Wave Generator Experiment Research Articles

Chiral Cationic Chromophores: A New Class of Efficient Ultrabroadband Organic THz Crystals

AbstractA strategic approach to develop efficient ultra‐broadband terahertz (THz) crystals involves the incorporation of different electron donating groups (EDGs) into cationic chromophores. In contrast to the widely utilized non‐cyclic 4‐(dimethylamino)phenyl (DA) EDG, cyclic 5‐membered 4‐(2‐(hydroxymethyl)pyrrolidine‐1‐yl)phenyl (PB) and cyclic 6‐membered 4‐(3‐(hydroxymethyl)piperidin‐1‐yl)phenyl (PN) EDGs exhibit an asymmetrical shape with a strongly interacting hydroxymethyl group at the chiral center. Notably, the PB EDG shows narrower intrinsic vibrational states with lower conformational flexibility and smaller ring flips compared to the PN EDG. In the crystalline state, introducing the PB EDG leads to the formation of a new class of nonlinear optical assembly, the so‐called stair‐type cation‐anion assembly. This assembly demonstrates optimal chromophore alignment with state‐of‐the‐art effective first hyperpolarizability (209 × 10−30 esu). Furthermore, the PB EDG‐based crystals exhibit significantly lower THz absorption compared to previously reported benchmark crystals. In THz wave generation experiments, PB EDG‐based crystals demonstrate state‐of‐the‐art efficiency and ultrabroad spectral bandwidth, featuring a cut‐off frequency of up to 16 THz.

Verification of 3D DDA-SPH coupling method and its application in the analysis of geological disasters

At present, discontinuous deformation analysis (DDA) is in an active development stage and its application in the field of geohazards is expanding. Recently, the DDA method has been coupled with smooth particle hydrodynamics (SPH) to analyze some geohazards involving fluid-solid interactions (FSI) and has shown good prospects in the field. However, the interaction between rocks and water in geological disasters is extremely complex and the comprehensive verifications of the simulation results between the block and fluid under different interactive modes are never found. To solve this problem, we first introduced the basic principle of the DDA and SPH method, followed by the coupling algorithm of the two methods. Then, the correctness of the coupling method was verified through analyses of fluid and solid movement by adopting the dam break test, Scott Russell's wave generator experiment, water entry test of a single sphere, and landslide-generated impulsive wave, which represent different interaction modes between blocks and fluids respectively. Finally, based on the 3D DDA-SPH method, this paper simulated the chain disasters of the water inrush induced by tunnel collapse and the dam instability induced by slope sliding, and analyzed the chain mechanism between the different disasters, which illustrates the feasibility of 3D DDA-SPH coupling method in simulating the FSI problem in geotechnical engineering.

Active Absorption of Random Waves in Wave Flume Using Artificial Neural Networks

Abstract A wave flume is primarily intended to reproduce actual sea conditions in order to provide a reliable means of testing for small-scale models. The realization of scaled tests is extremely important for the validation of a project on real scale, since, through the laws of similitude, such tests make it possible to predict the behavior of structures in the ocean as well as their performance during operation. This research aims to develop, test, and validate an active control algorithm for wave absorption in a two-dimensional wave channel—that is, when the waves propagate in only one direction—based on artificial neural networks (ANN). The ANN control algorithm relies on the linear wave theory and the principle of time-reversal of wave propagation; i.e., the phenomenon of wave absorption corresponds to the wave generation when observed in the reverse direction of time. Through this principle, data from wave generation experiments, after proper manipulation, are used to train an ANN capable of generating the control signal used to move the wave-generator device, this time as a wave absorber

Tsunami Squares modeling of landslide tsunami generation considering the ‘Push Ahead’ effects in slide/water interactions: Theory, experimental validation, and sensitivity analyses

