Observations of the atmospheric boundary layer across the land-sea transition zone using a scanning Mie lidar

Optical properties of the atmospheric boundary layer (ABL) above the land-sea transition interface were measured using a scanning Mie lidar located 30 km away from the Adriatic coast. Based on the two-dimensional range-height-indicator scans, detailed information on the ABL was obtained, including parameters such as atmospheric optical depth, aerosol extinction coefficient and the height of the ABL. The presented case study indicates that the height of the ABL in the land-sea transition zone and the adjacent mountainous region was changing rapidly due to highly variable atmospheric conditions.     

Accurate temperature profiling of the atmospheric boundary layer using an ultraviolet rotational Raman lidar

An ultraviolet rotational Raman lidar (RRL) system at an eye-safe wavelength of 354.7 nm is demonstrated for profiling the vertical temperature of the atmospheric layer. Two sets of narrow-band interference filters, which central wavelengths are located at 353.9 and 352.5 nm, respectively, are employed to block the elastic Mie- and Rayleigh-scattering signals and to separate two required rotational Raman signals for temperature retrieval. Experiments were carried out for verifying the feasibility of the prototype of the ultraviolet RRL from May to November in 2008, and the system calibrations were performed with the radiosonde data which were obtain from the local meteorological bureau, and good agreements were got in the experiment results. The observation results show that a statistical temperature error of less then 1 K was obtained up to the heights of 2.5 km for nighttime and 2.0 km for daytime with 300 mJ laser energy and ∼8 min observation time.     

大气边界层各向异性的室内模拟研究

 边界层的各向异性研究对于了解实际大气的湍流特性和经典理论的适用范围均具有较大的意义。利用室内水槽实验模拟了实际大气边界层,使用小波变换方法分析了对数光强起伏谱的特征,将获得的标度指数与理论值进行比较,结果表明混合层在水平方向较为接近各向同性,而垂直方向则呈现一定的各向异性。     

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.     

Scaling the characteristic time of the bursting process in the turbulent boundary layer

Characteristic time scales associated with bursting events in the turbulent boundary layer were examined over a very large range of Reynolds numbers based on momentum thickness of 1010, 2870, 4850, and 5×10⁶. Well-resolved hot-wire measurements were obtained at the lowest three Reynolds numbers in a low-speed wind tunnel with a long development length and compared to hot-wire and sonic anemometer measurements in the near-neutral atmospheric surface layer over the salt flats of Utah’s western desert. Bursting events were detected using the modified U-level threshold-crossing algorithm outlined by Luchik and Tiederman (1987) [1]. The same procedure and codes were used to analyze all time series records from both the wind tunnel and field experiments. The time between events, T_e, and the event duration, ΔT, were calculated and normalized using four different types of scalings: inner, outer, mixed, and Taylor microscales. It was found that both Reynolds number and wall-normal trends in the mode of T_e were eliminated when scaled by the Taylor microscale. Furthermore, constant (Reynolds number independent) values of the nondimensional meanT_e and ΔT were found in a narrow wall-normal region near the top of the buffer layer when the data were normalized by the Taylor microscale.     

Multi-scale interactions in a compressible boundary layer

The properties of spectral subranges of scales in a boundary layer at Mach=2.3 and friction Reynolds number Reτ = 570 are investigated by analysing DNS data. One major aim is to examine whether footprinting and modulation of small-scale near-wall motions by outer large structures, observed at high Reynolds numbers, also pertain to this low-Reynolds-number case, or whether the logarithmic layer simply contains a continuous hierarchy of motions without specific outer scales playing a distinctive role. To this end, the spectrum of scales is decomposed into modes by application of the “Empirical Mode Decomposition”. The properties of different scales are then investigated by means of spectra, maps of isotropy/anisotropy parameters, the premultiplied derivative of the second-order structure function, correlation coefficients and joint probability density function (PDF), the last constructed from conditionally sampled data for the small-scale motions within the large-scale footprints. A clear commonality is identified between interactions in high-Reynolds-number channel flow and the present low-Reynolds-number boundary layer.     

