碳纳米管应变传感测量与偏振拉曼控制方法研究

面向微尺度平面变形的精细测量对碳纳米管应变传感测量方法进行了理论与实验研究。利用碳纳米管的拉曼应变敏感性及其偏振选择性,针对各种典型的拉曼光谱系统偏振构型,推导建立了适于各种偏振拉曼构型的碳纳米管平面应变传感解析关系式。从测量学角度出发,对不同偏振构型下的应变传感进行了对比分析,得出入射与散射光偏振方向皆能够连续控制且始终保持平行的双偏协同构型最适合于平面应变传感,并进一步提出易于实现的双偏协同构型光路配置方式与控制方法。通过实验应用证明,所提出的方法能够有效的实现基于偏振拉曼的碳纳米管平面应变传感测量。     

A study on coherent structures and drag-reduction in the wall turbulence with polymer additives by TRPIV

An experimental measurement was performed using time-resolved particle image velocimetry (TRPIV) to investigate the spatial topological character of coherent structures in wall-bounded turbulence of polymer additive solution. The fully developed near-wall turbulent flow fields with and without polymer additives at the same Reynolds number were measured by TRPIV in a water channel. The comparisons of turbulent statistics confirm that due to viscoelastic structure of long-chain polymers, the wall-normal velocity fluctuation and Reynolds shear stress in the near-wall region are suppressed significantly. Furthermore, it is noted that such a behavior of polymers is closely related to the decease of the motion of the second and forth quadrants, i.e., the ejection and sweep events, in the near-wall region. The spatial topological mode of coherent structures during bursts has been extracted by the new mu-level criteria based on locally averaged velocity structure function. Although the general shapes of coherent structures are unchanged by polymer additives, the fluctuating velocity, velocity gradient, velocity strain rate and vorticity of coherent structures during burst events are suppressed in the polymer additive solution compared with that in water. The results show that due to the polymer additives the occurrence and intensity of coherent structures are suppressed, leading to drag reduction.     

TRPIV MEASUREMENT OF DRAG-REDUCTION IN THE TURBULENT BOUNDARY LAYER OVER RIBLETS PLATE

采用高时间分辨率粒子图像测速技术对沟槽壁面平板湍流边界层速度矢量场的时间序列及其统计量进行了实验测量,讨论了在同一来流速度下沟槽壁面对平均速度剖面﹑雷诺切应力及湍流强度的影响. 用流向速度分量的多尺度空间局部平均结构函数辨识壁湍流多尺度相干结构,用条件采样和相位平均技术提取壁湍流多尺度相干结构喷射和扫掠事件的脉动速度、展向涡量的二维空间拓扑形态. 结果表明,与同材料光滑壁面对比,沟槽壁面实现了10.73%的摩阻减小量;沟槽壁面湍流边界层湍流强度及雷诺切应力皆比光滑平板湍流边界层对应统计量小,说明沟槽壁面有效降低了湍流边界层内流体的脉动. 通过比较壁湍流相干结构猝发事件各脉动速度分量与展向涡量的空间分布特征,肯定了沟槽壁面的减阻效果,发现沟槽壁面通过抑制相干结构猝发事件实现减阻.     

Statistical scale of hairpin packets in the later stage of bypass transition induced by cylinder wake

The hairpin packet's structure and its statistical scale in the later stage of bypass transition induced by a cylinder wake are investigated by time-resolved particle image velocimetry from the side and top view, respectively. Linear stochastic estimation is used to achieve the conditionally averaged velocity fields. For the side view case, the conditionally averaged structure consists of a series of swirling motions located along a line inclining at a large angle (18°) from the wall and a low-speed region occupied by the cylinder wake appearing right above them. In the (x, z)-plane at the wall-normal height y/???=?0.2, the dominant structures are shown to be the large-scale regions of low momentum elongated almost over 3?? along the streamwise. The low-speed regions are consistently bordered by small-scale counter-rotating vortice pairs organized in the streamwise with a statistical spanwise width of 0.55??. The results suggest that in the later part of the transitional zone, the upward induction of the cylinder wake enhances both the wall-normal and spanwise extent of the hairpin packets.     

Boundary-layer disturbances subjected to free-stream turbulence and simulation on bypass transition

The phenomena associated with the entrainment of free-stream turbulence(FST) into boundary-layer flows are relevant for a number of subjects.It has been believed that the continuous spectra of the Orr-Sommerfeld(O-S)/Squire equations describe the entrainment process,and thus they are used to specify the inlet condition in simulation of bypass transition.However,Dong and Wu(Dong,M.and Wu,X.On continuous spectra of the Orr-Sommerfeld/Squire equations and entrainment of free-stream vortical disturbances.Journal of Fluid Mechanics,732,616–659(2013)) pointed out that continuous spectra exhibit several non-physical features due to neglecting the non-parallelism.They further proposed a large-Reynolds-number asymptotic approach,and showed that the non-parallelism is a leading-order effect even for the short-wavelength disturbance,for which the response concentrates in the edge layer.In this paper,the asymptotic solution is verified numerically by studying its evolution in incompressible boundary layers.It is found that the numerical results can be accurately predicted by the asymptotic solution,implying that the latter is adequate for moderate Reynolds numbers.By introducing a series of such solutions as the inflow perturbations,the bypass transition is investigated via the direct numerical simulation(DNS).The transition processes,including the evolution of streaks,the amplification of secondary-instability modes,and the emergence of turbulent spots,agree with the experimental observations.     

