Small Angle Range Research Articles

The analysis of Gurney flap applications for autonomous underwater gliders

ABSTRACT In this paper, the Gurney flap structure (GF) from aeronautical research was applied to underwater gliders for improving the lift-to-drag ratio. Firstly, the numerical simulation method was used to verify the lift enhancement principle of the GF structure and the feasibility of its application to autonomous underwater gliders (AUG). Secondly, the performance of different GF was simulated and analysed, and a variable GF structure was proposed with the requirements of the operating modes. The GF parameters were optimised by a genetic algorithm. The results show that the performance of GF in water is similar to that in air, so it is feasible to apply GF to AUG. When the height of GF is close to the turbulent boundary layer of the trailing edge of the wing, the lift-to-drag ratio is significantly improved, especially in the range of small angle of attack and the maximum increase is up to 30%.

Light Scattering From High‐Porosity 3D Simulants of the Lunar Regolith at Small Phase Angles

AbstractLunar regolith consists of unconsolidated grains with high porosity, called the fairy castle structure. It is closely linked to the lunar opposition effect, which is the effect where brightness sharply increases as the phase angle approaches 0. However, owing to the Earth's gravity, it is difficult to reproduce the structure to study the physical characteristics of the lunar fairy castle structure in the laboratory. We designed a lunar fairy castle structure model for 3D printing. These models had high porosity and were simplified to tree‐like shapes. Various porous conditions of the surface were considered, represented by the number of trees, maximum trunk length, and maximum branch angle. In this study, a laboratory experiment was conducted to measure the reflectance of simulants with a fairy castle structure within a small phase angle range from 1.4 to 5.0. The result is analyzed for the sample porosity with the tangential slope of the reflectance S, which denotes the strength of the opposition effect. In addition, the results of this study were compared with lunar observation data. The porous samples exhibited a relatively large S value. The influence of branch length and attachment angle was very weak in this study. Samples with a porosity between 0.78 and 0.82 represent the similar S values to the lunar observation data, a mean porosity of lunar regolith. In conclusion, our findings suggest a potential correlation between porosity and the opposition effect in printed samples, proposing a new research approach for understanding the lunar opposition effect.

Metalens optical receiver for multi-beam free-space optical communication

A conventional optical receiver with a single detector accepts optical beams from a small range of angles of arrival (AoAs) and hence is not suitable for simultaneous communication with nodes with angular diversity. Here, we demonstrate a novel, to our knowledge, two-metalens-single-detector optical receiver that can collect optical beams coming from a wide range of AoAs. We show that the capability to collect broad AoAs with high efficiency makes the proposed receiver suitable for multi-beam free-space optical communication. An analysis for an exemplary three-beam system shows more than 70% collection efficiency at 2° AoA, which is 35× higher than conventional receivers.

A model for predicting the unsteady hydrodynamic characteristics on the blades of a horizontal axis tidal turbine

Tidal current turbines operate in a highly turbulent environment, which results in significant fluctuations in unsteady loads and consequent fatigue and dynamic failures in the rotor. Therefore, accurate calculation of these unsteady loads is crucial. However, the existing models do not take into account the influence of varying relative blade section thickness on dynamic stall when predicting the unsteady characteristics of horizontal axis tidal turbine (HATT) blades. To forecast the unsteady characteristics of the HATT blades, this study developed a method that considers the influence of relative blade thickness on dynamic stall. The modified Leishman-Beddoes (L-B) dynamic stall model, 3D rotational augmentation model, and steady-state BEM model were combined to predict the unsteady hydrodynamic characteristics of blades by synthesizing the transient inlet velocities induced by waves, shear flow and turbulence. In this method, the static parameters required for the modified L-B dynamic stall model were calculated by XFOIL within a range of small angle of attack (AoA) and then extrapolated to cover the entire range of AoA using the Viterna extrapolation method. The required dynamic parameters were obtained by CFD numerical simulation. This method, that we call HATT-UCPM, can consider both the effects of a single element on the unsteady hydrodynamic characteristics of the blades, as well as the combined effects of shear flow, waves, and turbulence on the blades in the actual marine environment. The accuracy of the method was validate through CFD numerical simulation. The results showed that the proposed method has a high consistency with the numerical simulation results, indicating that it has a good accuracy in the prediction of unsteady characteristics of HATT blades. Finally, the study examined several cases to investigate the impacts of shear flow, waves, and turbulence on the unsteady loads of blades, both individually and in combination.

