Large Radius Of Curvature Research Articles

Fabrication and Design of Wood-Based High-Performance Composites

Delignified densified wood is a new promising and sustainable material that possesses the potential to replace synthetic materials, such as glass fiber reinforced composites, due to its excellent mechanical properties. Delignified wood, however, is rather fragile in a wet state, which makes handling and shaping challenging. Here we present two fabrication processes, closed-mold densification and vacuum densification, to produce high-performance cellulose composites based on delignified wood, including an assessment of their advantages and limitations. Further, we suggest strategies for how the composites can be re-used or decomposed at the end-of-life cycle. Closed-mold densification has the advantage that no elaborate lab equipment is needed. Simple screw clamps or a press can be used for densification. We recommend this method for small parts with simple geometries and large radii of curvature. Vacuum densification in an open-mold process is suitable for larger objects and complex geometries, including small radii of curvature. Compared to the closed-mold process, the open-mold vacuum approach only needs the manufacture of a single mold cavity.

Evolution of Carbides in Cr–Mo–Si–V Alloy Steel during the Spheroidization Annealing Process

The evolution of eutectic carbide and granular pearlite, including secondary carbide and matrix ferrite, in 1.12C–8.1Cr–2.3Mo–1.08Si–0.32V alloy steel under spheroidization annealing is investigated. This procedure can be divided into two stages: austenitization in the range of 780–900 °C for 0.5–5 h and second annealing in the range of 740–780 °C for 3–7 h. The area fraction, size, number per unit area, and sphericity of carbides are analyzed under different annealing processes. The result indicates that the mean diameter of secondary carbides increases and the number of carbides per area decreases with the increase in temperature in two ranges of 780–900 and 740–780 °C. At >860 °C, the secondary carbide sufficiently spheroidizes, the eutectic carbide dissolves at the tip with a larger radius of curvature, and ferrite grains of the matrix are basically equiaxed. It can be found that the austenitizing stage plays a crucial role in nucleation and coarsening of secondary carbides, dissolution and segmentation of eutectic carbides, and recrystallization of matrix ferrite grains. The microhardness of the sample presents a decreasing tendency from fast to slow with the increase of the average diameter of secondary carbides.

Frictional Characteristics and Electrical Conductivity of Electrical Sliding Contacts With Circular Grooved Porous Disk Under Lubricated Conditions

Abstract In this study, porous materials were applied to the surface materials of electrical sliding contacts to produce long and stable operations. The proposed sliding contacts consist of a rotating circular grooved porous disk and a stationary rider having a spherical surface. We conducted the experimental analysis from two perspectives. The first perspective was to inspect the effect of the design parameters on the electrical conductivity and the frictional characteristics; this was done by performing tests under various loads applied on the sliding contacts. The second perspective was to investigate the durability and stability of the sliding contacts by using the tests for a long operating time under a constant applied load. Our experimental results proved that the porous disk generated a lower and a more stable contact voltage and frictional force than the conventional nonporous solid disk. This effect is significant when a large curvature radius of grooves is provided on the disk surface. These results are attributable to the reduction of the hydrodynamic fluid force by the porous material and the grooves; the porous material yields lower hydrodynamic pressure due to the seepage of the lubricant oil across the interface between the oil film and the porous matrix and the grooves enhance the reduction of the hydrodynamic effect because of oil leakage to the downstream region. These factors reduce the oil film thickness between the disk and rider and facilitate the metal contact, and thereby a porous grooved disk generates higher electrical conductivity than a conventional solid disk.

