Time-domain Calculations Research Articles

Geometrically nonlinear vibrations of plate embedding a large circular acoustic black hole

The concept of acoustic black hole (ABH) designates a decrease in bending stiffness and a localized added damping within a thin structure aiming to localize and efficiently dissipate vibrations. The small thickness results in large amplitude vibrations, which suggests to take into account geometrical nonlinearities. The subject has been previously studied on a beam with a one dimensional ABH. Previous research studied a geometrically nonlinear plate embedding a one dimensional ABH. This study aims at solving the Von Karman equations for a variable thickness plate, via a modal projection using the eigenmodes obtained with a finite element model. The use of a finite element model in this context allows to take into consideration more complex geometries. The methodology is then applied to a rectangular plate embedding a large circular ABH with added damping. The convergence of the nonlinear coefficients is studied and time domain computations show the relative importance of the geometrical nonlinearity.

Neural Dynamics Associated with Biological Variation in Normal Human Brain Regions.

The processes involved in encoding and decoding signals in the human brain are a continually studied topic, as neuronal information flow involves complex nonlinear dynamics. This study examines awake human intracranial electroencephalography (iEEG) data from normal brain regions to explore how biological sex influences these dynamics. The iEEG data were analyzed using permutation entropy and statistical complexity in the time domain and power spectrum calculations in the frequency domain. The Bandt and Pompe method was used to assess time series causality by associating probability distributions based on ordinal patterns with the signals. Due to the invasive nature of data acquisition, the study encountered limitations such as small sample sizes and potential sources of error. Nevertheless, the high spatial resolution of iEEG allows detailed analysis and comparison of specific brain regions. The results reveal differences between sexes in brain regions, observed through power spectrum, entropy, and complexity analyses. Significant differences were found in the left supramarginal gyrus, posterior cingulate, supplementary motor cortex, middle temporal gyrus, and right superior temporal gyrus. This study emphasizes the importance of considering sex as a biological variable in brain dynamics research, which is essential for improving the diagnosis and treatment of neurological and psychiatric disorders.

Coherent Phonon Dynamics in Plasmonic Gold Tetrahedral Nanoparticle Ensembles.

Coherent phonon modes supported by plasmonic nanoparticles offer prospective applications in chemical and biological sensing. Whereas the characterization of these phonon modes often requires single-particle measurements, synthetic routes to narrow size distributions of nanoparticles permit ensemble investigations. Recently, the synthesis of highly monodisperse gold tetrahedral nanoparticles with tunable edge lengths and corner sharpnesses has been developed. Herein, we characterize a size series of these nanoparticles in colloidal dispersion via transient absorption spectroscopy to examine their mechanical and plasmonic responses upon photoexcitation. Oscillations of transient absorption signals are observed in the plasmon resonance and correspond to the lowest-order radial breathing modes of the nanoparticles, the frequencies of which are affected by the edge length and truncation of the corners. Homogeneous quality factor values ranging from 24 to 34 are observed for the oscillations that convey potential utility in mass-sensing and plasmon-exciton-coupling photonics schemes. Finite-difference time domain and finite element analysis calculations establish specific optically relevant phonon modes.

A Computer Model of a Marx Generator Charging a Coaxial Switched Wave Oscillator

The paper describes an axisymmetric Finite-Difference Time-Domain computer model of a coaxial Switched Wave Oscillator integrated with a dipole antenna. The model analyzes the operation of a realistic circuit model of a multistage Marx generator that charges the oscillator to a high voltage. The initial field distribution is calculated with an electrostatic finite-difference method solver to speed up the time-domain analysis. The work presents the results of our circuit model of a Marx generator’s simulations of the charging phase, followed by the results from the discharge phase, using our axisymmetric Finite-Difference Time-Domain model. Our work describes new and fast numerical solvers that can observe the operation of Switched Wave Oscillator systems (also considering the connected antenna). The codes could be used in the process of designing such a system. Advanced boundary conditions modeling the spark gap and oscillator’s excitation set our work apart from the other attempts in the literature.

