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Homogeneous isotropic turbulence (HIT) has been a useful theoretical concept for more than fifty years of theory, modelling, and calculations. Some exact results are revisited in incompressible HIT, with special emphasis on the 4/5 Kolmogorov law. The finite Reynolds number effect (FRN), which yields corrections to that law, is investigated, using both Kármán–Howarth-type equations and a statistical spectral closure of the Eddy-Damped Quasi-Normal Markovian (EDQNM)-type. This discussion offers an opportunity to give an extended review of such spectral closures, from weak turbulence, as in wave turbulence theory, to a strong one. Extensions of the 4/5 or 4/3 Kolmogorov/Monin laws to anisotropic cases, such as stably stratified and MHD turbulence, are briefly touched on. Before addressing more recent work on compressible isotropic turbulence, the simplest case of quasi-incompressible turbulence subjected to externally imposed isotropic compression or dilatation is presented. Rapid distortion theory is found to be a poor model in this isotropic case, in contrast with its relevance in strongly anisotropic flow cases. Accordingly, a fully nonlinear approach based on a rescaling of all fluctuating variables is used, in order to show its interplay with the linear operator. This opens the discussion on the cases of homogeneous incompressible turbulence, where RDT and nonlinear models are relevant, provided that anisotropy is accounted for. Finally, isotropic compressible flows of increasing complexity are considered. Recent studies using weak turbulence theory, modelling, and DNS are discussed. A final unpublished study involves interactions between the solenoidal mode, inherited from incompressible turbulence, and the acoustic and entropic modes, which are specific to the compressible problem. An approach to acoustic wave turbulence, with resonant triads, is revisited on this occasion. [ABSTRACT FROM AUTHOR]

Homogeneous isotropic turbulence (HIT) has been a useful theoretical concept for more than fifty years of theory, modelling, and calculations. Some exact results are revisited in incompressible HIT, with special emphasis on the 4/5 Kolmogorov law. The finite Reynolds number effect (FRN), which yields corrections to that law, is investigated, using both Kármán–Howarth-type equations and a statistical spectral closure of the Eddy-Damped Quasi-Normal Markovian (EDQNM)-type. This discussion offers an opportunity to give an extended review of such spectral closures, from weak turbulence, as in wave turbulence theory, to a strong one. Extensions of the 4/5 or 4/3 Kolmogorov/Monin laws to anisotropic cases, such as stably stratified and MHD turbulence, are briefly touched on. Before addressing more recent work on compressible isotropic turbulence, the simplest case of quasi-incompressible turbulence subjected to externally imposed isotropic compression or dilatation is presented. Rapid distortion theory is found to be a poor model in this isotropic case, in contrast with its relevance in strongly anisotropic flow cases. Accordingly, a fully nonlinear approach based on a rescaling of all fluctuating variables is used, in order to show its interplay with the linear operator. This opens the discussion on the cases of homogeneous incompressible turbulence, where RDT and nonlinear models are relevant, provided that anisotropy is accounted for. Finally, isotropic compressible flows of increasing complexity are considered. Recent studies using weak turbulence theory, modelling, and DNS are discussed. A final unpublished study involves interactions between the solenoidal mode, inherited from incompressible turbulence, and the acoustic and entropic modes, which are specific to the compressible problem. An approach to acoustic wave turbulence, with resonant triads, is revisited on this occasion.

