共查询到20条相似文献,搜索用时 54 毫秒
1.
表面台阶引起的高超声速湍流边界层分离 总被引:2,自引:1,他引:2
介绍了圆柱、方柱和二维台阶前干扰热流分布及油流和液晶热图的实验结果。来流马赫数M_1=5—9,雷诺数Re=(2—5)×10 ̄7/m,台阶高度与边界层厚度比h/δ=0.06— 2.5.实验发现干扰压力和热流高峰值出现在台阶前0.15倍台阶高度处的再附点附近,方柱台阶前压力和热流最高峰值不在中心线上,而在两侧角之内0.5倍台阶高度处附近,结果还表明干扰区几何特征参数,如分离距离、热流峰值和谷值点位置,与马赫数、雷诺数和台阶展宽无关,只随台阶高度线性增加。 相似文献
2.
应用于激波/边界层相互作用的非线性湍流模式 总被引:2,自引:1,他引:2
选择8个近年来有代表性的非线性湍流模式,研究2个跨声速激波/边界层相互作用问题.采用的非线性湍流模式包括4个二阶模式和4个三阶模式.2个跨声速激波/边界层相互作用的流动是轴对称圆弧突起绕流和二维管道突起流动.通过数值计算结果和实验结果的比较,对有关的非线性湍流模式进行评估和分析.计算结果表明,非线性模式的模化系数与平均流动应变不变量以及涡量不变量有关,反映了湍流的各向异性,比线性模式优越得多. 相似文献
3.
4.
利用一套以位移传感器为主的弹性平衡装置,测量几种不同表面形状设计的平板在充分发展的二维湍流边界层中的减阻效果,并对其减阻机理进行研究。结果表明,大涡破碎器 LEBU(Large Eddy Break-up)和其它减阻装置的形状和布置对表面摩擦阻力有较大影响,在有些设计状态下的平板得到了净减阻。 相似文献
5.
大气边界层的一些空气动力学特征 总被引:4,自引:0,他引:4
本文在定常和中性温度层结条件下,首先介绍了均匀平坦地面上大气边界层的一些空气动力学特征,并将大气边界层与一般空气动力学边界层作了比较。然后介绍了地面粗糙度分布有阶跃变化以及非平坦地形上的大气边界层特征。文中说明,气流的平均流场结构以及湍流特性与均匀平坦地面的情形相比有显著不同。最后,提出了一些有待今后进一步研究的问题。 相似文献
6.
7.
湍流场中二阶速度加速度结构函数 (velocity-acceleration structure function, VASF) 被认为与尺度间能量或者拟涡能的传递相关,其正负表明传递的方向. 三维湍流中,能量从大尺度向 小尺度传递,VASF 为负. 在二维湍流中,能量反向传递到大尺度,拟涡能正向传递到小尺度,因此理论上 VASF 无论在反向能量级串区还是在正向拟 涡能级串区均为正. 然而,相对于三维湍流中 VASF 的充分研究,二维湍流中 VASF 的正负性迄今尚无实验或数值模拟数据验证. 本文通过三维二维湍流中普适的公式推导,指出在空间非均匀湍流场中,VASF 除了尺度间传递,还受到非均匀项的影响. 一种常见的空间非均匀湍流场是在实验研究中常用的风洞或水洞中,湍流发生装置 (如栅格) 后的湍流. 该流场中,湍流强度随下游位置增大而逐渐衰减,这种衰减则带来空间上的非均匀性. 本文在基于竖直流动皂膜的二维衰减湍流场中,利用拉格朗日粒子追踪法测得在拟涡能级串区的 VASF,并分析各部分的影响. 结果表明,虽然尺度间传递项为正值,但由于衰减带来的非均匀项为负值,使得 VASF 的值为负,使之失去了表征拟涡能传递方向的意义. 因此,在类似风洞、水洞、水槽等衰减流场中对 VASF 的讨论不应忽略非均匀项. 最后对与 VASF 密切相关的弥散过程进行分析,发现后期弥散过程变慢是由于负的 VASF 导致. 相似文献
8.
