Search Results (19)

When hypersonic vehicles fly in near space, the flow field near the optical window leads to light displacement, jitter, blurring, and energy attenuation of the star sensor. This ultimately affects the imaging quality and navigation accuracy. In order to investigate the impact of aerodynamic optical effects on imaging, the fourth-order Runge–Kutta and the fourth-order Adams–Bashforth–Moulton (ABM) predictor-corrector methods are used for ray tracing on the density data. A comparative analysis of the imaging quality results from the two methods reveals their respective strengths and limitations. The influence of the optical system is included in the image quality calculations to make the results more representative of real data. The effects of altitude, velocity, and angle of attack on the imaging quality are explored when the optical window is located at the tail of the vehicle. The results show that altitude significantly affects imaging results, and higher altitudes reduce the impact of the flow field on imaging quality. When the optical window is located at the tail of the vehicle, the relationship between velocity and offset is no longer simply linear. This research provides theoretical support for analyzing the imaging quality and navigation accuracy of a star sensor when a vehicle is flying at hypersonic speeds in near space. Full article

During high-speed flight, the aircraft causes rapid compression of the surrounding air, creating a complex turbulent flow field. This high-speed flow field interferes with the optical transmission of optical imaging systems, resulting in high-frequency random displacement, blurring, intensity attenuation, or saturation of the target scene. Aero-optical effects severely degrade imaging quality and target recognition capabilities. Based on the spectral characteristics of aero-optical degraded images and the deep learning approach, this paper proposes an adaptive frequency selection network (AFS-NET) for correction. To learn multi-scale and accurate features, we develop cascaded global and local attention mechanism modules to capture long-distance dependency and extensive contextual information. To deeply excavate the frequency component, an adaptive frequency separation and fusion strategy is proposed to guide the image restoration. Integrating both spatial and frequency domain processing and learning the residual representation between the observed data and the underlying ideal data, the proposed method assists in restoring aero-optical degraded images and significantly improves the quality and efficiency of image reconstruction. Full article

This paper provides an overview of the three-year effort to design and implement a prototypical sense-and-avoid (SAA) system based on a multisensory architecture leveraging data fusion between optical and radar sensors. The work was carried out within the context of the Italian research project named TERSA (electrical and radar technologies for remotely piloted aircraft systems) undertaken by the University of Pisa in collaboration with its industrial partners, aimed at the design and development of a series of innovative technologies for remotely piloted aircraft systems of small scale (MTOW < 25 Kgf). The system leverages advanced computer vision algorithms and an extended Kalman filter to enhance obstacle detection and tracking capabilities. The “Sense” module processes environmental data through a radar and an electro-optical sensor, while the “Avoid” module utilizes efficient geometric algorithms for collision prediction and evasive maneuver computation. A novel hardware-in-the-loop (HIL) simulation environment was developed and used for validation, enabling the evaluation of closed-loop real-time interaction between the “Sense” and “Avoid” subsystems. Extensive numerical simulations and a flight test campaign demonstrate the system’s effectiveness in real-time detection and the avoidance of non-cooperative obstacles, ensuring compliance with UAV aero mechanical and safety constraints in terms of minimum separation requirements. The novelty of this research lies in (1) the design of an innovative and efficient visual processing pipeline tailored for SWaP-constrained mini-UAVs, (2) the formulation an EKF-based data fusion strategy integrating optical data with a custom-built Doppler radar, and (3) the development of a unique HIL simulation environment with realistic scenery generation for comprehensive system evaluation. The findings underscore the potential for deploying such advanced SAA systems in tactical UAV operations, significantly contributing to the safety of flight in non-segregated airspaces Full article

