激波/湍流边界层干扰压力脉动特性数值研究

激波/湍流边界层干扰问题广泛存在于高速飞行器内外流动中, 激波干扰会导致局部流场出现强压力脉动, 严重影响飞行器气动性能和飞行安全. 为了考察干扰区内脉动压力的统计特性, 对来流马赫数2.25, 激波角33.2°的入射激波与平板湍流边界层相互作用问题进行了直接数值模拟研究. 在对计算结果进行细致验证的基础上, 分析比较了干扰区外层和物面脉动压力的典型统计特征, 如脉动强度、功率谱密度、两点相关和时空关联特性等, 着重探讨了两者的差异及其原因. 研究发现, 激波干扰对外层和物面压力脉动的影响差异显著. 分离区内脉动以低频特征为主, 随后再附区外层压力脉动的峰值频率往高频区偏移, 而物面压力脉动的低频能量仍相对较高. 两点相关结果表明, 外层和物面脉动压力的展向关联性均明显强于其流向, 前者积分尺度过激波急剧增长随后缓慢衰减, 而后者积分尺度整体上呈现逐步增大趋势. 此外, 时空关联分析结果指出, 脉动压力关联系数等值线仍符合经典的椭圆形分布, 干扰区下游压力脉动对流速度将减小, 外层对流速度仍明显高于物面.      

Stochastic estimation of flow near the trailing edge of a NACA0012 airfoil

A stochastic estimation technique has been applied to simultaneously acquired data of velocity and surface pressure as a tool to identify the sources of wall-pressure fluctuations. The measurements have been done on a NACA0012 airfoil at a Reynolds number of Re _c  = 2 × 10⁵, based on the chord of the airfoil, where a separated laminar boundary layer was present. By performing simultaneous measurements of the surface pressure fluctuations and of the velocity field in the boundary layer and wake of the airfoil, the wall-pressure sources near the trailing edge (TE) have been studied. The mechanisms and flow structures associated with the generation of the surface pressure have been investigated. The “quasi-instantaneous” velocity field resulting from the application of the technique has led to a picture of the evolution in time of the convecting surface pressure generating flow structures and revealed information about the sources of the wall-pressure fluctuations, their nature and variability. These sources are closely related to those of the radiated noise from the TE of an airfoil and to the vibration issues encountered in ship hulls for example. The NACA0012 airfoil had a 30 cm chord and aspect ratio of 1.     

A silicon transducer for the determination of wall-pressure fluctuations in turbulent boundary layers

Small and sensitive silicon sensors for turbulent wall-pressure fluctuation measurements have been designed and fabricated using microelectronic technology. For the detection of the pressure fluctuations piezoresistive gauges are placed on a diaphragm and the resistance of these gauges depends on the stresses in the diaphragm. For the determination of the performance of these pressure transducers comparisons with conventional microphones were carried out in a well-defined two-dimensional boundary layer. Power spectra from the silicon pressure transducer revealed a slope in the inertial sublayer corresponding approximately to the 5/3-law of Kolmogorov, and the normalized wall pressure fluctuations agreed well with other direct measurements.     

水下爆炸冲击波壁压理论及数值计算方法改进研究

水下爆炸冲击波是舰船抗冲击评估中重要的载荷成分,也是水中结构物毁伤程度快速预报的关键和依据。通过小当量实验发现,由于传统 Taylor 平板理论公式忽略了冲击波波速的非线性变化 ,导致其在预报近距离水下爆炸冲击波壁压脉宽时出现偏差。为此,给出了比例爆距R/W1/3为0.11～5.30 m/kg1/3 (R为爆距,W为炸药质量)下的冲击波速度拟合公式,对传统Taylor理论公式进行修正。修正后,在R/W1/3=0.11 m/kg1/3下,壁压脉宽及冲量偏差大幅减小;在R/W1/3≥0.21 m/kg1/3下,两者偏差均小于12%。此外,在处理水下近场和中远场爆炸问题时,发现数值耗散会导致壁压峰值被明显削弱,于是提出了一种可行的数值策略消除计算中数值耗散导致的削弱效应,结果与修正的Taylor平板理论公式吻合良好,峰值偏差均小于9%。改进后的冲击波壁压理论公式及数值计算方法可为舰船抗爆抗冲击领域提供理论和技术支撑。     

