Equivalent Sand Research Articles

Effects of steepness on turbulent heat transfer over sinusoidal rough surfaces

We conducted a direct numerical simulation (DNS) study to investigate the impact of surface undulation steepness on rough wall turbulent heat transfer. The flow geometry was turbulent open-channel flow over three-dimensional sinusoidal rough surfaces. To examine the effects of steepness, we systematically varied the streamwise and spanwise wavelengths of the sinusoidal roughness while keeping the roughness height constant. The friction Reynolds number ranged from 180 to 600, and we considered a passive scalar with the fluid Prandtl number was 0.7, assuming air flow conditions. In the fully rough regime, the velocity roughness function is expressed as a function of the inner-scaled equivalent sand grain roughness ks+ independent of steepness, whereas the steeper surfaces with shorter wavelengths result in larger temperature roughness functions at the same ks+ value. Analysis of the physical mechanisms that increases the roughness function shows that the pressure drag primarily contributes to the increase in the velocity roughness function, while the temperature roughness function is mainly augmented by the roughness-induced wall heat transfer term, correlating with the steepness of the surface undulations. It is also suggested that the effective slope, which quantifies the steepness of rough surfaces, could improve the predictive accuracy of existing correlations for the temperature roughness function.

Extended analytical model of Tesla turbine with advanced modelling of velocity profile in minichannel between corotating disks with consideration of surface roughness

Consideration of roughness effects in the flow in a minichannel has been a major scientific problem for many decades. Roughness can alter the momentum diffusion, heat transfer conditions or laminar-turbulent transition. These effects are even more pronounced in minichannel flows where the roughness can occupy a large part of the cross-section. Modelling methods applied so far usually fall short in internal flows with high relative roughness. The presented paper pertains to the analytical flow model in Tesla turbine components with consideration of roughness on the rotor walls. The model uses the equivalent sand grain approach to modify dynamic viscosity in the governing equations to achieve a desired downward shift of the dimensionless velocity profile. The model solves two-dimensional equations of continuity, momentum and energy. The semi-empirical function was derived to consider the change in the shape of the radial velocity. The applied model incorporates Euler's turbomachinery equation to determine the influence of roughness on the turbine performance under varied operating conditions. Roughness shortens the streamlines inside the rotor, but the overall turbine's performance is improved. The roughness equal to 10−5 m increased the power generated and isentropic efficiency by factors of 3 and 2.5, respectively, compared to the smooth rotor.

Large eddy simulations of flow over additively manufactured surfaces: Impact of roughness and skewness on turbulent heat transfer

Additive manufacturing creates surfaces with random roughness, impacting heat transfer and pressure loss differently than traditional sand–grain roughness. Further research is needed to understand these effects. We conducted high-fidelity heat transfer simulations over three-dimensional additive manufactured surfaces with varying roughness heights and skewness. Based on an additive manufactured Inconel 939 sample from Siemens Energy, we created six surfaces with different normalized roughness heights, Ra/D=0.001,0.006,0.012,0.015,0.020, and 0.028, and a fixed skewness, sk=0.424. Each surface was also flipped to obtain negatively skewed counterparts (sk=−0.424). Simulations were conducted at a constant Reynolds number of 8000 and with temperature treated as a passive scalar (Prandtl number of 0.71). We analyzed temperature, velocity profiles, and heat fluxes to understand the impact of roughness height and skewness on heat and momentum transfer. The inner-scaled mean temperature profiles are of larger magnitude than the mean velocity profiles both inside and outside the roughness layer. This means, the temperature wall roughness function, ΔΘ+, differs from the momentum wall roughness function, ΔU+. Surfaces with positive and negative skewness yielded different estimates of equivalent sand–grain roughness for the same Ra/D values, suggesting a strong influence of slope and skewness on the relationship between roughness function and equivalent sand–grain roughness. Analysis of the heat and momentum transfer mechanisms indicated an increased effective Prandtl number within the rough surface in which the momentum diffusivity is larger than the corresponding thermal diffusivity due to the combined effects of turbulence and dispersion. Results consistently indicated improved heat transfer with increasing roughness height and positively skewed surfaces performing better beyond a certain roughness threshold than negatively skewed ones.