High-speed turbulence induces significant aero-optical effects that severely disrupt the functionality of imaging systems of hypersonic vehicles. In this study, the aero-optical correction of various jet cooling modes is investigated using a Terminal High Altitude Area Defense (THAAD)-like seeker model and the imaging impact of high-speed flow field and flow control on the optical window is analyzed by the Delayed Detached Eddy Simulation (DDES) method. The findings reveal that a jet mode parallel to the window exhibits better cooling effectiveness compared to a perpendicular jet mode along the body axis; however, it introduces additional wavefront distortion, leading to degraded imaging quality. Although micro-vortex generators (MVGs) can reduce density fluctuations near the window from a refractive index perspective, they do not effectively mitigate wavefront distortion or improve window cooling efficiency. Finally, incorporating suction control, a comprehensive flow control solution, significantly improves the flow field structure near the window, resulting in a more uniform temperature distribution and reduced wavefront distortion. Applying this flow control method results in a 14.7% reduction in wavefront distortion at 3 Ma and an approximately 20% maximum value reduction at 5 Ma. This study proposes a novel and comprehensive flow control method to effectively mitigate the aero-optical effect in hypersonic flows, providing a new avenue for subsequent researchers in this field. Full article

High-speed turbulence is generated when hypersonic vehicles fly in the atmosphere, which can create aero-optical effects and interfere with optical navigation and guidance systems. At the same time, the front end and optical window of hypersonic vehicles are exposed to an aerodynamic heating environment, leading to the head ablation and thermal deformation of the optical window. This further aggravates the turbulent transition process and makes the error of the aero-optical effects more difficult to predict. In this paper, the aero-optical effects under the condition of high-temperature ablation were analyzed. Ablation deformation models of both the head and optical window were established. Then, a high-speed flow field was simulated under different flight conditions. The distortion characteristics of the aero-optical effects were obtained through the photon transmission theory. The simulation results show that the ablation deformation of hypersonic vehicles under an aerodynamic heating environment aggravates the disturbance error of the aero-optical effects. Moreover, with the increase in the flight speed and the decrease in the flight altitude, the ablation deformation of the hypersonic vehicles and the aero-optical effects distortion both gradually increase. The research in this paper provides a reference for the prediction of aero-optical distortion in an aerothermal environment. Full article

Aero-optical effects caused by hypersonic turbulence will affect the accuracy of optical sensors on aircraft. Traditional analysis methods, which do not consider absorption and scattering effects, cannot easily be used to completely describe the transmission process of light in hypersonic turbulence. In this paper, an aero-optical effect analysis method based on photon Monte Carlo simulation (MC-AOEA) was proposed to explain the distortion characteristics of aero-optical effects from the perspective of photon statistics. The energy distribution of photons in the transmission process was determined by taking a photon packet as a unit, and the microscopic statistics of the photon dissipation energy for all photon packets were calculated. The effectiveness of this method was verified by comparing the photon statistical parameters with the traditional optical distortion physical quantities. MC-AOEA was used to analyze the distortion of aero-optical effects at different altitudes and speeds. Additionally, the simulation results showed that, with the reduction in flight altitude and the enhancement of speed, the distortion of aero-optical effects was aggravated, and the energy loss was more serious, which provides a reference for the evaluation of aero-optical effect errors. Full article

Aero-optical effects caused by high-speed flow fields will interfere with the transmission of starlight, reduce the accuracy of optical sensors, and affect the application of celestial navigation on hypersonic vehicles. At present, the research of aero-optical effects relies heavily on the flow field simulation of computational fluid dynamics (CFD), which requires a great deal of computing resources and time, and cannot satisfy the demand of the rapid analysis of aero-optical effects in the engineering design stage. Therefore, a quick simulation method for aero-optical effects based on a density proxy model (DP-AOQS) is proposed in this paper. A proxy model of the turbulent density field is designed to replace the density field in the CFD simulation, and the proxy model is parametrically calibrated to simulate the optical characteristics of the turbulent boundary layer (TBL) in the external flow field of the optical window. The performance of DP-AOQS in the visible light band is verified from the perspectives of density field distribution, optical path difference (OPD), and fuzzy star map. The simulation results show that the method can quickly provide the distortion results of aero-optical effects in different flight conditions on the premise of ensuring the simulation accuracy. The research in this paper provides a new analytical method for the study of aero-optical effects. Full article

