Turbulent pressure fluctuations in a long cylinder’s boundary layer in the longitudinal flow around it

Frequency spectra of the turbulent pressure’s pulsations measurements have been performed in the low-noise wind tunnel on the extended cylinder’s surface during the longitudinal flow around it. The measurement results for frequency spectra caused by the integral contribution of all the pressure field’s modes as well as only the zero (ring) mode are presented. The obtained results are compared with other known experimental data. The longitudinal cross-spectra of the pressure field for the zero mode have been measured. With the help of Fourier transformation for the cross-spectra the analytical expressions for dimensional and dimensionless wavenumber-frequency spectrum of the zero mode have been obtained. The frequency-wave’s spectrum features considering large wavenumbers are discussed.     

Wall pressure fluctuation spectra due to boundary-layer transition

Boundary-layer transition has been expected to be an important contributor to sensor flow-induced self-noise. The pressure fluctuations caused by this spatially bounded, and intermittent, phenomenon encompass a very wide range of wavenumbers and temporal frequencies. Here, we analyze the wavevector–frequency spectrum of the wall pressure fluctuations due to subsonic boundary-layer transition as it occurs on a flat plate under zero-pressure gradient conditions. Based on previous measurements of the statistics of the boundary-layer intermittency, it is found that transition induces higher low-streamwise wavenumber wall pressure levels than does a fully developed turbulent boundary layer that might superficially exist at the same location and at the same Reynolds number. The transition zone spanwise wavenumber pressure components are virtually unchanged from the fully developed turbulent boundary-layer case. The results suggest that transition may be more effective than the fully developed turbulent boundary layer in forcing structural excitation at low Mach numbers, and it may have a more intense radiated noise contribution. This may help explain increases in measured sensor self-noise when the sensors are placed near the transition zone. We believe, based on the presented analytical calculation and numerical simulation, that the rapid growth and subsequent decay of turbulent spots in the intermittent transition zone causes the higher low-(streamwise) wavenumber spectra.     

Mechanisms for the Formation of Electric-Field Pulsation Spectra in the Near-Surface Atmosphere

We present the results of detailed measurements of the spectrum of short-term (f 0.001-1 Hz) pulsations of the electric field of the near-surface atmosphere under the fair-weather and fog conditions. It is shown that the electric-field pulsations at frequencies 10^-2-10^-1 Hz have a power-law spectrum under both fair-weather and fog conditions. The spectral index varies in a range of from -1.23 to -3.36 depending on the experimental conditions, but the most probable values of the index fall in a range of from -2.25 to -3.0. The spectra corresponding to long time intervals of about a few hours are more steep. The relation of the spectral characteristics to the formation of aeroelectric structures (AESs) is studied. The distribution obtained for the structured spectra is bimodal, i.e., it exhibits two maxima in the ranges of spectral indices from -2.75 to -3.0 and from -2.25 to -2.5. The nonstructured-spectrum distribution is asymmetric and has a pronounced maximum corresponding to hard spectra with indices from -2.5 to -3.3. The intensity of the electric-field pulsations under fog conditions increases by about an order of magnitude compared to the case of fair-weather conditions. The mechanisms of spectrum formation of electric-field pulsations and their relations to the pulsation spectra of the electric-charge density with allowance for the neutral-gas turbulence and the presence of AESs are analyzed. We point out the key role of the nonlocal relation between the electric-field intensity and the space-charge density under conditions of spatially inhomogeneous turbulence. Model problems of the spectrum of electric-field fluctuations generated by a homogeneous and structured turbulence in the presence of charge-density fluctuations, considered as a passive tracer, are solved.     

Measurement of wind noise levels in streamlined probes

This paper investigates the wind noise pressure spectra measured by aerodynamically designed devices in turbulent flow. Such measurement probes are often used in acoustic measurements in wind tunnels to reduce the pressure fluctuations generated by the interaction of the devices with the incident flow. When placed in an outdoor turbulent environment however, their performance declines noticeably. It is hypothesized that these devices are measuring the stagnation pressures generated by the cross flow components of the turbulence. Predictions for the cross flow contribution to the stagnation pressure spectra based on measured velocity spectra are developed, and are then compared to the measured pressure spectra in four different probe type devices in windy conditions outdoors. The predictions agree well with the measurements and show that the cross flow contamination coefficient is on the order of 0.5 in outdoor turbulent flows in contrast to the published value of 0.15 for measurements in a turbulent jet indoors.     

