Progress of the unified wave-particle methods for non-equilibrium flows from continuum to rarefied regimes

We utilize the nonlinear acoustic solver (NLAS) and Ffowcs-Williams/Hawkings (FW-H) equation to investigate the noise generation and radiation due to shock (wave) and boundary layer interaction (SBLI) in the inlet duct. A classical benchmark for SBLI is chosen to validate the flow features and numerical results show good agreement with experimental results. In the simulation of the noise generated by SBLI, the inlet buzz phenomenon is successfully observed. The oscillation of the normal shock is a kind of little buzz and the oscillation of inner shocks is a kind of big buzz with a frequency around 100 Hz. In the far-field, frequency spectrums show a dominant frequency close to the frequency of inner shocks oscillation. This indicates that the oscillation of inner shocks determines the magnitude of the overall sound pressure level (OASPL) of the far-field noise.

     

Phase sensitivity for coherence resonance oscillators

Nonlinear Dynamics - The phase reduction approach has manifested its power in analyzing the rhythmic behaviors for limit cycle oscillators. For coherent oscillation purely induced by noise, e.g.,...     

Basins and bifurcations of a delayed feedback control system and its experimental verification for a DC bus circuit

Nonlinear Dynamics - Stochastic synchrony, also known as noise-induced synchronization that leads to phase coherence, arises when a set of uncoupled neurons synchronizes to a common white noise...     

Bifurcations in a birhythmic biological system with time-delayed noise

We study the effects of recycled noise on the dynamics of a birhythmic biological system. This noise is generated by the superposition of a primary Gaussian white noise source with a second component (its replicas delayed of time τ). We find that under the influence of this kind of noise, the dynamics of the birhythmic biological system can be well characterized through the concept of stochastic bifurcation, consisting in a qualitative change of the stationary probability distribution. Analytical results are obtained following the quasiharmonic assumption through the Langevin and Fokker–Planck equations. Comparing the analytical and numerical results, we find good agreement when the frequencies of both attractors are equal, while the predictions of the analytic estimates deteriorate when the two frequencies depart. We also find that the increase of noise intensity leads to coherence resonance.     

Local sensitivity of spatiotemporal structures

We present an index for the local sensitivity of spatiotemporal structures in coupled oscillatory systems based on the properties of local-in-space, finite-time Lyapunov exponents. For a system of nonlocally coupled Rössler oscillators, we show that variations of this index for different oscillators reflect the sensitivity to noise and the onset of spatial chaos for the patterns where coherence and incoherence regions coexist.     

Three Dimensional Numerical Flow Simulation Around Parallel Rectangular Cylinders

Noises of flow around parallel rectangular cylinders are likely to be caused by wind flow. According to the wind tunnel experiment, it is known that there are three kinds of special noises. Two kinds of noises occur when the wind comes from a perpendicular direction to the cylinders. These noises are caused by the vortex excited oscillation. Another noise occurs when the wind comes from an inclined direction to the cylinders. This noise is very high frequency, and its mechanism is not clear. Therefore in this study, the three dimensional numerical flow simulation is performed to clarify the high frequency noise of flow around parallel rectangular cylinders.     

On the precision of optical imaging to study free surface dynamics at high frame rates

The precision of optical imaging to study free surface dynamics is analyzed. The damping of a liquid bridge free surface oscillation is used to validate this method. Images are acquired with a digital camera at relatively high frame rates and processed by several techniques. Oscillations with amplitudes of about 20 times smaller than the pixel size are measured, which allows one to reach the nanometer scale in the analysis. The experimental results presented in this paper constitute the first quantitative validation of optical imaging to study free surface dynamics at the nanometer scale. As a secondary goal, we propose an image processing technique based on the local thresholding criterion to determine the free surface position with sub-pixel resolution. This yields more precision (less noise) than the standard technique when considering very small oscillations. Further improvement of the results is obtained by a simple smoothing technique.     

The planar jet-plate oscillator

The aeroacoustic noise generated by a high speed, planar gas jet impinging on a flat plate is investigated experimentally. The jet used in this study is typical of those commonly found in industrial applications such as in various coating control and heat transfer processes. Normal jet impingement on the plate is found to generate strong acoustic tones over a wide range of impingement distances and jet velocities. The characteristics of these tones, as a function of the jet velocity and impingement distance, are quantified. Phase and amplitude measurements of the pressure fluctuations on the impingement plate indicate that the acoustic tones are generated by an antisymmetric instability mode of the jet oscillation. The effect of plate inclination in both the transverse and span-wise directions, with respect to the incident jet, is also studied. The jet-plate tone is found to be much more sensitive to changes in the span-wise plate inclination than to changes in the transverse inclination, but in both cases, a complete suppression of the tone is found to be possible.     

