Stealth Performance Research Articles

Multi-Objective Optimization Design of Adaptive Cycle Engine with Serpentine 2-D Exhaust System Based on Infrared Stealth

In the overall design process of the turbofan engine, it has become crucial to address the challenge of selecting design parameters that not only meet the flight thrust demand but also enhance engine economy. As the demand for stealth performance in future fighter aircraft increases, it becomes imperative to consider infrared stealth indicators during the design process. The adaptive cycle engine possesses an adjustable thermal cycle, necessitating careful attention to the selection of design parameters to fulfill the requirements. Therefore, this paper proposes a multi-objective optimization design method for the adaptive cycle engine that integrates infrared stealth technology. Initially, the parameter cycle model of the adaptive cycle engine is established based on the principles of aerodynamic and thermodynamic calculations. Subsequently, the model incorporates a serpentine two-dimensional (2-D) exhaust system to achieve infrared suppression. Meanwhile, a method for predicting the infrared characteristics is proposed to calculate the infrared radiation intensity of the engine exhaust system. Finally, the sequential quadratic programming algorithm is applied to comprehensively optimize the engine’s performance. The simulation results reveal that the multi-objective optimization design can effectively select appropriate design parameters to im-prove the engine, thereby reducing fuel consumption while meeting thrust requirements. This approach combines the consideration of infrared stealth technology with the optimization of engine performance, thus contributing to the development of advanced adaptive cycle engines.

Underwater sound-absorbing performance of composite structure under hydrostatic pressure by considering constitutive models

Underwater vessels face significant challenges as the high hydrostatic pressure of deep water affects their stealth performance. Linear models may not accurately depict the deformation of structures under hydrostatic pressure due to its non-linear properties of rubber and therefore, influence the results. This study utilized uniaxial tension and compression test data of styrene-butadiene rubber to fit four constitutive models and the optimal was selected to calculate the sound-absorbing performance under hydrostatic pressures,. Results showed that the Yeoh model fitted by uniaxial compression with r2 of 0.9928. Viscous losing, waveform conversion and locally resonance are main sound-absorbing mechanisms. The maximum sound-absorbing coefficient of structure at 0 MPa is 0.8914 at 7.6 kHz and the sound-absorbing performance mostly decreases with increasing hydrostatic pressure, Maximum reduction of 32 % was achieved at 3.2 kHz. Experiments verified the accuracy of simulation results, and the mean relative errors between the simulated results and the experimental results are 15.28 % (0 MPa), 16.91(0.5 MPa) %, 16.67 % (1 MPa) and 16.38 % (2 MPa), respectively. These findings can provide guidance for development of underwater sound-absorbing structures in hydrostatic pressure conditions.

Study on noise induced by cavitation flow in orifice plates

In water supply pipelines, the installation of multiple orifice plates as throttling components can lead to significant noise generation, caused by the cavitation flow downstream, which in turn, can negatively impact the stealth performance of ships and human health. This study aims to investigate the noise reduction characteristics of orifice plates used for throttling. For this purpose, a test rig was established to measure the noise generated by cavitation flow through orifice plates with varying porosity rates, along with the corresponding pressure losses. Additionally, Large Eddy Simulation (LES) and the Zwart-Gerber-Belamri (ZGB) cavitation model were used to numerically simulate the cavitation flow through orifice plates with a porosity rate of β = 0.4. The results show that the pressure loss coefficient is approximately inversely proportional to the porosity rate and follows a power function with the Reynolds number, with the exponent closely related to the porosity rate. During the cavitation stage, the sound pressure level significantly increases in the high-frequency range of 500–8000 Hz, exhibiting broad-spectrum characteristics. During the initial cavitation, bubbles are mainly confined to the surface of the orifice wall. Nevertheless, under conditions of intense cavitation, the generation of bubbles in the shear layer and trail-flowing region is induced by vortex cavitation. Moreover, cavities form at the intersection of the jet and shear layer, which extend towards the orifice plate. To optimize the noise reduction performance of orifice plate throttling components, it is advisable to maintain the porosity rate within the range of 0.40–0.49.

A wideband acoustic cloak based on radar cross section reduction and sound absorption

In this paper, by analogy with the theory of radar cross section (RCS) reduction in electromagnetics, we propose a design method for an acoustic invisibility cloak, which can achieve wideband acoustic stealth performance. The cloak consists of two sets of subwavelength metasurface units with phase differences within a range, which can achieve good RCS reduction in a wide band. At the same time, it overcomes the shortcomings of narrowband and high directional dependence of previous metasurface stealth cloaks based on phase gradient distribution. Using a simple resonant cavity metasurface structure, the invisibility cloak based on wideband scattering reduction and sound absorption is realized. Because of its simple structure and wide working frequency band, the RCS stealth cloak has wide engineering application potential.

