Traveling Wave Model Research Articles

Development and Stiffness Optimization for a Flexible-Tail Robotic Fish

The integral flexible tail has the potential advantage of lifelike undulating motion. However, due to the complex manufacturing process and difficult modification of structural parameters, its application in robotic fish encounters many challenges. Combining rigid structure and flexible material, this letter proposes a passive flexible fish tail, which can achieve continuous movement and high swimming frequency with simple but effective structure. First, with the full consideration of bending deformation of the spring steels, a dynamic model is established. Next, a passive fitting method is particularly applied to imitate the traveling wave model of the carangiform fish. More importantly, a calculation model, which can be used to acquire the theoretical ranges of the stiffness of spring steels, is derived based on the bending model of the cantilever beam subjected to the concentrated force and moment. Finally, the extensive simulation and experiments validate the effectiveness of the proposed methods, and the designed robotic fish can achieve 0.77 m/s (i.e., 1.12 BL/s) at a swimming frequency of 2.5 Hz. The obtained results can provide a valuable sight for improving the swimming performance of the robotic fish.

Hydrodynamic numerical simulation and prediction of bionic fish based on computational fluid dynamics and multilayer perceptron

This paper proposes a prediction strategy for the hydrodynamic performance of bionic fish. The major challenges are meshing and building prediction model. The NACA0012 airfoil is used to replace the fish driven by the body and/or caudal fin (BCF), and a two-dimensional swimming geometric model is constructed. The geometric model is divided into hybrid meshes using the overset mesh method. The classical traveling wave model is studied using Matlab, and an improved self-propelled motion model is established. The hydrodynamic performance of the self-propelled model is numerically simulated based on computational fluid dynamics (CFD). In this strategy, the geometric model and the self-propelled motion model are integrated with user defined functions (UDF). The influences of the parameters such as inflow velocity, frequency, wavelength, and head fluctuation amplitude on the motion performance are studied. The results show that when inflow velocity is uniform, the self-propelled motion will eventually reach a quasi-steady state. According to the numerical simulation results, a hydrodynamic performance prediction model is established based on multilayer perceptron (MLP). The model is used to predict the performance of the optimized traveling wave parameters, and the error is within 3. The accuracy and generalization ability of the MLP prediction model is verified.

A Bragg reflection waveguide laser operating at an exceptional point

A vertical Bragg reflection waveguide (BRW) laser is proposed and simulated for reaching single-mode output, which operates at an exceptional point (EP). We study the properties of the EP-BRW laser using both the transfer matrix method and the traveling wave model. The mode selection mechanism for the EP-BRW laser and the key design parameters for optimum performance are analyzed. The effect of the EP on laser performance is investigated to achieve high confinement transversely of the optical field and low threshold gain. At the EP, such laser system exhibits exotic behaviors, such as high transverse gain, high single-mode stability, and high slope efficiency.

Analysis of Groundwater Recharge in Mongolian Drylands Using Composite Vadose Zone Modeling

Knowledge of groundwater recharge (GR) is important for the effective management of water resources under semi-arid continental climates. Unfortunately, studies and data in Mongolia are limited due to the constraints in funding and lack of research infrastructures. Currently, the wide accessibility of freely available global-scale digital datasets of physical and chemical soil properties, weather data, vegetation characteristics, and depths to the water table offers new tools and basic information that can support low-cost physically based and process-oriented models. Estimates of GR over 41 study sites in Mongolia were obtained using HYDRUS-1D in a 2-m-thick soil profile with root depths of either 0.30 or 0.97 m by exploiting the daily precipitation and biome-specific potential evapotranspiration values. The GR simulated by HYDRUS-1D arrives at the water table and becomes the actual GR with a lag time that has been calculated using a simplified form of the Richards equation and a traveling wave model. The mean annual precipitation ranges from 57 to 316 mm year−1, and on average about 95% of it is lost by mean annual actual evapotranspiration. In the steppe region, the vegetation cover induces higher-than-normal actual transpiration losses and consequently lower GR. The mean annual GR rates span between 0.3 and 12.0 mm year−1, while travel times range between 4 and 558 years. Model prediction uncertainty was quantified by comparing actual evapotranspiration and GR with available maps and by a sensitivity assessment of lag time to the soil moisture in the deep vadose zone. The partial least squares regression (PLSR) was used to evaluate the impact of available environmental properties in explaining the 47.1 and 59.1% variability of the spatially averaged mean annual GR and travel time, respectively. The most relevant contributors are clay content, aridity index, and leaf area index for GR, and depth to the water table and silt content for the lag time. In data-poor, arid, and semi-arid regions such as Mongolia, where the mean annual GR rates are low and poorly correlated to precipitation, the ever-increasing availability of world databases and remote sensing products offers promise in estimating GR.