Differential bank migration and the maintenance of channel width in meandering river bends

AbstractComparison of three bare-earth lidar data sets along 30 consecutive river bends on the Trinity River in Texas, USA, shows that differential migration of river banks in a channel bend counterbalances past bank migrations so that a statistically steady-state channel width is maintained. Two difference maps created from lidar flown in 2011, 2015, and 2017 capture this temporal variability in the relative amounts of inner versus outer bank migration. In 20 of the studied river bends, channel narrowing during 2011–2015 was counterbalanced by widening during the second interval, or vice versa. Only four bends recorded significant (>1%) progressive change in channel width during both measurement periods. Each of these four bends recorded progressive channel narrowing that was connected to floodplain complexity associated with past bend cutoffs or tributaries. Subaerial volumes of sediment deposited on inner banks of bends were smaller for 2015–2017 than for 2011–2015, while erosional volumes associated with the outer banks were similar despite 2015–2017 having had almost twice the number of days under flood conditions. Over time, channel width for the river appears roughly constant because differences in outer and inner bank migration at one time are counterbalanced by compensating differences at a later time. For the Trinity River, this compensation happened over time spans as short as 2–3 yr and would lead to the appearance of invariant channel width at the decadal scale. Tighter river bends with relatively smaller radii of curvature have smaller magnitudes of width change compared to broader bends with larger radii of curvature.

Determination of the allowance for grinding with flap wheels after shot peen forming

Shot peening is widely used in forming long panels and sheaths. Due to the impact of shot on the processed surface, the specific microgeometry is formed, the characteristic feature of this microgeometry is the numerous dimples of shot with different diameters and depths. The presence of these dimples causes deterioration of the surface roughness parameters. Therefore, after shot peening the mandatory requirement is implementation of surface grinding with flap wheels for partial removal of the shot dimples. Allowance for grinding depends on the requirements for the quality of the surface of the part. As a rule, the depth of dimples in magnitude significantly exceeds the valleys of micro-irregularities formed owing to previous treatment. The value of the assigned allowance for grinding depends on the requirements for the quality of the surface of the part. As a result of grinding on the treated surface, a new microrelief is formed in the shape of a combination of traces of the impact of abrasive grains of flap wheels and the remains of the dimples from the shot peening. Since the dimples have a spherical shape with a much larger radius of curvature than their depth, they have a special effect on the amount of metal removed and on the roughness of the treated surface. The paper presents an analytical description of the process of quantitative removal of metal and the formation of roughness during grinding with flap wheels of the surface treated with a shot peening. Based on the results of research, mathematical models are constructed to determine the amount of material removed from the surface treated with shot peening during grinding with flap wheels, and a numerical method for determining the allowance for obtaining the required roughness is proposed.

Effect of humidity and air pressure on the discharge modes transition characteristics of negative DC corona

Corona discharges on high-voltage transmission lines cause the problems of energy waste and electromagnetic interferences. Previous studies on negative DC corona discharges are mainly on the effects of various environmental factors on the corona inception voltage, Trichel pulse frequency etc. However, few researches are focused on Trichel pulses-pulseless glow mode transition, and their transition mechanisms are still lacking. In this paper, a vacuum vessel is built up in which the humidity and pressure can be accurately controlled to simulate corona discharges under different climatic conditions. The pressure range is 50–101 kPa and the relative humidity (RH) is 10–90%. A needle-plane electrode with different radius of curvature is used to produce negative corona discharges. It is found that the transition voltage increases with the RH and air pressure. A higher humidity results in the formation of hydrated ions with lower ion mobility and the increase in the gas temperature around the needle electrode. The experimental results demonstrate that the effect of reduced ion mobility is greater than the gas temperature rise. Furthermore, an interim process, in which the Trichel pulses and the pulseless glow exist simultaneously, appears during the discharge modes transition for the needle electrode with a larger radius of curvature.

Accuracy analysis and improvement of an algorithm-based stitching interferometry method for curved mirrors with large radii of curvature

An algorithm-based stitching interferometry method was developed for the measurement of mirrors with large radii of curvature, where the single-shot measurement of the surface figure is not possible. The global and adjacent-subaperture stitching algorithms were compared and the influence of the overlap ratio, the effect of the defocusing error, and methods for its reduction were studied. Using the global stitching algorithm, a root mean square repeatability error of the surface measurement of 0.24 nm was achieved. The centerlines of the stitching result showed good agreement with those from the Nanometer Optical Component Measuring Machine.

