Minimizing airfoil drag at low angles of attack with DBD-based turbulent drag reduction methods

Dielectric Barrier Discharge (DBD) based turbulent drag reduction methods are used to reduce the total drag on a NACA 0012 airfoil at low angels of attack. The interaction of DBD with turbulent boundary layer was investigated, based on which the drag reduction experiments were conducted. The results show that unidirectional steady discharge is more effective than oscillating discharge in terms of drag reduction, while steady impinging discharge fails to finish the mission (i.e. drag increase). In the best scenario, a maximum relative drag reduction as high as 64 % is achieved at the freestream velocity of 5 m/s, and a drag reduction of 13.7 % keeps existing at the freestream velocity of 20 m/s. For unidirectional discharge, the jet velocity ratio and the dimensionless actuator spacing are the two key parameters affecting the effectiveness. The drag reduction magnitude varies inversely with the dimensionless spacing, and a threshold value of the dimensionless actuator spacing of 540 (approximately five times of the low-speed streak spacing) exists, above which the drag increases. When the jet velocity ratio smaller than 0.05, marginal drag variation is observed. In contrast, when the jet velocity ratio larger than 0.05, the experimental data bifurcates, one into the drag increase zone and the other into the drag reduction zone, depending on the value of dimensionless actuator spacing. In both zones, the drag variation magnitude increases with the jet velocity ratio. The total drag reduction can be divided into the reduction in pressure drag and turbulent friction drag, as well as the increase in friction drag brought by transition promotion. The reduction in turbulent friction drag plays an important role in the total drag reduction.     

Geometrical and electrical optimization of NS-SDBD streamwise plasma heat knife for aircraft anti-icing

Ice accretion on aircraft encountering supercooled water droplets in clouds poses great risks to flight performance and safety. With the aim of optimizing the newly developed streamwise plasma heat knife method for anti-icing, a parametric investigation is carried out in this work. The influence of the detailed voltage profile on the heating effects of a Surface Dielectric Barrier Discharge driven by Nanosecond Pulses (NS-SDBD) is investigated, and a comparison of the anti-icing performance among different configurations of streamwise plasma heat knife is made. The results show that columnar high-temperature regions produced by a multi-streamer discharge appear at small pulse rise time, but become diffuse as the pulse rise time increases. An optimal pulse rise time exists to provide a wide range and high value of temperature, which is found to be 150 ns for the setup in the present study. The influence of the pulse fall time is much weaker than that of the rise time. The range and value of the temperature decrease with increasing pulse fall time. A greater pulse width is found to improve the heating effect by increasing the discharge power. When a spanwise electrode is placed connecting the streamwise electrodes of the streamwise plasma heat knife at the airfoil leading edge, the anti-icing performance becomes poorer, whereas good performance is achieved when the spanwise electrode is at the edge of the streamwise electrodes. Based on this, a three-level configuration of the plasma heat knife is proposed, and its anti-icing performance is found to be much better than that of the original configuration.     

Study of UV Ne II line shapes in the cathode sheath of an abnormal glow discharge

We report results of an experimental and theoretical study of complex UV line shapes of Ne II 369.421 nm, Ne II 370.962 nm, Ne II 371.308 nm, and.Ne II 372.711 nm lines in the cathode sheath (CS) region of an abnormal DC glow discharge in pure neon. Two sets of experimental profiles and electric field distributions, one for discharge with tungsten and the other with titanium cathode, were obtained by means of the optical emission spectroscopy (OES) and theoretically studied by the iterative CS kinetic model. It is shown that our theoretical model enables the determination of the most important CS parameters (e.g. the thickness of the CS region, and the theoretical distributions of electric field and gas temperature) and thereupon based accurate theoretical predictions of the experimentally observed profiles of the studied lines.     

Trajectory optimisation in electrical discharge machining of three-dimensional curved and twisted channels

Numerical control electrical discharge machining(NC EDM) is one of the most widely used machining technologies for manufacturing a closed blisk flow path, particularly for three-dimensional(3D) curved and twisted flow channels. In this process, tool electrode design and machining trajectory planning are the key factors affecting machining accessibility and efficiency. Herein, to reduce the difficulty in designing the electrode and its motion path in the closed curved and twisted channels, a heur...     

Breakthrough detection in electrochemical discharge drilling to enhance machining stability

Film cooling holes are widely used in aero-engine turbine blades. These blades feature large numbers of holes with complex angles and require a high level of surface integrity. Electrochemical discharge drilling(ECDD) combines the high efficiency of electrical discharge drilling(EDD) with high quality of electrochemical drilling(ECD). However, due to the existence of a variety of energy for material removal, accurate and timely detection of breakthroughs is fraught with difficulties. An insuffic...     

航空用不锈钢磨粒流光整加工表面完整性研究

为研究磨粒流光整加工不锈钢闭式整体叶环的表面完整性，通过对电火花制备的不锈钢试样进行加工时间、加工压强及磨料目数的单因素试验，研究3个因素对加工后表面完整性的影响。结果表明：表面粗糙度Ra随着加工时间的增加而降低，从初始时的1.459μm降至60 min时的0.193μm，降低速率越来越慢，最后趋于稳定。磨粒流加工有效去除了加工表面再铸层。显微硬度在加工后明显降低，由450～950 HV降至270～350 HV且分布均匀性得到改善。显微硬度随着加工时间的增加整体上呈降低趋势，并出现了一定的加工硬化现象。在使用不同目数的磨料进行磨粒流加工后，残余应力由拉应力变为压应力。随着加工时间和加工压强的增加，试样表面越平整，抛光效果越好。     

Time resolved study of laser triggered electric discharge spark in atmosphere: Machine learning approach

In this paper we analyze the possibilities of using machine learning algorithms for analysis of optical spectra of electric discharge spark in atmosphere. Breakdown in air can be initiated by intense laser pulse, making plasma which has a significant electrical conductivity. The formed plasma can be further maintained by electric current obtained from capacitor discharge. In such a case the capacitor voltage can be much lower than the striking voltage (the voltage needed to initiate the electric breakdown in air). Present setup has timing precision and low jitter of fast laser and arbitrary high energies corresponding to capacitance and voltage to which the capacitor is charged. We have used a streak camera equipped with a spectrograph to analyze optical emission of plasma obtained in this way. Q-switched Nd:Yag laser was used to achieve the initial breakdown in air. Machine learning methods were used in order to classify optical spectra of plasmas with different electron temperatures obtained with different excitation energies. We have shown that, instead of using the usual way of identifying the spectral peaks and calculating their intensity ratio, it is possible to train the computer software to recognize the spectra corresponding to different electron temperatures. Principal component analysis was used to reduce the dimensionality of problem. We present possibilities of plasma electron temperature estimation based on several clustering algorithms.     

Recent developments in thermal characteristics of surface dielectric barrier discharge plasma actuators driven by sinusoidal high-voltage power

Flow control using surface Dielectric Barrier Discharge(DBD) plasma actuators driven by a sinusoidal alternating-current power supply has gained significant attention from the aeronautic industry. The induced flow field of the plasma actuator, with the starting vortex in the wall jet,plays an important role in flow control. However, the energy consumed for producing the induced flow field is only a small fraction of the total energy utilized by the plasma actuator, and most of the total energy i...