Aero-optical effects are the key factors that restrict the accuracy of the optical sensors of hypersonic vehicles, and the numerical simulation of aero-optical effects is a powerful tool with which to analyze aero-optical distortion. Most existing research focuses on the simulation analysis of refraction distortion based on the density field at the macro level via the ray-tracing method. In this paper, a method for analyzing aero-optical effects based on the interaction between photons and gas molecules is proposed and can explain the optical distortion and energy dissipation caused by aero-optical effects at the micro level. By establishing a transmission model of photons in turbulence, a simulation method of aero-optical effects based on a microscopic mechanism is designed and breaks through the limitations of a traditional macro method in energy analyses. The optical distortion parameters based on photonics are compared with the physical quantities of traditional aero-optical effects, which verifies the effectiveness of the micro analysis on the macro scale and provides a new idea for studying the microscopic mechanism of aero-optical effects. Full article

Aiming at the problem of mechanical resonance faced by the servo control system of the aero-optical stabilization platform, based on the proportional integral and disturbance observer combination (PI+DOB) control algorithm, a state equalizer speed closed loop is proposed. Compared with the traditional PI+DOB control algorithm, this new control structure can suppress the resonance peak and the anti-resonance peak at the same time. The experimental results show that compared with the PI+DOB control algorithm, after adding the state equalizer speed closed-loop to compensate for the model, the closed-loop bandwidth is increased by 42%. The anti-disturbance capability of the control system has been significantly improved, and it has good robustness under vibration conditions. To sum up, adding the state equalizer speed closed loop on the basis of PI+DOB has an obvious effect on the suppression of mechanical resonance and the performance improvement of the control system. Full article

In order to overcome the shortcomings of the poor shear resistance of the bare optical fiber whose coating layer falls off due to harsh conditions, such as on aero-engines and the marine environment, the coaxial powder feeding laser cladding method (CPFLCM) is proposed to connect the optical fiber sensor and the substrate. The concentration field model of the powder flow is established in sections, the effective number model of particles and the corresponding laser attenuation rate are obtained. Through simulation, the influence of relevant parameters of laser cladding on the temperature field was analyzed, and the accurate parameters of laser cladding were optimized. Finally, the temperature rise trajectory of the substrate temperature field was verified by using the fiber grating temperature sensor. Through experiments, the quality of the molten pool and the optical transmission loss of the optical fiber sensor were analyzed, and the consistency of the simulation optimization parameters was verified. Through this paper, it can be concluded that the proposed CPFLCM can realize the effective connection of the optical fiber sensor to the substrate. It is of great significance in the application of optical fiber sensors in harsh environments of oceans and aerospace. Full article

The direct radiative effects of atmospheric aerosols are essential for climate, as well as for other societal areas, such as the energy sector. The goal of the present study is to exploit the newly developed ModIs Dust AeroSol (MIDAS) dataset for quantifying the direct effects on the downwelling surface solar irradiance (DSSI), induced by the total and dust aerosol amounts, under clear-sky conditions and the associated impacts on solar energy for the broader Mediterranean Basin, over the period 2003–2017. Aerosol optical depth (AOD) and dust optical depth (DOD) derived by the MIDAS dataset, along with additional aerosol and dust optical properties and atmospheric variables, were used as inputs to radiative transfer modeling to simulate DSSI components. A 15-year climatology of AOD, DOD and clear-sky global horizontal irradiation (GHI) and direct normal irradiation (DNI) was derived. The spatial and temporal variability of the aerosol and dust effects on the different DSSI components was assessed. Aerosol attenuation of annual GHI and DNI were 1–13% and 5–47%, respectively. Over North Africa and the Middle East, attenuation by dust was found to contribute 45–90% to the overall attenuation by aerosols. The GHI and DNI attenuation during extreme dust episodes reached 12% and 44%, respectively, over particular areas. After 2008, attenuation of DSSI by aerosols became weaker mainly because of changes in the amount of dust. Sensitivity analysis using different AOD/DOD inputs from Copernicus Atmosphere Monitoring Service (CAMS) reanalysis dataset revealed that using CAMS products leads to underestimation of the aerosol and dust radiative effects compared to MIDAS, mainly because the former underestimates DOD. Full article

