Surface Roughness Elements Research Articles

Embedded large eddy simulation of boundary layer transition behind a micro-ramp

Boundary layer transition induced by surface roughness elements plays an important role in aerodynamic and aero-thermodynamic design of subsonic/supersonic/hypersonic vehicles. However, the effect of three-dimensional isolated roughness element in the process of promoting/suppressing boundary layer transition is far from being fully understood, particularly in the incompressible laminar flow. In the present study, the laminar-to-turbulent transition induced by a three-dimensional isolated micro-ramp element immersed in an incompressible laminar boundary layer is investigated numerically. The embedded large eddy simulation (ELES) combining the intermittency transition model and the wall-modeled large eddy simulation (WMLES) S − Ω model is employed for the first time. Numerical results on the time-averaged/instantaneous flow field and the statistical flow fluctuations are analysed and validated thoroughly by the existing experimental measurements. It is found that the interaction of the secondary and the tertiary streamwise vortices causes a high level of wall shear and an inflectional velocity distribution in the near-wall region. An additional eddy system that develops nonlinearly from the unstable inflectional flow may trigger the boundary layer transition. The present study validates that the ELES combined with the WMLES S − Ω model is an efficient simulation tool for industrial wall-bounded flows allowing effective compromise between flexibility, cost, and accuracy.

Experimental study of the effects of isolated roughness elements on the stability and transition of a hypersonic boundary layer on a flat plate

Surface roughness elements on hypersonic vehicles can cause early boundary layer transition, increasing wall skin friction and heat flux and affecting aircraft range and thermal protection systems. Accurate prediction of the transition caused by these roughness elements is crucial for the design of hypersonic vehicles. In this work, wind tunnel experiments on isolated roughness-induced boundary layer transition at Ma = 6 are conducted. Infrared thermography and high-frequency pressure sensors are utilized to investigate the effects of different roughness element configurations (cylindrical, diamond, ramp) on the hypersonic boundary layer instability and transition. The experimental results show that all three roughness elements can effectively enhance the generation of second mode waves and promote boundary layer transition. Compared to smooth surfaces, they exhibit similar frequency band range, faster growth, and earlier saturation. Among them, the ramp roughness element most effectively triggers the boundary layer transition, with a relatively small heat flux increase. Furthermore, bispectral analysis illustrates that all three roughness elements undergo self-interactions that lead to spectral broadening, ultimately resulting in boundary layer transitions.

Machine learning algorithms to study the relative roles of jet convection and natural convection in the presence of surface roughness elements on enhancement of jet impingement heat transfer

Based on data-driven approaches, eight machine-learning algorithms have been proposed to characterize the heat transfer performance of turbulent jet impingement cooling. These algorithms are designed to cool high-heat-flux surfaces modified with single protrusions, multi-protrusions, or V-grooves. To train these algorithms, an experimental dataset was used, which included 184 experiments with Reynolds numbers ranging from 10,000 to 33,000, nozzle exit-to-plate distances ranging from 2 to 10 jet diameters, and two different heat inputs of 60 W and 90 W. Multiple linear regression algorithms in Python programming were used to model jet impingement cooling, showing that the proposed machine learning models provided more accurate predictions with a 20.4 % to 66.5 % relative residual improvement over the existing conventional regression model based on performance metrics like R2 and relative residual. This improvement was leveraged to quantify natural convection's impact on the effectiveness of heat transfer enhancement due to different types and sizes of roughness elements in high-temperature applications. The research found that as the protrusion surface area increased from 0.02 % to 0.188 % compared to the flat plate, the effectiveness of heat transfer enhancement decreased from 2.69 % to 8.37 % at similar operating conditions. These results revealed that natural convection, which opposes forced jet impingement, was strengthened with increased heat input values, roughness element surface area, and the local turbulence generated. The higher natural convection weakened the enhancement afforded by jet impingement on the modified plates relative to the flat plate. Additionally, the capabilities of machine learning algorithms were explored to unlock the full potential of the jet impingement technique and to overcome the associated complexity in jet impingement applications for next-generation electronics cooling. Finally, the Decision Tree Classifier algorithm was applied to classify the performance of the surface roughness element as either effective or ineffective.

