Defence Science Journal
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Direction of Arrival Estimation Using Underwater Acoustic Vector Sensor Array Towards Coastal Surveillance Applications
Full text linkThe objective of this paper is to present the performance of Direction of Arrival(DoA) estimation algorithms for underwater sound source localization using an acoustic Vector Sensor Array (VSA) that is developed by the National Institute of Ocean Technology, Chennai. Algorithms such as conventional beam forming, Multiple Signal Classification (MUSIC) with Eigen value decomposition, and MUSIC with Singular Value Decomposition (SVD) are used for estimation of DoA and performance study. An experiment has been conducted with the VSA at the Acoustic Test Facility of NIOT with the source transmission of 1 kHz to 5 kHz for different azimuth angles. The estimation of DoA using the above three algorithms and the comparison of the results on resolution and accuracy have been studied in detail in terms of the number of vector elements. Results reveal that the MUSIC method gives results with higher accuracy and resolution than the conventional method. The maximum deviation from the true angle in the conventional method is 4°; in MUSIC, it is 2°, whereas in MUSIC with SVD, it is 1°. While the standard MUSIC algorithm involves computing the eigenvectors of the covariance matrix, which can be computationally expensive, MUSIC with SVD provides a more efficient way to achieve better results. SVD enables straightforward computation of the signal subspace, making it more practical for real-time applications like coastal surveillance. Further to the laboratory experiment, the vector sensor system has been deployed in an open sea environment near the harbor and a known source experiment is carried out. The DoA estimated using MUSIC with SVD for the field data reveals that the results are in good comparison with the measured azimuth and elevation positions. The deviations in the field results are due to dynamic conditions of the ocean ,and more sea trials need to be carried out for further study
Effect of Thermodiffusion on Non Premixed Flame in MILD Regime Using a Modified Reacting Solver
Full text linkNumerical simulations for moderate and intense low oxygen dilution (MILD) combustion with essential solvers and detailed mechanisms involve more complications and computational time. Various advanced combustion modeling techniques have recently been developed to study MILD combustion characteristics. However, every combustion model has specific issues predicting the temperature and emissions of the MILD combustion flames. The diffusive nature of the MILD flame is considered, and individual Lewis numbers are investigated on a non-premixed flame. The current study analyzes the methane/hydrogen flame propagation with different Lewis number combinations in a hot co-flow environment.Individual Lewis numbers for methane and hydrogen are investigated from stochiometric to ultra-rich mixtures in non-premixed flames. Several numerical simulations are performed in the OpenFOAM9 environment using a modified EDC model with tuned turbulence and combustion model constants. The numerical simulation results with hydrogen and methane Lewis numbers of 0.4 and 0.9, respectively, show promising agreement with the experimental findings of Dally et al. [1]. Various combustion parameters are studied with different CH4 and H2 Lewis number combinations. In addition, the unity Lewis number case is simulated and compared to the situations that are taken into consideration
Development of Micromechanics Based Constitutive Model for Alumina Using Unified Mechanics Theory Role of Microcracks in Damage
Full text linkCeramic materials used in mechanical applications show variations in their properties due to the difference in the presence of cracks and various defects. The micro-crack length, orientation, geometry and wing crack formation and propagation within the ceramic material define the strength of the ceramic material. In this study, a micro-mechanics-based model that accounts for micro-cracks is developed. Unlike other micromechanics-based models, the current model defines failure based on entropy. Entropy generated with various micro-crack lengths, orientations and wing crack extensions is calculated using the energy approach.The Unified Mechanics Theory (UMT) is used to define the damage in the ceramic material, which can include all possible failure mechanisms. A representative volume element (RVE) with a pre-existing flaw is simulated to generate stress-strain curves. The effect of different initial crack lengths and orientations on alumina peak strength is also investigated
Effect of Porosity on the Free Vibration Analysis of a Rotating Pretwisted Sandwich Blade with a Functionally Graded Core in the Thermal Environment
Full text linkThis work deals with the investigation of the effect of porosity on the natural frequencies of the rotating pretwisted sandwich blade with a functionally graded core in the thermal environment. The top metallic and the bottom ceramic surfaces are exposed to ambient temperature and high inlet temperature, respectively. A finite element approach using a layerwise theory is developed. Two different porosity distributions are assumed here. The effect of the volume fraction index, rotational velocity, porosity model and temperature gradient on the natural frequencies of the pretwisted sandwich blade is studied
Development of Curved Shape Diffuser Duct for Aero Engines With a Backend Centrifugal Compressor
Full text linkIn the present study, a design of the curved shape diffuser-duct is proposed to apply to the compressor stage for aero-engines with a backend centrifugal compressor. For these configurations, the flow exiting radially from the centrifugal impeller needs to be changed from radial-to-axial direction to meet downstream combustion chamber requirements. Hence, the specially designed exit duct with a curved shape to manage the flow from the radial-to-axial direction was later diffused using the diffuser-shaped exit duct. This study aims to design and develop an efficient diffuser-duct for the adequate deceleration of flow in the exhaust diffuser and, as a result, a uniform and low-velocity flow at the exit. This configuration comprises a radial-to-axial turning annular passage at the centrifugal impeller exit. Then, the passage is directed to either a purely axial direction or is inclined to the axial direction by a slight angle. The results indicate a uniform total pressure at the stage exit with marginal variation along the span. It shows equivalent end wall effects near both the shroud and hub surfaces. This endwall effect may be occurring mainly due to the growth of the boundary layer across the diffuser passage
Numerical and Experimental Analysis of the Influence of Projectile Impact Angle on Armour Plate Protection Capability