Measurement of the thickness and refractive index of a thin film by analyzing reflected interference fringes.

We propose a novel, to the best of our knowledge, method to estimate the thickness and refractive index of a thin film by analyzing the reflectance as a function of the incidence angle. In most cases, interference fringes cannot be obtained from a film within a practical angular range unless it is much thicker than the wavelength. This problem was solved by adopting a high-index material as the medium of incidence, in which case several cycles of interference fringes were observed within a small range of incidence angles near the critical angle, allowing a fringe analysis. Consequently, the thicknesses, as well as the refractive indices of dielectric thin films, could be estimated. Our proposed method gave uncertainties of 20nm and 0.0004 for the thickness and refractive index measurements, respectively.

Research on Soil Moisture Estimation of Multiple-Track-GNSS Dual-Frequency Combination Observations Considering the Detection and Correction of Phase Outliers.

Soil moisture (SM), as one of the crucial environmental factors, has traditionally been estimated using global navigation satellite system interferometric reflectometry (GNSS-IR) microwave remote sensing technology. This approach relies on the signal-to-noise ratio (SNR) reflection component, and its accuracy hinges on the successful separation of the reflection component from the direct component. In contrast, the presence of carrier phase and pseudorange multipath errors enables soil moisture retrieval without the requirement for separating the direct component of the signal. To acquire high-quality combined multipath errors and diversify GNSS-IR data sources, this study establishes the dual-frequency pseudorange combination (DFPC) and dual-frequency carrier phase combination (L4) that exclude geometrical factors, ionospheric delay, and tropospheric delay. Simultaneously, we propose two methods for estimating soil moisture: the DFPC method and the L4 method. Initially, the equal-weight least squares method is employed to calculate the initial delay phase. Subsequently, anomalous delay phases are detected and corrected through a combination of the minimum covariance determinant robust estimation (MCD) and the moving average filter (MAF). Finally, we utilize the multivariate linear regression (MLR) and extreme learning machine (ELM) to construct multi-satellite linear regression models (MSLRs) and multi-satellite nonlinear regression models (MSNRs) for soil moisture prediction, and compare the accuracy of each model. To validate the feasibility of these methods, data from site P031 of the Plate Boundary Observatory (PBO) H2O project are utilized. Experimental results demonstrate that combining MCD and MAF can effectively detect and correct outliers, yielding single-satellite delay phase sequences with a high quality. This improvement contributes to varying degrees of enhanced correlation between the single-satellite delay phase and soil moisture. When fusing the corrected delay phases from multiple satellite orbits using the DFPC method for soil moisture estimation, the correlations between the true soil moisture values and the predicted values obtained through MLR and ELM reach 0.81 and 0.88, respectively, while the correlations of the L4 method can reach 0.84 and 0.90, respectively. These findings indicate a substantial achievement in high-precision soil moisture estimation within a small satellite-elevation angle range.

Giant flexoelectric response of uniformly dispersed BT-PVDF composite films induced by SDS-assisted treatment

Polymer-ceramic composites are commonly used as flexoelectric films. In existing studies, the flexoelectric effect of composites are generally improved by adjusting the material structures or adding ferroelectric materials. Further improvement of flexoelectric response has encountered a bottleneck. Considering from a new perspective, this study innovatively proposes to prepare the uniformly dispersed BT-PVDF composite films with giant flexoelectric response by surfactant SDS-assisted treatment. According tothe engineering applications, tilt sensors have been fabricated with the SDS/BT-PVDF composite films. The prepared tilt sensors can accurately sense the tilt change in a small-angle range (0-10°) between the coaxial connecting parts, the response signal changes significantly (49.25-72.35mV/°), and the response speed can reach 0.166 s. The research provides a new idea for improving the flexoelectric response and also paves a way for developing tilt sensors through a low-cost, facile, and reliable method, showing potential applications including bending sensing and structural health monitoring.

