空中飞行目标尾焰红外辐射信号的建模与仿真

本文通过对空中飞行目标高温尾焰的流场分布进行研究和对飞行目标尾焰的红外特性进行深入分析,以圆对称尾喷管尾焰的温度分布和各组分压强的分布理论为例,对其红外辐射强度进行了计算.首先,根据不同区域的温度分布公式计算出尾焰的等温线;然后,考虑了谱线的碰撞展宽效应和多普勒展宽效应,利用谱带模型C-G近似法,计算了尾焰各部分的吸收系数.最后,根据前面的计算得到了尾焰的红外辐射分布,建立了较准确的定量分析模型,并给出了仿真结果.     

卫星轨控推进器尾焰红外辐射的数值模拟

为全面了解卫星的红外辐射特性,需掌握卫星轨控推进器尾焰的红外辐射特性.按照先计算尾焰的流场、再计算尾焰的吸收系数、最后用有限体积法求解尾焰红外辐射亮度的先后顺序,较完整地建立了卫星轨控推进器尾焰红外辐射的求解模型.根据该模型计算得到的尾焰辐射亮度,进一步求解获得了1 000～4 500 cm-1 内的光谱和波段辐射强度.通过分析,光谱辐射强度在1 525 cm-1、1 700 cm-1、2 155 cm-1、2 175 cm-1、2 350 cm-1 和3 750 cm-1附近的数值较大;到喷管出口截面的垂直距离不同,最大的波段辐射强度可能对应不同的方位角.在波数2 350cm-1和3 750 cm-1 附近,光谱辐射强度与同类文献中的计算结果能够相互吻合.此结果表明,该模型具有较高的可行性和精确性.     

巡航导弹尾焰红外辐射特性建模及分析

对巡航导弹尾焰的流场分布进行了分析,建立了简化工程模型.该模型可在已知尾喷管压力、温度的条件下计算尾焰的温度场和压力场分布,与计算流体力学在相应条件下模拟的尾焰流场特性基本一致.进而以C.G谱带法计算简化模型的辐射亮度分布.计算结果表明,巡航导弹尾焰核心区的辐射亮度特征明显;尾焰辐射3～5鱩波段可作为探测尾焰的主要波段;辐射亮度在与尾焰垂直方向最强.本文的结果可为探测巡航导弹及空间相机的谱带选择提供依据.     

加装红外抑制器的直升机红外辐射的计算及测量

计算了光谱在3～5 μm及8～12 μm范围内,加装与机身结构一体化红外抑制器的某型直升机排气系统的红外辐射特性.计算中考虑了排气系统部分蒙皮的红外辐射、双喷管发动机喷气流的红外辐射、背景辐射及大气的吸收衰减对直升机排气系统红外辐射信号的影响.实验发现,一架加装与机身结构一体化红外抑制器的直升机排气系统红外辐射特性的计算结果,与实测结果基本一致.     

8~14μm辐射温度计标定公式误差分析北大核心CSCD

插值公式法是红外辐射温度计标定的一种常用方法。为选择使用8~14 μm波段辐射温度计的标定公式,采用MATLAB仿真的方法,基于8~14 μm矩形光谱响应假设进行定量分析,模拟了几种常用的标定公式在8~14 μm波段、-50~1 000 ℃的标定过程,根据仿真结果,比较了几种标定公式的准确度,并分析了标定温度点的选择对标定结果的影响,最终确定了该波段最适用的标定公式和标定点。     

辐射板强化喷管换热与红外抑制效果试验研究

试验研究了在发动机喷管中加装金属辐射板前后,喷管壁面温度、热喷流温度与喷管红外辐射特征的变化。结果表明,加装金属辐射板后,热喷流与喷管壁面之间的热量传递显著增强,热喷流中心温度降低,壁面温度明显升高,在90°方向上,热喷流3～5μm波段的红外辐射强度降低了38.5%。文中从热喷流、喷管壁面以及金属辐射板等相关部件的温度变化情况对红外辐射强度的变化原因进行了解释。      

辐射测温在钢铁工业中的应用及发射率对测量的影响

采用辅助吹气、侧面瞄准和底部瞄准3种辅助方法,并利用热电偶校正红外测温仪发射率的方法改进了测量钢板温度的辐射测温技术.研究结果表明:带氧化皮的钢板在1 μm附近的光谱发射率约为0.84,全波发射率约为0.85;若将工作波长为1 μm的红外测温仪的发射率设置在0.81～0.87之间,全辐射红外测温仪的发射率设置在0.83～0.87之间,则钢板在1 000 ℃时,单波长和全辐射红外测温仪因发射率波动而导致的测温偏差分别小于4 ℃和7.5 ℃.     