Selection of CVD Diamond Crystals for X-ray Monochromator Applications Using X-ray Diffraction Imaging

A set of 20 single crystal diamond plates synthesized using chemical vapor deposition (CVD) was studied using X-ray diffraction imaging to determine their applicability as side-bounce (single-reflection) Laue monochromators for synchrotron radiation. The crystal plates were of optical grade (as provided by the supplier) with (001) nominal surface orientation. High dislocation density was found for all samples. Distortions in the crystal lattice were quantified for low-index Laue reflections of interests using rocking curve topography. Maps of effective radius of curvature in the scattering plane were calculated using spline interpolation of the rocking curve peak position across the studied plates. For several selected plates, nearly flat regions with large effective radius of curvature were found ( R 0 ≳ 30 − 70 m, some regions as large as 1 × 4 mm 2 ). The average width of the rocking curve for these regions was found to be about 150 μ rad (r.m.s.). These observations suggest that the selected CVD diamond plates could be used as intermediate-bandwidth monochromators refocusing the radiation source to a specific location downstream with close to 1:1 distance ratio.

Characterization of the electrical and optical properties for a-IGZO/Ag/a-IGZO triple-layer thin films with different thickness depositions on a curved glass substrate

Nine specimens are prepared on the basis of the L9(33) orthogonal array design to evaluate the optical and electrical properties, morphology, and microstructure of a-IGZO/Ag/a-IGZO (IAI) triple-layer composite films deposited on curved glass substrates with a different radius of curvature efficiently. The experiments are arranged for the change in the three controlling factors, namely the IGZO and Ag thicknesses, and the substrate’s radius of curvature. The radius of curvature has the highest contribution for the RMS surface roughness (SRq) and the mean particle size (PS). The carrier mobility (CM) and carrier concentration (CC) are proportional to each, irrespective of the controlling factors in this study. An increase in the radius of curvature can result in the increase of SRq and PS, and therefore brings in a surface scattering effect that can cause the reduction of CM as well as the enhancement of resistivity (R). In addition, a sufficiently large radius of curvature can elevate the transmittance at 550 nm and Haack’s figure of merit (FOM) effectively, but it can also lower the reflectance for blue, green and red. Via the carrier injections, increasing the Ag thickness can elevate the carrier mobility and concentration significantly. The reflectance for blue, green, and red are also risen by increasing the Ag thickness. As a result of the Burstein-Moss effect, carrier concentration and optical bandgap are elevated by increasing the IGZO thickness. Additionally, the reflectance for blue, green, and red are also increased. Appropriate choices in the IGZO and Ag thicknesses and the radius of curvature can obtain the transmittance >80 %, and elevate the FOM significantly.

Meso-fracture mechanism and its fracture toughness analysis of Longmaxi shale including different angles by means of M-SENB tests

