三维边界层内定常横流涡的感受性研究

层流向湍流转捩的预测与控制一直是研究的前沿热点问题之一,其中感受性阶段是转捩过程中的初始阶段,它决定着湍流产生或形成的物理过程.但是有关三维边界层内感受性问题的数值和理论研究都比较少;实际工程问题中大部分转捩过程都是发生在三维边界层流中,所以研究三维边界层中的感受性问题显得尤为重要.本文以典型的后掠角45?无限长平板为例,数值研究了在三维壁面局部粗糙作用下的三维边界层感受性问题,探讨了三维边界层感受性问题与三维壁面局部粗糙长、宽和高之间的关系;然后,考虑在后掠平板上设计不同的三维壁面局部粗糙的分布状态、几何形状、距离后掠平板前缘的位置以及流向和展向设计多个三维壁面局部粗糙对三维边界层感受性问题有何影响;最后,讨论两两三维壁面局部粗糙中心点之间的距离以及后掠角的改变对三维边界层感受性的物理过程将会发生何种影响等.这一问题的深入研究将为三维边界层流中层流向湍流转捩过程的认识和理解提供理论依据.     

前缘曲率变化对平板边界层感受性问题的影响

边界层感受性问题是层流向湍流转捩的初始阶段,是实现边界层转捩预测和控制的关键环节.目前已有的研究成果显示,在声波扰动或涡波扰动作用下前缘曲率变化对边界层感受性机制有着显著的影响.本文采用直接数值模拟方法,研究了在自由来流湍流作用下具有不同椭圆形前缘平板边界层感受性问题,揭示椭圆形前缘曲率变化对平板边界层内被激发出Tollmien-Schlichting (T-S)波波包的感受性机制以及波包向前传播群速度的影响;通过快速傅里叶分析方法从波包中提取获得了不同频率的T-S波,详细分析了前缘曲率变化对不同频率的T-S波的幅值、色散关系、增长率、相速度以及形状函数的作用;确定了前缘曲率在平板边界层内激发T-S波的感受性过程中所占据的地位.通过上述研究能够进一步认识和理解边界层感受性机制,从而丰富和完善了流动稳定性理论.     

壁面局部吹吸边界层感受性的数值研究

目前理论、实验以及数值模拟主要研究自由来流中的小扰动与壁面局部粗糙相互作用激发边界层感受性问题. 但是, 针对自由来流湍流与壁面局部吹吸相互作用诱导边界层感受性的相关报道甚少. 本文采用直接数值模拟和快速傅里叶变换的方法, 数值研究了二维平板壁面具有局部吹吸的边界层感受性问题. 结果发现, 在二维边界层内能找到一组被激发产生的Tollmien-Schlichting(T-S)波波包的包络序列以及从波包中能够分离出一组稳定的、中性的和不稳定的T-S波, 证明了二维边界层内感受性现象的存在性. 经数值计算获得了T-S波波包传播的群速度; 并建立了自由来流湍流强度、壁面局部吹吸强度和长度与二维边界层感受性之间的关系, 获得了与Dietz感受性实验相类似的结论. 另外, 还发现在自由来流湍流与壁面局部吹、吸相互作用下能诱导二维边界层内产生相位相反的T-S波. 依据这一理论机理来优化设计局部吹吸装置, 不但能促使层流向湍流转捩的提前, 也可以延迟转捩过程的发生, 达到控制湍流运动的目的.     

粗糙壁面上湍流边界层的直接数值模拟

本文采用多重直接力内嵌边界法计算流体与粗糙壁面之间的相互作用力,直接数值模拟了粗糙壁面上湍流边界层的流动状况。粗糙壁面则通过流向和展向分别叠加正弦函数的方法产生。研究发现,峰值较大的位置优先诱导产生出涡结构,并且加大了涡结构的倾斜角度。此外,较大峰值处的粗糙度所产生出的"喷出"和"扫入"事件更加强烈,其对外层的作用范围也就更大。同时粗糙壁面的峰值大小对于雷诺应力的输运也起着至关重要的作用。     