Wavelet decomposition method decoupled boiling/evaporation oscillation mechanisms over two to three timescales: A study for a microchannel with pin fin structure

Boiling/evaporation heat transfer in a microchannel with pin fin structure was performed with water as the working fluid. Simultaneous measurements of various parameters were performed. The chip wall temperatures were measured by a high spatial-time resolution IR image system, having a sensitivity of 0.02 °C. The flow pattern variations synchronously changed wall temperatures due to ultra-small Bi number. The wavelet decomposition method successfully identified the noise signal and decoupled various temperature oscillations with different amplitudes and frequencies. Three types of temperature oscillations were identified according to heat flux q and mass flux G. The first type of oscillation occurred at q/G < 0.62 kJ/kg. The approximation coefficient of wavelet decomposition decided the dominant cycle period which was ∼3 times of the fluid residence time in the microchannel, behaving the density wave oscillation characteristic. The detail coefficients of wavelet decomposition decided the dominant cycle period, which matched the flow pattern transition determined value well, representing the flow pattern transition induced oscillation. For the second type of oscillation, the wavelet decomposition decoupled the three oscillation mechanisms. The pressure drop oscillation caused the temperature oscillation amplitudes of 5–10 °C and cycle periods of 10–15 s. The density wave oscillation and flow pattern transition induced oscillation are embedded with both the pressure rise and decrease stages of the pressure drop oscillation. The third type of oscillation happened at q/G > 1.13 kJ/kg, having the density wave oscillation coupled with the varied liquid film evaporation induced oscillation. The liquid island, retention bubble induced nucleation sites and cone-shape two-phase developing region are unique features of microchannel boiling with pin fin structure. This study illustrated that pressure drop oscillation and density wave oscillation, usually happened in large size channels, also take place in microchannels. The flow pattern transition and varied liquid film evaporation induced oscillations are specific to microchannel boiling/evaporation flow.     

Prediction of the dynamic oscillation threshold in a clarinet model with a linearly increasing blowing pressure: influence of noise

This paper presents an analysis of the effects of noise and precision on a simplified model of the clarinet driven by a variable control parameter. When the control parameter is varied, the clarinet model undergoes a dynamic bifurcation. A consequence of this is the phenomenon of bifurcation delay: the bifurcation point is shifted from the static oscillation threshold to a higher value, called dynamic oscillation threshold. In a previous work Bergeot et al. in Nonlinear Dyn. doi:10.1007/s11071-013-0806-y, (2013), the dynamic oscillation threshold is obtained analytically. In the present article, the sensitivity of the dynamic threshold on precision is analyzed as a stochastic variable introduced in the model. A new theoretical expression is given for the dynamic thresholds in presence of the stochastic variable, providing a fair prediction of the thresholds found in finite-precision simulations. These dynamic thresholds are found to depend on the increase rate and are independent on the initial value of the parameter, both in simulations and in theory.     

Heat and mass transfer of a fuel droplet evaporating in oscillatory flow

A numerical study of the heat and mass transfer from an evaporating fuel droplet in oscillatory flow was performed. The flow was assumed to be laminar and axisymmetric, and the droplet was assumed to maintain its spherical shape during its lifetime. Based on these assumptions, the conservation equations in a general curvilinear coordinate were solved numerically. The behaviors of droplet evaporation in the oscillatory flow were investigated by analyzing the effects of flow oscillation on the evaporation process of a n-heptane fuel droplet at high pressure.The response of the time history of the square of droplet diameter and space-averaged Nusselt numbers to the main flow oscillation were investigated in frequency band of 1–75 Hz with various oscillation amplitudes. Results showed that, depending on the frequency and amplitude of the oscillation, there are different modes of response of the evaporation process to the flow oscillation. One response mode is synchronous with the main flow oscillation, and thus the quasi-steady condition is attained. Another mode is asynchronous with the flow oscillation and is highly unsteady. As for the evaporation rate, however, in all conditions is more greatly enhanced in oscillatory flow than in quiescent air.To quantify the conditions of the transition from quasi-steady to unsteady, the response of the boundary layer around the droplet surface to the flow oscillation was investigated. The results led to including the oscillation Strouhal number as a criteria for the transition. The numerical results showed that at a low Strouhal number, a quasi-steady boundary layer is formed in response to the flow oscillation, whereas by increasing the oscillation Strouhal number, the phenomena become unsteady.     