Anomalous Hall metal and fractional Chern insulator in twisted transition metal dichalcogenides

We predict robust Ising ferromagnetism driven by Coulomb interaction in the metallic phase of twisted transition metal dichalcogenides homobilayers for a range of small twist angles. Due to the presence of spin-valley locking and Chern band, the completely spin polarized state -- a half metal -- has a spin gap and exhibits anomalous Hall effect. We also find that near a magic angle where the Chern band is predicted to be exceptionally flat, the anomalous Hall metal at $1/3$ filling may become unstable at low temperature to a $\sqrt{3}\times\sqrt{3}$ charge density wave, or a fractional Chern insulator.

Optimal Shortcuts to Adiabatic Control by Lagrange Mechanics.

We combined an inverse engineering technique based on Lagrange mechanics and optimal control theory to design an optimal trajectory that can transport a cartpole in a fast and stable way. For classical control, we used the relative displacement between the ball and the trolley as the controller to study the anharmonic effect of the cartpole. Under this constraint, we used the time minimization principle in optimal control theory to find the optimal trajectory, and the solution of time minimization is the bang-bang form, which ensures that the pendulum is in a vertical upward position at the initial and the final moments and oscillates in a small angle range.

Studies on Morphological Parameters, Radius of Gyration, Correlation Length and Invariant of HPMC Polymer Doped with Ferrous (II) Oxide Using SAXS Method

It shed light on morphological parameters which influence radius of gyration, correlation length and invariant of Hydroxy Propyl Methyl Cellulose (HPMC) polymer doped with ferrous (II) oxide (FeO) using small angle X-ray scattering (SAXS) method. SAXS patterns have been recorded for different composition and gives an information about the structure of the polymer on a scale of about 800Ǻ, in which the variation of scattered intensity with angles from 0o to 2.5o has been measured for different composition of FeO with host polymer of HPMC. Employing para-crystalline model and exponential distribution function of phase lengths (amorphous and crystalline), we find a good agreement between the measured and predicted scattering except for a very small range of angles on the low angle side of the peak intensity. The method enabled quantitative parameters describing morphology of the two phases, amorphous and crystalline, consistent with the SAXS intensity distribution. A stepping procedure has been employed to compute the final parameters which gave the best fit between the calculated and experimental data. This was done by minimizing the statistical goodness-of-fit parameter chi-square. It is observed that there are significant changes in distribution of phase lengths due to different percentage weight of FeO in HPMC contents. Additionally, SAXS data has been used to estimate (i) radius of gyration (Rg), ii) correlation length (Lcor) and (iii) invariant (Q) parameters. A functional representation of the long period and also radius of gyration at different dopant concentrations is adopted by viewing them as realizations of a continuous-time stochastic process observed with measurement error. This allows one to estimate the mean functional relationship between long period and the dopant concentration.

Visual encoder-based angle measurement method in low-frequency angular vibration calibration.

Angular vibration calibration is required to determine the sensitivity of sensors such as dynamic inclinometers, gyroscopes, and angular accelerometers, which are used for angular motion measurement in engineering applications. Additionally, the calibration performance depends on the accuracy of the angle measurement by laser interferometry or a circular grating (CG) method that is commonly used in vibration calibration. However, these methods usually own a complex and high-cost system or limited frequency and amplitude ranges. In this study, a novel, to the best of our knowledge, angle measurement method that combines a special visual encoder and an accurate angular position detection method is investigated; the method requires only a simple and flexible telecentric vision measurement system. Comparison experiments with the CG method demonstrate that the investigated method has the maximum measurement deviation of 0.0014° and 0.0138° for static angle measurement in the small-angle range and continuous full circle, respectively. The relative deviation of the angular vibration measurement in the range of 0.1-8 Hz with amplitudes 0-100° is less than 0.173%. Additionally, the relative deviation of calibrated sensitivity of a gyroscope by the investigated and CG methods is less than 0.096%.

