Waves Of Different Frequencies Research Articles

Uncertainty Assessment of CFD Investigation of the Nonlinear Difference-Frequency Wave Loads on a Semisubmersible FOWT Platform

Current mid-fidelity modeling approaches for floating offshore wind turbines (FOWTs) have been found to underpredict the nonlinear, low-frequency wave excitation and the response of semisubmersible FOWTs. To examine the cause of this underprediction, the OC6 project is using computational fluid dynamics (CFD) tools to investigate the wave loads on the OC5-DeepCwind semisubmersible, with a focus on the nonlinear difference-frequency excitation. This paper focuses on assessing the uncertainty of the CFD predictions from simulations of the semisubmersible in a fixed condition under bichromatic wave loading and on establishing confidence in the results for use in improving mid-fidelity models. The uncertainty for the nonlinear wave excitation is found to be acceptable but larger than that for the wave-frequency excitation, with the spatial discretization error being the dominant contributor. Further, unwanted free waves at the difference frequency have been identified in the CFD solution. A wave-splitting and wave load-correction procedure are presented to remove the contamination from the free waves in the results. A preliminary comparison to second-order potential-flow theory shows that the CFD model predicted significantly higher difference-frequency wave excitations, especially in surge, suggesting that the CFD results can be used to better calibrate the mid-fidelity tools.

Time-domain coupled analysis of curved floating bridge under wind and wave excitations

A floating bridge is an innovative solution for deep-water and long-distance crossing. This paper presents a curved floating bridge's dynamic behaviors under the wind, wave, and current loads. Since the present curved bridge need not have mooring lines, its deep-water application can be more straightforward than conventional straight floating bridges with mooring lines. We solve the coupled interaction among the bridge girders, pontoons, and columns in the time-domain and to consider various load combinations to evaluate each force's contribution to overall dynamic responses. Discrete pontoons are uniformly spaced, and the pontoon's hydrodynamic coefficients and excitation forces are computed in the frequency domain by using the potential-theory-based 3D diffraction/radiation program. In the successive time-domain simulation, the Cummins equation is used for solving the pontoon's dynamics, and the bridge girders and columns are modeled by the beam theory and finite element formulation. Then, all the components are fully coupled to solve the fully-coupled equation of motion. Subsequently, the wet natural frequencies for various bending modes are identified. Then, the time histories and spectra of the girder's dynamic responses are presented and systematically analyzed. The second-order difference-frequency wave force and slowly-varying wind force may significantly affect the girder's lateral responses through resonance if the bridge's lateral bending stiffness is not sufficient. On the other hand, the first-order wave-frequency forces play a crucial role in the vertical responses.

Fourier analysis: the simplification of a complex waveform into simple component sine waves of different amplitudes and frequencies

Fourier analysis is the simplification of a complex waveform into simple component sine waves of different amplitudes and frequencies. A discussion on Fourier analysis necessitates reiteration of the physics of waves. A wave is a series of repeating disturbances that propagate in space and time. Frequency: the number of oscillations, or cycles per second. It is measured in Hertz and denoted as 1/time or s-1. Fundamental frequency: the lowest frequency wave in a series. It is also known as the first harmonic. Every other wave in the series is an exact multiple of the fundamental frequency. Harmonic: whole number multiples of the fundamental frequency. Amplitude: the maximum disturbance or displacement from zero caused by the wave. This is the height of the wave. Period: time to complete one oscillation. Wavelength: physical length of one complete cycle. This can be between two crests or two troughs. The higher the frequency, the shorter the wavelength. Velocity: frequency x wavelength. Phase: displacement of one wave compared to another, described as 0°–360°. A sine wave is a simple wave. It can be depicted as the path of a point travelling round a circle at a constant speed, defined by the equation ‘y = sinx’. Combining sine waves of different frequency, amplitude and phase can yield any waveform, and, conversely, any wave can be simplified into its component sine waves. Fourier analysis is a mathematical method of analysing a complex periodic waveform to find its constituent frequencies (as sine waves). Complex waveforms can be analysed, with very simple results. Usually, few sine and cosine waves combine to create reasonably accurate representations of most waves. Fourier analysis finds its anaesthetic applications in invasive blood pressure, electrocardiogram (ECG) and electroencephalogram (EEG) signals, which are all periodic waveforms. It enables monitors to display accurate representations of these biological waveforms. Fourier analysis was developed by Joseph Fourier, a mathematician who analysed and altered periodic waveforms. It is done by computer programmes that plot the results of the analysis as a spectrum of frequencies with amplitude on the y-axis and frequency on the x-axis.