Turbidity currents on the open slope of the Fraser Delta

Turbidity currents have been observed and measured in submarine channels and canyons where the flows are confined, but with the exception of one serendipitous direct measurement, are only inferred to occur on unconfined delta slopes. In this paper, direct measurement of turbidity currents on the Fraser delta slope are reported and their dynamics compared to turbidity currents in channelized settings. Twelve turbidity current events occurring between May 2014 and June 2018 were identified from Acoustic Doppler Current Profiler instruments deployed at two sites (109 and 159 m water depth). All the events occurred in the months of May and June, in association with high spring river flow. Five of the events were recorded at both sites and the timing of first arrivals gives estimates of the current speed ranging from 0.4 to 3.6 m s−1. Direct current measurements confirm that most flow speeds exceed 1.5 m s−1, although one weak event did not exceed 0.4 m s−1. Current profiles indicate that the flows are of variable thickness, ranging from less than 3 m to 15 m. The turbidity current events varied in duration from 5 to 66 min and, in the cases where transit speeds were available, the durations were proportionally shorter when the transit speed was high. In stronger flows, the initial stage of the flow is highly variable, suggesting extreme turbulence, and the velocity structure of the flow is similar to previous experimental and field observations, with a distinctive relatively thin, high speed lower region being evident at times. Downslope translation of the instrument platform during two of the events suggest the presence of a dense basal layer up to 2.75 m thick, as suggested in the travelling wave model of Heerema et al. (2020). The fact that only approximately half the flows measured at the shallower site are observed at the deeper site suggest that the runout distances of open slope flows are limited. Weaker flows are barely distinguishable in speed and direction from ambient tidal velocities. Flow directions suggest that the open slope flows are unconfined and initiate in the upper slope area that is approximately the same as the area of upslope sediment waves. It is hypothesized that shallow gullies in the same area may be important for initiation and ignition of the flows that become unconfined on the lower part of the slope.

Low-complexity graph-based traveling wave models for HVDC grids with hybrid transmission lines: Application to fault identification

Faults affecting hybrid transmission lines comprising both overhead and underground parts, generate numerous traveling waves may be difficult to describe and evaluate. This paper proposes a combined knowledge-based and behavioral modeling approach to represent the voltage and current waveform after the fault occurrence. The grid is reprensente as a graph which facilitates the selection of waves traveling through the grid for a finite time-horizon simulation. A relatively compact description of the waves is then derived. The obtained model depends explicitly on the characteristics of the grid and on the fault parameters. An application of the model to the identification of the faulty portion of an hybrid line within an HVDC grid is proposed.

Photodetection via Optical Rectification of Terahertz-Modulated Optical Carriers

For fairly fundamental reasons the frequency response of conventional, absorption-based photodetectors decreases with RF modulation frequency, which means they are becoming increasingly hard pressed to meet the need for operation at terahertz (THz) frequencies. Optical rectification (OR) is a non-absorptive, parametric process based on the 2nd-order nonlinear electrical susceptibility, (2). When OR is used to perform photodetection (ORPD), the response time is limited fundamentally by the bandwidth of optical nonlinearity (>200 THz). The RF O-to-E efficiency of an ORPD increases with RF modulation frequency, which makes it increasingly attractive to consider as the frequency increases into the THz. Additionally, OR photodetectors are amenable to significant optical preamplification without saturating the response, allowing orders of magnitude increase in the output current. We develop a lumped-element model of an ORPD, which is confirmed by experiments, that shows the increase in RF O-to-E efficiency with frequency, but only gives an order-of-magnitude estimate of the RF O-to-E efficiency. We then develop a traveling-wave model of an ORPD, which is confirmed by experiments, that more accurately confirms the RF O-to-E efficiency. We measure world record RF O-to-E efficiencies at 16 GHz and 500 GHz of 5 x 10-5 A/W and 3.3 x 10-4 A/W, respectively.

