等离子体减阻

1、等离子体减阻一、 飞行器的升力、阻力、控制方法1. 飞行器升力的产生2. 飞行器的阻力阻力是与飞机运动方向相反的空气动力，起着阻碍飞行器前进的作用。低速飞行时的阻力种类：l 摩擦阻力（解释）l 压差阻力（解释）l 诱导阻力（专业性强，不解释）l 干扰阻力（专业性强，不解释）（a）摩擦阻力 当有粘性的空气流过飞机时，紧贴飞机表面的一层空气，与飞机表面发生粘性摩擦，这一层空气完全粘附在飞机表面上，气流速度降低为零。紧靠这静止空气层的外面第二气流层，因受这静止空气层粘性摩擦的作用，气流速度也要降低，但这种作用要弱些，因此气流速度不会降低为零。再往外，第三气流层又要受第二气流层粘性摩擦的作用，气流速度

2、也要降低，但这种作用更弱些，因此气流速度降低就更少些。这样，沿垂直于飞机表面的方向，从飞机表面向外，由于粘性摩擦作用的减弱，气流速度就一层一层的逐渐增大，到附面层边界，就和主流速度相等了。这层气流速度由零逐渐增大到主流速度的空气层，就是附面层，或边界层附面层内，气流速度之所以越贴近飞机表面越慢，这必然是由于这些流动空气受到了飞机表面给它的向前的作用力的作用的结果。根据作用和反作用定律，这些被减慢的空气，也必然要给飞机表面一个向后的反作用力，这就是飞机表面的摩擦阻力。（b）压差阻力生活历经：人在逆风中行走，会感到阻力的作用，这就是一种压差阻力。空气流过机翼时，在机翼前缘部分，受机翼阻挡，流速减慢

3、，压力增大；在机翼后缘，由于气流分离形成涡流区，压力减小。这样，机翼前后便产生压力差，形成阻力。这种由前后压力差形成的阻力叫压差阻力。机身、尾翼等飞机的其它部件都会产生压差阻力。生活观察例子：汽车开过，在车身后的灰尘之所以被吸起，就是由于车身后面涡流区内的空气压力小的缘故。3. 降低飞行器的阻力A) 降低摩擦阻力l 减少飞行器飞机同空气的接触面积，减少摩擦阻力；l 提高飞行器表面的光滑度；如有的高速飞机甚至将表面打 磨光。维护使用中，保持好飞机表面光洁。如上飞机，要求穿软底鞋，铺好脚踏布等。飞机要定期清洗。停放时加盖蒙布，以防风沙雨雪侵蚀。l 实验表明：湍流附面层的摩擦阻力要比层流附面层的摩擦

4、 阻力大得多。因此，尽可能在机翼上保持层流附面层，相应减小摩擦阻力。 层流区湍流区40%100%B) 降低压差阻力方法l 降低物体的迎风面积；l 采用流线形：降低压差阻力。（形状对升力、阻力影响的细致情况：最大厚度位置，对升阻力也有影响。最大厚度位置靠前，机翼前缘势必弯曲得更厉害些，导致流管在前缘变细，流速加快，吸力增大，升力较大。但因后缘涡流区大，阻力也较大。最大厚度位置靠近翼弦中央，升力较小，但其阻力也较小。因为，最大厚度位置靠后，最低压力点，分离点均向后移，层流附面层加长，湍流附面层减短，使摩擦阻力减小，所以阻力较小。）l 提高飞行器表面的光滑度，边界层分离点后移，层流段增 加，湍流段减

5、少，摩擦阻力降低，同时压差阻力也降低。l 降低飞行器的仰角（有时应用目的矛盾：如起飞，爬高） C) 降低压差阻力的重要性在通常条件下，压差阻力一般大于摩擦阻力，降低压差阻力重要。D) 降低飞行阻力的重要性提高飞行器速度和增加航程。飞机的阻力下降百分之几,对飞行器和民航机来说,每年可节省上千亿美元的燃料成本。 集中讲授内容àà4. 大仰角飞行及控制A) 大仰角飞行的必要性起飞，爬高，降落 B) 大仰角飞行时出现的问题l 迎面面积增加，压差差阻力增大；l 大迎角时，边界层分离点前移，机翼后部的涡流区扩大，压力减小，机翼前后的压力差增加，压差阻力增加。 （细致情况：在小迎角的情况

