Turbulence velocity spectra dependence on the mean wind at the bottom of a valley

A spectral analysis of the surface boundary layer turbulence at the bottom of a valley is presented. The spectra were classified according to wind speed, wind direction with respect to the valley axis and stability condition. The results reveal that the vertical velocity spectra have a well defined peak in all cases. The average spectra are better defined for mean winds parallel to the valley axis. For transversal winds, a large scatter occurs. The peak of the vertical velocity spectra happens at smaller frequencies for winds parallel to the valley axis than when the winds are transversal to the valley. The stability dependence of the spectral peak frequency of the vertical component is larger for winds larger than 1 m s⁻¹. The spectra of the horizontal components show low-frequency peaks, probably associated to mesoescala exchange. The frequency peak of the horizontal spectra does not depend on stability for convective conditions. For stable conditions, the peak frequency increases as it gets more stable. Local isotropy occurs at large frequencies for both wind directions with respect to the valley axis.     

Climatological and dynamical aspects of the wind regiome in the Strait of messina

Summary The anemometric measurements recorded for more than one year on two 232 m high towers supporting the power lines crossing the Strait of Messina are analysed. The results indicate several meteo-climatic characteristics of considerable interest to the construction of the bridge over the Strait. It is apparent that: 1) the surrounding orography, although several km from the points of measurement, located on the north side of the Strait, plays the main role in determining wind distribution, from ground level up to a height of several hundred metres. In practice the wind blows from four directions 30 degrees each. 2) Two of these (210 and 330 degrees) display velocity distributions of specific interest for the construction of the bridge. 3) The wind blowing up the Strait (210 degrees) is the strongest in all seasons; it has a logarithmic vertical profile, low turbulence and for the same altitude maintains a constant ratio between vertical velocity (always ascending) and horizontal velocity. 4) The NW wind is more turbulent, has a nonlogarithmic profile and also has a constant ratio between horizontal and vertical velocity.     

Effect of gravity on the development of homogeneous shear turbulence laden with finite-size particles

Fully resolved simulations of homogeneous shear turbulence (HST) laden with sedimenting spherical particles of finite size have been performed to clarify the effects of gravity on the development of particle-laden turbulent shear flows. We consider turbulence in a horizontal flow subjected to vertical or horizontal shear. Numerical results show that the development of HST laden with finite-size particles are significantly altered by gravity. The effects of gravity lead to a slower increase in the Taylor-microscale Reynolds number, whose value is found to be well correlated with the average particle Reynolds number. The gravity also causes a slower increase in the turbulence kinetic energy (TKE) through the enhancement of energy dissipation. The change in the Reynolds shear stress (RSS) due to particles also significantly contributes to the relative change in TKE. In vertically sheared cases, RSS has high values between counter-rotating trailing vortices behind the particles, which causes a transient relative increase in TKE. In horizontally sheared cases, on the other hand, RSS is reduced in the wakes of particles, which contributes to a significant relative reduction in TKE.     

超声速湍流边界层湍流统计与湍流结构分析

可压缩边界层转捩问题与湍流问题一直是制约高超声速飞行器发展的关键基础问题,也是近年来流体力学领域研究的热点问题.采用直接数值模拟方法,获得了空间发展的Ma=2.25超声速湍流边界层流场,通过对湍流边界层的发展状态进行评估,得出有效的Reynolds数Reθ 范围约为2600～4600.对壁面摩阻系数开展了分解,获得了各...     

Eulerian spatial and temporal autocorrelations: assessment of Taylor's hypothesis and a model

We present measurements of Eulerian longitudinal velocity autocorrelations in homogeneous, isotropic, high-intensity (～9%) free-stream turbulence behind an active grid. Spatial correlations are measured using particle image velocimetry as well as with two-point hot-wire anemometry (HWA), while temporal correlations are measured using HWA. The temporal correlations are transformed into spatial correlations by using Taylor's ‘frozen’ hypothesis with both the mean as well as instantaneous velocities. A model relating Eulerian spatial and temporal autocorrelations is also used for this purpose. The differences from the measured spatial correlation resulting from the use of Taylor's hypothesis on the temporal correlation is quantified; even at this moderately high level of turbulent intensity, the result from the use of the instantaneous velocity as convection velocity is practically indistinguishable from that obtained using the mean velocity. Use of the model produces a good agreement between the estimates of the spatial correlation function. A relation between Eulerian spatial and temporal integral scales is also derived.     

