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本文用子波变换检测了刻画壁湍流脉动信号自相似性的局部标度指数,研究了不同尺度的湍流结构的自相似性,发现在湍流边界层猝发过程中,喷射和扫掠发生时刻小尺度脉动速度信号的局部标度指数为负值,说明在大尺度猝发事件发生的时刻小尺度结构具有奇异的自相似性,在猝发过程中其作用不仅仅是对湍能的耗散. 相似文献
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研究了用条件采样方法检测壁湍流相干结构的门限值与壁湍流相干结构平均粹发周期检测结果之间的关系,根据用壁湍流流向脉动速度自相关函数检测壁湍流相干结构平均猝发周期的方法,提出了用自相关法确定壁湍流相干结构条件采样的门限值,从而检测壁湍流相干结构的方法 相似文献
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Enhanced Variable Interval Time Averaging (EVITA) is introduced as a method that incorporates structure information directly into the detection method. The paper tests VITA and EVITA against an ersatz turbulence signal (antisymmetric exponential time decay) inserted into a random noise distribution. It is shown that EVITA is much less dependent than VITA on the choice of both threshold level and averaging time. EVITA is then applied to particle image velocimetry turbulence data from channel flow [7] at Re = 2872, and is shown to effectively extract bursting events.This work forms part of the Ph.D. thesis of P. SullivanThe authors gratefully acknowledge Dr. R.J. Adrian for providing the channel data. The work is supported by Natural Science and Engineering Research Council of Canada, Queen's University Advisory Research Council and DANTEC, New Jersey. MD was supported through two NSERC summer undergraduate fellowships. 相似文献
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用IFA300恒温热线风速仪和×形二分量热线探针,以采样间隔小于最小湍流时间尺度的分辨率,精细测量了风洞中平板湍流边界层不同法向位置的瞬时流向、展向速度分量的时间序列信号.用子波分析辨识壁湍流相干结构猝发事件的能量最大准则,确定壁湍流相干结构猝发事件的时间尺度;用条件相位平均技术提取了相干结构猝发过程中流向、展向脉动速度分量条件相位平均波形,用互相关方法研究了相干结构猝发过程中流向、展向脉动速度分量条件相位平均波形的相位差关系及其对雷诺应力的影响,发现在缓冲层和对数律区,展向脉动速度与流向脉动速度的条件相位平均波形具有不同的相位;当两者相位基本一致时,雷诺应力达到正的最大值,此时湍流相干结构的产生非常活跃;当两者相位差分别集中在90°和270°附近时,雷诺应力的幅值减小并接近于零,此时湍流相干结构的产生和猝发都得到了抑制. 相似文献
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将壁湍流脉动速度信号子波分析技术引入壁湍流相干结构猝发检测的研究,基于子波分析能量最大准则确定相干结构猝发的时间尺度,对能量最大时间尺度的子波系数进行子波逆变换,获得能量最大时间尺度的脉动速度信号。根据其自相关函数的波长确定相干结构的平均猝发周期;根据能量最大时间尺度的子波系数的正极大值和负极小值检测壁湍流喷射和扫掠事件,获得壁湍流相干结构喷射和扫掠事件的相位平均波形。该方法可以不需要确定门限值,有效排除了小尺度脉动的干扰。 相似文献
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采用热线风速仪测量受吹吸扰动的壁湍流边界层的流向速度,用傅里叶变换和子波变换研究
吹吸扰动对壁湍流能谱的影响,结果显示施加的低频扰动使边界层内层大尺度结构的能量减
少,小尺度结构的能量有所增强,远离壁面时扰动强度逐步衰减直到在外层中消失;通过VITA
法和子波变换法检测猝发事件,表明该扰动降低了猝发强度,使猝发周期延长,条件平均速
度波形的幅值降低、持续时间变短,说明扰动明显抑制了相干结构的猝发过程. 利用子波变
换可以实现湍谱分析,能有效检测猝发中的湍流结构,是一种客观的分析工具. 相似文献
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IntroductionMuchworkhasbeendevotedinthelastfewdecadestothemeasurementandmodelingofthescalinglawofstructurefunctionofturbulentflows.Theso_called“velocitystructurefunctionofordern”forturbulentflowsisdefinedas〈ΔV(r) n〉 ,whereΔV(r) =V(x r) -V(x)isthevelocitycomp… 相似文献
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雷诺应力各向异性涡黏模型的层析TRPIV测量 总被引:1,自引:0,他引:1
利用层析TRPIV测量水洞中平板湍流边界层3D-3C速度场的高分辨率时间序列数据库. 提出了空间局部平均多尺度速度结构函数的新概念, 描述湍流多尺度涡结构的空间拉伸、压缩、剪切变形和旋转. 用空间局部平均多尺度速度结构函数对湍流脉动速度进行了空间多尺度分解. 用空间流向局部平均多尺度速度结构函数, 根据湍流多尺度涡结构在流向的拉伸和压缩物理特征, 提出了新的湍流相干结构条件采样方法, 检测并提取了层析TRPIV数据中相干结构“喷射”和“扫掠”事件中的脉动速度、平均速度变形率、雷诺应力等物理量的空间拓扑形态. 通过研究平均速度变形率各分量与雷诺应力各分量之间的空间相位差异,肯定了壁湍流相干结构雷诺应力各向异性复涡黏模型的合理性. 相似文献
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Velocity statistics along the stagnation line of an axi-symmetric wall stagnating turbulent flow are studied experimentally.
