scholarly journals Mutual inductance instability of the tip vortices behind a wind turbine

2014 ◽  
Vol 755 ◽  
pp. 705-731 ◽  
Author(s):  
Sasan Sarmast ◽  
Reza Dadfar ◽  
Robert F. Mikkelsen ◽  
Philipp Schlatter ◽  
Stefan Ivanell ◽  
...  
Keyword(s):  
Growth Rate ◽  
Wind Turbine ◽  
Numerical Study ◽  
Operating Conditions ◽  
Analytical Relationship ◽  
Dynamic Mode ◽  
Spatial Structures ◽  
Tip Vortices ◽  
Positive Growth ◽  
The Stability

AbstractTwo modal decomposition techniques are employed to analyse the stability of wind turbine wakes. A numerical study on a single wind turbine wake is carried out focusing on the instability onset of the trailing tip vortices shed from the turbine blades. The numerical model is based on large-eddy simulations (LES) of the Navier–Stokes equations using the actuator line (ACL) method to simulate the wake behind the Tjæreborg wind turbine. The wake is perturbed by low-amplitude excitation sources located in the neighbourhood of the tip spirals. The amplification of the waves travelling along the spiral triggers instabilities, leading to breakdown of the wake. Based on the grid configurations and the type of excitations, two basic flow cases, symmetric and asymmetric, are identified. In the symmetric setup, we impose a 120° symmetry condition in the dynamics of the flow and in the asymmetric setup we calculate the full 360° wake. Different cases are subsequently analysed using dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD). The results reveal that the main instability mechanism is dispersive and that the modal growth in the symmetric setup arises only for some specific frequencies and spatial structures, e.g. two dominant groups of modes with positive growth (spatial structures) are identified, while breaking the symmetry reveals that almost all the modes have positive growth rate. In both setups, the most unstable modes have a non-dimensional spatial growth rate close to $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}\pi /2$ and they are characterized by an out-of-phase displacement of successive helix turns leading to local vortex pairing. The present results indicate that the asymmetric case is crucial to study, as the stability characteristics of the flow change significantly compared to the symmetric configurations. Based on the constant non-dimensional growth rate of disturbances, we derive a new analytical relationship between the length of the wake up to the turbulent breakdown and the operating conditions of a wind turbine.

A Study of the Lateral Stability of Railroad Vehicles Using a Nonlinear Constrained Multibody Formulation

2001 ◽  
Author(s):  
Davide Valtorta ◽  
Khaled E. Zaazaa ◽  
Ahmed A. Shabana ◽  
Jalil R. Sany
Keyword(s):  
Stability Analysis ◽  
Linear Stability ◽  
Linear Theory ◽  
Linear Stability Analysis ◽  
Numerical Study ◽  
Operating Conditions ◽  
Lateral Stability ◽  
Railroad Vehicles ◽  
Contact Formulation ◽  
The Stability

Abstract The lateral stability of railroad vehicles travelling on tangent tracks is one of the important problems that has been the subject of extensive research since the nineteenth century. Early detailed studies of this problem in the twentieth century are the work of Carter and Rocard on the stability of locomotives. The linear theory for the lateral stability analysis has been extensively used in the past and can give good results under certain operating conditions. In this paper, the results obtained using a linear stability analysis are compared with the results obtained using a general nonlinear multibody methodology. In the linear stability analysis, the sources of the instability are investigated using Liapunov’s linear theory and the eigenvalue analysis for a simple wheelset model on a tangent track. The effects of the stiffness of the primary and secondary suspensions on the stability results are investigated. The results obtained for the simple model using the linear approach are compared with the results obtained using a new nonlinear multibody based constrained wheel/rail contact formulation. This comparative numerical study can be used to validate the use of the constrained wheel/rail contact formulation in the study of lateral stability. Similar studies can be used in the future to define the limitations of the linear theory under general operating conditions.

Study of stabilty analysis of a grid connected Doubly fed induction generator based on wind energy Application

2016 ◽  
Vol 3 (2) ◽  
pp. 305 ◽  
Author(s):  
Ghulam sarwar Kaloi ◽  
Jie Wang ◽  
Mazhar H Baloch
Keyword(s):  
Wind Turbine ◽  
Operating Conditions ◽  
Induction Generator ◽  
Doubly Fed Induction Generator ◽  
Grid Connection ◽  
Wind Turbine System ◽  
Space Modeling ◽  
Point Of Common Coupling ◽  
Doubly Fed ◽  
The Stability

<p><em> </em><em>     </em>The present paper formulates the state space modeling of doubly fed induction generator (DFIG) based wind turbine system for the purpose of the stability analysis. The objective of this study is to discuss the various modes of operation of the DFIG system under different operating conditions such as voltage sags with reference to variable wind speed and grid connection. The proposed control methodology exploits the potential of the DFIG scheme to avoid that grid voltage unbalances compromise the machine operation, and to compensate voltage unbalances at the point of common coupling (PCC), preventing adverse effects on loads connected next to the PCC. This methodology uses the rotor side converter (RSC) to control the stator current injected through the machine and the GSC to control the stator voltage to minimize the electromagnetic torque oscillations. Extensive simulation results on a 2MW DFIG wind turbine system illustrate the enhanced system performance and verify the effectiveness of the controller.</p>

scholarly journals Towards reduced order modelling for predicting the dynamics of coherent vorticity structures within wind turbine wakes

