Step Variation Research Articles

Analysis of compressed air storage caverns in rock salt considering thermo-mechanical cyclic loading

Exploring the material response of rock salt subjected to the variable thermo-mechanical loading is essential for engineering design of compressed air energy storage (CAES) caverns. Accurate design of salt caverns requires adequate numerical simulations which take into account the most important processes affecting the development of stresses and strains. To fulfill this objective, this paper presents a two-step simulation to analyze the thermo-mechanical behavior of rock salt in the vicinity of CAES caverns. In the first step, the changes in air temperature and pressure resulted from injection and withdrawal processes are estimated using an analytical thermodynamic model. Then, in the second step, the temperature and pressure variations obtained from the analytical model are utilized as the boundary condition for a finite element model of CAES cavern. An elasto-viscoplastic creep model is employed to describe the material behavior of rock salt. In the numerical section, a computational model to simulate the thermo-mechanical behavior of rock salt around the cavern is presented. Finally, the stability and long-term serviceability of the simulated cavern are evaluated considering two extreme loading scenarios: (1) low-pressure working condition and (2) high-temperature operation. Obtained results show that both stability and serviceability of the cavern are highly affected by the internal operating pressure. Dilatancy, damage propagation, tensile failure and increasing the rate of cavern closure are the unfavorable consequences of low-pressure working condition. Similarly, the increased creep rate due to the elevated temperature accelerates the volume convergence and subsequently endangers the serviceability of the system.

Tracking performance of optical phase locking loop with frequency discrimination and control subloop

Optical phase locking is a key technique in homodyne coherent optical communication, coherent optical detection, and active coherent laser beam combination. In these applications, environmental temperature variation and mechanical vibration would affect the accuracy of phase locking, or even cause losing lock. These disturbances are generally equivalent to introducing phase jitter, phase step, frequency ramp, and frequency step in the loop. A frequency discrimination and control subloop is introduced to improve the frequency acquisition, and the tracking performance is studied experimentally. The loop can track phase step in 0.2 ms, and precisely track ±π/2 sine phase jitter for jittering frequency lower than 1 kHz. For frequency ramp, the residual phase error is unaffected for ramping rates slower than 40 MHz/s. The frequency discrimination and control subloop makes the loop lock quickly under a frequency step larger than the pull-in frequency. The mean tracking time is 31 ms for a 1 MHz frequency step. The maximum trackable frequency step is around 160 MHz. Continuous or step variation of phase and frequency could be tracked by the loop with the frequency discrimination and control subloop.

A Digital Self-Sustained Phase Shift Modulation Control Strategy for Full-Bridge LLC Resonant Converters

A digital self-sustained phase shift modulation (DSSPSM) strategy that allows for good soft switching and dynamic response performance in the presence of step variations is presented in this paper. The working principle, soft switching characteristics, and voltage gain formulae of a LLC converter with DSSPSM have been provided separately. Furthermore, the method for realizing DSSPSM is proposed. Specifically, some key components of the proposed DSSPSM are carefully investigated, including a parameter variation analysis, the start-up process, and the zero-crossing capture of the resonant current. The simulation and experiment results verify the feasibility of the proposed control method. It is observed that the zero voltage switching of the switches and the zero current switching of the rectifier diodes can be easily realized in presence of step load variations.

Toward intensifying design of experiments in upstream bioprocess development: An industrial Escherichia coli feasibility study.

In this study, step variations in temperature, pH, and carbon substrate feeding rate were performed within five high cell density Escherichia coli fermentations to assess whether intraexperiment step changes, can principally be used to exploit the process operation space in a design of experiment manner. A dynamic process modeling approach was adopted to determine parameter interactions. A bioreactor model was integrated with an artificial neural network that describes biomass and product formation rates as function of varied fed-batch fermentation conditions for heterologous protein production. A model reliability measure was introduced to assess in which process region the model can be expected to predict process states accurately. It was found that the model could accurately predict process states of multiple fermentations performed at fixed conditions within the determined validity domain. The results suggest that intraexperimental variations of process conditions could be used to reduce the number of experiments by a factor, which in limit would be equivalent to the number of intraexperimental variations per experiment. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:1343-1352, 2016.