A new model for the cross spectrum and wavenumber-frequency spectrum of turbulent pressure fluctuations in a boundary layer

A model is developed for the statistical characteristics of the turbulent pressure field in a boundary layer. It is shown that the coherence scales are limited at low frequencies by the finiteness of the boundary layer thickness and at high frequencies by the effect of viscous forces acting in the flow. A relationship between the behavior of the coherence scales and that of the power spectra is demonstrated. The model is characterized by convertibility, which allows changes from cross spectra to wavenumber-frequency spectra and back via Fourier transformations. Calculations performed using the proposed model agree well with experimental results.     

Fourier Transform Spectroscopy of the O(2) Herzberg Bands. III. Absorption Cross Sections of the Collision-Induced Bands and of the Herzberg Continuum

Absorption spectra of molecular oxygen were measured in the laboratory under temperature and pressure conditions prevailing in the Earth's atmosphere. Spectra of pure O(2), O(2) + N(2), and O(2) + Ar were recorded in the 41 700 to 33 000 cm(-1) region (240-300 nm) at a maximal optical path difference of 0.45 cm using a Fourier transform spectrometer and a multiple reflection gas cell. The different components of the spectra, namely the discrete bands of the three Herzberg systems, the Herzberg continuum, and the collision-induced diffuse Wulf bands, were separated. The contribution of the Herzberg bands was first subtracted using the line parameters determined previously [A. Jenouvrier, M.-F. Mérienne, B. Coquart, M. Carleer, S. Fally, A. C. Vandaele, C. Hermans, and R. Colin, J. Mol. Spectrosc. 198, 136-162 (1999)] from high-resolution data. Spectra recorded at various pressures then made it possible to determine by linear regression the intensity of the Wulf bands and the Herzberg continuum. The characteristics of the Wulf bands have been investigated in details: vibrational analysis, pressure effect, foreign gas effect, and a simulated spectrum are reported. The Herzberg continuum cross section is determined below the dissociation limit. A comparison with literature data is given. The new O(2) absorption cross sections and O(2)-O(2) collision-induced absorption cross sections are useful in connection with atmospheric measurements of ozone and other trace gases in the UV spectral region. Copyright 2000 Academic Press.     

Experimental study of noise emitted by circular cylinders with large roughness

The aerodynamic noise generated by high Reynolds number flow around a bluff body with large surface roughness was investigated. This is a relevant problem in many applications, in particular aircraft landing gear noise. A circular cylinder in cross-flow and a zero-pressure-gradient turbulent boundary layer with various types of roughness was tested in a series of wind tunnel experiments. It has been shown that distributed roughness covering a circular cylinder affects the spectra over the entire frequency range. Roughness noise is dominant at high frequencies, and the peak frequency is well described by Howe?s roughness noise model when scaled with the maximum outer velocity. There are differences between hemispherical and cylindrical roughness elements for both the circular cylinder and the zero-pressure-gradient turbulent boundary layer cases, indicating a dependence on roughness shape, not described by the considered roughness noise models. Cylindrical roughness generates higher noise levels at the highest frequencies, especially for the zero-pressure-gradient turbulent boundary layer case. Cable-type roughness aligned with the mean flow does not generate roughness noise, and its spectrum has been found to collapse with the smooth cylinder at medium and high frequencies. At low and medium frequencies the noise spectra have the same features as the smooth cylinder, but with higher shedding peak levels and fall-off levels, despite the decrease in spanwise correlation length. Roughness induces early separation, and thus a shift of the spectra to lower frequencies.     

Wave membrane filters for estimating the wave spectra of near-wall pressure pulsations of a turbulent boundary layer in the subconvective region

We have obtained results on measuring the levels of the frequency-wave spectrum of the field of near-wall pressure pulsations of a turbulent boundary layer on the smooth wall of a wind tunnel with the use of stretched plastic membranes as wave filters. We compare the experimental data with the classical model.     

