Elastic Targets Research Articles

Finite Element–Boundary Element Acoustic Backscattering with Model Reduction of Surface Pressure Based on Coherent Clusters

Computing backscattering of harmonic acoustic waves from underwater elastic targets of arbitrary shape is a problem of considerable practical significance. The finite element method is commonly applied to the discretization of the target; on the other hand, the boundary element method naturally incorporates the radiation boundary condition at infinity. The coupled model tends to be expensive, primarily due to the need to manipulate large, dense, and complex matrices and to repeatedly solve systems of complex linear algebraic equations of significant size for each frequency of interest. In this article, we develop a model reduction transformation based on the notion of coherence applied to the surface pressures, which considerably reduces the size of the systems to be solved. We found that the proposed model reduction approach delivers acceptably accurate results at a fraction of the cost of the full model. A typical speedup of an order of magnitude was realized in our numerical experiments. Our approach enables backscattering computations with considerably larger models than have been feasible to date.

Automatic CHIEF Point Selection for Finite Element–Boundary Element Acoustic Backscattering

Computing the backscattering of harmonic acoustic waves from underwater elastic targets of arbitrary shapes is a challenging problem of considerable practical significance. The finite element method is well suited for the discretization of the target, while the boundary element method addresses the radiation boundary condition at infinity. A disadvantage of the boundary integral method is that it yields non-unique solutions at certain wavenumbers. This failure is associated with the existence of eigensolutions of the Helmholtz equation in the interior of the complement of the fluid domain (acoustic modes). The combined Helmholtz integral equation formulation (CHIEF) credited to Schenk is employed to combine the surface Helmholtz boundary integral with equations of the interior Helmholtz relation written down at selected points within the cavity of the scatterer (i.e., in the complement of the fluid domain).The difficulty associated with this approach has always been the lack of guidance on the necessary number of interior points and on their locations. The solution to this problem proposed here is to compute the acoustic modes using the finite element method to complement of the fluid domain and to identify locations of the peaks.This novel approach aids the decision as to how many points should be employed and where they should be located. Our numerical experiments demonstrate the robustness of the proposed automatic selection of the CHIEF points’ numbers and locations.

Validation and verification of three-dimensional finite element target scattering models and Green's functions

A configurable three-dimensional (3D) finite element framework is used for modeling acoustic scattering from various elastic targets on seabed and for the numerical determination of Green's functions used in far-field target scattering prediction. The 3D selection is chosen for possible inclusion of complex seafloor geometries, such as rough, rippled, cluttered, or variably layered seabeds, in which two-dimensional modeling approaches cannot capture the full acoustic interaction with the target and the seabed. Model results are verified and validated against analytic scattering theories and measured results from the 2013 Target and Reverberation Experiment (TREX) and the 2017 CLUTTEREX experiment.

Focusing late-time returns from elastic targets in bistatic collection geometries

Targets encountered during synthetic aperture sonar (SAS) surveys may exhibit elastic scattering behavior and re-radiate sound after initial interrogation. These re-radiated returns are often described as “late-time” energy, as they reach the sonar after the initial geometric returns. Range-specific focusing methods can enhance acoustic features such as late-time energy, but these methods typically make a monostatic sonar approximation that may not hold in very shallow water sensing geometries. This presentation will discuss the development of a late-time reconstruction algorithm applicable to bistatic sonar geometries. Using both simulated and experimental sonar data from monostatic and bistatic collection geometries, this work will first quantify the differences between traditionally beamformed imagery and late-time-focused imagery using common image quality metrics. Next, we will quantitatively compare the impact of collection geometry on the performance of late-time focusing algorithms. Finally, the work will describe methods that use information from a multi-static collection to compress late-time energy down to a high-contrast, spatiallycondensed point. The analysis of these signal processing strategies will focus on their potential applications to automatic target recognition.