Harmonic wideband simultaneous dual-frequency MR Elastography.

Magnetic resonance elastography (MRE) is used to non-invasively quantify viscoelastic properties of tissues based on the measurement of propagation characteristics of shear waves. Because some of these viscoelastic parameters show a frequency dependence, multifrequency analysis allows us to measure the wave propagation dispersion, leading to a better characterization of tissue properties. Conventionally, motion encoding gradients (MEGs) oscillating at the same frequency as the mechanical excitation encode motion. Hence, multifrequency data is usually obtained by sequentially repeating monochromatic wave excitations experiments at different frequencies. The result is that the total acquisition time is multiplied by a factor corresponding to the number of repetitions of monofrequency experiments, which is a major limitation of multifrequency MRE. In order to make it more accessible, a novel single-shot harmonic wideband dual-frequency MRE method is proposed. Two superposed shear waves of different frequencies are simultaneously generated and propagate in a sample. Trapezoidal oscillating MEGs are used to encode mechanical vibrations having frequencies that are an odd multiple of the MEG frequency. The number of phase offsets is optimized to reduce the acquisition time. For this purpose, a sampling method not respecting the Shannon theorem is used to produce a controlled temporal aliasing that allows us to encode both frequencies without any additional examination time. Phantom experiments were run to compare conventional monofrequency MRE with the single-shot dual-frequency MRE method and showed excellent agreement between the reconstructed shear storage moduli G'. In addition, dual-frequency MRE yielded an increased signal-to-noise ratio compared with conventional monofrequency MRE acquisitions when encoding the high frequency component. The novel proposed multifrequency MRE method could be applied to simultaneously acquire more than two frequency components, reducing examination time. Further studies are needed to confirm its applicability in preclinical and clinical models.

Microscopic fluid dynamics of a wire screen bound to a slit resonator excited by acoustic waves

The microscopic fluid dynamics of a wire screen bound to a slit resonator excited by incident sound waves of different intensity are investigated numerically. The microscopic flow features help in understanding the acoustic behavior. A normal impedance-tube model is used in this investigation, and the wire mesh is modeled as an array of eight identical tiny circular cylinders arranged in parallel. Tonal waves of different sound pressure levels and frequencies are introduced from the termination of the tube through a non-reflecting boundary condition. Direct numerical simulations are carried out to solve for the flow and acoustic fields simultaneously, and the velocity and vorticity fields around the resonator are resolved. Upon closer inspection, the tiny cylinders suppress the vortex shedding from the slit excited by high-intensity incident sound waves, thereby retarding the nonlinear acoustic behavior of the slit. Furthermore, the wire mesh contributes greatly to the absorption of acoustic energy through scrubbing loss and flow separation. The acoustic impedance and absorption coefficient are derived using a two-microphone method. The numerical results show that the wire mesh increases the resistance of the resonator significantly while hardly affecting its reactance.

Effects of second-order hydrodynamics on an ultra-large semi-submersible floating offshore wind turbine

Second-order wave loads on floating offshore wind turbine (FOWT) may excite severe resonance at the eigenfrequencies of the floating system and cause fatigue damage. The hydrodynamic coefficients including first- and second-order wave force transfer functions are calculated, and fully coupled time-domain simulation by FAST is conducted for selected environmental conditions. Each load case is simulated without second-order loads, and with difference-frequency full quadratic transfer functions (QTFs), sum-frequency full QTFs, and complete second-order solution. The effect of second-order difference- and sum-frequency wave loads on the dynamic behaviors of the 10 MW semi-submersible FOWT is discussed. Subsequently, the effect of second-order wave loads on the structural fatigue behavior of the 10 MW and 5 MW semi-submersible FOWTs is investigated and compared by computing ultimate and damage-equivalent loads (DELs). The results show that ignoring second-order difference-frequency wave loads may underestimate the low-frequency responses, and second-order sum-frequency wave loads significantly affect the structural dynamics of the 10 MW FOWT. In addition, the effect of second-order wave loads on the fatigue damage of the 5 MW and 10 MW semi-submersible FOWTs are significantly different.