Researches on fracture toughness and crack initiation, propagation influenced by bedding at mesoscale are quite beneficial to the understanding of hydraulic fracturing and wellbore stability analyses in shale gas exploitation. Three-point bending tests with Scanning Electron Microscopy (SEM) were applied on 29 Longmaxi shale samples with different pre-notch angle at mesoscale. Peak load, failure mechanism and fracture toughness of shale were analyzed and discussed in detail. Experimental results show that the peak load is directly proportional to the angle β between the bedding plane and the pre-notch. In addition, the crack propagation path is very zigzag with the increase of β. It is obvious that the bedding weak surface seriously affects the crack initiation and propagation under the action of the principal stress field. Mode I crack is normally initiated at a relatively large curvature radius of notch edge. However, mixed mode crack I/II is easily initiated at the location where the principal stress is severely concentrated. Two typical facture modes have been found in Longmaxi shale. At the beginning of the expansion, mode I crack perpendicularly passes through the weak bedding plane and then extends into the weak layer. Mode I/II mixed slant-crack directly passes through the bedding plane or goes zigzag on the bedding plane or become zigzag before reached to weak bedding plane, and then passes through the weak plane perpendicularly or just deflects perpendicularly. The branch crack occurs near the main crack propagates along or through the mineral particles. Meso-cracks are mainly propagated around the crack tip, natural fissures, crack turning point, and large mineral particles. Cracking direction changes during the propagation process and eventually parallel to the main crack and then stops, or it intersects with the main crack and forms exfoliated particles. By utilizing both FEM software and test results, the I/II mixed mode fracture toughness of shale has been calculated, and the pure mode I fracture toughness has been obtained as 0.7359 MPa m0.5 approximately. Furthermore, both stress intensity factor and the T stress change law of I/II mixed mode cracks were also obtained and discussed in detail.

Void-Free Copper Electrodeposition in Full Wafer Thickness through-Silicon Vias with 10:1 Aspect Ratios

The incorporation of copper-filled, mesoscale through-silicon-vias (TSVs) as a three-dimensional integration technique allows for increased input/output per unit volume compared to wire bonded microelectronic devices. Mesoscale TSVs that span the full thickness of a silicon wafer allow for preservation of a large radius of curvature during multi-layer device fabrication and for conservation of mass in the device, which is often a requirement in MEMS applications. The damascene process is commonly implemented for Cu electrodeposition in high aspect ratio features and typically involves a three-additive system in a CuSO4-H2SO4 electrolyte. However, a single-additive system has recently been shown capable of achieving bottom-up superfilling in features with higher aspect ratios than those commonly used in damascene interconnects.[i] This resistive electrodeposition system utilizes CuSO4, methane sulfonic acid (MSA), chloride, and a poloxamine suppressor additive. Cyclic voltammetry (CV) can be used to characterize the plating solution and identify a potential range that exhibits hysteresis in the voltage-current relationship that is caused by suppressor disruption at the cathode surface during electrodeposition. This hysteretic region serves as an operating window where void-free Cu filling of high aspect ratio features may be achieved. Using this chemistry, potentiostatic and galvanostatic plating conditions for void-free filling were developed for 100 μm diameter vias that are 600 μm deep, a 6:1 aspect ratio.[ii],[iii] Cu electrodeposition in TSVs with a higher aspect ratio of 10:1 is currently under investigation. These TSVs have a 62.5 μm diameter etched into a 625 μm thick silicon-on-insulator (SOI) wafer. Conditions that produce void free, bottom-up filling for 100 μm diameter TSVs have not directly translated to fill 62.5 μm diameter TSVs. In this work, the poloxamine suppressor concentration, current, potential, and sample rotation rate were varied to analyze their effect on the fill profile in these 62.5 μm diameter TSVs. Fill profiles were analyzed after mechanically cross-sectioning and optically imaging the samples. Cu fill profiles were also obtained through X-ray computed tomography (CT) scans. This work details the experimental approach associated with determining galvanostatic and potentiostatic plating conditions for 62.5 μm diameter TSVs and presents the resultant fill profiles of Cu in these vias. Sandia National Laboratories, a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government. [i]. Moffat, T. P., and D. Josell. "Extreme bottom-up superfilling of through-silicon-vias by damascene processing: suppressor disruption, positive feedback and turing patterns." Journal of the Electrochemical Society 159.4 (2012): D208-D216. [ii]. Menk, L. A. et. al. “Bottom-Up Copper Filling of Large Scale Through Silicon Vias for MEMS Technology.” Journal of the Electrochemical Society 166.1 (2019): D3066-D3071. [iii]. Menk, L. A. et. al. “Galvanostatic Plating with a Single Additive Electrolyte for Bottom-Up Filling of Copper in Mesoscale TSVs” Journal of the Electrochemical Society 166.1 (2019).

