Active control of turbulence for drag reduction based on the detection of near-wall streamwise vortices by wall information

The spatial relations between the measurable wall quantities （streamwise shear stress τwx, spanwise shear stress τwz, and pressure fluctuations Pw） and the near-wall streamwise vortices （NWSV） are investigated via direct numerical simulation （DNS） databases of fully developed turbulent channel flow at a low Reynolds number. In the stan- dard turbulent channel flow, the results show that all the wall measurable variables are closely associated with the NWSV. But after applying a stochastic interference, the relation based on τwx breaks down while the correlations based on Pw and τwz are still robust. Hence, two wall flow quantities based on Pw and τwz are proposed to detect the NWSV. As an appli- cation, two new control schemes are developed to suppress the near-wall vortical structures using the actuation of wall blowing/suction and obtain 16 % and 11% drag reduction, respectively.

Spatial relation between fluctuating wall pressure and near-wall streamwise vortices in wall bounded turbulent flow

A new view of the spatial relation between fluctuating wall pressure and near-wall streamwise vortices （NWSV） is proposed for wall bounded turbulent flow by use of the direct numerical simulation （DNS） database. The results show that the wall region with low pressure forms just below the strong NWSV, which is mostly associated with the overhead NWSV. The wall region with high pressure forms downstream of the NWSV, which has a good correspondence with the downwash of the fluids induced by the upstream NWSV. The results provide a significant basis for the detection of NWSV.

FORMATION AND EVOLUTION OF THE STREAMWISE VORTICES IN MIXING LAYERS

The evolution of the three-dimensional time-developing mixing layer was simulated numerically using the pseudo-spectral method. The initial perturbations consisted of the two-dimensional fundamental wave and the streamwise-invariant three-dimensional disturbance. A comparison of the formations of the streamwise vortices with different amplitude functions for three-dimensional disturbances was made. In one case the results are similar to that of Rogers and Moser (1992), whereas a different way in which the quadrupole forms and sudden expansion of the rib were observed in another case. The simulation also confirms that stretching by the forming roller rather than Rayleigh centrifugal instability is responsible for the formation of the rib. Finally, numerical flow visualization results were presented. (Edited author abstract) 9 Refs.

Physical mechanism for origin of streamwise vortices in mode A of a square-section cylinder

In the present paper, physical mechanism responsible for origin of streamwise vortices in mode A appeared in the threedimensional (3-D) wake transition of a square-section cylinder is investigated. Direct numerical simulations at a Reynolds number of 180 firstly show that such streamwise vorticity is not originated from lateral surfaces. Then through the analysis of local flow field in the immediate neighborhood of rear surface, based on the theory of vortex-induced vortex, a new physical mechanism is identified. At first, the vertical vorticity on rear surface is generated by the intrinsic three-dimensional instability with the same instability wavelength of mode A. Then the streamwise vorticity at a specific sign is induced by such vertical vorticity, convected and concentrated in the shear layers. Finally, streamwise vortices are formed and shed with alternatively shedding spanwise vortices in the near wake. Moreover, the effect of induced spanwise vorticity on original two-dimensional (2-D) spanwise vorticity is also presented in detail.

Relationship between wall shear stresses and streamwise vortices in turbulent flows over wavy boundaries

The relationship between wall shear stresses and near-wall streamwise vortices is investigated via a direct numerical simulation(DNS) of turbulent flows over a wavy boundary with traveling-wave motion. The results indicate that the wall shear stresses are still closely related to the near-wall streamwise vortices in the presence of a wave. The wave age and wave phase significantly affect the distribution of a two-point correlation coefficient between the wall shear stresses and streamwise vorticity. For the slow wave case of c/Um = 0.14, the correlation is attenuated above the leeward side while the distribution of correlation function is more elongated and also exhibits a larger vertical extent above the crest. With respect to the fast wave case of c/U_m=1.4, the distribution of the correlation function is recovered in a manner similar to that in the flat-wall case. In this case, the maximum correlation coefficient exhibits only slight differences at different wave phases while the vertical distribution of the correlation function depends on the wave phase.

Streamwise Vorticity Equation

A streamwise vorticity equation is derived in generalized natural coordinates. This equation reveals that the total change and local change of the streamwise vorticity are mainly determined by the curvature of streamline, unsteady feature of streamline and magnitude of velocity. This equation enables the study of mesoscale or small-scale systems since the term associated with pressure gradient force in the original vorticity equation is replaced by terms associated with streamlines and wind speed. With this modification the wind field rather than the pressure field is used in the calculation considering that 1) the pressure field is to adapt wind field. 2) Smoother and more consecutive streamline pattern is easier to obtain either by data analysis or by the numerical simulation. From this sense, this present study suggests the application of this equation to studying the evolution of severe storm system as well as other simplified cases. Key words Wind field instead of pressure field - Generalized natural coordinate - Streamwise vorticity This work was supported by the project on the study of the formative mechanism and predictive theory of the significant climate and weather disaster in China under Grant G 1998040907 and by the key project on the Dynamic Study of Severe Mesoscale Covective Systems sponsored by the National Natural Science Foundation of China under Grant No. 49735180.