Measuring wind speeds and turbulence with a wave-front sensor

We analyze wave-front sensor (WFS) measurements taken with the 1.5-m telescope at the Starfire Optical Range in Albuquerque, N.M., of wind speeds in the turbulent atmospheric layers that cause seeing. The frozen-flow hypothesis suggests that atmospheric turbulence is located in thin horizontal layers and that turbulent features do not change over short time scales but are drawn along by the prevailing wind. Exploiting autocorrelation properties of the WFS data that result from these characteristics of atmospheric turbulence, we are able to measure the movements of individual layers. We also test the validity of the frozen-flow hypothesis.     

On the use of spires for generating inflow conditions with energetic coherent structures in large eddy simulation

While it has long been a practice to place spires near the inlet of a wind tunnel to quickly develop a turbulent boundary layer with similarities to an atmospheric boundary layer, this has not been the case for creating turbulent boundary layer inflow in large eddy simulations (LESs) of turbulent flows. We carry out LES with the curvilinear immersed boundary method to simulate the flow in a wind tunnel with a series of spires in order to investigate the feasibility of numerically developing inflow conditions from a precursory spire LES and assessing the similarities of the turbulence statistics to those of an atmospheric boundary layer. The simulated mean velocity field demonstrates that a turbulent boundary layer with height equal to the spire height develops very quickly, within five spire heights downstream. The major attribute of using spires for precursory simulations is the spatially evolving coherent structures that form downstream of the spires offering a range of length scales at both the vertical and streamwise directions allowing multiple turbulent inflow conditions to be extracted from a single simulation. While the distribution of length scales far from the spires resembles an atmospheric boundary layer, some turbulence statistics have some significant differences.     

Effect of gap flow on the shallow wake of a sharp-edged bluff body – turbulence parameters

This experimental study was carried out to investigate the turbulent wake generated by a vertical sharp-edged flat plate suspended in a shallow channel flow with a gap near the bed. The objective of this study is to understand the effect of the gap flow on the turbulent wake by studying two different gap heights between the channel bed and the bottom edge of the bluff body. These two cases were compared to the no-gap case which is considered as a reference case. The maximum flow velocity was 0.45 m/s and the Reynolds number based on the water depth was 45,000. Extensive measurements of the flow field in the vertical mid-plane and in the horizontal near-bed, mid-depth, and near-surface planes were made using particle-image velocimetry (PIV). This paper is the second part of an extensive study to characterise the gap-flow effects and is primarily focused on the mean and instantaneous turbulence quantities as well as coherent structures.

The results revealed that the gap flow increased the transfer of the turbulent kinetic energy (TKE) from the streamwise to the vertical component along the vertical mid-plane. In addition, there is a corresponding increase and spread of the transverse component in the transverse direction as the flow evolves in the downstream direction. The momentum exchange by the Reynolds stress is significantly weak in the vertical mid-plane particularly in the lower half of the water depth, but the gap flow enhanced the momentum exchange in the upper half of the water depth by up to 1% of the freestream velocity squared. Furthermore, the intensity and bursting direction of the turbulence fluctuations in the far field are also affected by the gap flow when it is large. Furthermore, the proper orthogonal decomposition results revealed that the flow contains a large number of structures, and their interactions are responsible for deforming and/or tearing apart the structures, and transferring fluid throughout the velocity field.     

Effect of surface roughness element on near wall turbulence with zero-pressure gradient

In the present paper,we present an investigation on the effect of roughness elements onto near-wall kinematics of a zeropressure-gradient turbulent boundary layer.An array of spanwisely-aligned cylindrical roughness elements was attached to the wall surface to regulate the near-wall low-speed streaky structures.With both qualitative visualization and quantitative measurement,we found that the regularization only occurs in the region below the height of the roughness elements.Statistical analysis on the probability distribution of the streak spanwise spacing showed that the mean spanwise streak spacing is dominated by the roughness elements;however,the latter's effect is in competition with the intrinsic streak generation mechanisms of smooth wall turbulence.Below the top of the roughness elements,local streamwise turbulent fluctuation intensity can be reduced by about 10%.We used POD analysis to depict such regularization effect in terms of near-wall structure modulation.We further found that if the spanwise spacing of roughness elements increased to be larger than the mean streak spacing in the smooth wall turbulence,there is no streak-regularization effect in the buffer region,so that the near-wall streamwise turbulent fluctuation intensity doesn't reduce.     