Dynamic Measurements of Impulses Generated by the Separation of Adhered Bodies under Near-Zero Gravity Conditions

The present paper is aimed at investigating the dynamics of release of objects in free-falling conditions, which constitutes a typical phase of some space applications. In the presence of surface interaction forces, a quick separation of the released body from the constraining one will result in a momentum transfer, provided that the inertial forces exceed the maximum attractive force. The release conditions as well as the related parameters affecting the momentum acquired by the released body through the adhesion rupture play a fundamental role. An experimental technique aimed at measuring the momentum transfer has been developed. The basic concept of the measuring apparatus is to suspend both bodies from two pendulums. A position sensor detects the weakly damped oscillation of one object due to the momentum transferred upon pulling the other one away. Particular attention has been placed on the capability to accurately reproduce the stress status on the adhesive contact patch between the two bodies, on the noise sources affecting the measurement, and on the performances of a noise optimal-filtering technique. This paper presents measurements of momentum transfer between adhered surfaces upon quick separation.     

Dynamic behavior in a delayed stage-structured population model with stochastic fluctuation and harvesting

In this paper, the dynamic behavior of a stage-structured population model involving gestation delay is investigated within stochastically fluctuating environment and harvesting. Firstly, the stability and Hopf bifurcation condition are described on the delayed population model within deterministic environment. Secondly, the stochastic population model systems are discussed by incorporating white noise terms to the deterministic system model. Finally, numerical simulations show that the gestation delay with larger magnitude has ability to drive the system from stable to unstable within the same fluctuating environment and the frequency and amplitude of oscillation for the population density is enhanced as environmental driving forces increase. These indicate that the magnitude of gestation delay plays a crucial role to determine the stability or instability and the magnitude of environmental driving forces plays a crucial role to determine the magnitude of oscillation of the population model system within fluctuating environment.     

风力机叶片翼型动态试验技术研究

风力机叶片动态振荡过程往往伴随着俯仰和横摆同时进行, 以前对许多动态问题不清楚的阶段, 工程上不惜以增加叶片重量为代价而采用偏安全的设计, 通常忽略横摆振荡的影响; 大型风力机设计对获取翼型更加全面、准确的动态载荷提出了更高要求, 研究横摆振荡对翼型动态气动特性的影响规律具有重要意义. 本文首次开展翼型横摆振荡动态风洞试验研究, 采用“电子凸轮”技术代替机械凸轮实现了振荡频率和振荡角度的无级变化, 基于设计的电子外触发装置实现了对动态流场的实时测量, 实现了风洞来流、模型角位移和动态压力数据的同步采集, 分别开展了翼型静态测压、俯仰/横摆动态测压、粒子图像测速和荧光丝线等试验研究, 试验结果准度较高、规律合理; 分析了动态试验洞壁干扰影响机制. 研究表明, 横摆振荡翼型的气动曲线也存在明显迟滞效应; 随着振荡频率升高, 翼型俯仰和横摆振荡下的气动迟滞性均增强; 翼型俯仰振荡正行程的动态失速涡破裂有所延迟; 洞壁与模型端部交界处的强三维效应对翼型压力分布影响较大; 建立的横摆振荡试验技术可为风力机动态掠效应的研究提供技术支撑.      

缓变主流中三维气泡的非线性振动

空化现象和水下噪声机制与液体中气泡的动力学行为密切相关．在无粘势流的假定下，采用多参数摄动分析，研究了缓变主流中三维气泡的非线性体积模态振动．推导了关于缓变泡形展开的各阶扰动方程，获得了一阶振动的演化方程和一些特殊情况下的解析解；并采用高阶有限元离散的边界积分方程方法，对平面固壁和自由面附近三维气泡的固有频率进行了数值计算     

Spatial Coherence of the Heat Release Fluctuations in Turbulent Jet and Swirl Flames

