Reason For Oscillations Research Articles

Концентрационная конвекция в замкнутой области пористой среды при заданном вертикальном перепаде концентрации и учете иммобилизации примеси

This work is devoted to the study of stability of horizontal filtration flow of a mixture through a closed porous domain taking into account impurity immobilization. The instability arises due to the vertical concentration difference of heavy impurities, which creates unstable density stratification. A general mathematical model describing the transport of an impurity through a porous medium is presented. The equations are simplified for the case of low impurity concentration. Simplification made it possible to analytically obtain the solution corresponding to homogeneous horizontal filtration and to study its stability. It is known that, in the narrow regions of porous medium and at weak intensities of the external flow, convection is excited in a monotonous manner. On the contrary, in the case of an infinite horizontal layer, oscillatory instability is observed. A study of the transition between the instability modes is presented. It is shown that the oscillatory regime is observed in long regions or at significant intensity of the external horizontal flow. At low flow intensities, convective cells do not move relative to the region and, hence, there is no reason for oscillations. It has been established that the range of flow intensity values, in which oscillations are observed, grows with increasing length of the domain. Impurity immobilization leads to the stabilization of horizontal filtration with respect to convective perturbations. Critical curves and stability maps are obtained in a wide range of problem parameters and then analyzed. For limiting cases, a comparison is made with the known results obtained for an infinite layer.

The Physical Reasons to Have Underdamped or Oscillating Variable-Head (Slug) Tests: A Review and a Clarification

Variable-head permeability tests are performed in monitoring wells (MWs) or open holes between packers to assess the local value of hydraulic conductivity, K, of gravel, sand, and fractured rock. This article examines physical reasons for having an oscillating water level in the riser pipe, called underdamped (UD) response. The UD solutions are incomplete because they solve one half of the problem—for the water in the pipe—and ignore the second half, seepage in the tested material. This article shows that these half-solutions violate mathematical rules for the second half and thus, are incorrect. Four reasons for oscillations are examined: (1) solid matrix elasticity (storativity), (2) trapped gas, (3) sealing defect, and (4) hydraulic fracturing. Reason (1) yields small oscillations but not large oscillations. Reasons (2–4) can yield large oscillations and explain field data for occurrence, frequency, and damping. Thus, a free space along the casing or some trapped gas is needed to observe large oscillations. Hence, a UD slug test response only proves that the MW (or a packer) is poorly installed. This MW yields wrong values for hydraulic head, gradient, and water analyses. The data of oscillating slug tests are shown to depend upon parameters that are not easy to evaluate. This is why the UD slug tests are found to give poor estimates of K. It is recommended to use suitable drilling techniques to avoid overexcavation, to take 2–4 hours to correctly install a MW, to avoid using compressed gas to start a slug test, and to apply head differences that are high enough to have good results but lower than those producing hydraulic separation. For packer tests, it is recommended to monitor the water pressure between the packers, in the hole below the lower packer and above the upper packer, using three pressure sensors.

Design of Non-Interacting Controllers for PV Systems in Distribution Networks

This paper makes two contributions. Firstly, it is shown that there may exist oscillations in distribution networks with physically close photovoltaic (PV) units due to their control interactions. The main reason for oscillations is changes in the dynamics of PV units with varying solar irradiance and the decentralized tuning of control parameters for the nominal operating condition. Eigenvalue analysis, nonlinear interaction index, and time-domain simulations are used to assess the degree and impacts of dynamic interactions. Secondly, a decentralized robust control design is proposed to ensure a non-interacting and well-damped response under varying operating conditions for physically close PV units. The control design process uses an estimate of changes in the model due to variations in solar irradiations, changing electric loads, and the dynamics of the physically close PV units. A minimally conservative method is used to capture the estimate as a norm-bounded uncertainty. Time-domain simulations on a test distribution system verify the predictions of the interaction analysis and demonstrate the performance of the proposed robust controller under different system contingencies.