LDV measurements of boundary layer velocity profiles on flat plates with different surface roughnesses

The dynamics acting upon thin flat plates submerged in a fluid are chiefly governed by the delicate boundary layer enveloping their surfaces. Through a series of experiments, we investigated the impact of surface roughness elements on the boundary layer adjacent to a flat plate across a range of Reynolds numbers. The experiments were performed in the Chungnam National University-Cavitation Tunnel (CNU-CT). Three flat plates, each characterized by distinct surface roughness heights denoted by k, were subjected to scrutiny. One boasted a pristine smoothness, while the others bore the deliberate roughness of sandpaper, each with its own unique texture. With precision instrumentation, including Laser Doppler Velocimetry (LDV), we meticulously documented the axial velocity profile and the RMS (Root Mean Square) velocity at strategic points along the flat plates. Through these measurements, we unveiled the boundary layer's thickness, δ, and momentum thickness, θ, elucidating their variations under differing free-stream velocities. As our exploration deepened, the relationship between the local Reynolds number, Rnx, and the non-dimensional velocity profiles, u+ − y+, became apparent. A systematic shift along the log-law line ensued, with both u+ and y + increasing in tandem with the rise in Rnx. Yet, our inquiry did not conclude with observation alone. Employing empirical rigor, we quantified the drag forces acting upon flat plates of varying roughness heights, deriving them from the measured momentum thickness across a range of local Reynolds numbers, Rnx.

Experimental Study on the Influence of Surfactants on Ore Surface Wettability.

To reveal the evolution law of ore surface wettability under the condition of adding surfactant and chemical reaction, the test experiments of ore surface morphology, roughness, and chemical element distribution were carried out. The results show that the contour of the ore surface morphology was relatively flat in the early stage of leaching and the roughness factor increased slowly, while the contour fluctuated significantly in the late stage of leaching and the roughness factor increased sharply. With the leaching reaction, the roughness factor and spreading coefficient of the ore with a 0.008 mol/L surfactant (sodium dodecyl sulfate) concentration were always higher than those without surfactant and the surfactant enhanced the erosion effect of the solution on the ore surface. During the leaching process, the chemical elements Cu, Al, and K were dissolved on the ore surface, and the content of Cu decreased significantly from 2.01 to 0.86%; the leaching rate of copper was about 57.2%. According to the morphology characteristics of the ore surface during leaching and the distribution law of chemical elements on the ore surface before and after leaching, the leaching process was divided into four stages of "contact-penetration-reaction-disintegration". The surfactant had the most obvious effect on the ore surface morphology in the two stages of "reaction" and "disintegration".

Influence of engineered roughness microstructures on adhesion and turbulent resuspension of microparticles

From microplastics resuspending into the atmosphere to earth particles left behind during extraterrestrial explorations, the resuspension of microparticles by a turbulent gas flow occurs in many natural, environmental and industrial systems. Wall surfaces, onto which particles initially adhere, are rarely smooth and this surface roughness affects particle resuspension. Available experimental data on particle resuspension have been obtained with substrates, whose surfaces are either unaltered or manually abraded with, for instance, sand blasting. In these experiments, the roughness elements span a wide size range and are in-homogeneously distributed in space. Surface functionalization is a modern technique allowing the precise fabrication of a wall surface with well-characterized microstructures, hence reducing the asperity randomness associated with conventional abrasion techniques. Taking advantage of surface functionalization, we present here a new set of reference data, where the wall asperities are represented by a structured arrangement of micropillars and microcubes. Adhesion force measurements and particle remaining fraction against gas velocity, at Reynolds number up to 8000, are reported for one reference and two artificially roughened substrates. Laboratory measurements show that the microasperities have little to moderate effect on the mean adhesion force and the threshold velocity, at which half of the 100-μm particles resuspend. The standard deviations are, however, much more affected. The presented results will primarily contribute to the improvement of resuspension models, which until now rely on a simplified representation of the surface roughness elements. The presented measurements are highly compatible with such models, which involve elementary roughness features, such as hemispherical asperities superimposed with a flat plate.

A new process-based and scale-aware desert dust emission scheme for global climate models – Part I: Description and evaluation against inverse modeling emissions

