High Incidence Angles Research Articles

Long-range trajectory reconstructions using the point mass model.

In shooting incident reconstructions, forensic examiners usually deal with scenes involving short-range trajectories, typically ≤30 m. Insituations such as this, a linear trajectory reconstruction model is appropriate. However, a forensic expert can also be asked to estimate a shooter's position by reconstructing a long-range trajectory where the bullet's path becomes arced as a result of gravity and the greater time in flight. In this study, the point mass model (PMM) was used, because it is accessible and considered sufficiently accurate. A computer program using PMM can perform long-range trajectory reconstructions starting from an impact point. The reconstruction results in an area where the shot is expected to be fired from, not a single location. This is caused by varying the input parameters of the PMM. The aim of this study is to assess the accuracy of the method and discuss the influence of the most relevant parameters. The model has been validated by comparing its performance with 20 handgun bullet trajectories that were determined using Doppler radar measurements over long ranges, i.e. from 500 m to 1800 m. Comparison between the area calculated using the model and the actual shooter position demonstrates the limits of these reconstructions, particularly at high incident angles. The differences between the reconstructed deflections and the deflections measured by the tracking radar are rather large. This phenomenon is caused by either measurement errors in the cross wind as a function of height or inaccuracy of the radar's deflection measurements.

Flow control of blade-end oscillating jets on corner separation in a high-load compressor cascade

Based on unsteady numerical simulation, the feasibility of utilizing a fluid oscillator to generate oscillating jets for relieving the compressor cascade's corner separation was investigated. First, at design incidence angle, the optimal jet position is located where corner separation is not fully developed (74% axial chord length). Jets at more upstream and downstream positions are less effective due to premature dissipation of jet effects and the occurrence of high corner losses, respectively. The effectiveness of separation control through jet injection increases with higher jet mass flow rates, and the scheme with 0.66% relative jet flow rate exhibits a wide effective jet position range. However, excessively low jet flow rates are sensitive to jet position selection, while excessively high jet flow rates lead to significant mixing losses, resulting in high overall field losses and reduced engineering applicability. Second, the optimal jet scheme remains consistent at both design and high incidence angles and exhibits effective control at other off-design incidence angles. Finally, the oscillating jet suppresses the spanwise development of wall vortex and passage vortex within the blade passage by injecting high-momentum flow. Moreover, proper orthogonal decomposition analysis indicates that the oscillating jet redistributes the modal energy of the original flow field, exciting the vortex structures into high-frequency, small-scale oscillations at the jet frequency. Meanwhile, the oscillating jet primarily facilitates momentum exchange through strong mixing with passage vortex, wall vortex, and concentrated separation vortex, ultimately mitigating corner separation and reducing corner loss.

Is There a Relation Between High Pelvic Incidence and Sagittal Angle of Posterior Lumbar Facets?

In recent years, several studies have shown the presence of a linear correlation between the pelvic incidence (PI) and spondylolisthesis. However, no study has attempted to investigate a potential association between facet sagittal angle and spinopelvic parameters, especially PI in the normal population. Abdominopelvic computed tomography (CT) scans were collected. Inclusion criteria included age less than 40 years and CT done for non-orthopedic diagnostic purposes. All cases with any spinal pathology were excluded. Spinopelvic and lumbar spinal parameters were collected using the KEOPS software (SMAIO, Lyon, France), and the facet sagittal angle were evaluated on axial CT images from L1-L2 to L5-S1 using the institution PACS system (GE Centricity, Chicago, IL). A total of 450 patients' imaging were analyzed, with a mean age of 31.3 years (±4.9). Facet sagittal angle was found to be significantly correlated to an increase in PI but only at the L5-S1 level (odds ratio=2.3). The effect of sex on sagittal angle of facet joints was found to be non-significant compared with high PI. Finally, at the L5-S1 level, facet tropism was associated with a higher PI but was not found to play a direct role in the angle of facet joints. The PI seems to be correlated to the other spondylolisthesis risk factors: facet tropism and female sex. It carries the heaviest load in the progression towards sagittally oriented facet joints, which might lead to segmental instability and eventual spinal pathologies.