基于全局稳定性理论的eN方法对高超声速有迎角锥背风流向涡转捩分析

有迎角圆锥表面存在周向压力梯度,压力梯度驱使流体从迎风面的高压区向背风面的低压区流动,导致背风中心线附近出现大尺度流向涡结构,从而进一步失稳触发转捩。本文利用全局稳定性理论与e^(N)方法,研究3个不同工况下高超声速有迎角圆锥的流向涡转捩问题。研究发现,Mack模态在上游流向涡较弱时为主导模态,但当流场出现周向卷曲、流向涡逐渐增强时逐渐减弱,此时流向涡失稳模态出现,并逐渐成为主导不稳定性。基于全局稳定性得到不稳定模态的N值。结合风洞实验测量结果可判断,在迎角2°时转捩N值不到3,而在迎角4°、转捩完成时的N值可达6左右。

Wall temperature effects on the stability of leeward vortices of a blunt inclined cone

Streamwise vortex instability is one of the most potent mechanisms for the transition of the three-dimensional boundary layers.By using the global stability analysis methods,stability characteristics of the leeward vortex over a blunt cone with an angle of attack under a typical wind tunnel condition are studied and are compared to the case with a smaller wall temperature ratio(corresponding to a flight condition).The vortical structure features inward and outward vortices,similar to that in the flight con-dition.Unlike the flight condition,the outward vortices appear stronger than the inward vortices,resulting in stronger outer-mode instabilities.Although the inner mode is heavily stabilized compared to the flight condition,it can still radiate apparent acoustics.The acoustic sources are computed based on Lighthill's acoustic analogy,showing that the entropy term measuring the deviation from the isentropic relation is dominant.While Mack second mode is shown to most likely trigger the transition in the flight condition,it is absent in the wind tunnel condition,and a shear-layer mode turns out to be the most dangerous instead.Moreover,the instability frequencies and growth rates of the wind tunnel case are much smaller than those of the flight case,indicating that wall heating may stabilize the leeward vortices.

Shock Wave Induced Separation Control By Streamwise Vortices

Control of shock wave and boundary layer interaction finds still a lot of attention. Methods of this interaction control have been especially investigated in recent decade. This research was mostly concerned with flows without separation. However, in many applications shock waves induce separation often leads to strong unsteady effects. In this context it is proposed to use streamwise vortices for the interaction control. The results of experimental investigations are presented here. The very promising results were obtained, meaning that the incipient separation was postponed and the separation size was reduced for the higher Mach numbers. The decrease of the RMS of average shock wave oscillation was also achieved.

空气涡轮火箭发动机内外涵气流掺混研究

通过无化学反应、均匀进气条件下肼单组元空气涡轮火箭发动机混流燃烧室内流场的数值计算,得到了流向涡与正交涡系产生、衰减演变过程及其对内外涵气流掺混效率的影响规律。结果表明,大尺度阵列二次环流诱导形成的流向涡对内外涵气流掺混起主导作用,大波瓣穿透率的斜切波瓣混流器的综合性能较优。结合热试车结果,分析了包括波瓣混流器在内的两类掺混方案的强化掺混效率。分析表明,非均匀进气条件对小尺寸空气涡轮火箭发动机掺混燃烧效率影响很大。

空气涡轮火箭发动机掺混燃烧研究进展

在混流燃烧室内组织富燃燃气与空气的高效稳定掺混燃烧对于空气涡轮火箭发动机(ATR)性能至关重要。回顾了国外各研究机构关于ATR掺混燃烧方案的试验研究,对波瓣混流器在航空领域强化内、外涵气流掺混中的典型应用及研究成果进行总结评述,总结并提出ATR掺混燃烧的特点和关键问题,分析了后续掺混燃烧研究中需重点关注的问题。

波瓣混合器混合流场中涡结构的数值研究

以FLUENT程序为研究平台,分别采用标准k-ε、RNG k-ε、Realizable k-ε、SST k-ω以及S-A五种湍流模型,对航空发动机混合排气系统中常用的波瓣混合器的流场,进行了三维数值模拟。与实验测量数据对比可知,所采用的计算方法能够较好地描述波瓣混合器复杂流场中流向涡、正交涡形状以及涡量沿流动方向的变化规律。在各种湍流模型中,Realizable k-ε在流向涡以及正交涡涡量大小的描述方面较其他模型更为精确。

波瓣喷管混合流动的数值计算

借助Ｎ－Ｓ方程对波瓣喷管混合装置的流场进行了数值分析。文中采用三维贴体曲线坐标，非交错网格法和代数湍流模型，对波瓣喷管内外两侧的气流通道及波瓣喷管下游的混合段流场采用分块计算方法进行了数值计算。

