The influence of boundary layer on sound propagation in optical fibers

Physical models of two-layered and three-layered fiber were built. These models were used to calculate acoustic properties of optical fibers. Acoustic properties of fibers with boundary layer and without boundary layer calculated from these model were compared. The propagation of acoustic cylindrical symmetric waves in optical fibers was considered. This problem was treated analytically and numerically in continual approximation. The possibility of simultaneous propagation of two cylindrical symmetric waves in optical fibers with a boundary layer was shown (in a three-layered model). The physical phenomenon of the presence of two waves in the fiber with boundary layer is proposed for study of the boundary layer. Formulas for the calculation of the properties of the boundary layer from the acoustic experiment, verified by numerical calculations, are represented.     

Simulation of a morning air temperature inversion break-up in complex terrain and the influence on sound propagation on a local scale

A mesoscale atmospheric model is used to model the break up of a morning air-temperature inversion during a clear weather situation with low wind speeds at ground. Modified slope-radiation parameterization in the model results in more realistic predicted air temperature profiles when compared to profiles measured with a tethered balloon. A wave number integration code is used to demonstrate how the modelled atmospheric profiles can be used to predict the reduction of sound level along ground during inversion break-up.     

浅海起伏海面下气泡层对声传播的影响

分析了起伏海面下风浪引起的气泡层对海面反射损失和对声传播的影响.一方面,气泡层会改变原来水中的声速剖面;另一方面,气泡层会对声波产生散射和吸收作用.考虑以上两方面的因素,分析了不同风速下气泡层对海面反射损失和声传播损失的影响,仿真发现,在风速大于10 m/s时,对于2 k Hz以上频率时气泡层对小掠射角下海面反射损失的影响不可忽视.在给定的水声环境中,当声源深度和接收深度都为7 m时,风速为16 m/s的风浪下生成的气泡层,在10 km处对3 k Hz的声传播损失的影响达到8.1 d B.当声源深度和接收深度都为18 m时,风速为16 m/s的风浪下生成的气泡层,在10 km处对3 k Hz的声传播损失的影响达到4 d B.     

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.     

Characterizing the relative role of low-frequency and turbulent processes in the nocturnal boundary layer through the analysis of two-point correlations of the wind components

The study presents an analysis of two-point correlations between time series of nocturnal atmospheric wind, obtained from two micrometeorological towers, 45 m horizontally apart, each equipped with two sonic anemometers, 2.5 m vertically apart. It focuses on the scale dependence of the two-point correlations obtained from sensors vertically and horizontally separated. In particular, the role of low-frequency non-turbulent processes in the correlations is assessed, and compared to that of the turbulent scales of motion. The vertical correlations of the streamwise and vertical wind components show little dependence on the turbulence intensity, but those of the spanwise component decrease appreciably as it gets more turbulent. Multiresolution decomposition shows that the two-point correlations become increasingly dominated by low-frequency scales as it gets less turbulent, and that such large-scale processes are largely reduced in fully turbulent conditions. It is also shown that the vertical correlations of the spanwise wind component is negative for very small time scales. Horizontal two-point correlations obtained at the 45 m separation distance between the towers are almost entirely dominated by low-frequency motions, regardless of the turbulence intensity, but the magnitude of such correlations decreases with increasing turbulence intensity for any wind components. A comparison between the horizontal two-point correlations and autocorrelations taken with a time lag given by the ratio of the horizontal separation to the mean wind component in the direction that connects the two towers leads to the conclusion that the statistical properties of turbulence are often preserved over the horizontal distance, despite the lack of turbulence correlations for that separation.     

Effect of Mesoscale eddies on underwater sound propagation

The effects of sound speed variables induced by an anticyclonic eddy and a field of three cyclonic eddies on long-range sound propagation are investigated respectively. A deep-sea analytical eddy model [Henrick RF, Siegmann WL, Jacobson MJ. General analysis of ocean eddy effects for sound transmission applications. J Acoust Soc Am 1977;62:860-870] is used to determined sound speed distributions produced by warm-core ring in the southwest of South China Sea. Furthermore, the above analytical eddy model is generalize to include the azimuth angle variation and is used to determined sound speed distributions produced by Gulf Stream rings with different strengths. The theoretical temperature fluctuations induced by the warm eddy and a eddy field including three cyclonic eddies agree qualitatively with the in situ investigation data. The transmission loss of acoustic energy through the cross section of the warm-core ring center and three cyclonic eddy centers is simulated using 2-D parabolic-equation (PE) numerical modeling. It is found that the acoustic field has a significant change with variation of the location of SOFAR axis in the presence of the warm eddy and three cyclonic eddies comparing with the scenario of no eddies. When the source is located in the outside of the warm-core eddy and three cyclonic eddies respectively, and the receiver is located in outside of the eddy, the transmission loss as a function of range is investigated at different receive depth. It is shown that the changes of transmission loss caused by the warm-core eddy and three cyclonic eddies are as much as 20 dB than that of no-eddy situation. In the case of three cyclonic eddies, the largest discrepancy of transmission loss is about 40 dB near the range of 45 km for a 25-Hz source being located at a depth of 1500 m.     