Validation of drift motions for a semi-submersible floating wind turbine and associated challenges

In the EU H2020 project LIFES50+, a 1:36 scaled model test campaign was carried out for the NAUTILUS-DTU10 semi-submersible floating offshore wind turbine with active ballast. This paper concentrates on the modelling capabilities of a state-of-the-art time domain simulation model FAST8 for floating offshore wind turbines and specific challenges associated with the validation process. For the modelling, the platform is considered as a rigid body, and the frequency dependant radiation damping, added mass, and wave excitation are evaluated with a panel code using potential theory. The results from the scaled model are compared to the simulations, looking into the effects on the platform when the first order radiation-diffraction hydrodynamics through the Cummins equation, the mean drift coefficients from the nearfield solution for Newman’s approximation of the second-order difference-frequency wave forces, and full quadratic transfer function (QTF) are taken into account. Moreover, viscous forces on the floating platform are modelled through Morison elements with coefficients of drag. Frequency domain analysis of the motions showed good agreement after modifications of the coefficients of drag of the Morison elements for tests performed with a pink wave spectrum. On the other hand, the extreme wave test showed large discrepancies between results and simulations, which could not be overcome by the inclusion of the QTFs or tuning of the coefficients of drag which model the viscous forces. This is followed by a discussion of the challenges in the modelling approach, and other validation techniques are proposed for future research. The main goal is to define the Morison elements for the floater, and tune the drag coefficients to enable the numerical tool to capture the floater’s motions. The effect of the change of the coefficients on the simulation outputs are shown.

Formation and migration of ripple pattern due to pure wave

ABSTRACT This paper reports an experimental study carried out in a laboratory flume to study the ripple formation and migration for three different sediment sizes ranging from fine sand to coarse sand under the influence of surface waves of different frequencies. This study compares the ripple length, height, ripple patch formation and migration characteristics for different wave frequencies and sediment sizes. Three-dimensional velocity was measured by 3D micro-ADV (Acoustic Doppler Velocimeter) and the evolution of bed morphology was quantified by Ultrasonic Ranging System (URS), SeaTek Inc. It is found from experimental results that patches (bunch of ripples) do not appear all over the bed at a time, rather these patches appear at isolated locations and the separation between the patches scale with the wave length of surface waves. Further, the ripple migration rate is evaluated and its relationship with sediment grain size and frequency is presented.

Моdern technologies in the production of food products

Some forms of food preservation have been in use for thousands of years, though preservation as an industrial food processing has been discussed for the last 100 years. During this period, the basic principles underlying the preservation process were adopted. The main purpose of food preservation is to prevent the growth of microorganisms in food products, that is, to reduce the risk to human health of possible food poisoning. Some preservation technologies (processes based on the use of high temperatures) are effective in this respect, however, they can cause harm to the composition, nutritional and sensory properties. Consumers have therefore emphasized their desire to consume foods that have "natural properties" of taste and color, even when is not stored under special conditions. Foods of animal origin are particularly sensitive to the harmful effects of microorganisms. This review provides insight into contemporary research into new conservation technologies, during which it is possible to preserve nutritional and sensory properties and at the same time inactivate microorganisms causing food spoilage and foodborne pathogens. New technologies which are applied during the production and preservation of foods respect the principles relating to the conservation of resources and the reduction of energy consumption and processing time. Non-thermal procedures (so-called minimal food processing procedures) which are based on the application of pulsed electric fields, high hydrostatic pressure, high-intensity light, oscillating magnetic field, electromagnetic waves of different frequencies and ultrasound are presented in detail. The use of new technologies allows to reduce production costs, increase productivity, save energy and water, affect production reliability, improve product quality, improve sustainability and reduce emissions in the food/nutrition chain.

