Stability analysis of second- and fourth-order finite-difference modelling of wave propagation in orthotropic media

The stability of the finite-difference approximation of elastic wave propagation in orthotropic homogeneous media in the three-dimensional case is discussed. The model applies second- and fourth-order finite-difference approaches with staggered grid and stress-free boundary conditions in the space domain and second-order finite-difference approach in the time domain. The numerical integration of the wave equation by central differences is conditionally stable and the corresponding stability criterion for the time domain discretisation has been deduced as a function of the material properties and the geometrical discretization. The problem is discussed by applying the method of VonNeumann. Solutions and the calculation of the critical time steps is presented for orthotropic material in both the second- and fourth-order case. The criterion is verified for the special case of isotropy and results in the well-known formula from the literature. In the case of orthotropy the method was verified by long time simulations and by calculating the total energy of the system

Cosmology with Gamma-Ray Bursts Using k-correction

In the case of Gamma-Ray Bursts with measured redshift, we can calculate the k-correction to get the fluence and energy that were actually produced in the comoving system of the GRB. To achieve this we have to use well-fitted parameters of a GRB spectrum, available in the GCN database. The output of the calculations is the comoving isotropic energy E_iso, but this is not the endpoint: this data can be useful for estimating the {\Omega}M parameter of the Universe and for making a GRB Hubble diagram using Amati's relation.Comment: 4 pages, 6 figures. Presented as a talk on the conference '7th INTEGRAL/BART Workshop 14 -18 April 2010, Karlovy Vary, Czech Republic'. Published in Acta Polytechnic

Low power laser generated ultrasound : signal processing for time domain data acquisition

The use of low power modulated laser diode systems has previously been established as a suitable method for non-destructive laser generation of ultrasound. Using a quasi-continuous optical excitation amplified by an erbium-doped fibre amplifier (EDFA) allows flexible generation of ultrasonic waves, offering control of further parameters such as the frequency content or signal shape. In addition, pseudo-random binary sequences (PRBS) can be used to improve the detected impulse response. Here we compare two sequences, the m-sequence and the Golay code, and discuss the advantages and practical limits of their application with laser diode based optical excitation of ultrasound

Low power laser generated ultrasound : signal processing for time domain data acquisition

The use of low power modulated laser diode systems has previously been established as a suitable method for non-destructive laser generation of ultrasound. Using a quasi-continuous optical excitation amplified by an erbium-doped fibre amplifier (EDFA) allows flexible generation of ultrasonic waves, offering control of further parameters such as the frequency content or signal shape. In addition, pseudo-random binary sequences (PRBS) can be used to improve the detected impulse response. Here we compare two sequences, the m-sequence and the Golay code, and discuss the advantages and practical limits of their application with laser diode based optical excitation of ultrasound

Investigation of PDMS-gold nanoparticle composite films for plasmonic sensors

Poly(dimethylsiloxane) (PDMS)–gold nanoparticle composite films were synthetized in situ by using a simple method based on the reduction of chloroauric acid (HAuCl4) by the PDMS membrane. The technological parameters which affect the gold nanoparticle formation on the membrane (namely the concentration of the HAuCl4 solution, the ratio of curing agent, the incubation time and the temperature) were investigated, the resulting nanoparticle films were characterized with optical spectrophotometry. The possibility to utilize the nanocomposite membranes as sensing elements in plasmonics sensors (based on localised surface plasmon resonance – LSPR) and as surface enhanced Raman spectroscopy (SERS) substrates is discussed in detail

Comparison of numerical and effective-medium modeling of porosity in layered media

In this study, modeling approaches for porosity in layered media are presented and compared. First, an effective-medium model is used to account for the frequencydependent attenuation of the elastic waves. The effective-medium model is based on a single-scattering approach, i.e., it neglects multiple-scattering effects. Then, the effective-medium model is compared in time-domain finite element simulations. The numerical model allows the study of the scattering of the elastic waves on randomly distributed spherical cavities and also accounts for multiple-scattering effects. The models are compared to investigate the validity of the effective-medium model approach. The calculated reflected laminate responses and transmission spectra from the two models show a good agreement

