This paper deals with adaptive moving target detection for a forward scatter radar in complex Gaussian noise. The echoes received by the forward scatter radar include not only the noise and the ...possible target signals but also the direct signals. To suppress the direct signals and detect the target signals, Rao and Wald tests are derived in two cases: the secondary data which contain no target signal are available or not available. Different from monostatic radar, it is proved that the derived Rao and Wald detectors for the forward scatter radar have the same test statistics as the generalized likelihood ratio test-based detector in the complex Gaussian noise both when the secondary data are available or not available. The numerical evaluation further demonstrates the equivalence and the effectiveness of the proposed detectors.
In a radial bed, the uniformity of the pressure drop distribution is investigated by Euler single-phase flow and porous media models under different operating mode (CF-U/Z, CP-U/Z), gas flow rate ...(120–240 m
3
/h), particle diameter ((0.5–3)exp-3 m) and bed voidage (0.3–0.6). According to the nonuniform index
η
, the uniformity relates to these parameters and improves with increasing total pressure drop of particle bed Δp
s
(sum of the pressure drops of particle bed and gas perforation) or decreasing main channel pressure drop Δp
g
. Comparing the flow fields with/without particles, Δp
s
is approximately equal to the pressure drop of the particle bed with high-porosity Johnson net, which is well calculated by the Ergun equation. Δp
g
can be calculated by the modified momentum equation containing k. After changing the wall shear stress and gas-solid axial resistance, it is found that the internal generation factors for k include the influence of gas perforation on boundary layer and the existence of gas axial velocity after perforation. Besides, the global/local k hardly changes with the investigated parameters. The local k is a function of axial position or velocity ratio, which changes obviously at the end of the main channel for the existence of a gas stagnation zone.
The problem of accurately measuring the motion parameters of low radar cross-section (RCS) maneuvering targets has long been a hurdle in the radar technology landscape. Small targets, due to their ...elusive characteristics, are particularly difficult to detect with conventional radar systems. In this paper, we investigate the capabilities of dechirp-receiving stepped-frequency radar, a modern system using a linear frequency modulation signal for downconversion. This permits the radar to function at reduced sampling rates while maintaining the transmission of large-bandwidth signals and achieving synthetic imaging. Our primary contribution is introducing the stepped dechirp generalized Radon–Fourier transform (stepped DGRFT) algorithm. This novel approach allows the radar system to perform coherent accumulation, enhancing the accuracy of motion parameter estimates for low-RCS maneuvering targets. Results from our simulations and measured data analysis validate the effectiveness of our proposed algorithm, demonstrating its superiority over other methods.
With the increasing complexity of radar jamming threats, accurate and automatic jamming recognition is essential but remains challenging. Conventional algorithms often suffer from sharply decreased ...recognition accuracy under low jamming-to-noise ratios (JNR).Artificial intelligence-based jamming signal recognition is currently the main research directions for this issue. This paper proposes a new radar jamming recognition framework called Diff-SwinT. Firstly, the time-frequency representations of jamming signals are generated using Choi-Williams distribution. Then, a diffusion model with U-Net backbone is trained by adding Gaussian noise in the forward process and reconstructing in the reverse process, obtaining an inverse diffusion model with denoising capability. Next, Swin Transformer extracts hierarchical multi-scale features from the denoised time-frequency plots, and the features are fed into linear layers for classification. Experiments show that compared to using Swin Transformer, the proposed framework improves overall accuracy by 15% to 10% at JNR from −16 dB to −8 dB, demonstrating the efficacy of diffusion-based denoising in enhancing model robustness. Compared to VGG-based and feature-fusion-based recognition methods, the proposed framework has over 27% overall accuracy advantage under JNR from −16 dB to −8 dB. This integrated approach significantly enhances intelligent radar jamming recognition capability in complex environments.
Celotno besedilo
Dostopno za:
CEKLJ, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
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•PVDF-HFP/SiNPs hybrid nanofibrous membranes were electrospun for DCMD.•The influence of hydrophobic SiNPs on membrane properties was investigated.•The hybrid nanofibrous membrane ...presented a superhydrophobic surface.•A high permeate flux was obtained for the hybrid nanofibrous membrane.•The hybrid membrane exhibited stable permeability and superhydrophobicity.