The application of smart materials and metastructures has been rapidly increasing in advanced multiphysical systems because of their ability to modify mechanical responses by adding circuits in a programmable way. This paper proposes to exploit functional gradation and programmed disorder for flexural wave manipulation to enhance broadband vibration control, leading to a new application of smart metamaterials. The graded metamaterial configuration involves arranging the shunted piezoelectric patches with algorithmically obtained spatially varying parameters, resulting in wideband wave attenuation and mode trapping. The considered locally grading parameter here is the shunt resonant frequency of the unit cells, designed following the rainbow trap idea and referred to as 'rainbow' metamaterials. Two metastructures are developed in this article by tuning the shunted piezoelectric electrical circuit in single and multiple configurations, each related to the unit cell. The computationally efficient spectral element method is employed to calculate the dynamic response, and the spectral transfer matrix method is integrated therein to obtain the dispersive diagram. Subsequently, effective vibration mitigation in a wider frequency band is realized through wave manipulation based on the concept of rainbow metamaterials. To this end, we have considered a unimorph beam hosting an array of piezoelectric unit cells with single and multiple resonant shunts for obtaining the numerical results, which demonstrate that the vibration attenuation zone of the multi-resonant rainbow arrangement becomes significantly wider than the single shunt configuration. The programmed disorder in the elastic waves imposes the veering effect, which generates an interaction between two dispersion curves showing a coupling phenomenon for the waves. It involves relevant energetic exchanges between the wave modes and strongly affect the undamped forced response of the system that can influence the wave trapping generated by the proposed metamaterial. Such outcomes lead to the realization of the benefit of rainbow smart metastructures compared to conventional locally resonant metamaterials on vibration and elastic bandwidth manipulation. • Design of smart rainbow metamaterials aiming to enhance single- and multi-broadband vibration and bandgap control. • Exploitation of functional gradation and programmed disorder for enhanced flexural wave manipulation. • Graded shunted piezoelectric patches to obtain spatially varying parameters for wideband wave attenuation and mode trapping. • Veering effect in the elastic waves involving energetic exchanges between the wave modes improving the vibration control. [ABSTRACT FROM AUTHOR]

The paper is devoted to the problem of additive fault detection observer design for discrete LTI plants with scalar measurement and external disturbance with the known spectral features. Choice of the observer-filter parameters should maximize its sensitivity to the faults effect and minimize its response to the external disturbance signal. These features are provided by the special filter, generating the corrective signal. The specific spectral approach to discrete H2 optimization in frequency domain, based on the polynomial factorization, is applied with the aim to improve computational effectiveness of the synthesis. Some theoretical aspects are discussed and the novel algorithm of discrete adaptive fault detection observer analytical design is formulated and its effectiveness is demonstrated by the numerical example with implementation of MATLAB package.

• Diffusion and multi-species heterogeneous reactions in porous media are studied numerically. • Homogenized properties of diffusion and reaction are determined numerically according to the Damköhler number. • For high Damköhler number, complex behavior at short time, is successfully captured and validate numerically. • The macroscopic homogenized equations are validated numerically by comparing to the direct numerical simulation results. In our previous work, macroscopic models for multi-species diffusion/heterogeneous reaction with different diffusion coefficients in porous media were derived from the homogenization technique based on a spectral approach (Le et al. , Applied Mathematical Modelling, 104 , 666–681, 2022). This work aims to analyze numerically the properties of the eigenvalues and eigenfunctions of the spectral problem, the solution of the closure problems and the effective coefficients as a function of key parameters such as the Damköhler number, the ratios of the reaction rates and diffusion coefficients. This provides in-depth understanding of the role of the coefficients involved in the macroscopic models in the whole range of the Damköhler number. Simulation results of the macroscopic equations are compared with direct numerical simulations in unsteady regime, showing a very satisfactory agreement and thus numerically validating the proposed models. [ABSTRACT FROM AUTHOR]

Presented here is the application of the spherical harmonic expansion to the nonclassical transport equation to derive a system of equations for the nonclassical flux moments, namely nonclassical spherical harmonic approximations (NSHA). The nonclassical transport equation is a recently developed mathematical model that allows the modeling of transport problems wherein the particle flux does not undergo exponential attenuation. We employ a Spectral Approach technique to depict the nonclassical flux by expressing it as a truncated series of Laguerre polynomials with respect to the free-path variable s. This yields a system of equations for the nonclassical flux moments structured similarly to the classical P N equations. We show that the NSHA is reduced to the classical P N equation in a special case. Numerical results for slab-geometry test problems are given to verify the derivation. • Spherical harmonic approximations to the nonclassical transport equation. • We use a spectral approach to deal with the s-dependency of the nonclassical flux. • The NSHA reduces to the classical PN equation in a special case. • We present numerical results in slab-geometry to verify the derivation. [ABSTRACT FROM AUTHOR]

