雷诺应力对平均和脉动压力影响的数值模拟

采用增加雷诺应力边界条件方法考察了雷诺应力对流场的影响。当雷诺应力边界条件存在时,流场内平均和脉动压力的变化体现了雷诺应力的影响。本文首先在没有雷诺应力边界条件和有雷诺应力边界条件的情况下得到TTU低矮建筑标准模型表面的平均压力系数,结果表明:考虑雷诺应力可使建筑物表面的平均压力系数的绝对值增大。本文还利用Selvam提出的脉动压力系数数学模型,计算了TTU模型表面的脉动压力系数,并对计算结果进行了分析。     

脉动压力分布同步测量的探讨

动态气动力的研究是实验工作者极为关注的问题。特别对于二元模型动态气动力的测定,必须从脉动的压力分布通过面积加权积分来得到气动力的时间历程,而其先决条件为各测压点压力的时间历程必须是同步的,也就是要测得同一时刻诸多测点的压力。本文介绍了一种比较经济的多点同步测压方法,并对该方法的实施进行了探讨。分别对A/D卡采样的同步性、整套测压系统的同步性以及频响特性进行了测试和验证。经相关性分析,相关系数分别达到0. 9999和 0. 99。最后以圆柱表面脉动压力系数分布的测试为例,给出了单圆柱不同瞬间的压力向量图、按照瞬间压力沿表面积分得到的单个圆柱脉动升力的时间历程,以及升力的脉动频率,表明整套同步测压系统具有很好的实用性,能满足实验的需要。     

飞艇尾翼脉动压力特性实验研究

在中航气动院FL-9低速风洞中,进行了飞艇尾翼脉动压力特性实验研究。测量了尾翼的脉动压力,着重分析了各测量点的脉动压力系数、频谱和相关性系数等特性。结果表明:脉动压力系数由尾翼前缘向后缘逐渐增大。尾翼的中部和前缘脉动压力系数随迎角无明显变化,在迎角超过8°以后,尾翼后缘的脉动压力系数随迎角增大而急剧增加。连续变迎角测量结果与固定迎角测量结果相比,脉动压力系数产生了明显的迟滞特性。尾翼上表面脉动压力的自相关系数和互相关系数从前缘到后缘逐渐降低,且前缘表现出显著自相关性,下翼面脉动压力基本互不相关。     

串列双圆柱同步测压实验结果的相关分析

本文在雷诺数2×104下,同步测量了12个不同间距下串列双圆柱的表面压力分布,积分得到脉动升、阻力的时间历程,并对前、后柱之间的脉动升、阻力以及脉动升阻力和圆柱表面的脉动压力进行了相关分析.在本实验中,串列双圆柱流态切换的临界间距比在3.5～4.0之间,在临界间距前后,相关曲线的形态差别很明显.当串列双圆柱之间的距离小于临界间距时,前后柱之间的影响显著,造成脉动升/阻力和脉动压力之间的相关程度下降.串列双圆柱之间的距离大于临界距离时,前、后柱的脉动升力之间的相位差随间距线性增加.分离点的脉动压力和脉动升力之间以及背压和脉动阻力之间都有较好的相关性,可以用分离点的脉动压力和背压来分别表征脉动升力和脉动阻力.     

边孔弧形闸门水流脉动压力特性研究

弧形泄水闸是水利枢纽工程中常见的挡水、泄水建筑物,其在关闭或开启过程中或者局部开启运行时,在水流脉动压力作用下会产生振动,容易引起闸门结构破坏。本文针对边孔闸门与中孔闸门不同的水流边界条件特点,制作了带导流墙与无导流墙两个弧形闸门模型。在实验室内开展了不同库水位、不同闸门开度下的物理模型试验,并对两个模型上大量测点的水流脉动压力数据进行比较分析。试验表明,导流墙的存在对脉动压力主频的影响较小,但对脉动压力均方根值影响较大;在较高库水位下相对开度位于0.2至0.3区间内时,导流墙的存在不但不会改善闸门的受力状况,反而还会导致受力增强。     

侧柱对串列双柱脉动压力的干扰

研究了串列双圆柱旁加上一个等直径的圆柱，组成一个等边三角形排列的三圆柱的脉动压力分布．着重研究侧柱对串列双圆柱脉动压力分布的影响．研究的结果表明，绕流等边三角形排列的三圆柱，受影响最严重的是后柱．脉动压力分布出现了严重的不对称，外侧的压力脉动极其强烈，内侧的压力脉动较弱，与时均压力分布，很好的对应关系．另外，侧柱对于串列双圆柱是否达到超过临界间距的绕流流态，有很大的影响     

