secondary currents
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Geophysics ◽  
2021 ◽  
pp. 1-48
Author(s):  
Gurban Orujov ◽  
Andrei Swidinsky ◽  
Rita Streich

Controlled-source electromagnetic (CSEM) methods have the potential to be used in reservoir monitoring problems, due to their sensitivity to subsurface resistivity distribution. For example, time-lapse electromagnetic (EM) measurements can help to determine reservoir changes during enhanced oil recovery (EOR) processes such as water/steam injection or CO2 sequestration. Although metal infrastructure such as pipelines and casings can strongly influence EM data and mask the underlying geological response, one may presume that these effects cancel out during time-lapse surveys. In this paper, we analyze the effects of well casings on time-lapse surface-to-surface EM measurements. First, using a synthetic example of an onshore 1D hydrocarbon reservoir we quantify the effect of single and multiple casings at several source and receiver locations. We show that time-lapse responses are distorted significantly when a source or receiver is located near a casing. Next, we study a more realistic scenario where we approximate the hydrocarbon reservoir as a thin bounded resistive sheet. We present a Method of Moments (MoM) algorithm to calculate the secondary currents and charges on a well casing and resistive sheet combination and validate the electric fields these secondary sources generate against finite element modeling. Finally, we calculate and explicitly demonstrate time-lapse amplitude changes in the well casing-thin sheet interaction matrix, secondary currents, charges, and surface electric fields. Our 3D modeling results show that the conductive casing reduces the ability of the resistive sheet to impede the current flow and distorts time-lapse responses. Therefore, one cannot fully eliminate casing effects by subtraction of time-lapse data and must fully incorporate such infrastructure into forward models for time-lapse EM inversion.


2021 ◽  
Vol 930 ◽  
Author(s):  
Markus Scherer ◽  
Markus Uhlmann ◽  
Aman G. Kidanemariam ◽  
Michael Krayer

The role of turbulent large-scale streaks or large-scale motions in forming subaqueous sediment ridges on an initially flat sediment bed is investigated with the aid of particle resolved direct numerical simulations of open channel flow at bulk Reynolds numbers up to 9500. The regular arrangement of quasi-streamwise ridges and troughs at a characteristic spanwise spacing between 1 and 1.5 times the mean fluid height is found to be a consequence of the spanwise organisation of turbulence in large-scale streamwise velocity streaks. Ridges predominantly appear in regions of weaker erosion below large-scale low-speed streaks and vice versa for troughs. The interaction between the dynamics of the large-scale streaks in the bulk flow and the evolution of sediment ridges on the sediment bed is best described as ‘top-down’ process, as the arrangement of the sediment bedforms is seen to adapt to changes in the outer flow with a time delay of several bulk time units. The observed ‘top-down’ interaction between the outer flow and the bed agrees fairly well with the conceptual model on causality in canonical channel flows proposed by Jiménez (J. Fluid Mech., vol. 842, 2018, P1, § 5.6). Mean secondary currents of Prandtl's second kind of comparable intensity and lateral spacing are found over developed sediment ridges and in single-phase smooth-wall channels alike in averages over ${O}(10)$ bulk time units. This indicates that the secondary flow commonly observed together with sediment ridges is the statistical footprint of the regularly organised large-scale streaks.


2021 ◽  
Vol 927 ◽  
Author(s):  
Carlo Camporeale ◽  
Fabio Cannamela ◽  
Claudio Canuto ◽  
Costantino Manes

This paper presents some results coming from a linear stability analysis of turbulent depth-averaged open-channel flows (OCFs) with secondary currents. The aim was to identify plausible mechanisms underpinning the formation of large-scale turbulence structures, which are commonly referred to as large-scale motions (LSMs) and very-large-scale motions (VLSMs). Results indicate that the investigated flows are subjected to a sinuous instability whose longitudinal wavelength compares very well with that pertaining to LSMs. In contrast, no unstable modes at wavelengths comparable to those associated with VLSMs could be found. This suggests that VLSMs in OCFs are triggered by nonlinear mechanisms to which the present analysis is obviously blind. We demonstrate that the existence of the sinuous instability requires two necessary conditions: (i) the circulation of the secondary currents $\omega$ must be greater than a critical value $\omega _c$ ; (ii) the presence of a dynamically responding free surface (i.e. when the free surface is modelled as a frictionless flat surface, no instabilities are detected). The present paper draws some ideas from the work by Cossu, Hwang and co-workers on other wall flows (i.e. turbulent boundary layers, pipe, channel and Couette flows) and somewhat supports their idea that LSMs and VLSMs might be governed by an outer-layer cycle also in OCFs. However, the presence of steady secondary flows makes the procedure adopted herein much simpler than that used by these authors.