An experimental investigation of unsteady surface pressure on an airfoil in turbulence—Part 2: Sources and prediction of mean loading effects

An experimental investigation into the response of an airfoil in turbulence was undertaken and the results are presented in a two part series of papers. The effects of mean loading on the airfoil response are investigated in Part 1 with the likely origins discussed in this paper (Part 2). Unsteady pressure measurements were made on the surface of a NACA 0015 airfoil immersed in grid turbulence (λ/c=13%) for angles of attack α=0-20°. This paper (Part 2) presents the causes of the low-frequency reduction and high-frequency increase observed in measured lift and pressure spectral levels. Scaling lift spectra on the mean lift reveals the increase in lift spectral level for reduced frequencies greater than 10 is closely related to the airfoils mean pressure field. Based on analysis of the chordwise and spanwise pressure correlation length scale, the reduction in lift spectral level at low reduced frequency appears to result from distortion of the inflow by the mean velocity field. A possible model is developed that accurately predicts mean loading effects on lift spectra. This model uses a circular cylinder fit to the airfoil to compute effects of distortion on the inflow turbulence. The distorted inflow velocity spectrum is then used with Amiet's theory to predict the unsteady loading. This model successfully captures the reduction observed in measured lift spectra at low reduced frequencies. Furthermore, it is shown that the angle of attack effects arising from inflow distortion are significant only when the relative scale of the inflow turbulence to airfoil chord is sufficiently small (λ/c=13% for present experiment).     

Density Fluctuation Spectrum of Solar Wind Turbulence between Ion and Electron Scales

We present a measurement of the spectral index of density fluctuations between ion and electron scales in solar wind turbulence using the EFI instrument on the ARTEMIS spacecraft. The mean spectral index at 1?AU was found to be -2.75±0.06, steeper than predictions for pure whistler or kinetic Alfvén wave turbulence but consistent with previous magnetic field measurements. The steep spectra are also consistent with expectations of increased intermittency or damping of some of the turbulent energy over this range of scales. Neither the spectral index nor the flattening of the density spectra before ion scales were found to depend on the proximity to the pressure anisotropy instability thresholds, suggesting that they are features inherent to the turbulent cascade.     

太湖水体反射率的光谱特征波长分析

水体反射率的光谱特征波长分析是解决内陆水色遥感难题的一项重要的基础工作.该文分析了2006-2009年七次太湖综合实验获取的312个采样点遥感反射率,找出反射率曲线出现极大值、极小值、由凹向凸转变的拐点、由凸向凹转变的拐点对应的特征波长,给出了太湖水体反射率350～900nm范围内的光谱特征波长:359,440,464,472,552,566,583,628,636,645,660,676,689,706,728,791,806和825 nm.最后用浮游植物色素的吸收光谱、太湖水体特有的组成成分解释了特征波长.文章分析特征波长的方法,对于各种光谱曲线的特征波长分析都适用,该方法能够有效地区分曲线峰值所在波长和谷值所在波长重叠的情况.该文的研究成果有利于建立反演水质参数的算法,从而提高反演算法的精度.     

Optical characteristics of a low-density electric discharge bromine vapor plasma in the UV-VUV spectral region

The emission characteristics of a glow discharge in bromine vapor have been investigated in the spectral region 130–350 nm. The current-voltage characteristics and the emission spectra of the glow discharge with an interelectrode gap of 10 cm and a discharge tube 14 mm in inner diameter have been studied. The emission characteristics have been optimized as functions of the bromine vapor pressure and the power deposited into the plasma. It has been shown that, at a low pressure of the bromine vapor, the emission spectrum of the lamp is determined by the spectral lines of atomic bromine in the range 158–164 nm, which are analogous to the known lines (such as those at 206.2 nm) of atomic iodine in an iodine-containing glow discharge plasma. As the pressure of the bromine vapor increases above 100 Pa, the intensity of these emission lines of the bromine atom decreases and the lamp spectrum is formed by bromine molecular bands in the form of a continuum with sharp boundaries (λ = 165–300 nm).     