Defect-Free SiGe/Si Multistacks for Next-Generation CFET Architectures

The complementary FET (CFET) is the leading architecture choice for upcoming logic applications. Leveraging a stacked pMOS and nMOS design, this architecture is expected to enhance device performance with a reduced footprint [1,2]. The fabrication of the channels in these devices requires the deposition of SiGe/Si multistacks. The difference in etching rate of Si and SiGe is exploited, allowing for the selective removal of the SiGe layers and the resulting formation of free-standing Si channels.In this contribution, we present fully strained pseudomorphic SiGe/Si epitaxial multistacks for the use as channel regions in CFET devices. All layers were grown in a 300 mm ASM reduced-pressure chemical vapor deposition (RPCVD) reactor on Si(001) substrates.A targeted multistack structure for 1x1 nanosheet channel is displayed in Fig. 1(a). Two types of SiGe layers are present in the stack, having Ge concentration of 22% and 38%, respectively. Fig. 1(b) shows the X-ray diffraction (XRD) Omega-2Theta scan acquired for the deposited stack at the Si(004) Bragg reflection. The experimental spectrum is accurately simulated using a model closely resembling the nominal structure, thereby confirming that the thicknesses and germanium concentrations of the stack’s constituent layers are well within the targeted specifications. The X-ray reflectivity (XRR) spectrum acquired on the same sample and shown in Fig. 1(c) exhibits a high signal-to-noise ratio even at high incidence angles, suggesting a low surface roughness of the sample. Fig. 1(d) displays the asymmetric reciprocal space map (RSM) acquired around the Si(113) reflection. The stack's diffraction peaks are very narrow and horizontally aligned, indicating the absence of strain-relaxation in the sample. The within-wafer non-uniformity (WWNU) of thickness and Ge concentration was first minimized for single SiGe/Si bilayers. The optimized growth parameters were then successfully applied to the growth of multi-layered structures. Fig.2 depicts the haze map measured via deep-UV (DUV) laser scattering for the same multistack considering a 3 mm wafer edge-exclusion. The measured intensity ranges between 0.59 and 0.69 ppm, indicating a highly smooth surface morphology.AFM inspections, as shown in Fig. 3(a), confirm that the stack is fully strained, as evidenced by the absence of cross-hatch pattern in the examined areas with dimension 20x20 μm2. The RMS roughness (Rq) of the sample surface is found to be as low as 0.1 nm, which reduces to 0.07 nm on a 2x2 μm2 inspection area (Fig. 3(b)).The cross-sectional TEM analysis of a 3x3 nanosheet channel stack, as shown in Fig. 4(a), indicates that the layer thicknesses are highly consistent and in good agreement with the targeted nominal values. The energy-dispersive X-ray spectroscopy (EDS) elemental mapping of the same cross-section also reveals a close match between the measured Ge concentration and the expected value. Furthermore, the high-resolution HAADF-TEM image presented in Fig. 4(b) demonstrates the excellent crystalline quality of the epitaxial stack and indicates the presence of sharp compositional transitions between the layers.Finally, Fig. 5 displays the Omega-2Theta spectra measured for the first and last 1x1 nano-sheet channel stacks fabricated in a batch processing of 25 wafers. Notably, the two spectra exhibit perfect overlapping, with identical peak positions and intensities, showcasing ideal run-to-run repeatability of the process.In this work, we have demonstrated the fabrication of complex SiGe/Si multistacks that exhibit excellent agreement with the targeted specifications, are free of relaxation-defects and have extremely low surface haze values. These results highlight the effectiveness of our growth process in overcoming typical challenges associated with the development of advanced CFET devices.[1] P. Schuddinck et al., 2022 IEEE Symposium on VLSI, pp. 365-366.[2] E. Park et al., IEEE Transactions on VLSI Systems, 31(2), , 2022, pp.177-187. Figure 1

Analysis of Scattering Mechanisms in SAR Image Simulations of Japanese Wooden Buildings Damaged by Earthquake

