Robust airfoil optimization based on improved particle swarm optimization method

A robust airfoil optimization platform is constructed based on the modified particle swarm optimization method (i.e., the second-order oscillating particle swarm method), which consists of an efficient optimization algorithm, a precise aerodynamic analysis program, a high accuracy surrogate model, and a classical airfoil parametric method. There are two improvements for the modified particle swarm method compared with the standard particle swarm method. First, the particle velocity is represented by the combination of the particle position and the variation of position, which makes the particle swarm algorithm a second-order precision method with respect to the particle position. Second, for the sake of adding diversity to the swarm and enlarging the parameter searching domain to improve the global convergence performance of the algorithm, an oscillating term is introduced to the update formula of the particle velocity. At last, taking two airfoils as examples, the aerodynamic shapes are optimized on this optimization platform. It is shown from the optimization results that the aerodynamic characteristic of the airfoils is greatly improved in a broad design range.     

Rolling/sliding of a particle on a flat wall in a linear shear flow at finite Re

Recently Lee and Balachandar proposed analytically-based expressions for drag and lift coefficients for a spherical particle moving on a flat wall in a linear shear flow at finite Reynolds number. In order to evaluate the accuracy of these expressions, we have conducted direct numerical simulations of a rolling particle for shear Reynolds number up to 100. We assume that the particle rolls on a horizontal flat wall with a small gap separating the particle from the wall (L = 0.505) and thus avoiding the logarithmic singularity. The influence of the shear Reynolds number and the translational velocity of the particle on the hydrodynamic forces of the particle was investigated under both transient and the final drag-free and torque-free steady state. It is observed that the quasi-steady drag and lift expressions of Lee and Balachandar provide good approximation for the terminal state of the particle motion ranging from perfect sliding to perfect rolling. With regards to transient particle motion in a wall-bounded shear flow it is observed that the above validated quasi-steady drag and lift forces must be supplemented with appropriate wall-corrected added-mass and history forces in order to accurately predict the time-dependent approach to the terminal steady state. Quantitative comparison with the actual particle motion computed in the numerical simulations shows that the theoretical models quite effective in predicting rolling/sliding motion of a particle in a wall-bounded shear flow at moderate Re.     

Drag amplification of long flexible riser models undergoing multi-mode VIV in uniform currents

Measurement data of long flexible riser models are used to study the drag amplification due to multi-mode vortex induced vibration (VIV). The riser model was towed in a towing tank, and vibration along its length was measured indirectly at a set of discrete points using accelerometers, along with the total drag and lift forces on the model. These data were used to fit expressions for the drag amplification taking into account the spatial variation of the vibration amplitude along the riser length. The results were compared with the existing empirical expression used in the VIV analysis tool SHEAR7 as well as other test data. It was found that the expression in SHEAR7 agrees well with the test data. However, for larger vibration amplitudes, it appears to under-predict the drag amplification.     

Experimental study on vibrations of a nonwoven fabric cylindrical filter

Unexpected vibrations on a cylindrical filter made of nonwoven fabrics were clarified experimentally. Two types of filter with length L=1.8 m and 3.7 m, both 45 cm in diameter and 1.08 mm in thickness, were used. This is a new type of aeroelastic vibration phenomenon because the filter is a closed cylindrical vessel. In addition, the flow velocity of inner air was very slow and inner air flowed out slowly from the filter surface. The velocity distribution of air flow from a fan duct as well as the frequency and amplitude of the filter vibration were measured for two types of filter. By setting up a roll core panel at the outlet of the fan duct, we could rectify the outflow and suppressed the vibration of the shorter filter with L=1.8 m. However, this method was not adequate for the longer filter with L=3.7 m, and we could suppress the vibration by shielding the inner surface of the filter at the top region.     