The poly(vinylidene fluoride-co-hexafluropropylene) (PVDF-HFP)/Silica nanoparticles (SiNPs) flat-sheet hybrid membranes were fabricated for membrane distillation via electrospinning, and the effects of SiNPs on the resultant membranes properties were investigated. The electrospun nanofibers membranes presented a three-dimension interconnected open morphology. Although the SiNPs had no significant effect on the crystalline form of the PVDF-HFP polymer in the hybrid membranes, the membrane surface became rougher and exhibited a hierarchical stratum structure due to overlapping nanofibers and the presence of beads-on-string with the SiNPs addition, which made the superhydrophobic membrane surface formation. The SiNPs addition declined membrane porosity, while enlarge pore diameter and increased membrane thickness. However, the liquid entry pressure of water of the hybrid membrane was not significantly influenced by the SiNPs addition. The electrospun membranes were tested through direct contact membrane distillation using 35g/L NaCl solution as feed, and the highest permeate flux bout 48.6kg/m2h was obtained and the salt rejection maintained 99.99%. The 240h continuous desalination application demonstrated the stable permeability of the electrospun hybrid membrane, and the membrane hydrophobicity can also maintained well with long-time immersion in salt solution. The results indicated the potential of the electospun superhydrophobic PVDF-HFP/SiNPs hybrid membrane for membrane distillation application.
Membrane distillation (MD) is a promising membrane-based thermal process capable of desalinating highly saline water. However, its application is limited by fouling and wetting of commercial ...hydrophobic membranes. Inspired by the lotus leaf, we developed a biomimetic superhydrophobic polyvinylidene fluoride (PVDF) membrane for robust MD via self-assembly method. The hierarchical micro-nanoscale texture on the membrane surface was constructed by grafting the spherical polyvinylsilsesquioxane (PVSQ) nanoparticles onto micron-sized silica particles (SiPs). The membrane surface energy can be simultaneously lowered due to the hydrophobic groups including vinyls and methoxyls created from the condensation reaction of the vinyltrimethoxysilane (VTMOS). The resulting membrane showed a very high water contact angle (~160°) and a low water sliding angle (<15°), demonstrating the strong hydrophobicity that is expected for a lotus-leaf-like surface. Compared to the commercial PVDF membrane, the fabricated superhydrophobic membrane exhibited superior resistance against wetting by a surfactant (sodium dodecyl sulfate) and fouling by humic acid during MD experiments. Our results suggest that biomimetic superhydrophobic membranes can enhance the operational robustness of MD and facilitate the efficient use of MD for desalinating saline wastewaters in favor of beneficial water reuse and resource recovery.
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•A novel superhydrophobic membrane for robust membrane distillation was developed.•The biomimetic membrane surface was constructed via self-assembly method.•The modified membrane surface had hierarchical micro-nanoscale rough structures.•The membrane surface energy can be lowered by using a fluoride-free modifier.•The modified membrane showed superior anti-wetting and anti-fouling performance.
•The film thickness of the corrugated tube increased with the increasing of both film Reynolds number and was the thinnest β = 90°−120°.•The film thickness of the corrugated tube decreased when the ...tube spacing increases or the tube diameter/ corrugated radius decreases.•The falling film thickness was thinner at the wave crests and thicker at the wave troughs.•The phenomenon of dryout hardly appears in the falling film flow of the corrugated tube.•A new correlation has been given to predict the falling film thickness of the horizontal corrugated tube.
Falling-film evaporation has an essential method for desalination. The heat transfer tube is the core component of falling film evaporator. Liquid film thickness is one of the most important parameters for predicting heat and mass transfer coefficient of falling film evaporation. In this paper, a set of experimental apparatus for measuring the film thickness of a horizontal tube is set up. The experiments have been undertaken under different parameters, i.e. the tube diameter varying from 25 to 45 mm, the tube spacing ranging from 10 to 60 mm, the corrugated radius varying from 0.25 to 1 mm, and the film Reynolds number ranging from 150 to 1000. The liquid film falling around a horizontal tube is performed to explore the distribution characteristics of the film thickness. The liquid film thickness is measured by the conductance probe measurement method. The experimental results showed that the film thickness of the corrugated tube increases with the increasing of the film Reynolds number, which is the thinnest from 90° to 120°. It decreases as the tube spacing increases and increases with the tube diameter and corrugated radius. The liquid in the wave troughs can be replenished to the wave crests at any time because of the corrugated structure. The falling film thickness was thinner at the wave crests and thicker at the wave troughs. The phenomenon of dryout hardly appears in the falling film flow of the corrugated tube. Based on the experimental data, a new correlation has been given to predict the film thickness of the corrugated tube.
