Roughness Height Research Articles

Noise reduction on low Reynolds number rotors by boundary layer transition

Following force balance and acoustic measurements in an anechoic chamber, we have shown that promoting earlier boundary layer transition on an idealised low Reynolds number rotor using 3-D cylindrical roughness elements could lead to an interesting decrease in the noise emitted by the rotor. For most tripped cases, the aerodynamic performance of the rotor was impaired by tripping, but a reduction of the broadband noise and the high amplitude blade passing frequency harmonics were observed. On the contrary, the blade passing frequency and its first harmonics did not seem to be significantly affected by the presence of the tripping. The tripping was more effective at low rotational speeds, where the broadband noise contribution dominates the overall sound pressure level (OASPL), whereas the effect on OASPL is almost negligible at high rotational speeds, where the blade passing frequency noise becomes dominant. Careful selection of the roughness height with respect to the boundary thickness and inter-roughness spacing showed that aerodynamic performance loss can be minimised while preserving the gain in noise reduction.

Aerodynamic Roughness Height of Gravel-covered Plains in the Puna of Argentina

Plains covered by gravel-dominated desert pavement in the Puna of Argentina have an aerodynamic roughness height (or length) z 0 of ∼1 cm, likely representing a skimming flow regime above the closely spaced gravel particles. Aerodynamic roughness height locally may transition from that of skimming flow over the gravels to a z 0 that includes the effects of obstacles considerably larger than those of the gravel particles alone. Among large (>60 cm tall) megaripples, z 0 is elevated beyond that of the gravels alone to values of 2–4 cm. These results represent an analog for an improved understanding of the aerodynamics of gravel-dominated desert pavement and megaripples documented by multiple rovers on Mars.

The Impact of High-Speed and Thermal-Assisted Machining on Tool Wear and Surface Roughness during Milling of SKD11 Steel

This research investigates the impact of high-speed and thermal-assisted machining (HS-TAM) on tool wear and surface roughness during the milling of SKD11 steel. The goal is to identify high-speed and elevated temperature zones that can improve machining efficiency, enhance surface quality, minimize costs, and extend tool life. The study involves the high-speed milling of SKD11 steel at various temperature conditions to evaluate the effect of temperature on tool wear and surface roughness. Additionally, experiments are conducted at the highest allowable support temperature with increased high-speed cutting to examine the effect of high speed on tool wear and surface roughness. The study demonstrates the correlation between cutting-tool wear and surface roughness at various high-speed cutting conditions and TAM environments and provides recommendations for cutting speeds and heating temperatures for different quality and productivity objectives. The findings indicate that high-speed milling of SKD11 at 600 m/min and 500 °C can decrease cutting tool-wear height (wear volume) and surface roughness by 82.47% (95.74%) and 91.08%, respectively, compared to machining at room temperature. Furthermore, the higher-speed modes at 500 °C result in a slight increase in wear height and surface roughness for high-speed cutting below 800 m/min, but reduces surface roughness for high-speed cutting beyond 800 m/min, reaching a value of 0.158 µm at the high-speed cutting limit of 1000 m/min.

Оптимизация конструктивных параметров рекуперативного сцепного устройства, установленного в лесовозном автомобиле с прицепом