An Overview of Impact of Nonlinear Friction in Oscillating Control Loops: from Diagnosis to Compensation

Among the prominent reasons for oscillations in control loops such as poor controller tuning, inappropriate process design, oscillating load and disturbances with a high-frequency, the presence of static friction or stiction in the control valve is the most common reason. As a consequence of the friction, the control valves may present an oscillatory behavior, affecting the regulatory control performance and therefore it increases the variability of the control loop, causing loss of product quality and increasing energy consumption. Modeling of stiction using clear mathematical definition is hard due to its nonlinearity properties such as hysteresis, deadband or deadzone. This comparative study identifies the relative strengths and weaknesses of the different approaches of diagnosing of stiction in control loops and the compensation techniques to mitigate this effect. The aim is to determine the appropriate and precise model of stiction in order to design a proper controller to prevent control loops from oscillations. Numerous algorithms and methods which are either qualitative or quantitative in the literature show that despite these attempts to modeling, diagnosing and compensation of this phenomenon, lack of a reliable methodology to predict occurrence and properties of such oscillations and therefore lack of proper model is quite sensible. Furthermore exclusive of lack of precise model, even solving the problem were limited to some and different assumptions and applications and there is not one method which can cover all cases reliably. From this study too, it is seen that Data Driven model because of providing a simpler solution with fewer parameters is the most common method in contrast to physical based models with the presence of a number of unknown physical parameters. It is our view that hybrid approaches where different methods work in conjunction to solve parts of the problem are attractive and more effective methods. Lastly, numerous methods of stiction detection are available. Hence, it is suggested that the future research emphasis should be more on compensation of stiction especially for nonlinear processes.

Pattern recognition for valve stiction detection with principal component analysis

Static friction (stiction) in control valves is an often unrecognized problem which can lead tooscillating process variables. Therefore it is important to detect stiction at an early stage as the reason for oscillation. This work presents a new and robust method which uses the pattern recognition with principal component analysis for stiction detection.

A Method to Reduce the Oscillations of the Solution of Time Domain Integral Equation Using Laguerre Polynomials

In this paper, the reason of oscillations appeared in late time low energy region of time domain electric flled integral equation using laguerre polynomials is analyzed. And then, we propose an approach to reduce the oscillations by combining frequency and time domain. According to the Fourier transform relation between frequency responses and time responses, the solution of TDIE method with Laguerre expansion is absolutely convergent. Replace the spectrum of oscillation parts of time domain data with accurate frequency data by MOM. The stable time domain data can be obtained by performing IFFT to the revised frequency data. Numerical results are presented to illustrate the e-ciency of this approach. 1. INTRODUCTION For solving the time domain integral equation, the marching on in time (MOT) method (1) is usually employed. However, as pointed by many researchers, some of the TD-EFIE formulations associated with the MOT method sufiers from late-time instability, which usually takes the form of an expo- nentially increasing high frequency oscillation. In order to eliminate the instability of the MOT methods, the approximate averaging method (2) and the implicit scheme based on backward flnite difierence (3) approximation or central flnite difierence (4) methodology are proposed. Even though employing these methods, the solution obtained by using MOT has still exponentially increased late-time oscillations but only extend the span of the stable region. Young-seek Chung and Sarkar (5{7) proposed a marching on in order method of using the expansion by weighted Laguerre poly- nomials for temporal coe-cients of Hertz vector. The advantages of using the weighted Laguerre polynomials are the solution is independent of the time discretization, and all the computations are carried out using spatial variables. So the exponentially increasing solution will not emerge in late time. But there still exist some oscillations in late-time low energy region due to oscilla- tions characteristic of weighted Laguerre polynomials. Oscillation phenomenon efiects on obtaining other electromagnetic characteristic from the late time scattering data. In this paper, the reason of oscillations appeared in late time low energy region of time domain electric flled integral equation using laguerre polynomials is analyzed, and then, we consider absolute convergence of the time so- lution and the Fourier transform relation between temporal response and frequency response. An approach based on discrete Fourier transforms to improve the stabilities in late time is proposed.

AUTOWAVES ON CATALYST SURFACE CAUSED BY CARBON MONOXIDE OXIDATION KINETICS: IMITATION MODEL

We constructed and studied an imitation model of CO oxidation reaction on platinum metals. The model takes into account oxygen dissociative adsorption, monomolecular adsorption and desorption of carbon monoxide, active oxygen transformation into a less active oxygen of the second type, adsorbed CO reaction with the two types of oxygen, and CO diffusion over the surface. We found that adsorbate coverages and reaction rate undergo self-oscillations attended by the autowave behavior of adsorbates on the surface. With respect to oxygen partial pressures the interval of oscillations splits into three parts: nonregular relaxation oscillations, regular oscillations, and chaos, when the period and amplitude of adsorbate concentration oscillations change stochastically. The model shows that nonuniform distribution of the less active second type oxygen on the surface and the difference in the orders of reagents adsorption kinetics is the reason of oscillations. We suggest as well the reasons for the autowaves formation and development on the catalyst surface.