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Frequency regulation of wind turbines can improve the stability of the power system. However, it would cause generator torque fluctuation, increasing the risk of fatigue load. Previous research works were restricted to inertial and droop responses. Less attention has been paid to striking a balance between frequency regulation and fatigue load. To overcome these, a hybrid control strategy is proposed to consider both frequency response characteristic and fatigue load mitigation. First, a small signal linearization model is built to reveal the impact of the correlation mechanism of frequency regulation on drive train torque load. Second, a multivariable cost function is constructed to optimize the proportional integral (PI) controller, which combines the total fluctuation with the dispersion of the fatigue load and frequency. Then, a hybrid controller based on PI control optimized by particle swarm optimization algorithm and active disturbance rejection control is designed to restrain rapid frequency changes as well as fatigue torque fluctuation simultaneously. Several experiments are performed to verify the significance of the proposed method under different scenarios. Compared with the existing methods, the proposed hybrid control exhibits superiority in improving frequency response and fatigue load mitigation. [ABSTRACT FROM AUTHOR]

Large eddy simulation (LES) has the potential to predict turbulent combustion phenomena in modern practical combustors. As errors from sub-grid models may be comparable to the numerical errors in the LES approach, mitigating the impact of the numerical errors is as important as constructing accurate sub-grid models. Therefore, a low-dissipative, structure-preserving ROUND (Reconstruction Operators on Unified Normalized-variable Diagram) scheme is tested for the LES of reacting flows in this study. The high efficiency of this scheme is demonstrated by evaluating its accuracy, central processing unit cost, and structure-preserving property by simulating the convection–diffusion process of a passive scalar. Simulations of two bluff body stabilized flames are studied using this scheme. For low turbulence intensity, the current scheme improves the numerical resolution of the instantaneous and averaged flow fields. The prediction of flow statistics is also improved by the ROUND schemes compared to the conventional schemes. Moreover, the ROUND schemes preserve the axisymmetry of the averaged flow better than the conventional schemes for the cases investigated here. For the high turbulence intensity case, the ROUND scheme avoids nonphysical numerical oscillations. The flow and flame statistics obtained using this scheme compare well with measurements. Therefore, this work demonstrates the advantages of using ROUND schemes for LES of reacting flows. [ABSTRACT FROM AUTHOR]

A Mach 1.5 non-reactive flow in a cavity-stabilized combustor of a model scramjet is studied via a direct-numerical simulation approach, and the analysis is focused on the interaction among boundary layer, free shear-layer above the cavity and shock wave. It is found that the impingement of the free shear-layer on the aft wall of the cavity leads to strong turbulence kinetic energy, high local pressure, and a fan of compression waves. The compression waves evolve into an oblique shock, which reflects between the upper and lower walls and interacts with the boundary layers attached to the two walls. The analysis of the turbulence production reveals that the amplification of turbulence in the core of the shear-layer and around the reattachment point is mainly due to the shear production, but the deceleration production mechanism presents a significant impact in the regions above the aft wall of the cavity and around the shock interaction points. The very low frequency commonly observed in shock wave/boundary layer interactions is not observed in the present research, which might be due to the low Reynolds number of the studied case. [ABSTRACT FROM AUTHOR]

In this work, we developed high-fidelity numerical solvers for turbulent cavitation flows and presented numerical simulations of cavitation and supercavitation behind an axisymmetric projectile and a conical cavitator. The proposed numerical solver is based on the homogeneous equilibrium model where the mass transfer rate between vapor and liquid is dependent on a volume of fraction (VOF) function. As a result, the numerical results of cavitation turbulent flow are heavily affected by the accuracy of the VOF evolution prediction. Thus, the proposed solver employs the Tangent of Hyperbola for INterface Capture method with Quadratic surface representation and Gaussian Quadrature scheme to accurately resolve the interfacial structures of cavitation bubbles. To effectively model the turbulent effects, the solver utilizes two approaches, respectively, i.e., the Smagorinsky model of large eddy simulation and the shear stress transport model of the Reynolds-averaged Navier–Stokes equation. The homogeneous turbulence equation systems are then discretized by the linear second-order schemes in space and time. The phase transition including both evaporation and condensation is described by the finite-rate mass transfer models such as Schnerr–Sauer and Kunz models. Numerical simulations and comparison studies are performed with the proposed solvers. Compared with previous simulation works, the current simulation results of cavity shape patterns and related hydrodynamic characteristics are in better agreement with experimental data and analytical theories, as well as reference solutions. These results validate that the proposed solver can produce high-fidelity predictions for solving the flow structures of cavitation and reentrant jet in the turbulent cavitation simulations. [ABSTRACT FROM AUTHOR]

