A swept fin-induced flow field with different height mounting gaps

In order to apply the air fin successfully and ensure the maneuverability of hypersonic vehicle, a key problem to be studied urgently is the heat flux brought by the fin mounting gap. The appearance of mounting gap and fin shaft can induce many complex flow structures which need more attentions to be investigated. Under Ma 6, Nano-tracer-based Planar Laser Scattering (NPLS) and Temperature Sensitive Paints (TSP) were applied to visualize and measure transient flow structures and heat flux distribution of a swept fin-induced flow field with different height mounting gaps. Complementarily, Reynolds-averaged N-S equations were solved with k-ω SST turbulent model. The heat flux distribution results of numerical simulation and TSP observed the change of high heat flux region with different mounting gap, both in position and magnitude. The streamlines based on Computational Fluid Dynamics (CFD) and flow visualization results obtained by NPLS revealed the cause of high heat flux region. The high heat flux region in this flow field is mainly related to the reattachment of vortex and flow stagnation. The increase of gap height can lead to stronger gap overflow and shaft-induced horseshoe vortex, which are source of the high heat flux around the fin. The case with the highest mounting gap (4 mm) en-counters the most severe aerodynamic heating, both on the surface of fin and plate. Thus, under the premise of ensuring the flexibility of the fin, the gap should be set as small as possible.     

超声速尾迹-剪切流的混合增强

超声速条件下燃料和空气之间的高效混合是超然冲压发动机技术上的主要挑战。基于大涡模拟和流动稳定性分析,针对超声速尾迹-剪切流动开展了混合增强方法研究。尾迹的存在改变了混合层流动的速度剖面,对流动稳定性产生了重要影响,使混合层由三维最不稳定变为二维最不稳定,最不稳定扰动波频率和增长率增大。基于流动稳定性结果引入扰动的混合增强方式依然有效,根据稳定性结果设计了波纹隔板。数值结果表明：二维波纹壁引入的扰动未能增长,不具备混合强化效果,而三维波纹壁引入的扰动能够快速增长,具有混合强化效果,且波纹壁参数越接近最不稳定扰动波参数,混合强化效果越明显。     

展向离散抽吸法控制边界层转捩实验研究

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。     

采用长细管法进行脉动压力转捩探测的实验研究

为了简便地使用测量模型表面脉动压力特征的方法探测边界层转捩位置,需要研究脉动压力传感器接在传统测压模型外的适用性,即通过长细管将模型表面的脉动压力信号传递到脉动压力传感器上的方式是否可得到转捩的特征信号。首先采用信号发生器驱动扬声器,在无风条件下,测量了长细管对不同频率声压信号的传递损失情况。证明了所采用的长细管系统具有合适的工作频带。然后在西北工业大学NF-3低速风洞二元实验段、实验风速为30m/s的条件下,对弦长为800mm、展长为1.6m的翼型模型沿弦向进行了脉动压力信号测量,并通过改进的数据处理技术判断了模型表面的转捩位置。研究结果表明,采用长细管系统进行脉动压力方法转捩探测具有一定应用价值,值得进一步深入研究。     

Numerical investigation of the impact of asymmetric fuel injection on shock train characteristics

Numerical simulations are carried out to investigate the impact of asymmetric fuel injection on shock train characteristics using the commercial-code FLUENT. The asymmetry of fuel injection is examined by changing the fuel flow rates of the upper and lower wall fuel injectors. The numerical approach solves the two-dimensional Reynolds-averaged Navier–Stokes (RANS) equations, supplemented with a k-ω model of turbulence. As a result, different ways of fuel injections will always lead to shock train transitions, with the variations of shock train structure, strength and leading edge position. For symmetric fuel injection, the flowfield of the isolator is quite asymmetric with the boundary layer of the upper wall side developing much stronger than that of the lower wall, which is due to the heterogeneity of the incoming flow. Regarding to asymmetric fuel injection with more of lower wall side, though the pressures in the combustor are nearly the same, the first shock of the shock train converts between ‘Distinct symmetric X type shock’ and ‘Obscure and weaker asymmetric shock’ and the shock train leading edge moves upstream with the increase of the asymmetry level. With regard to asymmetric fuel injection with more of upper wall side, ‘incomplete asymmetric X type shock’ occurs and the shock train structures keep nearly the same with low level of fuel injection asymmetry. Unexpected results like unstart will happen when increasing the level of fuel injection asymmetry. And the isolator will come back to normal state by decreasing the differential of upper and lower wall sides fuel injections.     

Features of the occurrence of the additional stratification on the bottom-side F region over the equatorial location of Trivandrum

The general features of occurrence of an additional layer on the bottom side of F region, referred to as F0.5 layer in the pre noon period, over the magnetic equatorial location of Trivandrum (8.5° N; 77° E; dip lat of 0.5° N) in India during the period from 2004 to 2007 are presented using ionosonde observations. The F0.5 layer has a June (northern summer) solsticial maximum probability of occurrence with secondary maxima during December (northern winter) solstice. The seasonal as well as the day-to-day variability in the occurrence of F0.5 layer as mentioned in this paper seems to be a result of the variations in the amplitude and phases of the tides and gravity waves, and inventory of the metallic ions of meteoric origin. This study brings out an important manifestation of morning time F layer base region dynamics.     