Landslide tsunami generation is a complex physical process that involves solid materials interacting with water bodies and other 3D nonlinear wave effects. Modeling of this process ranks as an urgent task because wave source features are crucial clues for subsequent tsunami motions and hazards threatening coastlines. Herein, we introduce a concept of ‘Push Ahead’ (PA) in landslide/water interactions and propose a PA formulation integrated in a meshless numerical procedure named ‘Tsunami Squares’ (TS) for modeling of landslide tsunami generation. We draw comparisons on the physical significances of PA acceleration versus ‘Drag Along’ (DA) acceleration and conventional aerodynamic and hydrodynamic solutions. We carry out physical experiments of wave generation by block-dragging and gravel flows and corresponding TS simulations to study PA effects in landslide/water interactions. Wave features in TS simulations match the experimental observations in a reasonable range, providing a firm validation of the PA formulation in the TS approach. Sensitivity analysis that compares simulations with various ratios of PA acceleration is carried out both for the block-dragging and gravel flow experiments. The comparisons indicate that PA acceleration plays an important role in landslide wave generation and that larger PA accelerations make greater contributions to the initial wave source. We discuss intrinsic relations between PA formulation and the empirical equations derived from previous experiment studies that support the validity and applicability of the PA solution. We also insist that the complex 3D wave effects such as dispersion, shoaling, refraction, reflection and superposition, occurring in the experiments can be well reproduced in the TS simulations. The newly proposed PA acceleration that works together with our previously presented ‘Drag Along’ and ‘Lift Up’ effects can excellently illustrate the mechanical process of landslide water interactions. We conclude that TS simulations that do not include the ‘Push Ahead’ acceleration can seriously underestimate landslide tsunami size. • Development of a ‘Push Ahead’ acceleration in TS modeling of landslide/water interactions. • Model validation with well-designed experiments of waves generated by block slide and gravel flow. • Sensitivity analysis of the ‘Push Ahead’ acceleration that affects landslide tsunami generation. • Discussion on intrinsic relations between PA formulation and the empirical equations.

Single Crystals Based on Hydrogen‐Bonding Mediated Cation–Anion Assembly with Extremely Large Optical Nonlinearity and Their Application for Intense THz Wave Generation

AbstractNew molecular salt crystals based on linear‐shaped polymer‐like cation–anion assembly exhibiting extremely large nonlinear optical response and high THz generation efficiency are reported. Two hydroxy benzothialzolium PMB (2‐(4‐(4‐(hydroxymethyl)piperidin‐1‐yl)styryl)‐3‐methylbenzo[d]thiazol‐3‐ium) crystals with different benzenesulfonate counter anions provide isomorphic crystal structure with acentric monoclinic Cc space group symmetry. In contrast to previously reported benchmark nonlinear optical salt crystals with a parallel‐type cation–anion assembly, newly developed PMB‐based crystals exhibit a series‐type cation–anion assembly mediated by strong bidentate‐like hydrogen‐bonds. Such series‐type cation–anion assembly results in perfect alignment of highly nonlinear PMB cations in the crystalline state, leading to extremely large diagonal component of the second‐order nonlinear optical coefficient exceeding that of the state‐of‐the‐art nonlinear optical crystals. In THz wave generation experiments based on optical rectification, a 0.33 mm thick PMB crystal generates intense THz pulses with peak‐to‐peak THz electric field of 430 kV cm−1 and extremely broad flat spectrum with upper cut‐off frequency of above 8.0 THz. In addition, compared to inorganic standard 1.0 mm thick ZnTe crystals, the PMB crystal delivers a 24 times higher THz electric field and about 3 times broader bandwidth. Therefore, hydroxy benzothialzolium PMB crystals are highly desired novel materials for various nonlinear optical applications including THz photonics.

Unified particle method with application to the 2013 Huangping landslides and tsunamis event of China

In this study, a unified particle method is presented to simulate the fluid-solid coupling problem in broad range of scales such as landslide and tsunamis. First, a general overview of the method is addressed, and the governing equations are solved in a Lagrangian form. Second, the method is used to simulate the Scott Russell wave generator experiment containing simple fluid-solid coupling, and the relationship between the simulation results and the experimental results is analyzed to verify the validity of the model. Then, the method is applied to the 2D processes simulation of Huangping landslide generated impulse waves in large scale. The results show that: in the verification test, the wave amplitude error between the simulation and the experimental data is almost 0. In the application case of Huangping landslides and tsunamis, the maximum impulse wave height obtained by the simulation is close to the local observations, which indicates that the method has high accuracy and credibility.

