Adaptive flow control of low-Reynolds number aerodynamics using dielectric barrier discharge actuator

Aerodynamic performance of low-Reynolds number flyers, for a chord-based Reynolds number of 10⁵ or below, is sensitive to wind gusts and flow separation. Active flow control offers insight into fluid physics as well as possible improvements in vehicle performance. While facilitating flow control by introducing feedback control and fluidic devices, major challenges of achieving a target aerodynamic performance under unsteady flow conditions lie on the high-dimensional nonlinear dynamics of the flow system. Therefore, a successful flow control framework requires a viable as well as accessible control scheme and understanding of underlying flow dynamics as key information of the flow system. On the other hand, promising devices have been developed recently to facilitate flow control in this flow regime. The dielectric barrier discharge (DBD) actuator is such an example; it does not have moving parts and provides fast impact on the flow field locally. In this paper, recent feedback flow control studies, especially those focusing on unsteady low-Reynolds number aerodynamics, are reviewed. As an example of an effective flow control framework, it is demonstrated that aerodynamic lift of a high angle-of-attack wing under fluctuating free-stream conditions can be stabilized using the DBD actuator and an adaptive algorithm based on general input–output models. System nonlinearities and control challenges are discussed by assessing control performance and the variation of the system parameters under various flow and actuation conditions. Other fundamental issues from the flow dynamics view point, such as the lift stabilization mechanism and the influence on drag fluctuation are also explored. Both potentiality and limitation of the linear modeling approach are discussed. In addition, guidelines on system identification and the controller and actuator setups are suggested.     

Sunspot cycle prediction using Warped Gaussian process regression

Solar cycle prediction is a key activity in space weather research. Several techniques have been employed in recent decades in order to try to forecast the next sunspot-cycle maxima and time. In this work, the Gaussian process, a machine-learning technique, is used to make a prediction for the solar cycle 25 based on the annual sunspot number 2.0 data from 1700 to 2018. A variation known as Warped Gaussian process is employed in order to deal with the non-negativity constraint and asymmetrical data distribution. Tests using holdout data yielded a root mean square error of 10.0 within 5 years and 25.0–35.0 within 10 years. Simulations using the predictive distribution were performed to account for the uncertainty in the prediction. Cycle 25 is expected to last from 2019 to 2029, with a peak sunspot number about 117 (110 by the median) occurring most likely in 2024. Thus our method predicts that solar Cycle 25 will be weaker than previous ones, implying a continuing trend of declining solar activity as observed in the past two cycles.     

热激励在超声速进气道内对激波诱导的边界层分离的控制机理

通过数值模拟的方法研究了马赫5的超声速进气道内,热激励对激波/边界层相互作用的控制机理。研究了热激励器放热功率(E)、热激励器展向放置数目(N)和热激励器到控制激波的距离(S)三个参数在超声速进气道内激波控制和边界层分离改善中的表现。分别针对以下四种条件进行了数值模拟:1)E=2k W、N=2、S=0.02m;2)E=3k W、N=2、S=0.02m;3)E=2k W、N=3、S=0.02m;4)E=2k W、N=2、S=0m。分析发现:在以上四种条件下,均可观察到热激励在控制激波和改善激波诱导边界层分离上有着显著的效果。热激励器的放热功率对激波的控制效果有着明显的影响,在本文所考虑的热激励能量范围内,放热功率越大,原激波角的改变越大,最终分离区的改变越明显;展向放置的热激励器数目N影响着输入到流场的能量密度,并且N越大,壁面附近的激波面越趋于平面。尽管N对上壁面沿展向的压力分布无明显影响,但对上壁面分离区大小有明显的影响;对比条件1和4下的计算结果,发现S=0.02m可以得到很好的控制效果,而S=0m时流场结构几乎没有变化,这就表明热激励器必须放置在控制激波上游的一定距离处才会有预期的效果。     

高马赫数超燃冲压发动机性能数值研究

考虑水平起降要求,构造了一种采用流线追踪内转式进气道、圆形截面燃烧室的双侧布局高马赫数超燃冲压发动机,设计点马赫数为7。对Ma 7~10范围内的发动机性能进行了数值模拟,给出了发动机进气道性能、整机性能,对燃烧室内的燃料掺混和燃烧情况进行了分析。此外,采用一维性能计算方法对燃烧室性能进行了预估。研究表明,此发动机性能可满足飞行器推阻匹配需求;一维性能结果与三维数值模拟的压力分布处于15%的误差范围内,可用于发动机性能的快速预估。     