Data-model comparison of elastic target scattering for a shallow water sonar system for buried UXO detection

Successful buried unexploded ordnance (UXO) detection in shallow water (<3 m) has been demonstrated using a sonar system deployed from a shallow-draft surface vessel. Data was collected over several years and during different seasons in a freshwater reservoir near Howard, PA, USA. The target test field contains a variety of science (e.g., sphere, cylinder) and other objects. Previously, a point-based sonar scattering model that simulates calibrated time-series sonar data due to sediment surface and volume scattering effects was shown to provide simulated data of the environment. Recently, object scattering models have been added to the simulation. The results of a model-data comparison of objects in the environment will be presented. Realistic simulation results containing objects will increase understanding of detection capability in different geometries (e.g., depth) and environments (e.g., sediments, layering).

3D finite element modeling techniques and application to underwater target scattering

Underwater acoustic target scattering measurements rely on high fidelity modeling for experimental comparison and understanding. Three-dimensional (3D) finite element models are well suited for this purpose, as they can account for arbitrary or unknown target properties and configurations/orientations within complex and asymmetrical seafloor environments. High acoustic frequencies and large physical distances associated with in situ scattering measurements pose challenges to 3D modeling efforts in terms of model sizes and runtimes. Certain model considerations must be made to keep the 3D model computationally efficient, yet accurate in predictive capability. Numerically determined Green’s functions are demonstrated to permit 3D model reduction, while still preserving far-field scattering prediction capability through the Helmholtz-Kirchhoff integral. By determining Green’s functions within the model, they need not be known or estimated for complex ocean environments a priori. Nontraditional scattering formulations and a survey of boundary truncation methods also are explored and implemented for maximal accuracy within small 3D computational domains. Model results for canonical elastic targets within varying seafloor environments are shown and compared to theory and experimentation. [This work has been supported by the Strategic Environmental Research and Development Program, and by the Office of Naval Research, Ocean Acoustics.]

Liquid droplet impact pressure on (elastic) solids for prediction of rain erosion loads on wind turbine blades

Leading edge erosion of wind turbine blades is causing high maintenance cost and therefore higher levelized cost of energy (LCOE). In order to lower the LCOE and to design reliable blades, the causes of rain erosion have to be understood. Traditionally, the (modified) waterhammer pressure equation is used to determine the contact pressure between the liquid droplet and the (elastic) target. This equation however is independent of time, droplet size and distance to the center of impact which have been identified as important parameters. This paper focuses on the fundamental impact mechanisms and proposes a numerical model that conforms to these mechanisms to determine the dynamic contact pressure between the liquid droplet and the elastic target. The effect of variations in droplet diameter, impact velocity and target elasticity on the resulting contact pressure profiles was investigated. The output of the model can be used for low cost 2D axisymmetric finite element simulations to determine the mechanical performance of coating systems for wind turbine blades and allows for material and geometry optimization.

SOSIALISASI PENDIDIKAN SEKSUAL TERHADAP REMAJA OLEH KELUARGA DESA GELAP KECAMATAN LAREN KABUPATEN LAMONGAN

The increase in cases of premarital pregnancy and cases of submission is very difficult for parents in Desa Gelap to protect their children from deviant sexual behavior. This issue is a point of interest discussed by analysts. The purpose of this study was to find out what types of socialization of sex training for young adults in the elastic target family in the Gelap Village, Laren District, Lamongan Regency. To test this information, his research analyst used George Herbert Mead's role theory. The methodology in this research is qualitative which is a kind of contextual analysis. This research uses a process of observation and in-depth interviews with 6 informants. The validity of the data utilizes a triangulation strategy. The information obtained was investigated using Miles and Huberman's intuitive examination model with three stages, namely information decline, information display, withdrawal/final confirmation. Considering the impact of the examination in the field of socialization of young adult sex training carried out by parents in the Dark Village, the type of primary socialization regarding adolescent sex is separated into three aspects, in particular 1) biological socialization, presenting actual and new changes, things that can be done by children -children 2) socialization of sexual norms and boundaries, showing children there are standards that must be maintained as human beings 3) socialization of social roles and functions, helping children to experience their work as women and men. Barriers experienced by parents to foster free association are the association of children and the use of cell phones.

Inferring the characteristics of marine sediments from the acoustic response of a known, partially buried object.