深海表面层声速剖面变化对声传播的影响

在我国部分海域，表面声速常常在较短的时间内从弱正梯度变为弱负梯度，这将对表面附近的声传播产生巨大影响。本文通过数值模拟的方法，讨论了表面层声速梯度变化对声传播的影响，同时解释了某次海上实验出现的异常现象：在同一地点同一航向，前后两天海况相同，水文相差不大，第一天的实验可以在50km之外接收到信号，而第二天只能在30km处才能收到信号。     

Intratympanic manganese administration revealed sound intensity and frequency dependent functional activity in rat auditory pathway

The cochlear plays a vital role in the sense and sensitivity of hearing; however, there is currently a lack of knowledge regarding the relationships between mechanical transduction of sound at different intensities and frequencies in the cochlear and the neurochemical processes that lead to neuronal responses in the central auditory system. In the current study, we introduced manganese-enhanced MRI (MEMRI), a convenient in vivo imaging method, for investigation of how sound, at different intensities and frequencies, is propagated from the cochlear to the central auditory system. Using MEMRI with intratympanic administration, we demonstrated differential manganese signal enhancements according to sound intensity and frequencies in the ascending auditory pathway of the rat after administration ofintratympanicMnCl₂.Compared to signal enhancement without explicit sound stimuli, auditory structures in the ascending auditory pathway showed stronger signal enhancement in rats who received sound stimuli of 10 and 40 kHz. In addition, signal enhancement with a stimulation frequency of 40 kHz was stronger than that with 10 kHz. Therefore, the results of this study seem to suggest that, in order to achieve an effective response to high sound intensity or frequency, more firing of auditory neurons, or firing of many auditory neurons together for the pooled neural activity is needed.     

Acoustic boundary layer influence on scale model simulation of sound propagation: Theory and numerical examples

Scale model simulation of sound propagation above a solid surface will give a systematic and calculable error in the predicted sound field, because the acoustic boundary layer above the surface has an apparent admittance which is not invariant under scaling. The typical error is approximately 5 dB depending on the geometrical configuration, scale and frequency. The effect of the acoustic boundary layer admittance is negligible for sound propagation above an acoustically soft surface (e.g., grassland). One may moreover note, with reference to scale model simulation of concert hall acoustics, that the absorption coefficient of a solid surface increases with frequency.     

Applicability of locally reacting boundary conditions to porous material layer backed by rigid wall: Wave-based numerical study in non-diffuse sound field with unevenly distributed sound absorbing surfaces

To clarify the applicability of locally reacting boundary conditions in wave-based numerical analyses of sound fields in rooms, we numerically analyzed a non-diffuse sound field in a room with unevenly distributed sound absorbing surfaces and investigated the differences between the extended and local reactions. Each absorbing surface was a porous material layer backed by a rigid wall. Simulations were performed by the fast multipole boundary element method, a highly efficient boundary element method using the fast multipole method. At low frequencies, the extended and local reactions yielded similar reverberation decay curves because of the influence of the room. However, when the random incidence absorption coefficients were small at low frequencies or frequencies were high, the difference was greater than expected from the corresponding Eyring decay lines. We conclude at high frequencies, the locally reacting boundary conditions lead to a longer reverberation time than that expected from the absorption coefficient differences between the extended and local reactions. These differences were similar in sound-pressure-level and sound-intensity-level distributions, and in the oblique incidence absorption coefficient of the absorbing surfaces, but were increased at low frequencies.     