As a promising thermally driven separation process, membrane distillation (MD) is capable of treating challenging wastewaters. However, the traditional hydrophobic membranes are vulnerable to fouling ...by non-polar contaminants owing to the strong hydrophobic-hydrophobic interactions. To address this problem, we developed novel anti-oil-fouling MD membranes in this study. The composite membranes with asymmetric wettability were fabricated through electrospinning polyacrylonitrile (PAN) fibrous coating on a hydrophobic polytetrafluoroethylene (PTFE) membrane, followed by hydrolyzing the PAN coating with ethylenediamine (EDA) and NaOH, respectively. These two composite membranes exhibited excellent underwater superoleophobicity, with the underwater oil contact angle >150°, which can be attributed to the fibrous and re-entrant surface structure and the optimized surface hydrophilicity of the electrospun coating. During MD process using saline and oily emulsion as feed, the composite membranes presented robust anti-oil-fouling performance, indicating by stable permeate flux and salt rejection. A novel oil-droplet adhesion force probe was introduced to quasi-quantitatively elucidate oil-membrane interaction and evaluate membrane fouling propensity, the force spectroscopy indicated that the fabricated composite membranes had fairly less attractive to crude oil compared with the PTFE membrane. Our research results suggest that the novel composite membranes with asymmetric wettability were competent to serve as an anti-oil-fouling MD membrane for desalinating challenging saline and oily wastewaters.
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•Novel anti-oil-fouling composite membranes were fabricated for robust MD.•The modified fibrous coatings of hydrophobic PTFE substrate were superoleophobic.•Composite membranes showed superior anti-oil-fouling performance in MD application.•Oil-droplet force probe was introduced to evaluate membrane fouling propensity.
A novel omniphobic membrane was fabricated for anti-surfactant-wetting membrane distillation (MD) by electrospinning hybrid nanofibers comprising cellulose acetate (CA) and silica nanoparticles ...(SiNPs) to create hierarchical re-entrant surface structures, followed by surface fluorination to lower the surface energy of the fibrous membrane. The effects of surface morphology and surface energy on membrane wettability were investigated through evaluating wetting resistance to various liquids with different surface tension, it was demonstrated that both ultralow surface energy and hierarchical re-entrant surface structure were indispensable for membrane wetting resistance to low-surface-tension liquids. Specifically, the secondary re-entrant structure induced by SiNPs inlayed in the cylindrical nanofibers played a critical role in achieving surface omniphobicity. The fabricated omniphobic fibrous membrane exhibited superior anti-wetting properties, as evidenced by the contact angles for water and decane, which were as high as 155.6 ± 3.9° and 95.3 ± 2.5°, respectively. Compared with the hydrophobic membranes, the omniphobic membrane possessed robust wetting resistance to the low-surface-tension feed and maintained stable permeate flux and salt rejection during direct contact MD experiments, while all the tested hydrophobic membranes can be easily wetted. Our results suggest that the fabricated omniphobic membrane is promising and feasible for MD desalinating the wastewaters with low-surface-tension contaminants.
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•A novel fibrous omniphobic membrane for anti-surfactant-wetting MD was fabricated.•Effects of surface energy and morphology on membrane wettability were investigated.•Hierarchical re-entrant structure and surface fluorination created omniphobicity.•The omniphobic membrane showed superior anti-surfactant-wetting performance in MD.
In this study, we developed a new type of composite membrane to mitigate oil fouling in membrane distillation (MD). The composite membrane consists of a polytetrafluoroethylene (PTFE) hydrophobic ...substrate and a hydrophilic poly(vinyl alcohol)/silica nanoparticles (PVA-Si) hybrid fibrous coating prepared via sol-gel and electrospinning. We characterized the pristine PTFE membrane and the modified membrane using contact angle measurements and tensiometer-based oil probe force spectroscopy. While the hydrophilic coating presented excellent oil fouling resistance, the glutaraldehyde cross-linking treatment augmented the in-air hydrophilicity and underwater superoleophobicity of the electrospun coating surface. The cross-linking treatment also prevented the formation of PVA-Si hydrogels for the electrospun hybrid fibers, which made availability to utilize the water-soluble polymer to modify hydrophobic membrane for anti-oil-fouling MD. By comparing the performance of the composite membrane and the pristine PTFE membrane in MD desalination experiments using a saline emulsion with 1000mg/L crude oil, it showed that the fabricated composite membrane was significantly more resistant to oil-fouling compared to the PTFE membrane. The results from this study suggest that underwater superoleophobic coating can effectively mitigate oil fouling in MD, and that the fabricated composite membrane can enable MD to desalinate hypersaline wastewater with high concentrations of hydrophobic contaminants.
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•A dual-layer composite membrane was fabricated to mitigate oil fouling in MD.•The electrospun hybrid fibrous coating was underwater superoleophobic.•Glutaraldehyde cross-linking treatment augmented oil-repellency of the coating.•Oil probe force spectroscopy can quantitatively evaluate oil-membrane interaction.
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