Large-Eddy-simulation prediction of indirect combustion noise in the entropy wave generator experiment

Compact and non-compact analytical solutions of the subsonic operating point of the entropy wave generator experiment are compared with detailed numerical results obtained by large Eddy simulations. Two energy deposition methods are presented to account for the experimental ignition sequence and geometry: a single-block deposition as previously used and a delayed deposition that reproduces the experimental protocle closely. The unknown inlet acoustic reflection coefficient is assumed to be fully reflective to be more physically consistent with the actual experimental setup. The time delay between the activation of the heating modules must be considered to retrieve the temperature signal measured at the vibrometer and pressure signals at the microphones. Moreover, pressure signals extracted from the large Eddy simulations in the outlet duct using the delayed ignition model clearly reproduce the experimental signals better than the analytical models. An additional simulation with actual temperature fluctuations directly injected at the inlet of the computational domain clearly shows that the pressure fluctuations produced by the acceleration of the hot slug yields indirect noise almost entirely. Finally, the entropy spot is shown to be distorted when convecting through the turbulent flow in the entropy wave generator nozzle. Its amplitude decreases and its shape is dispersed, but hardly any dissipation occurs. The distortion appears to be negligible through the nozzle and become important only when convected over a long distance in the downstream duct. As the dominant frequencies of the entropy wave generator entropy forcing are very low, the effects of dispersion by the mean flow are however weak.

Theory and application of reverberated direct and indirect noise

The generation of a temperature disturbance in a flow is accompanied by the production of acoustic waves (direct noise) and of an entropy perturbation. If this entropy perturbation is accelerated or decelerated (as is the case through a nozzle or flow restriction), additional acoustic waves are generated (indirect noise). Several studies have demonstrated this mechanism in controlled conditions by using entropy wave generators, in which entropy waves are generated and convected through a nozzle, leading to direct and indirect noise. An analytical analysis of the direct and indirect noise produced by the generation and acceleration of entropy waves in a reflective environment is presented. The effect of reverberation (repeated acoustic reflections) on low-frequency perturbations (characteristic of entropy wave generators) is determined analytically. These results are then implemented in a set of limit cases, showing the limit behaviours of such systems. The analytical model is applied to the case of the Cambridge entropy wave generator experiment, in which entropy waves are generated by an electric heater and accelerated through a subsonic orifice plate. Due to the clear time separation of direct and indirect noise in the experimental results, direct and indirect noise transfer functions can be extracted from the experimental data for the first time and compared directly with existing theoretical models. The backward-propagating indirect noise generated at an orifice plate is shown to be significantly higher than predicted by existing theoretical models for isentropic nozzles.

Resonant scattering of energetic electrons in the outer radiation belt by HAARP-induced ELF/VLF waves

Several extremely low-frequency (ELF)/very low-frequency (VLF) wave generation experiments have been performed successfully at High-Frequency Active Auroral Research Program (HAARP) heating facility and the artificial ELF/VLF signals can leak into the outer radiation belt and contribute to resonant interactions with energetic electrons. Based on the artificial wave properties revealed by many of in situ observations, we implement test particle simulations to evaluate the effects of energetic electron resonant scattering driven by the HAARP-induced ELF/VLF waves. The results indicate that for both single-frequency/monotonic wave and multi-frequency/broadband waves, the behavior of each electron is stochastic while the averaged diffusion effect exhibits temporal linearity in the wave–particle interaction process. The computed local diffusion coefficients show that, the local pitch-angle scattering due to HARRP-induced single-frequency ELF/VLF whistlers with an amplitude of ∼10pT can be intense near the loss cone with a rate of ∼10−2 rad2 s−1, suggesting the feasibility of HAARP-induced ELF/VLF waves for removal of outer radiation belt energetic electrons. In contrast, the energy diffusion of energetic electrons is relatively weak, which confirms that pitch-angle scattering by artificial ELF/VLF waves can dominantly lead to the precipitation of energetic electrons. Moreover, diffusion rates of the discrete, broadband waves, with the same amplitude of each discrete frequency as the monotonic waves, can be much larger, which suggests that it is feasible to trigger a reasonable broadband wave instead of the monotonic wave to achieve better performance of controlled precipitation of energetic electrons. Moreover, our test particle scattering simulation show good agreement with the predictions of the quasi-linear theory, confirming that both methods are applied to evaluate the effects of resonant interactions between radiation belt electrons and artificially generated discrete ELF/VLF waves.