Chiral 3D structures through multi-dimensional transfer printing of multilayer quantum dot patterns

Three-dimensional optical nanostructures have garnered significant interest in photonics due to their extraordinary capabilities to manipulate the amplitude, phase, and polarization states of light. However, achieving complex three-dimensional optical nanostructures with bottom-up fabrication has remained challenging, despite its nanoscale precision and cost-effectiveness, mainly due to inherent limitations in structural controllability. Here, we report the optical characteristics of intricate two- and three-dimensional nanoarchitectures made of colloidal quantum dots fabricated with multi-dimensional transfer printing. Our customizable fabrication platform, directed by tailored interface polarity, enables flexible geometric control over a variety of one-, two-, and three-dimensional quantum dot architectures, achieving tunable and advanced optical features. For example, we demonstrate a two-dimensional quantum dot nanomesh with tuned subwavelength square perforations designed by finite-difference time-domain calculations, achieving an 8-fold enhanced photoluminescence due to the maximized optical resonance. Furthermore, a three-dimensional quantum dot chiral structure is also created via asymmetric stacking of one-dimensional quantum dot layers, realizing a pronounced circular dichroism intensity exceeding 20°.

Design and Implementation of Crowbar and STATCOM for Enhanced Stability of Grid-Tied Doubly Fed Induction Wind Generators

These days, one of the most used layouts in the wind power industry is a variable-speed doubly-fed induction wind generator (DFIWG). For providing active power (P) and reactive power (Q) control during grid failures, this research examines the DFIWG. The system's transient behavior is examined under normal and abnormal circumstances. Through control of rotor side (RSC) and grid side (GSC) converters, Q assistance for the grid, and power converter stress reduction, the suggested control approach achieves system stability while enabling DFIWG to operate smoothly during grid failures. The DFIWG is exposed to three- and two-phase faults to analyze the machine's performance. The crowbar and STATCOM tools are implemented to enhance the system performance under faults and compared with the base case. The implemented tools successfully suppress rotor and stator overcurrent, over voltage at the DC link (DCL), and power oscillations, as well as supporting the grid voltage understudied cases. The obtained results prove that both STATCOM and crowbar not only enhance the system's effectiveness and performance but also enable the system to achieve the fault ride-through capacity (FRTC). MATLAB/SIMULINK 2017b is used for time-domain computer simulations.

A Time-Domain Calculation Method for Gust Aerodynamics in Flight Simulation

Gusts have a significant impact on aircraft and need to be analyzed through flight simulations. The solution for time-domain gust aerodynamic forces stands as a pivotal stage in this process. With the increasing demand for flight simulations within gusty environments, traditional methods related to gust aerodynamics cannot fail to balance computational accuracy and efficiency. A method that can be used to quickly and accurately calculate the time-domain gust aerodynamic force is needed. This study proposes the fitting strip method, a gust aerodynamic force solution method that is suitable for real-time flight simulations. It only requires the current and previous gust information to calculate the aerodynamic force and is suitable for different configurations of aircraft and different kinds of gusts. Firstly, the fitting strip method requires the division of fitting strips and the calculation of the aerodynamic force under calibration conditions. In this study, the double-lattice method and computational fluid dynamics are used to calculate the aerodynamic force of the strips. Then, the amplitude coefficients and time-delay coefficients are obtained through a fitting calculation. Finally, the coefficients and gust information are put into the formula to calculate the gust aerodynamic force. An example of a swept wing is used for validation, demonstrating congruence between the computational results and experimental data across subsonic and transonic speeds, which proves the accuracy of the fitting strip method in both discrete gusts and continuous gusts. Compared with other methods, the fitting strip method uses the shortest time. Furthermore, the results of a calculation for normal-layout aircraft show that this method avoids the shortcomings of the rational function approximation method and is more accurate than the gust grouping method. Concurrently, gust aerodynamic force calculations were performed on aircraft with large aspect ratios and used in a real-time flight simulation.

Facile Fabrication of SERS Substrates by the Electrodeposition Method to Detect Pesticides with High Enhancement Effect and Long-Term Stability.