Abstract. Desert dust accounts for most of the atmosphere's aerosol burden by mass and produces numerous important impacts on the Earth system. However, current global climate models (GCMs) and land-surface models (LSMs) struggle to accurately represent key dust emission processes, in part because of inadequate representations of soil particle sizes that affect the dust emission threshold, surface roughness elements that absorb wind momentum, and boundary-layer characteristics that control wind fluctuations. Furthermore, because dust emission is driven by small-scale (∼ 1 km or smaller) processes, simulating the global cycle of desert dust in GCMs with coarse horizontal resolutions (∼ 100 km) presents a fundamental challenge. This representation problem is exacerbated by dust emission fluxes scaling nonlinearly with wind speed above a threshold wind speed that is sensitive to land-surface characteristics. Here, we address these fundamental problems underlying the simulation of dust emissions in GCMs and LSMs by developing improved descriptions of (1) the effect of soil texture on the dust emission threshold, (2) the effects of nonerodible roughness elements (both rocks and green vegetation) on the surface wind stress, and (3) the effects of boundary-layer turbulence on driving intermittent dust emissions. We then use the resulting revised dust emission parameterization to simulate global dust emissions in a standalone model forced by reanalysis meteorology and land-surface fields. We further propose (4) a simple methodology to rescale lower-resolution dust emission simulations to match the spatial variability of higher-resolution emission simulations in GCMs. The resulting dust emission simulation shows substantially improved agreement against regional dust emissions observationally constrained by inverse modeling. We thus find that our revised dust emission parameterization can substantially improve dust emission simulations in GCMs and LSMs.

Microstructural evolution and biological properties of PEO coating on SLM-prepared NiTi alloy

Selective laser melting (SLM)-prepared NiTi alloy is considered to have great application potential in the fabrication of complex-structured metal implant materials. However, the SLM-NiTi alloy still suffers from some disadvantages, such as poor bio-corrosion resistance, bio-compatibility, and health hazards caused by the release of nickel ions. In order to improve the surface biological properties of SLM-NiTi alloys, bio-active coatings rich in Ca and P elements were prepared on SLM-NiTi alloys by PEO process in this work. Confocal laser scanning microscopy, SEM, EDX, XRD, and XPS were utilized to feature the micro-structure, thickness, element information, and composition of SLM/PEO coatings at different oxidation times. The adhesion strength was determined using scratch testing. A potential polarization test and a simulated body fluid (SBF) immersion test were applied to assess the corrosion resistance and detect the biological activity of SLM/PEO coatings at varied oxidation periods, respectively. The results showed that the growth of SLM/PEO coatings was hindered due to the overflow of Ni elements at the spark discharge stage (stage II). However, with the PEO process, the inhibition effect of Ni element was gradually eliminated. The pore size, thickness and the TiO2 rutile phase content of the SLM/PEO coatings gradually increased over time. However, the coating density, roughness and adhesion strength increased first and then decreased, and the SLM/PEO coatings were maximized at 15 min. The main composition of SLM/PEO coatings was TiO2, CaO, CaHPO4 and Ca3(PO4)2, and hydroxyapatite (HA) was gradually formed in the coating with time. The SLM/PEO-15 min coating with high thicknesses and rutile phase content, as well as optimum density provided optimum corrosion resistance (1.615 × 10−6 A cm−2). Meanwhile, the SLM/PEO-15 min coating possessed the best surface roughness and higher active element content, providing an excellent nucleation point and growth environment for the deposition of apatite. And the Ni element on the SLM-NiTi alloy substrate surface (oxidation time from 0 to 20 min) was significantly reduced from 12.49 % to 0.52 % after immersing in SBF solution for 24 h. Therefore, the PEO process can effectively improve the corrosion resistance and biological activity of SLM-NiTi alloy and inhibit the release of Ni elements.

Lagrange Multiplier-Based Optimal Control Technique for Streak Attenuation in High-Speed Boundary Layers

High-amplitude freestream turbulence and surface roughness elements can excite a laminar boundary-layer flow sufficiently to cause streamwise-oriented vortices to develop. These vortices resemble elongated streaks having alternate spanwise variations of the streamwise velocity. Downstream, the vortices “wobble” through an inviscid secondary instability mechanism and, ultimately, transition to turbulence. We formulate an optimal control algorithm to suppress the growth rate of the streamwise vortex system. Considering a high-Reynolds-number asymptotic framework, we reduce the full compressible Navier–Stokes equations to the nonlinear compressible boundary-region equations. We then implement the method of Lagrange multipliers via an appropriate transformation of the original constrained optimization problem into an unconstrained form to obtain the disturbance equations in the form of the adjoint compressible boundary-region equations (ACBREs) and corresponding optimality conditions. Numerical solutions of the ACBRE approach for high-supersonic and hypersonic flows reveal a significant reduction in the kinetic energy and wall shear stress for all considered configurations. We present contour plots to demonstrate the qualitative effect of increased control iterations. Our results indicate that the primary vortex instabilities gradually flatten in the spanwise direction thanks to the ACBRE algorithm.