Fluid motion in a corrugated curved channel

In this paper, pressure driven flow between two corrugated curved walls is analyzed. The combined effects of the corrugations and the radius of curvature of the channel on the fluid flow are investigated using the boundary perturbation method. The results show that the peak of the velocity increases with the radius of curvature and the width of the channel for a constant pressure gradient. The flow rate is increased by the corrugations for any phase difference between the corrugated curved walls depending on the corrugation wavenumber and the channel radius of curvature. For a sufficiently large corrugation wavenumber, the flow rate decreases, and the phase difference becomes irrelevant. However, the reduction in flow can be minimized by decreasing the channel radius of curvature. In general, a smooth curved channel will give the maximum flow rate for large corrugation wavenumber. The results of this study are consistent with those of a corrugated straight channel flow for sufficiently large radius of curvature.

Macroscopic and Microscopic Non-Uniform Deformations of Polycrystalline Pure Copper during Uniaxial Tensile Test with High Stress Gradient

Metallic materials usually have microscopically heterogeneous structures, such as polycrystalline structures, affecting macroscopic mechanical characteristics. Both macroscopic and microscopic non-uniform deformations of polycrystalline pure copper under a moderate stress gradient were investigated. In this study, macroscopic and microscopic non-uniform deformations under higher stress gradients are investigated. Uniaxial tensile tests using three-curve specimens with different curvatures and grain sizes were performed. In order to evaluate the heterogeneous strain field in the specimen surface, the development of the displacement field was measured using the digital image correlation method (DIC). The stress field was evaluated by coupling the DIC and finite-element methods. In smaller-grain specimens, a strong strain concentration was generated in the minimum cross-section area. Although a strong strain concentration was also confirmed in a larger-grain specimen, the strain field depended not only on the specimen shape but also on the microscopic heterogeneity. This microstructure-driven non-uniform deformation was also observed in the specimen with a larger curvature radius. These results indicated that the macroscopic non-uniform deformation should be estimated by the material parameter related to the microscopic heterogeneity.

Development of Forming Technology to Reduce Dimensional Scattering of Automotive Parts with Cambers by Using Bauschinger Effect

Dimensional scattering is a severe problem in press forming of ultrahigh-strength steels (UHSS), because of material strength scattering in mass production. In this study, camber back which occurs in longitudinally curved parts was focused on and a new forming method whereby dimensional scattering of camber back can be suppressed by the Bauschinger effect was developed. The new method consists of two processes. In the 1st process, a blank is formed to with a small radius of curvature compared with the 2nd-process radius of curvature. In the 2nd process, the 1st-formed part is formed to a large radius of curvature compared with the 1st-process radius of curvature. Then the Bauschinger effect works to decrease the amount of camber back. The new method was applied to a hat-shaped model of 590, 980, and 1180 MPagrade steels, where the radius of curvature in the longitudinal direction was 1600 mm. The experimental results showed that the difference in the amount of camber back between 590 MPa and 1180 MPa steels formed by the developed method decreased 95 % compared with the case of the conventional method.

Optical characteristics of overstressed nanosecond discharge in atmospheric pressure air between chalcopyrite electrodes

The results of the investigation of the optical characteristics of a high-current nanosecond discharge in atmospheric pressure air between chalcopyrite electrodes (CuInSe2 and CuSbSe2 compounds) in conditions of overstressed discharge gap are presented. When using electrodes with a relatively large radius of curvature of the working part (3 mm) and a special system for isolating a part of the surface of the electrodes, a relatively uniform nanosecond discharge was obtained in millimeter intervals. Electrical and optical (spectral and temporal) discharge characteristics were investigated. Based on the solution of the Boltzmann equation for the electron energy distribution function, the parameters of the air plasma with small additions of the vapor of easily ionized impurity, in the form of copper vapor, were simulated.