The difficulty in identifying collapsed houses and damaged structures in synthetic aperture radar (SAR) images after natural disasters represents a significant challenge in the monitoring of urban structural deformation using SAR. SAR image simulation was conducted on a three-dimensional model of a typical wooden building in Japan to analyze the scattering mechanism of the structure in collapsed and uncollapsed states. Based on the physical properties of the buildings, a correlation was established between the simulated SAR image feature signals and the geometric structures of the buildings. The findings indicate that SAR scattering is more uniform for uncollapsed structures, which is predominantly influenced by their geometry. At low incidence angles, single reflections were the predominant phenomenon, whereas at high incidence angles, multiple reflections became more prevalent. The uncollapsed building’s facade formed a dihedral angle, exhibiting bright lines in the SAR image. Multiple reflections occurred at the edges of the building and floor junctions. These findings follow the theoretical predictions. In the case of the collapsed buildings, multiple reflections occurred with greater frequency, and irregular scattering was observed. Notwithstanding the augmented scattering pathways, some walls nevertheless manifested single reflections. The collapsed structures demonstrated a reduced sensitivity to alterations in the angle of incidence.

Airborne lidar intensity correction for mapping snow cover extent and effective grain size in mountainous terrain

ABSTRACT Differentially mapping snow depth in mountain watersheds from airborne laser altimetry is a valuable hydrologic technique that has seen an expanded use in recent years. Additionally, lidar systems also record the strength of the returned light pulse (i.e. intensity), which can be used to characterize snow surface properties. For near-infrared lidar systems, return intensity is relatively high over snow and inversely related to the effective grain size, a primary control on snow albedo. Raw intensity is also sensitive to laser range and incidence angle, however, and requires a correction for snow property retrieval that is especially pertinent in mountainous terrain. Here, we describe a workflow to correct the intensity using the plane trajectory, lidar scan angle, and lidar-derived topography. As a proof of concept for snow retrievals, we apply the workflow to an airborne 1064 nm lidar flight over a snow-covered mountain basin in the Colorado Rockies. Corrected intensity was empirically related to reflectance before delineating snow extent and retrieving grain size. Relative to the traditional snow classification derived from optical imagery, the lidar-derived snow extent covered 5.4% more area due to the fine resolution point cloud and absence of shadows common in optical imagery. The lidar-derived grain size retrievals had a MAE of 32 µm compared to those from field spectroscopy, which translated to a 1% error in snow albedo. We found high incidence angles yielded an overcorrection in intensity that introduced a high bias in the grain size distribution and, therefore, suggest using an incidence angle threshold (40°). Developing methods specifically for quantitative snow surface property retrievals from lidar intensity is timely and relevant as aerial lidar is increasingly being used to map snow depth for hydrologic and cryospheric studies.

Modelling П-Shaped Concentrating Optics for Lcpv Solar Cells Using Fresnel Lens

Abstract Most concentrating optics do not show good performance at higher incidence angles and have low acceptance angles and, therefore, they require a high-accuracy solar tracking system, which is costly. In this study, by detailed investigation of optics of the proposed П-shaped concentrating optics, it was found that system remained high in terms of optical efficiency and its concentration ratio at certain higher incidence angles. During the work, ray path through the concentrating optics, width of the light spot at different incidence angles were calculated. Optical efficiency, geometrical concentration ratio, concentration ratio at different incidence angles were found by the results of COMSOL Multiphysics calculations. It was found that the system had a high optical efficiency of approximately 95% and its concentration ratio of 3x-5x was at the range of ±0-20 degrees of incidence angle, and it could reduce the work of a solar tracking system. As well, an increase in the optical efficiency could be seen from 0 to 5 degrees of the incidence angle and an increase in the concentration ratio could be seen from 0 to 12 degrees of the incidence angle in terms of the reflective mirrors which helped redirect the rays to the solar cells. Optical systems with such a high incidence angle could reduce the performance of the solar tracker system, and it reduced the overall cost and energy consumption of the LCPV.