Measured wavenumber: frequency spectrum associated with acoustic and aerodynamic wall pressure fluctuations

Direct measurements of the wavenumber-frequency spectrum of wall pressure fluctuations beneath a turbulent plane channel flow have been performed in an anechoic wind tunnel. A rotative array has been designed that allows the measurement of a complete map, 63×63 measuring points, of cross-power spectral densities over a large area. An original post-processing has been developed to separate the acoustic and the aerodynamic exciting loadings by transforming space-frequency data into wavenumber-frequency spectra. The acoustic part has also been estimated from a simple Corcos-like model including the contribution of a diffuse sound field. The measured acoustic contribution to the surface pressure fluctuations is 5% of the measured aerodynamic surface pressure fluctuations for a velocity and boundary layer thickness relevant for automotive interior noise applications. This shows that for aerodynamically induced car interior noise, both contributions to the surface pressure fluctuations on car windows have to be taken into account.     

基于极值统计的拉曼光谱信噪比评估方法与应用

随着近年来便携式光谱仪技术的迅速发展,CCD光谱仪相对于传统光谱仪在光谱收集方式上发生了很多变化：（1）采集到的光谱对信号进行叠加积分,传统信噪比评估方法无法通过单次检测获得探测器波动;（2）对于谱图噪声（谱线随机波动）,由探测器响应随机波动和扫描重复误差转变为CCD探测器像素响应差异、探测器随机噪声和与光学系统分辨力有关的模式噪声。噪声类型发生改变,导致原有的光谱质量评价方法适用性变差,基于实测光谱提出更具适应性的光谱质量评价方法具有很强的现实意义。根据拉曼光谱仪检测器的变化,对采集光谱信号的成分进行分析,在该分析的基础上提出了CCD光谱仪的噪声模型假设,根据该假设使用不同的信号极值点频率对不同的噪声进行像素分离,并对噪声频率模式进行了数值模拟,模拟结果与假设相符;在此基础上提出并实验验证了通过谱线极值间距评估谱线噪声的拉曼光谱信噪比评估方法,该方法包括以下两个步骤：（1）通过采集多次实测光谱进行叠加,叠加过程中对对应不同频次的光谱极值点数量进行统计,得到统计结果后基于文中规律分离光谱仪中的环境噪声和暗噪声;（2）应用上述分离结果,对实测光谱中对应暗噪声的谱线极值点作统计平均,再将该值应用于文中公式,计算得到信噪比。该方法在进行了步骤（1）的前期准备后,可以通过单张谱图评估CCD拉曼光谱仪的随机噪声,并用于评估光谱的信噪比。基于光学构架相同、CCD探测器不同的三个拉曼光谱系统进行实验,采用该方法通过设定信噪比阈值对谱图质量进行控制,获得了一致的光谱曲线;基于该方法对同步叠加平均法进行信噪比拟合,拟合优度达到98%。该方法可用于拉曼光谱仪的性能评估和获取拉曼光谱谱图的质量实时控制。理论和实验表明：对于基于CCD探测器的拉曼光谱仪器,当确定样品和特征峰时,可以基于此方法获得信噪比。该方法还可用于比对不同配置的拉曼光谱设备,以及作为控制谱图质量一致性的标准,并对基于拉曼光谱技术的智能鉴别系统的开发具有指导意义。     

Scattering of acoustic waves by small crustaceans

Features of underwater sound scattering by small crustaceans are considered. The scattering data are obtained with the use of unique instrumentation that allows one to measure quantitative scattering characteristics (backscattering cross sections and angular scattering patterns) for crustaceans of different sizes, at different frequencies (20–200 kHz) and different insonification aspects. A computational model of crustaceans is considered with allowance for both the soft tissues of the main massive part of the animal's body and the stiff armour. The model proves to be advantageous for explaining some scattering features observed in the experiments. The scattering cross sections of crustaceans measured by other researchers are presented in a unified form appropriate for comparison. Based on such a quantitative comparison, relatively simple approximate empirical formulas are proposed for estimating the backscattering cross sections of small (within several centimeters) marine crustaceans in a broad frequency range.     

Mixing in a model gas turbine combustor studied by panoramic optical techniques

Using planar optical methods based on laser-induced fluorescence and particle image velocimetry instantaneous velocity fields and passive tracer concentration are measured simultaneously in a model of GT-combustor at realistic flow rates. Spatial distributions of velocity pulsations and passive tracer concentration pulsations are measured at air flow rate about 0.4 kg/s. Correlations of velocity and concentration pulsations are measured. The most intense turbulent mass flux in the region of swirling flow mixing layer was observed. The contribution of advective and turbulent components in the transfer of a passive tracer in the axial direction was estimated.     