This article solves the problem of increasing the efficiency of the functioning of logging road trains in conditions of insufficiently equipped logging roads. Experience in the operation of road trains on such logging roads shows that on these roads the transport and operational qualities of logging road trains are low. The relevance of the scientific direction, aimed at achieving the fuel efficiency of logging road trains and thus increasing the efficiency and competitiveness of logging enterprises, is substantiated. An analysis of the work of foreign scientists is presented, which made it possible to identify significant factors that largely determine the efficiency of the process of hauling timber by timber road trains. A promising design of the coupling device is proposed, which allows converting and usefully using the kinetic energy of the mass of the trailer with timber, which occurs during the movement of a road train along insufficiently equipped logging roads in unsteady and transient traffic modes. The study was based on mathematical and simulation modeling, numerical methods, as well as modern methods for obtaining and processing information with computer support. A mathematical model and a computer program for the movement of a timber road train under changing road conditions have been developed. Computer simulation made it possible to carry out a preliminary assessment of the performance of the proposed pneumohydraulic coupling device with recuperative and damper mechanisms by identifying and analyzing dependencies characterizing the change in the studied performance indicators on the driving conditions of the road train and the design parameters of the device. It has been established that the coupling device during the movement of a logging road train in difficult road conditions at a speed of 30 km/h makes it possible to recuperate up to 7 kW, while the value of the longitudinal acceleration of the trailer link relative to the logging vehicle does not exceed 0.83 m/s2. It is determined that the value of the optimal diameter of the recuperative hydraulic cylinder is 80 ... 100 mm, and the optimal piston stroke is 83 ... 100 mm. In this case, the average power recuperated by the device will be at least 7 kW, and the average longitudinal acceleration of the trailer will be no more than 0.8 m/s2. It was found that the developed recuperation system remains effective in a wide range of roughness heights of 0.2 ... 0.4 m, providing a recuperated power of 2.3 ... 19.7 kW, respectively, with an acceptable average longitudinal acceleration of 0.3 ... 2.2 m/s2. The results obtained will be used as recommendations for designers to refine the proposed coupling device at the design stage.

An experimental investigation of new roughness patterns (dimples with alternative protrusions) for the performance enhancement of solar air heater

Conventional solar air heater (SAH) has poor performance and in this work, an initiative has been taken to improve their overall performance with the help of a combined roughness pattern over the absorber plate. The experimental setup has been designed and fabricated having a rectangular duct with one heat-absorbing side (with roughness). The combination of dimple and protrusion roughness elements are arranged alternatively over the absorber plate for performance improvement of SAH. The precise placement of these roughness elements over the heat-absorbing side has been ensured with relative roughness pitch (RRP; P/e), print diameter to hydraulic diameter (d/D), and relative roughness height (RRH, e/D) which ranged from 10 to 20, 0.025–0.045, and 0.25 to 0.5, respectively. It is discovered with the experimental results that the thermal performance increases along with the pressure loss in the roughened SAH (rSAH). The heat flow increment is comparatively higher at the trailing and the leading edge of the dimple and the protrusion because of flow separation, reattachment, and flow impingement. The thermal performance is the highest while RRP, RRH, d/D are 10, 0.036, and d/D = 0.35, respectively. The proposed roughness pattern has resulted in a 2.1 and 1.95 times higher thermal augmentation and overall performance of SAH over the conventional.

Numerical analysis of a novel solar air heater design with V-ribs and jet cooling

The application of solar air heaters (SAHs) is an effective method of utilizing abundant solar energy for heating purposes and reducing the consumption of fossil fuels. However, the performance of SAHs is characteristically low owing to a sublayer formation over the heated surface. To address this low performance, numerical examinations on a novel configuration of SAH have been performed in this study. The proposed design encompasses a combined arrangement of both impinging air jets and rectangular sectioned V-ribs on the absorber surface. The effect of varying roughness height, pitch, and angle of attack from 0.016 to 0.024, 0.264 to 0.792, and 30° to 75° respectively is investigated while keeping impingement parameters fixed and varying Re from 3000 to 18000. Nusselt number (Nu) and friction factor (ff) characteristics as functions of roughness parameters are evaluated. The results exhibit a considerable enhancement of 3.36 and 5.31 in Nu and ff respectively in contrast to conventional SAHs under similar boundary conditions. Moreover, the proposed model performs best at Re = 18000 with the highest thermohydraulic performance (ηcombined) of 1.92. The study concludes that the proposed arrangement of jet cooling with rectangular sectioned V-ribs is an efficacious way for improving the performance of SAHs.

Capabilities and limits of surface roughness measurements with monochromatic speckles.