6、下增加迎角时，由于升力的增加和涡流区的扩大都很慢，故压差阻力和诱导阻力增加都很少，这时机翼的阻力主要是摩擦阻力，因此整个机翼阻力增加不多。当迎角逐渐变大以后，再增大迎角时，由于机翼升力的增加和涡流区的扩大都加快，故压差阻力和诱导阻力的增加也随之加快。特别是诱导阻力，在大迎角时，随着迎角的增大而增加更快。因此，整个机翼的阻力随着迎角的增大而增加较快。这时，诱导阻力是机翼阻力的主要部份。超过临界迎角以后，虽然诱导阻力要随着升力的降低而减小，但由于压差阻力的急剧增加，结果使整个机翼阻力增加更快。简单说：迎角增大，阻力增大；迎角越大，阻力增加越多；超过临界迎角，阻力急剧增大。） l 大迎角时，机翼升力

7、下降。大迎角飞行阻力大的不良后果：升力减小，阻力大。飞行器失速，即只爬高，前行速度慢。 附：小迎角飞行问题飞机以小迎角进行跨音速和超音速飞行时，随着M数增大,翼面上激波与边界层相互干扰增强到一定程度，也会使边界层分离，使翼面上空气动力随时间强烈振荡，引起飞机抖振、操纵面嗡鸣等现象，从而限制飞机巡航速度的提高，降低飞机的机动性能，因此边界层分离的实验与理论计算一直是人们关注的重要课题。C) 大仰角飞行问题的解决方法思路：问题由边界层的过早分离产生，问题解决途径是尽量增加层流面，避免边界层过早分离，降低湍流层面积。解决方法：l 优化机翼形状，可变的机翼形状，提高表面光滑度（前面有叙述），但效果有限

8、。l 气流控制：通过物面上的喷孔(狭缝)吹出流体，以增加表面滞流的能量；通过物面上的狭缝，吸走滞流，使边界层变薄，以抑制分离；用不同气体喷射，加速滞流。 新思路：若等离子体能产生气流，即能控制边界层 plenum chamberouter skininner skinBoundary layer thins and becomes fuller across slot采用打孔表面、横向隙缝吸走滞流，降低边界层厚度。二、 等离子体减阻方法1. 基本知识回顾电磁学尖端放电中的“离子风”。2. 等离子体控制飞行器边界层和减阻l 离子风对边界层的影响； l 为什么只强调离子的作用？在电场中，电子、离子

9、均被加速。当电子、离子与中性气体粒子时，离子与中性气体粒子碰撞中传递给中性气体的动量大，而电子传递给中性气体粒子的动量小（电子的动能可有效地转化为中性气体粒子的内能，产生分解、激发、电离。），后者是前者的百分之几。l 放电形式 电晕，辉光。问题：电弧放电适合边界层控制？l 等离子体减阻优点 直接将能量转化为动能，无需运动的机械装置（如风扇等）。l 离子风速度与放电电压、电流的关系l 电晕放电对边界层影响的视觉图像l 进一步提高离子风速的方法多相放电串联加速多相串联加速电极之一串联加速电极之二（同相）离子风边界层控制实验系统之一离子风边界层控制实验系统之二3. 其他等离子体减阻方法等离子体逆向喷

10、流减阻等离子体喷出时组里减少Assumed paper is devoted to the problem of high-speed dense flow management by the plasma ofelectrical discharges excited inflow. The results of wind tunnel experiments with the discharges of differenttypes, analytical, and CFD efforts are going to be presented.There are no doubts now

11、that the plasma methods based on electrical discharges generation have apractical potential for a flow/flight control 1-8. An idea of the method can be formulated on the most simplemanner as following: modification of flow-field structure and, consequently, changing a pressure and tangentialtension

12、near surfaces by means bulk forces excitation in EM fields and heat release into predefined space areawith predefined parameters distribution and at predefined tempering.Conventional methods to advance aerodynamic characteristics of aero-vehicles and its parts or to adjustthe trajectory are based on

13、 application of mechanical elements, which use energy of approach airflow for redistributionof pressure on surfaces, and application of jets power in local areas near the surfaces. Among otherknown methods for flow characteristics control the plasma generation is, probably, the most prospective. Sev