Numerical and analytical investigation of the influence of porosity on the frequency response of GLARE composite

In this study, the validity of the effective-medium model approach to model the ultrasonic response of porosity is investigated with the help of time-domain Finite Element Method simulations. The effective-medium model is based on a single scattering approach i.e. by neglecting the rescattering of the waves and assuming a complex wave number to account for the frequency-dependent attenuation of the elastic waves. The numerical model, on the other hand, allows the study of the scattering of the elastic waves on randomly distributed spherical cavities and also accounts for the multiple scattering effects

Toward the 3D characterisation of GLARE and other fibre-metal laminate composites

Fibre-metal laminates such as GLARE (alternating glass-fibre composite and aluminium layers) are seeing increasing usage on critical aircraft structures due to their enhanced fatigue resistance compared with unreinforced metal. They can be inspected for overall quality using through-transmission ultrasound, but it is very difficult to determine the depth or nature of any defect in the structure in order to assess its importance or severity. As a result, manufacturing scrap rates are higher than desirable and designed components are heavier in order to mitigate risk due to inadequate information. Defect-depth information is buried in the ultrasonic response but is difficult to extract due to the high reflection coefficients of the interfaces and the variable glass-fibre layer thicknesses. This paper presents the potential for using model-based multi-dimensional optimisation to determine the layer thicknesses and depth locations of anomalies in the ultrasonic response due to delaminations or porosity. Numerical (FEM) and analytical methods are presented to model the ultrasonic response of fibre-metal laminates, calculated as the steady-state harmonic response of the layered medium. These frequency-domain responses can be used to determine the individual layer thicknesses and depth locations of anomalies by multi-dimensional optimisation. Investigations on the accuracy and the limitations of the method for the 3D characterisation of laminates will be presented. In addition, the evaluated frequency-domain responses show that the high reflection coefficients in combination with the periodic arrangement of the layup effectively mimic the behaviour of a one-dimensional phononic crystal. In the through-transmission ultrasound response, stop bands arise where the transmission is close to zero. None of the resonance frequencies of a laminate - even one with a finite number of layers - can lie within a stop band. However, the presence of a defect in a layer, or different material properties or thickness, can cause the defect modes, i.e. eigenmodes, to shift into the expected stop bands. This might open new possibilities in the nondestructive testing of fibre-metal laminates, which will be elaborated in the presented paper

Investigation of the Performance of Thermally Generated Au/Ag Nanoislands for SERS and LSPR Applications

In this work the performance of Au/Ag nanoislands was investigated for Surface Enhanced Raman Spectroscopy (SERS) and Localized Surface Plasmon Resonance (LSPR) applications. The nanoislands were generated by thermally annealing thin layers of silver and gold (having thickness in the 5-15 nm range), which were previously sputtered onto glass surfaces. Both pure (silver and gold nanoparticles – AuNP and AgNP) and composite metallic systems (silver-gold core-shell structures – Ag-Au core-shell) were evaluated based on their plasmonic and SERS sensitivity. Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) were used to measure the size, shape and distribution of the nanoparticles to correlate them with the obtained plasmonic/Raman capabilities. The technological parameters of nanoisland fabrication for optimal sensitivities are presented

Traffic Classification over Gbit Speed with Commodity Hardware

This paper discusses necessary components of a GPU-assisted traffic classification method, which is capable ofmulti-Gbps speeds on commodity hardware. The majority of the traffic classification is pushed to the GPU to offload the CPU, which then may serve other processing intensive tasks, e.g., traffic capture. The paper presents two massively parallelizable algorithms suitable for GPUs. The first one performs signature search using a modification of Zobrist hashing. The second algorithm supports connection pattern-based analysis and aggregation of matches using a parallel-prefix-sum algorithm adapted to GPU.The performance tests of the proposed methods showed that traffic classification is possible up to approximately 6 Gbps with a commodity PC