The nonlinear behavior of generic tail fins for small wind turbines

This paper describes analysis and measurements of the yaw response of tail fins for small wind turbines. It is based on an extension of unsteady slender body theory (USBT) to cover non-slender fins and high angles of incidence, both of which make the theory nonlinear. We provide three main additions to the substantial literature on linearized USBT for tail fins. First, USBT is extended to high angles by modeling the nonlinear vortex dynamics. Second, the restriction to slender bodies is removed by modeling the chordwise load variation. Third, we consider the effect of time-varying wind speed. The extended theory is compared to wind tunnel measurements of the yaw behavior of delta, elliptical, and rectangular tail fins without a rotor and nacelle. The fins were released from initial yaw angles of −40° and −80°; the latter is of sufficient magnitude to show the importance of the nonlinear yaw dynamics. Generally good agreement was found between the theory and measurements, and the theory was shown to be more accurate than a “polar” or quasi-steady model which uses only the lift and drag of a delta planform. Of the three planforms, the rectangular one showed the lowest accuracy in terms of frequency but the damping was accurately predicted. Overall, the results demonstrate the importance of nonlinearity in the response of a yawing tail fin, particularly for the higher aspect ratio fins at large yaw angles.

Effect of radial inlet distortion on aerodynamic stability in a high load axial flow compressor

Radial inlet distortion induced by boundary layer separation can significantly affect the aerodynamic performance of the compressor. The effect of radial inlet distortion on the flow structure is numerically investigated in a high load axial flow compressor. The inlet boundary is determined by the experimental results at the downstream of the distortion generator. The results reveal that tip distortion leads to a reduction in the stall margin, whereas hub distortion extends it. Radial distortion redistributes the main flow toward undistorted region due to the obstructive effect of lattice ring. The deficit in axial velocity with tip radial distortion causes the rotor to operate at a higher incidence angle near the casing, while hub radial distortion alleviates the tip blade loading. The detailed three-dimensional flow field analysis indicates that increased blade loading with tip distortion shifts the trajectories of both primary and secondary tip leakage flow toward the leading edge, thereby expanding the blockage region. Conversely, hub radial distortion unloads the rotor tip region, thereby reducing the blockage region induced by tip leakage flow. Additionally, with hub distortion, the location of separation line on the blade suction surface moves closer to the leading edge, and the flow separation around the trailing edge is intensified.

Design and Implementation of a Dual-Axis Solar Tracking System with IoT-Enhanced Monitoring Using Arduino

The position of the sun varies over the day and with the seasons, making it difficult for conventional fixed solar panel systems to achieve optimum energy production. By reorienting the panels to face the sun, solar tracking systems solve this problem. This study suggests a dual-axis solar tracker system that continuously adjusts the panels to stay perpendicular to sunlight in order to maximize energy capture. The system uses sensors to monitor the sun's elevation and azimuth angles. Light sensors, motor controllers, microcontrollers, control algorithms, and an Internet of Things (IoT) monitoring system are some of the system's essential parts. The microprocessor determines the ideal angles for panel alignment based on the location of the sun as detected by the light sensors. The motor driver then adjusts the panels appropriately. The sun's position is accurately and smoothly tracked using dual-axis tracking systems, which employ sensors to detect solar position and actuators controlled by sophisticated algorithms to adjust the panels accordingly. Real-time parameter monitoring of the solar panel is done using an IoT-enabled webpage. The study's findings show that the dual-axis solar tracker system performs noticeably better in terms of energy capture efficiency than fixed installations, especially in areas with high solar incidence angles and fluctuating sunshine.

Improving the experimental estimation of the incident angle modifier of evacuated tube solar collectors with heat pipes

This article focuses on the thermal performance testing of evacuated tube solar collectors with heat pipes (ETC-HP) using the ISO 9806:2017 standard test methods: Steady-state testing (SST) and Quasi-dynamic testing (QDT). The main objective of this work is to improve the experimental estimation of the incident angle modifier (IAM) for these types of solar collectors in both test methods. For the QDT method, a novel model for the IAM is presented and validated against SST results. This IAM model, recently developed for flat plate collectors under the SST framework, has demonstrated superior performance compared to other available models. This study marks its first application to ETC-HP technology, showcasing its adaptability across different technologies and test methods. While this work primarily focuses on ETC-HP collectors, the results are applicable to evacuated tubes in general. Thus, the generality of this model and its consistency with the SST method make it suitable for implementation in test standards as a general-purpose model. Regarding the SST method, and aiming to enhance consistency between testing methods, an improved parameter conversion from SST to QDT is also proposed, reducing IAM differences between test methods by 1 to 19 percentage points, with greater improvement at higher incidence angles.