Stability of the seasonal variations in diurnal and semidiurnal components of mid-latitude F2 layer parameters

We report work utilizing 15-min resolution ionospheric data obtained with DPS-4 digisonde in 2003–2011 to study the seasonal variations in amplitudes and phases of the most powerful spectral components of the F2 layer critical frequency (foF2) and peak height (hmF2) fluctuations over Irkutsk (52.5°N, 104.0°E). We show that fluctuations of both parameters contain quasi-harmonic components with periods of T_n = 24/n h (n = 1–7). The number of distinct spectral peaks varies from 3 in summer to 7 in winter. Amplitude and phase characteristics of the diurnal (n = 1) and semidiurnal (n = 2) components is studied using the data sets extracted from the original data sets with band-pass filter. It has been found that the amplitudes of diurnal/semidiurnal foF2 and diurnal hmF2 components are maximum in winter and minimum in summer. Amplitudes of the diurnal components vary gradually; those of the foF2 semidiurnal one, abruptly, thus forming a narrow winter maximum in November–January. The phase (local time of maximum) of the diurnal foF2 component increases gradually by 4–6 h from winter to summer. The phase of the semidiurnal foF2 component is nearly stable in winter/summer and sharply decreases (increases) by 2–3 h near the spring (autumn) equinox. The phase of the diurnal component of hmF2 (local time of minimum) varies slightly between 1130 and 1300 LT; that of the semidiurnal one decreases (increases) by 4–6 h from January to March (from September to November). The results obtained show that the main features of seasonal variations in the diurnal and semidiurnal components of the mid-latitude F2 layer parameters recur consistently during the solar activity growth and decline phases.     

A comparative sporadic-E layer study between two mid-latitude ionospheric stations

Hourly systematic measurements of the highest frequency reflected by the sporadic-E layer (foEs) recorded from January 1976 to June 2009 at the ionospheric stations of Rome (Italy, 41.8°N, 12.5°E) and Gibilmanna (Italy, 37.9°N, 14.0°E) were considered to carry out a comparative study between the sporadic E layer (Es) over Rome and Gibilmanna. Different statistical analysis were performed taking into account foEs observations near the periods of minimum and maximum solar activity. The results reveal that: (1) independently from the solar activity, Es develops concurrently over extended regions in space, instead of being a spatially limited layer which is transported horizontally by neutral winds over a larger area; especially during summer months, when an Es layer is present at Rome, there is a high probability that an Es layer is also present over Gibilmanna, and vice versa; (2) Es layer lifetimes of 1–5 h were found; in particular, Es layers with lifetimes of 5 h both over Gibilmanna and Rome are observed with highest percentages of occurrence in summer ranging between 80% and 90%, independently from the solar activity; (3) latitudinal effects on Es layer occurrence emerge mostly for low solar activity during winter, equinoctial, and summer months, when Es layers are detected more frequently over Gibilmanna rather than Rome; (4) when the presence of an Es layer over Rome and Gibilmanna is not simultaneous, Es layer appearance both over Rome and Gibilmanna confirms to be a locally confined event, because drifting phenomena from Rome to Gibilmanna or vice versa have not been emphasized.     

亚/超声速楔状流层流边界层速度与温度相似解及拟合解

利用相似变换获得了楔状流层流边界层无量纲流函数的3阶非线性常微分方程,用Runge-Kutta法求解微分方程获得了不同楔形角楔状流层流边界层无量纲速度随相似变量的变化曲线;推导了亚声速和超声速楔状流层流边界层无量纲温度关于相似变量的2阶线性齐次和非齐次微分方程,获得了温度分布的通解,恒壁温条件下亚声速楔状流和绝热壁面条件下超声速楔状流层流边界层无量纲温度解析解及指数函数形式的拟合解.以楔形角为0为例利用相似变换研究了超声速条件下气体压缩性及黏度随温度变化等因素对层流边界层速度与温度的影响,得出不可压缩常物性与可压缩变物性条件下无量纲速度相对误差绝对值小于9.8%的结论.研究表明:Pr越大贴近壁面处无量纲温度变化越剧烈;超声速条件下壁温低于绝热壁温时黏性耗散作用可以使层流边界层气体温度从壁面到主流间出现先升高后降低的变化.      

吸附式低反动度轴流压气机气动设计方法在多级轴流压气机中的应用

随着对吸附式低反动度轴流压气机内部流动细节的逐步深入,进一步完善了多级吸附式低反动度轴流压气机气动设计思想,完成了3级吸附式低反动度轴流压气机气动设计.三维黏性的数值计算结果表明:在第1级动叶入口叶尖切线速度为370m/s的前提下,通过只在首级静叶和末级静叶中进行附面层抽吸,实现了总压比为6.1的3级吸附式低反动度轴流压气机气动设计.附面层总抽吸流量占入口流量的11.3%.在不考虑附面层抽吸对流动效率影响的前提下,3级效率达到88.1%.