Low-frequency Phenomena Research Articles

Investigation of Low-Frequency Phenomena within Flow Pattern in Standard Mixing Vessel Induced by Pitched Blade Impeller

An experimental study on the flow pattern dynamics in a standard mixing vessel with radial baffles filled with water and induced by a pitched blade impeller pumping downward is presented. Investigation is mainly focused on detection and analysis of quasi-periodical or periodical low-frequency phenomenon connected with time- and length-scales considerably exceeding the Blade Passage Frequency (hereinafter BPF) and common turbulent eddies. This phenomenon, which is expressed as large-scale mean-flow variations, is generally known as flow Macro-Instability (hereinafter MI). It could break-down just below the liquid surface, or it crashes to the liquid surface and causes its Macro-Swelling (hereinafter MS). Our investigation was based on classical two-dimensional (2D) Particle Image Velocimetry (hereinafter PIV) measurement within 3 selected vertical planes in the vessel and subsequent analysis of the velocity field. The dominant frequencies evaluated in the selected points and overall analysis of the quasi-periodical macro-flow pattern behavior is to be shown. Identification of the quasi-periodical substructures appeared within the flow pattern was performed using the Oscillation Pattern Decomposition (hereinafter OPD) method. Observation of the macro-flow patterns confirmed presence of the macro-flow structures detected within flow pattern at the identical mixing pilot plant setup by previous investigations of the MIs phenomenon, i.e., the primary circulation loop and strong impeller discharge jet located in the lower vessel segment and the strong ascending wall current at the baffle, which break-down below the surface very often. A further important contribution of the presented work is the investigation of both flow pattern within the baffles vicinity and in the middle of the sector far from the baffle, showing a significant difference. Low-frequency periodical (or quasi-periodical) behavior of the investigated macro-structures was qualitatively confirmed by the presented results and it was quantified using the velocity dominant frequencies evaluation, as noted below. This frequency analysis brings insight into the supposed and detected interconnections between dynamics of the adjacent flow structures. Detected different flow patterns within the main plane near the baffles and in the inclined plane reveal a strong influence of the baffle presence to the local vertical flow, especially within the upper part of the vessel. Quite a different flow pattern appears tangentially in front of and behind the baffle, where a wake is revealed, indicating significant influence of the baffle on the local tangential flow. The new findings represent a contribution to better understanding the physical phenomena behind the standard mixing process.

Establishing Legal <i>Force Majeure</i> Case for the Interruption of Hydro Energy Supply Related to an Extreme Hydrological Drought Event

The management of water resources and specifically the energy supply from hydropower systems are strongly affected by drought events. This paper is devoted to the unfolding of legal consequences caused by an extreme hydrological drought event. The legal litigation occurred when – due to a severe deficit of water – the hydropower company that manages the hydro energy resources over the entire territory of Romania was unable to deliver the amount of electricity stipulated in the power purchase agreements. The studypresents an extensive statistical analysis of extreme hydrological drought events occurrence, using the threshold level method. The statistical analysis was applied on large historical records of the flow rates (50-90 years) and revealed the occurrence of extreme drought events, i.e. extremely low occurrence frequency phenomena. The characteristics of the historical data were studied to highlight the area where the forecast normally used in the hydro generation planning is not possible. In order to evaluate the occurrence of extreme drought phenomena, the natural monthly and annually flows of the Danube and other seven major interior rivers in Romania (Argeș, Bistrița, Olt, Lotru, Sebeș, Someș, Siret and Raul Mare) were analyzed, reflecting fairly a general picture of the flow distribution on the entire territory. The analyzed cases refer to periods when – due to a severe deficit of water – the Romanian hydropower company was unable to generate electricity according to the power purchase agreements (PPA). The results of the statistical analysis were further used as the base for the legal cases that confirmed the <i>force majeure </i>situations claimed by the hydropower company.

Low-frequency spillover phenomenon in NC-piezoelectric shunt damping

Low-frequency spillover phenomenon in NC-piezoelectric shunt damping

Visual experimental investigation on stable operating process of the plane-radial rotating detonation engine