The Elusive Cochlear Filter: Wave Origin of Cochlear Cross-Frequency Masking.

The mammalian cochlea achieves its remarkable sensitivity, frequency selectivity, and dynamic range by spatially segregating the different frequency components of sound via nonlinear processes that remain only partially understood. As a consequence of the wave-based nature of cochlear processing, the different frequency components of complex sounds interact spatially and nonlinearly, mutually suppressing one another as they propagate. Because understanding nonlinear wave interactions and their effects on hearing appears to require mathematically complex or computationally intensive models, theories of hearing that do not deal specifically with cochlear mechanics have often neglected the spatial nature of suppression phenomena. Here we describe a simple framework consisting of a nonlinear traveling-wave model whose spatial response properties can be estimated from basilar-membrane (BM) transfer functions. Without invoking jazzy details of organ-of-Corti mechanics, the model accounts well for the peculiar frequency-dependence of suppression found in two-tone suppression experiments. In particular, our analysis shows that near the peak of the traveling wave, the amplitude of the BM response depends primarily on the nonlinear properties of the traveling wave in more basal (high-frequency) regions. The proposed framework provides perhaps the simplest representation of cochlear signal processing that accounts for the spatially distributed effects of nonlinear wave propagation. Shifting the perspective from local filters to non-local, spatially distributed processes not only elucidates the character of cochlear signal processing, but also has important consequences for interpreting psychophysical experiments.

Genealogical structure changes as range expansions transition from pushed to pulled

Range expansions accelerate evolution through multiple mechanisms, including gene surfing and genetic drift. The inference and control of these evolutionary processes ultimately rely on the information contained in genealogical trees. Currently, there are two opposing views on how range expansions shape genealogies. In invasion biology, expansions are typically approximated by a series of population bottlenecks producing genealogies with only pairwise mergers between lineages-a process known as the Kingman coalescent. Conversely, traveling wave models predict a coalescent with multiple mergers, known as the Bolthausen-Sznitman coalescent. Here, we unify these two approaches and show that expansions can generate an entire spectrum of coalescent topologies. Specifically, we show that tree topology is controlled by growth dynamics at the front and exhibits large differences between pulled and pushed expansions. These differences are explained by the fluctuations in the total number of descendants left by the early founders. High growth cooperativity leads to a narrow distribution of reproductive values and the Kingman coalescent. Conversely, low growth cooperativity results in a broad distribution, whose exponent controls the merger sizes in the genealogies. These broad distribution and non-Kingman tree topologies emerge due to the fluctuations in the front shape and position and do not occur in quasi-deterministic simulations. Overall, our results show that range expansions provide a robust mechanism for generating different types of multiple mergers, which could be similar to those observed in populations with strong selection or high fecundity. Thus, caution should be exercised in making inferences about the origin of non-Kingman genealogies.

Hybrid modeling approach for mode-locked laser diodes with cavity dispersion and nonlinearity

Semiconductor-based mode-locked lasers, integrated sources enabling the generation of coherent ultra-short optical pulses, are important for a wide range of applications, including datacom, optical ranging and spectroscopy. As their performance remains largely unpredictable due to the lack of commercial design tools and the poorly understood mode-locking dynamics, significant research has focused on their modeling. In recent years, traveling-wave models have been favored because they can efficiently incorporate the rich semiconductor physics of the laser. However, thus far such models struggle to include nonlinear and dispersive effects of an extended passive laser cavity, which can play an important role for the temporal and spectral pulse evolution and stability. To overcome these challenges, we developed a hybrid modeling strategy by unifying the traveling-wave modeling technique for the semiconductor laser sections with a split-step Fourier method for the extended passive laser cavity. This paper presents the hybrid modeling concept and exemplifies for the first time the significance of the third order nonlinearity and dispersion of the extended cavity for a 2.6 GHz III–V-on-Silicon mode-locked laser. This modeling approach allows to include a wide range of physical phenomena with low computational complexity, enabling the exploration of novel operating regimes such as chip-scale soliton mode-locking.