2017 ◽  
Vol 375 (2091) ◽  
pp. 20160108 ◽  
Author(s):  
M. Debnath ◽  
C. Santoni ◽  
S. Leonardi ◽  
G. V. Iungo
Keyword(s):  
Boundary Layer ◽  
Velocity Field ◽  
Atmospheric Boundary Layer ◽  
Wind Turbine ◽  
Turbine Blades ◽  
Dynamic Mode ◽  
Decomposition Techniques ◽  
Modal Decomposition ◽  
Tip Vortices ◽  
Reduced Order

The dynamics of the velocity field resulting from the interaction between the atmospheric boundary layer and a wind turbine array can affect significantly the performance of a wind power plant and the durability of wind turbines. In this work, dynamics in wind turbine wakes and instabilities of helicoidal tip vortices are detected and characterized through modal decomposition techniques. The dataset under examination consists of snapshots of the velocity field obtained from large-eddy simulations (LES) of an isolated wind turbine, for which aerodynamic forcing exerted by the turbine blades on the atmospheric boundary layer is mimicked through the actuator line model. Particular attention is paid to the interaction between the downstream evolution of the helicoidal tip vortices and the alternate vortex shedding from the turbine tower. The LES dataset is interrogated through different modal decomposition techniques, such as proper orthogonal decomposition and dynamic mode decomposition. The dominant wake dynamics are selected for the formulation of a reduced order model, which consists in a linear time-marching algorithm where temporal evolution of flow dynamics is obtained from the previous temporal realization multiplied by a time-invariant operator. This article is part of the themed issue ‘Wind energy in complex terrains’.

Experimental Voltage Stabilization of a Variable Speed Wind Turbine Driving Synchronous Generator using STATCOM based on Genetic Algorithm

2016 ◽  
Vol 17 (5) ◽  
pp. 541-546 ◽  
Author(s):  
Helmy M. El-Zoghby ◽  
Ahmed F. Bendary
Keyword(s):  
Genetic Algorithm ◽  
Power System ◽  
Wind Turbine ◽  
Reactive Power ◽  
Power Grid ◽  
Synchronous Generator ◽  
Voltage Regulation ◽  
Operating Conditions ◽  
Static Synchronous Compensator ◽  
The Stability

Abstract In this paper Static Synchronous Compensator (STATCOM) is used for improving the performance of the power grid with wind turbine that drives synchronous generator. The main feature of the STATCOM is that it has the ability to absorb or inject rapidly reactive power to grid. Therefore the voltage regulation of the power grid with STATCOM device is achieved. STATCOM also improves the stability of the power system after occurring severe disturbance such as faults, or suddenly step change in wind speed. The proposed STATCOM controller is a Proportional-Integral (PI) controller tuned by Genetic Algorithm (GA). An experimental model was built in Helwan University to the proposed system. The system is tested at different operating conditions. The experimental results prove the effectiveness of the proposed STATCOM controller in damping the power system oscillations and restoring the power system voltage and stability.

Fabrication of GaN Films in a Chemical Vapor Deposition Reactor

2013 ◽  
Author(s):  
J. Meng ◽  
Y. Jaluria ◽  
S. Wong
Keyword(s):  
Growth Rate ◽  
Film Growth ◽  
Numerical Study ◽  
Three Dimensional ◽  
Chemical Vapor ◽  
Rotating Disk ◽  
Transport Processes ◽  
Operating Conditions ◽  
Fluid Loss ◽  
Flow Recirculation

A three-dimensional numerical study has been carried out on the rotating disk GaN MOCVD process, and it is also coupled with an experimental study on the flow and thermal transport processes in the system. An impingement type reactor, with a rotating base, is considered. The dependence of the thin film growth rate and uniformity on operating conditions such as inflow velocity, rotational speed, and susceptor temperature are investigated in detail. Similarly, the effect of the geometry and configuration of the reactor are studied. The study also considers the effect of thermal and solutal buoyancy on the resulting flow. The flow and the associated transport processes are discussed in detail on the basis of the results obtained to suggest approaches to improve the uniformity of the film, minimize fluid loss and reduce flow recirculation that could affect growth rate and uniformity.

scholarly journals Computational fluid dynamic (CFD) of vertical-axis wind turbine: mesh and time-step sensitivity study

2019 ◽  
Vol 13 (3) ◽  
pp. 5604-5624
Author(s):  
S. Ashwindran ◽  
A. A. Azizuddin ◽  
A. N. Oumer
Keyword(s):  
Wind Turbine ◽  
Numerical Study ◽  
Computational Cost ◽  
Vertical Axis ◽  
Sensitivity Study ◽  
Dynamic Mode ◽  
Vertical Axis Wind Turbine ◽  
Time Step ◽  
Sliding Mesh ◽  
Mesh Resolution