Using visual observation methods, assisted by synchronous measurement of dynamic pressure and ion tracks, the flow field structures of stable operating modes in plane-radial rotating detonation engine (RDE) are revealed, and the influences of injection conditions on the RDE stable operating range are analyzed. The flow field structures of single-wave mode, symmetric dual-wave mode and triple-wave mode are observed. Typical structures such as rotating detonation wave (RDW), oblique shock wave, combustion products, deflagration surface and contact discontinuity are obtained in the flow fields. Obvious reactants pre-combustion ahead of detonation wave is found in the combustor, and low-frequency oscillation phenomenon is gained under specific conditions. With the increase of wave-head number in combustor, the detonation-wave height and parameters decrease. The RDW propagating velocity and pressure peak are higher in single-wave operating mode, but the flow fields in multi-wave mode are more stable. Both the RDW velocity and pressure peak increase generally with equivalence ratio. Keeping the equivalent ratio (ER) fixed, as the mass flow rate increases, the number of detonation wave in combustor increases, and the operating mode transforms from single-wave mode to multi-wave mode. Keeping the mass flow rate fixed, when the equivalent ratio deviates from stoichiometric ratio, the operating mode varies from multi-wave mode to single-wave mode. Additionally, with the mass flow rate increasing, the ER upper and lower limits of multi-wave mode decrease linearly, and the operating range of dual-wave mode remains unchanged. Mass flow rate has little influence on the ER lower limit of the RDE stable operation, which is approximately ER=0.55 in this structure.

Time reversed optical waves by arbitrary vector spatiotemporal field generation

Lossless linear wave propagation is symmetric in time, a principle which can be used to create time reversed waves. Such waves are special “pre-scattered” spatiotemporal fields, which propagate through a complex medium as if observing a scattering process in reverse, entering the medium as a complicated spatiotemporal field and arriving after propagation as a desired target field, such as a spatiotemporal focus. Time reversed waves have previously been demonstrated for relatively low frequency phenomena such as acoustics, water waves and microwaves. Many attempts have been made to extend these techniques into optics. However, the much higher frequencies of optics make for very different requirements. A fully time reversed wave is a volumetric field with arbitrary amplitude, phase and polarisation at every point in space and time. The creation of such fields has not previously been possible in optics. We demonstrate time reversed optical waves with a device capable of independently controlling all of light’s classical degrees of freedom simultaneously. Such a class of ultrafast wavefront shaper is capable of generating a sequence of arbitrary 2D spatial/polarisation wavefronts at a bandwidth limited rate of 4.4 THz. This ability to manipulate the full field of an optical beam could be used to control both linear and nonlinear optical phenomena.

Constraining natural gas pipeline emissions in San Juan Basin using mobile sampling

Quantifying methane (CH4) leaks of pipeline systems is critical to ensure accurate emission factors in regional and global atmospheric models. The previous emission factors in the United States Environmental Protection Agency (EPA) Greenhouse Gas Inventory (GHGI) are from 1996 and do not reflect the modern gathering pipeline system. Additional data from different basins across the United States are urgently needed to improve the emission factors. The National Energy Technology Laboratory conducted a ground-based vehicle survey at Carson National Forest in the San Juan Basin, New Mexico, in September 2019. 187 km of natural gas gathering pipeline systems were surveyed. The mobile CH4 survey system was efficient in identifying CH4 plumes and pinpointing the leak sources. Gaussian dispersion modeling suggested our survey system had a minimum detection limit of 1.5 LPM. No leaks were found from the pipelines while a leak of 7.1 +/− 0.2 LPM was on a pig launcher door and another leak of 0.7 +/− 0.1 LPM on a block valve. Limited access to the gathering pipeline system prevented us from quantifying all potential leaks detected by the CH4 sensors. The low leak frequency phenomenon was also observed in the sole existing study of natural gas gathering pipelines in the Fayetteville Shale.

Investigation of the Low-Frequency Oscillations in the Flowfield About an Airfoil

The present investigation shows that a triad of three vortices is behind the quasi-periodic self-sustained low-frequency flow oscillation phenomenon. Large-eddy simulations are carried out at , , and sixteen angles of attack at near-stall conditions. It is shown that the best description of the underlying mechanism is that of a button whirligig. A globally oscillating flow whirls around the airfoil in the clockwise direction and stores energy in the triad of vortices. When the oscillating flow loses momentum and comes to an equilibrium, the energy stored in the triad of vortices reaches a threshold. Thus, the triad of vortices reverses the direction of rotation of the oscillating flow, gives it a pulse of energy to whirl in the anticlockwise direction, and starts a new disequilibrium. When the oscillating flow rotates in the clockwise direction (in the streamwise direction on the suction surface of the airfoil), it adds momentum to the boundary layer and helps it to remain attached, and vice versa. Consequently, the instantaneous flowfield switches between an attached phase and a separated phase in a periodic manner with some disturbed cycles.