Thermal Response of Multi-Microchannel Evaporators During Flow Boiling of Refrigerants Under Transient Heat Loads With Flow Visualization

Multi-microchannel evaporators with flow boiling, used for cooling high heat flux devices, usually experience transient heat loads in practical applications. These transient processes may cause failure of devices due to a thermal excursion or poor local cooling or dryout. However, experimental studies on such transient thermal behavior of multi-microchannel evaporators during flow boiling are few. Thus, an extensive experimental study was conducted to investigate the base temperature response of multi-microchannel evaporators under transient heat loads, including cold startups and periodic step variations in heat flux using two different test sections and two coolants (R236fa and R245fa) for a wide variety of flow conditions. The effects on the base temperature behavior of the test section, heat flux magnitude, mass flux, inlet subcooling, outlet saturation temperature, and fluid were investigated. The transient base temperature response, monitored by an infrared (IR) camera, was recorded simultaneously with the flow regime acquired by a high-speed video camera. For cold startups, it was found that reducing the inlet orifice width, heat flux magnitude, inlet subcooling, and outlet saturation temperature but increasing the mass flux decreased the maximum base temperature. Meanwhile, the time required to initiate boiling increased with the inlet orifice width, mass flux, inlet subcooling, and outlet saturation temperature but decreased with the heat flux magnitude. For periodic variations in heat flux, the resulting base temperature was found to oscillate and then damp out along the flow direction. Furthermore, the effects of mass flux and heat flux pulsation period were insignificant.

Improvement of economic profit by optimal allocation of TCSC & UPFC with wind power generators in double auction competitive power market

This paper presents a simple and reliable optimization approach to optimally allocate the Thyristor Controlled Series Compensator (TCSC) and Unified Power Flow Controller (UPFC) with wind generator under deregulated power system. The proposed approach is based on step by step variation in control parameters of TCSC and UPFC devices. Results have been determined for all possible locations, compensation level and reactance of TCSC & UPFC, reactive power injection or absorption, maximization of social welfare, maximization of profit with minimization of objective function. The double auction bidding model has been incorporated in this paper. The impacts on the locational marginal pricing and system voltage have been also investigated in this work. The effectiveness of the proposed approach for optimal placement of TCSC and UPFC has been tested and analyzed on modified IEEE 14-bus and modified IEEE 118-bus systems.

Complexity of Chlorophyll Fluorescence Dynamic Response as an Indicator of Excessive Light Intensity

The controllability of LED lighting systems for greenhouses and plant factories offers a possibility for light induced diagnose of plant status. Here, a novel method for proximal remote detection of plant light tolerance is investigated. The method is based on an identification of a transfer function model for the measured chlorophyll fluorescence response to a small step variation in blue LED light. It is postulated that the least required model order decreases as the plants become light stressed due to saturation effects at excess light conditions. We apply this method to basil and lettuce plants under different background light intensities, and the results are compared to measured effective quantum yield (y(II)), relative electron transport rate through PSII (ETR(II)) and non-photochemical quenching (NPQ), all reflecting the photosynthetic performance. For both species it is indeed found that the required model order decreases with increasing background light intensity at the same time as the measured reference parameters indicates a decreased photosynthetic efficiency. It is suggested that the light intensity should be such that the chlorophyll fluorescence response requires a model order of 3 or higher to avoid ineffective irradiation of the plants.