A CFD Analysis of Oscillating Airfoil with Leading Edge Roughness: Comparing Correlations for Equivalent Sand Grain Roughness

Numerical simulations express surface roughness as equivalent sand grain roughness values, so the correlation is required to convert realistic roughness into equivalent sand grain roughness values. The correlations were designed to match the wall velocity distributions for static cases with roughness, and they accurately predicted roughness effects when applied to URANS simulations. Several studies have examined the roughness effect on dynamic pitching airfoils. Still, they have only considered the presence of roughness, neglecting the process of converting the actual roughness to equivalent sand-grain roughness. Thus, in the present study, we apply a static case-based equivalent sand-grain roughness correlation to dynamic pitching analysis to validate the suitability of each correlation and verify the significance of correlation parameters. It was observed from a comparison study between numerical aerodynamic coefficients and experimental data from Ohio State University that physical parameters such as skewness and roughness height were important parameters to predict the dynamic stall behaviour more accurately. It was also observed that dynamic stall prediction accuracy is affected by the frequency and type of airfoil, and through this, it was confirmed that there are additional factors to consider when using an equivalent sand grain roughness correlation on pitching airfoils.

Analyses of non-clean surface compressor blade cascades’ aerodynamic performance deterioration

Abstract The efficiency and dependability of the engine are directly impacted by the compressor’s performance, which is a crucial part of gas turbines. The blade cascade is significantly influenced by surface roughness in terms of flow characteristics and aerodynamic performance. In this study, the NACA65-410 airfoil is selected as the research object to investigate the variations in blade cascade performance and flow characteristics under different inflow and roughness conditions. The results show that the smooth surface blade cascade performs optimally when the Reynolds number increases from 120,000 to 200,000. As the angle of attack increases from -5° to 8°, the blade cascade performance deteriorates, and the degree of performance degradation increases with the increase in angle of attack. For the roughened blade cascade surface, the performance initially improves and then decreases as the equivalent sand grain density increases, and there exists a critical equivalent sand grain density that maximizes the blade cascade performance. When the Reynolds number is 120,000, the critical equivalent sand grain density is 225 μm, and when the Reynolds number is 160,000, the critical equivalent sand grain density is 111 μm. Based on the findings of this study, the degradation pattern of blade cascade performance can be predicted, and it can be utilized to enhance the overall performance of the compressor.

Effectiveness of the Concrete Equivalent Mortar Method for the Prediction of Fresh and Hardened Properties of Concrete

Modern concrete mix design is a complex process involving superplasticisers, fine powders, and fibres, requiring time and energy due to the high number of trial tests needed to achieve rheological properties in the fresh state. Concrete batching involves the extensive use of materials, time, and the testing of chemical admixtures, with various methodologies proposed. Therefore, in some instances, the required design properties (physical and mechanical) are not achieved, leading to the loss of resources. The concrete equivalent mortar (CEM) method was introduced to anticipate concrete behaviour at fresh and hardened states. Moreover, the CEM method saves time and costs by replacing coarse aggregates with an equivalent sand mass, resulting in an equivalent specific surface area at the mortar scale. This study aims to evaluate the performance of fibre in CEM and concrete and determine the relationships between the CEM and the concrete in fresh and hardened states. Steel and polypropylene fibres were used to design three series of mixtures (CEM and concrete): normal-strength concrete (NSC), high-strength concrete (HSC), high-strength concrete with fly ash (HSCFA), and equivalent normal-strength mortar (NSM), high-strength mortar (HSM), and high-strength mortar with fly ash (HSMFA). This study used three-point bending tests and digital image correlation to evaluate load and crack mouth opening displacement (CMOD) curves. An analytical mode I crack propagation model was developed using a tri-linear stress–crack opening relationship. Post-cracking parameters were optimised using inverse analysis and compared to actual MC2010 characteristic values. The concrete slump is approximately half of the CEM flow; its compressive strength ranges between 78% and 82% of CEM strength, while its flexural strength is 60% of CEM strength. The post-cracking behaviour showed a significant difference attributed to the presence of aggregates in concrete. The fracture energy of concrete is 28.6% of the CEM fracture energy, while the critical crack opening of the concrete is 60% of that of the CEM.