This paper presents mesh and time-step dependence study of newly designed drag type vertical axis wind turbine. Ansys FLUENT a commercially available CFD solver was used to perform CFD numerical study on the drag type wind turbine. In computational analysis, 2D models was simulated under unsteady flow fields using SST k-ω to achieve stabilized numerical convergence. The model was analyzed at static and dynamic mode, where sliding mesh technique was used to analyze the turbine in dynamic mode. Three main parameters were taken under careful consideration: mesh resolution, turbulence model and time-step. Aerodynamic force was used in mesh sensitivity study for both static and sliding mesh. A small discrepancy in results of 2D sliding mesh result at different time-step and mesh resolution was observed. The generated results showed good agreement between fine and medium mesh with small difference in the initial initialization. In time-step dependency study for static mesh, dt=0.0002 time-step size was chosen for economical computational cost.

scholarly journals Modeling the Dynamics of Predator-Prey Community with Age Structures

2019 ◽  
Vol 14 (1) ◽  
pp. 77-93 ◽  
Author(s):  
G.P. Neverova ◽  
O.L. Zhdanova ◽  
E.Ya. Frisman
Keyword(s):  
Numerical Study ◽  
Classical Model ◽  
Interaction Coefficient ◽  
Individual Development ◽  
Dynamic Mode ◽  
Population Fluctuations ◽  
Predator Prey ◽  
Sustainable Community Development ◽  
Periodic Dynamics ◽  
The Stability

A model of the predator-prey community has been proposed with specific stages of individual development and the seasonality of breeding processes. It is assumed each of the species has an age structure with two stages of development. The case typical for the community “Arctic fox – rodents” is modeled. An analytical and numerical study of the model proposed is made. It is shown that periodic, quasi-periodic and chaotic oscillations can occur in the system, as well as a shift in the dynamics mode as a result of changes in the current sizes of the community’s populations. The model proposed demonstrates long-period oscillations with time delay like auto-oscillations in the classical model of Lotka-Volterra. It is shown that a transition from stable dynamics to quasi-periodic oscillations and vise verse is possible in the system, while an increase in the values of the half capturing saturation coefficient reduces the possibility of quasiperiodic oscillation emergence. Simulations demonstrate the growth in predator’s consumption of the prey average number expands the zones of multistability and quasi-periodic dynamics in the stability area of nontrivial fixed point. Therefore, the variation of the current population size of the community can lead to a change in the dynamic mode observed. The scenarios of transition from stationary dynamics to community’s population fluctuations are analyzed with different values of population parameters determining the dynamics of both species and their interaction coefficient. The model shows both sustainable community development and various complex fluctuations of interacting species. At the same time, the prey dynamics affects the predator one: the prey population fluctuations initiate predator oscillations like prey’s fluctuations, while the intrapopulation parameters of the predator can give to both stationary and fluctuating dynamic modes.

Coverage and Stability of NHx Terminated Cobalt and Ruthenium Surfaces: a First Principles Investigation

2019 ◽  
Author(s):  
Ji Liu ◽  
Michael Nolan
Keyword(s):  
Metal Surfaces ◽  
High Vacuum ◽  
Atomic Layer ◽  
Nitrogen Plasma ◽  
Operating Conditions ◽  
Ultra High Vacuum ◽  
Bridge Site ◽  
Stable Surface ◽  
Saturation Coverage ◽  
The Stability

<div>In the atomic layer deposition (ALD) of Cobalt (Co) and Ruthenium (Ru) metal using nitrogen plasma, the structure and composition of the post N-plasma NHx terminated (x = 1 or 2) metal surfaces are not well known but are important in the subsequent metal containing pulse. In this paper, we use the low-index (001) and (100) surfaces of Co and Ru as models of the metal polycrystalline thin films. The (001) surface with a hexagonal surface structure is the most stable surface and the (100) surface with a zigzag structure is the least stable surface but has high reactivity. We investigate the stability of NH and NH2 terminations on these surfaces to determine the saturation coverage of NHx on Co and Ru. NH is most stable in the hollow hcp site on (001) surface and the bridge site on the (100) surface, while NH2 prefers the bridge site on both (001) and (100) surfaces. The differential energy is calculated to find the saturation coverage of NH and NH2. We also present results on mixed NH/NH2-terminations. The results are analyzed by thermodynamics using Gibbs free energies (ΔG) to reveal temperature effects on the stability of NH and NH2 terminations. Ultra-high vacuum (UHV) and standard ALD</div><div>operating conditions are considered. Under typical ALD operating conditions we find that the most stable NHx terminated metal surfaces are 1 ML NH on Ru (001) surface (350K-550K), 5/9 ML NH on Co (001) surface (400K-650K) and a mixture of NH and NH2 on both Ru (100) and Co (100) surfaces.</div>

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