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文摘为研究电磁波在吸波涂层中的干涉现象,选取了X型羰基铁粉吸收剂,采用同轴法测试了材料电磁参数,通过多次反射方法求解了X型吸波涂层在厚度为1/4波长时反射系数公式;另外,求解了不同厚度X-1型吸波涂层对频率为9.5 GHz平面电磁波反射系数公式,取公式前6项计算了反射系数Γa和反射率Ra。结果表明,多次反射波之间产生了干涉行为,发生了干涉相消现象。Γa、Ra分别与传输线方法计算的Γ和R的值非常接近,表明Γa和Ra可以近似代表X-1型吸波涂层对9.5 GHz电磁波的反射系数和反射率。当X-1型吸波涂层厚度小于1.9 mm或在3.2~6.6 mm时,第二反射波对第一反射波起到干涉相长作用,降低吸波性能,表明优选的吸波涂层厚度应该介于1.9~3.2 mm。 相似文献
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文摘为研究点阵结构工艺参数对透波平板电磁波谱的影响,基于体心立方结构(BCC)点阵基本构型,建立透波平板仿真模型,运用CST仿真软件分别从孔隙率、蒙皮厚度、胞元尺寸、入射角度4方面探究不同结构工艺参数对透波平板共振频率及透射率的影响。并运用选区激光烧结(SLS)制备点阵透波平板结构,用于2~18 GHz透射率对比测试。结果表明,仿真数据与实验结果高度一致;透波平板共振频率随孔隙率增大,第一频率点向低频偏移,第二频率点向高频偏移;随着蒙皮厚度的增加,透射率降低;胞元尺寸与蒙皮厚度决定透射谱的通带共振频率,孔隙率影响较小;入射角增大时,水平极化入射波透射率均大于75%,影响较小;竖直极化入射波透射率在高频处逐渐降低。 相似文献
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针对悬臂梁结构振动控制问题,开展基于内嵌式颗粒阻尼(embedded particle damper, EPD)减振方法的理论与实验研究。应用有限元法分析悬臂梁振动特性,围绕梁前三阶模态频率开展振动控制实验,通过改变填充颗粒的参数(粒径、填充率)和激励力,比较悬臂梁在不同填充情况下的振幅,并使用半功率法计算阻尼比。采用离散元法分析不同情况下颗粒的流变行为,以确定阻尼器最优设计参数。结果表明:颗粒填充率为90%时EPD减振效果最佳;填充颗粒的粒径与系统所受激励有关,本文模型中,激励振幅为80μm时,梁前三阶模态频率下分别填充直径为8、6、1 mm颗粒时效果最好,减振率分别为47.5%、48.7%及71.2%,阻尼比分别提高1.7、3.1及2.1倍。 相似文献
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法向与展向电磁力可以有效调制湍流近壁流动及减少壁面摩擦阻力.为进一步揭示该方法的减阻机理,利用Fourier-Chebyshev谱方法,通过直接数值模拟(DNS),对槽道湍流的法向与展向电磁力控制和减阻问题进行了研究.结果表明,对于确定的流向波长λx+,存在最佳的电磁力强度St,使阻力降最大,最佳St与λx+成反比.法向与展向电磁力对湍流的控制过程实质上是一种由电磁力诱导的调制波对壁湍流的调制过程.在优化参数控制下,当法向与展向电磁力诱导的流场被用来调制固有的近壁湍流流场时,固有流场和诱导流场同时受到调制.在这种调制波作用下,调制流场逐渐主宰壁面边界层,这导致了壁面阻力的下降,平均减阻率最高可达8%. 相似文献
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本文为对当量比乙炔氧气混合气体中爆轰波与激波正面对撞产生稳定波系的实验和理论研究.实验主要以高速扫描摄影获取两波对撞的x-t纹影图,一维理论分析则基于三种热完全的组分求解两波对撞的稳定解并探寻它们的规律.实验发现透射波系包括一道激波和爆轰波,以及紧随爆轰波后的稀疏波区,这种波系情况与一维理论分析中CJ解一致.透射CJ爆轰与人射爆轰相比马赫数降低,而相对波前来流的传播速度有轻微提高,但在实验室坐标下其速度显著降低.透射波系受初始压强影响不大;初始温度提高使得爆轰波速度降低,而透射激波速度增加;对波系起实质影响作用的是入射激波强度,激波越强,则整个透射流场呈现偏向激波的趋势;理论分析还指出,稀疏波区的出现不可避免,当激波强度趋于声波时稀疏波区趋于消失,激波越强则疏波区趋于扩大. 相似文献
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采用超声衍射时差法(TOFD)检测铝合金板底面缺陷时,受直通波脉冲宽度影响存在近表面盲区。提出了TOFD半跨模式波法进行盲区抑制,利用一次底面反射波在缺陷端点处衍射波的特征及其在B扫查图像中的对称性推导底面缺陷定深公式。仿真和实验结果表明,对于厚度为7.0 mm的铝合金板,在探头中心距为40 mm、中心频率为10 MHz的检测条件下TOFD半跨模式波法能将近表面盲区抑制64%以上,且深度不小于2.0 mm底面缺陷的定位误差不超过6.32%。与模式转换波及TOFD-W波相比,半跨模式波不会与底波混叠且离直通波较近,在铝合金板底面缺陷检测中适用性较强。 相似文献
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涡轮盘材料低周叠加高周复合疲劳断裂特性研究 总被引:1,自引:0,他引:1
航空发动机的载荷谱,包含起飞和着陆间的各种巡航状态,可简化为梯形波表示。在涡轮盘和压气机盘的关键区域,如连接叶片的盘缘,离心力和热应力的变化,以起动——停车作为一个工作循环,称为主循环,是低周疲劳的主要损伤源。在载荷保持期间(巡航状态),局部高应力区可能产生蠕变或应力松弛,实际上还会有低幅值高周振动(称为次循环)应力叠加在主循环上,简化载荷谱如图1所示。这种状态除航空发动机外,在多数动力机械如原子能装置、内燃机等都可能发生。在评定承受低周叠加高周复合作用的构件及材料疲劳断裂性能时,应该考虑高周损伤的重要作用。本文介绍近年来所作初步研究的部分结果。 相似文献
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In the vicinity of reflecting objects, a VOR receiver picks up both the direct and reflected signals. This summation adds both amplitude and phase modulation to the signal that were not present at the transmitter. The error due to this phase modulation is fairly evident since the desired directional information is transmitted as a relative phase. What is not so evident is the fact that the amplitude modulation can also produce a phase error. This happens when asymmetrical filters are used to process the amplitude-modulated signal. The asymmetrical filter converts some of the amplitude-modulated sidebands into phase modulation which is recorded as a direct phase error. 相似文献