Acceleration and wall pressure fluctuations generated by an incompressible jet in installed configuration

In this work, the cross-statistics of acceleration and wall pressure fluctuations generated by an incompressible jet interacting with a tangential flat-plate are presented. The results are derived from an experimental test campaign on a laboratory-scale model involving simultaneous velocity and wall pressure measurements. The pressure footprint of the jet on the surface was measured through a cavity-mounted microphone array, whereas pointwise velocity measurements were carried out by a hot wire anemometer. The time derivative of the velocity signal has been taken as an estimation of the local acceleration of the jet. The multivariate statistics between acceleration and wall pressure are achieved through cross-correlations and cross-spectra, highlighting that the causality relation is more significant in the potential core where the Kelvin–Helmholtz instability is dominant. The application of a conditional sampling procedure based on wavelet transform allowed us to educe the acceleration flow structures related to the energetic wall-pressure events. The analysis revealed that, unlike the velocity, the acceleration signatures were detected only for positions where the jet had not yet impinged on the plate, their shape being related to a convected wavepacket structure.     

船舶与海洋平台结构冰载荷的高性能扩展多面体离散元方法

船舶与海洋平台结构的冰载荷是寒区海洋工程结构物设计中的关键参数,而离散元方法是有效计算结构冰载荷的重要手段. 本文采用基于闵可夫斯基和原理的扩展多面体离散元方法模拟船舶与海洋平台结构的相互作用过程. 其中,构造扩展多面体的近似包络函数并建立了基于优化模型的快速接触搜索算法;考虑单元间粘结作用的刚度软化过程建标识码元间的粘结-破碎模型. 同时,发展了 CPU-GPU 协同异构环境下的高性能并行算法. 为分析海冰与海洋结构作用中的冰载荷,采用ISO标准验证了扩展多面体离散元分析结构冰载荷的准确性. 采用离散元方法计算了船舶结构的冰载荷,研究了船舶结构表明的线载荷分布特点,并采用船舶结构冰阻力经验公式验证了计算结果的合理性. 采用离散元方法计算了平整冰区与多桩腿平台结构的相互作用,分析各桩腿上的冰载荷特点. 针对碎冰区的海冰管理过程,采用离散元方法分析了船舶结构绕行过程中的船舶和海洋平台结构冰载荷. 本文方法可有效应用于海洋结构冰载荷分析,能为极地船舶与海洋平台结构的设计和安全运行提供科学的分析手段.      

Small silicon based pressure transducers for measurements in turbulent boundary layers

Small silicon based sensors for the measurement of wall-pressure in turbulent flows have been designed and fabricated using microelectronic technology. The sensor diaphragms have a side length of 100 and 300 μm, and polysilicon piezoresistive gauges were used for detection of the deflection. A two-dimensional flat plate boundary layer was employed to determine the performance of the pressure transducers, and comparisons with established data from the literature were made. A threshold value for the pressure fluctuations of about the double Kolmogorov length scale was estimated independently from the probability distribution as well as from the power spectra. In good agreement with theoretical predictions of Blake (1986), the slope of the power spectra was found to beω ^?1 in the intermediate andω ^?5 in the high frequency range.     

Computational and experimental investigation of flow over a transient pitching hydrofoil