We investigate the feasibility of using a known elastic target located near the seabed for the purpose of inferring characteristics of marine sediment. In the problem considered the object position and its burial depth are not known with precision. First, the admittance matrix of the elastic object is determined (numerically or experimentally) over a wide frequency range in the structural acoustic regime. Then, the equivalent source method (ESM) coupled with a spectral representation of the Green's functions in stratified domains is used to predict the object acoustic signature in various environments and experimental configurations. The resulting solver takes into account all multiple scattering between target (buried or not), sea floor, and sea surface and is not limited to short distances. After presenting the solution to the forward problem several synthetic inversions for sediment characteristics are shown. They are based upon a resonance-based misfit function we describe. The Bayesian procedure also infers object burial and source-object range, broadening its range of application.

Simulating elastic targets for sonar algorithm development

Targets interrogated during synthetic aperture sonar (SAS) applications may exhibit elastic scattering behavior and re-radiate sound after the initial geometric returns. This behavior can be a discriminating feature in target identification, but may also obscure nearby targets and make the full scene difficult to comprehend. Advancing algorithms to leverage elastic scattering for target localization and identification requires data with accurate ground truth. There is a shortage of field data containing elastic targets with sufficient quality for algorithm development. Stanton’s 1984 model presents the scattering amplitude of an elastic cylinder as a function of frequency and spherical angles. This presentation uses Stanton’s scattering model to simulate the time series of pressure scattered by the cylinder. This approach allows the target scattering model to be readily coupled with time series models for the environment to generate high fidelity simulated data. Simulated time series are generated for SAS collection geometries. The presentation will explore this modeling approach as a substitute for field data during initial stages of algorithm development for improved target localization. It will investigate the impact of elastic effects on imagery and discuss the limitations of methods used to isolate such effects.

3D finite element modeling techniques and application to underwater target scattering

Underwater acoustic target scattering measurements rely on high-fidelity modeling for experimental comparison and understanding. Three-dimensional (3D) finite element models are well suited for this purpose, as they can account for arbitrary or unknown target properties and configurations/orientations within complex and asymmetrical seafloor environments. High acoustic frequencies and large physical distances associated with in situ scattering measurements pose challenges to 3D modeling efforts in terms of model sizes and runtimes. Certain model considerations must be made to keep the 3D model computationally efficient, yet accurate in predictive capability. Numerically determined Green’s functions are demonstrated to permit 3D model reduction, while still preserving far-field scattering prediction capability through the Helmholtz–Kirchhoff integral. By determining Green’s functions within the model, they need not be known or estimated for complex ocean environments a priori. Nontraditional scattering formulations and a survey of boundary truncation methods also are explored and implemented for maximal accuracy within small 3D computational domains. Model results for canonical elastic targets within varying seafloor environments are shown and compared to theory and experimentation. [Work supported by the Strategic Environmental Research and Development Program and by the Office of Naval Research, Ocean Acoustics.]

Ultrasonic horn contact-induced transient anharmonic resonance effect on vibro-thermography

We present some new insights into the impact-separation contact dynamics induced spatial localization of anharmonic in a cavity-like feature. Origin of different nonlinear anharmonics is identified with the help of an inelastic/plastic collision signal model, a nonlinear Duffing oscillator model, and a detailed 3D finite element simulation, with increasing granularity and physical assumptions regarding contact dynamics. A rigid contact on the target elastic structure generates super-harmonics when the target structure resonates precisely with the excitation frequency. It generates sub-super harmonics when there is a slight mistuning in the motion of the target structure. The response amplitude and the energy in the primary excitation frequency reduce due to the energy partitioning over the different anharmonics generated. The ratio of Power Spectral Density (PSD) of primary harmonics to input excitation decreases almost linearly with decreasing separation distance between the actuator horn tip and the stationary target. The ratios of PSD of all anharmonics to that of primary excitation decreases quadratically with decreasing the relative amplitude of an elastic target structure moving in phase with the actuator. Different anharmonics further evolve as elastic waves in the target structure. The non-stationary vibration at the beginning of the excitation governs the evolving patterns of heat generation, which is of great interest in applications such as vibro-thermographic inspection. The effect of the initial transient state on time required to reach the steady-state excitation is analyzed, and related computational aspects are discussed. Spatio-temporal anharmonic modal distribution and the thermal signature obtained from numerical simulations are compared with the experimental results. The results strongly correlate the anharmonic modal contribution with the transient thermal signature due to a cavity-like feature in the target structure.

