A Wave Modulation Model of Ripples over Long Surface Waves

A study is presented on the modulation of ripples induced by a long surface wave (LW) and a new theoretical modula-tion model is proposed. In this model, the wind surface stress modulation is related to the modulation of ripple spectrum. The model results show that in the case of LW propagating in the wind direction with the wave age parameter of LW increasing, the area with enhanced shear stress shifts from the region near the LW crest on the upwind slope to the LW trough. With a smaller wave age parameter of LW, the ripple modulation has the maximum on the upwind slope in the vicinity of LW crest, while with a larger parameter the enhancement of ripple spectrum does not occur in that region. At low winds the amplitude of ripple modulation transfer function (MTF) is larger in the gravity wave range, while at moderate or high winds it changes little in the range from short gravity waves to capillary waves.

Fabrication of large-scale ripples on fluorine-doped tin oxide films by femtosecond laser irradiation

The large-scale uniform self-organized ripples are fabricated on fluorine-doped tin oxide （FTO） coated glass by femtosecond laser. They can be smoothly linked in a horizontal line with the moving of XYZ stage by setting its velocity and the repetition rate of the laser. The ripple-to-ripple linking can also be realized through line-by-line scanning on a vertical level. The mechanism analysis shows that the seeding effect plays a key role in the linking of ripples.

Molecular dynamics study of temperature-dependent ripples in monolayer and bilayer graphene on 6H-SiC surfaces

Using classical molecular dynamics and a simulated annealing technique, we show that microscopic corrugations occur in monolayer and bilayer graphene on 6H-SiC substrates. From an analysis of the atomic configurations, two types of microscopic corrugations are identified, namely periodic ripples at room temperature and random ripples at high temperature. Two different kinds of ripple morphologies, each with a periodic structure, occur in the monolayer graphene due to the existence of a coincidence lattice between graphene and the SiC terminated surface （Si- or C-terminated surface）. The effect of temperature on microscopic ripple morphology is shown through analysing the roughness of the graphene. A temperature-dependent multiple bonding conjugation is also shown by the broad distribution of the carbon-carbon bond length and the bond angle in the rippled graphene on the SiC surface. These results provide atomic-level information about the rippled graphene layers or~ the two polar faces of the 6H-SiC substrate, which is useful not only for a better understanding of the stability and structural properties of graphene, but also for the study of the electronic properties of graphene-based devices.

A New Switching Scheme for Z-source Inverter to Minimize Ripples in the Z-source Elements

This paper presents a modification in pulse width modulation （PWM） scheme with unequal shoot-through distribution for the Z-source inverter （ZSI） which can minimize ripples in the current through the Z-source inductors as well as the voltage across the Z-source capacitors. For the same system parameters, the proposed control technique provides better voltage boost across the Z-source capacitor, DC-link, and also the AC output voltage than the traditional PWM. The ripples in the Z-network elements are found to be reduced by 75 % in the proposed modulation scheme with optimum harmonic profile in the AC output. Since the Zqnetwork requirement will be based on the ripple profile of the elements, the Z-network requirements can be greatly reduced. The effectiveness of the proposed modulation scheme has been simulated in Matlab/Simulink software and the results are validated by the experiment in the laboratory.

Single Phase Induction Motor Drive with Restrained Speed and Torque Ripples Using Neural Network Predictive Controller

In industrial drives, electric motors are extensively utilized to impart motion control and induction motors are the most familiar drive at present due to its extensive performance characteristic similar with that of DC drives. Precise control of drives is the main attribute in industries to optimize the performance and to increase its production rate. In motion control, the major considerations are the torque and speed ripples. Design of controllers has become increasingly complex to such systems for better management of energy and raw materials to attain optimal performance. Meager parameter appraisal results are unsuitable, leading to unstable operation. The rapid intensification of digital computer revolutionizes to practice precise control and allows implementation of advanced control strategy to extremely multifaceted systems. To solve complex control problems, model predictive control is an authoritative scheme, which exploits an explicit model of the process to be controlled. This paper presents a predictive control strategy by a neural network predictive controller based single phase induction motor drive to minimize the speed and torque ripples. The proposed method exhibits better performance than the conventional controller and validity of the proposed method is verified by the simulation results using MATLAB software.

Influence of PWM Modes on Commutation Torque Ripples in Sensorless Brushless DC Motor Control System

This paper introduces four PWM modes used in the sensorless brushless DC motor control system, analyzes their different influences on the commutation torque ripple in detail, and selects the best PWM mode in four given types to reduce commutation torque ripple of Brushless OC(BLDC) motors. Simulation and experimental results show that the selection is correct and practical.

Influence of Skewing Design for Reduction of Force Ripples in DSL-SynRM using 3D FEA

Double Sided Linear Synchronous Reluctance Motors(DSL-SynRM)are being increasingly used in high force density applications.The force ripples are one of the major issue in machine which is due to nonlinear nature of current in the machines.This paper focuses on the reduction of force ripples for increasing the force density of the motor.In order to reduce the force ripples,DSL-SynRM with a skewed translator is proposed.The proposed structure is designed and developed by using computational magnetic tools.This concept is effective for reduction of the force ripples and improves the force density of the machine.The proposed design has been reduced the percentage of force ripples by 21.62%,improved the force density by 10.32 N/mm³and efficiency by 0.89%.