Suppression of surface waves with the ensemble empirical mode decomposition to enhance ultrasound images of reinforced concrete elements

The surface waves were suppressed to enhance ultrasound images of reinforced concrete elements with no hard assumption about the surface waves’ time-domain characteristics. The surface waves yield artifacts that degrade image quality and generally hinder the recognition of shallow inclusions and defects. The Ensemble Empirical Mode Decomposition (EEMD) was used to adaptively decompose the signals into their Intrinsic Mode Functions (IMFs). The signals were acquired in a concrete element with rebars at 54, 74, and 94 mm from the concrete element’s surface. The surface waves were generally decomposed to their own IMFs. The IMFs which comprised noise signal components, including the surface waves, were labeled as such based on empirical data. The remaining IMFs were used to construct new signals that generally comprised signal components from reflection at a rebar and the backwall and other low-frequency phenomena. The new signals yielded images in which the surface waves artifacts were not present and the rebars were represented correctly. The Synthetic Aperture Focusing Technique was used for imaging. The method has two main drawbacks. The EEMD performs according to parameters which require extensive ad hoc testing to be selected. The IMFs comprising the surface waves suffered from mode mixing with the backwall at times and hence the backwall was not fully represented in the new images.

Gathering Pipeline Methane Emissions in Utica Shale Using an Unmanned Aerial Vehicle and Ground-Based Mobile Sampling

The United States Environmental Protection Agency Greenhouse Gas Inventory only recently updated the emission factors of natural gas gathering pipelines in April 2019 from the previous estimates based on a 1990s study of distribution pipelines. Additional measurements are needed from different basins for more accurate assessments of methane emissions from natural gas midstream industries and hence the overall climate implications of natural gas as the interim major energy source for the next decade. We conducted an unmanned aerial vehicle (UAV) survey and a ground-based vehicle sampling campaign targeting gathering pipeline systems in the Utica Shale from March to April in 2019. Out of 73 km of pipeline systems surveyed, we found no leaks on pipelines and two leaks on an accessory block valve with leak rates of 3.8 ± 0.4 and 7.6 ± 0.8 mg/s. The low leak frequency phenomenon was also observed in the only existing gathering pipeline study in Fayetteville Shale. The UAV sampling system facilitated ease of access, broadened the availability of pipelines for leak detection, and was estimated to detect methane leaks down to 0.07 g/s using Gaussian dispersion modeling. For future UAV surveys adopting similar instrument setup and dispersion models, we recommend arranging controlled release experiments first to understand the system’s detection limit and choosing sampling days with steady and low wind speeds (2 m/s).

Understanding single-phase water-management signatures in fuel-cell impedance spectra: A numerical study

Electrochemical impedance spectroscopy is a powerful technique used in fuel-cell diagnosis and characterization. A low-frequency inductive behavior may appear in the fuel-cell-impedance spectrum and is believed to be related to the electrolyte-hydration dynamics. Since water management plays a crucial role in achieving high performance, understanding the electrolyte-hydration-related signatures of the impedance spectrum may help better interpret experimental data and design future fuel cells. In this work, an open-source, transient, single-phase 2D model is presented that is suitable for analyzing impedance spectra of proton-exchange-membrane fuel cells. Special care is taken to compare the model to transient polarization, ohmic-resistance, and impedance data measured in-house. The model reveals that the low-frequency inductive phenomenon is related to the finite-rate exchange of water between the electrolyte and the pores. Two inductive phenomena are observed, at 0.1–200 mHz and at 0.2–5 Hz, that are attributed to the water-transport dynamics in the electrolyte phase of the catalyst layers and the membrane, and just the catalyst layers, respectively. This work also shows an ohmic-resistance breakdown study that demonstrates that the high-frequency resistance is comprised of the ohmic resistance of the membrane and the electronically conductive components of the cell, but does not include protonic resistance of the carbon-supported catalyst layers.

Poroelastic property analysis of seismic low-frequency shadows associated with gas reservoirs

Abstract Seismic low-frequency amplitude shadows have been widely used as a hydrocarbon indicator. This study investigates the effect of reservoir properties and seismic wave mode conversion on the characteristics of the low-frequency amplitude shadows in gas-bearing reservoirs. The target gas reservoirs are typically related to the lithology of tight sandstone with strong heterogeneity. Pore-fluid distribution within the reservoirs presents patchy saturation in the vertical and horizontal directions, and this patchy saturation easily induces low-frequency shadows beneath gas-bearing reservoirs. These low-frequency shadows are validated by using a poroelastic simulation method. The results of our field case-based study indicate that pore-fluid property, plus the thickness and heterogeneity of reservoirs are the key elements in the generation of low-frequency shadows. The results also indicate that the poroelastic simulation method can be used to effectively predict the spatial distribution of gas-bearing reservoirs, by directly verifying the low-frequency shadow phenomenon existing in the seismic data.