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Information matrices are derived for estimates of the range parameters of moving targets as obtained by combining a priori information (if available) with reflected radar signals observed in the presence of additive white Gaussian noise. The inverse of the information matrix provides a lower bound on the covariance matrix of any unbiased parameter estimates. This bound can be approached with a high signal-to-noise ratio and optimum data processing (matched filters). Arbitrary frequency modulation, amplitude modulation, and target motion as well as various assumptions on processing the RF phase are considered. The multiple-target case makes possible investigation of a signal's resolution ability, as well as its accuracy potentials. Results for a carrier frequency much greater than the effective signal bandwidth are obtained as a special case. A main purpose of the paper is the reduction of the original radar problem to a linear model which is equivalent in the sense of having the same information matrix. These models provide valuable insight into the relative effects of multiple targets, choice of modulation, a priori information, and assumptions regarding RF phase and bandwidth. The linear equivalent model also leads to a valuable computational algorithm for investigations using digital or hybrid computers. The various special cases of interest are obtained by simple modifications of the general case, and thus the algorithm can provide a very versatile tool for evaluating and designing radar signals. 相似文献
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Film cooling experiments with sonic injection were conducted to investigate the effects of the number of the injection holes, the mass flow ratio, and the hole spacing on the film cooling effectiveness. The mainstream was obtained by the hydrogen-oxygen combustion, entering the experimental section at a Mach number of 2.0. The nitrogen with ambient temperature was injected into the experimental section at a sonic speed. The measured mainstream recovery temperature was approximately 910K. The mass flow ratio was regulated by varying the nitrogen injection pressure. The experimental results show that for the investigated cooling surface, the cooling effectiveness increases with the increase in the number of the injection holes with other parameters held constant. For a fixed cooling configuration, the cooling effectiveness increases with the increase in the mass flow ratio. Different from the subsonic film cooling, the optimal mass flow ratio is not observed. When the hole spacing is less than 4, no obvious difference is observed on the cooling effectiveness and lateral uniformity. With the mass flow ratio increasing further, this difference becomes much smaller. The shock wave also has an effect on the cooling effectiveness. Downstream the incident point of the shock wave, the cooling effectiveness is lower than that in the case without the shock wave. 相似文献