In this study, surface-enhanced Raman scattering substrates were investigated by the electrodeposition method to detect low concentrations of pesticides via the electrodeposition method with different agents from silver and gold precursors on APTES-modified ITO glass. A dual-potential method supplied three electrodes and was performed with a nucleation potential of 0.7 V for 2 s and a growth potential of -0.2 V for 500 s. The Ag film produced by the electrodeposition approach has great surface uniformity and good SERS signal amplification for the thiram insecticide at low concentrations. Interestingly, the ITO/APTES/Ag substrate extensively increased the sensitivity than the other investigated ones, thanks to the adequate assistance of amino groups of APTES in the denser and hierarchical deposition of Ag NPs. These observations were additionally elucidated via finite-difference time-domain (FDTD) calculation. For thiram, the detection was set at 10-8 M with an enhancement factor of up to 3.6 × 107 times. Comparing the SERS spectra of thiram at concentrations of 10-3, 10-4, and 10-5 M with a relative standard deviation (RSD) of less than 7.0% demonstrates excellent reproducibility of the ITO/APTES/Ag substrate. In addition, the special selectivity of the ITO/APTES/Ag substrate for thiram demonstrates that these nanostructures can identify pesticides with extreme sensitivity.

Improving diagnostic precision in amyloid brain PET imaging through data-driven motion correction

BackgroundHead motion during brain positron emission tomography (PET)/computed tomography (CT) imaging degrades image quality, resulting in reduced reading accuracy. We evaluated the performance of a head motion correction algorithm using 18F-flutemetamol (FMM) brain PET/CT images.MethodsFMM brain PET/CT images were retrospectively included, and PET images were reconstructed using a motion correction algorithm: (1) motion estimation through 3D time-domain signal analysis, signal smoothing, and calculation of motion-free intervals using a Merging Adjacent Clustering method; (2) estimation of 3D motion transformations using the Summing Tree Structural algorithm; and (3) calculation of the final motion-corrected images using the 3D motion transformations during the iterative reconstruction process. All conventional and motion-corrected PET images were visually reviewed by two readers. Image quality was evaluated using a 3-point scale, and the presence of amyloid deposition was interpreted as negative, positive, or equivocal. For quantitative analysis, we calculated the uptake ratio (UR) of 5 specific brain regions, with the cerebellar cortex as a reference region. The results of the conventional and motion-corrected PET images were statistically compared.ResultsIn total, 108 sets of FMM brain PET images from 108 patients (34 men and 74 women; median age, 78 years) were included. After motion correction, image quality significantly improved (p < 0.001), and there were no images of poor quality. In the visual analysis of amyloid deposition, higher interobserver agreements were observed in motion-corrected PET images for all specific regions. In the quantitative analysis, the UR difference between the conventional and motion-corrected PET images was significantly higher in the group with head motion than in the group without head motion (p = 0.016).ConclusionsThe motion correction algorithm provided better image quality and higher interobserver agreement. Therefore, we suggest that this algorithm be adopted as a routine post-processing protocol in amyloid brain PET/CT imaging and applied to brain PET scans with other radiotracers.

Analyses of gas pulsation characteristics in the discharge pipeline of a hyper compressor

For the analysis of gas pulsation characteristics in the pipeline systems of low density polyethylene (LDPE) hyper compressors, a time-domain calculation method for compressor pipeline gas pulsation based on real gas properties is proposed. First the thermophysical property tables are prepared with gas state parameters as independent and dependent variables. Then one-dimensional unsteady flow equations and characteristic equations for pipeline flow are established based on real gas properties. After setting pipeline parameters and boundary conditions, the flow equations are discretized using a two-step method at the inner points of the pipeline. The characteristic equations are discretized using the trapezoidal integration method at the boundary points. Thus, the time-domain solving of pulsating flow field in the pipeline is realized. A self-developed numerical code was used to conduct the time-domain calculation and analysis of gas pulsation in the discharge pipeline of a two-stage hyper compressor. The calculated pressure pulsation curves based on real gas properties are in good agreement with experimental data, with a maximum difference lower than 4% of the local average pressure, which is smaller than the difference between the results calculated based on the plane wave theory and the measured data. The analysis of gas pulsation characteristics shows that the pressure pulsation in the main pipeline is mainly contributed by the fundamental and second harmonics. The pressure pulsation in the safety valve branch is higher than that in the main pipeline, and the harmonic components falling into the acoustic resonance frequency range of this branch have significant amplitudes. The acoustic resonance frequency of the branch pipeline is not affected by the main pipeline. Pressure pulsation varies with changes in rotational speed and back pressure, and the pulsation level is more significantly affected by rotational speed compared to back pressure. The renovation scheme of connecting the safety valve branch and expansion tube in series can effectively reduce the pressure pulsation levels of the main pipeline and branch pipeline.