Dynamics of a Q-switched bismuth-doped fibre laser: simulation and comparison with experiment

Using an improved travelling wave model, we have numerically investigated the dynamics of a Q-switched bismuth-doped fibre laser. Comparison of simulation results with previously reported experimental data demonstrates good agreement of the multipeak lasing structure and the shape of the strongest pulses at various levels of pumping. We have assessed the effect of the main laser parameters on output laser characteristics and quantitatively evaluated the effects of the nonsaturable loss in the active fibre and amplified spontaneous emission on output power saturation. Based on analysis of time-dependent light intensity and gain distributions in the gain medium at different instants in time, we have formulated recommendations how to improve output laser characteristics.

Obtaining an Operating Point Solution of a Traveling Wave Laser Model

This paper presents a method of obtaining an operating point configuration for a laser model based on a traveling wave model (TWM), which can then be used in a circuit-level simulator. The method first finds an approximate distributed single-mode stationary solution, this solution is then iterated using the traveling wave equations to an accurate single-mode solution, and finally a short pre-simulation is used to add harmonic content to create a multi-mode configuration of the laser approximating its behavior at an operating point. The effectiveness of this approximation is tested by initiating transient simulations from this operating point and comparing them to the output of the model started from an off state. The stochastic variation in the operating point for a particular configuration is also well predicted. Included in the formulation are gain compression and dispersion effects, laser chirp due to variation in the effective index of the laser mode, and spontaneous emission. Finally, the use of the three-stage process of finding the operating point in a circuit-level simulator is discussed. Not only does the three-stage method provide a quick, accurate operating point for the circuit simulator, but the ability to provide an orders of magnitude faster estimate for the initial circuit-level operating point is critical to the practicality of its use in the simulator. The first stage of the three-stage method does just this.

Delay differential equations enriched with nonlinear gain compression for passively mode-locked semiconductor lasers

Non-linear gain compression is well-known to play an important role in the dynamics of short-pulse generation and propagation in semiconductor lasers. Here, a previously reported delay differential equation model for passively mode-locked semiconductor lasers is enhanced with nonlinear gain compression terms in gain and absorber sections. We report the modified model equations and show the impact in gain/absorption dynamics with respect to the original model. In addition, we perform an extended comparison between the enriched delay differential equation model applied on a ring cavity and a travelling wave model applied on an equivalent Fabry-Perot cavity, highlighting the limits of quantitative and qualitative agreement between the two approaches.

Optimization of the Parameters of a Compact Laser Microchip Based on Nd:LSB with Cr:YAG Saturable Absorber

Two models of Nd:LSB microchip laser with Cr:YAG saturable absorber have been compared: the travelling wave model and the point model which is deduced from the first one by averaging of all characteristics over the resonator length. It has been shown that the point model is applicable only for a narrow range of laser parameters, in other cases the results of simulations based on these two models differ significantly.

Integration of Traveling Wave Optical Device Models Into an MNA-Based Circuit Simulator

This article presents a method for integrating traveling wave models (TWMs) into an optical circuit simulator based on modified nodal analysis (MNA) techniques previously developed for electrical circuit simulation. TWMs of optical propagation through waveguides provide flexible distributed descriptions of optical devices from simple waveguides to lasers. The model is, however, by definition an explicit time marching method and does not fit naturally into an MNA formulation. This article will show a method of integrating the TWM into a fully implicit multidomain (electrical/optical) circuit simulator based on MNA techniques. It will first describe the TWM for simple optical propagation and then its extension for optical gratings, amplifiers, and lasers. The integration of the TWM model into the MNA-based engine will then be specified, showing how the model is implemented using nonlinear optical sources. Finally, a set of examples, including waveguides, filters, and electro-optic devices will be used to show the effectiveness of the technique.

Mutation bias can shape adaptation in large asexual populations experiencing clonal interference.