Using slowness and azimuth fluctuations as new observables for four‐dimensional reservoir seismic monitoring

ABSTRACTFour‐dimensional imaging using geophysical data is of increasing interest in the oil and gas industries. While travel‐time and amplitude variations are commonly used to monitor reservoir properties at depth, their interpretation can suffer from a lack of information to decipher the parts played by different parameters. In this context, this study focuses on the slowness and azimuth angle measured at the surface using source and receiver arrays as complementary observables. In the first step, array processing techniques are used to extract both azimuth and incidence angles at the source side (departure angles) and at the receiver side (arrival angles). In the second step, the slowness and angle variations are monitored in a laboratory environment. These new observables are compared with traditional arrival‐time variations when the propagation medium is subject to temperature fluctuations. Finally, field data from a heavy‐oil permanent reservoir monitoring system installed onshore and facing steam injection and temperature variations are investigated. The slowness variations are computed over a period of 152 days. In agreement with Fermat's principle, strong correlations between the slowness and arrival‐time variations are highlighted, as well as good consistency with other techniques and field pressure measurements. Although the temporal variations of slowness and arrival time show the same features, there are still differences that can be considered for further characterization of the physical changes at depth.

Design and implementation of a linear quadratic regulator controlled active power conditioner for effective source utilisation and voltage regulation in low‐power wind energy conversion systems

Power electronic converters play a major role in wind energy conversion systems (WECS). A typical variable speed WECS includes wind turbine, permanent magnet synchronous generator (PMSG) and power conditioners. In this study, a three-phase modular boost converter supplied from a PMSG and controlled using a linear quadratic regulator (LQR) is proposed for battery charging applications. Each phase of PMSG is connected to a single phase diode rectifier and a DC–DC boost converter. All boost converters have a common output capacitor and load. The proportional–integral-based voltage controller provides voltage regulation on the DC side. A fixed frequency LQR-based current controller applied for the individual phases provides a better three-phase source utilisation. The instantaneous pq theory is adopted for the reference current calculation. Extensive simulation studies based on the models developed using MATLAB/SIMULINK reveal that the proposed system performs satisfactorily for step variations in wind speed and load. The performance of the low power prototype developed and controlled using a dSPACE 1104 digital signal processor shows good output voltage regulation and better source utilisation for wide variations in load.

Effects of 12-week supervised treadmill training on spatio-temporal gait parameters in patients with claudication

The purpose of the study was to evaluate selected temporal and spatial gait parameters in patients with intermittent claudication after completion of 12-week supervised treadmill walking training. The study included 36 patients (26 males and 10 females) aged: mean 64 (SD 7.7) with intermittent claudication. All patients were tested on treadmill (Gait Trainer, Biodex). Before the programme and after its completion, the following gait biomechanical parameters were tested: step length (cm), step cycle (cycle/s), leg support time (%), coefficient of step variation (%) as well as pain-free walking time (PFWT) and maximal walking time (MWT) were measured. Training was conducted in accordance with the current TASC II guidelines. After 12 weeks of training, patients showed significant change in gait biomechanics consisting in decreased frequency of step cycle (p < 0.05) and extended step length (p < 0.05). PFWT increased by 96% (p < 0.05). MWT increased by 100% (p < 0.05). After completing the training, patients’ gait was more regular, which was expressed via statistically significant decrease of coefficient of variation (p < 0.05) for both legs. No statistically significant relation between the post-training improvement of PFWT and MWT and step length increase and decreased frequency of step cycle was observed (p > 0.05).Implications for RehabilitationTwelve-week treadmill walking training programme may lead to significant improvement of temporal and spatial gait parameters in patients with intermittent claudication.Twelve-week treadmill walking training programme may lead to significant improvement of pain-free walking time and maximum walking time in patients with intermittent claudication.

High-throughput glycomics: optimization of sample preparation.

Glycosylation affects structure, folding, and function of numerous proteins. Aberrant glycosylation has been shown to be associated with different diseases. A wide range of analytical methods is available for glycan analysis of antibodies (mainly IgG), but analysis of plasma glycans is less established due to additional challenges encountered with higher complexity of the sample. Here we describe development and optimization of a high-throughput sample preparation method for hydrophilic interaction liquid chromatography and ultra-performance liquid chromatography analysis of plasma N-glycans. Clean-up of labeled glycans was found to be the largest source of variation, and we tested cellulose, silica gel, Bio-Gel, and hydrophilic GHP filter as stationary phases for solid-phase extraction. All stationary phases were shown to be suitable for purification of labeled glycans, but GHP filter plate in combination with cold 96% acetonitrile had the highest reproducibility and was easiest to work with. The method was further optimized with Plackett-Burman screening design and validated in terms of analysis of major step variation and between-day and between-person variation. The developed method is fast, cost-effective, and easy to perform, and it has very good reproducibility during long period of time, enabling the detection of biological variability of the plasma N-glycome.