Atmospheric wind pressure distribution on an isolated agricultural silo / Distribuição da pressão atmosférica do vento em um silo agrícola isolado

Storage silos installations are very common in the agricultural areas of Argentina and Brazil. Accidents caused by the action of the wind on the structural components of silos are usually reported. For this reason, the UTN - Regional Rafaela and the Laboratorio de Aerodinámica of the UNNE began to develop experimental studies in a wind tunnel with the objective of analyzing the external pressure distribution on isolated silos and complementing the procedures established in the Argentine Wind Code CIRSOC 102 and in the Brazilian Code NBR 6123. The tests were carried out in the UNNE “Jacek P. Gorecki” wind tunnel, which is a boundary layer tunnel of the Eiffel-type. The total length of the tunnel is 39.65 m, the test section is 2.4 m wide × 1.8 m high × 22.8 m long and an axial fan generates a maximum speed of 25 m/s. Turbulence generators and surface roughness elements were used to simulate atmospheric wind and then a 1:100 scale model of the silo was tested. In this work, results of mean and fluctuating pressure values obtained by wind tunnel tests are presented. The turbulent wind actions defined by the fluctuating pressures are evaluated through the distributions of RMS and peak values.

Estimating the sand saltation thresholds from Sentinel-1 SAR data in the Gobi Desert, Mongolia

Spatial information about the sand saltation threshold is essential to quantify sand transport and dust emission. Quantitatively estimating the threshold over a wide area is challenging, however, because its main influencing factors (surface roughness elements) are unknown. Here, we explored the use of Sentinel-1 Synthetic Aperture Radar (SAR) data to quantify surface roughness conditions and their temporal changes and map the threshold. Thresholds were obtained by in situ observations in dry spring periods during 2018–2020 at three year-round and eight temporary sites during natural sand and dust storms. We found a linear relationship (r = 0.91, p < 0.0001) between the SAR gamma naught VV intensity and the observed threshold where the site surface was stony or vegetated, indicating that SAR data can be used to quantify the spatial-temporal distribution of the threshold during dry periods. We then estimated thresholds from SAR data for spring 2017–2021 and found high thresholds on slopes and mountains, mainly due to stone and vegetation effects, whereas thresholds in the topographic depression varied because of changes in surface roughness conditions. Thresholds estimated from SAR data should greatly improve wide-area simulations of sand horizontal transport and dust emission fluxes by numerical dust models.

A comparison of the aerodynamic characteristics of four kinds of land surface in wind erosion areas of northern China

Aerodynamic characteristics is a crucial factor influencing soil wind erosion, which is closely related to the properties of surface roughness elements. In the expansive arid and semi-arid northern China, grassland, farmland, mobile sandy land, and gobi are the main land surface types that suffer wind erosion to varying degrees. To investigate the aerodynamic characteristics of different surfaces, we used three-dimensional ultrasonic anemometers to observe the near-surface wind speed above grassland, farmland, mobile sand, and two types of gobi surfaces in areas of northern China (a total of five observation sites). We compared the aerodynamic characteristics of the five surfaces and their causes (i.e., the surface roughness length, wind speed pulsation, turbulence intensity, and drag coefficient), and analyzed how surface roughness elements weakened the airflow. The roughness length was greatest for grassland, followed by farmland, sand, fine gobi, and black gobi. The pulsation intensity of near-surface wind speed increased linearly with increasing wind speed at all sites. At a given wind speed, increasing roughness length increased the pulsation intensity of wind speed. The probability distributions for the drag coefficient and turbulence intensity of the five surfaces followed a positively skewed distribution. The drag coefficient and turbulence intensity were greatest for grassland, followed by farmland, sand, fine gobi, and black gobi surfaces. The drag force exerted on the roughness elements increased with increasing wind speed, but the ratio of drag force exerted on the roughness elements to the total drag force (τR/τ) for flexible roughness elements (i.e., plants) increased as the wind speed increased; for rigid roughness elements, τR/τ decreased with increasing wind speed. This confirms that the shelter capability of flexible roughness elements against soil wind erosion increases with the increase of wind speed, while that of rigid roughness elements is opposite.