On the scale-to-scale coupling between a full-scale wind turbine and turbulence

The scale-dependent response of an instrumented full-scale wind turbine is studied under neutrally stratified conditions. The analysis is focused on the linkage between the incoming flow, turbine power output and foundation strain. Wind speed, measured from sonic anemometers installed on a meteorological tower, and foundation strain were sampled at 20 Hz, while the turbine power was sampled at 1 Hz. A wavelet framework and structure function are used to obtain cross correlations among flow turbulence, turbine power and strain across scales as well as to quantify intermittent signatures in both flow and turbine quantities. Results indicate that correlation between the streamwise velocity component of the wind flow and turbine power is maximised across all scales larger than the rotor radius for wind measured at the turbine hub height. The characteristic time lag associated with maximum correlation is shown to be consistent with the Taylor’s hypothesis for turbulent scales smaller than the separation between the meteorological tower and the turbine. However, it decreases with increasing scale size and diminishes to zero at scales on the order of the boundary layer thickness. Turbine power and strain fluctuations exhibited practically the same behaviour at scales larger than two rotor diameters. At those scales, the cross correlation between these quantities resulted ～0.99 and remains still over 0.9 at the scale of rotor radius. Below this scale, the correlation decreases logarithmically with scale. The strong linkage between power and strain for all the relevant scales would eventually allow the analysis of dynamic forcing on the foundation based on the power output. Intermittency on the flow is shown to be transferred and amplified by the turbine, leading to highly intermittent power output.     

随机分布粒子侧向散射光特性的实验研究

两种不同直径(0．22μm和0．494μm)的粒子与过滤的蒸馏水制作不同体积分数的悬浮液作为散射介质。比较了微粒子群侧向散射光中垂直与水平两种偏振光的强度分布。实验结果表明，直径大的粒子其侧向散射光中水平方向线偏振强度远远小于直径小的粒子，而垂直方向线偏振强度却完全相反，远远大于直径小的粒子。     

扰流板作用下湍流/非湍流界面特性

在零压梯度平板湍流边界层流动中安装垂直流向高度为h的扰流板,诱导流场产生横向涡,研究横向涡影响下湍流/非湍流界面特性沿流向的发展。结果表明,在本实验条件下,整个流动经历了从湍流边界层到流动分离和再附,再向湍流边界层恢复的过程。在扰流板下游约18h距离后,扰流板尾迹的影响逐渐衰退,壁面剪切对湍流强度的贡献开始逐渐恢复,在扰流板下游约55h距离后,湍流边界层再次充分发展。与此同时,由于扰流板后流场流动结构拟序性的增强,湍流/非湍流界面的分形维度受扰流板影响而减小,表明脱落涡有使界面多尺度特性、三维性减弱的趋势。此外,界面高度的概率密度分布受扰流板影响呈现显著的右偏,主要与扰流板增强喷射运动强度,使得界面更容易抬升相关。流动结构及界面特性受扰流板影响后的流向演化有同步变化的模式,扰流板对界面特性影响主要集中于（-5~18）h的流向范围。      

Phenomenology of two-dimensional stably stratified turbulence under large-scale forcing

In this paper, we characterise the scaling of energy spectra, and the interscale transfer of energy and enstrophy, for strongly, moderately and weakly stably stratified two-dimensional (2D) turbulence, restricted in a vertical plane, under large-scale random forcing. In the strongly stratified case, a large-scale vertically sheared horizontal flow (VSHF) coexists with small scale turbulence. The VSHF consists of internal gravity waves and the turbulent flow has a kinetic energy (KE) spectrum that follows an approximate k^?3 scaling with zero KE flux and a robust positive enstrophy flux. The spectrum of the turbulent potential energy (PE) also approximately follows a k^?3 power-law and its flux is directed to small scales. For moderate stratification, there is no VSHF and the KE of the turbulent flow exhibits Bolgiano–Obukhov scaling that transitions from a shallow k^?11/5 form at large scales, to a steeper approximate k^?3 scaling at small scales. The entire range of scales shows a strong forward enstrophy flux, and interestingly, large (small) scales show an inverse (forward) KE flux. The PE flux in this regime is directed to small scales, and the PE spectrum is characterised by an approximate k^?1.64 scaling. Finally, for weak stratification, KE is transferred upscale and its spectrum closely follows a k^?2.5 scaling, while PE exhibits a forward transfer and its spectrum shows an approximate k^?1.6 power-law. For all stratification strengths, the total energy always flows from large to small scales and almost all the spectral indicies are well explained by accounting for the scale-dependent nature of the corresponding flux.     