基于人工红外光照时地面目标红外特性的分析

针对目标的探测、识别以及目标红外特征的模拟,本丈提出采用人工红外光源照射目标的方法,该方法以人工红外光源照射目标的物理模型为基础,综合考虑自然环境与人工红外光源对目标的影响,建立起目标表面温度的计算模型和目标红外辐射的计算模型.在分析以碳硅棒为红外光源的光谱特性基础上,对合肥某建筑物表面照射时目标在3～5岬和8～14 μm波段的红外辐射进行了仿真计算与分析.结果表明:采用不同功率的光源照射时,目标的红外辐射将发生改变,相同条件下8～14 μm波段的功率利用率大于3～5 μm波段的功率利用率.     

微结构特征对微结构零件注射成型流场均匀性的影响

在Navier-Stokes方程基础上,配合适当边界条件,借助计算流体力学软件CFD-ACE实现了单侧分布矩形微结构、梯形微结构和双侧分布矩形微结构零件模型的流场仿真,得到了三种零件的速度场和剪切速率场.结果表明,单侧矩形微结构零件各微结构截面的速度和剪切速率都有较大差异;单侧梯形微结构零件各微结构截面的剪切速率分布一致,速度分布不一致;双侧矩形微结构零件三个微结构截面的速度和剪切速率都一致.单侧分布梯形微结构零件流场均匀性好于单侧分布矩形微结构零件流场均匀性,双侧分布的微结构零件流场均匀性好于单侧布置的微结构零件流场均匀性.     

飞行器表面温度和发射率分布对红外辐射特征的影响

为了考察飞行器表面8～14μm波段内红外辐射特征的控制规律,建立了包括蒙皮外气动热与蒙皮内部传热的数学物理模型,分别采用有限体积法求解蒙皮温度分布和反向蒙特卡罗法计算飞行器表面8～14μm波段内红外辐射特征.通过数值模拟,计算分析了在水平方向上飞行器各主要部件对红外辐射强度的贡献.最后,改变飞行器各主要部件的表面温度和发射率,获得了对飞行器水平方向上总的红外辐射特征的影响和控制规律.     

Three-Dimensional Model of the Spectral Emissivity of Light- Scattering Exhaust Plumes

A three-dimensional model of radiation of rocket exhaust plumes is given, which is based on the Monte Carlo simulation method. Two problems associated with the determination of spectral directional radiation of rocket exhaust plumes are treated, namely, the self-radiation of multiple plumes and the scattering of solar radiation incident on a single or multiple plume at an arbitrary angle by the condensed phase of the combustion products. The suggested model enables one to calculate the spectral emissivity with averaging of the rotational structure of molecular emission spectrum by diatomic and triatomic molecules of combustion products in the spectral range from 0.1 to 10 m. The results are given of calculations of the spectral and integral emissivity of light-scattering rocket exhaust plumes of various configurations, containing Al₂O₃ particles with an average radius of 2 m, in the infrared spectral region of 2-5 m.     

Numerical simulation of micro-nozzle and micro-nozzle-array flowfield characteristics

Preliminary numerical simulation using a Direct Simulation Monte Carlo (DSMC) method was conducted to elucidate the internal flowfield and external plume characteristics of micro-single-nozzles and micro-nozzle-arrays, since these small-sized nozzles generally undergo a severe viscous loss due to the low Reynolds numbers. This study also contains the investigation on optimization of the geometry and configuration of the micro-nozzles and micro-nozzle-arrays to achieve the improved propulsive performance. Typical sizes of each rectangular nozzle element were 0.1 mm in throat height, 0.36 mm in exit height, and 0.35 mm in length of the divergent part. For the micro-single-nozzles, calculated specific impulses were fairly in good agreement with our previous experimental data, showing a poor nozzle efficiency due to the viscous loss of low Reynolds number. Also, mechanisms of exhaust jet interaction of multi-nozzle-array jets, bringing a significant improvement in thrust performance, were investigated. As a result, it was shown that pressure and temperature increased at the exit and jet boundaries, and then the exhaust multi-jets were not expanded after the exit, or rather being confined, showing possibilities to realize the higher propulsive performance due to the augmented effect of the pressure thrust.     