The present study is developed within the framework of marine structure design operating in transient regimes. It deals with an experimental and numerical investigation of the time–space distribution of the wall-pressure field on a NACA66 hydrofoil undergoing a transient up-and-down pitching motion from 0° to 15° at four pitching velocities and a Reynolds number Re = 0.75 × 10⁶. The experimental investigation is performed using an array of wall-pressure transducers located on the suction side and by means of time–frequency analysis and Empirical Modal Decomposition method. The numerical study is conducted for the same flow conditions. It is based on a 2D RANS code including mesh reconstruction and an ALE formulation in order to take into account the foil rotation and the tunnel walls. Due to the moderate Reynolds number, a laminar to turbulent transition model was also activated. For the operating flow conditions of the study, experimental and numerical flow analysis revealed that the flow experiences complex boundary layer events as leading-edge laminar separation bubble, laminar to turbulent transition, trailing-edge separation and flow detachment at stall. Although the flow is relatively complex, the calculated wall pressure shows a quite good agreement with the experiment provided that the mesh resolution and the temporal discretization are carefully selected depending on the pitching velocity. It is particularly shown that the general trend of the wall pressure (low frequency) is rather well predicted for the four pitching velocities with for instance a net inflection of the wall pressure when transition occurs. The inflection zone is reduced as the pitching velocity increases and tends to disappear for the highest pitching velocity. Conversely, high frequency wall-pressure fluctuations observed experimentally are not captured by the RANS model. Based on the good agreement with experiment, the model is then used to investigate the effects of the pitching velocity on boundary layer events and on hydrodynamic loadings. It is shown that increasing the pitching velocity tends to delay the laminar-to-turbulence transition and even to suppress it for the highest pitching velocity during the pitch-up motion. It induces also an increase of the stall angle (compared to quasi-static one) and an increase of the hysteresis effect during pitch-down motion resulting to a significant increase of the hydrodynamic loading.     

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.     

Shock-vortex shear-layer interaction in the transonic flow around a supercritical airfoil at high Reynolds number in buffet conditions

This paper provides a conceptual analysis and a computational model for how the unsteady ‘buffeting’ phenomenon develops in transonic, low incidence flow around a supercritical aerofoil, the OAT15A, at Reynolds number of 3.3 million. It is shown how a low-frequency buffet mode is amplified in the shock-wave region and then develops upstream and downstream interaction with the alternating von Kármán eddies in the wake past the trailing-edge as well as with the shear-layer, Kelvin–Helmholtz vortices. These interactions are tracked by wavelet analysis, autoregressive (AR) modelling and by Proper Orthogonal Decomposition. The frequency modulation of the trailing-edge instability modes is shown in the spectra and in the wall-pressure fluctuations. The amplitude modulation of the buffet and von Kármán modes has been also quantified by POD analysis. The thinning of the shear layers, both at the outer edge of the turbulent boundary layers and the wake, caused by an ‘eddy-blocking’ mechanism is modelled by stochastic forcing of the turbulent kinetic energy and dissipation, by small-scale straining of the higher-order POD modes. The benefits from thinning the shear-layers by taking into account the interfacial dynamics are clearly shown in the velocity profiles, and wall pressure distribution in comparison with the experimental data.     

MPS-FEM方法模拟弹性船体在规则波中的运动

船舶在海洋中航行时经常会受波浪的作用,在波浪的作用下,船体可能会发生六自由度的运动.在船体运动幅度较小时,可以简单地将船体运动视为刚体运动.但当波浪环境较为剧烈、船体运动幅度较大时,船体可能会发生变形,此时船舶弹性的影响无法忽略.因此,研究弹性船体在波浪中的运动对船舶运动性能和航行安全具有重要的意义.移动粒子半隐式方法 MPS方法是一种基于拉格朗日方法表示的无网格粒子类方法,该方法在模拟具有自由面大变形特征的问题时具有其独特的优势.有限元方法 FEM作为一种传统的并且已被广泛应用的结构求解方法,具有很好的稳定性、准确性和鲁棒性.本文将MPS方法与FEM方法二者的优势结合,基于MPS-FEM耦合方法,使用自主开发的MPSFEM-SJTU流固耦合求解器,模拟刚性船体和弹性船体在规则波中的运动,并分析船体的弹性对船体运动响应的影响.首先模拟刚性船体在不同波长的规则波中的运动,研究规则波波长对船体运动响应的影响.接着分别模拟了刚性和弹性船体在规则波中的运动,结果表明,刚性船体的运动幅值大于弹性船体的运动幅值,而弹性船体船舯附近的压力大于刚性船体.     