In this paper, we study the exact controllability of a system of two wave equations coupled by velocities with boundary control acted on only one equation. In the first part of this paper, we consider the -d case. Then, using a multiplier technique, we prove that, by observing only one component of the associated homogeneous system, one can get back a full energy of both components in the case where the waves propagate with equal speeds (i.e. in (1)) and where the coupling parameter is small enough. This leads, by the Hilbert Uniqueness Method, to the exact controllability of our system in any dimension space. It seems that the conditions and small enough are technical for the multiplier method. The natural question is then : what happens if one of the two conditions is not satisfied? This consists the aim of the second part of this paper. Indeed, we consider the exact controllability of a system of two one-dimensional wave equations coupled by velocities with a boundary control acted on only one equation. Using a spectral approach, we establish different types of observability inequalities which depend on the algebraic nature of the coupling parameter and on the arithmetic property of the wave propagation speeds. [ABSTRACT FROM AUTHOR]

The presented paper is devoted to the problem of the fault estimation observers design in case of plants with the Lipschitz system nonlinearity. Insensitivity of the observer to external disturbances, including ones with the given spectral properties, is to be guaranteed by the choice of its parameters. The proposed method is based on the special spectral approach in frequency domain, using parametrization of the set of transfer functions of the closed-loop system, Nevanlinna-Pick rational function interpolation and the condition of asymptotic stability of the zero equilibrium position of the estimation error dynamics. A novel approach to analytical synthesis of the adaptive nonlinear fault estimation observer is proposed, and its effectiveness is demonstrated on the practical example – the marine ship horizontal motion with the constant longitudinal speed under the sea waves action. The MATLAB package is used for numerical simulation of the ship dynamics.

A spectral algorithm is considered for describing nonlinear generation of an acoustic wave of difference frequency formed as the interaction of two high-intensity pump waves close in frequency. To correctly simulate the full wave spectrum involved in such two-frequency interaction, including formation of shock fronts in the acoustic pressure waveform, it is necessary to retain about several thousand spectral components in the numerical algorithm. This paper proposes a method that enables to significantly reduce this number while maintaining the accuracy of calculating the difference-frequency wave field. The method combines the limitation of the high-frequency part of the spectrum and the combination frequencies located between the spectral components that are multiples of frequencies of the initial pump waves. Examples of the interaction of close frequencies typical for the operation of an underwater parametric array are considered in the approximation of the plane one-dimensional wave propagation. It was shown that the method makes it possible to reduce the number of spectral components included in the nonlinear algorithm by more than 100 times and thereby reduce the number of operations by four orders of magnitude. This makes the algorithms for simulating parametric interactions in ultrasound wave fields feasible, including three-dimensional acoustic beams. [ABSTRACT FROM AUTHOR]

This paper addresses a spectral approach for evaluating the fatigue damage in a floating offshore wind turbine (FOWT) structure accurately by taking account of the nonlinear coupling effects between wind and wave loads. It is shown that the spectral moments under the combined loads can be estimated by a sum of those under wind-only cases and wave only-cases, however, with some corrections to account for the nonlinear coupling effects. A formula to estimate the variance of the stress under combined loads as a function of wind speed and wave height is developed based on a mechanical model. The formula is extended to even higher spectral moments. Then, the developed method is applied to the fatigue damage evaluation in a semi-submersible FOWT. The results from the developed method in terms of the fatigue damage are compared with those from the direct approach for validation. An efficient method for long-term fatigue damage evaluation of FOWT is envisaged based on the proposed method. [ABSTRACT FROM AUTHOR]