Design Optimization of Lambda-Wing Planform and Vortex Generators for Longitudinal Instability Alleviation

This paper focuses on optimizing the planform and vortex generators (VGs) to improve longitudinal stability of the lambda-wing aircraft by alleviating pitchup. The optimizations are performed in two stages using a Reynolds-averaged Navier–Stokes (RANS) solver that can accurately capture the vortical flow structure, affecting the pitchup. First, the planform configuration is optimized to minimize the rise in the pitching moment while maintaining aerodynamic and stealth performances. The designed planform delays the pitchup by 4 deg and increases the usable lift by 31% due to the leading-edge vortex (LEV) flow over the outboard wing. Second, the VGs are installed and optimized to reduce the sudden increase in the pitching moment at high angles of attack. The designed VGs partially eliminate the separated flow and recover the LEV on the outboard wing, suppressing the radical change in the pitching moment by 75%. Some quantitative difference in aerodynamic coefficients is observed in unsteady RANS computations, but the vortical flow unsteadiness minimally affects the flow structure, and the stability improvement remains over 80%. Overall, the generation and sustainability of the LEV are critical aerodynamic factors to secure longitudinal stability in designing the lambda-wing aircraft.

Control method of line spectrum excitation force for pump-jet propeller: Random unevenly spaced rotor blades

The control of the excitation force of pump-jets is of great significance for the stealth performance of submarines. In a previous study, the authors found that the sinusoidal unevenly spaced rotor blades (SUSR) can control the excitation force of the rotor, but the suppression effect is limited by the low degree of freedom. This paper proposes random unevenly spaced rotor blades (RUSR) with higher degrees of freedom, achieving twice the suppression effect of the SUSR. A method for batch generation of RUSRs is proposed in this paper, and a large number of RUSRs are generated. The numerical simulation study was carried out by selecting the RUSRs with better suppression effects through theoretical methods, and the numerical simulation method used was improved based on the previous study. The RUSR also has an impact on the excitation force of the duct. The conclusion of this paper has guiding significance for the low excitation design of pump-jets and has the potential to promote the use of unevenly spaced rotor blades in the field of pump-jets.

Research on the Modified Echo Highlight Model for Underwater Vehicles with Combined Structures

Abstract The highlight model is widely used as a simple and convenient method in the radar field but its accuracy is not high. Based on the traditional highlight model, the algorithm has been improved to address the acoustic scattering problems of underwater vehicles with more complex line shapes. The basic idea is to partition the model into micro-bodies to calculate the scattered sound pressure, consider the phase interference of each part, and then synthesise the scattered sound pressure to approximate the target’s actual shape. A computational model of the wedge-shaped convex structure on the back of the underwater vehicles is developed using a highlight model of a trapezoidal plate. The results of the calculations using the highlight model approach are consistent with those of the planar element method. Utilising the modified highlight model method, the accuracy of acoustic scattering characteristics calculations for the stern and overall structures of underwater vehicles has proven satisfactory. Finally, fast acoustic scattering prediction software is developed for underwater vehicles, enabling the calculation of the acoustic scattering characteristics for individual structures, combined structures, and coated silent tiles. This software provides algorithmic support for the fast prediction of the acoustic stealth performance of underwater vehicles.

3D Architectural MXene‐based Composite Films for Stealth Terahertz Electromagnetic Interference Shielding Performance

AbstractThe terahertz frequency range is gaining popularity in security, stealth technology, and the future 6G network communication. For the control of severe terahertz electromagnetic interference (EMI) pollution, frequency‐selective stealth‐capable shielding materials are being explored to mask terahertz signals. For the realization of masking terahertz signals, the robustness, lightweight, and shape‐conformable materials with excellent terahertz EMI shielding/absorption are crucial. Here, the study reports the fabrication of 3D symmetric pyramidal architectural MXene composite films with frequency‐selective stealth performance characteristics via the facile drop casting method. With the high absorption capability of 2D MXene layers, the MXene composite films exhibit substantial terahertz stealth performance. 3D pyramidal microstructure design leads to frequency selective surface‐assisted reflection resonance in the frequency range of 0.6–1.1 THz. The MXene composite film demonstrates an outstanding maximum terahertz shielding effectiveness (SE) of up to 70.4 dB and a specific SE of 0.55 dB µm−1. These terahertz SE values exceed all of those for MX‐based shielding material designs reported in the literature. The investigation will open a new direction toward developing terahertz EMI shielding thin films with easy integration into any surface for stealth capabilities.