二维和三维湍流的分形模型 总被引:1,自引:0,他引:1
<正> 1 引言 目前分形物体已广泛引入物理学和自然科学各种现象的分析中。湍流的几何形态特别需用“自相似”方法、而不是用“欧几里德”方法来研究。人们通常考虑的是均匀分形(即具有整体严格的膨胀不变性),但我们想引入自相似性仅在平均意义下保持的一个简单模型, 相似文献
9.
低湍流度风洞中湍流度对平板边界层转捩影响的试验研究 总被引:2,自引:0,他引:2
本文报告了在西北工业大学壁低流度风洞中进行了平板边界层转捩试验研究的简况及初步结果,试验湍流度为0.02%、0.1%及0.33%,用恒温热线风速仪测量时均速度型,求得边界层沿流向的位移厚度分布,并用示波器观察速度脉动脉形变化,从而确定起始转捩点和完全转捩点位置。结果表明,转捩的规律性和国外经典结果极为吻合。 相似文献
10.
11.
A turbulent separation-reattachment flow in a two-dimensional asymmetrical curved-wall diffuser is studied by a two-dimensional
laser doppler velocimeter. The turbulent boundary layer separates on the lower curved wall under strong pressure gradient
and then reattaches on a parallel channel. At the inlet of the diffuser, Reynolds number based on the diffuser height is 1.2×105 and the velocity is 25.2m/s. The results of experiments are presented and analyzed in new defined streamline-aligned coordinates.
The experiment shows that after Transitory Detachment Reynolds shear stress is negative in the near-wall backflow region.
Their characteristics are approximately the same as in simple turbulent shear layers near the maximum Reynolds shear stress.
A scale is formed using the maximum Reynolds shear stresses. It is found that a Reynolds shear stress similarity exists from
separation to reattachment and the Schofield-Perry velocity law exists in the forward shear flow. Both profiles are used in
the experimental work that leads to the design of a new eddy-viscosity model. The length scale is taken from that developed
by Schofield and Perry. The composite velocity scale is formed by the maximum Reynolds shear stress and the Schofield-Perry
velocity scale as well as the edge velocity of the boundary layer. The results of these experiments are presented in this
paper. 相似文献
12.
湍流边界层流场与噪声实验研究 总被引:5,自引:0,他引:5
在重力式水洞中进行了水翼及半翼湍流边界层流场与噪声的实验研究。测量了水翼及半翼边界层附近的湍流脉动速度场;测量了半翼翼型表面三点处的压力脉动及其辐射噪声,测量了水翼内部测点的噪声及外部辐射噪声,在不同流速、不同攻角、光滑和粗糙翼面的情况下都进行了测量分析。试验结果发现,上述因素对模型的边界层湍流速度场有显著影响,15度攻角时,翼面附近湍流强度要比0度时大得多,粗糙翼面附近的湍流强度比光滑的大,而湍流强度随来流速度的变化不大,u(来流)方向和v方向的湍流强度量级相当;一般地,翼面压力脉动、翼内部噪声及外部噪声都是随来流速度的增大而增大,随攻角的增大而增大,粗糙翼面时的结果要比光滑翼面的大。从压力脉动与噪声测量结果与相应的流场测量结果比较可知,可以从湍流区域的湍流强度来判断出声源强度的定性变化。 相似文献
13.
V. V. Larichkin S. N. Yakovenko 《Journal of Applied Mechanics and Technical Physics》2003,44(3):365-372
Results of physical and numerical experiments on investigating the effect of the depth of immersion of a two-dimensional obstacle with a square cross section into a developed turbulent boundary layer on the length of the separated flow region are presented. The numerical simulation is based on solving averaged Navier–Stokes equations with the use of the k– model of turbulence. The near-wall flow is visualized in the experiments, and the fields of mean and fluctuating velocities are measured. Flow regions where the results of numerical simulation agree with experimental data are determined. It is shown that the length of the recirculation flow region in the near wake increases with decreasing depth of immersion of the two-dimensional obstacle into the turbulent boundary layer. 相似文献
14.