This article proposes a control strategy to stabilize the axial–torsional dynamics of a distributed drill-string system. An infinite-dimensional model for the vibrational dynamics of the drill string is used as a basis for controller design. In this article, both the cutting process and frictional contact effects are considered in the bit–rock interaction model. Moreover, models for the top-side boundary conditions regarding axial and torsional actuation are considered. The resulting model is formulated in terms of neutral-type delay differential equations that involve constant state delays, state-dependent state delays, and constant input delays arising from the distributed nature of the drill-string dynamics and the cutting process at the bit. Using a spectral approach, the stability and stabilizability of the associated linearized dynamics are analyzed to support controller design. An optimization-based continuous pole-placement technique has been employed to design a stabilizing controller. Since the designed state-feedback control law needs state prediction, a predictor with observer structure is proposed. Both the controller and the predictor only employ top-side measurements. The effectiveness of the control strategy, in the presence of measurement noise, is shown in a representative case study. It is also shown that the controller is robust to parametric uncertainty in the bit–rock interaction. [ABSTRACT FROM AUTHOR]

Determining a minimum separation distance required to avoid seismic pounding between adjacent buildings is a capacity‐demand design issue. This study developed a spectral approach to evaluate the expected minimum separation demand (referred to as the critical separation distance, [CSD]) to prevent pounding between buildings during strong earthquake motions. Presented firstly in this paper is an analytical procedure for evaluating the relative displacement process at the potential pounding location between two adjacent buildings during earthquake motions. The procedure is based on random vibration theory and modal analysis with application of the well‐known CQC rule. Subsequently, an approximate nonlinear closed‐form solution for evaluating CSD is derived, by means of a simplified spectral‐based procedure with introducing the concept of equivalent linearization of nonlinear hysteretic behavior and the effective correlation coefficients of the generalized coordinates of equivalent linearized systems to consider inelastic vibration phase between buildings. Finally, the CSDs between two buildings having different heights and the same height for various combinations of buildings with different fundamental periods are separately examined as verification examples of the proposed approach. The accuracy of solution is demonstrated by comparing computed results with corresponding numerical results from nonlinear response history analyses (RHAs). The results show that the proposed approach provides a reliable solution in predicting the separation demand of buildings during earthquake motions. [ABSTRACT FROM AUTHOR]

This paper presents an iterative spectral approach for the determination of the maximum heat pump capacity that ensures the longevity of a specified ground heat exchanger field in hybrid ground source heat pump systems. Fast fluid calculation is obtained by spectral convolution of the incremental ground load signal and the ground heat exchanger transfer function. A correction function is used in an iterative scheme to determine the maximum heat pump capacity that maintains fluid temperature within the operation limits. Results show that the maximum capacity is case dependent, and that long-term simulation is important for its determination. The proposed approach is fast, convergent, general and can manage different building load profiles, either balanced or imbalanced, and different heat pump controls. • A spectral approach for the determination of the heat pump capacity is proposed. • An iterative procedure is used to account for nonlinearity of the problem. • The proposed algorithm can manage different types of building load profiles. • The proposed algorithm is compared against a reference algorithm. • The algorithm can simulate the dynamic operation of the system in under 1 s. [ABSTRACT FROM AUTHOR]

This paper presents a modification of the successive flux estimation algorithm of Pasquier and Marcotte (2013) [1] for computation of the so-called g-functions of small to large-scale ground heat exchanger fields. Fast calculation times are obtained by spectral convolution of the incremental flux signal and the borehole transfer function in an iterative scheme. An initial guess that is close to the solution and easily computable is also suggested for fast convergence. Further gains are obtained through the preprocessing of the transfer functions and the sub-sampling of the g-function. The proposed method was compared against the original successive flux estimation method, as well as two other rapid methods in the literature. Results show that the algorithm proposed in this work is by far the fastest and can generate a 40 year g-function of a large ground heat exchanger field composed of 400 randomly positioned boreholes in less than 4 s. The proposed approach is general and can manage fields composed of different number of boreholes and configurations. • An algorithm for construction of g-function of ground heat exchanger is proposed. • Fast calculation times are obtained by spectral convolution. • Efficiency is improved with the successive flux estimation method. • The algorithm can generate a g-function of 400 boreholes in 4 s. [ABSTRACT FROM AUTHOR]