Solar radiation on naturally ventilated double skin facade in real climates: The impact of solar incidence angle

Although the angular dependence of optical properties has long been recognised in transparent glazing facades, the impact of such dependence on natural ventilation between glazing has not been widely studied, especially in building facades with high solar incident angles. To better cope with such properties with the prevailing implementation of transparent building envelopes, this study investigated the effects of high solar incidence angles for vertical building facades through theoretical and numerical methods. The theoretical model validates the features of the optical properties simulated by the numerical model, supporting the numerical model's capability of revealing the radiation reflection, absorption, and transmission when multi-reflection is present. Then, such angular impact is studied with typical seasonal data in three cities, namely Shanghai, Melbourne and Chicago. Results firstly revealed the discrepancies between considering or ignoring multi-reflection between two glazing facades at varying incidence angles. Due to the opposite variation trend in reflectivity and transmissivity over angles, there is a critical solar incidence angle at 75° when multi-reflection's impact on natural ventilation reaches its highest. After that, the influence is reduced to its minimum. Identifying this angle helped highlight the enhanced natural flow due to realistic angular-dependent reflections between transparent panes, namely, under-estimation occurs if the angular dependence is not considered in the setting of optical properties. The analysis in three cities also revealed the climatic and locational impact of the angular impacts on ventilation rates. By addressing the impact on the optical and thermal performance of transparent facades, the dynamic variation of solar conditions can be more accurately integrated into passive building designs.

Atomistic wave packet investigation of phonon scattering at rough surfaces

Investigating the phonon-surface scattering mechanism is essential for the evaluation of thermal transport in nanostructures. The theoretical description to quantify this mechanism remains to be developed at the atomic scale. This work presents a phonon wave packet method to study the phonon-surface scattering behavior at rough surfaces. We obtain the specularity distribution dependent on phonon polarization, wavelength, and surface roughness. The reflection of the diffuse wave packet is primarily attributed to the surface shadowing effect at a higher incident angle, surface disorder, and surface-induced localized modes. Taking the wavevector-dependent specularity data as input, the thermal conductivity of silicon nanowires is calculated based on the Boltzmann Transport Equation. Our specularity model provides an accurate evaluation for predicting thermal conductivity. This work offers an atomic-level analysis for phonon-surface interaction, which is helpful for the understanding of thermal transport in nanostructures.

Modification of algorithm for accurate estimation of Hp(3) based on response of head TLD badge calibrated using ISO cylindrical phantom.

In India, the prevailing approach to eye lens dosimetry is the placement of an existing dosemeter on the forehead region after slight modification serves as a dedicated Eye Lens Dosemeter. A methodology for estimating the eye lens dose in terms of the Hp(3) has been previously explored employing an algorithm based on the response characteristics of this dosemeter using ISO slab phantom. It was observed that the performance of the dosemeter in terms of Hp(3) using previous algorithm showed under response at higher angles of incidence and photon beams of energy < 200keV. Further, study was conducted to modify the algorithm following the latest ISO recommendations. This involved generation of data from the response of existing dosemeter on a cylindrical phantom. The results of this study revealed better performance of the newly established algorithm in estimating eye lens dose in terms of Hp(3) when compared to the earlier algorithm.

Effect of branch-like structures created by vortex generators on cavitation dynamics at high angle of attack

When hydraulic machines operate away from their design condition, the angle between the inflow and the blade's leading edge increases significantly, causing severe cavitation. To address this, this investigation focuses on cavitation flow around hydrofoil with a high incidence angle. The effects of the vortex generators (VGs) on cavitation evolution, pressure fluctuations, and flow-induced noise were discussed. Experiments and simulations were jointly employed in this work. The results indicate that under current conditions, cavitation initiates upstream of the VGs, closer to the leading edge. The branch-like vortex cavitation induced by the VGs enhances the stability of the shedding cavities in the midstream of the hydrofoil, leading to a 15.24% reduction in the primary frequency of cavitation shedding. With the addition of the VGs, the amplitude of pressure fluctuations on the hydrofoil surface is reduced. Also, the acoustic power drops over the entire spectrum, especially in the high-frequency range. The sound pressure corresponding to the main frequency of cavitation noise is reduced by 7 dB.