Flight control of a flying wing aircraft based on circulation control using synthetic jet actuators

To achieve the nice stealth performance and aerodynamic maneuverability of a Flying Wing Aircraft (FWA), a longitudinal aerodynamic control technology based on circulation control using trailing-edge synthetic jet actuators was proposed without the movement of rudders. Effects on the longitudinal aerodynamic characteristics of a small-sweep FWA were investigated. Then, flight tests were carried out to verify the control abilities, providing a novel technology for the design of a future rudderless FWA. Results show that synthetic jets could narrow the dead zone area, improve the flow velocity near the trailing edge, and then move the trailing-edge separation point and the leading-edge stagnation point downwards, which make the effective Attack of Angle (AOA) increase, thereby enhancing the pressure envelope area. Circulation control based on synthetic jets could improve the lift, drag and nose-down moment. The variations of lift and nose-down moment decrease with the growth of AOA caused by the improved reverse pressure gradient and the weakened circulation control efficiency. Finally, synthetic jet actuators were integrated into the trailing edge of a small-sweep FWA, which could realize the roll and pitch control without deflections of rudders during the cruise stage, and the maximum roll and pitch angular velocity are 12.64 (°)/s and 8.51 (°)/s, respectively.

Layered metamaterials with Sierpinski triangular fractal metasurface: Compatible stealth for S-band radar and infrared

Due to the completely opposite stealth mechanisms of radar and infrared waves, and the limitation of the Planck–Rozanov limit, compatible stealth of S-band radar and infrared is extremely difficult to realize at thin thicknesses. In order to solve the problem, a layered metamaterial consisting of infrared stealth layer (IRSL) with aluminum foil and radar stealth layer (RASL) with carbonyl iron powder is designed. The excellent infrared stealth performance (infrared emissivity < 0.3) is achieved because of the low infrared emissivity aluminum foil and reasonable area design. Meanwhile, the Sierpinski triangular fractal structure of IRSL not only serves to reduce the reflection of radar waves, but also introduces the electromagnetic double resonance mechanism of one-dimensional electric resonance and two-dimensional magnetic resonance to lose waves. Therefore, the effective absorption (RL < −10 dB) of 2.12–3.5 GHz in S-band is achieved with a thickness of only 2.24 mm. The double-layer model with fractal structure in this work realizes the perfect compatible stealth for S-band radar and infrared at extremely thin thickness, which provides a new design idea for the development of multi-band compatible stealth.

Dynamic radar jamming metasurface with amplitude and phase modulation

In this paper, a one-bit coding metasurface comprised of 20 columns is proposed to achieve amplitude and phase modulation. Each column is dynamically controlled by p-i-n diodes to modulate the incident electromagnetic waves. By synthesizing the code sequence, the radar cross section (RCS) of the surface can be reduced to specific values. With shift coding and inverse code sequence, the metasurface can also achieve phase modulation function with almost the same amplitude responses. A metasurface prototype is fabricated and measured, showing an obvious RCS reduction in a 35.2% bandwidth from 7 to 10 GHz. Specifically, a high RCS reduction of 23 dB can be achieved at 8.5 GHz. In addition, the metasurface exhibits the capability of phase modulation with responses of and , while the amplitude responses are almost the same. Overall, the proposed dynamic metasurface is useful for radar jamming, which is expected to further improve the stealth performance of the target.

Research on the Friction Noise Generation Mechanism and Suppression Method of Submarine Rubber-Based Propeller Bearings-A Review.

This article introduces the main mechanisms of friction noise generated by submarine rubber-based propeller bearings and analyzes their respective scope of application and limitations. Then, the research on suppressing friction noise through the optimization of the structure and improvement of materials of rubber-based propeller bearings is discussed. Finally, the article summarizes a promising research direction aimed at eliminating friction noise in submarine rubber-based propeller bearings. By improving the structure and materials, the friction noise of propeller bearings can be effectively suppressed, thereby improving the deterrence and stealth performance of submarines.