This paper is devoted to slowly varying additive fault detection with implementation of the observer-filter to be designed. Sensitivity of the observer to the external disturbance is to be minimized by the choice of its parameters. There are a lot of papers, devoted to fault detection issues, but such problems with initially given spectral features of external disturbances are not a subject of serious attention, in our opinion. External disturbances, which are considered in this research, can be presented as a sum of harmonic oscillations with given central frequency (sea wave disturbance) and a constant signal (ocean currents and wind). A suppression of the polyharmonical oscillations influence is considered as H2-optimization problem, which can be solved with application of the specific spectral approach in frequency domain. This approach is based on polynomial factorization that can improve computational effectiveness of the observer design procedures. This also guaranties nonuniqueness of the optimal solution that makes possible to provide integral action of the residual signal relatively to the external disturbance for the case when at least two sensors are used. The novel algorithm of adaptive fault estimation observer analytical synthesis is proposed and its effectiveness is demonstrated by the numerical example of the fault detection process with implementation of MATLAB package.

We generalize a previously published numerical approach for the one‐dimensional (1D) nonlinear Schrödinger (NLS) equation based on a multidomain spectral method on the whole real line in two ways: first, a fully explicit fourth‐order method for the time integration, based on a splitting scheme and an implicit Runge‐Kutta method for the linear part, is presented. Second, the 1D code is combined with a Fourier spectral method in the transverse variable both for elliptic and hyperbolic NLS equations. As an example we study the transverse stability of the Peregrine solution, an exact solution to the 1D NLS equation and thus a y‐independent solution to the 2D NLS. It is shown that the Peregine solution is unstable agains all standard perturbations, and that some perturbations can even lead to a blow‐up for the elliptic NLS equation. [ABSTRACT FROM AUTHOR]

A new modification of the minimum-contrast estimator (the weighted MCE) of drift parameter in a linear stochastic evolution equation with additive fractional noise is introduced in the setting of the spectral approach (Fourier coordinates of the solution are observed). The reweighing technique, which utilizes the self-similarity property, achieves strong consistency and asymptotic normality of the estimator as number of coordinates increases and time horizon is fixed (the space consistency). In this respect, this modification outperforms the standard (non-weighted) MCE. Compared to other drift estimators studied within spectral approach (e.g., maximum likelihood, trajectory fitting), the weighted MCE is rather universal. It covers discrete time as well as continuous-time observations and it is applicable to processes with any value of Hurst index H ∈ (0 , 1). To the author's best knowledge, this is so far the first space-consistent estimator studied for H < 1 / 2. [ABSTRACT FROM AUTHOR]

The spectral method of Elnagar and Kazemi (J. Comp. Appl. Math. 76(1–2):147–158, 1996), which yields spectral convergence rate for the approximate solutions of Volterra-Hammerstein integral equations, is generalized in order to solve the larger class of functional integral equation control systems with spectral accuracy. The proposed method is based on the idea of relating spectrally constructed grid points to the structure of projection operators. These operators will be used to approximate the control vector and the associated state vector. Numerical examples are included to demonstrate the accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

