with the correlation here presented.

Pore-scale study of droplet settling on a heterogenous surface structure

Equilibrium contact angle of a droplet is influenced by surface characteristics and fluid properties. In addition to increasing the solid–liquid contact line, surface roughness also alters the surface free energy, which has a significant influence on contact angle values. Droplets are more likely to impinge on vertices as surface roughness increases. Anisotropic wetting of chemically heterogeneous surfaces further controls the total surface free energy. The free energy Lattice Boltzmann method is utilized to study the effects of wettability heterogeneity and roughened surfaces. Initial model comparisons with experiments showed excellent agreement. The rough surface is modeled with different pillar shapes on a smooth wall, with surface wettability ranging from hydrophilic to neutral conditions. The length scale of surface patterns matches the droplet size, making the Cassie–Baxter and Wenzel equations inapplicable. Results indicate that droplets pin on the vertices of rectangular pillars, while frustum shapes facilitate movement. Studies cover nearly neutral wet, moderately wet, and strongly wet conditions. The effects of relative surface roughness, roughness distribution, mixed wetting surfaces, and body force on equilibrium contact angle are examined. Additionally, the interaction between fluid flow and surface roughness elements shows that smaller spacing and greater height of roughness elements enhance thermal performance, with Nusselt numbers fluctuating significantly. Findings suggest that the ratio of droplet size to pillar surface area is crucial for minimizing surface free energy. On superhydrophilic surfaces, droplet pinning at pillar edges causes the surface to behave hydrophobically. In mixed-wet rough surfaces, pillar wettability significantly influences the equilibrium contact angle.

Bottom Stress and Drag on a Shallow Coral Reef

AbstractCoral reef roughness produces turbulent boundary layers and bottom stresses that are important for reef metabolism monitoring and reef circulation modeling. However, there is some uncertainty as to whether field methods for estimating bottom stress are applicable in shallow canopy environments as found on coral reefs. Friction velocities () and drag coefficients () were estimated using five independent methods and compared across 14 sites on a shallow forereef (2–9 m deep) in Palau with large and spatially variable coral roughness elements (0.4–1 m tall). The methods included the following: (a) momentum balance closure, (b) log‐fitting to velocity profiles, (c) Reynolds stresses, (d) turbulence dissipation, and (e) roughness characterization from digital elevation models (DEMs). Both velocity profiles and point turbulence measurements indicated good agreement with log‐layer scaling, suggesting that measurements were taken within a well‐developed turbulent boundary layer and that canopy effects were minimal. However, estimated from the DEMs, momentum budget and log‐profile fitting were consistently larger than those estimated from direct turbulence measurements. Near‐bed Reynolds stresses only contributed about 1/3 of the total bottom stress and drag produced by the reef. Thus, effects of topographical heterogeneity that induce mean velocity fluxes, dispersive stresses, and form drag are expected to be important. This decoupling of total drag and local turbulence implies that both rates of mass transfer as well as values of fluxes inferred from concentration measurements may be proportional to smaller, turbulence‐derived values of rather than to those based on larger‐scale flow structure.

Relative size matters: Spawning substrate roughness size and spacing affect egg dislodgement and retention in the benthos

AbstractThe consequences of hydrodynamic effects on survival of embryonic benthic organisms remain unknown. This is because the interaction of hydrodynamics and substrate roughness is complex, but it can be conceptualized in two dimensions by roughness flow regimes (isolated, wake interference, and skimming flow) or roughness types (k‐ and d‐type). We examined the effect of different roughness flow regimes/roughness types and roughness heights (k) on the dislodgement of walleye (Sander vitreus) eggs (2 mm diameter, ⌀) using a wall jet apparatus. We expected that lower wall shear stress (τw) would be required for dislodgement in more‐exposed/less‐sheltered roughness type/flow regimes (i.e., k‐type/isolated roughness flow) and when k ≤ ⌀. Surprisingly, the opposite was found, indicating that other mechanism(s) are also responsible for egg dislodgement on these rough surfaces. Hydrodynamic sheltering occurred when flow was directed over exposed eggs (k ≤ ⌀) adjacent to roughness elements effectively increasing their width, and thus requiring higher τw for rolling and then ejection. When k > ⌀, eggs dislodgement likely occurred via the lower pressure in the eddies recirculating in the groove width between roughness elements, rather than due to τw. These results indicate the relevance of heterogeneous sorting and sizing of substrate elements to provide suitable spawning habitats in benthic environments.

Exploring nusselt number and friction factor correlations for sphere-shaped roughness elements on the absorber to enhance solar air heater efficiency

ABSTRACT This investigation is conducted to analyze the artificial surface roughness influence on the heat flow and friction characteristics within the ducts of solar air heaters (SAHs). This research aims to investigate the consequences of applying spherical-shaped surface roughness to the absorber in a linear and staggered manner with the intention of enhancing the heat transfer efficiency. The utilization of roughness elements in the shape of spheres is aimed at augmenting the heat transfer characteristics; however, it is crucial to acknowledge that this enhancement is concomitant with an elevation in pumping power due to heightened friction. An experimental campaign encom- passes a diverse set of operational and device parameters. These parameters include the Reynolds number (Re), which varies from 3,000 to 8,000. Additionally, the roughness pitch-to-height ratio (p/h) ranges from 10 to 20, while the roughness gap-to-height ratio (w/h) ranges from 4 to 8. Throughout the trials, a constant value of 0.06 is maintained for the roughness height-to-hydraulic diameter ratio (h/D) and an amount of 5 is maintained for the duct width-to-height ratio (W/H). The outcomes of this study indicate a considerable increase in the Nusselt number, ranging from 50.47% to 69.51%, concurrently with a substantial rise in the friction factor, ranging from 27.76% to 144.75%, when compared to designs using a smooth absorber surface in the context of SAHs. By utilizing the experimental data, relationships between the friction factor and the Nusselt number are established based on the artificial roughness parameters and the operating conditions. The current study’s findings significantly advance our knowledge of and ability to improve SAH systems’ heat transfer and friction characteristics. Thus, this investigation improves our understanding of these systems operational behavior in many situations.