Tomographic PIV investigation on coherent vortex structures over shark-skin-inspired drag-reducing riblets

Nature has shown us that the microstructure of the skin of fast-swimming sharks in the ocean can reduce the skin friction drag due to the well-known shark-skin effect.In the present study,the effect of shark-skin-inspired riblets on coherent vortex structures in a turbulent boundary layer(TBL) is investigated.This is done by means of tomographic particle image velocimetry(TPIV) measurements in channel fl ws over an acrylic plate of drag-reducing riblets at a friction Reynolds number of 190.The turbulent fl ws over drag-reducing riblets are verifie by a planar time-resolved particle image velocimetry(TRPIV) system initially,and then the TPIV measurements are performed.Two-dimensional(2D) experimental results with a dragreduction rate of around 4.81% are clearly visible over triangle riblets with a peak-to-peak spacing s+of 14,indicating from the drag-reducing performance that the buffer layer within the TBL has thickened;the logarithmic law region has shifted upward and the Reynolds shear stress decreased.A comparison of the spatial topological distributions of the spanwise vorticity of coherent vortex structures extracted at different wall-normal heights through the improved quadrant splitting method shows that riblets weaken the amplitudesof the spanwise vorticity when ejection(Q2) and sweep(Q4) events occur at the near wall,having the greatest effect on Q4 events in particular.The so-called quadrupole statistical model for coherent structures in the whole TBL is verified Meanwhile,their spatial conditional-averaged topological shapes and the spatial scales of quadrupole coherent vortex structures as a whole in the overlying turbulent fl w over riblets are changed,suggesting that the riblets dampen the momentum and energy exchange between the regions of near-wall and outer portion of the TBL by depressing the bursting events(Q2 and Q4),thereby reducing the skin friction drag.     

Elastic waves guided by a material interface

The propagation of interfacial small-amplitude waves along a rectilinear thin film separating two pre-stressed, incompressible, elastic media is addressed. The film is modelled as a material surface possessing its own mass density and normal and flexural stiffnesses. It is shown that these features induce dispersion as the obtained secular equations are polynomials of the second degree in the wavenumber when bending stiffness is absent (membrane-like interface), and of the fourth degree otherwise (plate-like interface). In both case, beyond the modified Stoneley mode, a bending mode for the interface, an additional propagating wave can exist, with amplitude polarized along the interface (extensional mode). The associated bifurcation problem is analyzed with focus on the effects of compressive residual forces at the interface. The buckling strain of a compressed metal layer embedded in an elastomeric medium is computed also with an exact approach, to provide the range of validity of the proposed simplified model of material interface.     

Visualization of Lagrangian fluid transport of a vortex ring based on time-resolved PIV

Journal of Visualization - We carry out experiments on vortex ring flows submerged underwater which are generated by a pulsatile circular jet with the stroke ratio of 10. The pulsatile Reynolds...     

On the pricing of American contingent claims under transaction costs and multiple risky assets

This paper addresses the hedging problem of American Contingents Claims (ACCs) in the framework of continuous-time Itô models for financial market. The special feature of this paper is that in the financial market the investor has to face fixed and proportional transaction costs when trading multiple risky assets. By using the auxiliary martingale approach and extending the results of Cvitanic and Karatzas [Cvitanic J, Karatzas I. Hedging and portfolio optimization under transaction costs: a martingale approach. Math Finance 1996;6:135–65] on pricing European contingent with transaction costs in the single-stock market, an arbitrage-free interval [h_low, h_up] is identified, and the end points are characterized by auxiliary martingales and stopping times in terms of auxiliary stochastic control problems. Here h_up and h_low are so-called the upper hedging price and the lower hedging price.     

基于双线性位移模式数字图像相关方法的误差分析及应用

探讨数字图像相关方法测试结果的误差分布规律对提高该方法的测试精度具有重要意义。本文从理论上分析了基于双线性位移模式数字图像相关方法的子区变形场测试误差分布规律,结果表明,当试件的真实变形可由双线性位移模式描述时,变形场最大测试误差通常出现在子区的边界或节点处。因此,若试件发生均匀拉伸等常应变变形,可利用相关系数选取一个最优子区,认为测得最优子区中心应变为试件真实应变。零变形实验验证了该测试方法的可靠性。最后对手机导光板试件在单轴拉伸载荷下的数字图像进行分析,并利用本文方法测试了其弹性常数。