The extended evolutionary synthesis invokes a role for development in shaping adaptive evolution, which in population genetics terms corresponds to mutation-biased adaptation. Critics have claimed that clonal interference makes mutation-biased adaptation rare. We consider the behaviour of two simultaneously adapting traits, one with larger mutation rate U, the other with larger selection coefficient s, using asexual travelling wave models. We find that adaptation is dominated by whichever trait has the faster rate of adaptation v in isolation, with the other trait subject to evolutionary stalling. Reviewing empirical claims for mutation-biased adaptation, we find that not all occur in the 'origin-fixation' regime of population genetics where v is only twice as sensitive to s as to U. In some cases, differences in U are at least ten to twelve times larger than differences in s, as needed to cause mutation-biased adaptation even in the 'multiple mutations' regime. Surprisingly, when U > s in the 'diffusive-mutation' regime, the required sensitivity ratio is also only two, despite pervasive clonal interference. Given two traits with identical v, the benefit of having higher s is surprisingly small, occurring largely when one trait is at the boundary between the origin-fixation and multiple mutations regimes.

Vector demography, dispersal and the spread of disease: Experimental epidemics under elevated resource supply

Abstract The spread of many diseases depends on the demography and dispersal of arthropod vectors. Classic epidemiological theory typically ignores vector dynamics and instead makes the simplifying assumption of frequency‐dependent transmission. Yet, vector ecology may be critical for understanding the spread of disease over space and time and how disease dynamics respond to environmental change. Here, we ask how environmental change shapes vector demography and dispersal, and how these traits of vectors govern the spatiotemporal spread of disease. We developed disease models parameterised by traits of vectors and fit them to experimental epidemics. The experiment featured a viral pathogen (CYDV‐RPV) vectored by aphids Rhopalosiphum padi among populations of grass hosts Avena sativa under two rates of environmental resource supply (i.e. fertilisation of the host). We compared a non‐spatial model that ignores vector movement, a lagged dispersal model that emphasises the delay between vector reproduction and dispersal, and a travelling wave model that generates waves of infections across space and time. Resource supply altered both vector demography and dispersal. The lagged dispersal model fit best, indicating that vectors first reproduced locally and then dispersed globally among hosts in the experiment. Elevated resources decreased vector population growth rates, nearly doubled their carrying capacity per host, increased dispersal rates when vectors carried the virus, and homogenised disease risk across space. Together, the models and experiment show how environmental eutrophication can shape spatial disease dynamics—for example, homogenising disease risk across space—by altering the demography and behaviour of vectors. A free Plain Language Summary can be found within the Supporting Information of this article.

Parameter extraction and inverse design of semiconductor lasers based on the deep learning and particle swarm optimization method.

A deep-learning artificial neural network (NN) combined with the particle swarm optimization (PSO) method has been proposed to inversely design the semiconductor laser with high accuracy and computational speed. This method is exempt from the single-solution problem of tandem NN and can be highly useful to extract the possible problematic parameters in the failure analysis of a device. The light-current curves and small signal responses have been tested against the benchmarks calculated by the traveling-wave model to demonstrate the NN's robustness and efficiency in simulating the laser behavior for further use in the inverse design by PSO.

Analysis of solitonic pulse propagation in metamaterials implemented in photonic crystals

ABSTRACT In this paper we present the analysis of soliton propagation in photonic crystal metamaterials. Current analysis considers the case of the complex refractive index for double negative (DNG) materials and for regular metamaterials. The analytical solution of the nonlinear Schrödinger (NLS) partial differential equation is realized through the traveling wave model. The analysis reveals the existence of soliton pulse propagation under specific constraint conditions necessary for the existence of the soliton solutions. The analytical solutions are validated by the results obtained through simulations. The analysis results give design guidance of waveguide structures able to support undistorted soliton wave propagation.

Narrow-linewidth distributed feedback moiré-grating laser for high-speed optical communications.

A time-domain traveling wave model is described to simulate a novel distributed feedback moiré-grating (DFM) laser, which can suppress the longitudinal spatial hole burning (LSHB) effect and improve the spectral linewidth for high-speed optical communications. The longitudinal photon distribution, linewidth, and dynamic modulation characteristics of the DFM lasers are calculated and compared with the state-of-the-art distributed feedback lasers. We found that the DFM laser can also improve the light-current slope efficiency and modulation bandwidth by introducing multiple $\unicode{x03C0} $π-phase shifts at the moiré envelope index changes while effectively suppressing the LSHB.