Impact of Weather Regimes on the Wind Power Ramp Forecast in Portugal

Short-term forecasting and diagnostic tools for severe changes of wind power production (power ramps) may provide reliable information for a secure power system operation at a small cost. Understanding the underlying role of the synoptic weather regimes (WRs) in triggering the wind power ramp events can be an added value to improve and complement the current forecast techniques. This work identifies and classifies the WRs over mainland Portugal associated with the occurrence of severe wind power ramps. The most representative WRs are identified on compressed surface level atmospheric data using principal component analysis by applying K-means clustering. The results show a strong association between some synoptic circulation patterns and step variations of the wind power production indicating the possibility to identify certain WRs that are prone to trigger severe wind power ramps, thus opening the possibility for future development of diagnostic warning systems for system operators use.

Gate-to-source voltage response in high-sensitivity amorphous InGaZnO4 thin-film transistor pH sensors

In this paper, we discuss our top-gate-effect-based high-sensitivity amorphous InGaZnO4 thin-film transistor (a-InGaZnO TFT) pH sensor from the viewpoint of gate-to-source voltage (Vgs) response to small pH step variations. The a-InGaZnO TFT pH sensor, whose sensitivity is as high as 450 mV/pH, shows Vgs response to a pH step change of 0.1 with negligible hysteresis and good linearity. Because the high sensitivity is based on the enhancement of parallel shift in the transfer characteristics through the top-gate effect, the Vgs range for detecting is randomly selected.

Dynamic Response Analysis of the Planar and Tubular Solid Oxide Fuel Cells to the Inlet Air Mass Flow Rate Variation

The purpose of present study is to investigate the dynamic response of two conventional types of solid oxide fuel cells to the inlet air mass flow rate variation. A dynamic compartmental model based on CFD principles is developed for two typical planar and tubular SOFC designs. The model accounts for transport processes (heat and mass transfer), diffusion processes, electrochemical processes, anode and cathode activation and ohmic polarizations, among others. Using developed model the dynamic response of the cell to the step change of the air feed stream conditions is investigated. The results show an almost slow electrical response of the cell to the air mass flow rate step variation that is estimated to be about one hour. Moreover, it can be concluded that the effect of the inlet air flow conditions on a tubular solid oxide fuel cell performance is more noticeable than its effects on a planar SOFC. However, the electrical response time of the tubular type SOFC is calculated about ten times more than the planar type.

Revisiting Some Developments of Boundary Elements for Thick Plates in Brazil

This work reviews the developments of Boundary Element Method formulations to solve several types of plate bending problems, including non-linear bending. The formulation is developed and solved using the standard BEM procedure, and different integration approaches were discussed and tested. Object oriented implementation issues are commented. Results were obtained for linear and non-linear elastic bending as well as buckling of selected cases of thick plates, including cases of step variation in thickness under large displacements regime.

Impact of previous one-step variation in positively long-range correlated processes

In a positively long-range correlated process, variations among consecutive steps are interdependent, especially the influence of previous one-step variation on next steps. How to quantify this kind of impact is of great importance to predict the future variations. In this paper, we demonstrate that this kind of impact depends on the memory strength of underlying processes from two aspects based on the theoretical and observational calculations. More precisely, the conditional calculations and the marginal distribution of the next step variation with given distribution of the previous one-step variation. Both the theoretical and observational calculations demonstrate that the previous one-step variation affect greatly the variation for the next one-step, and the expectation of next step variation will shift to larger value as the increase of memory strength but with a much smaller uncertainty. This is beneficial for our one-step ahead prediction, and will be especially beneficial for multi-step ahead prediction.