An improved approach of dry snow density estimation using C-band synthetic aperture radar data

Snow density is one of the important indicators of snow cover hydrological potential. The application of existing algorithms for retrieving dry snow density using synthetic aperture radar (SAR) data is limited by single scattering mechanism, small terrain fluctuation or narrow incidence angle range. In the study, an improved approach was proposed to retrieve dry snow density from C-band SAR data with a wide range of roughness and local incidence angles. Both the snow-ground interface scattering and volume scattering were considered in the approach. First, the relationship between the backscattering at the snow-ground interface and relative permittivity was obtained based on simulation using the Advanced Integral Equation Model (AIEM) and regression analysis. Then the classical relationship between the volume backscattering and relative permittivity obtained by the first-order volume scattering model was incorporated into the approach. For comparison, the coefficients of the Shi algorithm were redefined by the AIEM model and regression analysis, and the Shi algorithm initially developed for L-band was modified for C-band. In experiments, the RADARSAT-2 data obtained in the Manasi River Basin on December 12–17, 2013 and the C-band GaoFen-3 data obtained in the Kelan River Basin on January 17, 2018 were selected to validate the applicability of the proposed approach under different conditions. The inversion results in the Manasi River Basin using the proposed approach, Singh algorithm, and modified Shi algorithm were compared. The results in the Manasi River Basin show that the correlation coefficients (Rs) between the measured and estimated dry snow density are 0.868, 0.694, and 0.653 for the three methods, respectively. The root mean square errors (RMSEs) are 31.1 kg m−3, 59.1 kg m−3, and 64.7 kg m−3, respectively, and the mean relative errors (MREs) are 12.9%, 21.9%, and 25.5%, respectively. The corresponding R, RMSE, and MRE in the Kelan River Basin using the proposed approach are 0.717, 57.2 kg m−3, and 27.1%, respectively. The results prove that the dry snow density under different C-band SAR data and different areas can be effectively retrieved using the proposed approach superior to the other two algorithms.

MEMS mirror based omnidirectional scanning for lidar optical systems

Lidar 3D vision system is currently one of the most effective technical means for high-precision perception of three-dimensional image information of the surrounding environment. Microelectron-mechanical systems (MEMS) micromirror lidar has advantages of miniaturization, fast scanning speed, and relatively low price. The MEMS micromirror scanning method has become the best choice for laser scanning 3D image sensors. However, due to the small size of the MEMS micromirror, it can only scan a small angle range, which limits its industrial applications. In this paper, combined with MEMS micro-mirror spiral scanning form, we design a new type of omnidirectional optical system that can realize MEMS lidar omnidirectional scanning imaging. The overall optical system is composed of the catadioptric prism, the MEMS micromirror, and the compensation prism. The overall system can achieve 360° omnidirectional scanning imaging with a horizontal resolution of 0.1°; a vertical field view with a 0.5° resolution. The design results are analyzed by the optical design software (Zemax). The point diagram shows that the emitted light spots are circular and evenly distributed. The divergence angle is less than 1mrad, which can well meet the actual needs and solve the problems of solid-state, miniaturization, and omnidirectional scanning imaging of lidar.

Inverse Synthetic Aperture LiDAR Imaging of Rough Targets under Small Rotation Angles

Inverse synthetic aperture LiDAR (ISAL) breaks through the limitations of the diffraction limit and achieves ultra-long-distance radar imaging with centimeter-level resolution. However, because ISAL obtains a high resolution, it is accompanied by a high sampling rate and a large data volume, and the processing process is complicated, which is not conducive to fast real-time imaging of ISAL targets. At the same time, considering that actual non-cooperative targets cannot obtain full-angle ISAL images during movement, in this paper, the Kirchhoff approximation method based on two-dimensional Fourier transform is used to calculate the scattering echo of the rough surface of the target, and then, the rough surface scattering echo of the target coordinate system is obtained through coordinate transformation. After vector superposition, the scattered echo and ISAL image of the rough target are finally obtained, and then the influence of the rotation angle on the ISAL imaging of the rough plate and the rough target is discussed. It is found that a small rotation angle range can also achieve clear ISAL imaging of rough targets, and the influence of different roughness on the ISAL imaging results of different rough targets under a small rotation angle is analyzed. When the roughness is decreased, the target scattering mainly comes from coherent scattering, and the target edge becomes sharper. As the roughness increases, the image energy distribution becomes more uniform. Theoretical and simulation experiments verify the feasibility of ISAL imaging of rough targets under small rotation angles.