前缘曲率对三维边界层内被激发出非定常横流模态的影响研究

三维边界层感受性问题是三维边界层层流向湍流转捩的初始阶段,是实现三维边界层转捩预测与控制的关键环节.在高湍流度的环境下,非定常横流模态的失稳是导致三维边界层流动转捩的主要原因;但是,前缘曲率对三维边界层感受性机制作用的研究也是十分重要的课题之一.因此,本文采用直接数值模拟方法研究在自由来流湍流作用下具有不同椭圆形前缘三维（后掠翼平板）边界层内被激发出非定常横流模态的感受性机制;揭示不同椭圆形前缘曲率对三维边界层内被激发出非定常横流模态的扰动波波包传播速度、传播方向、分布规律、感受性系数以及分别提取获得一组扰动波的幅值、色散关系和增长率等关键因素的影响;建立在不同椭圆形前缘曲率情况下,三维边界层内被激发出非定常横流模态的感受性问题与自由来流湍流的强度和运动方向变化之间的内在联系;详细分析了不同强度各向异性的自由来流湍流在激发三维边界层感受性机制的物理过程中起着何种作用等.通过上述研究将有益于拓展和完善流动稳定性理论,为三维边界层内层流向湍流转捩的预测与控制提供依据.     

两相可压缩边界层的颗粒运动特性分析

本文采用高精度直接数值模拟方法,研究了马赫数为2的可压缩湍流边界层中,两种不同粒径颗粒的运动特性.研究发现近壁面的颗粒流向平均速度要小于流体;颗粒具有法向指向壁面的平均速度;近壁面颗粒的流向雷诺正应力要略大于流体:其中大颗粒具有更加显著的上述趋势.小颗粒的法向和展向雷诺正应力基本等于流体,而大颗粒则都要远小于流体.条件...     

粗糙壁面湍流边界层流动和拟序结构的大涡模拟研究

采用LES方法和虚拟粗糙元模型,,模拟了Rer=180有粗糙壁面的充分发展槽道湍流流动.从流向平均速度、脉动速度均方根、雷诺剪切应力、脉动速度相关,以及瞬时近壁准流向涡结构等多个方面,对粗糙壁面湍流边界层流动和拟序结构进行了分析.结果表明,粗糙壁面处雷诺剪切应力、脉动速度均方根以及近壁条带平均距离和准流向涡尺度都大于光滑壁面,且随着粗糙元高度的增加这种趋势更加明显.本工作为进一步研究颗粒在近壁区域的弥散规律奠定了基础.     

湍流边界层内流向涡的数值模拟研究

采用准二维共振三波作为湍流边界层近壁区相干结构初值,用直接数值模拟方法计算了流动从二维结构发展到三维结构并且伴随流向涡生成的整个过程,分析结果显示流向涡对湍流动能和质量传输有着重要作用,是湍流边界层相干结构的重要特征和运动形式.     

湍流边界层对数律区相干结构的Tomo-TRPIV实验研究

应用层析高时间分辨率粒子图像测速技术（tomographic time-resolved particle image velocimetry,Tomo-TRPIV）,测量了Re_θ=2 460的平板湍流边界层三维3分量瞬时速度场的时间序列.提出湍流空间多尺度局部平均速度结构函数的概念,应用流向脉动速度沿流向的空间多尺度局部平均速度结构函数的正负过零点法,从瞬时脉动速度场中检测相干结构的喷射和扫掠事件,对检测到的喷射和扫掠事件的瞬态局部速度场、速度梯度场、涡量场、速度变形率场进行空间相位对齐叠加平均,获得喷射和扫掠事件的局部速度场、速度梯度场、涡量场、速度变形率场的典型特征.研究发现,相干结构喷射和扫掠时,速度梯度、速度变形率、涡量均表现为空间反对称分布的4极子结构.特别是流向涡量是沿流向、法向、展向均为反对称分布的法向多层4极子结构,表明法向各层相干结构是紧密联系,互相关联的.这种法向多层的4极子反对称结构导致强烈的动量、质量和能量交换,维持了相干结构的演化和发展过程.      

水平槽道气固两相湍流边界层的PIV测量

本文运用粒子图像速度场测量仪(PIV)对水平槽道气固两相湍流边界层进行研究.实验雷诺数为Rer=444,选用两种粒径分别为60μm和110μm的聚乙烯颗粒作为固相,质量载荷控制在1×10-3.实验结果表明即便在较低载荷下,固相颗粒的存在仍然能比较明显地改变边界层的湍流属性以及拟序结构,而且可以发现在近壁面颗粒会削弱流向的湍流强度以及雷诺剪切应力.     