The noise generation of turbulent flames is governed by temporal changes of the total flame volume due to local heat release fluctuations. On the basis of the wave equation an expression for the relationship between the acoustic power and the heat release fluctuations is derived and a correlation function is obtained which reveals that the sound pressure level of flames is governed by the spatial coherence. Noise models rely on precise coherence information in terms of characteristic length scales, which are the measure of the acoustic efficiency of the flame. Since the published length scale information is scarce and inconsistent, length scales were measured for a number of laboratory flames using two measurement techniques developed for this purpose: A planar LIF-system with a repetition rate of 1 kHz acquires the instantaneous flame front position and heat release, whereas two chemiluminescence probes with an optics confining the measurement volume to a line of sight provide further spatial correlation data. For all flames investigated the length scales are smaller than the height of the burner exit annulus and they are of the order of the local flame brush thickness. Using the measured length scales, the coherent volume and the efficiency of the noise generation are calculated, which are three orders of magnitude higher than measured. However, the proper order of magnitude is obtained, if only the measured fluctuating part of the thermal power is used in the model and if the periodic formation of local zones with heat release overshoot and deficit are properly incorporated.     

Time-resolved stereo PIV measurements of shock–boundary layer interaction on a supercritical airfoil

Time-resolved stereo particle-image velocimetry (TR-SPIV) and unsteady pressure measurements are used to analyze the unsteady flow over a supercritical DRA-2303 airfoil in transonic flow. The dynamic shock wave–boundary layer interaction is one of the most essential features of this unsteady flow causing a distinct oscillation of the flow field. Results from wind-tunnel experiments with a variation of the freestream Mach number at Reynolds numbers ranging from 2.55 to 2.79 × 10⁶ are analyzed regarding the origin and nature of the unsteady shock–boundary layer interaction. Therefore, the TR-SPIV results are analyzed for three buffet flows. One flow exhibits a sinusoidal streamwise oscillation of the shock wave only due to an acoustic feedback loop formed by the shock wave and the trailing-edge noise. The other two buffet flows have been intentionally influenced by an artificial acoustic source installed downstream of the test section to investigate the behavior of the interaction to upstream-propagating disturbances generated by a defined source of noise. The results show that such upstream-propagating disturbances could be identified to be responsible for the upstream displacement of the shock wave and that the feedback loop is formed by a pulsating separation of the boundary layer dependent on the shock position and the sound pressure level at the shock position. Thereby, the pulsation of the separation could be determined to be a reaction to the shock motion and not vice versa.     

基于水平集方法的毛管上升特征研究

毛管上升现象与许多行业密切相关，系统地对此现象进行研究具有重大意义。与传统理论研究方法不同，本文使用N-S方程耦合水平集方法模拟毛管气液上升行为。通过与简化条件的解析解对比，验证了模拟方法的可靠性。此外，详细地研究了毛管振荡现象，并分析了影响毛管振荡行为的主要因素。结果表明，水平集方法能够精确地表征毛管振荡现象，与数值解相比具有更高的精度。毛管长度的增加能够减弱液柱振荡，主要归结于非湿相气体的粘滞力作用；湿相密度和湿相粘度同样对毛管振荡现象影响显著。湿相密度越大，惯性力越大，促进了毛管振荡；而湿相粘度变大，会增大粘滞力作用，因此减弱了毛管振荡现象。毛管振荡是由多种影响因素共同控制的，流体的惯性力是造成毛管振荡的主要原因，而粘滞力是减弱毛管振荡行为的主要因素，使液柱振荡逐渐衰减，并稳定至平衡高度。     

离心式风机气流系统离散频率声学特性及计算方法的研究

本文利用稳态气流的变更方程,建立了风机叶轮的转移矩阵。这是多级叶轮气流系统离散性频率声学特性的一个基衣公式。为了讨论离散性频率噪声,本文提出了多叶叶道流速激发的概念和计算公式。与风机负荷的波动相关,风机转子的角速度变化及相伴的低频拍振是不可避免的。本文还推荐了一些降低拍振振幅的措施。     

FREQUENCY DOMAIN CALCULATION METHOD FOR WIND-INDUCED RESPONSE OF MONOLAYER CABLE NET

以随机振动理论为基础,采用频域法对单层索网结构风振响应进行了研究,主要探讨不同计算方法和空间相干函数的选取等问题对索网结构风振响应的影响. 依据不同的计算方法和空间相干函数,建立了3 种分析模式进行了分析. 分析结果表明：对于频率密集的索网结构,需要考虑模态交叉项对计算结果的影响;空间相干函数的选取对计算结果影响较大,采用与风频率相干的空间相干函数更为合理.