Energies ◽  
2021 ◽  
Vol 14 (14) ◽  
pp. 4297
Author(s):  
Taufik Taluo ◽  
Leposava Ristić ◽  
Milutin Jovanović

The Brushless Doubly-Fed Reluctance Generator (BDFRG) is a potential alternative to the Doubly Fed Induction Generator (DFIG) in wind power applications owing to its reasonable cost, competitive performance, and high reliability. In comparison with the Brushless Doubly-Fed Induction Generator (BDFIG), the BDFRG is more efficient and easier to control owing to the cage-less rotor. One of the most preferable advantages of BDFRG over DFIG is the inherently better performance under unbalanced grid conditions. The study conducted in this paper showed that conventional vector control of the BDFRG results in excessive oscillations of the primary active/reactive power, electromagnetic torque, and primary/secondary currents in this case. In order to address such limitations, this paper presented a new control strategy for the unbalanced operation of BDFRG-based wind generation systems. A modified vector control scheme was proposed with the capability to control the positive and the negative sequences of the secondary currents independently, thus greatly reducing the adverse implications of the unbalanced supply. The controller performance has been validated by simulations using a 1.5 MW BDFRG dynamical model built upon the positive and negative sequence equations. The main benefits of the new control strategy are quantified in comparison with conventional PI current control design.


2021 ◽  
Vol 3 (7) ◽  
Author(s):  
João N. Fernandes

AbstractOverbank flows occur in alluvial valleys during flood events when the conveyance of main channel of rivers is exceeded. Once floodplains are inundated and the so-called compound channel flow is observed, the faster flow in the main channel interacts with the slower flow in the floodplain featuring a much more pronounced 3D flow structure compared to single channel flow. These flow mechanisms comprise a shear layer near the interface, lateral momentum transfer and strong secondary currents due to the non-isotropic turbulence. This paper starts by giving an overview of the main flow mechanisms in compound channels pointing out the importance of taking into account the apparent shear stress generated between the main channel and the floodplain flows due to the interaction of these flows. A new simple model was developed to include the apparent shear stress concept as a correction of the Manning roughness coefficient of main channel and floodplains. The proposed method for predicting stage–discharge relationships was calibrated and validated by experimental data from several compound channel facilities. A significant improvement in prediction of the compound channel conveyance in comparison with the traditional methods was achieved.


2021 ◽  
Vol 5 (3) ◽  
pp. 87-91
Author(s):  
LUO Ching- Ruey

Braided river reaches and alluvial systems are characterized by their multi-threaded planform and agents of sediment transport due to eroding and deposing to form the bars and riffles. In braided river, frequent sediment transport and the quick shifting of the positions about the river channel induce many attentions discussion and relating a complicated consideration of the combinations of disciplines. In this article we introduce its fundamental characteristics and further the complicated mechanism in the literature and methodologies. The braided channel ecology and the management of braided river are mentioned and discussed, especially, the secondary currents, in this paper we explain in detail, the combinations on multiplying of 2-D flow of the velocity fluctuations. The interdisciplinary approach on linking engineers, earth scientists and social scientists concerned with environmental economics, planning, and societal and political strategies in order to fully evaluate the validity and reliability of different selections to various timescales is really sensitive. Furthermore, the requirements of public education on reinforcing about the mechanism of braided river formation will be obviously important and necessary.


2021 ◽  
Vol 2021 ◽  
pp. 1-11
Author(s):  
Younes Menni ◽  
Giulio Lorenzini ◽  
Ravinder Kumar ◽  
Babak Mosavati ◽  
Saeed Nekoonam

A numerical study of an especial heat exchanger (HE) equipped with complicated geometry baffles was performed in this research study. This shell-and-tube HE could be applied in various engineering applications like solar collectors. It can be acknowledged that generating longitudinal vortices in the flow results in enhancing the turbulent convective heat transfer. In order to generate these vortices, S-shaped baffles can be applied. It should be noted that computational analysis of shell-and-tube HEs is considered a challenging task due to these vortices. So, in this study, a commercial CFD software has been used for solving the problem and important equation and numerical approach used in the simulation have been explained. The aerodynamic aspect with respect to stream function, mean, axial, and transverse velocities, dynamic pressure, turbulent dissipation rate, turbulence kinetic energy, turbulent viscosity, and turbulence intensity fields was included in this research. This study reports many physical phenomena, such as the turbulence, instability, flow separation, and the appearance of reverse secondary currents. The average speed changed in different areas, where it is low next to the baffles. Velocity amounts are paramount around the upper channel’s wall, starting from the upper left side of the last baffle to the exit. This increase in velocity can be justified by a reduction in flow area and pressure augmentation.