Numerical Analysis of Indirect Combustion Noise Generation Within a Subsonic Nozzle

Indirect noise is analyzed by numerical simulations of the Entropy Wave Generator experiment, which has been set up to study the fundamental mechanism of indirect combustion noise generation. Previous studies of this test case show the major importance of modeling the impedance terminations of the test rig to obtain accurate results. So far, characteristic boundary conditions have been used to model the impedance for numerical calculations. Within this paper, different proposals for setting up the characteristic boundary conditions are reviewed. In addition, a higher accuracy of the impedance model is achieved by applying time-domain impedance boundary conditions. As part of the Entropy Wave Generator experiment, indirect noise is generated by electrical heating wires. It has been found in this study that the modeling of these heat sources is crucial for obtaining accurate results. An advanced heat source model is proposed that is in excellent agreement with the experimental data. Both the advanced impedance and heat source model account for a better agreement between experimental and numerical data than reported in literature to date.

100 days of ELF/VLF generation via HF heating with HAARP

Extremely low frequency/very low frequency (ELF/VLF) radio waves are difficult to generate with conventional antennas. Ionospheric high frequency (HF) heating facilities generate ELF/VLF waves via modulated heating of the lower ionosphere. HF heating of the ionosphere changes the lower ionospheric conductivity, which in the presence of natural currents such as the auroral electrojet creates an antenna in the sky when heating is modulated at ELF/VLF frequencies. We present a summary of nearly 100 days of ELF/VLF wave generation experiments at the 3.6 MW High Frequency Active Auroral Research Program (HAARP) facility near Gakona, Alaska, at a variety of ELF/VLF frequencies, seasons, and times of day. We present comprehensive statistics of generated ELF/VLF magnetic fields observed at a nearby site, in the 500–3500 Hz band. Transmissions with a specific HF beam configuration (3.25 MHz, vertical beam, amplitude modulation) are isolated so the data comparison is self‐consistent, across nearly 5 million individual measurements of either a tone or a piece of a frequency‐time ramp. There is a minimum in the average generation close to local midnight. It is found that generation during local nighttime is on average weaker but more highly variable, with a small number of very strong generation periods. Signal amplitudes from day to day may vary by as much as 20–30 dB. Generation strengthens by ∼5 dB during the first ∼30 min of transmission, which may be a signature of slow electron density changes from sustained HF heating. Theoretical calculations are made to relate the amplitude observed to the power injected into the waveguide and reaching 250 km. The median power generated by HAARP and injected into the waveguide is ∼0.05–0.1 W in this baseline configuration (vertical beam, 3.25 MHz, amplitude modulation) but may have generated hundreds of watts for brief durations. Several efficiency improvements have improved the ELF/VLF wave generation efficiency further.

High time resolution observations of HF cross‐modulation within the D region ionosphere

High‐frequency cross‐modulation is employed to probe the D region ionosphere during HF heating experiments at the High‐frequency Active Auroral Research Program (HAARP) observatory. We have adapted Fejer's well‐known cross‐modulation probing method to determine the extent of ionospheric conductivity modification in the D region ionosphere with high (5 μsec) time resolution. We demonstrate that the method can be used to analyze D region conductivity changes produced by HF heating both during the initial stages of heating and under steady state conditions. The sequence of CW probe pulses used allow the separation of cross‐modulation effects that occur as the probe pulse propagates upward and downward through the heated region. We discuss how this probing technique can be applied to benefit ELF/VLF wave generation experiments and ionospheric irregularities experiments at higher altitudes. We demonstrate that large phase changes equivalent to Doppler shift velocities >60 km/s can be imposed on HF waves propagating through the heated D region ionosphere.

Dual‐beam ELF wave generation as a function of power, frequency, modulation waveform, and receiver location

Dual‐beam ELF wave generation experiments performed at the High‐frequency Active Auroral Research Program (HAARP) HF transmitter are used to investigate the dependence of the generated ELF wave magnitude on HF power, HF frequency, modulation waveform, and receiver location. During the experiments, two HF beams transmit simultaneously: one amplitude modulated (AM) HF beam modulates the conductivity of the lower ionosphere at ELF frequencies while a second HF beam broadcasts a continuous waveform (CW) signal, modifying the efficiency of ELF conductivity modulation and thereby the efficiency of ELF wave generation. We report experimental results for different ambient ionospheric conditions, and we interpret the observations in the context of a newly developed dual‐beam HF heating model. A comparison between model predictions and experimental observations indicates that the theoretical model includes the essential physics involved in multifrequency HF heating of the lower ionosphere. In addition to the HF transmission parameters mentioned above, the model is used to predict the dependence of ELF wave magnitude on the polarization of the CW beam and on the modulation frequency of the modulated beam. We consider how these effects vary with ambientD‐region electron density and electron temperature.