Infering the characteristics of marine sediments from the acoustic response of a known object

The acoustic response of an object depends on the surrounding environment. We show here how to take advantage of a known elastic target located near the seabed to infer characteristics of marine sediment. The approach consists of several steps. First, the structural admittance matrix of the elastic object is determined (numerically or experimentally). Then, the Equivalent Source Method (ESM) coupled with a spectral representation of the Green's functions in stratified domains makes it relatively inexpensive in terms of computational time to predict the object acoustic signature in various environments and experimental configurations. The resulting solver takes into account all multiple scattering between target (buried or not), sea floor and sea surface and is not limited to short distances. After presenting the solution to the forward problem we then show several synthetic inversions for sediment characteristics.

Soft impact of an elongated elasto-plastic missile

Motivated by the grave consequences of an aircraft impact into robust engineering structures like nuclear power plants, we investigate the time-dependent reaction force during the impact of a crushing, elongated elasto-plastic missile into a robust elastic target. We derive a set of partial differential equations on a time-dependent domain to describe the impact. The simplest case of a homogeneous, cylindrical aircraft fuselage impacting a rigid target is solved analytically by using conformal mapping. After developing a numerical scheme for the case of an elastic target, we find that a simple dimensionless number, the damage potential, gives an accurate prediction of the impact force. The peak reaction force during the impact is found to exceed the initial impact force by up to 20%; hence the often applied strategy to estimate the peak force as the initial impact force is not a conservative approach. This effect is due to a resonant vibration in both the missile and the target. Predicted and computed parameters at which this counter-intuitive, resonance-like behavior occurs are shown to agree well.

Prediction of Sound Scattering from Deep-Sea Targets Based on Equivalence of Directional Point Sources

A fast prediction method is proposed for calculating the sound scattering of targets in the deep-sea acoustic channel by equating the sound scattering field of a complex elastic target to the acoustic field excited by a directional point source. In deep-sea conditions, the effects of the sea surface on the impedance characteristics of the elastic target surface can be ignored. Through the finite element simulation of the acoustic scattering of the target in the free field, the sound scattering field is equated to the radiation field of a directional point source. Subsequently, the point source is placed in the channel, and the acoustic ray method is used to calculate the distribution of the scattering field. On the basis of theoretical modelling, the method of obtaining the directional point source and the influence of the sea surface on the impedance of the scattering field are analysed. Subsequently, the proposed method is compared with the finite element method in terms of computational efficiency. The result shows that the method considers the multiple complex coupling effects between the elastic structure and marine environment. The influence of the boundary is approximately negligible when the distance from the ocean boundary to the elastic structure is equal to the wavelength. The method only performs finite element coupling calculation in the free field; the amount of mesh size is greatly reduced and the calculation efficiency is significantly improved when compared with the finite element calculation in the entire channel, the. The calculation time in the example can be reduced by more than one order of magnitude. This method organically combines the near-field calculation with acoustic ray theory and it can realise the rapid calculation of the large-scale acoustic scattering field in complex marine environments.

Damage potential: A dimensionless parameter to characterize soft aircraft impact into robust targets

To investigate numerically the effect of all parameters on the outcome of an aircraft impact into robust engineering structures like nuclear power plant containments is a tedious task. In order to reduce the problem to a manageable size, we propose a single dimensionless parameter, the damage potential, to characterize the main features of the impact. The damage potential, which is the ratio of the initial kinetic energy of the aircraft to the work required to crush it, enables us to find the crucial parameter settings that need to be modelled numerically in detail. We show in this paper that the damage potential is indeed the most important parameter of the impact that determines the time-dependent reaction force when either finite element (FE) modelling or the Riera model is applied. We find that parameters that do not alter the damage potential, like elasticity of the target, are of secondary importance and if parameters are altered in a way that the damage potential remains the same then the course of the impact remains similar. We show, however, that the maximum value of the reaction force can be higher in case of elastic targets than in case of rigid targets due to the vibration of the target. The difference between the Riera and FE model results is also found to depend on the damage potential.