大湍流度高雷诺数时并列双圆柱的平均和脉动压力分布

本文通过风洞实验研究了来流湍流度,Iu=10％雷诺数分别为Re=1.95×10~9和Re=6.5×10~5时单个圆柱和不同间距比下并列双圆柱的平均和脉动压力分布。结果表明:在Re=1.95×10~5时单个圆柱的平均压力分布类似于低湍流度高超临界雷诺数时的压力分布;当雷诺数增大至6.5×10~5时,绕圆柱表面流动的分离点前移和背压绝对值提高,总的阻力系数随之增加。并列双圆柱的间距比变化对圆柱表面压力分布影响很大,在极小间距比(N/d=1.05)时,双圆柱间的缝隙流使附近柱面产生高达-5的压力系数峰值(Re=6.5×10~5),同时脉动压力也大为增加;在较小间距比时(1.5     

孔板消减气流脉动的数值模拟及实验研究

添加孔板是一种消减压缩机管道系统内气流脉动有效而简便的方法,尽管在工业生产中已被广泛应用,但是其设计和制作所需的各个参数尚处于靠经验取值的阶段.针对这种情况,首先阐述了孔板消减管道内气流脉动的机理;然后使用流体仿真计算软件Fluent建立了管道内气体的二维非稳定流动模型,计算了孔板对管道内气流的压力脉动的影响;并在数值模拟的基础上,搭建了往复式压缩机管道系统实验平台,在进气管线研究了孔板对气流脉动的消减作用.通过数值模拟和实验研究分析了孔板孔径比对气流脉动的影响,并指出选用恰当孔径比的孔板不仅能有效降低主管线和缓冲器至孔板段管线的压力脉动幅度而且对压缩机进口段管线内压力脉动同样具有良好的消减效果.     

沙岭子电厂冷却塔群风荷载的风洞研究

本文通过风洞实验研究了沙岭子电厂接近直线布置的四个双曲线型冷却塔在主导风向和最大风速风向角下的平均压力和脉动压力分布。研究结果表明:在较大风向角时,主要由于塔群间的邻近作用,使塔面最小平均风压系数绝对值有所增加,对塔面的脉动压力分布影响较小;在较小风向角时,主要由于上游塔的尾流影响,使下游塔塔面的平均压力分布变化较大,特别是脉动压力,与孤立塔相比可增大几倍。同时,塔群影响使得下游塔压力谱中能量分布相对集中,其峰值频率较孤立塔有成倍的提高。     

油水两相渗流压力脉动的实验及数值分析

本文用高精度的压差传感器和高速度的数据采集设备同时测量出油水两相流体在多孔介质流动时产生的较大压力降和微小压力脉动,应用自行研制的大型数据处理软件对恒流速水驱油的两相渗流压力脉动实验数据进行了分析,发现不同阶段压力脉动具有明显的频谱特性和时间相关特性的不同,在第三阶段（油为主,水增加阶段）谱能增加最大,时间正相关程度最强。     

Generation of locked-on flow tones: Effect of damping

The overall objective of this investigation is to determine the effect of variable damping on the pressure response of a deep cavity. The pressure fluctuations arise from coupling between the unsteady shear layer along the cavity opening and a resonant mode of the cavity. The damping of the cavity is tuned to desired values without changes of geometry or other parameters.The amplitude of the cavity pressure fluctuation as a function of flow velocity is characterized for the first, second and third acoustic modes of the cavity. For each mode, variation of the value of damping over a relatively wide range yields corresponding attenuation of the pressure amplitude. For higher acoustic modes and sufficiently large damping, abrupt decreases of the pressure amplitude occur at threshold values of flow velocity.The variable damping of the deep cavity does not significantly alter the eigenfrequencies of the system. The peak response amplitude of the pressure fluctuation, however, occurs at a value of Strouhal number that increases with increasing values of damping. Moreover, this peak response amplitude, when normalized by the free stream dynamic head, generally shows a linear variation with the value of damping, for three acoustic modes of the cavity.The strength of lock-on of the pressure oscillation, as a function of the degree of damping, is evaluated in terms of the coherent and broadband pressure amplitudes. Both amplitudes are attenuated for increased damping; the difference between them, however, remains relatively large (40 dB minimum), thereby indicating well-defined lock-on, even when the amplitude of the spectral peak of the coherent component is substantially attenuated.     