Understanding solute segregation thermodynamics is the first step in investigating grain boundary (GB) properties, such as strong yttrium (Y) effects on grain growth and texture evolution in micro-scale polycrystalline magnesium (Mg) alloys. To estimate the average GB segregation behavior in low-solute-concentration Mg alloys (e.g., 2 at.% Y), a state-of-the-art spectral approach is applied based on a per-site segregation energy spectrum for Y solute atoms at zero K obtained from molecular statistics (MS) simulations of ∼ 1 0 4 GB sites in Mg symmetric tilt GBs (STGBs). Although selected MS simulation results are consistent with verification by density functional theory (DFT) calculations, estimates of average segregation tendency based on the zero-K energy spectrum deviate from experimental observations. To resolve this problem, thermodynamic integration (TI) methods based on molecular dynamics (MD) simulations are used to determine the per-site segregation free energies of Y at representative GB sites, which show contributions beyond harmonic approximations can be important for certain GB sites at high temperatures. A surrogate model of per-site segregation free energy is constructed from a small set of TI data points using stacking cross-validation regressors and physics-informed descriptors. This model is applied to predict the Y segregation free energy spectra for thousands of GB sites in Mg STGBs with uncertainty quantification. The average segregation tendency predicted by the spectral approach based on the free energy spectra agrees well (within the uncertainty range) with experimental observations for micro-scale polycrystalline Mg alloys at typical thermomechanical processing temperatures (500 ∼ 800 K), where thermodynamic equilibrium states are likely to be achieved due to fast diffusion. [Display omitted] [ABSTRACT FROM AUTHOR]

We developed a spectral technique that is independent of the light transport modality (diffusive or nondiffusive) to separate optical changes in scattering and absorption in the cat's brain due to the hemodynamic signal following visual stimulation. We observe changes in oxyhemoglobin and deoxyhemoglobin concentration signals during visual stimulation reminiscent of the functional magnetic resonance imaging (fMRI) blood oxygenation level dependence (BOLD) effect. Repeated measurements at different locations show that the observed changes are local rather than global. We also determine that there is an apparent large decrease in the water concentration and scattering coefficient during stimulation. We model the apparent change in water concentration on the separation of the optical signal from two tissue compartments. One opaque compartment is featureless (black), due to relatively large blood vessels. The other compartment is the rest of the tissue. When blood flow increases due to stimulation, the opaque compartment increases in volume, resulting in an overall decrease of tissue transmission. This increase in baseline absorption changes the apparent relative proportion of all tissue components. However, due to physiological effects, the deoxyhemoglobin is exchanged with oxyhemoglobin resulting in an overall increase in the oxyhemoglobin signal, which is the only component that shows an apparent increase during stimulation.

This paper investigates on the effect of story lateral stiffness variation on the maximum elastic interstory drift ratio (IDRmax) for existing reinforced concrete (RC) buildings. Several classes of existing gravity load designed RC buildings are obtained via a simulated design approach. The presence of infills in the perimeter frames as well as different opening percentages along the height are considered. A simplified elastic analysis is performed, adopting an equivalent multistory cantilever system to represent the stiffness variation along the buildings height. IDRmax is significantly influenced by the ratio of the lateral stiffness at the second and upper stories over the lateral stiffness of the first story. Such a ratio has been found dependent on a number of geometric and configuration factors, including the variation of the opening percentage ratio. Two regression formulas are proposed to estimate, given the spectral displacement at the fundamental period T, the roof displacement and IDRmax as a function of the building height and the opening percentage at first and upper stories. Suitable modification of the formulas is also introduced to account for possible cracking of RC elements and infill panels even at very low levels of lateral drift. These expressions could be used for the simplified evaluation of the expected drift demands for buildings of existing RC typologies. Finally, the proposed formulations are applied to a number of permanently monitored buildings, comparing the calculated IDRmax with the one resulting from record processing, obtaining a fair good agreement. [ABSTRACT FROM AUTHOR]

Multi-species diffusion/heterogeneous reaction coupled problem in porous media involving a boundary layer problem is studied. The spectral approach developed in our previous work allows to derive a macroscopic model in the high Damköhler number regime combining homogenization technique and spectral approach. Such a homogenized model cannot describe the perturbation at the external boundary where the chemical equilibrium is not necessarily satisfied. We construct herein a modified model involving additional variables, which are rapidly-decaying functions, to capture the complex physics in the boundary layer. Numerical simulations underline the accuracy of the proposed correction in both steady and transient states. • Boundary layer problem of diffusion/reaction process in porous media is studied. • Macro-model derived from homogenization technique and spectral approach is recalled. • Corrective terms are introduced to modify the concentration's expansion. • These terms decay rapidly from the boundary and are solved on several unit cells. • Numerical simulations highlight the role of the corrective terms in boundary layer. [ABSTRACT FROM AUTHOR]