Angle-tunable two-photon absorption in twisted graphene systems

We theoretically study the two-photon absorption of twisted bilayer, trilayer and double bilayer graphene with Bernal stacking and demonstrate the layer and twist angle dependent two-photon absorption properties based on both the density functional theory calculations and our developed continuum model for small angle range. The expressions of two-photon absorption for above mentioned three twisted graphene layers around Dirac points are obtained. For small rotation angles which are less than 15 ° , the rotation angle well tunes the two-photon absorption coefficient in terms of the angle-dependent renormalized Fermi velocity. • Energy bands of twisted graphene systems around K point were investigated using DFT calculation. • Two-photon absorption spectra in twisted graphene systems were simulated on the basis of the second-order perturbation theory. • Twist angle effectively tuned the two-photon absorption coefficient within the small angle range ( θ < 15 ° ).

Estimation of Angles of Attack and Sideslip of Unmanned Aerial Vehicle in the Absence of Aerodynamic Angle Sensors

A method for estimating aerodynamic angles in the absence of appropriate sensors is proposed, using measurements of three projections of flight speed carried out by the navigation system and the values of the orientation angles. The relevance of the problem being solved is determined by the fact that on unmanned aerial vehicles (UAVs) sensors of aerodynamic angles, that is, angles of attack and slip, are often not installed due to restrictions on dimensions and mass. The proposed method is based on the joint use of mathematical models of aircraft motion, known from flight dynamics, and the theory of parametric identification of dynamic systems. The key factor ensuring the accuracy of the proposed method is the use of very accurate measurements of three UAV velocity projections performed by a satellite navigation system or an inertial navigation system with satellite correction. To account for the influence of wind, parametric identification of three projections of wind speed is provided. Another feature of the method is that instead of the missing aerodynamic angle sensors, it is proposed to use information about the aerodynamic coefficients of the lifting and lateral forces of the UAV. If these coefficients are known with errors, their values are also specified by identification methods. The dimension of the identification problem turns out to be low in the range of small and medium angles of attack when the aerodynamic dependencies are linear. The results of testing the proposed method based on simulation data on the flight test bench of a modern training aircraft for nine different flight modes under conditions of simulating random errors of onboard measurements corresponding to the flight experiment are presented.

The Aerodynamic Development of the New Range Rover Evoque

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;The Range Rover Evoque is a compact luxury SUV, first introduced by Land Rover in 2012. Almost 800,000 units of the first-generation vehicle were sold. This paper explores some of the challenges entailed in developing the next generation of this successful product, maintaining key design cues while at the same time improving its aerodynamic efficiency.&lt;/div&gt;&lt;div class="htmlview paragraph"&gt;A development approach is outlined that made use of both numerical simulation and full-scale moving ground wind tunnel testing. A drag coefficient of 0.32 was obtained for the best derivative by paying particular attention to:&lt;ul class="list disc"&gt;&lt;li class="list-item"&gt;&lt;div class="htmlview paragraph"&gt;the integration of active grille shutters;&lt;/div&gt;&lt;/li&gt;&lt;li class="list-item"&gt;&lt;div class="htmlview paragraph"&gt;the front bumper and tyre package;&lt;/div&gt;&lt;/li&gt;&lt;li class="list-item"&gt;&lt;div class="htmlview paragraph"&gt;brake cooling;&lt;/div&gt;&lt;/li&gt;&lt;li class="list-item"&gt;&lt;div class="htmlview paragraph"&gt;underfloor design;&lt;/div&gt;&lt;/li&gt;&lt;li class="list-item"&gt;&lt;div class="htmlview paragraph"&gt;wake control strategy; and&lt;/div&gt;&lt;/li&gt;&lt;li class="list-item"&gt;&lt;div class="htmlview paragraph"&gt;detail optimization.&lt;/div&gt;&lt;/li&gt;&lt;/ul&gt;&lt;/div&gt;&lt;div class="htmlview paragraph"&gt;This approach delivered the most aerodynamic Range Rover at the time of its introduction. The impact of these design changes on the aerodynamic flow field and consequently drag is highlighted. An interaction between front wheel deflectors and different tyres of the same nominal dimensions is explored over a range of small yaw angles, illustrating the need to develop vehicles using a range of boundary conditions. In addition, a relationship is shown between wake state defined in terms of the vertical pressure gradient and reduced aerodynamic drag.&lt;/div&gt;&lt;div class="htmlview paragraph"&gt;Finally, changes in regulations governing the assessment of emissions and fuel economy are driving a need for vehicle manufacturers to go beyond the traditional focus on the drag of the best vehicle derivative and manage the spread of values arising from customers’ choice of powertrain, wheel and tyre fitment along with exterior trim options. For this vehicle, we highlight that the average drag coefficient for sales in the European market over a year was 15% lower than the outgoing car, showing that improved aerodynamic efficiency has actually been delivered to customers.&lt;/div&gt;&lt;/div&gt;