Underwater metagratings for sub-kilohertz low frequency and broadband sound absorption

The anechoic coating capable of absorbing sound energy in low frequency and broadband is essential to conceal underwater vehicles. In this work, metagrating has been demonstrated as a tunable quasi-perfect absorber in low frequency, which is constructed with the single layer viscoelastic medium embedded with the periodic cylindrical cavities and the steel plate. Upon effective medium approximation and genetic algorithm optimization, such tunable absorbers could be put forward via deep subwavelength thicknesses. Broadband underwater absorption would be further approached in the multi-layer metagratings while the longitudinal waves convert into shear waves efficiently via the local resonance coupling and multiple scattering effects. Distinct from previous studies that limit the configuration of anechoic coatings to a given law and inefficiently alter numerous geometric parameters for optimal acoustic performance, the metagrating comprising multi-layer voids with random periods is adopted to precisely modulate the surface impedance in the target spectrum, leading to quasi-perfect absorption performance in broadband. Our results will contribute to designing lightweight underwater absorbers to improve the stealth performance of vehicles.

An environmentally friendly chitosan-derived VO2/carbon aerogel for radar infrared compatible stealth

With the advancements in military detection technology, stealth materials that only operate in a single frequency band are no longer efficient in dealing with detection weapon interference. Hence, there is an urgent need to develop multifunctional material to achieve multi-band compatible stealth. In this study, a composite porous aerogel was synthesized utilizing a chitosan-derived carbon aerogel as the scaffold loaded with VO2 (VO2/CA), which demonstrated exceptional radar/infrared compatible stealth performance. Under the action of loss mechanisms conductivity loss, dipole polarization and interface polarization, the composite material can reach the minimum RL of −52.0 dB at a thickness of 2.4 mm, and the EAB at 1.6 mm is 5.2 GHz, covering almost the whole Ku band. Meantime, VO2/CA achieves excellent thermal stealth characteristic. In terms of infrared stealth, after the bottom is heated to 120 °C for 180 s, the surface temperature of the sample is only 2.6 °C. This work provides a feasible idea and scheme for the research of radar infrared stealth compatibility materials.

Mechanical Performance Analysis and Experimental Study of Four-Star-Type Crank-Linkage Mechanism

Mechanical performance analysis and experimental study of the mechanics of the four-star-type crank-linkage mechanism of the marine air compressor were carried out to improve the submerged floating performance and stealth performance of the underwater vehicle and to meet the demand of its large diving depth. Firstly, we analyzed the forces on the four-star-type crank-linkage mechanism, derived the inertia forces and moments of the crank-linkage mechanism, and proved the advantages of the four-star-type crank-linkage mechanism in balancing the second-order reciprocating inertia forces. The static strength of some parts was calibrated, and the modal analysis of the four-star-type crank-linkage mechanism was carried out. Second, a flexible crankshaft dynamics model was established to study the influence of kinematic pair parameters on the excitation characteristics of the main bearing. The mechanical performance analysis of the mechanics of the four-star-type crank-linkage with clearance was carried out. Finally, a test bench of four-star-type crank-linkage mechanism was designed and built independently to analyze the effects of clearance size and rotational speed on the acceleration of the motion and vibration acceleration of the slider. The results show that the first-order reciprocating inertia force and second-order reciprocating moment of inertia mainly exist in the four-star-type crank-linkage mechanism. The friction force of the revolving pair can suppress part of the resonance peak, but it will broaden the excitation band and excite high-frequency vibration. The larger the clearance of the four-star-type crank-linkage mechanism kinematic pair, the higher the crankshaft speed, the larger the acceleration amplitude, and the more concentrated the phase trajectory. The research results of this paper can guide the low-noise design of other types of air compressors, help to improve the overall level of marine air compressors, and show future directions for the vibration control of four-star-type air compressors.

Theoretical investigation on the underwater sound absorption of a functionally graded anechoic coating under the phased arc array incidence

Laying anechoic coatings on the surface of underwater vehicles is an effective technology to improve the acoustic stealth performance. Meanwhile, the detection waves may have complex directivity, and it is important to study their influence on the sound absorption of the coating. This article introduces a functionally graded coating under a phased arc array incidence, which simulates a novel anechoic coating on an underwater vehicle to absorb sound waves emitted by active sonars. The incident field of the phased array is constructed by spherical harmonic expansion and the superposition principle. The wave propagation of the functionally graded coating is analyzed through an equivalent laminated model and the reverberation matrix method. Utilizing the angular spectrum method, the reflected sound field and absorption coefficient of the coating under the phased arc array incidence are predicted and the theoretical method is formed. Then the selections of parameters in Fourier transforms of the angular spectrum method are discussed for achieving converged results. A finite element analysis is applied to verify the effectiveness and calculation speed of the proposed method through the establishment of detailed quarter or half model. The effect of phased arc array parameters and the coating on the absorption and reflection performances are further studied. The angular spectrum decomposition of the incident sound field explains the changing trends of the sound absorption curves. The present work provides an important way to deal with this complex underwater sound propagation problem.