A New Method for Reducing Commutation Torque Ripples in BLDC Motors

A new compensation method and an algorithm for compensating for the commutation torque ripples of the trapezoidal EMF brushless DC motor are put forward. Simulation and experimental results show that this method is correct and practical.

Applying Sliding Mode Control to Suppress Double Frequency Voltage Ripples in Single-phase Quasi-Z-source Inverters

The inherent double-line frequency ripple(DFR)of the single-phase quasi-Z-Source inverter(qZSI)strongly affects performance and design of the whole system,which requires hardware improvement methods such as additional semiconductor components or increasing the volume of capacitors and inductors.As a result,hardware solutions increase the price,and the volume and also reduces reliability of the system.In this paper,as a non-hardware solution,a sliding mode control(SMC)method is presented to control the voltage of the qZSI capacitor.To suppress the DFR,a second-order harmonic is injected into the capacitor reference voltage.Simulation and experimental results show that the introduced controller will have satisfactory results in reducing the DFR without any hardware changes in the qZSI,such as increasing the volume of the capacitor and inductor,adding a semiconductor,or increasing the frequency.A simulation model and experimental prototype are provided to demonstrate the validation of the effectiveness of the proposed controller.

Large-eddy simulation of formation of three-dimensional aeolian sand ripples in a turbulent field

With the method of large-eddy simulation,the equation of spherule motion and the method of immersed boundary condition,numerical simulations of three-dimen-sional turbulent aeolian motion and the formation of sand ripples under three-di-mensional turbulent wind and the mutual actions of saltation and creeping motion were carried out. The resulting sand ripples have the form that is flat on the upwind side and steep on the leeward,which is identical to the sand ripples in nature. We also realized the self-restoration process of three-dimensional sand ripples,which shows the correctness of the method of numerical simulation and the models of saltation and creeping. Finally,We analyzed the influence of sand ripples on the three-dimensional turbulent wind field,and found that due to the appearance and development of sand ripples,in the normal direction of ground there exists stronger energy exchange,and moreover,there is close correspondence between the forms of sand ripples and the vorticity close to the ground surface.

Stability characteristics of the open channel flow above the asymmetrical irregular sand ripples

Sandy bed cannot keep its original smoothness as the flows pass. With the increase of the flow intensity, the bed forms will appear as sand ripples and dune in turn. Among these morphologies, the sand ripple scale is the smallest, which is generally symmetrical when it just appears, but as time goes on, the asymmetrical form gradually develops. Just because of this sand ripples asymmetry, it manifests the influence of the flow on the bed morphology and also the impact on the laminar flow dynamical process, especially the stability characteristics. The stability features of laminar flow on open channels with the asymmetrical sand ripples are discussed, and also the results on the symmetrical sand ripples are compared in detail.

Numerical study of alpha particle loss with toroidal field ripple based on CFETR steady-state scenario

Effects of plasma equilibrium parameters on the alpha particle loss with the toroidal field ripple based on the CFETR steady-state scenario have been numerically investigated by the orbit-following code GYCAVA. It is found that alpha particle losses decrease and loss regions become narrower with the plasma current increasing or with the magnetic field decreasing. It is because the ripple stochastic transport and the ripple well loss of alpha particle are reduced with the safety factor decreasing. Decrease of the plasma density and temperature can reduce alpha particle losses due to enhancement of the slowing-down effect. The direction of the toroidal magnetic field can significantly affect heat loads induced by lost alpha particle. The vertical asymmetry of heat loads induced by the clockwise and counter-clockwise toroidal magnetic fields are due to the fact that the ripple distribution is asymmetric about the mid-plane, which can be explained by the typical orbits of alpha particle. The maximal heat load of alpha particle for the clockwise toroidal magnetic field is much smaller than that for the counter-clockwise one.

Airgap-harmonic-oriented Partitioned Design Method of PMV Motor with Improved Torque Performances

Here,we introduce a partitioned design method that is oriented toward airgap harmonic for permanent magnet vernier(PMV)motors.The method proposes the utilization of airgap flux harmonics as an effective bridge between the torque design region and the torque performances.To illustrate the efficacy of this method,a partitioned design PMV motor is presented and compared with the initial design.Firstly,the torque design region of the rotor is artfully divided into the torque enhancement region and ripple reduction region.Meanwhile,the main harmonics that generate output torque are chosen and enhanced,optimization.Moreover,the harmonics that generate torque ripple are selected and reduced based on torque harmonics optimization.Finally,the functions of the partitioned PMV motor torque are assessed based on the finite element method.By the purposeful design of these two regions,the output torque is strengthened while torque ripple is inhibited effectively,verifying the effectiveness and reasonability of the proposed design method.