Roughness sub-layer investigation of a turbulent boundary layer

Particle image velocimetry (PIV) measurements at relatively low Reynolds numbers in a turbulent boundary layer over a three-dimensional roughened surface, consisting of pyramidal rows, have been presented. Measurements have been taken in a streamwise wall-normal plane intercepting the apex of a row of pyramids and the diagonal of the square base. The results shown in this paper point out the non-homogeneity of the flow in the roughness sub-layer. The different flow behaviour along the ascendant and the descendent part of the pyramids and in the region between two consecutive pyramids has been visualised. Low values of the streamwise component of the mean velocity and high values of the streamwise and wall-normal component of the Reynolds normal stresses, turbulent kinetic energy and Reynolds shear stresses are present in the downstream part of the pyramids, near their base. The mean representation of the flow shows swirling patterns in correspondence of the top of the pyramids. It is suggested that this representation is produced mainly by vortical structures travelling along the wall, retarded and intensified when interacting with the flow around the roughness element and by swirling patterns originating from the interaction of the incoming flow with the pyramids. In order to understand the origin of the high turbulence activity observed downstream the pyramids, near their base, a conditional analysis based on the quadrant method has been performed. It has been shown that sweep events are the major contributor to the very high values of the Reynolds shear stresses observed in this region and are due to the combination of the vortical flow at the top of the pyramid and a large scale (with respect to the roughness element) inflow motion.     

Harmonics of Tollmien — Schlichting waves in a compressible boundary layer

The harmonics of Tollmien—Schlichting waves in a compressible boundary layer of a plate are computed with the aid of nonlinear parabolized stability equations. At the (downstream) growth of the second harmonic amplitude up to the values of the order of the basic harmonic amplitude, the amplification rate of the latter is shown to increase abruptly. A similar rapid deviation from the results of the linear theory characterizes the onset of the boundary-layer transition to turbulent state. Computations are carried out for the Mach numbers M = 0.01 and 2. The work was financially supported by the Russian Foundation for Basic Research (Grant No. 05-01-00079a).     

DNS of compressible turbulent boundary layer over a blunt wedge

Wall turbulence is more complicated than free turbulence, and the direct numerical simulation (DNS) of wall turbulence is more difficult. In recent years, most of DNS cases for wall turbulence are simplified by using temporal mode, where streamwise pe- riodic boundary condition is imposed. In temporal mode, spatial transition will be con- sidered as an analogue of time-evolving transition. For the channel turbulence, an equivalent body force can substitute for the mean gradient of pressure, …     

The evolution of synthesised vortices in turbulent boundary layer

Particle image velocimetry technique has been used to investigate the evolution of synthesised vortical structures in the turbulent boundary layer. Synthetic jet actuator is implemented on the flat plate surface to synthesise various vortical structures by operating the actuator at varying operating parameters. The vortices are issued into the boundary layer and their evolution and subsequent interaction with the relatively less energetic near wall fluid is studied. The investigation is based on the quantitative measurements that are made both on the central and parallel lateral planes. Finally, the enhancement of the wall shear stress resulted in by the passing vortices is measured to evaluate the effectiveness of the actuator towards flow separation control.     

Characteristics of large-scale and superstructure motions in a turbulent boundary layer overlying complex roughness

The structural attributes of turbulent flow over a complex roughness topography are explored using high-frame-rate stereo particle-image velocimetry measurements in the wall-normal–spanwise plane. The roughness under consideration was replicated from a turbine blade damaged by deposition of foreign materials and contains a broad range of topographical scales arranged in a highly irregular manner. Previous results from Barros and Christensen [Observations of turbulent secondary flows in a rough-wall boundary layer. J Fluid Mech. 2014;748] revealed strong spanwise heterogeneity in the flow attributed to the formation of roughness-induced turbulent secondary flows identified by spanwise-alternating low- and high-momentum flow pathways (HMP & LMP, respectively) in the mean flow marked by enhanced Reynolds stresses and turbulent kinetic energy. Frequency spectra of streamwise velocity at fixed wall-normal location presented herein also display strong dependence on spanwise position. In particular, the roughness promotes enhanced energy content of the large-scale and smaller-scale motions (as opposed to very-large-scale ones). Depending on spanwise position, pre-multiplied spectra highlight significant modification of the energy content of the very large-scale motions (superstructures) due to roughness compared to smooth-wall flow. Of note, a shift in both TKE and RSS content to shorter streamwise scales at an LMP was noted, while less of an impact was found coincident with an HMP.     