顺桨风力机塔叶干涉翼型非稳气动力时频特性

由于风力机叶片与塔筒流场相互干涉,实际气动力与理想情况存在较大差异,这种干涉作用造成的气动力差异给叶片与塔筒结构可靠性带来不可忽视的影响.以翼型DU91-W2-250为研究对象,采用瞬态数值分析与本征正交分解方法,考虑叶片和塔筒流场相互干涉作用,分析顺桨工况翼型非稳气动力时频特性及其影响规律,量化不同雷诺数下塔叶相对位置及几何参数对气动力均值、波动幅度和频率的影响程度,通过流场模态能量分布形态分析,揭示流场干涉对气动力的影响机制.结果表明,翼型气动中心至塔筒几何中心的垂直距离、水平距离以及塔筒直径相对于翼型弦长的无量纲参数y*,x*和D*对气动力均有不同程度影响,其中y*对升阻力系数均值影响最大,对频率无明显影响,y*绝对值越大,C_l均值越接近单翼型C_l值,y*绝对值越小升阻力系数波动幅度越大,y*从-12增大到12,升力系数均值最小值为-0.48,最大值为1.16;x*减小和D*增大,反向阻力均值增大,波动幅度增大,波动频率略有下降,当x*小于临界值5时,带塔翼型阻力均值反向;在计算范围内,带塔翼型升力系数均值相对于单翼型升力系数最大偏差为...     

Coupling Relation Between Air Shockwave and High-Temperature Flow from Explosion of Methane in Air

After a methane-in-air explosion in a coal mine tunnel, a secondary explosion of coal dust is prone to happen. The shockwave in the gas explosion produces a coal dust suspension, and the peak temperature band may detonate that suspension. This secondary detonation depends on the space-time relation between the shockwave and the peak temperature band. This paper presents a methodology to estimate the coupling relation between the air shockwave and high-temperature flow from the explosion of methane in air. The commercial software package AutoReaGas was used to carry out the numerical simulation for the explosion processes of methane in air in the tunnel. Based on the numerical simulation and its analysis, the coupling relation between the leading shock wave and high-temperature flow was demonstrated for a methane-in-air explosion in a tunnel. In the near field of the ignition point, the deflagration wave transmits energy by heat, and the temperature load is in the front of the pressure wave. With development of deflagration and deflagration-to-detonation transition, the corresponding mechanism of energy transmission is changed from heat conduction to shock compression, and a precursor pressure wave is formed gradually. The time interval between the precursor pressure wave and high-temperature flow behind the wave increases with distance. Attenuation of the precursor shock wave and high-temperature flow depends on the length of the methane-in-air space in a tunnel. Beyond the methane-in-air space, the quantitative relation of the time interval between the precursor shock wave and high-temperature flow with axial distance from ignition and the length of methane-in-air space was proposed.     

爆炸载荷特征参数对无限长圆柱壳弹性动态响应的影响

采用含有三角脉冲载荷和准静压载荷的爆炸载荷加载,利用单自由度模型对无限长圆柱壳体（即等效平面应变圆环）的弹性动态响应进行了力学分析,获得了径向位移响应解析解及准静压阶段弹性响应振幅的解析解。基于所得解析解,通过控制变量法分析了载荷压力及载荷分界点时刻（即三角脉冲载荷与准静压载荷作用的分界点时刻）对径向位移最大值、准静压阶段弹性响应振幅的影响规律,更加深入地研究了爆炸载荷对结构响应的影响。本文主要从准静压幅值与三角脉冲峰值的比值以及载荷分界点时刻两个主要特征参数入手,结合结构的呼吸振动频率来研究爆炸载荷对无限长圆柱壳弹性动态响应的影响。在研究中发现存在临界时刻:当载荷分界点时刻早于临界时刻时,径向位移最大值出现在准静压阶段;当载荷分界点时刻晚于临界时刻时,获得了便于直观判断径向位移达到最大值时所处载荷阶段的分区图。基于前述解析解的分析,还获得了不同影响因素导致的振幅变化的单调性分区图,便于判别载荷压力的变化所致的准静压阶段振幅的增减趋势。通过研究获得的爆炸压力载荷对结构响应的影响规律,可为爆炸容器设计以及结构防护基础研究提供参考。     

Stall control at high angle of attack with plasma sheet actuators

We analyzed the modifications of the airflow around an NACA 0015 airfoil when the flow was perturbed with electrohydrodynamic forces. The actuation was produced with a plasma sheet device (PSD) consisting in two bare electrodes flush mounted on the surface of the wing profile operated to obtain a discharge contouring the body in the inter-electrode space. We analyze the influence of different parameters of the actuation (frequency, input power, electrodes position) on the aerodynamic performance of the airfoil, basing our study on measurements of the surface pressure distribution and of the flow fields with particle image velocimetry technique. The experiments indicated that at moderate Reynolds numbers (150,000 < Re < 333,000) and at high angles of attack, steady or periodic actuations enabled large improvement of the lift and drag/lift aerodynamic coefficients by reattaching the flow along the extrados. However, to attain the same results steady actuations required larger power consumption. When exciting the flow with a moderate value of non-dimensional power coefficient (ratio of electric power flow with the kinetic power flow), a frequency of excitation produced a peak on the coefficients that evaluate the airfoil performance. This peak in terms of a non-dimensional frequency was close to 0.4 and can be associated to an optimal frequency of excitation. However, our work indicates that this peak is not constant for all stalled flow conditions and should be analyzed considering scale factors that take into account the ratio of the length where the forcing acts and the cord length.     