2021 ◽  
Author(s):  
Tatiana Fedorova ◽  
Vitaly Belikov ◽  
Andrei Alabyan

<p>The retrospective simulation of the Pyoza river (Arkhangelsk region, Russia) meander cut-off in 2003-2008 has been undertaken. As a result of the river bend straightening two large villages were cut off from the road network of the region.</p><p>The initial data for modeling were obtained by analyses of archive satellite images for the period from 1997 together with the runoff data, as well as by the field survey of September 2019. The simulation was performed by the latest version of the STREAM_2D CUDA software, using a new method for the numerical solution of two-dimensional Saint-Venant equations [1]. It was adapted for the complicated bottom topography typical for a wide floodplain with a meandering channel flooded in high water stage.</p><p>The mass-exchange equations for three layers of sediment over the unerodible bed were solved together with the hydrodynamic equations. When calculating channel deformations, the gravitational effect due to bottom slope and the influence of secondary currents on the sediment shift were taken into account [2].</p><p>The Pyoza river is the lowest large tributary of the Mezen’ river flowing into the White sea. It is distinguished by a typical alluvial channel, meandering along wide floodplain composed by sands and sandy loams. The maximum runoff usually corresponds to spring snow-melting and can reach 1500-2000 m<sup>3</sup>/s.</p><p>To schematize the computational domain of the Pyoza river section of 13 km long, a hybrid grid of irregular structure was constructed, consisting of 37 329 cells of a quadrangular shape for the channel and a triangular one for the floodplain.</p><p>The simulation started at the year 1997 when where was no any rill across the meander neck. The time step of calculation was taken to be one day.</p><p>Modeling made it possible to simulate realistically the essential steps and mechanisms of the meander cut-off: the development of a pioneer straightening rill, its widening and deepening, as well as blocking of the old channel by a point bar in its upper reaches, as well as its further silting and aggradation.</p><p>1. Aleksyuk A.I., Belikov V.V. (2017): Simulation of shallow water flows with shoaling areas and bottom discontinuities. Computational Mathematics and Mathematical Physics, Volume 57, issue 2, pp. 318–339. https://doi.org/10.1134/S0965542517020026</p><p>2. Aleksyuk А. I., Belikov V. V., Borisova N. M., Fedorova T. A. (2018): Numerical modeling of non-uniform sediment transport in river channels. Water Resources, Volume 45, Special Issue S1, pp. 11–17. http://dx.doi.org/10.1134/S0097807818050275</p>


Water ◽  
2021 ◽  
Vol 13 (3) ◽  
pp. 319
Author(s):  
Mouldi Ben Meftah ◽  
Diana De Padova ◽  
Francesca De Serio ◽  
Michele Mossa

Most studies on local scouring at grade control structures have principally focused on the analysis of the primary flow field, predicting the equilibrium scour depth. Despite the numerous studies on scouring processes, secondary currents were not often considered. Based on comprehensive measurements of flow velocities in clear water scours downstream of a grade control structure in a channel with non-cohesive sediments, in this study, we attempted to investigate the generation and turbulence properties of secondary currents across a scour hole at equilibrium condition. The flow velocity distributions through the cross-sectional planes at the downstream location of the maximum equilibrium scour depth clearly show the development of secondary current cells. The secondary currents form a sort of helical-like motion, occurring in both halves of the cross-section in an axisymmetric fashion. A detailed analysis of the turbulence intensities and Reynolds shear stresses was carried out and compared with previous studies. The results highlight considerable spatial heterogeneities of flow turbulence. The anisotropy term of normal stresses dominates the secondary shear stress, giving the impression of its crucial role in generating secondary flow motion across the scour hole. The anisotropy term shows maximum values near both the scour mouth and the scour bed, caused, respectively, by the grade control structure and the sediment ridge formation, which play fundamental roles in maintaining and enhancing the secondary flow motion.


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