14、eralmain mechanisms of aerodynamic effect due to plasma release can be described: (1) change of thermodynamicproperties of medium, (2) modification of flow-field structure, (3) local artificial separation and (4) boundarylayer modification. Technically the effects are become apparent in bow shocks t

15、ransformation, wave dragreduction (thermodynamic and form-factor effects), base drag reduction, skin friction change, thermal fluxreduction (redistribution), adjustment of flow-field structure in inlets/diffusers, etc. Such possibilities can berealized by means of electrical discharge plasma generat

16、ion; free-localized plasma generation in electromagneticwave beams; blowing out of high enthalpy plasma jets; and by the other similar phenomena.Several model aerodynamicconfigurations are proposed for an analysisand the testing under the condition of inflowplasma excitation, as it shown in illustra

17、tion.The first scheme is related to forestreamdischarge generation, the next ones to nearwallplasma where the oblique shocksgeneration and separation take place. The fifthone (transversal discharge) can be applied forflow modification and mixing intensification.In the next case the flowfield modific

18、ationnear plane and profiled plates is explored. Thelast scheme reflects the situation below abackwise wall step.Experiments. The first group of experiments were fulfilled in frames of supersonic drag reduction ideadue to discharges generation ahead a simple body. Several types of discharges were ex

19、plored and described.The next experiments were conducted in a short-duration blowdown wind tunnel with a closed testsection at Mach number M=1.2-2.0 and static pressure Pst=100-500Torr. The experimental setup has beenequipped with a Schlieren system with short time exposure, high-speed video camera,

20、 fast line-scan camera, IRcamera, set of fast-response pressure transducers, spectroscopic system, photo-sensors, current-voltage sensors,thermocouples and a set of control-measurement devices. The electrodes have been flush mounted on an insertmade of a dielectric thermo-refractory material. Typica

21、lly, a quasi-continuous multi-electrode surface dischargewas used for the plasma excitation. The electric energy input to the plasma volume was 0.1-2kW over a width ofthe discharge plate of 2-10cm. The plasma temperature was measured in the region of the discharge cords usingspectroscopic techniques

22、 and was 1.5-4 kK in depending on the experimental conditions.Appropriate experimental results on flowfield control by plasma generation near wall are planed to bepresented. It can be considered the following. The plasma of the surface electric discharge changes the structureand the properties of me

23、dium in boundary overlayer sufficiently. The transversal surface discharge is moreeffective than longitudinal discharge for energy input and flow structure control. The transversal discharge is anunstable system of relaxation type with hot plasma filaments, which moves with the flow. A local separat

24、ionoccurs in plasma generation area. The generation of plasma overlayer is a method for the shocks position control2near the surface as well as flow parameters in whole duct. The value of input power has a direct effect on theamplitude of the plasma influence. The energetic threshold for global boun

25、dary layer separation are defined.An addition of large amount of the thermal energy might lead to modification of flow structure. Fromthe other side such an addition can change the parameters of flow significantly and not in the desired direction.The efficiency of the plasma influence is very import

26、ant at the diminishing of the possible penalties. Last time afresh point of view is being crystallized that the plasma technology makes a sense for the gentle correction offlow-field under the off-design operation modes. The idea of plasma strong non-uniformity in space structure,non-equilibrium com

27、position and unsteady temporal behavior gives chance to get a quite sufficient effect inflowfield structure under high-speed flows.The non-thermal mechanisms of the plasma effects are going to be discussed in two domains:electrostatic and magnetic volumetric forces generation. It is easily to estima

28、te that Electro-Hydro-Dynamic/Electrostatic (EHD) effects might be appeared under low-speed conditions or in distances comparedwith Debays radius, that is equivalent to low-density conditions. But under the conditions of boundary layer andstrong non-homogeneity of the medium parameters the mechanism

29、s of charge separation and thermalelectromotive force generation should be taking into account. The first effect can be realized due to strongtransversal gradient of gas velocity; the second occurs due to temperature gradients: ). ( 1e tg kT gradeE × = . Aresonant effect of dielectric barrier discharge on transonic flow is shown in experiment as well as calculatednumerically.Vise verse to that the magnetic forces on current in the gas can be quite valuable. The velocity ofdischarge-induced flow exceeds sonic level at electric current Ipl=102-10