Slip flow and thermal characteristics in gas thrust bearings with rough surfaces

PurposeThis study aims to investigate the rarefaction effects on flow and thermal performances of an equivalent sand-grain roughness model for aerodynamic thrust bearing.Design/methodology/approachIn this study, a model of gas lubrication thrust bearing was established by modifying the wall roughness and considering rarefaction effect. The flow and lubrication characteristics of gas film were discussed based on the equivalent sand roughness model and rarefaction effect.FindingsThe boundary slip and the surface roughness effect lead to a decrease in gas film pressure and temperature, with a maximum decrease of 39.2% and 8.4%, respectively. The vortex effect present in the gas film is closely linked to the gas film’s pressure. Slip flow decreases the vortex effect, and an increase in roughness results in the development of slip flow. The increase of roughness leads to a decrease for the static and thermal characteristics.Originality/valueThis work uses the rarefaction effect and the equivalent sand roughness model to investigate the lubrication characteristics of gas thrust bearing. The results help to guide the selection of the surface roughness of rotor and bearing, so as to fully control the rarefaction effect and make use of it.

Atterberg limits of baseball infield soils containing over-size particles, Part II: effects of sand angularity and uniformity

Context Atterberg limit tests may be useful for evaluating baseball infield soils because these tests directly link soil behaviour to water content. Prior research has demonstrated that the liquid and plastic limits (LL and PL) of sand-clay mixtures are affected by sand properties. However, these studies have used sand exclusively <425 μm and little attention has been devoted to sand angularity or sand-size uniformity. Aims This research tested the effects of sand angularity and sand-size uniformity on the Atterberg limits of infield mixes containing 0–80% sand with much of the sand 425–2000 μm. Methods Experiment 1 compared the effect of mixing angular or round sand of equivalent size with a kaolinitic clay. Experiment 2 compared the effect of mixing one of two sands having a similar average particle size but varying uniformity with an illitic clay. Key results For mixes having equivalent sand content and sand size, the shape of the sand particles did not affect LL (P = 0.47) or PL (P = 0.80). Mixtures with non-uniform sand yielded higher LL than those with uniform sand (mean difference ~0.6% water content g g−1). The mixtures with non-uniform sand also remained plastic at higher sand content (~72.5%) than mixtures with uniform sand (~67.5%). Calculated threshold sand contents for the two sets of mixtures agreed closely with the experiments. Conclusions Sand angularity was shown to be unimportant in this context. When average particle size was held constant, sand uniformity affected the LL water content and the sand content corresponding to a transition between plastic and non-plastic behaviour. Implications This research suggests that baseball field managers need not consider the angularity of sand in an infield mix but should be aware of the uniformity of the sand used to produce the mix as this may influence the mixture’s plasticity.

Flow resistance over heterogeneous roughness made of spanwise-alternating sandpaper strips

The Reynolds number dependent flow resistance of heterogeneous rough surfaces is largely unknown at present. The present work provides novel reference data for spanwise-alternating sandpaper strips as one idealised case of a heterogeneous rough surface. Experimental data are presented and analysed in direct comparison with drag measurements of homogeneous sandpaper surfaces and numerical simulations. Based on the homogeneous roughness data, the related challenges and sensitivities for the evaluation of roughness functions from experiments and simulations are discussed. A hydraulic channel height is suggested as an alternative measure for the drag impact of rough surfaces in internal flows. For the investigated heterogeneous roughness, it is found that turbulent flow does not exhibit a fully rough flow behaviour, indicating that the assignment of an equivalent sand grain height as commonly applied for homogeneous roughness is not possible. A prediction of the drag behaviour of rough strips based on an average between rough and smooth drag curves appears promising, but requires further refinement to capture the impact of turbulent secondary flows and spatial transients linking smooth and rough surface parts. While turbulent secondary flow induced by the roughness strips yield significant spanwise variation of the mean velocity profile for the investigated rough strips, we show that the spanwise averaged velocity profiles collapse reasonably well with a smooth or homogeneous rough wall flow. This allows to extract a global roughness function from the spanwise averaged flow field in good agreement with the one deduced from global pressure drop measurements.

On predictive models for the equivalent sand grain roughness for wall-bounded turbulent flows

One of the long-standing goals of rough wall fluid dynamics research is to determine the drag penalty of surfaces based solely on their topographical parameters. The most important length scale or roughness parameter that best describes a surface in relation to friction drag has not been agreed upon yet, despite the many studies that, over the years, have attempted to identify the most appropriate surface parameter. The concept of an equivalent sand-grain roughness (ks) was introduced to standardize different types of roughness in wall-bounded turbulence, serving as an input parameter for predicting the roughness function ΔU+. To anticipate ΔU+ generated by a rough surface under turbulent flow conditions, experts use roughness correlations that establish a correspondence between the topographical characteristics of the surface and ks. Therefore, a chronological compilation of roughness correlations is presented, detailing the parameter ranges and types of roughness used in their development. This study evaluates the effectiveness of predictive correlation functions and aims to formulate a universal function by exploring a comprehensive assortment of three-dimensional (3D) surface textures available in the literature. The results suggest that a correlation based on surface height skewness (ksk) and streamwise effective slope (ESx) can predict the ratio (ks/kq), where kq is the root mean square roughness height for 3D roughness in the fully rough regime. Despite the fact that the correlation is restricted to 3D surface roughness, which is a more realistic representation, the model demonstrated a high level of accuracy in predicting ks for over 120 distinct rough surfaces.