Phase-relationships between scales in the perturbed turbulent boundary layer

The phase-relationship between large-scale motions and small-scale fluctuations in a non-equilibrium turbulent boundary layer was investigated. A zero-pressure-gradient flat plate turbulent boundary layer was perturbed by a short array of two-dimensional roughness elements, both statically, and under dynamic actuation. Within the compound, dynamic perturbation, the forcing generated a synthetic very-large-scale motion (VLSM) within the flow. The flow was decomposed by phase-locking the flow measurements to the roughness forcing, and the phase-relationship between the synthetic VLSM and remaining fluctuating scales was explored by correlation techniques. The general relationship between large- and small-scale motions in the perturbed flow, without phase-locking, was also examined. The synthetic large scale cohered with smaller scales in the flow via a phase-relationship that is similar to that of natural large scales in an unperturbed flow, but with a much stronger organizing effect. Cospectral techniques were employed to describe the physical implications of the perturbation on the relative orientation of large- and small-scale structures in the flow. The correlation and cospectral techniques provide tools for designing more efficient control strategies that can indirectly control small-scale motions via the large scales.     

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.     

Group and phase velocities in the free-surface visco-potential flow: New kind of boundary layer induced instability

Water wave propagation can be attenuated by various physical mechanisms. One of the main sources of wave energy dissipation lies in boundary layers. The present work is entirely devoted to thorough analysis of the dispersion relation of the novel visco-potential formulation. Namely, in this study we relax all assumptions of the weak dependence of the wave frequency on time. As a result, we have to deal with complex integro-differential equations that describe transient behaviour of the phase and group velocities. Using numerical computations, we show several snapshots of these important quantities at different times as functions of the wave number. Good qualitative agreement with previous study [D. Dutykh, Eur. J. Mech. B/Fluids 28 (2009) 430] is obtained. Thus, we validate in some sense approximations made anteriorly. There is an unexpected conclusion of this study. According to our computations, the bottom boundary layer creates disintegrating modes in the group velocity. In the same time, the imaginary part of the phase velocity remains negative for all times. This result can be interpreted as a new kind of instability which is induced by the bottom boundary layer effect.     

激光雷达探测北京城区夏季大气边界层

为了研究北京城区夏季大气边界层结构变化特征及大气边界层内气溶胶消光特性,2004年8月利用便携式米散射激光雷达对北京城区夏季大气边界层进行了系统观测。反演了观测站上空大气气溶胶的消光特性垂直分布以及大气边界层的高度。分析了气象条件和人类活动对大气边界层结构的影响。观测数据表明： 北京城区夏季大气边界层有明显的日变化特征,早晚比较低,日间有一个从低变高再变低的过程,中午前后达到最高。结合气象参数对测量数据进行的统计分析表明：北京城区夏季大气边界层高度相对稳定,多分布在1.8km以下,平均值为0.68km;大气边界层内存在浓度较高的气溶胶粒子,平均光学厚度（3km以内）在0.30左右。     

利用湍流廓线获得斜程大气相干长度的实验研究

 用两种方法获得有限高度斜程大气相干长度。一种方法用安置在测量铁塔上的温度脉动仪以及多普勒声雷达测量了大气边界层下部折射率结构常数廓线，用以计算有限高度球面波斜程大气相干长度；另一种方法用系留气球搭载一个点光源，在地面用差分光学系统接收球面波到达角起伏法测量了实时斜程大气相干长度。对实验结果进行了比较和分析，二者都反映了实际大气斜程相干长度的时间变化特点。两种方法结果的相关性比较好，因此两种方法都可以在大气光学传输研究中应用。最后在实验的基础上对湍流廓线计算球面波斜程大气相干长度作了讨论。     

Local equilibrium in the inertial layer of wall bounded turbulence

Recently Brouwers [Dissipation equals production in the log layer of wall-induced turbulence. Phys Fluids. 2007;19:101702] carried out an asymptotic analysis using the RANS based turbulence energy transport equation and showed that the energy dissipation equals its production in the inertial layer of wall-induced turbulence. Assuming log-law profile to the mean velocity, pressure, viscous and energy diffusion terms were estimated and shown to be negligibly small compared to the production and dissipation terms thereby proving local equilibrium. However, based on scale relations Tennekes and Lumley [A first course in turbulence. Cambridge (MA): MIT Press; 1994] have already established that the pressure and energy diffusion terms appearing in the energy transport equation are of the same order of magnitude especially in the inertial layer thus leading to a contradiction. Hence we have attempted here to re-estimate the turbulence energy budgets in a different way by invoking the Kolomogrov’s similarity hypotheses and (4/5)th law. Magnitudes of pressure and energy diffusion terms are determined explicitly and found to match well with the DNS data. The striking point of the present analysis is that no prior assumption is enforced on the mean velocity profile. Further, two main advantages of the present study are, (i) reasonable estimates for both the diffusion terms are obtained explicitly that were unavailable before and (ii) these estimates help us to tweak the production/dissipation terms to reflect the influence of turbulent diffusion mechanisms without the necessity to model them as in the case of elliptic relaxation and Reynold stress RANS models.     