Patterned Liquid Crystal Polymer Thin Films Improved Energy Conversion Efficiency at High Incident Angles for Photovoltaic Cells.

In this report, micro-patterned silicon semiconductor photovoltaic cells have been proposed to improve the efficiency in various incident sunlight angles, using homeotropic liquid crystal polymers. The anisotropic liquid crystal precursor solution based on a reactive mesogen has good flowing characteristics. It can be evenly coated on the silicon solar cells' surface by a conventional spreading technique, such as spin coating. Once cured, the polymers exhibit asymmetric transmittance properties. The optical retardation characteristics of the coated polymer films can be eventually determined by the applicable coating and curing parameters during the processes. The birefringence of light then influences the optical path and the divergence of any encountered sunlight. This allows more photons to enter the active semiconductor layers for optical absorption, resulting in an increase in the photon-to-electron conversion, and thus improving the photovoltaic cell efficiency. This new design is straightforward and could allow various patterns to be created for scientific development. The experimental results have evidenced that the energy conversion efficiency could be improved by 2-3% for the silicon photovoltaic cells, under direct sunlight or at no inclination, when the liquid crystal polymer precursor solution is prepared at 5%. In addition, the efficiency could be much more significantly improved to 14-16% when the angle is inclined to 45°. The unique patterned liquid crystal polymer thin films provide enhanced energy conversion efficiency for silicon photovoltaic cells. The design could be further evaluated for other solar cell applications.

Effects of ocean wave spectra on the polarized bidirectional reflectance distribution function matrix at Ku‐band and its implications on satellite backscattering simulations

A polarized bidirectional reflectance distribution function (pBRDF) matrix is developed from two-scale roughness theory with the aim of providing more accurate simulations of microwave emissions and scattering required for ocean–atmosphere coupled radiative transfer models. The potential of the pBRDF matrix is explored for simulating the ocean backscatter at Ku-band. The effects of ocean wave spectra including the modified Durden and Vesecky (DV2), Elfouhaily, and Kudryavtsev spectra on the pBRDF matrix backscatter simulations are investigated. Additionally, the differences in backscattering normalized radar cross-section (NRCS) simulations between the Ku-band geophysical model function and pBRDF matrix are analyzed. The results show that the pBRDF matrix can reasonably reproduce the spatial distribution of ocean surface backscattering energy, but the distribution pattern and numerical values are influenced by ocean wave spectra. The DV2 spectrum is the best one for the pBRDF matrix to simulate horizontally polarized NRCSs, with the exception of scenarios where the incidence angle is below 35°, the wind speed is less than 10 m s−1, and in the cross-wind direction. Also, the DV2 spectrum effectively characterizes the wind speed and relative azimuth angle dependence for vertically polarized NRCSs. The Elfouhaily spectrum is suitable for simulating vertically polarized NRCSs under conditions of low wind speed (below 5 m s−1) and incidence angles under 40°. The Kudryavtsev spectrum excels in simulating vertically polarized NRCSs at high incidence angles (> 40°) and horizontally polarized NRCSs at low incidence angles (< 35°).摘要为提供微波海洋-大气耦合辐射传输模型所需的精确发射和散射, 基于双尺度粗糙度理论发展了极化双向反射分布函数 (pBRDF) 矩阵. 本研究探讨了pBRDF矩阵是否能够表征ku波段洋面后向散射, 对比了三种常用海浪谱模型对pBRDF矩阵后向散射模拟的影响, 分析了ku波段地球物理模型函数 (GMF) 与pBRDF矩阵在后向散射归一化雷达截面 (NRCS) 模拟中的差异. 结果表明, pBRDF矩阵能够准确再现洋面后向散射能量的空间分布, 但后向散射能量分布受海浪谱影响较大.