For coherent light illumination, surface roughness leads to speckles in the scattered light image. By evaluating the speckle contrast or image auto-correlation, a measurement of the roughness parameter S q is possible. While these measurement principles have been well known for decades, a fundamental understanding of the minimal achievable measurement uncertainty is missing. Therefore, the measurement uncertainty limits for four unavoidable sources of uncertainty are derived by means of theoretical and numerical approaches. The study is focused on the case of monochromatic speckles, which provide the highest sensitivity, as well as on the case of planar surface and isotropic surface roughness with a Gaussian height distribution and Gaussian correlation function. The considered uncertainty sources are the natural randomness of surface roughness itself, speckle noise, quantum shot noise, and camera noise. As a result, for the studied experimental configuration, speckle noise is determined as the largest contribution to measurement uncertainty, which leads to a minimal achievable relative uncertainty of 1%-2% for S q =(0.03-0.15)λ. According to theoretical studies, the speckle noise limit of the relative uncertainty is inversely proportional to four times the square root of the independent number of evaluated speckles. In addition, an absolute uncertainty limit is reached for ever-smoother surfaces, which amounts to λ divided by 64 times the square root of the independent number of evaluated speckles. Furthermore, systematic errors due to cross-sensitivity with respect to other parameters of surface roughness (height distribution, correlation length) as well as the surface position and shape (axial offset, tilt, curvature) are quantified and discussed. For the considered small deviations of different influencing quantities, the quantified errors are one order of magnitude smaller than the speckle noise limit.

Scaling laws for two- and three-dimensional wall jet scour based on the phenomenological theory of turbulence

The scour induced by wall jets may cause serious bed erosion and thus damage to hydraulic structures. Previous studies are largely empirical, providing only correlations of experimental data. At present, it is not clear what difference exists in the physical mechanisms of two-dimensional (2D) and three-dimensional (3D) wall jet scour. This study first summarizes previous efforts for predicting wall jet scour depths. Then, a scaling analysis is presented for investigating scour depths by applying the phenomenological theory of turbulence. It is shown that for wall jet cases, the dimensionless equilibrium scour depth can generally be expressed as a power function of the densimetric Froude number and relative roughness height, with the power index depending on jet configurations. The predictions of scour depth using the resulting formulas agree well with published data. The present analysis provides new insights into the understanding of the underlying physical mechanisms of wall jet scour as well as the difference between 2D and 3D configurations.

Extremely High-Quality Periodic Structures on ITO Film Efficiently Fabricated by Femtosecond Pulse Train Output from a Frequency-Doubled Fabry-Perot Cavity.

This study developed a novel frequency-doubled Fabry-Perot cavity method based on a femtosecond laser of 1030 nm, 190 fs, 1 mJ, and 1 kHz. The time interval (60-1000 ps) and attenuation ratio (0.5-0.9) between adjacent sub-pulses of the 515 nm pulse train were able to be easily adjusted, while the efficiency was up to 50% and remained unchanged. Extremely high-quality low-spatial-frequency LIPSS (LSFL) was efficiently fabricated on an indium tin oxide (ITO) film using a pulse train with a time interval of 150 ps and attenuation ratio of 0.9 focused with a cylindrical lens. Compared with the LSFL induced by the primary Gaussian pulse, the uniformity of the LSFL period was enhanced from 481 ± 41 nm to 435 ± 8 nm, the divergence of structural orientation angle was reduced from 15.6° to 3.7°, and the depth was enhanced from 74.21 ± 14.35 nm to 150.6 ± 8.63 nm. The average line edge roughness and line height roughness were only 7.34 nm and 2.06 nm, respectively. The depths and roughness values were close to or exceeded those of resist lines made by the interference lithography. Compared with the common Fabry-Perot cavity, the laser energy efficiency of the pulse trains and manufacturing efficiency were enhanced by factors of 19 and 25. A very colorful "lotus" pattern with a size of 30×28 mm2 was demonstrated, which was covered with high-quality LSFLs fabricated by a pulse train with optimized laser parameters. Pulse trains can efficiently enhance and prolong the excitation of surface plasmon polaritons, inhibit deposition particles, depress ablation residual heat and thermal shock waves, and eliminate high-spatial-frequency LIPSS formed on LSFL, therefore, producing extremely high-quality LSFL on ITO films.

Flow measurement by vegetated weirs: An investigation on discharge coefficient reduction by crest vegetation

Flow measurement using weirs as an engineered hydraulic control has been well studied and understood, but the presence of vegetation on the weir crest or top of weir-like structures in natural waterway has rarely been considered. This technical note presents a quantitative assessment of the effects of crest vegetation roughness on flow estimation. Experiments are carried out to record the discharge coefficient reduction of finite-crest-length weirs when the weir crests are covered by flexible artificial vegetation. The effects of equivalent roughness height of the vegetation canopy on the weir discharge capacity are evaluated, in addition to the variation of flow rate and weir crest length. According to the ratio of upstream head to vegetated crest length, the tested flows are classified as broad-crested and narrow-crested weir flows, and some change in discharge coefficient deterioration rate is observed for the two types of weir flows. Empirical equations are proposed for prediction of discharge coefficient thus flow estimate correction by taking into account the vegetation resistance effects. The results may contribute to improve the accuracy of hydrological modeling and flood forecasting, although vegetation degradation during floods is not considered.