Surface Roughness in RANS Applied to Aircraft Ice Accretion Simulation: A Review

Experimental and numerical fluid dynamics studies highlight a change of flow structure in the presence of surface roughness. The changes involve both wall heat transfer and skin friction, and are mainly restricted to the inner region of the boundary layer. Aircraft in-flight icing is a typical application where rough surfaces play an important role in the airflow structure and the subsequent ice growth. The objective of this work is to investigate how surface roughness is tackled in RANS with wall resolved boundary layers for aeronautics applications, with a focus on ice-induced roughness. The literature review shows that semi-empirical correlations were calibrated on experimental data to model flow changes in the presence of roughness. The correlations for RANS do not explicitly resolve the individual roughness. They principally involve turbulence model modifications to account for changes in the velocity and temperature profiles in the near-wall region. The equivalent sand grain roughness (ESGR) approach emerges as a popular metric to characterize roughness and is employed as a length scale for the RANS model. For in-flight icing, correlations were developed, accounting for both surface geometry and atmospheric conditions. Despite these research efforts, uncertainties are present in some specific conditions, where space and time roughness variations make the simulations difficult to calibrate. Research that addresses this gap could help improve ice accretion predictions.

Influence of wall roughness on pressure distribution and performance of centrifugal dredge pumps

Wall roughness is an important factor that affects pump head and efficiency. Based on the open source program OpenFOAM, the wall function method and the equivalent sand roughness model were used to study the influence of wall roughness on the performance of centrifugal dredge pumps. Then, the steady state of the dredge pump under 10 different types of wall roughness was calculated using the k-omega SST turbulence model. The pressure distribution under different flow rates and rotation speeds was also analyzed. The results reveal that the relationship between the increase of wall roughness and the performance of the dredge pump can be roughly divided into three stages, which may be related to the link between roughness and the viscous layer. In particular, the pump performance decreases significantly, and the shaft power increases linearly when it is in the third stage (complete rough region). Finally, different degrees of roughness have minimal effects on the pressure distribution of the impeller.

Effects of varied roughness coverage area on drag in a turbulent boundary layer using numerical simulations

Turbulent boundary layer (TBL) over various area coverages of rough surfaces have been studied via computational fluid dynamics using the steady Reynolds Averaged Navier–Stokes (RANS) technique. The rough surfaces are modelled by randomly distributed hemispheres with a 1 mm radius, covering 5%, 10%, 20%, 30%, 40%, and 50% of the wall surface area designed to represent a typical three-dimensional roughness and also replicate the biofouling growth of a ship’s hull. The results of this study show the relationships between the roughness area coverage and the skin friction drag within a fully rough regime. The flow experiences maximum drag and the highest equivalent sand grain roughness height at 30% roughness area coverage. However, beyond this area coverage value, the drag gradually decreases. Further analysis indicates that the 30% roughness area coverage corresponds to a roughness frontal density of λf=0.15. Recent reports indicate that when λf≲0.15, the pressure drag to total drag ratio increases with increased roughness frontal density.

Comparative study of surface roughness models in the hydro-thermal characterization of flow over rough walls

Surface roughness is responsible for the localized turbulence, which disrupts the viscous sublayer, affecting pressure drop and heat transfer. Thus, the numerical modeling of the effect of roughness on the fluid flow and heat transfer is quite essential. In this work, a numerical model is developed in OpenFOAM to incorporate the effect of surface roughness by modifying the wall function. Its accuracy is validated with available semi-empirical correlations and experimental results. The efficacy of available models for evaluating equivalent sand–grain roughness height (ks) based on surface statistics is investigated. A correlation for the dimensionless near-wall cell center distance (y+) is developed as the function of the Reynolds number and the equivalent sand–grain roughness height. The developed numerical model is validated with the semi-empirical relation and experimental results from the literature with average deviations of 7%. It is found that the equivalent sand–grain roughness height, evaluated using expressions reported by Flack et al. [ “Skin friction measurements of systematically-varied roughness: Probing the role of roughness amplitude and skewness,” Flow Turbul. Combust. 104, 317–329 (2020)], shows the lowest average deviation of 3.48% with the experimental data among all the considered formulas of ks. The proposed correlation of y+ well predicts the minimum dimensionless near-wall distance that gives near-wall spacing independent result with a mean absolute deviation of 2.1% compared to that obtained from the numerical results. The correlation of y+ developed based on the fluid flow analysis is further used to predict the Stanton number, which reasonably agrees with the experimental results.