Characterization of low and high frequency phenomena in a PEM fuel cell using singularity analysis of stack voltage

The aim of this work is to propose a new development of the fuel cell characterization tool based on Voltage Singularity Spectrum (VSS), a pattern that can advantageously be estimated from the ``free'' evolution of the stack voltage. A Polymer Electrolyte Membrane Fuel Cell (PEMFC) designed to operate in micro Combined Heat and Power units is investigated to create an experimental database made of normal and abnormal operating conditions. Some VSS signatures based on Wavelet Leader Multifractal Analysis (WLMA) are computed on low- and high-frequency filtered stack voltage signals reflecting different physical phenomena dynamics. The resulting low- and high-frequency VSS computed for six different operating conditions are then compared to the Electrochemical Impedance Spectra (EIS) recorded in the same conditions. In this way, we intend to establish some links between the VSS patterns and the EIS based on well-known physical analyses. We also show that the dominant singularity strengths of the low (and possibly also high) frequency VSS can be considered as relevant clustering features to identify various PEMFC operating states (linked with poorer/better air diffusion, with lower/higher MEA hydration), as it is usually done with the use of the characteristic resistances intercepted on the EIS graph x-axis.

Effect of Reactive Power Characteristic of Offshore Wind Power Plant on Low-Frequency Stability

Oscillation phenomena of offshore wind power plant (OWPP) in a wide frequency range can be caused due to impedance interactions between grid-connected inverters (GCIs) and transmission cables. In this article, impedance model of GCI with outer power control loop, inner current control loop and phase-locked loop is first established in dq reference frame. The correctness is validated by frequency scanning method. Then, the effects of active and reactive power/current references on dq impedance characteristics of GCI with/without consideration of power control loop are investigated using complex space vectors and complex transfer functions. Furthermore, RLC circuit model of transmission cable considering frequency-dependent characteristics is also established for dq-domain IBSC. On the basis of them, it's found that low-frequency oscillation phenomena of OWPP under power control mode may occur if active power reference exceeds a certain threshold value, which can be mitigated by injecting a certain amount of negative reactive power. Impacts of PLL parameters, length of transmission cable and number of paralleled GCIs on required negative reactive power for low-frequency stabilization are further investigated. Both Matlab/Simulink-based simulation and OPAL-RT-based real-time verification are implemented in an OWPP with four permanent magnet synchronous generators to validate the correctness of the reactive power characteristic analysis results and the feasibility of mitigating low-frequency oscillation phenomena by negative reactive power injection.

Masked by annotation: Minor declarative complementizers in parsed corpora of historical English

This article discusses some of the potential problems derived from the syntactic annotation of historical corpora, especially in connection with low-frequency phenomena. By way of illustration, we examine the parsing scheme used in the Penn Parsed Corpora of Historical English (PPCHE) for clauses introduced by so-called ‘minor declarative complementizers’, originally adverbial links which come to be occasionally used in complementizer function. We show that the functional similarities between canonical declarative complement clauses introduced by the major declarative links that and zero and those headed by minor declarative complementizers are not captured by the PPCHE parsing, where the latter constructions are not tagged as complement clauses, but rather as adverbial clauses. The examples discussed reveal that, despite the obvious advantages of parsed corpora, annotation may sometimes mask interesting linguistic facts.

A balanced state consistent with planetary-scale motion for quasi-geostrophic dynamics

The role of planetary-scale zonally-asymmetric thermal forcing on large-scale atmospheric dynamics is crucial for understanding low-frequency phenomena in the atmosphere. Despite its paramount importance, good theoretical foundation for the understanding is still lacking. Here, we address this issue by providing a general framework for including planetary-scale thermal forcing in large-scale atmospheric dynamics studies. This is accomplished by identifying two distinct geostrophic motions of horizontal length scale L in terms of the external Rossby deformation length scale LD : i) and ii) where ϵ is the Rossby number. In addition, via multi-scale analysis, we show that the large-scale atmospheric dynamics can be described by mutual interaction between the two scales. The analysis results in planetary geostrophic equations with large-scale thermal forcing that provide the basic balanced states for processes such as the growth of synoptic waves. In the long-time limit, the continuous growth and decay of synoptic waves provide the convergence of horizontal heat and vorticity fluxes, which contributes to the energy flux balance in the planetary geostrophic scale with planetary-scale advection and thermal forcing.