低雷诺数下进气道异常起动现象及其影响因素探析

结合激波风洞实验和数值模拟分析,对一种二元混压式进气道在实验中低单位雷诺数下反而呈现出自起动特征的异常现象进行了研究。根据激波风洞的反复实验观察,表明随着来流单位雷诺数的降低,在继进气道进入不起动状态之后又会重新出现自起动特征的异常起动现象。该结果与层流模型计算得到的流场结构相符,而与湍流模拟结果差异显著;分析表明,层流情况下,由于分离区向前体压缩面大范围地延伸,缓解了进气道入口的逆压梯度,从而在喉道处可以形成主体为超声速的通畅流道,而湍流情况下,进气道入口处激波/边界层干扰形成过分集中的分离泡则呈现明显的壅塞状态;尽管层流情况下进气道流场结构呈现出较为通畅的类似起动的特征,但其流量系数仍明显低于湍流的情况。因此,实验上所观察到的这种异常起动现象严格地说并不属于真正意义上的起动状态。     

大容量光通信系统中规则LDPC码的研究

为研究规则LDPC码在大容量光通信系统的性能,对不同列重和不同迭代次数的短规则LDPC码在大容量光信道中的误码率性能进行了数据仿真。发现确定码长的规则LDPC码,存在最优的列重,使得在相同的冗余度时,最大程度地改善大容量光通信系统的性能;此外,误码率随迭代次数的增加具有收敛的特性,LDPC码译码时应合理地选取迭代次数。     

某型燃气轮机进气系统内流场的数值分析

为了揭示燃机进气系统内的流动特性,并为进气系统的性能预测、优化设计提供理论依据,以某型燃气轮机的进气系统为研究对象,基于混合网格的SIMPLE算法和标准的k-ε湍流模型,运用FLUENT软件对燃气轮机进气系统内的流场进行数值模拟,并从压力分布、气流角分布2个方面进行分析。结果表明:对气动性能产生重要影响的损失区域主要集中在进气外环上方和支板附近,蜗壳内锥的绕流分离也是损失源之一。应进行尽可能消除进气外环上方的气流分离,同时将绕流分离控制在一定范围内的优化设计和改进。     

Birthdates of patients affected by mental illness and solar activity: A study from Italy

Purpose

This epidemiologic study tested an hypothesized association between the year of birth of persons with major mental illnesses and solar activity over the past century.

Methods

We collected data on diagnoses and birthdates of psychiatric patients born between 1926 and 1975 (N = 1954) in south Italy for comparison to yearly solar activity as registered by the International Observatories.

Results

We found a strong inverse correlation between high solar activity (HSA) and incidence of schizophrenia and bipolar disorder in a 20-year period whereas the incidence of non-affective/non-psychotic disorders was moderately associated with HSA in the same period.

Conclusions

Interpretation of the observed correlations between HSA during years of birth and the incidence of mental illnesses remains unclear, but the findings encourage further study.     

组合发动机可调进气道气动性能

为使涡轮/冲压组合发动机能在宽广的飞行高度、速度内飞行,设计一种组合发动机可调进气道气动结构.编制带黏性修正激波系结构预估程序,实现不同来流马赫数及来流攻角下,进气道内激波系结构布置及强度预估,并指导进气道可调楔板尺寸选取.考虑进气道附面层抽吸后,对来流在马赫数为1.6～4.0范围内,-2°,0°和+2°三种攻角下进气...     

基于遗传算法的高超声速曲面压缩进气道反设计

结合计算流体力学和遗传算法,建立了一种高超声速曲面压缩进气道的反设计方法。根据压力分布反设计了压缩型面。结果表明,该曲面压力分布与目标压力分布符合良好,从而验证了反设计方法的正确性。采用此反设计方法,设计了某高超声速曲面压缩进气道,并和等熵压缩二维进气道进行了比较。研究发现,在其它性能参数几乎相等情况下,曲面压缩进气道总压恢复较等熵压缩基准进气道提高9.7%,长度缩短5.6%。吞入23mm前体附面层后,基准进气道不起动,而曲面压缩进气道总压恢复系数仅下降5.4%,表现出良好的吞附面层能力。