Turbulent boundary layer profiles in airflow over young wind waves in co- and counter-wind water current

Understanding the air velocity profile over random and three-dimensional wind-waves is crucial for evaluating momentum and energy transfer between air and water. Unfortunately, determining accurate air velocity profiles in field conditions is nearly impossible. It is well accepted, though, that the airflow above water waves has a logarithmic profile usually expressed in terms of the effective roughness parameter. However, this parameter cannot be evaluated directly from the wave measurements at the water surface. For young wind waves, it was recently shown that the airflow over the waves maintains wall similarity. In this case, the airflow vertical velocity distribution can be described by the Nikuradse fully rough logarithmic profile, using the root mean square value of the water surface elevation as the equivalent sand grain roughness height. The existence of mean current in water in either co- or counter-wind direction may significantly modify the wave field, the vertical wind-velocity profile and thus the momentum and energy transfer from air to water. The effect of the water current on the spatially developing boundary layer over young wind waves and the wall similarity is examined. Combined laboratory measurements at several fetches of finely resolved mean air velocity profile above the water surface and of the characteristics of the wind-wave field are performed at multiple wind-forcing and mean water current conditions. The shear stress at the air–water interface estimated using two independent approaches is weakly dependent on current, while the resultant wave field differs significantly.

Hydrodynamic Forces on a Near-Bottom Pipeline Subject to Wave-Induced Boundary Layer

Abstract Hydrodynamic forces on small diameter subsea pipelines and cables placed near seabed are important for their on-bottom stability design. In offshore environments, these pipelines are usually subjected to extreme wave conditions. The present study investigates hydrodynamic forces acting on a pipeline near a flat seabed subjected to a wave-induced boundary layer flow. The Keulegan–Carpenter numbers of the wave-induced boundary layer flow are 20, 140, and 200, defined based on the pipeline diameter (D), the maximum velocity of the undisturbed near-bed orbital velocity (Uw), and the period of the incoming oscillatory flow (Tw). Reynolds number is 1 × 104 based on Uw and D. A seabed roughness ratio ks/D (ks is the Nikuradse equivalent sand roughness) of up to 0.1 and different gap ratios of G/D = 0.05–0.5 between the pipeline and the seabed are considered. Numerical simulations have been carried out based on two-dimensional (2D) unsteady Reynolds-averaged Navier–Stokes equations combined with the k–ω shear stress transport turbulence model. A preliminary one-dimensional (1D) simulation is carried out to obtain a fully developed wave-induced boundary layer velocity profile, which is used as inlet flow for the 2D simulations. The numerical model is validated against the experimental data reported by Sumer et al. [1991, “Effect of a Plane Boundary on Oscillatory Flow Around a Circular Cylinder,” J. Fluid Mech., 225, pp. 271–300] at KC = 10. Influences of KC, ks/D, and G/D on the hydrodynamic forces and the surrounding flows are discussed in detail.

Modeling the inhibition effect of straw checkerboard barriers on wind-blown sand

Abstract. Straw checkerboard barriers (SCBs) are usually laid to prevent or delay desertification caused by eolian sand erosion in arid and semiarid regions. Understanding the impact of SCBs and their laying length on eolian sand erosion is of great significance to reduce damage and laying costs. In this study, a three-dimensional wind-blown-sand model in the presence of SCBs was established by introducing the splash process and equivalent sand barriers into a large-eddy simulation airflow. From this model, the inhibition effect of SCBs on wind-blown sand was studied qualitatively, and the sensitivity of eolian sand erosion to the laying length was investigated. The results showed that the decrease in the wind speed in the SCB area oscillates along the flow direction. Moreover, the longer the laying lengths are, the lower the wind speed and the sand transport rate in the stable stage behind SCBs will be. We further found that the concentration of sand particles near the side of SCBs is higher than that in its central region, which is qualitatively consistent with previous research. Our results also indicated that whether the wind speed will decrease below the impact threshold or the fluid threshold is the key factor affecting whether sand particles can penetrate the SCBs and form stable wind-blown sand behind the SCBs under the same conditions. Although our model does not include the collision between sand particles and SCB walls, which makes the suppression of wind-blown sand by SCBs obtained from the current model conservative, our research still provides theoretical support for the minimum laying length of SCBs in anti-desertification projects.