Practical method for parametric array source evaluation at 120 kHz frequency range in the near-field region

A parametric array is a non-linear conversion process that can generate a narrow beam of low-frequency sound with a small aperture. One of the challenging issues with a parametric array is precise measurement of the sound field generated. In particular, near the transducer, it is not easy to measure the sound field generated by a parametric array precisely, because the amplitude of the difference frequency wave is much lower than the amplitude of the primary wave. In this study, the practical issues that should be considered in the design of near-distance experiments with a parametric array are examined. Limiting effects were examined, and their associated characteristics were identified by numerical simulations. Experiments were performed in a water tank (18 x 12 x 10 m) to assess these characteristics, using a custom-designed acoustic filter; the beam pattern and propagation curve of the difference frequency wave generated by the parametric array were measured and compared with simulated data.

Pulsed Air-Flow Thermography for Natural Crack Detection and Evaluation

Thermography is one of advanced non-destructive test and evaluation for imaging defects. This paper proposes pulsed air-flow thermography for sample surface crack detection and evaluation. The proposed approach combines the advantages of both convection heat transfer of air-flow and solid heat conduction. By controlling the pulse width, the heat waves of different frequencies can be modulated to penetrate materials with different thermal conductivity. And the cleaning degree of specimen surface can be improved by high-speed air-flow convection. The theoretical model and experimental facilities of air-flow thermography are presented. Meanwhile, the offline detection of rail surface defects and the dynamic test of the defects on the outer cylindrical surface of the turn-plate are completed. The offline experiments are conducted to identify the characteristics of the proposed method for the detection of artificial and natural cracks. And the dynamic experimental results show that the defects on the outer cylindrical surface of the turn-plate can be identified when the linear velocity at the edge of the turn-plate is 1.356km/h and 6.782km/h. The results demonstrate the proposed method on testing and evaluation for rail surface cracks is effective at the low speed.

On Efficient Surrogate Model Development for the Prediction of the Long-Term Extreme Response of a Moored Floating Structure

Abstract This study focuses on the development of efficient surrogate models by polynomial chaos expansion (PCE) for the prediction of the long-term extreme surge motion of a moored floating offshore structure. The structure is subjected to first-order and second-order (difference-frequency) wave loading. Uncertainty in the long-term response arises from contrasting sea state conditions, characterized by significant wave height, Hs, and spectral peak period, Tp, and their relative likelihood of occurrence; these two variables are explicitly included in the PCE-based uncertainty quantification (UQ). In a given sea state, however, response simulations must be run for the associated Hs and Tp; in such simulations, typically, a set of random amplitudes and phases define an irregular wave train consistent with that sea state. These random amplitudes and phases for all the frequency components in the wave train introduce additional uncertainty in the simulated waves and in the response. The UQ framework treats these two sources of uncertainty—from Hs and Tp on the one hand, and the amplitude and phase vectors on the other—in a manner that efficiently yields long-term surge motion extreme predictions consistent with more expensive Monte Carlo simulations (MCS) that serve as the “truth” system. To reduce uncertainty in response extremes that result from sea states with a low likelihood of occurrence, importance sampling is employed with both MCS- and PCE-based extreme response predictions. Satisfactory performance with such efficient surrogate models can help in assessing the long-term response of various offshore structures.

Coupled dynamic analysis for wave action on a tension leg-type submerged floating tunnel in time domain

A full time-domain two-dimensional (2-D) numerical model was established for the coupled dynamic analysis of submerged floating tunnel (SFT) structures. For hydrodynamic loads, a second-order time-domain potential flow numerical model was developed and a higher-order boundary element method (HOBEM) was applied to discretize the body boundary and the free surface boundary. The elastic rod theory was used to analyse the cable force to the SFT. Hinged boundary conditions were used at the junction of the mooring line and the structure. The viscous damping coefficient of the submerged cylinder was obtained through a decay physical model test, which was employed in the numerical simulation. In the coupled dynamic analysis, the motion equation for the hull and dynamic equations for the mooring lines were solved simultaneously using the Newmark-β method. The coupled analysis numerical model was applied in an SFT simulation with waves of different frequencies. The influences of viscous damping, buoyancy weight ratios (BWRs), and mooring stiffness distributions on the first- and second-order motion responses and tension of the mooring lines are analysed in detail, and some significant conclusions are presented.