Research on the CNC incremental forming method based on the surface partition and multi-size extrusion tool head

For the problem that a single-size extrusion tool head cannot meet the high-precision and high-efficiency forming requirement in the CNC incremental forming of the complex-shaped sheet metal parts, a new CNC incremental forming method based on the surface partition and multi-size extrusion tool head is proposed. This method divides the inner surface of the sheet metal parts into the different forming regions according to the curvature of the inner surface, thereby to select the extrusion tool heads with different radius for the different forming regions. That is, the extrusion tool head with large radius is used for the forming region with large curvature radius so as to effectively improve the surface quality and formability of the sheet metal parts, and the small radius of the extrusion tool head is used for the forming region with small curvature radius so as to improve the profile accuracy of the sheet metal parts. The results of the numerical simulation and forming experiment showed that the method can partition the surface of the given sheet metal part and ascertain the corresponding multi-sized extrusion tool heads to the partitioned surface, which can effectively form the complex-shaped sheet metal parts and improve the forming quality and forming efficiency of the sheet metal parts.

Ultrastructure of Bone: Hierarchical Features from Nanometer to Micrometer Scale Revealed in Focused Ion Beam Sections in the TEM.

The ultrastructure of bone has been widely debated, in part due to limitations in visualizing nanostructural features over relevant micrometer length scales. Here, we employ the high resolving power and compositional contrast of high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) to investigate new features in human bone with nanometer resolution over microscale areas. Using focused ion beam (FIB)-milled sections that span an area of 50μm2, we have shown how most of the mineral of cortical human osteonal bone occurs in the form of long, thin polycrystalline plates (mineral lamellae, MLs) which are either flat or curved to wrap closely around collagen fibrils. Close to the collagen fibril (< 20nm), the radius of curvature matches that of the fibril diameter, while at greater distances, MLs form arcs with much larger radii of curvature. In addition, stacks of closely packed planar (uncurved) MLs occur between fibrils. The curving of mineral lamellae both around and between the fibrils would contribute to the strength of bone. At a larger scale, rosette-like clusters of fibrils are noted for the first time, arranged in quasi-circular arrays that define tube-like structures in alternating osteonal lamellae. At the boundary between adjacent osteonal lamellae, the orientation of fibrils and surrounding mineral lamellae changes abruptly, resembling the "orthogonal" patterns identified by others (Reznikov et al. in Acta Biomater 10:3815-3826, 2014). These features spanning nanometer to micrometer scale have implications for our understanding of bone structure and mechanical integrity.

Mega-meander paleochannels of the southeastern Atlantic Coastal Plain, USA

Paleodischarge estimates based on the slope-area method and channel boundaries determined from stratigraphic cross-sections indicate that large, terminal Pleistocene meandering paleochannels (“mega-meanders”) in river valleys of the southeastern Atlantic Coastal Plain of the United States represent bankfull flows that were at least double the magnitude of those on modern rivers. Correlation of radiocarbon- and luminescence-dated paleomeanders with previously reported pollen and eolian sedimentary records suggests that greater discharge was driven by seasonally wetter conditions resulting from dynamic changes to regional precipitation and runoff that occurred in association with global warming at the end of the Pleistocene. While reflecting larger channel-forming flows, the exceptionally large widths and radii of curvature of mega-meanders were nonetheless maintained by a relatively modest discharge magnitude that was between two and four times larger than modern bankfull flow and within the size range of the present two- to five-year flood. Despite their large planform, the paleochannels conveyed relatively modest bankfull discharges because a wide, shallow shape limited their cross-sectional area and hydraulic radius. Within the late Quaternary evolution of fluvial systems in the region, scrolled mega-meanders constitute a transitional meandering planform that remained influenced by large volumes of sandy bedload following the sand-bed braided channels of the late Wisconsin interval, circa 30–17 ka. In addition to greater discharge, the large planimetric dimensions of paleochannels reflect a lack of cohesive vertical accretion facies on paleomeander floodplains, a sediment regime that transported large quantities of bedload sand, and the influence of these factors on channel boundary composition, bank stability, and channel shape. Findings underscore the importance of reconstructing channel cross-sectional dimensions and slope when estimating discharge for infilled paleomeanders. This approach reduces uncertainties surrounding channel cross-sectional area, gradient, and boundary composition inherent to studies lacking subsurface data that rely upon meander geometry to retrodict discharge.