Numerical Determination of the Equivalent Sand Roughness of a Turbopump’s Surface and Its Roughness Influence on the Pump Characteristics

The correct computation of flows over rough surfaces in technical systems, such as in turbomachines, is a significant issue for proper simulations of their performance data. Once the flow over rough surfaces is adequately computed in these machines, simulations become more trustworthy and can replace experimental prototyping. Roughness modelling approaches are often implemented in a solver to account for roughness effects in flow simulations. In these approaches, the equivalent sand roughness ks must be defined as a characteristic parameter of the rough surface. However, it is difficult to determine the corresponding ks-value for a surface roughness. In this context, this paper shows a novel and time-efficient numerical method, the discrete porosity method (DPM), which can be used to determine the ks-value of a rough surface. Applying this method, channel flow simulations were performed with an irregularly distributed cast iron surface from a turbopumps volute. After identifying the fully rough regime, the equivalent sand roughness was determined and a match with ks-values from literature data was found. Subsequently, the established ks-value for cast iron was used in a turbopump simulation with rough walls. The performance data of the pump were validated by experiments and a good agreement between the experimental and simulated performance data was found.

Effects of wall disturbances on the statistics of supersonic turbulent boundary layers

In the present study, we perform direct numerical simulations to investigate the spatial development and basic flow statistics in the supersonic turbulent boundary layers at the free-stream Mach number of 2.0 over smooth and disturbed walls, the latter of which enforces extra Reynolds shear stress in the streamwise direction to emulate the drag increment and mean streamline curvature effects of rough walls. Such disturbances escalate the growth rate of turbulent boundary layer thickness and the shape factor. It is found that under the rescaled global coordinate, the mean velocity, Reynolds stress, and pressure fluctuation variance manifest outer-layer similarity, whereas the average and fluctuation variances of temperature and density do not share such a property. Compressibility effects are enhanced by the wall disturbances, yet not sufficiently strong to directly impact the turbulent kinetic energy transport under the presently considered flow parameters. The generalized Reynolds analogy that relates the mean velocity and temperature can be satisfied by incorporating the refinement in modifying the generalized recovery coefficient, and that associates the fluctuating velocity and temperature work reasonably well, indicating the passive transport of temperature fluctuations. The dispersive motions are dominant and decay exponentially below the equivalent sand grain roughness height ks, above which the wall disturbances are distorted to form unsteady motions responsible for the intensified density and pressure fluctuations in the free-stream traveling isentropically as the acoustic radiations.

Modeling rough walls from surface topography to double averaged Navier-Stokes computation

The discrete element method was recently revisited using a double averaged Navier-Stokes formulation [Chedevergne F. A double-averaged navier-stokes turbulence model for wall flows over rough surfaces with heat transfer. J Turbul. 2021 Sep;22(11):713–734. doi:10.1080/14685248.2021.1973014] and a new closure relation for the drag coefficient [Chedevergne F, Forooghi P. On the importance of the drag coefficient modelling in the double averaged navier-stokes equations for prediction of the roughness effects. J Turbul. 2020 Aug;21(8):463–482. doi:10.1080/14685248.2020.1817465]. The developed model lies on the notion of representative elementary roughness whose characterisation needs to be generalised to provide a rigorous definition for randomly distributed rough configurations. From 3D scans of rough surfaces and simple image processing, a procedure was proposed to compute the blockage factor and the elementary diameter, the two main parameters of the representative elementary roughness. The procedure was successfully applied to two experimental configurations [Squire D, Morrill-Winter C, Hutchins N, et al. Comparison of turbulent boundary layers over smooth and rough surfaces up to high reynolds numbers. J Fluid Mech. 2016;795:210–240; Croner E, Léon O, Chedevergne F. Industrial use of equivalent sand grain height models for roughness modelling in turbomachinery. In: 55th 3AF International Conference on Applied Conference; Poitiers, France; Apr 2021. https://hal.archives-ouvertes.fr/hal-03228846]. Computed velocity profiles match experimental ones when the Reynolds number is varied, showing at the same time the relevance of the procedure and the validity of the double averaged Navier-Stokes model across the different rough regimes.