The large-caliber steady-state magnetic-field testing system is an important device for the International Thermonuclear Experimental Reactor, which is mainly used for electromagnetic compatibility tests in a strong magnetic field environment. The uniform magnetic field area needs to be measured to ensure the correctness of the design after the completion of the platform. This paper mainly analyzes the relationship between the magnetic field probe and error, studies the detection effect of different arrangement methods of magnetic field probe points, and obtains a more accurate probe arrangement method through comparative analysis. Then, the principle and structure of the acquisition and detection system are studied, and the multipoint synchronous Hall array magnetic field detection system of the magnetic field is built by using the Hall probe to realize the omnidirectional measurement of the magnetic field. Finally, the Hall probe is calibrated and tested by experiments, and a test platform is built to test the magnetic field. The results verify the correctness of the analysis. [ABSTRACT FROM AUTHOR]

This study proposes a new formulation for the Harten, Lax, and van Leer (HLL) type Riemann solver which is capable of solving contact discontinuities accurately but with robustness for strong shock. It is well known that the original HLL, which has incomplete wave structures, is too dissipative to capture contact discontinuities accurately. On the other side, contact-capturing approximate Riemann solvers such as HLL with Contact (HLLC) usually suffer from spurious solutions, also called carbuncle phenomenon, for strong shock. In this work, a new accurate and robust HLL-type formulation, the so-called HLL-BVD (HLL Riemann solver with boundary variation diminishing) is proposed by modifying the original HLL with the BVD algorithm. Instead of explicitly recovering the complete wave structures like the way of HLLC, the proposed method restores the missing contact with a jump-like function. The capability of solving contact discontinuities is further improved by minimizing the inherent dissipation term in HLL. Without modifying the original incomplete wave structures of HLL, the robustness for strong shock has been reserved. Thus, the proposed method is free from the shock instability problem. The accuracy and robustness of the new method are demonstrated through solving several one- and two-dimensional tests. Results indicate that the new formulation based on the two-wave HLL-type Riemann solver is not only capable of capturing contact waves more accurately than the original HLL or HLLC but, most importantly, is free form carbuncle instability for strong shock. [ABSTRACT FROM AUTHOR]

Stoichiometric acetylene-oxygen mixtures diluted by argon are widely used in normal detonation waves but have not yet been used in oblique detonation waves (ODWs). The present study simulates ODWs in acetylene-oxygen-argon mixtures, with a dilution ratio Φ = 50%-90%, using the reactive Euler equations with a detailed chemistry model, and describes the characteristics of acetylene ODWs. Similar to ODWs in hydrogen-air mixtures, the transition from the oblique shock wave to ODW may be either abrupt or smooth and the effects of Φ are investigated with a variable incident Mach number M0. When M0 changes between 8 and 10, the transition is abrupt in the case of Φ = 50%, while it is smooth in the case of Φ = 90%. In the case of Φ = 70%, a high M0 corresponds to a smooth transition, while a low M0 corresponds to an abrupt transition. A further study on the initiation mechanism was performed by comparing the numerical initiation length with the length obtained via the constant-volume combustion calculation, which demonstrated that the initiation is kinetic-controlled in the cases of Φ = 70% and 90%, but wave-controlled in the case of Φ = 50% with M0 below 8.5. Moreover, an initiation structure featured by two-shock in the product was observed for the first time, and its formation mechanism is qualitatively discussed and compared to hydrogen ODWs. [ABSTRACT FROM AUTHOR]

Thermal rectification in thickness asymmetric graphene nanoribbons connecting single-layer with multi-layer graphene is investigated by using classical nonequilibrium molecular dynamics. It is reported that the graphene nanoribbons with thickness-asymmetry have a good thermal rectification. The thermal rectification factor depends on temperature as well as the thickness-ratio of the two-segment. Our results provide a direct evidence that the thermal rectifier can be achieved in a nanostructure crossing two- and three-dimension. [ABSTRACT FROM AUTHOR]