5,503 research outputs found
A reduced-order model of three-dimensional unsteady flow in a cavity based on the resolvent operator
A novel reduced-order model for nonlinear flows is presented. The model
arises from a resolvent decomposition in which the nonlinear advection terms of
the Navier-Stokes equation are considered as the input to a linear system in
Fourier space. Results show that Taylor-G\"ortler-like vortices can be
represented from a low-order resolvent decomposition of a nonlinear lid-driven
cavity flow. The present approach provides an approximation of the fluctuating
velocity given the time-mean and the time history of a single velocity probe
Simulations of Kinetic Electrostatic Electron Nonlinear (KEEN) Waves with Variable Velocity Resolution Grids and High-Order Time-Splitting
KEEN waves are nonlinear, non-stationary, self-organized asymptotic states in
Vlasov plasmas outside the scope or purview of linear theory constructs such as
electron plasma waves or ion acoustic waves. Nonlinear stationary mode theories
such as those leading to BGK modes also do not apply. The range in velocity
that is strongly perturbed by KEEN waves depends on the amplitude and duration
of the ponderomotive force used to drive them. Smaller amplitude drives create
highly localized structures attempting to coalesce into KEEN waves. These cases
have much more chaotic and intricate time histories than strongly driven ones.
The narrow range in which one must maintain adequate velocity resolution in the
weakly driven cases challenges xed grid numerical schemes. What is missing
there is the capability of resolving locally in velocity while maintaining a
coarse grid outside the highly perturbed region of phase space. We here report
on a new Semi-Lagrangian Vlasov-Poisson solver based on conservative
non-uniform cubic splines in velocity that tackles this problem head on. An
additional feature of our approach is the use of a new high-order
time-splitting scheme which allows much longer simulations per computational e
ort. This is needed for low amplitude runs which take a long time to set up
KEEN waves, if they are able to do so at all. The new code's performance is
compared to uniform grid simulations and the advantages quanti ed. The birth
pains associated with KEEN waves which are weakly driven is captured in these
simulations. These techniques allow the e cient simulation of KEEN waves in
multiple dimensions which will be tackled next as well as generalizations to
Vlasov-Maxwell codes which are essential to understanding the impact of KEEN
waves in practice
Research and Implement of PMSM Regenerative Braking Control for Electric Vehicle
As the society pays more and more attention to the environment pollution and energy crisis, the electric vehicle (EV) development also entered in a new era. With the development of motor speed control technology and the improvement of motor performance, although the dynamic performance and economical cost of EVs are both better than the internal-combustion engine vehicle (ICEV), the driving range limit and charging station distribution are two major problems which limit the popularization of EVs. In order to extend driving range for EVs, regenerative braking (RB) emerges which is able to recover energy during the braking process to improve the energy efficiency. This thesis aims to investigate the RB based pure electric braking system and its implementation.
There are many forms of RB system such as fully electrified braking system and blended braking system (BBS) which is equipped both electric RB system and hydraulic braking (HB) system. In this thesis the main research objective is the RB based fully electrified braking system, however, RB system cannot satisfy all braking situation only by itself. Because the regenerating electromagnetic torque may be too small to meet the braking intention of the driver when the vehicle speed is very low and the regenerating electromagnetic torque may be not enough to stop the vehicle as soon as possible in the case of emergency braking. So, in order to ensure braking safety and braking performance, braking torque should be provided with different forms regarding different braking situation and different braking intention.
In this thesis, braking torque is classified into three types. First one is normal reverse current braking when the vehicle speed is too low to have enough RB torque. Second one is RB torque which could recover kinetic energy by regenerating electricity and collecting electric energy into battery packs. The last braking situation is emergency where the braking torque is provided by motor plugging braking based on the optimal slip ratio braking control strategy.
Considering two indicators of the RB system which are regenerative efficiency and braking safety, a trade-off point should be found and the corresponding control strategy should be designed. In this thesis, the maximum regenerative efficiency is obtained by a braking torque distribution strategy between front wheel and rear wheel based on a maximum available RB torque estimation method and ECE-R13 regulation. And the emergency braking performance is ensured by a novel fractional-order integral sliding mode control (FOISMC) and numerical simulations show that the control performance is better than the conventional sliding mode controller
Development of Braking Force Distribution Strategy for Dual-Motor-Drive Electric Vehicle
In the development of the optimal braking force distribution strategy for a dual-motor-drive electric vehicle (DMDEV) with a series cooperative braking system, three key factors were taken into consideration, i.e. the regenerative force distribution coefficient between the front and the rear motor (β), the energy recovery coefficient at the wheels (α3), and the front-and-rear-axle braking force distribution coefficient (λ). First, the overall power loss model of the two surface-mounted permanent magnetic synchronous motors (SMPMSMs) was created based on the d-q axis equivalent circuit model. The optimal relationship of β and the overall efficiency of the dual-motor system were confirmed, where the latter was quite different from that obtained from the traditional look-up table method for the motors\u27 efficiency. Then, four dimensionless evaluation coefficients were used to evaluate braking stability, regenerative energy transfer efficiency, and energy recovery at the wheels. Finally, based on several typical braking operations, the comprehensive effects of the four coefficients on braking stability and energy recovery were revealed. An optimal braking force distribution strategy balancing braking stability and energy recovery is suggested for a DMDEV with a series cooperative braking system
Development of Braking Force Distribution Strategy for Dual-Motor-Drive Electric Vehicle
In the development of the optimal braking force distribution strategy for a dual-motor-drive electric vehicle (DMDEV) with a series cooperative braking system, three key factors were taken into consideration, i.e. the regenerative force distribution coefficient between the front and the rear motor (β), the energy recovery coefficient at the wheels (α3), and the front-and-rear-axle braking force distribution coefficient (λ). First, the overall power loss model of the two surface-mounted permanent magnetic synchronous motors (SMPMSMs) was created based on the d-q axis equivalent circuit model. The optimal relationship of β and the overall efficiency of the dual-motor system were confirmed, where the latter was quite different from that obtained from the traditional look-up table method for the motors' efficiency. Then, four dimensionless evaluation coefficients were used to evaluate braking stability, regenerative energy transfer efficiency, and energy recovery at the wheels. Finally, based on several typical braking operations, the comprehensive effects of the four coefficients on braking stability and energy recovery were revealed. An optimal braking force distribution strategy balancing braking stability and energy recovery is suggested for a DMDEV with a series cooperative braking system
Large Grid-Connected Wind Turbines
This book covers the technological progress and developments of a large-scale wind energy conversion system along with its future trends, with each chapter constituting a contribution by a different leader in the wind energy arena. Recent developments in wind energy conversion systems, system optimization, stability augmentation, power smoothing, and many other fascinating topics are included in this book. Chapters are supported through modeling, control, and simulation analysis. This book contains both technical and review articles
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