Underwater acoustic non-reciprocal manipulation based on dynamic-modulation structures

Abstract Underwater acoustic non-reciprocal transmission via dynamic-modulation structures with time-varying mass and stiffness is studied. The model system consists of spatiotemporally modulated discrete lattices immersed in the water background. Based on the transfer matrix method, an analytic model for the coupled continuum-discrete system is developed to calculate acoustic scattering responses in the frequency domain. Finite-difference time-domain computation is conducted for the coupled system to verify the theoretical model. Results show that acoustic non-reciprocal transmission in opposite directions appears at frequencies where there are asymmetric bandgaps in dispersion diagrams. Asymmetric transmission can be enhanced in magnitude by engineering the modulating amplitudes of time-varying parameters or increasing the number of lattice elements, while the frequency bandwidth can be broadened by cascading structural elements with different modulating frequencies due to the gap-combining effect. The model may find potential applications in underwater acoustic isolation and sonar communication.

The importance of first-principles tools for safety enhancement in the design of passenger ships in the case of flooding events

The design of a passenger ship is a complex process covering multiple aspects of naval architecture and marine engineering to address performance, functionality safety, and cost as primary objectives. Between them, safety is a key element focusing on the people on board. In this sense, ship safety in the case of flooding events needs proper estimation from the first stages of the design process employing an appropriate metric. To this end, safety can be evaluated as a risk by calculating the Potential Loss of Life. Thanks to a multi-level framework developed during project FLARE, it is possible to calculate the risk associated with an accident, increasing the level of reliability as the design process advances. The framework aims at employing first-principles tools from the early stages of the design process, abandoning static calculations and empirical formulae as soon as data is available to set up advanced calculation techniques. Then, the framework adopts rigid-body time-domain calculations for the flooding simulations, advanced evacuation analysis tools, and direct crash simulation to evaluate collision damages. The process allows for testing alternative design solutions for the ship to enhance safety. Investigating risk control options is also possible, considering active or passive systems such as fixed foam installations, deployable barriers, or crashworthiness. Such an approach allows for evaluating safer solutions, respecting other design constraints and cost-related aspects. The present work describes the risk assessment framework for the case of flooding events, together with the different levels of accuracy that can be achieved, showing the improvements that could be reached by employing alternative risk control options.

Detecting of Barely Visible Impact Damage on Carbon Fiber Reinforced Polymer Using Diffusion Ultrasonic Improved by Time-Frequency Domain Disturbance Sensitive Zone.

Based on the decorrelation calculation of diffusion ultrasound in time-frequency domain, this paper discusses the repeatability and potential significance of Disturbance Sensitive Zone (DSZ) in time-frequency domain. The experimental study of Barely Visible Impact Damage (BVID) on Carbon Fiber Reinforced Polymer (CFRP) is carried out. The decorrelation coefficients of time, frequency, and time-frequency domains and DSZ are calculated and compared. It has been observed that the sensitivity of the scattered wave disturbance caused by impact damage is non-uniformly distributed in both the time and frequency domains. This is evident from the non-uniform distribution of the decorrelation coefficient in time-domain and frequency-domain decorrelation calculations. Further, the decorrelation calculation in the time-frequency domain can show the distribution of the sensitivity of the scattered wave disturbance in the time domain and frequency domain. The decorrelation coefficients in time, frequency, and time-frequency domains increase monotonically with the number of impacts. In addition, in the time-frequency domain decorrelation calculation results, stable and repetitive DSZ are observed, which means that the specific frequency component of the scattered wave is extremely sensitive to the damage evolution of the impact region at a specific time. Finally, the DSZ obtained from the first 15 impacts is used to improve the decorrelation calculation in the 16-th to 20-th impact. The results show that the increment rate of the improved decorrelation coefficient is 10.22%. This study reveals that the diffusion ultrasonic decorrelation calculation improved by DSZ makes it feasible to evaluate early-stage damage caused by BVID.