湍流边界层激励下高速列车车窗参数研究*

高速列车运行过程中,由于结构表面的不平整性产生的湍流边界层激励（TBL）对车厢内部的声场环境产生了一定的影响。采用非相关壁面平面波技术（UWPWs）将随机激励（TBL）转化为一组壁面平面波,利用该激励模型模拟获取了TBL激励下高速列车车窗表面的壁面压力,并将其加载到车窗有限元结构上构建了TBL激励下高速列车车窗模型。分别调查了空腔厚度、双侧玻璃厚度比以及空腔阻尼损耗因子三个参数对车窗声振特性的影响。结果表明：当空腔厚度为20mm,双侧玻璃厚度比为9:5,空腔阻尼损耗因子为0.05时,分别为各自参数下调查工况中的最优解。基于参数调查的结果对车窗结构进行了优化,优化后车窗结构的速度级响应在分析频段内较之前有了显著的削弱,辐射声功率级整体下降了2.8dB。     

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     

大气边界层各向异性的室内模拟研究

边界层的各向异性研究对于了解实际大气的湍流特性和经典理论的适用范围均具有较大的意义。利用室内水槽实验模拟了实际大气边界层,使用小波变换方法分析了对数光强起伏谱的特征,将获得的标度指数与理论值进行比较,结果表明混合层在水平方向较为接近各向同性,而垂直方向则呈现一定的各向异性。     

New expressions for the surface roughness length and displacement height in the atmospheric boundary layer

An alternative model for the prediction of surface roughness length is developed. In the model a new factor is introduced to compensate for the effects of wake diffusion and interactions between the wake and roughness obstacles. The experiments are carried out by the use of the hot wire anemometry in the simulated atmospheric boundary layer in a wind tunnel. Based on the experimental data, a new expression for the zero-plane displacement height is proposed for the square arrays of roughness elements, which highlights the influence of free-stream speed on the roughness length. It appears that the displacement height increases with the wind speed while the surface roughness length decreases with Reynolds number increasing. It is shown that the calculation results based on the new expressions are in reasonable agreement with the experimental data.     

确定大气边界层顶高度的新方法及数值实验

提出了一种确定大气边界层顶高度的数值微分新方法,该方法使用了正则化技术,把对弯角廓线求导数的数值微分问题转化为求目标泛函极小值的问题,采用双参数模型函数方法来选择正则化参数,最后利用最大梯度法确定边界层顶高度.首先通过两个数值实验验证了新方法的有效性,实验结果显示,随着掩星资料噪音的增多,由差分法和结合L曲线方案的数值微分方法得到的边界层顶高度波动增大,而通过双参数模型函数方法得到的高度很稳定,这说明新方法能够很好地过滤噪音,从而保留廓线的主要信息.随后基于2007-2011年1,4,7,10月的COSMIC弯角数据,利用新方法分析了全球海洋大气边界层顶高度的季节特征,并与用掩星资料自带的大气边界层顶高度数据zbalmax得到的季节分布进行对比.结果表明,两者的季节分布特征十分一致:海温相对周围海域高的区域,边界层顶高度较高,反之,边界层顶高度较低;在暖流经过的海域,边界层顶高度较高,在寒流经过的海域,边界层顶的高度相对较低.     

Drag reduction in spatially developing turbulent boundary layers by spatially intermittent blowing at constant mass-flux

A series of large-eddy simulations of spatially developing turbulent boundary layers with uniform blowing at moderate Reynolds numbers (based on free-stream velocity, U_∞, and momentum thickness, θ) up to Re_θ ≈ 2500 were performed with the special focus on the effect of intermittent (separated in streamwise direction) blowing sections. The number of blowing sections, N, investigated is set to be 3, 6, 20, 30 and compared to N = 1, which constitutes the reference case, while the total wall-mass flux is constrained to be the same for all considered cases, corresponding to a blowing amplitude of 0.1% of U_∞ for the reference case. Results indicate that the reference case provides a net-energy saving rate of around 18%, which initially decreases at most 2% points for N = 3 but recovers with increasing N, where the initial reduction of the drag reduction is found to be related to the shorter streamwise length of the intermittent blowing sections. The physical decomposition of the skin friction drag through the Fukagata-Iwamoto-Kasagi (FIK) identity shows that the distribution of all components over each blowing section has similar trends, resulting in similar averaged values over the whole control region.