平面滑移铰内发生自激振动的充分条件

自激振动是含摩擦机械系统最严重也最具破坏性的运动形式. 目前, 通常以增加润滑的方式 尽可能减少自激振动. 本文以平面滑移铰为研究对象, 假设铰连接的物体为刚体, 并且认为 铰内的间隙为小量, 推导了平面滑移铰内产生不符合物理事实解的条件. 这可看作是导致系 统产生自激振动的一种诱因. 该条件适用于任何含平面滑移铰的机械系统.     

高细对称结构正交耦合振动响应的试验研究

高细对称结构是指具有较大的高度／直径比,且横向和纵向完全对称的结构形式。由于其典型的对称电磁场特性,在星载天线上得到了一定的应用。有研究发现：高细对称工程结构在振动试验过程中往往表现出正交耦合响应较大的现象,这些现象相当于产品单方向振动激励时在两个互相正交方向同时进行了振动载荷,从而造成了产品的过试验问题。但是,工程实物模型相对较为复杂,无法通过仿真直观地揭示这种特殊现象的本质。本文针对在实践工程高细对称结构中发现的正交耦合振动响应现象,对这类工程结构进行了简化（将某一类星载天线简化为板梁弯曲结构）。在此基础上提出了试验验证模型,并完成了正弦激励载荷下的振动响应试验。通过分析试验数据对比了对称结构和非对称结构的振动耦合响应异同,剖析了各种产生正交耦合振动响应的原因;指出正交耦合振动响应是高细对称结构的一种固有特性,采用非对称设计可以减小正交耦合响应;最后提出了一种针对这种高细对称结构进行准确仿真计算的方法。     

一种新型SPH-FEM耦合算法及其在冲击动力学问题中的应用

为了充分发挥光滑粒子流体动力学方法(smoothed particle hydrodynamics,SPH)在处理大变形和有限元(finite element method,FEM)问题时计算精度高的优势,提出了一种新型SPH-FEM耦合算法.该耦合算法在大变形区域使用SPH粒子离散,其余区域使用有限元离散.在耦合界面...     

圆柱装药在偏心定向起爆时水中近场压力特性

对UHL-5装药在侧向9点偏心定向起爆和几何中心点起爆时的水下爆炸近场峰值压力分布特性进行实验研究和数值模拟,得到2种起爆条件下不同距离与方位角测点处冲击波峰值压力和侧向9点起爆时的定向增益区域.结果表明,对于5 kg圆柱形UHL-5装药,采用侧向9点偏心定向起爆方式时,在爆距750 mm、方位角90°范围内的流场区域...     

摩擦振动信号谐波小波包特征提取

在销-盘摩擦磨损试验机上进行了船用柴油机缸套-活塞环摩擦配副摩擦磨损试验,应用谐波小波包变换分析了不同磨损状态下的摩擦振动信号.结果表明:谐波小波包变换可提取微弱的摩擦振动特征.当磨损机制为轻微的黏着磨损时,摩擦振动信号的频谱以边频为主,没有明显的固有频率,平均幅值小;当磨损机制以磨粒磨损为主时,摩擦振动信号的频谱出现较大峰值的固有频率,但仍存在大量的边频,平均幅值变大;当磨损机制以疲劳磨损为主时,摩擦振动信号频谱的固有频率更加明显,边频显著减弱,平均幅值最大.因此,谐波小波包变换可实现微弱摩擦振动信号的特征提取.     

接触表面不平度对摩擦尖叫噪声的影响

针对摩擦尖叫的间歇性特点,建立了考虑摩擦接触表面不平度的3自由度摩擦自激振动机理模型.应用数值方法分析了接触表面不平度倾角对系统稳定性及瞬态动力学特性的影响,并进行了基于销-盘装置的摩擦尖叫噪声试验研究.结果表明:系统的稳定性主要取决于系统自身的耦合特性,但也与摩擦特性和接触表面不平度倾角的综合作用有关.当接触表面不平度倾角增大时,系统趋向不稳定,会激发尖叫噪声.