Reentrant ferroelectric phase induced by a tilting high magnetic field in Ni3V2O8

The canonical multiferroic material ${\mathrm{Ni}}_{3}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$ with a kagome staircase structure of $S=1\phantom{\rule{0.16em}{0ex}}{\mathrm{Ni}}^{2+}$ spins has attracted much attention for its complex magnetic phase diagram, as well as the interplay between spin orders and ferroelectricity. In the present work, we report a comprehensive angular dependent ferroelectric polarization study on ${\mathrm{Ni}}_{3}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$ with the field rotating within the $ac$ plane at low temperatures. The low field induced ferroelectric (LF) state along the crystallographic $b$ axis, which has been observed when the field is applied parallel to the easy axis ($a$ axis), is revealed to vanish suddenly as the field is rotating close to the $c$ axis. Of particular interest is the observation of the reentrant ferroelectric (HF) phase at high field for various orientations when the field is between the $a$ and $c$ axes within a small angle range. The field-angle phase diagrams are built based on the low-temperature angular dependent polarization results and composed of a continuous LF phase and a discontinuous HF phase both exhibiting symmetry with respect to the $a$ axis. The microscopic origins of the two phases are different due to the two decoupled sublattices in the kagome staircase structure, where the low-field polarization is proposed to originate from the spiral structure of the spine spins, while the high-field phase is ascribed to the ordering of the cross-ties under a tilting high magnetic field.

Observation of laser-assisted electron-impact ionization in ultrashort intense laser fields

A laser-assisted ($e, 2e$) [LA($e, 2e$)] process of Ar in an ultrashort (1.2 ps) near-infrared intense laser field, $\mathrm{Ar}+{e}^{\ensuremath{-}}+h\ensuremath{\nu}\ensuremath{\rightarrow}{\mathrm{Ar}}^{+}+{e}^{\ensuremath{-}}+{e}^{\ensuremath{-}}$, was observed by coincidence measurements of the fast electron scattered forward within the small-angle range and the slow electron emitted to the wide-angle range. The spectrum of the sum of the kinetic energies of fast and slow electrons exhibited a shoulder structure at the one-photon energy shift from the ($e, 2e$) peak for the Ar $3p$ orbital, which was unambiguously assigned to the LA($e, 2e$) process with the one-photon energy gain. The determined triply differential cross section of the LA($e, 2e$) process with one-photon gain from the laser field was approximately twice as large as a theoretical estimate and this enhancement was regarded as an evidence of the formation of light-dressed states of Ar.

A preconditioned landweber iteration scheme for the limited-angle image reconstruction.

The limited-angle reconstruction problem is of both theoretical and practical importance. Due to the severe ill-posedness of the problem, it is very challenging to get a valid reconstructed result from the known small limited-angle projection data. The theoretical ill-posedness leads the normal equation AT Ax = AT b of the linear system derived by discretizing the Radon transform to be severely ill-posed, which is quantified as the large condition number of AT A. To develop and test a new valid algorithm for improving the limited-angle image reconstruction with the known appropriately small angle range from [0,π3]∼[0,π2]. We propose a reweighted method of improving the condition number of AT Ax = AT b and the corresponding preconditioned Landweber iteration scheme. The weight means multiplying AT Ax = AT b by a matrix related to AT A, and the weighting process is repeated multiple times. In the experiment, the condition number of the coefficient matrix in the reweighted linear system decreases monotonically to 1 as the weighting times approaches infinity. The numerical experiments showed that the proposed algorithm is significantly superior to other iterative algorithms (Landweber, Cimmino, NWL-a and AEDS) and can reconstruct a valid image from the known appropriately small angle range. The proposed algorithm is effective for the limited-angle reconstruction problem with the known appropriately small angle range.