Investigations of the near-millimeter-wave propagation on surface boundary layer path

Variations of the millimeter waves amplitude and angle-of-arrival caused by an influence of the atmospheric surface layer turbulence and striation were investigated on the 14-km propagation path, where radiotelescope RT-7.5 was used as a receiver antenna operating at the frequency of 90 GHz. In experiments, performed during two-year cycle, a set of statistical characteristics of the vertical and horizontal angle-of-arrival components were studied. Particularly, temporal spectra at the bandwidth 10^–5–10¹ Hz were obtained and an essential anisotropy of turbulence was found.     

Simulations of turbulent asymptotic suction boundary layers

A series of large-eddy simulations of a turbulent asymptotic suction boundary layer (TASBL) was performed in a periodic domain, on which uniform suction was applied over a flat plate. Three Reynolds numbers (defined as ratio of free-stream and suction velocity) of Re = 333, 400 and 500 and a variety of domain sizes were considered in temporal simulations in order to investigate the turbulence statistics, the importance of the computational domain size, the arising flow structures as well as temporal development length required to achieve the asymptotic state. The effect of these two important parameters was assessed in terms of their influence on integral quantities, mean velocity, Reynolds stresses, higher order statistics, amplitude modulation and spectral maps. While the near-wall region up to the buffer region appears to scale irrespective of Re and domain size, the parameters of the logarithmic law (i.e. von Kármán and additive coefficient) decrease with increasing Re, while the wake strength decreases with increasing spanwise domain size and vanishes entirely once the spanwise domain size exceeds approximately two boundary-layer thicknesses irrespective of Re. The wake strength also reduces with increasing simulation time. The asymptotic state of the TASBL is characterised by surprisingly large friction Reynolds numbers and inherits features of wall turbulence at numerically high Re. Compared to a turbulent boundary layer (TBL) or a channel flow without suction, the components of the Reynolds-stress tensor are overall reduced, but exhibit a logarithmic increase with decreasing suction rates, i.e. increasing Re. At the same time, the anisotropy is increased compared to canonical wall-bounded flows without suction. The reduced amplitudes in turbulence quantities are discussed in light of the amplitude modulation due to the weakened larger outer structures. The inner peak in the spectral maps is shifted to higher wavelength and the strength of the outer peak is much less than for TBLs. An additional spatial simulation was performed, in order to relate the simulation results to wind tunnel experiments, which – in accordance with the results from the temporal simulation – indicate that a truly TASBL is practically impossible to realise in a wind tunnel. Our unique data set agrees qualitatively with existing literature results for both numerical and experimental studies, and at the same time sheds light on the fact why the asymptotic state could not be established in a wind tunnel experiment, viz. because experimental studies resemble our simulation results from too small simulation boxes or insufficient development times.     

Analysis of the turbulent boundary layer in the vicinity of a self-excited cylindrical Helmholtz resonator

This study investigates the changes in the structure of a turbulent boundary layer downstream of a flow-excited Helmholtz resonator. To this end, a fully developed turbulent boundary layer over a resonator mounted flush with a flat plate was simulated by implementing a large eddy simulation (LES). To assist in understanding the effect of the resonator on the flow structure, a sensitivity study was undertaken by changing the main geometrical parameters of the resonator. The results demonstrated that when the boundary layer thickness equals the orifice length, the cross-stream component of velocity fluctuations penetrates the boundary layer, resulting in a reduction of the turbulence intensity by up to 12%. Therefore, it is concluded that a Helmholtz resonator has the potential to reduce the instabilities within the boundary layer. These investigations also assist in identifying the optimal parameters to delay turbulence events within the grazing flow using Helmholtz resonators.     

平板边界层中湍流的发生与混沌动力学之间的联系

通过对平板边界层流动中的测量数据的仔细分析,证实了在平板边界层的湍流发生的过程中存在着奇怪吸引子.将这一结论与先前的转捩动力学分析结果相比较,证明了湍流的发生本身具有着混沌动力学本质,从而在平板边界层中的湍流发生与混沌之间建立了联系 关键词： 边界层 转捩 湍流的发生 混沌 奇怪吸引子