In numerous turbomachinery applications, e.g., in aero-engines with regenerators for improving specific fuel consumption (SFC), heat exchangers with low-pressure loss are required. Pil low-plate heat exchangers (PPHE) are a novel exchanger type and promising candidates for high-speed flow applications due to their smooth profiles avoiding blunt obstacles in the flow path. This work deals with the overall system behavior and gas dynamics of pillow-plate channels. A pillow-plate channel was placed in the test section of a blow-down wind tunnel working with dry air, and compressible flow phenomena were investigated utilizing conventional and focusing schlieren optics; furthermore, static and total pressure measurements were performed. The experiments supported the assumption that the system behavior can be described through a Fanno–Rayleigh flow model. Since only wavy walls with smooth profiles were involved, linearized gas dynamics was able to cover important flow features within the channel. The effects of the wavy wall structures on pressure drop and Mach number distribution within the flow path were investigated, and a good qualitative agreement with theoretical and numerical predictions was found. The present analysis demonstrates that pressure losses in pillow-plate heat exchangers are rather low, although their strong turbulent mixing enables high convective heat transfer coefficients. Full article

Aero-optical effect correction has become a crucial issue in airborne infrared imaging. However, it is impractical to test the correction algorithm using flight tests and numerical simulation because of its high cost. This study proposes a dynamic imaging simulation method for the infrared aero-optical effect based on a continuously varying Gaussian superposition model. The influence of infrared image degradation under different high-speed aerodynamic flow fields was investigated in detail. A continuously varying Gaussian superposition model was established for flight speed, altitude, and attitude. A dynamic infrared scene simulation model was constructed. Experimental results show that the proposed method can realistically simulate actual aero-optical effects of any flight case. Moreover, it can simulate continuous frames of aerodynamically degraded infrared images. The method uses a simpler model than numerical simulation and provides more data for multitype tasks. Full article

With the increase of combustion temperatures, the thermal radiation effect for hot components in the new generation of aero-engines has become a key factor in the combustion process, cooling structure design, and thermal protection. A radiation loading system can be used as an external heat source to simulate the real thermal environment of hot components in aero-engines. Total receiving power, as well as 3-D heat flux distribution, should better coincide with real conditions. With the aid of freeform optics and the feedback optimization method, the current study develops a concentrating-type radiation heating system fit for the leading-edge surface of a C3X turbine vane. A xenon lamp combined with a freeform reflector was optimized for controllable heat flux. A design method in the area of illumination engineering was innovatively extended for the current model. Considering the effect of polar angular radiative flux distribution of a xenon lamp, a Monte Carlo ray tracing (MCRT) method was adopted to evaluate the optical performance. Feedback modifications based on Bayesian theory were adopted to obtain the optimal shape of the FFS for target heat flux. The current study seeks a feasible way to generate 3-D heat flux distribution for complex curved surfaces, such as turbine vane surfaces, and helps to simulate the real thermal environment of hot components in aero-engines. Full article

Aerosol Optical Depth (AOD) is a crucial parameter for various environmental and climate studies. Merging multi-sensor AOD products is an effective way to produce AOD products with more spatiotemporal integrity and accuracy. This study proposed a conditional generative adversarial network architecture (AeroCGAN) to improve the estimation of AOD. It first adopted MODIS Multiple Angle Implication of Atmospheric Correction (MAIAC) AOD data to training the initial model, and then transferred the trained model to Himawari data and obtained the estimation of 1-km-resolution, hourly Himawari AOD products. Specifically, the generator adopted an encoder–decoder network for preliminary resolution enhancement. In addition, a three-dimensional convolutional neural network (3D-CNN) was used for environment features extraction and connected to a residual network for improving accuracy. Meanwhile, the sampled data and environment data were designed as conditions of the generator. The spatial distribution feature comparison and quantitative evaluation over an area of the North China Plain during the year 2017 have shown that this approach can better model the distribution of spatial features of AOD data and improve the accuracy of estimation with the help of local environment patterns. Full article