A mathematical model of turbulent transport processes is modified to make allowance for the turbulent energy gradient and the presence of walls. The modification consists in making the variance tensor in the Gaussian probability density distribution for the initial mole velocities anisotropic for nonzero turbulent energy gradient and a ratio of the turbulence scale to the distance from the wall of the order of unity. Formulas for the variance tensor components are derived and the empirical coefficients of these formulas are determined. The expression for the dimensionless turbulent friction stress is compared with experimental data for three boundary-layer-type flows, namely, in the wake of a cylinder, in the boundary layer on a flat plate, and in a channel with parallel walls. 相似文献
15.
Turbulent separation limits the performance in many engineering applications, for example creating pressure losses in diffuser
like flows or stall on aircraft wings. In the present study the turbulent boundary layer flow over a flat plate separating
due to an adverse pressure gradient is studied as a model problem and the effect of periodic excitation in both time and space
is investigated through direct numerical simulations. Linear stability analysis is used to analyse the sensitivity of the
flow with respect to time-periodic excitations. The dependence on position, amplitude and frequency of the forcing is investigated.
For a certain frequency range at sufficiently high amplitudes, it is possible to eliminate the separated region. Furthermore,
three-dimensional effects are studied by applying a steady spanwise forcing as well as a both time-dependent and spanwise
varying forcing. A forcing varying in spanwise direction is shown to be the most effective in eliminating the separated region,
whereas two-dimensional time-periodic excitation was not as efficient as it was expected. 相似文献
16.
Several studies of enclosed turbulent flows within rotating discs or cylinders (e.g. [6, 7]) have revealed that, while the
geometry may be strictly axisymmetric, it is possible for non-axisymmetric flow patterns to be created within the space. Here
we report a visualization study off low induced in the cavity formed between two discs, one rotating, the other stationary.
This is an idealization of the flow configuration that occurs between successive stages in the `hot section' of a gas turbine.
Such rotor-stator cavities have hitherto been regarded as creating asymmetric flow pattern but Owen [8] has conjectured that
the failure to predict heat transfer coefficients accurately for certain radius-to-height ratios may indicate that here, too,
organized rotating vortex structures were playing a crucial role. The present study has made an experimental visualization
of this flow over a range of conditions in order to test this conjecture and to help guide future numerical explorations.
The apparatus comprised a rotating disc over which is fitted a Perspex stationary disc and shroud. The lower disc was rotated
for a number of distinct speeds between 30 and 120 rpm and for two ratios of gap-height to radius (H/R). The spin Reynolds number based on gap height and maximum rotational speed, ρΩRh/μ, ranged from 3.7 × 10E4 to 2.24 × 10E5. The flow structures were visualized by injecting ink through a small hypodermic
tube at various radii and depths within the cavity and recording the ensuing dye streaks with a video camera mounted above
the discs. The results show that, for a wide range of conditions,structured flow with large-scale vortices does indeed arise,
the number of vortices diminishing as the spin Reynolds number is increased. The two-vortex S-shaped pattern is stable over
a wide range of conditions but three, five and seven vortices have also been observed.
These results suggest that an accurate numerical simulation of the flow within rotor-stator disc cavities may require unsteady,three-dimensional
CFD modelling over at least certain ranges of flow parameters.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
17.
In this paper, experiments to detect turbulent spots in the transitional boundary layers, formed on a flat plate in a free-piston
shock tunnel flow, are reported. Experiments indicate that thin-film heat-transfer gauges are suitable for identifying turbulent-spot
activity and can be used to identify parameters such as the convection rate of spots and the intermittency of turbulence. 相似文献
18.