碳氢燃料流向涡掺混超燃模型燃烧室冷态流场数值研究

对一种超声速燃烧模型燃烧室的冷态流场进行了数值研究。为稳定超燃火焰并提高燃烧速率和效率,取少量的冲压空气进行稳定的亚燃预燃烧,并以之作为超燃室的一个稳定火炬,其高温燃气将吸热型碳氢燃料预先蒸发汽化并部分裂解为短链小分子,最后通过城墙波瓣掺混器形成的流向涡带入超声速主流,以期实现快速高效的超燃燃烧。计算结果表明:首先,城墙波瓣掺混器能够在较小总压损失下得到显著掺混效果;其次,本文从气动结构的角度专门探究了亚/超燃接力阶段较低的隔离段进口马赫数存在热堵塞的可能性,发现较难适宜超燃启动点火。

波瓣混合流场流向涡的数值分析

借助Ｎ－Ｓ方程对波瓣喷管混合装置的流场进行了数值分析。采用三维贴体曲线坐标，非交错网格法和ｋ－ε两方程湍流模型，对波瓣喷管内外两侧的气流通道及波瓣喷管下游的混合段流场统一进行了数值计算。得出了速度场和流向涡及正交涡的分布，并对流向涡的发展规律及其对正交涡分布规律的影响作了分析。并对混合管壁面的压力分布和出口速度型与实验值进行了比较。

圆柱尾迹涡的三维演化及结构特征

应用无粘涡丝运动学理论和局部诱导近似(LIA)方法,以Lagrange观点跟踪涡丝在背景流场中运动,用数值方法研究了中等Re数(≈10~3)下圆柱分离尾迹中Kármán涡和涡辫区涡丝的三维演化的机制和动力学过程,及其结构特征。背景流场考虑为尾迹时间平均速度流场和Kármán涡街流场。初始展向小扰动为指数形式和谐波形式。结果指出:Kármán涡和涡辫区中的涡丝具有展向不稳定性,形成流向涡量。在尾迹的初期输运过程中,表现出有序的大、小尺度涡结构。并进一步分析了其产生的机理。

引射混合器内流场的数值分析

计算了以波瓣为主喷管的三维引射掺混流场；分析了包括复杂型面和引射二次流在内的边界条件处理；再现了流向涡的产生，计算结果与实验结果作了对比分析，吻合程度较好．

马赫数2.5超燃室中超/亚声流流向涡掺混冷态流场数值研究

结合实验,对一种带亚声速预燃室和流向涡掺混器的超声速燃烧模型燃烧室,在其进口马赫数为2.5的来流条件下,进行了冷态流场的数值研究。计算与实验得到的燃烧室沿程压力分布相当一致,计算与实验得到的激波结构也基本吻合。计算结果表明:首先,在马赫数2.5的来流条件下,亚燃预燃室易于达到启动状态;其次,流向涡掺混器增强超/亚声速流之间的掺混的效果明显,但其掺混深度尚有限;最后,流向涡掺混的超燃室掺混段有着复杂的激波膨胀波波系,波涡干涉和激波附面层干涉结构。

掺氢比对微波瓣燃烧器燃烧特性的影响

为解决微尺度非预混燃烧中燃料同氧化剂混合慢、燃烧不稳定的问题,基于实验的方法研究了燃料掺氢以及微波瓣燃烧器对燃烧的影响,并将微波瓣燃烧器的燃烧温度同微圆管燃烧器进行了对比。在此基础上建立了微波瓣燃烧器以及微圆管燃烧器的三维仿真模型,研究了掺氢比对微波瓣燃烧器和微圆管燃烧器流场、火焰长度、燃料消耗率、燃烧产物以及燃烧温度的影响。研究结果表明:随着掺氢比的增加,微波瓣燃烧器中流向涡涡量以及射流核心速度不断提高,火焰长度、燃烧温度不断增加,OH,H2O质量分数的峰值逐渐增加,CO2质量分数的峰值不断下降,CO质量分数峰值变化极小。在同一掺氢比下,相比于微圆管燃烧器,微波瓣燃烧器能够使燃料同氧化剂提前反应且具备更长的火焰、更大的OH质量分数峰值以及更高的燃烧效率和燃烧温度。在掺氢比为50%时,微波瓣燃烧器的最高燃烧温度较微圆管燃烧器提高了110K。这表明微波瓣燃烧器和燃料掺氢的结合能促进微尺度燃烧,提高燃烧的稳定性。

同轴射流突片激励混合的数值研究

通过数值方法对同轴射流的突片激励混合特性进行研究,分析了突片数目、突片顶角和安装角对同轴射流混合过程的流向涡和温度分布的影响.研究结果表明,突片的激励作用使得同轴射流之间的混合流在突片对应位置下游形成阵列涡对,导致温度分布等值线发生"指型"局部变形,增强了两股气流之间的混合.随着突片数目的增加,单个突片诱导的流向涡强度和影响范围存在较大的差异,过多的突片数导致相邻涡对的"挤压",流向涡的强度有很大的减弱;在相同的堵塞比下,突片顶角为90°时或安装角为30°时,单个突片诱导的流向涡强度和作用范围均体现最为显著,从而有利于改善同轴射流的混合程度.