Comparison of low Reynolds number turbulence and conjugate heat transfer modelling for pin-fin roughness elements

This study contrasts Reynolds-Averaged Navier-Stokes (RANS) and (Numerical) Large-Eddy Simulation (NLES) for their use in predicting conjugate heat transfer for a low Reynolds number flow over a surface roughness element. The (N)LES predictions are in good agreement with experimental data, the heat transfer estimate is within 7.7% error. The linear RANS model shows larger errors up to 40%, especially in modelling the turbulent stresses. Using a one-equation LES turbulence model slightly increases the heat transfer prediction compared to a numerical LES. This indicates that more advanced turbulence models might not give more accurate heat transfer predictions.Additionally, this study investigates the time dependant development of the flow and temperature fields and how long data needs to be collected for statistically stationary results. The flow needs to develop for approximately 1800 through-flow times, TL, and 150 TL is required for collection of statistics. A significant range in turbulence length scale prediction between (N)LES and RANS was found. (N)LES scales appeared physically reasonable and should inform mesh resolution in future studies. Two additional cases were run with different cube heights showing changes in the time dependant development of the temperature field and greater sensitivity to turbulence modelling.

Nonthermal Plasma Brush Treatment on Titanium and Zirconia To Improve Periabutment Epithelium Formation.

The peri-implant soft tissue with inferior adhesion takes a long healing period to form, which is undesirable for the reaction around the peri-implant soft tissues. The aim of this study is to improve the physicochemical properties of titanium (Ti) and zirconia (ZrO2) implant abutments and shorten the formation period of periabutment epithelium tissue. A nonthermal atmospheric plasma brush (NTAPB, N) was employed for Ti and ZrO2 activation. The surface topographies, roughness, crystallinity, wettability, and chemical elements of the abutment materials were examined. The epithelial cell behavior analysis and tissue remodeling of the periabutment epithelial tissue were performed in vitro and in vivo. N-Ti and N-ZrO2 had a similar good surface wettability, with a 65 and 70% increase in oxygen content and a 70 and 75% decrease in carbon content, respectively. Both N-Ti and N-ZrO2 showed excellent adhesion, spread, and proliferation of epithelial cells in vitro, with improved adhesion molecule expression levels compared to untreated samples. N-Ti and N-ZrO2 abutments were placed in the implantation sites of rats. From week 2 to week 6 after implantation, N-Ti and N-ZrO2 had similar periabutment epithelium tissue formation, and both had increased plectin-positive and laminin γ2-positive cell numbers compared to Ti and ZrO2. The NTAPB shows promising abutment modification abilities. It promotes the expression levels of adhesion molecules and the epithelial cell performance, which later leads to a quicker formation and remodeling of the important periabutment epithelial tissue.

Interactions between boulders and aeolian ridges on Mars

AbstractHere we present new observations of two different interactions between aeolian ridges and boulder fields on Mars that provide insight into past wind conditions. First, an analysis of ridge and boulder interactions at two test sites in Proctor Crater and an additional site ~430 km to the northeast shows that ridge geometry can be affected by changes in surface roughness elements generated by boulder fields. Second, a detailed examination of some of the boulder fields found that individual boulders can generate multi‐armed ‘wakes’ that have no clear proxy on Earth. The ridge/boulder dynamics suggest that transverse aeolian ridges acted as roughness during their development, indicating that they formed at a length scale greater than wind ripples. The boulder wakes seem to represent an unusual interaction between flow separation and pre‐exiting ridges; why this pattern is not observed on Earth remains uncertain.

Comparative Performance of New Surface Roughness Element and Pin fin in Converging Channel for Gas Turbine Application

Thermal performance of a novel surface roughness element, named as Double 45 Dimple (D45D), is compared with pin-fin element in a converging channel with rectangular cross section and presented. The Surface Roughness Element (SRE) is derived by combining protrusion &amp; dimple in a particular fashion such that area available for transfer of heat increases. The objective of this study is to demonstrate the applicability of D45D element channel for trailing edge channel of a typical nozzle guide vane where typically pin-fin element is used. New cooling configuration of Nozzle Guide Vane (NGV) with D45D element is also proposed. All thermal and flow related results are derived using validated CFD approach with EARSM turbulence model for a typical value of Reynolds number. From this investigation, it is found that D45D element provides remarkable improvement in the averaged as well as heat transfer in local region for the corresponding surface which makes it a candidate for trailing edge channel cooling application.