Seismic low-frequency amplitude analysis for identifying gas reservoirs within thinly layered media

AbstractSeismic low-frequency amplitude anomalies can be correlated with the existence of hydrocarbon reservoirs, but the generation mechanism of these low-frequency amplitude anomalies is very complicated. We investigated the effect of the Biot slow P-waves on the seismic reflection amplitude at low frequencies from fluid-saturated porous media using techniques of both theoretical calculation and physical modelling. First, we used theoretical calculation to study the effects of the frequency, the fluid content and the heterogeneity of layered media on the conversion of the Biot slow waves, and the reflection coefficients. Then, we used physical modelling data to show a low-frequency shadow of the air-saturated layered model. The phenomenon was similar to that in the theoretical calculation. A combination of the theoretical calculation and physical modelling results provided a reasonable explanation of the low-frequency shadow. The conclusion is that whether the actual hydrocarbon reservoirs exhibit low-frequency shadow phenomena depends on the frequency, the consolidation, the thickness of the hydrocarbon reservoirs, the homogeneity of the porosity and the fluid distributions of the reservoirs. The proposed method can be used to analyse the low-frequency anomalies and to identify the fluid properties using seismic data.

Admittance Modeling and Stability Analysis for Inverter-Based Microgrid With Voltage-Source-Rectifier Load

Instability and low-frequency oscillation phenomenon of inverter-based microgrid (IBM) with voltage-source-rectifier load have obtained increasing concern. However, the influence of instantaneous angle of operation point on IBM is usually neglected during voltage faults, and it may generate the uncertain impact of voltage faults on the stable and safe operation of IBM. Besides, the influence of instantaneous angle of operation point may cause varying degrees of system oscillation and stability at a different angle of system states' change, including fault recovery. To solve this issue, a systematic approach of small-signal admittance modeling for IBM in the αβ frame is proposed in this article. The small-signal admittance model of each submodule (SM) of IBM is established and formed as a multi-input-multi-output (MIMO) system considering the actual closed-loop controls and decoupled strategies in the dq frame. Based on the proposed admittance models of SMs, the complete model of IBM is integrated and linearized, and the influence of both amplitude and instantaneous angle of operation point on the IBM stability is analyzed in small-signal model comprehensively. Besides, using the generalized Nyquist stability criterion (GNC), the influence of instantaneous angle in large-signal operation point on oscillation amplitude and stability of IBM is investigated. The proposed analysis approach can analyze and determine the responded oscillation and stability of the system more accurately during system states' variation, such as voltage sag and sudden changes in load. It also reduces the uncertainty of IBM behaviors during states' variation to ensure that IBM operates smoothly, flexibly, and stably. Finally, experimental results are presented to validate the correctness of the proposed analysis method.

A study of grounding arrangements composed by vertical electrodes for two-layered stratified soil models

This paper presents a study of a grounding arrangement proposed for transmission line (TL) towers, particularly for conditions of a stratified soil where the electric resistivity of the first layer is higher than the second medium. Techniques based on TL-theory are presented to estimate the harmonic grounding impedance of such configurations and are compared to the solution obtained with a rigorous electromagnetic (EM) method. The results show that the grounding resistance, important to low frequency frequency phenomena, and the impulse impedance, to fast transients, decrease significantly with the use of vertical electrodes. For a 230 kV TL, the critical towers can be improved by the proposed arrangement, which includes a significant economy of the conductor material applied to the grounding.

On the Low Frequency Pressure Fluctuation in a 3/4 Open Jet Automotive Wind Tunnel

In the present research, a possible generation mechanism of low-frequency buffeting phenomenon based on a 1:15 open jet automotive wind tunnel was investigated. Evolution of vortex structures and pressure field in the plenum chamber have been visualized and analyzed by Large-eddy simulation (LES). It is shown that the low frequency pressure fluctuation is caused by the large-scale structures and their interaction. Multiple proper orthogonal decomposition was adopted to analyze the flow field in the plenum chamber. The characteristic frequencies of the vortex-rings after pairing is the same as the dominant resonance of buffeting at this wind velocity.

The Very Low Frequency Phenomenon During Sleeping

Fun-Jou Chen* Author Affiliations School of Chinese Medicine, China Received: August 19, 2019 | Published: August 26, 2019 Corresponding author: Fun-Jou Chen, School of Chinese Medicine, China DOI: 10.26717/BJSTR.2019.20.003515