Local water slamming impact on sandwich composite hulls

The local water slamming refers to the impact of a part of a ship hull on stationary water for a short duration during which high local pressures occur on the hull. We simulate slamming impact of rigid and deformable hull bottom panels by using the coupled Lagrangian and Eulerian formulation included in the commercial software LS-DYNA. We use the Lagrangian formulation to describe plane-strain deformations of the hull panel and consider geometric nonlinearities. The Eulerian formulation is used to analyze deformations of the water. Deformations of the hull panel and of the water are coupled through the hydrodynamic pressure exerted by water on the hull, and the velocity of particles on the hull wetted surface affecting deformations of the water. The continuity of surface tractions and the inter-penetrability of water into the hull are satisfied by using a penalty method. The computer code is verified by showing that the computed pressure distributions for water slamming on rigid panels agree well with those reported in the literature. The pressure distributions computed for deformable panels are found to differ from those obtained by using a plate theory and Wagner's slamming impact theory. We have also delineated jet flows near the edges of the wetted hull, and studied delamination induced in a sandwich composite panel due to the hydroelastic pressure.     

Proper orthogonal decomposition of wall-pressure fluctuations under the constrained wake of a square cylinder

The proper orthogonal decomposition (POD) analysis of the wall-pressure fluctuations below the constrained wake of a two-dimensional square cylinder in proximity to a plane wall was made on two systems, i.e., G/D = 0.25 and 0.5, which corresponds to the wakes with and without suppression of the vortex shedding, respectively. Here, G is the gap distance and D is the width of the square cylinder. Synchronized measurements of wall-pressure fluctuations were made using a microphone array. For the system G/D = 0.5, the first two energetic modes contribute 34.7% and 23.4% to the total fluctuation energy, respectively; however, the fluctuation energy corresponding to the third mode are relatively small and less than 10%. This sharp variation in eigenvalue is due to the presence and dominance of the Karman-like vortex shedding. However, for the system G/D = 0.25, the considerable reduction in the eigenvalues of the first several modes is due to the suppression of the Karman-like vortex shedding. The spatial wavy pattern of the first several energetic eigenmodes was shown to be a good reflection of convective vortices superimposed in the wakes. The spectra of the POD coefficients determined the frequency of the dominant structures. Based on the coherence of the POD coefficients, an effective method of determining the number of POD modes for reconstruction of the low-order wall-pressure field was proposed. Accordingly, the low-order wall-pressure fluctuations in the systems G/D = 0.5 and 0.25 were reconstructed by using the first four and five POD modes, respectively. The coherence and cross-correlation analysis of the reconstructed wall-pressure fluctuations, which excluded the influence of the small-scale structures and background ‘noise’, gave an insight view of the footprints of the dominant flow structures, which otherwise could not be effectively captured by using the original wall-pressure fluctuations.     

Unsteady separated and reattaching turbulent flow over a two-dimensional square rib

The spatio-temporal characteristics of the separated and reattaching turbulent flow over a two-dimensional square rib were studied experimentally. Synchronized measurements of wall-pressure fluctuations and velocity fluctuations were made using a microphone array and a split-fiber film, respectively. Profiles of time-averaged streamwise velocity and wall-pressure fluctuations showed that the shear layer separated from the leading edge of the rib sweeps past the rib and directly reattaches on the bottom wall (x/H=9.75) downstream of the rib. A thin region of reverse flow was formed above the rib. The shedding large-scale vortical structures (fH/U₀=0.03) and the flapping separation bubble (fH/U₀=0.0075) could be discerned in the wall-pressure spectra. A multi-resolution analysis based on the maximum overlap discrete wavelet transform (MODWT) was performed to extract the intermittent events associated with the shedding large-scale vortical structures and the flapping separation bubble. The convective dynamics of the large-scale vortical structures were analyzed in terms of the autocorrelation of the continuous wavelet-transformed wall pressure, cross-correlation of the wall-pressure fluctuations, and the cross-correlation between the wall pressure at the time-averaged reattachment point and the streamwise velocity field. The convection speeds of the large-scale vortical structures before and after the reattachment point were U_c=0.35U₀ and 0.45U₀, respectively. The flapping motion of the separation bubble was analyzed in terms of the conditionally averaged reverse-flow intermittency near the wall region. The instantaneous reattachment point in response to the flapping motion was obtained; these findings established that the reattachment zone was a 1.2H-long region centered at x/H=9.75. The reverse-flow intermittency in one period of the flapping motion demonstrated that the thin reverse flow above the rib is influenced by the flapping motion of the separation bubble behind the rib.     