Construction of an Au/g-C3N4/GO/Au substrate by a layer-by-layer assembly approach for surface-enhanced Raman spectroscopy

In this work, an Au/g-C3N4/GO/Au hybrid nanofilm for surface-enhanced Raman scattering (SERS) substrate application was prepared by a facile layer-by-layer assembly technique using nanosheet-structured graphitic carbon nitride (g-C3N4), graphene oxide (GO) and 40 nm-gold nanoparticles (Au NPs). The effects of composition, assembly sequence and assembly cycles on the SERS performance were optimized to achieve the SERS-active substrate. The detection limit of Au/g-C3N4/GO/Au substrate for crystal violet and rhodamine 6 G probe molecules reached as high as 10−8 M, with good stability and uniformity. Its further application in food colorants detection (erythrosine, carmine and temptation red) had successfully met the national food safety standards and the quantitative detection for erythrosine and temptation red had also been established. Based on finite-difference time-domain element simulation and density functional theory quantum chemical calculation, the mechanism of the SERS effect of the substrate was unveiled, which is attributed to the combination of electromagnetic enhancement effect of Au nanoparticle aggregates and chemical enhancement effect of GO and g-C3N4. Notably, water droplet evaporation induced Raman enhancement was observed on the Au/g-C3N4/GO/Au substrate. This can be ascribed to the inhomogeneous evaporation of the droplet that promotes the enrichment of the probe molecules into the center of the droplet. This novel enhancement strategy provides a new idea for further improving detection sensitivity of SERS substrate materials.

A DC fault current fast-computing method of MMC-HVDC grid with short circuit protection equipment

The multi-terminal modular multi-level converter-based high voltage direct current (MMC-HVDC) grid with short circuit protection equipment (SCPE) is so complex that it is difficult to estimate its fault current and analyze the performance of SCPE by conventional time-domain numerical calculation method, it meets three big obstacles. This paper has made significant progress in overcoming these obstacles. 1). By applying the modern electrical circuit theory, a systematic formulation of the differential equation set for fault current calculation is developed to avoid a lot of complex and cumbersome matrix manual calculations. 2). A novel Y-Delta transformation in the s-domain is proposed to develop an eliminating virtual node approach for a complex MMC-HVDC grid, including the ring, radial, and hybrid topologies. 3). It is difficult to solve the equivalent circuit of MMC-HVDC grid with SCPE since SCPE is a time-variable-nonlinear circuit. A canonical voltage source model of SCPE is established to transform the time-variable-nonlinear circuit into a piecewise linear circuit. Based on the three significant progresses, a DC fault current fast-computing method of MMC-HVDC grid with SCPE is put forward to deal with all kinds of MMC-HVDC grids with several kinds of SCPEs. Then, the performance of several kinds of SCPE is analyzed and compared by this method. Consequently, the proposed DC fault current fast-computing method is a new powerful tool to estimate the fault current of MMC-HVDC grid and analyze the performance of SCPE.

Modern Approaches for Historical Seismograms: Moment Tensor Inversion of the 1947 Squillace Basin, South Italy, Earthquake

Abstract The scientific community has become increasingly aware of the importance of preserving and recovering historical seismic data, also because of their possible use in combination with modern techniques of analysis. Seismograms coming from the analog recording era cover more than 100 yr of seismic activity and may have a great relevance, especially for seismic risk evaluations in regions struck by destructive events in the past centuries but characterized by minor activity in the last decades. In this study we used analog seismograms to investigate an earthquake of presumed magnitude 5.7 that occurred in 1947 in central Calabria, south Italy, a high-seismic risk region framed in a complex geodynamic setting led by northwest-trending Nubia–Eurasia convergence and southeastward Ionian slab rollback. According to seismic catalogs, the 1947 is the only M &amp;gt; 5.5 earthquake instrumentally recorded in an area where the presence of the lateral edge of the Ionian slab has been suggested and an intense debate is still open concerning possible existence, and proper location, of a subduction-transform edge propagator (STEP) fault zone. To study this earthquake, we selected 15 medium- to long-period analog seismograms with related instrumental parameters, and we proceeded with vectorization process and proper waveform corrections. A technique specifically developed for time-domain moment tensor computation through waveform inversion of analog seismograms has been applied to the digitized recordings. The moment tensor solution estimated for the 1947 earthquake indicates strike-slip mechanism, focal depth of 28 km and Mw 5.1. The obtained hypocentral depth and left-lateral kinematics on about west-northwest–east-southeast-oriented fault fit well with the local seismotectonic framework and are compatible with STEP fault activity in central Calabria, furnishing a new seismological constraint to the debate concerning slab edge kinematics. Moreover, the presented analysis is useful for sharing with the scientific community new data and methodological issues related to historical seismogram management.