Effects of spanwise system rotation on turbulent stripes in a plane Poiseuille flow

In the transitional channel flow, the large-scale intermittent structure of localised turbulence, which is called the turbulent stripe pattern, can be found in the form of stripe arrangement. The structure of the turbulent stripe pattern is an oblique laminar–turbulent banded pattern and is inclined with respect to the streamwise direction. We performed direct numerical simulation at a transitional Reynolds number and very low-rotation numbers, and focused on the turbulent stripe pattern in the plane Poiseuille flow subjected to spanwise system rotation. We captured the turbulent stripe pattern in a rotating channel flow and found the augmentation and diminution of the turbulent stripe pattern were affected by the spanwise rotation. The contents of the discussion are the spatial size of the turbulent stripe pattern on the basis of the instantaneous flow fields, the energy spectra, and various statistics relating to the spanwise velocity component that characterise the turbulent stripe pattern. The turbulent stripe pattern was found to contain kinetic energy that was larger in very weakly rotating flows than in the static system. It was also found that the magnitude of the spanwise secondary flow increases, while the quasi-laminar region is wider at a very lowrotation number.     

Anisotropic third-moment estimates of the energy cascade in solar wind turbulence using multispacecraft data

The first direct determination of the inertial range energy cascade rate, using an anisotropic form of Yaglom's law for magnetohydrodynamic turbulence, is obtained in the solar wind with multispacecraft measurements. The two-point mixed third-order structure functions of Els?sser fluctuations are integrated over a sphere in magnetic field-aligned coordinates, and the result is consistent with a linear scaling. Therefore, volume integrated heating and cascade rates are obtained that, unlike previous studies, make only limited assumptions about the underlying spectral geometry of solar wind turbulence. These results confirm the turbulent nature of magnetic and velocity field fluctuations in the low frequency limit, and could supply the energy necessary to account for the nonadiabatic heating of the solar wind.     

Direction finding and suppression of vector-scalar sound signals in shallow water taking into account their correlation and mode structure

The correlation of low-frequency sound signals from towed tonal low-frequency sources at the output of the scalar and vector channels is studied in shallow water. The correlation of the scalar field and signal received by a horizontally oriented vector receiver on average is 0.92–0.99; correlation with the signal received by a vertical vector receiver decreases to 0.66–85. When scalar fields or horizontal projections of the vibration velocity vector with application of the aperture synthesis algorithm are used, 3–5 normal waves are isolated; when the vertical component is used, 7–9 modes. It is demonstrated that the high signal correlation ensures direction-finding accuracy and suppression of strongly noise-emitting moving sources by 20–30 dB or more if the cardioid is directed at the source according to the zone of the minimum.     

A computational study of the effect of windscreen shape and flow resistivity on turbulent wind noise reduction

In this paper, numerical simulations are used to study the turbulent wind noise reduction effect of microphone windscreens with varying shapes and flow resistivities. Typical windscreen shapes consisting of circular, elliptical, and rectangular cylinders are investigated. A turbulent environment is generated by placing a solid circular cylinder upstream of the microphone. An immersed-boundary method with a fifth-order weighted essentially non-oscillatory scheme is implemented to enhance the simulation accuracy for high-Reynolds number flow around the solid cylinder as well as at the interface between the open air and the porous material comprising the windscreen. The Navier-Stokes equations for incompressible flow are solved in the open air. For the flow inside the porous material, a modified form of the Zwikker-Kosten equation is solved. The results show that, on average, the circular and horizontal ellipse windscreens have similar overall wind noise reduction performance, while the horizontal ellipse windscreen with medium flow resistivity provides the most effective wind noise reduction among all the considered cases. The vertical ellipse windscreen with high flow resistivity, in particular, increases the wind noise because of increased self-generation of turbulence.     

Dynamics and structure of unstably stratified homogeneous turbulence

We characterise the properties of unstably stratified homogeneous turbulence by means of high-resolution direct numerical simulations and a two-point statistical spectral model based on a quasi-normal closure proposed by Burlot et al. Both approaches agree very well regarding the evolution of one- and two-point turbulent statistics, showing that the model is valid at even higher Reynolds numbers than previously considered. From a parametric study with different initial conditions, we confirm that the energy distribution at large scale influences strongly the late time dynamics of the flow. In particular, we assess the existence of backscatter transfer of energy, and evaluate its role in the growth rate of several turbulent quantities. Moreover, thanks to the statistical model, we analyse the scale-by-scale anisotropy of the flow through the decomposition of turbulent spectra in terms of directional anisotropy and polarisation anisotropy, for a refined characterisation of the structure of the flow which is strongly anisotropic in the large scales. This also allows us to study how isotropy is restored in the inertial scales.