Servo-hydraulic System for Controlled Velocity Water Impact of Marine Sandwich Panels

Slamming, the impact between a marine craft’s hull and the water surface is a critical load case for structural design of marine vessels. The importance of hull slamming has led to a significant body of work to understand, predict and model these impacts. There is however, a lack of experimental data for validation, particularly for deformable panels and sandwich structures. This paper describes a high-velocity panel slamming test system that enables the generation of comprehensive and reliable experimental data on slamming impacts for both rigid and flexible panel structures. The pressure magnitudes, time-histories and spatial distributions resulting from testing of a nominally rigid panel have been compared with previous analytical, semi-empirical and experimental studies. Slamming impacts of a deformable sandwich panel are shown to cause different pressures to those from a rigid panel impact, resulting in increased transverse shear loading at the panel edge.     

DDES and Acoustic Prediction of Rudimentary Landing Gear Experiment Using Unstructured Finite Volume Methods

The far-field noise produced by a Rudimentary Landing Gear (RLG) was investigated using Delayed Detached Eddy Simulation (DDES) coupled with a Ffowcs-Williams Hawkings (FWH) integral. Computational results are presented for the RLG geometry using three unstructured grids of quasi-nested refinement. The geometry consists of a square post attached to an axle supporting four wheels, which is entirely suspended from an inviscid flat plate. The Reynolds number was fixed at 10⁶ based on the wheel diameter and freestream properties. The freestream Mach number was fixed at 0.115. The effect of grid resolution using a low dissipation scheme is examined in terms of both aerodynamic forces as well as acoustic spectra. Flow visualizations reveal rich three-dimensional unsteady content and the pattern of wall-pressure fluctuations are consistent with experimental observations. In general the FWH equation predictions of the far-field noise are in very good agreement with acoustic microphone measurements. From these preliminary results it appears that DDES has the potential to become a viable tool for evaluating low noise gear designs during the early design phases before wind tunnel and flight testing have commenced. Efforts are underway to extend the approach to more complex landing gear geometries.     

On the Mach- and Reynolds-Number Dependence of the Flat-Plate Turbulent Boundary Layer Wall-Pressure Spectrum

Wall-pressure measurements are performed using an array of pinhole microphones mounted into the test wall of a low noise, low turbulence flow facility. The frequency–wave-number spectra for both streamwise and spanwise directions are obtained through the spatial Fourier transformation of cross-spectral measurements in the said directions. The results of the wind-tunnel measurements reveal an interesting behavior of the low wave-number portion of the wall-pressure spectrum. Namely, this portion of the spectrum does not exhibit any Mach-number dependence for nearly incompressible flows. A low Mach-number analysis accounting for finite Reynolds-number effects is performed. This analysis shows the wall-pressure spectrum to have a leading term independent of the Mach number. The frequency dependence of the theoretical spectrum agrees well with that of the experimental spectrum. The physical mechanism for the Mach-number independence of the spectrum is attributed to a hydrodynamic dipole contribution to the wall pressure resulting from the Stokes layer induced by the turbulent eddies. The higher differential order of the viscous equations of motion allows for the existence of such Stokes layers. They have no counterpart in invisicd theories wherein no such Mach-number independence of the spectrum has been observed. A simple model problem is devised which isolates the viscous mechanism responsible for the modification of the wall pressure and the corresponding sound field. It is argued that the physical process involved is akin (albeit inversely) to that operative in the boundary-layer receptivity problem for nearly incompressible flows. Received 1 June 1998 and accepted 25 November 1998     

基于光纤光栅技术的船体变形修正技术

为进一步优化航天测量船变形测量系统,提出了基于光纤Bragg光栅技术的应变误差测量方法。首先通过对测量船变形系统的结构和光路进行几何分析,建立了应变量与误差角之间的转换模型。然后基于光纤光栅技术设计了测量船变形系统的应变误差测量方案。由应变与误差角的转换公式得到,运用常用的Si425型光纤光栅解调仪,其理论测量精度可达0.01″。