A Design of Modular Interior Ferrite Magnet Fluxswitching Linear Motor for Track Transport

A novel topology of modular ferrite magnet fluxswitching linear motor(FMFSLM)use for track transport is presented in this paper,which enables more ferrite magnets to be inserted into the primary iron core.The motor has a significant low-cost advantage in long-distance linear drive.The proposed FMFSLM’s structure and working principle were introduced.Further,the thrust force expression of the motor was established.The thrust force components triggering thrust force ripple were investigated,and their expressions can be obtained according to the inductances’Fourier series expressions.Resultantly,the relationship between the harmonics of thrust force and that of self-and mutual inductances was revealed clearly.Based on the relationship,a skewed secondary should be practical to reduce the thrust force ripple.Thus,the effect of employing a skewed secondary to the proposed FMFSLM was investigated,and an optimized skewing span distance was determined.Finite element analysis(FEA)was conducted to validate the exactness of the theoretical analysis.The simulation results indicate that the strategy of suppressing thrust force ripple has a significant effect.Meanwhile,the motor maintains a good efficiency characteristic.The results of the prototype experiment are in good agreement with FEAs,which further verifies the proposed modular interior FMFSLM’s practicability.

Ripples From The Arab World

Turmoil in the Middle East sparks global economic concerns The massive upheavals sweeping the Arab world started on January 14 this year when Tunisian President Zine El Abidine Ben Ali fled abroad after protesters demanded he step down as thecountry’s leader.Egyptian President

NUMERICAL STUDY FOR EVOLUTION OF COHERENT VORTEX IN UNIDIRECTIONAL OSCILLATORY BOUNDARY LAYER OVER SAND RIPPLES

Ripples are widely formed in river channels and coastal regions, the bed-forms and vortex dynamics accompanied with them are quite important because they are responsible for frictions and sediment transports. The coherent vortex structure play a key role in the process, so how to model them is needed for understanding the formation process of sand ripples. In this paper, 2D Large-Eddy-Simulation (LES) was used to predict the flow structure and the dynamics of coherent vortex. The numerical results show a complete process of formation, evolvement and disappearance for the cases of unidire ctional and oscillatory flows over symmetic and asymmetric ripples

WAVE BOTTOM LAYERS DYNAMIC WITH SUSPENDED SEDIMENT OVER VORTEX RIPPLES

Vortex ripple is widely formed in the coastal region, and the dynamic ofvortex is quite important because it is responsible for sediment transport. The flow structurearound the vortex ripples can be modeled as 2D flow due to the geometry of the flow boundaries. Inthis paper, 2D Large-Eddy-Simulation (LES) method was used to predict the flow structure and thedynamic of vortex in the bottom layers under the action of the wave, the numerical simulationresults show a completely process of vortex formation, evolvement and disappearance. Based on thestudy of flow structure, the suspended sediment transport was modeled in present paper. Thesimulated sediment concentrations were compared to measurements from the literature. The agreementbetween the time averaged simulated concentration profiles and measurements is satisfactory. For ahigh setting velocity, the suspended sediment is confined to the vicinity of the bed, and it isdominated by the local bottom shear stress. For a small setting velocity, the suspension is moredominated by the characteristic of vortex. There are two suspended sediment transport peaks observedin the cross-section at the trough and crest in the half period, the second peak is due to theseparation bubble taking the sediment.

Experimental Study on Pyramid Sand Ripples With Wind Tunnel Simulating Technique

The pyramid dune was named after its morphology similar to pyramids on the banks of the Nile River. They are also called star dunes as their plane figures are similar to multi-angular stars. In recent decades there still remained considerable arguments on the formation mechanism of the pyramid dunes although scientists have conducted a lot of work in the field of eolian sand geomorphology.

A STUDY OF OSCILLATING FLOWS OVER PROPAGATING RIPPLES:PART I. STEADY STREAMING

Oscillating flows over periodic ripples are numerically investigated by solving the two dimensional time dependent incompressible Navier Stokes equations in curvilinear coordinates for a wider range of the parameters of KC, Re and ε, in particular for the oscillatory flow over propagating ripples. In the oscillating flows over periodic ripples, circulation cells due to the streaming effect are formed, which can help redistribute suspended sediments. In the present study, the circulation patterns for both stationary rigid ripples and propagating ripples are investigated. The energy dissipation in the flow field is also discussed.

A STUDY OF OSCILLATING FLOWS OVER PROPAGATING RIPPLES:PART Ⅱ. VORTICITY DYNAMICS

Vortex shedding for an oscillating flow over ripples is numerically investigated by solving 2 D time dependent incompressible Navier Stokes equations in curvilinear coordinates. The dynamics of vortex structures generated by flow separation is studied in order to understand some phenomena which take place near the bottom and in particular to get insight into the mechanism through which sediment grains are lifted up and maintained in suspension. Based on our calculated results, some important phenomena, such as vortex shedding, vortices pairing, vortices coalescence, vortex interaction with the free shear layer and so on, can be found. The behaviors of energy dissipation in the flow field and of shear stress along the ripple surface are also discussed for studying the dynamics of sediment grains in suspension and to obtain new information on the suspended load over the ripples.