Biomechanical comparison of glenoid implants with adaptable and fixed backside curvatures in anatomic total shoulder arthroplasty

We evaluated the biomechanical effects and potential advantages of glenoid implants with adaptable backside curvature radii and compared them with standard implants having fixed backside curvatures in anatomic total shoulder arthroplasty (aTSA) for primary glenohumeral osteoarthritis with uniconcave glenoids. A glenoid implant with adaptable backside curvatures (Aequalis PerFORM, Tornier SAS, Montbonnot, France) was compared with its previous model having a fixed curvature radius. Virtual aTSAs were performed in 24 patients from preoperative shoulder computed tomography data sets, using both implants in each patient. For all 48 simulated aTSAs, we first measured the glenoid bone reaming depth, subchondral bone quality after reaming, and implant backside surface and then the predicted cement stress, bone-cement interfacial stress, and bone strain at 60° of arm abduction. These biomechanical quantities were tested for differences between adaptable and fixed implants and for correlations between preoperative measurements and postoperative predictions. Adaptable glenoid implants induced a significant decrease in cement stress (P = .008), bone-cement interfacial stress (P = .045), and bone strain (P = .039), particularly for glenoids with curvature radii larger than 40 mm. However, these biomechanical effects were not significantly correlated with an increase in subchondral glenoid bone quality. Our study confirms the presumed biomechanical advantages of adaptable glenoid implants, even though the effects were not directly due to the adaptation of the backside curvature radius. Benefits were more pronounced for glenoids with large curvature radii. Our initial biomechanical findings should now be corroborated with large-scale clinical studies.

On the leading edge noise and aerodynamics of thin aerofoil subjected to the straight and curved serrations

This paper presents the results of an experimental study into the effect of add-on type leading edge serrations on the aeroacoustic and aerodynamic performances of a symmetrical NACA0008 aerofoil. The aeroacoustic part of this paper studies the reduction of interaction broadband noise in the presence of elevated freestream turbulence by employing serrated leading edges. For non-dimensional frequencies f′ < 1, the resulting sound pressure level reduction (ΔSPL) was found to be a linear function of f′ and the ΔSPL depends only on the serration amplitude, serration wavelength and freestream velocity. Leading edge with a large serration amplitude was found to be very effective in the reduction of broadband noise where up to ΔSPL ≈ 8 dB is achievable. It is generally more beneficial to choose a leading edge with a smaller serration wavelength, although the most effective configuration actually combines the largest serration amplitude and the largest serration wavelength. Interestingly, for a curved-serration, the most optimised configuration (with small serration wavelength, large serration amplitude, small inclination angle and large curvature radius) was found to outperform its straight-serration counterpart by a further 5 dB reduction of broadband noise at the same frequency. Concerning the aerodynamic part of the study, to effectively suppress boundary layer stall without incurring severe drag penalties for low freestream turbulence intensity, the most effective leading edge serration should possess a large serration wavelength and small serration amplitude. Hence, the serration geometry that works very well for a low noise aerofoil is usually inferior in the aerodynamic performance, and vice versa. The best compromise for the serration geometry that can still harness good performances in both the aeroacoustic and aerodynamic should possess the largest serration amplitude and the largest serration wavelength. This paper demonstrates that, when optimised properly, the add-on type leading edge serration can be very effective in both the reduction of the interaction broadband noise, and the suppression of the boundary layer separation at high angle of attack.