Coherent structures in a tripped turbulent boundary layer havebeen analysed by applying a new conditional sampling algorithm tolarge-eddy simulation (LES) data. The space-time development of theevents and structural characteristics were examined. The new conditionalsampling scheme is shown to be very effective in the eduction of thecoherent eddy structures, allowing the simultaneous detection, trackingand averaging of several three-dimensional (3-D) structures. Alignmentof the triggering events in all spatial directions and flip-averaging(spatial reflection of the samples) enhance significantly the extractionof detailed features of the structures. The detection method minimisesthe smearing of the spatial details and avoids imposing artificialsymmetry, which is often intrinsic to other conditional samplingschemes.The results show the existence of cigar-shapedstreamwise vortices which are directly associated with negative pressurefluctuation peaks (positive source term of the pressure Poissonequation). They are inclined at 12° to the wall andtilted laterally at an angle of 7° to the streamwisedirection. The streamwise vortices induce ejections and sweeps throughan advection mechanism due to the tilting or inclination of thevortices. There is no evidence of hairpin vortices in either theconditional averages or instantaneous flow fields. Near-wall shearlayers are found to be related to the positive pressure fluctuationpeaks as a result of complex interactions between ejection and sweepevents. The shear layer structure has an inclination of10°, being located between two tilted, streamwisevortices of opposite direction of rotation.The presentresults are very close to other Direct Numerical Simulation studiesusing different conditional sampling schemes. Conceptual models for thestreamwise vortices and shear layer structures are proposed to accountfor the results. 相似文献
19.
A thre-dimensional direct numerical simulation is combined with a laboratory study to describe the turbulent flow in an enclosed
annular rotor-stator cavity characterized by a large aspect ratio G = (b − a)/h = 18.32 and a small radius ratio a/b = 0.152, where a and b are the inner and outer radii of the rotating disk and h is the interdisk spacing. The rotation rate Ω considered is equivalent to the rotational Reynolds number Re = Ωb
2/ν= 9 .5 × 104 (ν the kinematic viscosity of water). This corresponds to a value at which experiment has revealed that the stator boundary
layer is turbulent, whereas the rotor boundary layer is still laminar. Comparisons of the computed solution with velocity
measurements have given good agreement for the mean and turbulent fields. The results enhance evidence of weak turbulence
by comparing the turbulence properties with available data in the literature (Lygren and Andersson, J Fluid Mech 426:297–326,
2001). An approximately self-similar boundary layer behavior is observed along the stator. The wall-normal variations of the structural
parameter and of characteristic angles confirm that this boundary layer is three-dimensional. A quadrant analysis (Kang et
al., Phys Fluids 10:2315–2322, 1998) of conditionally averaged velocities shows that the asymmetries obtained are dominated by Reynolds stress-producing events
in the stator boundary layer. Moreover, Case 1 vortices (with a positive wall induced velocity) are found to be the major
source of generation of special strong events, in agreement with the conclusions of Lygren and Andersson (J Fluid Mech 426:297–326,
2001). 相似文献
20.
A. Yu. D’yachenko V. I. Terekhov N. I. Yarygina 《Journal of Applied Mechanics and Technical Physics》2006,47(5):671-678
The process of vortex formation in a cavity with inclined walls, which has a moderate aspect ratio, is experimentally studied,
and the distribution of pressure coefficients is measured. The angle of inclination of the side walls ϕ is varied from 30
to 90°. It is found that the flow in the cavity becomes unstable in the range of inclination angles ϕ = 60–70°. Flow reconstruction
occurs, which substantially alters the surface-temperature and static-pressure distributions. Large changes in these characteristics
and their nonuniform distributions for these angles are observed across the cavity on its frontal wall and on the bottom.
For small angles (ϕ = 30 and 45°), the pressure on the rear wall drastically increases, which leads to a small increase in
pressure averaged over the entire cavity surface.
__________
Translated from PrikladnayaMekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 5, pp. 68–76, September–October, 2006. 相似文献