Effect of wall temperature on hypersonic turbulent boundary layer

Turbulent boundary layers at Mach 4.9 with the ratio of wall temperature to recovery temperature from 0.5 to 1.5 are investigated by means of direct numerical simulation. Various fundamental properties relevant to the influence of wall temperature on Morkovin’s scaling, standard and modified strong Reynolds analogies, and coherent vortical structures have been studied. It is identified that the scaling relations proposed for cool and adiabatic wall conditions, such as Morkovin’s scaling and the modified strong Reynolds analogy, are also applicable for hot wall condition. Moreover, the relation between the density and temperature fluctuations under the second-order approximation is derived and verified to provide a reliable prediction. Based on the analysis of coherent vortical structures, it is found that the orientation of vortex core can be quantitatively determined by means of the vector with its direction and modulus using the local strain direction and the imaginary part of the eigenvalue of velocity gradient tensor, respectively. As the increase of wall temperature, the spanwise distance between the two legs of hairpin vortex increases, and the mean swirling strength and the angle of vortical structure with respect to the wall plane also increase in the inner layer. The statistical properties relevant to vortical structures are nearly insensitive to the wall temperature in the outer layer.     

利用海洋环境噪声估计浅海海底表层声速

为提高反演效率,提出一种快速估计浅海海底表层声速的方法。根据噪声能流理论,垂直阵接收的海洋环境噪声数据能够用于无源提取海底反射损失,反射损失曲线中具有明显的临界角效应,从而接估计海底表层声速。以射线模型为基础,推导了噪声提取的反射损失与理论值之间的差异,并讨论实际阵列波束形成在不同角度和频率下的性能。考虑到非等声速环境下声线会发生弯曲,需对角度进行修正以提高方法的广泛适用性。不同频率的临界角与有效深度之间存在对应关系,黄海某海区数据处理结果表明,在临界角不变的有效深度内,海底表层可以视为等声速层,该海区海底表层0.5 m内声速估计结果为1547 m/s,与有源反演结果相近。     

Atmospheric stability of surface boundary layer in coastal region of the Wol–Ryong site

In order to provide statistically reliable information of a wind energy site, accurate analysis on the atmospheric stability and climate characteristics in a certain area is a prerequisite. Two 2-D ultrasonic anemometers and one cup anemometer, located perpendicular to the prevailing wind direction, were used to measure the atmospheric wind environment at a height of 4.5 m in coastal region of the Wol–Ryong, Jeju, South Korea. The study is aiming to understand the atmospheric stability about a coastal region, and the effect of roughness length. We calculate the Monin–Obukhov length for division of atmospheric stability about unstable regime, neutral regime and stable regime. The distribution of diurnal Monin–Obukhov length is highly sporadic in the coastal region due to the effect of radiant heat from the surface or other environmental effects. In order to calculate the roughness length in coastal region, three different methods are applied in terms of the surface roughness, flow fluctuation and gust wind, which are called logarithmic profile, standard deviation and gust factor methods. In the study, the atmospheric stability was insignificant when applying these three methods. In the results, three different roughness length scales sufficiently showed the effect of obstacle and surface conditions around the measurement position. On the basis of an overall analysis of the short-term data measured in the Wol–Ryong area, Jeju Island, it is concluded that for the development of future wind energy resources, the Wol–Ryong site could be a good candidate for a future wind energy site.     

The effect of multiple branches on sound propagation in long enclosures

Since the classical room acoustics can not be used for long enclosures due to the inhomogeneous sound field, much work has been carried out recently to investigate the sound propagation in long enclosures, which are helpful to the acoustic design of practical long enclosures, such as the high-speed railway tunnels. However, most of these works focuses on the straight long enclosures without branches or with one branch. In this paper, the effects of the multiple branches on sound propagation of long enclosures are studied. The sound pressure level (SPL) attenuation, early decay time (EDT), and reverberation time (T30) of long enclosures with multiple branches have been investigated by physical scale models based acoustic experiments. Several interesting results have been obtained concerning the sound propagation of long enclosures with multiple branches. It shows that the sound field of long enclosures with multiple branches is more complex and inhomogeneous than that of the long enclosures without branches or with one branch.     