Remote Sensing of Source Currents of Narrow Bipolar Events Using Measured Electric Fields

AbstractIn this work the electric field of narrow bipolar events (NBEs) measured at a remote location is used to extract the current waveform of the source discharge. All calculations correspond to a vertical linear current source above a perfectly conducting ground plane. The current study uses the well established formulation of electromagnetic fields in the frequency domain, and develops a deconvolution based technique to obtain exact reconstruction of the source current, improving upon previous modeling of NBEs, which often require tuning several inter‐dependent parameters to determine the current that best reproduces the observed electric field. Our proposed solution, although readily available in standard electromagnetic textbooks, has never been employed in the context of lightning related discharges, and offers a simple and efficient alternative to previous conventional time domain calculations.

Far-Field Petahertz Sampling of Plasmonic Fields.

The response of metal nanostructures to optical excitation leads to localized surface plasmon (LSP) generation with nanoscale field confinement driving applications in, for example, quantum optics and nanophotonics. Field sampling in the terahertz domain has had a tremendous impact on the ability to trace such collective excitations. Here, we extend such capabilities and introduce direct sampling of LSPs in a more relevant petahertz domain. The method allows to measure the LSP field in arbitrary nanostructures with subcycle precision. We demonstrate the technique for colloidal nanoparticles and compare the results to finite-difference time-domain calculations, which show that the build-up and dephasing of the plasmonic excitation can be resolved. Furthermore, we observe a reshaping of the spectral phase of the few-cycle pulse, and we demonstrate ad-hoc pulse shaping by tailoring the plasmonic sample. The methodology can be extended to single nanosystems and applied in exploring subcycle, attosecond phenomena.

Semi-Analytical Formula for Estimating Variance Response of Megawatt Wind Turbine Tower Under Parked Condition

As the supporting structure of the wind turbine, the tower is important for the safety of the wind turbine. The accurate estimation of tower response is critical for the wind turbine tower design. In the preliminary design, lots of calculations are necessary to determine design parameters. The traditional time domain calculation suffers its low efficiency and hinders the design progress. To overcome this difficulty, the frequency domain analysis should be developed, especially for the large wind turbine which may be significantly affected by the aerodynamic damping. In this study, a semi-analytical formula for the variance of the tower base bending moment is developed from the frequency domain analysis at parked condition. First, based on the quasi-steady theory and the three-component (mean, background and resonant components) method commonly used in the wind-resistant design of structure, the analytical formula of the variance for the tower base bending moment is presented considering the three components of the wind velocity. Second, the analytical formula is corrected based on the comparison with the calculation result by FAST. In the correction, the empirical aerodynamic damping is adopted. Finally, the applicability of the developed semi-analytical formula is verified by 5[Formula: see text]MW, 10[Formula: see text]MW and 15[Formula: see text]MW wind turbines.

Efficient time domain response computation of massive wave power farms

A potential future challenge in the wave energy sector will involve the design and construction of massive wave power farms. That is, collections of several (> 100) wave energy converters (WEC) operating in identical environmental conditions at a distance comparable with typical water wave lengths. In this context, the WECs are likely to be influenced by each another by radiation force effects that are associated with the radiated wave field propagated by WECs operating in the surrounding wave field. These effects are commonly captured by the Cummins’ equation, where the radiation force is expressed as a convolution integral depending on the past values of the WEC response. Due to this mathematical representation, the time domain computation of the wave farm response can become computationally daunting. This article proposes one approach for computing efficiently the wave farm response in the time domain. Specifically, it demonstrates that the values of the radiation force components can be determined at each time step from their previous values by approximating the retardation function matrix elements via the Prony method. A notable advantage of this approach with respect to the ones available in the open literature is that it does not require either the storage of past response values or additional differential equations. Instead, it uses simple algebraic expressions for updating at each time instant the radiation force values. Obviously, this feature can induce significant computational efficiency in analyzing an actual wave farm facility.The reliability and efficiency of the proposed algorithm are assessed vis-à-vis direct time domain comparisons and Monte Carlo data concerning a wave farm composed by an array of U-Oscillating Water Columns. Notably, the proposed methodology can be applied to any linear or nonlinear dynamics problem governed by differential equations involving memory effects.