Surface pressure on a cubic building exerted by conical vortices

Experiments were performed to study surface pressure on a cubic building underlying conical vortices, which are known to cause severe structural damage and failure. The focus is on the effects of turbulence in the incident flow. Three turbulent boundary layers were created in a boundary layer wind tunnel. A wall-mounted cube, i.e. a cube situated on the horizontal ground floor surface of the wind-tunnel test section, was used as an experimental model. The cube was subjected to the incidence flow at 40°. Steady and unsteady pressure measurements were performed on the cube surface. The analysis suggests that conical vortices developed above the top surface of the wall-mounted cube. A larger mean suction was observed on the top cube surface in the less turbulent boundary layer. With an increase in turbulence in the incoming flow, the strong suction zones decreased in size. The fluctuating pressure coefficient profiles retained their shape when the turbulence in the upstream flow of the cube increased. The fluctuating pressure coefficient was observed to be larger in more turbulent flows. The pressure fluctuations were larger on the cube surface underlying outer boundaries of the conical vortex. The fluctuating pressure coefficient under the conical vortex was three to four times larger than in the weak suction zone on the central area of the top cube surface. Close to the leading cube corner, the pressure spectra were dominated by a single low frequency peak. As the conical vortex developed, this primary peak weakened and a secondary peak emerged at a higher reduced frequency. There is a general trend of shifting the pressure spectra towards higher reduced frequencies when the turbulence in the undisturbed incident flow increases.     

On solute dispersion in pulsatile flow through a channel with absorbing walls

The paper presents the longitudinal dispersion of passive tracer molecules released in an incompressible viscous fluid flowing through a channel with reactive walls under the action of a periodic pressure gradient. A finite-difference implicit scheme is adopted to solve the unsteady advection-diffusion equation based on the Aris-Barton method of moments for all time period. Here it is shown how the spreading of tracers is influenced by the shear flow, lateral diffusion about its mean position due to the action of absorption at both the walls. The analysis has been performed for three different cases: steady, periodic and the combined effect of steady and periodic currents, separately. The results show that for all cases the dispersion coefficient asymptotically reaches a stationary state after a certain critical time and it achieves a stationary state at earlier instant of time, when absorption at the walls increases. The axial distributions of mean concentration are determined from the first four central moments by using Hermite polynomial representation for all three different flow velocities.     

Ensemble-Empirical-Mode-Decomposition method for instantaneous spatial-multi-scale decomposition of wall-pressure fluctuations under a turbulent flow

This work illustrates the possibilities of the Ensemble-Empirical-Mode-Decomposition (E-EMD) technique for a detailed analysis of the time and space characteristics of the wall-pressure fluctuations under a turbulent flow. Pressure fluctuations are measured with a linear microphone array, for the cases of a turbulent boundary layer and for a diffuse airborne acoustic field. The E-EMD technique is shown to be an efficient tool for representing the spatial scales of the turbulent fluctuations at each instant. In particular, this representation is obtained without any particular assumption or a priori information on the data (e.g. temporal or spatial stationarity of the wall pressure data is not required), and acts, when applied to wide-band turbulent signals, as a wavenumber filter. Finally, it is shown how, to some extent, the E-EMD technique can separate at each instant the acoustic (propagative) from the hydrodynamic (convective) energy.     

Unsteadiness of transonic convex-corner flows

An experimental investigation was conducted to study the transonic convex-corner flow in a turbulent boundary layer. The unsteadiness of the interaction was characterized by a local peak pressure fluctuation and the deviation of higher order moments from an effective Gaussian. The peak pressure fluctuations are correlated and compared with results from other studies.     

受限颗粒体对制动系统非线性振动的影响

制动系统在工作时,往往受到沙粒、尘土以及磨屑等受限颗粒体的影响,这些受限颗粒体在摩擦副中的高度分布具有较强的随机性,一定程度诱发了制动系统的非线性振动. 本文中基于制动片切向振动模型,引入了新的受限颗粒体摩擦模型,提出了用波动系数来描述受限颗粒体高度分布随机性的强弱. 发现在特定参数下,当此系数为0时,制动片切向振动为周期运动;但是当此系数不为0时,制动片切向振动呈现拟周期或混沌运动,此时的切向振动分岔特性图的稳定轨道也会出现数量或分布的变化,甚至表现出混沌特性. 同一时变信号内,受限颗粒体引发制动片切向非线性振动包括发散、收敛以及拟周期运动等多种形式.