Beam displacement as a function of temperature and turbulence length scale at two different laser radiation wavelengths

Narrow laser beams directed from aircraft may at times pass through the exhaust plume of the engines and potentially degrade some of the laser beam characteristics. This paper reports on controlled studies of laser beam deviation arising from propagation through turbulent hot gases, in a well-characterized laboratory burner, with conditions of relevance to aircraft engine exhaust plumes. The impact of the temperature, laser wavelength, and turbulence length scale on the beam deviation has been investigated. It was found that the laser beam displacement increases with the turbulent integral length scale. The effect of temperature on the laser beam angular deviation, σ, using two different laser wavelengths, namely 4.67 μm and 632.8 nm, was recorded. It was found that the beam deviation for both wavelengths may be semiempirically modeled using a single function of the form, σ=a(b+(1/T)(2))(-1), with two parameters only, a and b, where σ is in microradians and T is the temperature in °C.     

Low frequency oscillations in turbulent Rayleigh-Benard convection

A locally time-periodic flow in Rayleigh-Benard convection in the range of Rayleigh number R of 10⁷ to 10⁸, Prandtl number 7, and aspect ratio 12 is reported. This time periodicity is associated with organized clusters of tilted plumes travelling steadily across the fluid layer. A scavenging plume model is presented in which prior passage of a plume has depleted the thermal boundary layer in a history-dependent way: The boundary layer is very thin where the plume has just passed, but has had time to thicken where the plume passed some time ago. Thus at any moment there is a thermal boundary layer of varying thickness, and the pressure gradient in it drives flow towards the thicker regions. There is then a shear and a down-gradient momentum flux at the wall. This boundary layer is supposed to erupt when and where its thickness reaches some critical value. Then, assuming that the interior momentum flux is entirely by Reynolds stresses, we can, by matching interior and boundary layer heat and momentum fluxes, determine the period which is shown to be in reasonable agreement with the observations. This is a contribution #386 of the Geophysical Fluid Dynamics Institute, Florida State University.     

Buoyant plane plumes from heated horizontal confined wires and cylinders

Two-dimensional computations are reported for time-dependent laminar buoyancy-induced flows above a horizontal heated source immersed in an air-filled vessel. Two kinds of heated source were considered: a line heat source, modelled as a heat source term in the energy equation, and a heat-flux cylinder of small diameter. First, comparisons are presented for the results obtained for these two heated sources. Rather large discrepencies between the velocity fields appeared in the conduction regime due to the weak plume motion, while close agreements were found in the boundary layer regime. Nevertheless, same types of bifurcations occur with almost identical frequencies, whatever the Rayleigh number. It is concluded that for dimensions of the enclosures, which largely compared with the cylinder radius, the heat source term model is a promising way to study the behaviour of unsteady plumes owing to its simplicity, flexibility, and low computational costs. Second, transitions to unsteady flows were studied through direct flow simulations for various depths of immersion of a line heat source in the central vertical plane of a vessel. Different routes to chaos were shown to occur according to the aspect ratio of the vessel and the depth of immersion of the line source. Three distinct regimes were detected with different underlying physical mechanisms called natural swaying motion, penetrative convection and Rayleigh-Benard-like convection. The first bifurcations associated with these regimes are supercritical Hopf bifurcation, pitchfork bifurcation and subcritical Hopf bifurcation. Comparisons with experimental results of confined buoyant plumes above heated wires show very good agreement with laminar frequency correlations.     