Self-Raman Nd-doped vanadate laser: a pump source of organic crystal based difference frequency generation

We analyse a method of high repetition rate, efficient terahertz wave difference frequency generation (DFG) by using a self-Raman Nd-doped vanadate laser to pump 4’-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST). A semi-analytical model is introduced that combines the rate equations (for the self-Raman dual-wavelength laser) and the coupled-wave equations (for the DAST-based DFG). Calculations were performed on a diffusion-bonded Nd:YVO4-YVO4 laser with an acousto-optic Q-switch. Parameters (including transmittances at output mirror and crystal lengths) were optimized to obtain high output. Sub-milliwatt average power is predicted under current pump condition.

De Broglie wave in vacuum, matter and nanostructures

Properties of de Broglie waves and their differences from any other waves, for example, electromagnetic, are discussed. Their little-known properties are given: 1) the propagation velocity is greater than the speed of light V > c ; 2) dispersion even in vacuum (waves of different frequencies propagate at different speeds, which results in the spreading of any wave packets associated with the particles); 3) the connection of the de Broglie wave with another physical wave moving with a speed v less than the speed of light, but so always v · V = c 2, and for the photons v = V = c.According to later views of de Broglie, this second wave is not the group wave. Possible mechanisms of generating the de Broglie waves based on spherical and cylindrical hollow quantum resonators, which can be specially created in matter using specific nanostructures are considered. Perhaps their nature itself realizes them for any micro-objects, regardless of their electrical, magnetic and gravitational properties. That is, this phenomenon is universal. Applications and realizations of this idea are discussed in the part II of our article.

Gamma-Rhythmic Input Causes Spike Output

The gamma rhythm has been implicated in neuronal communication, but causal evidence remains indirect. We measured spike output of local neuronal networks and emulated their synaptic input through optogenetics. Opsins provide currents through somato-dendritic membranes, similar to synapses, yet under experimental control with high temporal precision. We expressed Channelrhodopsin‑2 in excitatory neurons of cat visual cortex and recorded neuronal responses to light with different temporal characteristics. Sine waves of different frequencies entrained neuronal responses with a reliability that peaked for input frequencies in the gamma band. Crucially, we also presented white-noise sequences, because their temporal unpredictability enables analysis of causality. Neuronal spike output was caused specifically by the input’s gamma component. This gamma-specific transfer function is likely an emergent property of in‑vivo networks with feedback inhibition. The method described here could reveal the transfer function between the input to any one and the output of any other neuronal group.

An Acoustic Metamaterial-Based Sensor Capable of Multiband Filtering and Amplification

Dispersive acoustic metamaterials (AMMs) with graded refractive index is promising in acoustic sensing enhancement by providing acoustic rainbow trapping (ART) effect. However, previous solutions to this type of AMMs are based on 2D unit cells that require infinite or very large size in the third dimension; they cannot integrate with conventional acoustic sensors without deteriorating the ability of acoustic wave manipulation. In order to advance their practical applications, this paper proposes an axisymmetric, microphone embedded AMM device with gradient profile as well as gradient plate thickness and gradient gap width along the axis. Both numerical simulations and experiments are performed to examine the ART effect of the proposed AMM device and demonstrate its excellent capacity of acoustic pressure amplification and multi-band acoustic filtering. Deploying a microphone in the gap does not disturb the acoustic field, so the proposed AMM sensor experimentally achieves remarkable acoustic gains over one order of magnitude. Sound waves of different frequencies are picked up by microphones at different gaps, realizing the real-time spatially splitting of the sound spectrum. The proposed AMM provides excellent immunity to broadband noise and a little weak suppression of the single tone noise while significantly amplifying acoustic signals of a given frequency.