Study on the effect of sound duration on the annoyance of helicopter noise by applying a technique of time compression and expansion of sound signals

The number of helicopter operations has rapidly increased during the last 20 years in Japan. Helicopter noise sounds different from other aircraft; the waveform of the sound pressure is impulsive and the signal duration is relatively long. The Environmental Agency of Japan implemented new guidelines for evaluating noise exposure around small airports, including heliports, in 1990. This study was executed in connection with the development of provisional guidelines. Psychoacoustic experiments were carried out to identify an evaluation index for helicopter noise. In order to examine the effect of duration independently, we not only used original sound recordings, but also synthesized sound signals. The durations of these sound signals were time compressed or expanded without degrading the quality of the original sound recording. The test results show that the effect of duration is significant, and that the A-weighted sound exposure level is a better index than the maximum A-weighted sound pressure level for the evaluation of helicopter noise.     

高超声速条件下7°直圆锥边界层转捩实验研究

在Ma=6低噪声风洞中开展了半锥角7?的直圆锥边界层转捩相关实验研究.利用响应频率达到MHz量级的高频压力传感器对圆锥壁面脉动压力进行了测量,研究了高超声速圆锥边界层中扰动波的发展过程.结果表明:高超声速圆锥边界层中第二模态扰动波产生的位置以及扰动波特征频率和波长等参数受雷诺数影响较大,当单位雷诺数从2×106m~(-1)增加到8×106m~(-1)时,第二模态波的特征频率从55 k Hz增加到226 k Hz;随着单位雷诺数增加,边界层中扰动增长速度加快,第二模态波出现在圆锥表面更靠近上游的位置;相同单位雷诺数条件下,随着第二模态波的向下游传播,其特征频率逐渐减小.通过对比发现自由来流湍流度对边界层中扰动波的发展同样有较大影响,自由来流湍流度降低,边界层中的第二模态波的特征频率明显减小.利用互相关分析得出第二模态扰动波在边界层中的传播速度大约为当地主流速度的0.8—0.9倍.在1?小攻角条件下,圆锥迎风面和背风面边界层发展呈现出明显的差异,背风面边界层中扰动发展提前,第二模态波出现在更靠近上游的位置,而迎风面中扰动发展受到抑制,第二模态波特征频率更大.     

Study on sound propagation in long enclosures with a vertical or inclined branch of different dimensions

As there may be some branches in long enclosures, such as high-speed railway and freeway tunnels, it will make a difference in the sound propagation in long enclosures if we give more consideration to different designs of the branches at the onset of their construction. However, most researches on the sound propagation in long enclosures available are concerned with straight long enclosures. In this paper, the sound pressure level (SPL) attenuation, early decay time (EDT), and reverberation time (RT30) of long enclosures with a vertical or inclined branch of different dimensions have been studied by comparing experimental results from physical scale models of such enclosures with those of the straight long enclosure. This experimental investigation gives interesting results on the behavior of sound propagation in long enclosures with a vertical or inclined branch of different dimensions. When conducting engineering design for long tunnels of the high-speed railway or freeway, it would be appropriate to consider designing the branch of the tunnels into a widened left inclined or right inclined one to provide a better relative SPL than that of other branches with different dimensions. This study further reveals that with an inclined branch of different dimensions, the EDT of the long enclosure will be different. At both 500 and 1000 Hz 1/3 octave bands, the EDT is the shortest when the long enclosure has a left inclined branch and an extended left inclined branch, while the EDT is the longest when the long enclosure has a widened vertical branch. Generally speaking, in the near field (i.e. the source-receiver distance is shorter than the threshold distance), the EDT measurements are similar, while in the far field (i.e. the source-receiver distance exceeds the threshold distance), the EDT is the longest for the straight long enclosure, second longest for the long enclosure with a widened inclined branch, third longest for the long enclosure with a vertical branch, and the shortest for the long enclosure with an extended inclined branch, which is similar to the trend with RT30.     

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.     

机体表面湍流边界层噪声特性及预测方法研究

湍流边界层噪声是飞机巡航过程中的主要外部噪声源,对舱内噪声水平的影响尤为重要。因此,对飞机机体表面湍流边界层噪声的研究具有重要意义。本文通过试验获得了某型民机巡航过程中的湍流边界层噪声,试飞工况为3500ft/0.78、3500ft/0.7、2500ft/0.67、1500ft/0.66。对实测数据进行分析,发现湍流边界层噪声与动压、边界层厚度等参数有关。同时,利用计算流体力学的方法得到了飞机机体表面的压力分布,并分析了压力梯度对湍流边界层噪声的影响。最后,基于工程预测方法对湍流边界层噪声进行了预测,对于不存在逆压梯度的区域,预测结果与试验结果吻合较好,仅部分频段存在一定偏差。通过对模型的参数进行优化,改善了预测结果。