Effect of Pressure Gradient on Local Excitation of Boundary-layer Instability Due to Free-stream Turbulence and Micro Surface Roughness

The prediction and control of the laminar-turbulent transition is crucial to the designs of vehicles, turbines, etc. The initial condition of transition depends on the exciting process of boundary-layer instability, which is the key to implement its prediction and control. The current researches confirm that the exciting process of boundary-layer instability, namely receptivity, is affected not only by different types of free-stream disturbances and shape parameters of surface roughness elements, but also by the pressure gradient of mean flow. Hence, we study the effect of pressure-gradient on local excitation of boundary-layer instability under the interaction of the low-level, isotropic free-stream turbulence and micro surface roughness in this work. The numerical results reveal the pressure-gradient effect on the receptive process and the group speed of excited wave packets in the Falkner-Skan boundary layer. The favorable/adverse pressure gradients (FPG/APG) are found to be able to promote/suppress the excitation and subsequent evolution of Tollmien–Schlichting (T-S) waves. Then the relations of the pressure gradient with the amplitude, growth rate, wave number, phase speed and shape function of excited T-S waves are studied.

High-temperature oxidation behaviour of refurbished (Ni,Pt)Al coating on Ni-based superalloy at 1100 °C

A single-phase (Ni,Pt)Al coating was removed by soft chemical stripping method successfully after preceding oxidation at 1100 °C for 300 h, and a fresh one was refurbished subsequently. Oxidation behaviour of the refurbished (Ni,Pt)Al coating was studied in comparison with the virgin one at 1100 °C. The results show that the refurbished coating exhibited superior isothermal and cyclic oxidation resistance than the virgin one in terms of oxidation rate and surface rumpling. The effects of surface roughness and residual elements in the refurbished coating on oxidation rate, surface rumpling and interdiffusion behaviour were discussed.

Light scattering by fractal roughness elements on ice crystal surfaces

Atmospheric ice crystals scatter sunlight, affecting Earth’s climate through the radiation properties of cirrus clouds. Naturally occurring surface roughness and its effect on the scattering properties of ice crystals remain largely unknown. Scattering by ice crystals with rough surfaces is studied by placing a finite, thin surface-roughness element on an infinitely large, planar vacuum-ice boundary. The elements are generated using a statistical model based on fractional Brownian motion. The horizontal roughness scale is described by the Hurst exponent H and the vertical roughness scale with the root-mean-square roughness parameter Rq. The computations are performed with the surface mode of the Discrete Dipole Approximation software ADDA (version 1.34b). Several incident directions for wavelength of 0.5 µm from both above and below the planar surface are studied. A refractive index for ice m=1.313+i5.889×10−10 is used throughout the computations. Results are averaged over ten rough surface realizations for a specific H,Rq-pair.Scattering by the rough elements is compared to that by the corresponding smooth elements. The rougher the element is, the more of the scattered intensity is transmitted through the surface. The rough elements have distinctively smoother angular distributions for the degree of linear polarization than their smooth counterparts. Also, it is found that while roughness itself affects polarization, the exact surface morphology does not seem to have a significant effect. The vertical roughness scale Rq has a larger effect on the light scattering results than the horizontal scale H. Enhanced angular scattering is detected in directions nearly parallel to the vacuum-ice boundary within the ice medium. The phenomenon is explained with a strong internal reflection mechanism.The model for surface roughness, along with the light scattering methodology used here, could be incorporated into geometric optics ray-tracing computations for large ice crystals and other particles.

Effect of Surface Roughness Geometry on Boundary-Layer Transition and Far-Field Noise

Surface roughness elements are often used to force laminar to turbulent transition in aerodynamic and aeroacoustic wind-tunnel experiments. The statistical features and spectral content of the pressure fluctuations in the resulting turbulent boundary layer at the trailing edge can affect far-field noise. To elucidate this dependence, boundary-layer transition induced by randomly distributed roughness elements and a zigzag strip of the same height over a NACA 0012 airfoil is investigated experimentally. The effects of roughness geometry on the near-field flow topology, transition location, and far-field noise are addressed in the common experimental setting for the first time. For a fixed roughness height, distributed roughness elements are less effective in forcing transition than the zigzag strip at low freestream velocity (). As increases, the transition front for the distributed roughness elements moves closer to the roughness location, reaching the same or even further upstream locations compared to the transition onset in the presence of the zigzag strip. The far-field noise depends on the transition location. For , a higher noise level is measured for the distributed roughness elements with respect to the zigzag strip. In contrast, for , the earlier onset of transition with the distributed surface roughness leads to a lower noise level than that with the zigzag strip. The data confirm that an adequate characterization of the boundary-layer transition is necessary when measuring the far-field noise during wind-tunnel experiments.