A waste textiles-based multilayer composite fabric with superior electromagnetic shielding, infrared stealth and flame retardance for military applications

The challenge of developing multifunctional military tent fabrics with low-cost, stable electromagnetic shielding and infrared stealth in complicated environments is enormous. Herein, a novel aluminium-flammability carbonized waste cotton-carbon felt (A-FCWCF) composite fabric consisting of hydrophobic flame-retardant fabric, carbonized waste cotton felt and carbon fiber felt was prepared via a simple yet low-cost stitching process. The resulting composite fabric was extensively investigated for its stability and applicability in a complicated environment. Due to the significant conductivity and impedance mismatches, the A-FCWCF composite fabric displayed exceptional EMI-shielding properties, plus an X-band EMI rating of 82.63 dB. Moreover, the A-FCWCF composite fabric presented excellent infrared stealth performance at room and extremely cold outdoor temperatures, owing to its high porosity and low emissivity. The functional finishing on the A-FCWCF composite fabric also made it stable flame retardate and hydrophobic. Furthermore, the EMI-shielding and infrared stealth performances of A-FCWCF composite fabric were almost unaffected even if subjected to environmental stability, mechanical properties and weather resistance tests, making it a promising candidate for tough natural environments. Overall, this low-cost and simple preparation strategy of multi-layer stitched multifunctional fabric can revolutionize the production of military materials, improving functionality even in the most complex environments.

Enhancement of infrared stealth performance of ultra-high molecular weight polyethylene-based composites through the multilayer structural construction

According to the infrared stealth mechanism, combining the low emissivity material and temperature control material is an effective way to improve the infrared stealth performance. In this paper, ultra-high molecular weight polyethylene (UHMWPE)-based multilayer composites with low emissivity and thermal conductivity are prepared through the combination of the low emissivity layers and the thermal insulation layers. The infrared stealth performance of the composites is varied by changing the layer numbers and the thickness of the functional layers as well as the structural design. And the orientation of the low emissivity fillers is realized to further reduce the emissivity of the low emissivity layers. The infrared emissivity of the low emissivity layer is as low as 0.297 and 0.247 at 3–5 μm and 8–14 μm, respectively, which is 25.9% and 46.4% lower than that before modification. The surface temperature of the multilayer sample reaches 26.8 °C after being placed on a 60 °C hot stage for 30 min, exhibiting excellent infrared stealth performance. Meanwhile, the density of the sample is 0.48 g/cm3, and the thickness is 2.8 mm, which is permissible for the infrared stealth applications.

Fabrication of sandwich-structured infrared camouflaged and flexible anti-aging composite for thermal management

The design and preparation of infrared stealth materials that meet the application in complex environments is urgently demanded in military fields. In this paper, composites with low emissivity and thermal insulation are designed to be used for indoor and outdoor infrared stealth. Thereinto, nanoPE-MXene, yttrium oxide coated hollow glass microspheres (HGMs@Y2O3) and polypropylene nonwoven (PPF) were used as low emission, insulation, and antifouling layers, respectively. The sandwich-structured composite reduce the infrared radiation temperature of the 80 °C heat source object to 33.9 °C, and the average infrared emissivity of 3-15 μm is only 0.17. In addition, the infrared stealth performance of the composite is not affected by environmental changes. More importantly, the composite exhibits excellent anti-fouling and ageing resistance performance, which lays the foundation for the practical application of efficient stealth infrared materials.

Research on the Vibration Behavior of Ring–Block Friction Pair Made of Materials of Water-Lubricated Rubber Bearing under Special Operating Conditions

A water-lubricated rubber bearing (WLRB) is prone to generate frictional vibration noise under special operating conditions, which seriously affects the acoustic stealth performance of warships and threatens their navigation safety. Meanwhile, the main factor affecting the frictional vibration behavior of a WLRB is the materials of the friction pair. Therefore, this work selects a friction pair composed of a copper ring and a rubber block as the research object and studies the frictional vibration behavior of the ring–block friction pair under low-speed and starting conditions. The real friction coefficient curve is used to establish a transient dynamic finite element analysis model for the ring–block friction pair. The effects of the load, friction coefficient, and Young’s modulus on the frictional vibration behavior under special operating conditions are studied. The analysis’s results show that the frequency of the medium-high frequency friction-induced vibration disappears under low-speed operating conditions when the friction coefficient is below 0.1. During the startup process, even if the friction coefficient is very low, the medium-high frequency friction-induced vibration still exists. The research results provide ideas for future theoretical research and guidance suggestions for engineering practice.