Recursive solutions for multi-group neutron kinetics diffusion equations in homogeneous three-dimensional rectangular domains with time dependent perturbations

Abstract In the present work we solve in analytical representation the three dimensional neutron kinetic diffusion problem in rectangular Cartesian geometry for homogeneous and bounded domains for any number of energy groups and precursor concentrations. The solution in analytical representation is constructed using a hierarchical procedure, i. e. the original problem is reduced to a problem previously solved by the authors making use of a combination of the spectral method and a recursive decomposition approach. Time dependent absorption cross sections of the thermal energy group are considered with step, ramp and Chebyshev polynomial variations. For these three cases, we present numerical results and discuss convergence properties and compare our results to those available in the literature.

Dynamic Analysis of Cross Flow Heat Exchangers in Data Centers Using Transient Effectiveness Method

Heat exchangers are important facilities used in data center cooling systems. The effectiveness of heat exchangers strongly influences the thermal performance of cooling systems. Rear door heat exchangers, in-row, overhead coolers, and fully contained cabinets are some examples of executing liquid and hybrid cooling systems used in data centers. A liquid to liquid heat exchanger is an important component of the coolant distribution unit, which supplies chilled water to the aforementioned heat exchangers. Computer room air handler units are also typically liquid to air cross flow heat exchangers. It is important to characterize the dynamic behavior of these heat exchangers for the thermal management of data centers, to improve the design of control strategies, and energy efficiency. The transient response of a 2-D unmixed-unmixed cross flow heat exchanger, with a finite wall capacitance, is investigated. The transient effectiveness concept is used to build the mathematical models that will be used to solve and analyze the transient problems involving cross flow heat exchangers. Several transient cases are investigated numerically for step, ramp, and exponential variations in the inlet temperature of the minimum capacity rate fluid, as well as step and ramp changes in the flow rate of hot and cold fluids. The transient effectiveness characteristics are tested in parametric studies. The results obtained show that transient effectiveness is a useful method of analyzing and predicting the dynamic performance of cross flow heat exchangers.

A fractional-N frequency synthesizer for wireless sensor network nodes

This paper presents a fractional-N frequency synthesizer for wireless sensor network (WSN) nodes. The proposed frequency synthesizer adopts a phase locked loop (PLL) based structure, which employs an LC voltage-controlled oscillator (VCO) with small VCO gain (KVCO) and frequency step (fstep) variations, a charge pump (CP) with current changing in proportion with the division ratio and a 20-bit ΔΣ modulator, etc. To realize constant KVCO and fstep, a novel capacitor sub-bands grouping method is proposed. The VCO sub-groups' sizes are arranged according to the maximal allowed KVCO variation of the system. Besides, a current mode logic divide-by-2 circuit with inside-loop buffers ensures the synthesizer generates I/Q quadrature signals robustly. This synthesizer is implemented in a 0.13 μm CMOS process. Measurement results show that the frequency synthesizer has a frequency span from 2.07 to 3.11 GHz and the typical phase noise is −86.34 dBc/Hz at 100 kHz offset and −114.17 dBc/Hz at 1 MHz offset with a loop bandwidth of about 200 kHz, which meet the WSN nodes' requirements.

Multiple Threats Sense and Avoid Algorithm for Static and Dynamic Obstacles

This paper presents a new computationally efficient S&A algorithm for implementation in real-time applications for UAV. Based on a simplified optimisation approach, the proposed algorithm aims to provide a reliable resolution manoeuvre (horizontal and vertical) for multiple threat scenarios which include both air and ground threats/obstacles. In presence of a conflict risk, the avoidance manoeuvre is defined as step variation in the heading angle or altitude variation of the autopilots command. This step command is optimised in order to keep a minimum distance of separation between the ownship and all threats during the overall manoeuvre. The algorithm computes the separation distance between the UAV and the threats by calculating the future trajectories at each time step of both the ownship and the threat, while always taking into account the ownship performance envelope constraints. The algorithms were validated in simulation, where the ground threats were derived from the ground elevation maps, while for the aerial threats the aircraft communicate their flight data through an ADS-B mode S transponder. The resolution manoeuvre optimisation technique takes about 0.1 second to compute. Hence enabling the algorithm to cope with any rapid changes in the aerial threat trajectory.