Evaluation of The Dispersion Uniformity of Footprint of The Magnus Rotor Type Dispersive Submunition

Dispersion munitions are often equipped with dispersive submunitions used to scatter bombs over a wide area, and one of the types of dispersive submunitions is the Magnus rotor, commonly referred to as a self-rotating flying body. The Magnus rotor is designed to be dispered over a wide area by utilizing the principle of the Magnus effect through self-rotation, and has various trajectories depending on the initial conditions from the mother dispersion munition. In this paper, an index to evaluate the dispersion uniformity of footprint of the dispersive submunition is presented and the dispersion uniformity according to various initial release conditions is evaluated, and it is getting larger with high incidence angle and get max value at certain initial angular velocity.

Optical polarization studies of latex beads in aqueous solution: An analog for radar scattering in icy regolith

This study presents low phase angle 0°–5° measurements of polarimetric properties of icy planetary regolith analog materials acquired using the custom-built Multi-Axis- Goniometer-Instrument (MAGI). We present same sense (SC), and opposite sense (OC) backscatter circular polarization coefficients, circular polarization ratio (CPR), and degree linear polarization (DLP) of spherical latex (non-dye) beads of varying sizes and volume concentrations (v/v) in aqueous solutions (λ=0.8μm) in water. We also present measurements of alumina powder in air at λ=1.064μm. Measurements showed that at a low incidence angle (i=0°), backscatter is dominated by surface specular single-bounce scattering, that hides other scattering processes. At high (i=15°) incidence angle, surface single-bounce surface scattering becomes negligible, allowing for the detection of diffuse, dihedral (multiple bounces) scattering. We find that classical Mie alumina particles (2.1μm,4.0μm) enhance subsurface scattering due to a larger void space relative to larger Mie particles (30μm), that cause the radar signal to scatter forward off small imperfections, maintaining the polarization properties of the signal and generating high >1CPR. Latex beads, representing impurities, demonstrate the impact of isotropic and anisotropic scattering on radar signatures. This study also found that the scattering medium’s anisotropy correlates to the size of the beads, while the void space of the medium inversely correlates with the bead size and the volume concentration (v/v) of the beads. Rayleigh beads, due to isotropic scattering from the reduced scattering cross-section and higher transparency relative to larger impurities, generate subsurface single bounce scattering from the sample platform, producing OC≫SC and a low (<0.5)CPR across all v/v. Rayleigh impurities in transparent water-ice simulate single bounce scattering from underlying layers. Conversely, classical and large Mie beads generate anisotropic scattering that intensifies scattering in the forward direction with high CPR, inversely proportional to the volume concentration. This study aids in interpreting radar observations of icy bodies in the solar system, providing insights into the interplay between radar waves and icy regolith compositions.

Connecting transonic buffet with incompressible low-frequency oscillations on aerofoils

Self-sustained, low-frequency, coherent flow unsteadiness over rigid, stationary aerofoils in the transonic regime is referred to as transonic buffet. This study examines the role of shock waves in sustaining this transonic phenomenon and its relation to low-frequency oscillations (LFO) that occur in flow over aerofoils in the incompressible regime (Zaman et al., J. Fluid Mech., vol. 202, 1989, pp. 403–442). This is investigated by performing large-eddy simulations of the flow over a NACA0012 profile for a wide range of flow conditions under free-transition conditions. At low Reynolds numbers, zero incidence angle and sufficiently high free-stream Mach numbers, $M$ , transonic buffet occurs with shock waves present in the flow. However, when $M$ alone is lowered, self-sustained, periodic oscillations at a low frequency are observed even though shock waves are absent and the entire flow field remains subsonic at all times. At higher incidence angles, the oscillations are sustained at progressively lower $M$ and are present even at $M=0.3$ , where compressibility effects are low. A spectral proper orthogonal decomposition (SPOD) shows that the spatial structure of these oscillations is consistent for all cases. The SPOD modes are topologically similar, suggesting a connection between transonic buffet and LFO in the incompressible regime. Comparisons with other studies examining transonic buffet on various aerofoils, under forced-transition and fully turbulent conditions support this hypothesis. Future studies using tools of global linear stability analysis, especially at high free-stream Reynolds numbers are required to examine whether the underlying mechanisms of transonic buffet and incompressible LFO are the same.