A low turbulence, uniform air flow from a nozzle type air supply with an exit diameter of 50 mm stagnates at a wall located
50 mm downstream. A flow velocity is set to 3 m/s, 10 mm downstream from the exit of the air supply. Instantaneous values
of streamwise and radial velocities are measured by laser-Doppler velocimetry. The turbulence level in the air flow is changed
by use of turbulence generator. When the turbulence generator is not installed in the air supply, the mean velocity profile
in the streamwise direction fits well with that of a laminar viscous flow with the rms value of velocity fluctuations low
near the wall. With the turbulence generator installed, a significant turbulence structure appears near the wall. When the
wall is approached, the rms value of velocity fluctuations in the streamwise direction decreases monotonically while the profile
of the rms value in the radial direction reaches a maximum near the wall. The increase in the rms value of velocity fluctuations
in the radial direction near the wall is attributed to the bi-modal histogram of the fluctuating velocity in the radial direction.
Near the wall, the instantaneous stagnation streamline fluctuates and the probability of the mean location of the stagnation
point reaches a maximum not at the stagnation line but on a circle around the stagnation line, resulting in the bi-modal histogram.
Turbulence statistics, the rms value of velocity fluctuation and the turbulent kinetic energy, can be normalized successfully
by similarity parameters based on the strain rate and the reference turbulent kinetic energy introduced by Champion and Libby.
Received: 7 April 1995/Accepted: 27 September 1996 相似文献
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可压缩各向同性衰减湍流直接数值模拟研究 总被引:5,自引:3,他引:2
采用五阶有限差分WENO格式直接模拟了高初始湍流Mach数的可压缩均匀各向同性湍流,主要分析了湍流的统计特性 和压缩性的影响,包括能谱特征、激波串、耗散率、标度律等. 研究表明,湍动能主要来自于速度场螺旋分量的贡献;各向同性湍流的小尺度脉动对压缩性更为敏感,并且压缩性的增强加快了湍流大 尺度脉动向小尺度脉动的湍动能输运;随着湍流Mach数的升高,胀量(压缩)耗散率所占比率也显著增长. 标度律分析表明,强可压缩湍流的横向速度结构函数仍然具有扩展自相似性;当阶数较高(p ≥ 5)时,纵向速度结构函数的扩展自相似性则不再成立. 对于压缩性较弱的湍流,与不可压缩湍流一致,横向湍流脉动的间歇性要强于纵向湍流脉动;而对于强可压缩湍流,纵向湍流脉动的 间歇性要强于横向湍流脉动. 相似文献
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Dissipation rates of the turbulent kinetic energy and of the scalar variance are underestimated when the measurement resolution of the small scales of a turbulent flow field are insufficient. Results are presented of experiments conducted in a salt-stratified water tunnel (Schmidt number ∼700). Dissipation rates are determined to be underestimated, and thus correction techniques based on velocity structure functions and mixed-moment functions are proposed. Dissipation rates in laboratory experiments of shear-free, grid-generated turbulence are determined from balance calculations of the kinetic energy and scalar variance evolution equations. Comparisons between the structure function and balance estimates of dissipation show that the corrections are O(1) for the kinetic energy dissipation rate, and are O(100) for the scalar variance dissipation rate. This difference is due to the lack of resolution down to the Batchelor scales that is required for a high Schmidt number flow. Simple correction functions based on microscale Reynolds numbers are developed for both turbulent kinetic energy and scalar variance dissipation rates. Application of the technique to the results of laboratory experiments of density stratified turbulence, sheared turbulence, and sheared density stratified turbulence yields successful corrections. It is also demonstrated that the Karman–Howarth equality (and the analogous Yaglom equation) that relates second and third-order structure functions to dissipation rates is valid for both unstrained (decaying grid-generated turbulence) and density stratified and sheared turbulence at least up to the magnitudes of strains of the current experiments Nt∼10, St∼10, respectively. This is helpful for it allows the use of these equations in the analysis of turbulence even when the large scale background profiles of velocity and scalar are unknown. 相似文献
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用三维超声风速计测量了合肥地区风速脉动和温度脉动数据, 从Kolmogorov
的4/5定律估算湍流动能耗散率\varepsilon, 从Yaglom的4/3定律估算温度方差
耗散率\varepsilon _\theta, 指出现有文献提供的公式不适合计算温度场的内
尺度. 提出了一种温度场内尺度计算公式, 式中含有温度方差耗散率\varepsilon
_\theta , 得到的内尺度大小正确反映了与C_n^2 强弱的对应关系. 相似文献