Full text linkThis research manuscript describes the process of degradation of an Armox 600 armour plate during the impact of a 5.56×45 mm SS109 projectile. The creation process of the numerical FEM model of the projectile is presented. The projectile impact angle is set between 15° and 90°, and this phenomenon is investigated via numerical and experimental approaches. The experiment is conducted under the same conditions as the numerical approach to validate the FEM model. The experiments are conducted using a high-speed camera. This research manuscript presents the influence of the projectile impact angle on the degradation of the armour plate and its protection capability for different angles. The results demonstrate the dependence of the transferred energy on the armour plate, speed of the particles after impact, and trace dimensions on the armour plate for different impact angles
High Strain Rate Modeling of CFRP Composite Under Compressive Loading
Full text linkAn in-depth understanding of how carbon fiber-reinforced plastics (CFRP) respond to intense strain rates is essential, particularly in non-linear deformation and dynamic loading situations. The researchers undertook a computational study to examine the behavior of CFRP composites when exposed to high strain rates under compressive loading. Specifically, they employed Split Hopkinson Pressure Bar models for cohesive interfacial simulations, continuum shell analysis, and laminated composites oriented at 0° at a strain rate equivalent to 900 s-1. The Finite Element model utilized a custom Hashin damage model and a vectorized user material (VUMAT) sub-routine to identify degradation damage within the CFRP composite model. The quasi-isotropic composite demonstrated a significant enhancement in dynamic strength compared to static values, attributed to its intense sensitivity to strain. As confirmed by experimental test results, numerical simulations accurately predicted stress (σ)-strain (ε) and strain rate (ἐ) curves. Additionally, it was observed that the relationship between damage behavior varied depending on the element type used
Identification of Hand Tremor Levels in Shooting Activities Under Different Shooting Positions Using a Low Cost and Portable System
Full text linkThe accuracy level is important in shooting activities and depends on many factors, such as hand tremors as body vibration and shooting position. Achieving high accuracy in different shooters is challenging, especially in the case of different shooting positions. However, there is a lack of information about the influence of shooting positions and experiences on a shooter’s body vibration and accuracy levels. Thus, this study aims to develop a portable and low-cost hand tremor measurement device (as a function of body vibration) to identify the influence of hand movement on shooting accuracy. For this purpose, low-cost accelerometer sensors and a microcontroller were used as the measurement kit. Three different shooting positions (squatting, standing, and prone) were analyzed. The shooters were classified into novice and expert groups. Each group had five participants with standard fire guns and accelerometer kits. These participants were asked to shoot the target to get their best accuracy. Besides, the hand tremor level data from the self-developed kit were recorded to investigate the hand tremors. The results show that the novice participants have more hand tremors in all shooting positions. There are significant differences between the squatting, standing, and prone positions in hand tremors for novice and expert participants. In the expert group, the prone and squatting positions have the least vibration level, indicated by the least acceleration (0.01 - 0.04 m/s2 for the expert group and 0.02 - 0.11 m/s2 for the novice group). The best accuracy for all positions is also obtained from expert shooters. It can be concluded that different shooting positions are related to the body vibrations. The expert shooters have a lower body vibration than the novice participants. The hand tremor levels may influence the accuracy level since different shooting positions and experiences have different vibration and accuracy level
Unveiling the Impact of Extreme Learning Machine in the Defence and Military Sector
Full text linkAmong the most well-known machine learning algorithms, Extreme Learning Machine (ELM) has seen widespread use across a variety of fields, including the defence and military industries. For problems like sluggish technique and iteratively altering the hidden layer’s parameters to optimise the efficiency of the gradient descent approach, a cutting-edge machine learning algorithm known as the ELM has been developed. Depending on the specific objective and circumstance, the Extreme Learning Machine (ELM) may be more appropriate than Deep Neural Network (DNN) techniques. The models constructed in this manner perform quite well in generalisation. The following three goals are emphasised in its unconventional structure: 1) A great degree of accuracy in learning 2) less need for direct human involvement 3) an extremely rapid rate of learning, and moreover, it provides an optimal response for the whole world. As a result of its quick training, flexibility, and resilience, the Extreme Learning Machine (ELM) has several uses in this field, including target detection and tracking, image and signal processing, cybersecurity and intrusion detection, decision support systems, pattern recognition and classification, etc. According to our findings, the ELM approach was used with low training time and the testing accuracy is excellent. Also, this study presents the contribution of the revolutionary machine learning algorithm ELM to the defence and military sectors
Impact of Blowing Location on the Aerodynamic Characteristics Over the Delta Wing
Full text linkThe performance of an aircraft can be enhanced by altering the flow field favourably by adopting flow control techniques. The present study deals with the application of the active flow control methods on a sharp-edged delta wing with a wing sweep of 65°. The concept of blowing was employed as an active flow control technique. The blowing technique is applied on the suction surface of the delta wing by varying its location. The various identified locations of the blowing holes are 1.62 %, 3.24 % and 4.86 % of root chord from the leading edge to the centre of the blowing holes. The computation is performed using the commercial software ANSYS Fluent. An unsteady, incompressible Reynolds-averaged Navier–Stokes equation and the shear-stress transport k-ω turbulence model are employed. The angles of attack varied in the range of 0°<α<35° and Reynolds number is 2.64×106 and the jet momentum coefficient is fixed at 0.05. The blowing of air from the injection region enhances the strength of the leading-edge vortices, resulting in a delay in the vortex breakdown. The performance of the delta wing is greatly improved while using the blowing method specifically for the blowing holes located at 3.24 % of root chord from the leading edge compared to without the blowing method