The Low-Frequency Acoustic Scattering Characteristics Study on Underwater Elastic Targets by the Coupled FEM-BEM Method

A mathematical model about calculating acoustic scattering of elastic shell with air inside unbounded fluid was developed using a coupled finite element and boundary element technique. The coupling of elastic shell with air and shell with fluid was included in this model. To validate the scheme, the scattering characteristic of spherical shell with air submerged in water was calculated. Comparison of the numerical results with the corresponding analytical solutions shows that they are in good agreement in low frequency. On this basis, the low-frequency acoustic scattering characteristic of underwater elastic target was calculated with this method. The results showed the shell thickness and material have little effect on the target strength in the transverse direction, when the frequency is 100-300 Hz. In the omni-direction, the target strength of titanium alloy shell is higher than that of steel shell, and the larger the shell thickness, the greater the target strength. As the frequency is 200 Hz, the steel shell with a thickness of 28 mm has the phenomenon of acoustic resonance, resulting in a sharp increase in the target strength.

Study of Underwater Target Recognition Using Feature Extraction Based on Wavelet Technology

The characteristic information in target echo’s that using geometry and elastic information is possibly the only way to recognize the underwater target. The findings indicate that if the material of the elastic target is different, the vertical wave velocity and the horizontal wave velocity’s relative value is different. Even if the shape is the same, its scattering difference in frequency characteristic is also very obvious, therefore the elastic information may be the only way to recognize the target of the same geometrical shape but material to be different. This paper studies three feature extraction methods based on wavelet technology to obtain the elastic information from the target echo. It can obtain the essential eigenvector and the characteristic dimension is lower than the conventional way. Using the three methods to analyse the target echoes of two different material targets in pond, the recognition ratio shows that the feature extraction method is effective.

Fluid Dynamics of Ballistic Strategies in Nematocyst Firing

Nematocysts are stinging organelles used by members of the phylum Cnidaria (e.g., jellyfish, anemones, hydrozoans) for a variety of important functions including capturing prey and defense. Nematocysts are the fastest-known accelerating structures in the animal world. The small scale (microns) coupled with rapid acceleration (in excess of 5 million g) present significant challenges in imaging that prevent detailed descriptions of their kinematics. The immersed boundary method was used to numerically simulate the dynamics of a barb-like structure accelerating a short distance across Reynolds numbers ranging from 0.9–900 towards a passive elastic target in two dimensions. Results indicate that acceleration followed by coasting at lower Reynolds numbers is not sufficient for a nematocyst to reach its target. The nematocyst’s barb-like projectile requires high accelerations in order to transition to the inertial regime and overcome the viscous damping effects normally encountered at small cellular scales. The longer the barb is in the inertial regime, the higher the final velocity of the projectile when it touches its target. We find the size of the target prey does not dramatically affect the barb’s approach for large enough values of the Reynolds number, however longer barbs are able to accelerate a larger amount of surrounding fluid, which in turn allows the barb to remain in the inertial regime for a longer period of time. Since the final velocity is proportional to the force available for piercing the membrane of the prey, high accelerations that allow the system to persist in the inertial regime have implications for the nematocyst’s ability to puncture surfaces such as cellular membranes or even crustacean cuticle.

Seismic Performance Evaluation of RC Frame Designed Using Ethiopian and Chinese Seismic Codes

This study presents the seismic performance evaluation of the reinforced concrete (RC) frame designed as per Ethiopian (based on EN1998‐1) and Chinese seismic codes to realize best practices within them. In the study, three‐model RC frames with 4‐, 8‐, and 12‐story are designed with the respective codes. Then, their seismic performances are evaluated using the nonlinear static (pushover) procedures of FEMA 356 and ATC 40 provisions. To validate the analysis result, dynamic nonlinear time history analysis is also made. The comparison parameters include elastic stiffness, peak strength, target displacement, and plastic hinge formation patterns in the structures. The results display many similarities in the global and local performances of the structures. Despite these, some noteworthy discrepancies are also noted. Besides, the performance point analysis revealed a significant difference in target displacement that reflects the two codes’ demand spectrum essential disagreements, particularly for the period of vibration greater than 2.0’s. In conclusion, the study highlighted the beneficial aspects of both codes, which will be useful for the future studies.