In this work, we discuss and compare three methods for the numerical approximation of constant- and variable-coefficient diffusion equations in both single and composite domains with possible discontinuity in the solution/flux at interfaces, considering (i) the Cut Finite Element Method; (ii) the Difference Potentials Method; and (iii) the summation-by-parts Finite Difference Method. First we give a brief introduction for each of the three methods. Next, we propose benchmark problems, and consider numerical tests—with respect to accuracy and convergence—for linear parabolic problems on a single domain, and continue with similar tests for linear parabolic problems on a composite domain (with the interface defined either explicitly or implicitly). Lastly, a comparative discussion of the methods and numerical results will be given. [ABSTRACT FROM AUTHOR]

In this study, we numerically investigate the transport properties of a two‐dimensional (2D) complex plasma crystal using diffusion of coplanar dust lattice waves. In the limit where the Hamiltonian interactions can be decoupled from the non‐Hamiltonian effects, we identify two distinct types of wave transport: Anderson‐type delocalization and long‐distance excitation. We use a recently developed spectral approach to evaluate the contribution of the Anderson problem and compare it to the results of the simulation. The benefit of our approach to transport problems is twofold. First, we employ a highly tuneable macroscopic hexagonal crystal, which exhibits many‐body interactions and allows for the investigation of transport properties at the kinetic level. Second, the analysis of the transport problem in 2D is provided using an innovative spectral approach, which avoids the use of scaling and boundary conditions. The comparison between the analytically predicted and numerically observed wave dynamics allows for the study of important characteristics of this open system. In our simulations, we observe long‐distance lattice excitation, which occurs around lattice defects even when the initial perturbation does not spread from the centre to the exterior of the crystal. In the decoupled Hamiltonian regime, this many‐body effect can be attributed to the dust lattice interaction with the plasma environment. [ABSTRACT FROM AUTHOR]

A 3D finite element model of circular borehole cluster connected in series is used as a response model to generate normalized transfer functions of a borehole thermal energy storage system under the influence of ground surface temperature variations and groundwater flow. Two response functions are obtained by convolving transfer functions of the system's outlet fluid temperature with two incremental input functions describing the temperature variation of the inlet and outlet fluid and variations of the ground surface temperature. Superposition is applied to obtain the resulting fluid temperature with respect to thermal inputs at the inlet fluid and the ground surface over the course of ten years and results are compared for various groundwater velocities. This work demonstrates that the combined effect of groundwater flow and ambient air temperature variations can significantly decrease the performance of a BTES system. The methodology used can be extended to simulate complex system. [ABSTRACT FROM AUTHOR]

The key contribution of the paper is in the modification of the well-known delayed resonator concept for vibration suppression using both delayed and non-delayed acceleration feedback control laws together. We further improve the classical resonator setting with lumped delay introducing a distributed delay structure into the control law. It is known that the operable frequency ranges for the delayed resonators are relatively narrow, taking into account both the stability and implementation constraints. As the key result of this paper, in order to widen the operable frequency range, the resonator's feedback is extended by including a non-delayed part to adjust virtually the mass and thus the natural frequency of the active absorber. The proposed extension takes into consideration also the existence of feedback delay and filters to reduce the measurement noise, and retain the simplicity of active controlled resonator tuning against excitations with time varying frequencies. The properties and performance of the resulting algorithms are compared with the delay free PI (proportional and integral) feedback control law. It is shown that the delay-based resonators are easier to implement and can better deal with the measurement noise compared to PI. Spectral theory of retarded and neutral time delay systems are applied to perform a thorough analysis of the resonators. The key theoretical results are validated on a laboratory set-up with voice-coil magnetic shakers. [ABSTRACT FROM AUTHOR]