Study of non-reactive isothermal turbulent asymmetric jet with variable density

In the present study, the effects of inlet isothermal jet geometry on the mixing process with variable density have been investigated numerically. Four density ratios were considered, namely R_ρ =7.2, 1.8,1 and 0.66 for He-air, CH4-air, Air-air and CO2-air mixtures respectively. The jets are produced through rectangular and elliptic nozzles with aspect ratio Ar = 2:1. The elliptic and rectangular nozzles have approximately the same exit area as the circular nozzle. A second-order Reynolds stress model (RSM) is used to investigate variable density effects in asymmetric turbulent jets. Comparative studies are presented for the calculations of the variables such as the longitudinal velocity, the longitudinal fluctuating velocity and the turbulent kinetic energy. The results indicate that the asymmetric geometry noticeably enhances mixing in comparison with the axisymmetric case. Typical phenomenon of 3D jets is observed.     

Experimental and numerical simulation of the flow in a driving module with an annular and linear double-slot nozzle

This paper presents the results of numerical and experimental studies of the flow and comparison of propulsion performance characteristics of a model of a jet engine exhaust system equipped with an annular or (equivalent in gas consumption) linear double-slot nozzle with an inner cavity and circular segment deflector in the axial section. Calculations performed for the annular nozzle and double-slot nozzle corresponding to it in geometric parameters demonstrate that a flow similar to the flow in nozzles with a central body is formed in the exhaust system. According to the data obtained, the initial turn of the flow takes place in the oblique shock wave. In the double-slot nozzle, the final turn of the flow in the direction of the thrust vector occurs in a configuration of four shock waves positioned downwards in the flow; in the annular nozzle, it is in the intense barrel shock wave. It was established that the exhaust system with an annular of the linear doubleslot nozzle develops a thrust and specific impulse that exceed the corresponding values for the sonic nozzle equivalent in gas consumption by almost a factor of 2.     

Three-dimensional measurement of a gas flow temperature field using far-infrared band CT techniques

Emission spectral tomography (EST) can be utilized to reconstruct three-dimensional (3D) physical parameter distributions of gas flow fields. Mostly, the radiant energy of the visual and near-infrared bands is received by a video camera in EST, so it is difficult to examine a low/medium-temperature gas flow field by normal EST. However, the far-infrared radiant energy of a low/medium-temperature gas flow field is strong enough to be received by a far-infrared detector. Based on EST, a far-infrared band computed tomography (FICT) approach is proposed that focuses on far-infrared spectral emission and band emission tomography. Both low- and medium-temperature blackbody furnaces were adopted to calibrate the relation between infrared thermal image intensity and radiant exitance. The 3D temperature reconstruction of an alcohol blow lamp was carried out. According to the results of multiple measurements, the relative error of the FICT approach is less than 20%. The experimental results prove the feasibility of the FICT approach.     

大空间竖向热羽流对横向冷射流的干扰作用盐水模拟实验研究

以相似理论为依据,设计搭建了用于模拟大空间建筑室内气流组织的液态模型实验台,对大空间建筑中常见的竖向热羽流对横向冷射流的影响规律进行盐水模拟实验研究。借助速度比例尺解决了冷射流与热羽流系统的相似同步性问题。实验结果表明:在竖向热羽流作用下,横向冷射流的运动轨迹将发生向上的偏转。单股竖向热羽流对单股横向冷射流对作用点上游的冷射流轨迹没有明显干扰作用,冷射流轨迹的抬升主要表现在作用点下游的冷射流末段。在两个热源形成的两股竖向热羽流作用下,单股横向冷射流的运动轨迹发生的偏转量增大;竖向热羽流与横向冷射流的作用点不同,冷射流运动轨迹受到的干扰作用大小也不同,在冷射流初段,冷射流的惯性力作用较强,不易受热羽流干扰,而在冷射流末段,冷射流惯性力减弱,易受热羽流干扰;相对于单股冷射流在两股热羽流作用下的运动来说,两股冷射流叠加后,热羽流的干扰作用有所减弱。     

Two-phase flow in a horizontal rectangular microchannel

Two-phase flow in a rectangular short horizontal channel 200 μm high was studied experimentally. The use of the fluorescent method made it possible to reveal flow of liquid in the channel and to determine its characteristics quantitatively. The existence of the regime of separate (stratified) flow is established. Based on analysis of previous investigations and newly obtained data, it is shown that a change in the height of the horizontal channel has a substantial effect on the boundaries between the regimes. The region of the churn regime increases with decreasing thickness of the channel.