Analysis of Surface Texture and Roughness in Composites Stiffening Ribs Formed by SPIF Process

Studying roughness parameters and the topography of stiffening ribs in composite sandwich structures is important for understanding these materials' surface quality and mechanical properties. The roughness parameters describe the micro-geometry of the surface, including the average height deviation, roughness depth, and waviness. The topography of the surface refers to the spatial arrangement and distribution of features such as bumps, ridges, and valleys. The study investigated the roughness parameters under three scenarios based on two SPIF process parameters: tool rotational speed(N) and feed rate (f). The vertical step was held constant at 0.4 mm across all scenarios. In scenario A, the process parameters were set at f = 300 mm/min and n = 300 rpm; in scenario B, f = 1500 mm/min and n = 3000 rpm; and in scenario C, f = 1500 mm/min and n = 300 rpm. The experimental research topography analyses revealed that the surface roughness of the stiffened ribs was highly dependent on the SPIF process parameters. The highest feed rate and tool rotational speed produced the smoothest surface texture with the lowest maximum height (Sz) value. In contrast, the lowest feed rate and tool rotational speed resulted in a rougher surface texture with a higher maximum height (Sz) value. Furthermore, the contour plots generated from the topography analyses provided a good visual representation of the surface texture and roughness, allowing for a more comprehensive analysis of the SPIF process parameters. This study emphasizes optimizing the SPIF process parameters to achieve the desired surface quality and texture of stiffened ribs formed in Litecor® panel sheets.

Experimental study of surface roughness effects on hydrodynamic characteristics of a submerged floating tunnel

Marine biofouling is a major concern in the operational performance of submerged floating tunnels (SFTs). The objective of this research is to investigate the effects of marine fouling (represented by surface roughness) on the hydrodynamic behavior of SFTs, including the hydrodynamic forces on the SFT subject to current-only, wave-only, and combined current-wave flow conditions. The effects of increased surface roughness induced by marine fouling on the dynamic response of an SFT are characterized by hydrodynamic force coefficients, including drag and inertia coefficients. At the Water Lab of Delft University of Technology (TU Delft), experiments have been performed in a wave-current flume to compare the SFTs’ behaviors as affected by different roughness characteristics. In addition, a parametric cross-section for an SFT is presented, and the hydrodynamic performance associated with surface roughness effects on the parametric shape and circular SFT cross-section shape are compared. The results show that the parametric shape can effectively reduce the drag coefficient (Cd) under current-only conditions and lower the inertia coefficient (Cm) when waves are present. As roughness height and coverage ratio increase, Cd generally increases while Cm decreases. However, small differences in Cd and Cm can be observed with regard to roughness parameters for wave-only conditions. The Morison coefficients adapted for a marine-fouled SFT measured in the experiments are compared to predictions from engineering standards and are recommended for engineering practice.

Thermal performance investigation of frustum roughened solar air heater

Present work reports the findings of experimental investigation performed outdoors in real time conditions on a novel roughness geometry frustrum roughened solar air heater. The test rig was designed and fabricated at rooftop of Mechanical Engineering Department, MANIT Bhopal, India. The geometrical specification of frustrum geometry machined to the absorber was varied and tested. Two new roughness parameters to determine its impact on thermal performance is introduced namely relative diametral ratio (d1/d2) and roughness height to base diameter ratio (e/d1) and varied respectively as 1.33–3.00 and 0.16–0.75. Relative roughness pitch (p/e) and height (e/Dh) is respectively varied as 8–14 and 0.013–0.054. Smooth and roughened absorbers are tested under varying range of geometrical and flow parameters (Reynolds number 3500–12500). The investigation revealed that thermal performance is strongly dependent upon flow Reynolds number and temperature rise parameter. Thermal efficiency is expressed in terms of collector efficiency factor and heat removal factor varying respectively in range of 0.76–0.81 and 0.60–0.75. Thermal efficiency varied in range of 42–79 % for different mass flow rates and roughness parameters. The maximum thermal efficiency obtained is 79 % at p/e = 12, e/Dh = 0.040, e/d1 = 0.75, d1/d2 = 3 for mass flow rate of 0.0486 kg/s.