Obtaining absorptive line shapes in two-dimensional infrared vibrational correlation spectra

Absorptive line shapes in two-dimensional infrared (2D IR) vibrational spectra are important for an intuitive interpretation of molecular structure and dynamics. We obtain an absorptive 2D IR correlation spectrum by summing complementary spectra from experiments sampling vibrational coherences that oscillate with conjugate frequencies in the initial evolution time period. The 2D correlation spectrum of a coupled vibrational system reveals certain spectral features with tilted line shapes that are explained in terms of unequal contributions of Liouville-space pathways.     

Single-pulse stimulated Raman scattering spectroscopy

We demonstrate the acquisition of stimulated Raman scattering spectra with the use of a single femtosecond pulse. High-resolution vibrational spectra are obtained by shifting the phase of a narrow band of frequencies within the input pulse spectrum, using spectral shaping. The vibrational lines are resolved via amplitude features formed in the spectrum after interaction with the sample. Using this technique, low-frequency Raman lines (<100 cm?1) are observed on both the Stokes and anti-Stokes sides.     

玻璃温室环境下番茄叶绿素含量敏感光谱波段提取及估测模型

为了快速、准确估测番茄叶片叶绿素含量,利用光谱分析技术研究了玻璃温室环境下番茄叶绿素含量敏感光谱波段提取及其估测模型。番茄以基质方式栽培,在结果期使用ASD FieldSpecTM HH型便携式光谱辐射仪采集叶片光谱,并采用752型紫外-可见分光光度计测定其叶绿素含量。从原始光谱、吸光度光谱、一阶微分光谱、去除包络线光谱出发,进行光谱预处理,分析了净化图谱信息、突出作物叶绿素含量光谱特征的有效性。其中,吸光度光谱在可见光部分增强了光谱响应特征,去除包络线光谱和一阶微分光谱均具有较强的蓝光、红光吸收谷和绿光反射峰。又结合波段间自相关分析和多重共线性诊断提取了番茄叶绿素含量敏感光谱波段,原始光谱特征波段为639,672,696,750,768 nm;吸光度光谱特征波段为638,663,750,763 nm;去包络线光谱特征波段为436,564,591,612,635,683,760 nm;一阶微分光谱特征波段为516,559,778 nm。最后,应用4种预处理下的番茄叶绿素含量敏感光谱波段分别建立多元线性回归模型,模型精度由高至低分别为去包络线、吸光度、原始、一阶微分,其中去包络线模型校正集决定系数R2_c为0.88,验证集决定系数R2_v达到0.82,具有较好的预测能力。     

On the hydrodynamic and acoustic wall pressure fluctuations in turbulent pipe flow due to a 90° mitred bend

When the fully-developed turbulent flow in a pipe of circular cross-section is forced to negotiate a 90° mitred bend, flow separation occurs at the inner and outer corners of the bend, with random switching of the separation regions from one side of the plane of symmetry of the bend to the other (as was previously observed by Tunstall and Harvey). The resulting disturbance to the fluctuating pressure field consists of intense non-propagating fluctuations over the region of the inner-wall separation, which near re-attachment have a maximum rms value of about 33% of the undisturbed centre-line dynamic pressure, but are rapidly attenuated with downstream distance from the bend. Beyond about 12 diameters downstream the only remaining disturbance is an acoustic field comprising propagating higher order modes and plane waves, the latter making the larger contribution to the overall mean square pressure. Extensive spectral measurements of the wall pressure field for flow Mach numbers in the range 0·2-0·5 are presented, and regions where higher order modes are detectable are identified. Downstream of the bend, wall pressure spectra generally have two local maxima at frequencies below those at which higher order acoustic modes can propagate. They occur at Strouhal numbers of about 0·4 and 1·6, and turbulent fluctuations at these frequencies in the vicinity of the bend appear to be mainly responsible for the generation of plane acoustic waves. The former Strouhal number does not vary significantly with streamwise position but decreases slightly with increasing flow speed; the latter is somewhat more sensitive to both flow speed and streamwise position. Upstream of the bend wall pressure spectra exhibit only the maximum at a Strouhal number of about 0·4.