Effect of Disease Severity on Respiratory Impedance in Parkinson's Disease.

Background:Parkinson’s disease (PD) is a progressive neurodegenerative disorder. PD has been traditionally perceived as a motor disorder. However, it is frequently associated with pulmonary dysfunction which has been assessed by Spirometry, an effort-dependent technique.Purpose:To evaluate in patients with PD the effect of disease severity on respiratory impedance using Impulse Oscillometry (IOS) and to correlate with Spirometry.Methods:The study was conducted on 30 patients diagnosed with PD. Pulmonary function was assessed by IOS and spirometer. IOS is an effort-independent technique that uses sound waves of different frequencies to measure airway resistance. Spirometer measures the lung volume and generates flow–volume and volume–time relationship.Results:The mean age of patients was 60.1±9.45. Resistance at 5 Hz (R5) was found to be negatively correlated with forced expiratory volume in the first second of the FVC manoeuver (FEV1) (r = –0.628, P = .002), FEV1/FVC (forced vital capacity) (r = –0.487, P = .025), and PEF (r = –0.599, P = .004), and resistance at 20 Hz (R20) with FEV1 (r = –0.474,P = .029) and PEF (r = –0.522, P = .015). There was significant increase in R5 (0.32(0.36–0.28) vs 0.47(0.60–0.36); P = .04) and R20 (0.25(0.28–0.20) vs 0.30(0.40–0.25); P = .04) in stage II as compared to stage I of Hoehn–Yahr scale.Conclusion:IOS might be a promising tool for diagnosis of respiratory dysfunction in addition to Spirometry, especially in cases where patients are not able to perform forced manoeuvers.

Scattering characteristics of an anisotropic medium sphere

Based on the invariable displacement current of anisotropic media, a theoretical model for electromagnetic reconstructing anisotropic media and a basic method for selecting electromagnetic field are proposed. Taking the gypsum crystal as an example, the analytical solution of bi-axial medium scattering is presented by using the spherical vector wave function of electromagnetic field. The electromagnetic wave scattering of the anisotropic medium spheres are simulated in radiation of electromagnetic waves of different frequency. The electromagnetic analysis of the simulation results shows that the electromagnetic mechanism is consistent with the results of relevant literature. When the anisotropic medium degenerates to the isotropic medium, the results obtained by the model are completely consistent with those obtained by Mie theory.

Lévy noise-induced near-death spikes and phase transitions of a biological neural network

Near-death spikes or near-death surges are a sudden increase in neuron activity in the human brain before neurons end their firings. Just before a person is clinically dead, such spikes are observed in certain cases, so the name is near-death spikes. The reason for this behavior is the lack of oxygen in brain (Chawla et al. in J Palliat Med 12(12):1095–1100, 2009). In this study, it is demonstrated that a particular type of noise called Levy noise can generate such activity in the neural network of the worm Caenorhabditis elegans. The study identified different parameter regions of noise at which the network makes transitions from one synchronous state to another and the mechanism behind them. Such transitions are already reported in cortical regions of brain (Canavero et al. in Surg Neurol Int 7(Suppl 24):S623–S625, 2016). During the transition period between asynchronous and synchronous firing states, network is more susceptible to changes in firing pattern of individual neurons (Zandt et al. in PLoS ONE 6(7):e22127, 2011; Uzuntarla et al. Neural Netw 110:131–140, 2019). In this work, it is demonstrated that the recognized parameter regions can be used to control the network dynamics. The study also identified Levy noise values at which the network displays generation of waves of different frequencies. This result suggests a new method for neurostimulation in the case of traumatic brain injury. The study reveals that the characteristic exponent ($$\alpha $$) of the noise has better influence on the network dynamics than the scale parameter of noise (D) and the synaptic coupling constant (Gsyn) of the network. The neuronal network even displayed Gamma oscillations for large values of $$\alpha $$. If the parameters of the neurons are made chaotic, the network firing rate is diminished and it displayed Delta and Theta oscillations.