Modeling the Flow Response to Surface Heterogeneity during a Semi-Idealized Diurnal Cycle

Abstract To characterize the effects of subgrid surface heterogeneity, the blending-height concept has been developed as a coupling strategy for surface parameterization schemes used in numerical weather prediction models. Previous modeling studies have tested this concept using stationary conditions with one-dimensional strips of surface roughness. Here, large-eddy simulations are used to examine the response of the blending height and effective surface roughness to tiled land-cover heterogeneity, or a two-dimensional chessboard pattern of alternating high and low vegetation given a diurnal cycle of solar irradiance in subarctic conditions. In each experiment, the length scale of the roughness elements is increased while the total domain fraction of each vegetation type is kept constant. The effective surface roughness was found to decrease with increasing length scale of surface cover heterogeneity, which is shown to have a significant impact on estimated wind turbine power calculated from logarithmic wind profiles. In stable conditions, the blending height in cases with large heterogeneity length scales was found to exist well above the surface layer. Because the behavior of the blending height has implications for coupled models, a simple model for the blending height as a function of heterogeneity length scale is introduced.

Large eddy simulations of fully developed turbulent flows over additively manufactured rough surfaces

In the last decade, with the growing demand for efficient and more sustainable products that reduce our CO2 footprint, progresses in Additive Manufacturing (AM) have paved the way for optimized heat exchangers, whose disruptive design will heavily depend on predictive numerical simulations. Typical AM rough surfaces show limited resemblance to the artificially constructed rough surfaces that have been the basis of most prior fundamental research on turbulent flow over rough walls. Hence, current wall models used in steady and unsteady three-dimensional (3D) Navier–Stokes simulations do not consider such characteristics. Therefore, a high-fidelity Large Eddy Simulation (LES) database is built to develop and assess novel wall models for AM. This article investigates the flow in rough pipes built from the surfaces created using AM techniques at Siemens based on Nickel Alloy IN939 material. We developed a code to generate the desired rough pipes from scanned planar surfaces. We performed high-fidelity LES of turbulent rough pipe flows at Reynolds number, Re = 11 700, to reveal the influence of roughness parameters on turbulence, mainly the average roughness height and the effective slope. The equivalent sand-grain roughnesses, ks, of the present AM rough surfaces are predicted using the Colebrook correlation. The main contributors to the skin friction coefficient are found to be turbulence and drag forces. In the present study, the existence of a logarithmic layer is marked even for high values of ks. The mean flow, the velocity fluctuations, and the Reynolds shear stresses show turbulence's strong dependence on the roughness topography. Profiles of turbulence statistics are compared by introducing an effective wall-normal distance defined as zero-plane displacement. The effective distance collapses the shear stresses and the velocity fluctuations outside the roughness sublayer; thus, Townsend's similarity of the streamwise mean velocity is marked for the present roughnesses. Furthermore, a mixed scaling is introduced to improve the collapse of turbulence statistics in the roughness sublayer. In addition, an attempt to investigate the impact of surface roughness on flow physics using the acquired LES results based on quadrant analysis of the Reynolds shear stresses and anisotropy of turbulence is made.

Vapor Deposited Zeolitic Imidazolate Framework-8 Derived from Porous ZnO Thin Films