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In this paper, separation of stress waves passing through the solid-gas media boundary is considered. The kinematic and dynamic conditions characterizing the interrelationship between the particle and wave velocities as well as the values of deformations in solid and gaseous media are found. Also presented are the equations for reflected and transmitted waves. 相似文献
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通过实验成功发展了一种生成大振幅稳定条带的有效方法,即展向离散抽吸方法。在此基础上,开展条带控制边界层转捩的研究。实验在水洞中进行,以零压力梯度平板边界层为研究对象,利用氢气泡时间线法观测引入条带前后人工激发转捩边界层中扰动发展变化,分析条带对转捩的控制效果及参数影响。结果表明,展向离散抽吸方法生成的稳定条带最大振幅可达28.4%U(U为自由流速度);实验中引入的宽度14和28mm的稳定条带都能起到抑制转捩的作用;条带振幅越大、宽度越窄,抑制效果越明显。研究结果为探索降低水/空气中航行器摩阻的新技术提供有价值的参考。 相似文献
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激波与火焰的相互作用常发生在超声速燃烧和燃烧转爆轰过程中,为深入了解这一现象,采用带化学反应的三维Navier-Stokes方程,对平面入射激波及其反射激波与火焰的作用进行了计算研究,其中燃烧过程使用单步反应模型描述。研究结果显示:在受限空间内模拟激波与火焰作用,能更好地符合实验结果,从而体现出受限空间的三维效应;火焰在入射激波的作用下主要经Richtmyer-Meshkov不稳定而发生变形,此时火焰变形以物理作用为主,燃烧膨胀效果相对不明显;当反射激波与变形火焰再次作用后,火焰迅速膨胀变形,放热率维持在较高水平,此时化学反应过程起主要作用;在反射激波的作用下,变形火焰复杂三维涡结构的形成能强化热量与质量的输送,提高燃烧速率。 相似文献
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Mismatched Filtering of Sonar Signals 总被引:1,自引:0,他引:1
A replica correlator (matched filter) is an optimum processor for a receiver employing a pulse of continuous wave (CW) signal in a white Gaussian noise background. In an active sonar, however, when the target of interest has low Doppler shift and is embedded in a high reverberation background, this is not so. High sidelobes of the correlator frequency response pass a significant portion of the signal contained in the mainlobe of the reverberation spectrum. In order to reduce the sidelobes of the correlator output spectrum and at the same time keep the increase in its 3 dB bandwidth to a small amount, we propose lengthening of the replica of the transmitted signal and weighting it by a Kaiser window. It is demonstrated that by extending the weighted replica by 50 percent compared with the transmitted signal, it is possible to reduce the sidelobe levels to at least 40 dB below the mainlobe peak, with the concomitant increase of the 3 dB band-width by less than 5 percent. The degradation in signal-to-noise ratio (SNR) performance for such a ?mismatched? filter receiver with respect to the matched filter is less than 1.5 dB. 相似文献
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为满足多电飞机(MEA)的大功率用电需求,系统工作电压需要进一步提高,而较高电压会增加相关部件的绝缘失效风险。面向多电飞机特定工作场景和参数,搭建了模拟飞机电作动器中的绕组间绝缘故障测试平台,开展了1 kHz范围内的局部放电(PD)大量重复实验,研究了特定电压幅值、正弦波和方波脉冲波形下局部放电幅值、放电重复率和放电相位等统计特征,并计算评估了不同频率值下多电飞机中的局部放电行为。实验结果表明:在设定频域范围内,方波脉冲下的起始放电电压(PDIV)都低于正弦,方波脉冲波形对绝缘影响更大;随着频率升高,放电幅值逐渐降低,但放电重复率几乎呈线性增长;放电时刻集中于上升沿/下降沿末端。以50 Hz作为对比基准频率,1 kHz时的放电幅值降低80%,而放电重复率增加11.92倍,较高频率下多次累积的小幅值击穿成为威胁绝缘失效的主要原因。计算分析认为高频下空间电荷场强变化导致的放电延迟时间减少和周期数目增加分别导致局部放电脉冲幅值降低和放电重复率增加。本实验结果有助于针对多电飞机电气系统和相关装备开展针对性绝缘测试和评估,并有望为多电飞机向大功率高电压方向的设计提供参考和借鉴。 相似文献
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锯齿尾缘叶片气动特性和绕流流场的数值研究 总被引:2,自引:1,他引:1
以基于NACA 0018翼型的锯齿尾缘仿生叶片为研究对象,采用大涡模拟的方法研究锯齿相对齿宽与相对齿高对锯齿尾缘叶片的气动特性和非定常绕流流场的影响规律和机制.研究表明,尾缘锯齿参数对叶片气动性能的影响是复杂的非线性过程,在一定来流攻角范围内能提高升阻比,但失速提前.如在9.4°~14.8°来流攻角范围内,不同相对齿宽系列叶片的升阻比高于原始叶片,升阻比与锯齿相对齿宽之间没有线性关系.研究还表明,锯齿尾缘能延迟边界层分离,加速尾迹的流动掺混和能量扩散,改变非定常涡结构和涡脱落频率.相对齿高的变化对非定常流动特性的影响更为显著.尾缘锯齿诱导的二次湍流射流和吸力面侧反向涡对改变了原始叶片的绕翼环量,进而影响锯齿尾缘叶片的气动特性和绕流流场特性. 相似文献