Analytical and Numerical Study of Combustion Noise Through a Subsonic Nozzle

Two mechanisms control sound generated by combustion in gas turbines: direct combustion noise in which acoustic waves generated by the flame propagate from the combustion chamber to the outlet through turbine stages, and indirect combustion noise (or entropy noise) in which entropy waves created by unsteady combustion generate noise as they are accelerated through the turbine stages. These mechanisms can be studied in laboratories by sending acoustic and entropy waves through a nozzle as done in the Entropy Wave Generator experiment. Previous studies have addressed the case where the nozzle is choked and have demonstrated that indirect noise was large compared to direct noise, suggesting that indirect noise only could be retained for gas turbine studies. In the present study, subsonic cases are analysed using, first, a full numerical resolution of the unsteady Euler equations; second, an analytical method based on the work of Marble and Candel in the low-frequency limit, and, finally, the one-dimensional linearized Euler equations in the frequency domain. Results show that direct noise cannot be neglected in these situations and will have to be included for real gas turbines where the flow remains mostly subsonic.

Analysis of time-of-arrival observations performed during ELF/VLF wave generation experiments at HAARP

[1] Modulated high frequency (HF) heating of the lower ionosphere in the presence of auroral electrojet currents has become an important method for generating electromagnetic waves in the extremely-low frequency (ELF) and very-low frequency (VLF) bands. Recent research efforts focus on improving the efficiency of ELF/VLF wave generation. One method to do so involves the spatial mapping of modulated currents that result from HF heating for comparison with HF heating models. As a first step toward providing a spatial map of the modulated ionospheric currents, we introduce time-of-arrival (TOA) observations performed during a series of experimental research campaigns conducted at the High-frequency Active Auroral Research Program (HAARP) in Gakona, Alaska. The TOA method provides a measurement of the ELF/VLF amplitude and phase detected at a ground-based receiver as a function of time, and this information may be used to estimate the distribution of ELF/VLF source currents within the HF heated region. In an effort to test and improve the TOA method, the University of Florida conducted ELF/VLF wave generation experiments using the HAARP HF transmitter under varying ionospheric conditions and using various transmission formats. In this paper, we summarize our experimental results and compare observations with the predictions of a theoretical model.

Experimental investigation of impact generated tsunami; related to a potential rock slide, Western Norway

Two-dimensional experiments of wave generation from the possible Åkneset rock slide have been performed using solid block modules in a transect with a geometric scaling factor of 1:500. The width of the slide model was kept fixed at 0.45 m. The length of the blocks spanned from 1 to 2 m, the thickness was either 0.12 or 0.16 m and the front angle was 45°. Maximum water depth was 0.6 m with the slide plane having an angle of 35°. Three different scenarios were studied. Only the run out side was modelled. Surface elevations at three locations outside the sloping region were measured with ultra sonic wave gauges and discussed in light of hydrodynamic wave theory. Particle Image Velocimetry (PIV) was used to extract instantaneous velocity fields. Comparison is made between experimental velocity profiles and profiles consistent with a Boussinesq theory. High speed video of the impact was recorded and used to determine qualitative aspects of the forward collapse of the impact crater (backfill wave).

Broadband Ultrasound Transducers Using Piezoelectrically Graded Materials

Broadband ultrasound transducers are presented that utilize piezoelectric materials having an effectively graded piezoelectric parameter and/or applied electric field. These transducers are constructed by forming a number of fine V-grooves on one surface of a piezoelectric plate. Broadband frequency characteristics of the proposed transducer is demonstrated through experiments of wave generation in water. The finite element method analysis has proved that even order resonant modes are also excited in the plate with a grooved configuration.

Broadband Transducers Using Effectively Graded Piezoelectric Plates for Generation of Short-Pulse Ultrasound

Broadband ultrasound transducers that utilize piezoelectric materials having an effectively graded piezoelectric parameter and/or an excitation electric field applied across the piezoelectric material are investigated. These transducers are constructed by forming a number of fine V-grooves on one surface of a piezoelectric plate. The broadband frequency characteristic of the proposed transducer is demonstrated by experiments of wave generation in water. Analysis by the finite-element method proves that even-order resonance modes are also excited in the plate with a grooved configuration.