In recent years, the vapor deposition of zeolitic imidazolate framework-8 (ZIF-8) has gained high attraction due to its good scalability, conformality, and thickness control. The present study provides new fundamental insights regarding the vapor deposition of ZIF-8 from zinc oxide (ZnO). During synthesis, ZnO thin films with different percentages of open porosity (14.5%–24%) were subjected to a 2-methylimidazole vapor for different conversion times (20 min–24 h). For the first time, the impact of the porosity of ZnO thin films onto the converted ZIF-8 is investigated. Grazing incidence X-ray diffraction reveals randomly oriented crystallites of ZIF-8, which already appear after 20 min of conversion. The thickness, roughness, and average particle height of the ZIF-8 layers increase with the conversion time, reaching values up to (172 ± 20) nm, (29 ± 3) nm, and (113 ± 8) nm, respectively, for ZIF-8 obtained from ZnO with 14.5% open porosity. At long conversion times (i.e., 24 h), the results hint at greater precursor porosities resulting in lower thicknesses of ZIF-8, as the thickness, roughness, and average particle height for ZIF-8 obtained from 24%-porous ZnO show values of (132 ± 20) nm, (25 ± 3) nm and (80 ± 8) nm, respectively. Additionally, the potential of the ZIF-8 layers as a photocatalyst for the degradation of the organic dye methylene blue was studied. The ZIF-8 enhances the degradation by approximately 8% when compared to degradation without a photocatalyst.

Scaling of the roughness effects in turbulent flows over systematically-varied irregular rough surfaces

To clarify the effects of surface topology on the scaling of roughness effects, we conduct experiments on turbulent rough-walled channel flows from transitionally to fully rough regimes. We considered three-dimensional irregular rough surfaces with different effective slope ES, skewness factor Sk, and fixed roughness height scales. The ES values are varied from 0.09 to 0.72 for positively and negatively skewed surfaces with Sk=±0.4. It is revealed that the transitional behavior to the fully rough regime is not influenced by Sk, but rather by ES. The transition to the fully rough regime for surfaces with ES≥0.18 is insensitive to the ES values, whereas wavy surfaces with ES=0.09 lead to a moderate transition. The equivalent sand-grain roughness ks steeply increases with increasing ES values from 0.09 to 0.36, whereas a further increase in the ES value does not significantly influence ks. It is also found that positively skewed surfaces with Sk=+0.4 yield larger ks values compared to surfaces with Sk=−0.4; this Sk effect is more pronounced for wavy surfaces with small ES values.

Identification of gear wear damage using topography analysis

Gear failure modes and their underlying mechanisms are usually identified by visual inspection, which relies on the skills and experience of the human observer and hence is prone to subjectivity and bias. Therefore, it is essential to design and implement objective methods to improve the identification of gear failure modes. In the present study, the 3D topography of gear surfaces affected by different types of wear modes, i.e., micropitting, pitting, and scuffing, was measured by means of white light interferometry. The surfaces were evaluated in terms of height, spatial, and function roughness parameters, according to ISO 25178-2. Besides, a new roughness parameter, named surface motion orientation (Smo), was proposed. Three features were found relevant to identify the differences between the gear surfaces affected by different failure modes: the shape of asperities distribution, the severity of damage, and the surface texture orientation with respect to the motion direction. The combination of the roughness parameters selected to quantify each of these features (Ssk,Sq,Smo) resulted in an objective classification of the assessed gear failure modes.

Designing of low cost solar air heater equipped with roughness of streamlined cross-section

In the current investigation an attempt is made to design a low cost solar air heater (SAH) using numerical simulation and exergy analysis approach. The thermo-hydraulic behaviour of SAH duct (aspect ratio = 10) with rib roughness of rectangular and symmetric half NACA 0020 (reverse orientation) cross-sections on one of its broad walls are analysed numerically. This novel design of the symmetric half NACA 0020 has been tailored to maximise heat transmission by capitalising on the benefits offered by secondary flows. For Reynolds numbers (Re) ranges from 6000 to 18000, fixed relative roughness pitch (P/e = 13), and relative roughness height (e/D = 0.06), the Nusselt number (Nu) and friction factor (f) are calculated. Ribs with NACA 0020 cross-section have higher heat transfer and lower flow friction. With NACA 0020 rib roughness, the maximum thermo-hydraulic performance parameter (THPP) is observed at Re = 6000. Contours of temperature, pressure, turbulence kinetic energy and velocity streamlines provides essential information for explanation of phenomenon in the SAH. Experimental results are used to validate the numerical outcomes. Exergy analysis is also carried out for the comparison of performance of SAH roughened with bluff (rectangular) and streamlined (reverse NACA 0020) roughness elements.

Aerodynamic Roughness Height of Gravel-Covered Plains in the Puna of Argentina

Aerodynamic Roughness Height of Gravel-Covered Plains in the Puna of Argentina