Large-eddy simulation of flows past a flapping airfoil using immersed boundary method

The numerical simulation of flows past flapping foils at moderate Reynolds numbers presents two challenges to computational fluid dynamics: turbulent flows and moving boundaries. The direct forcing immersed boundary (IB) method has been developed to simulate laminar flows. However, its performance in simulating turbulent flows and transitional flows with moving boundaries has not been fully evaluated. In the present work, we use the IB method to simulate fully developed turbulent channel flows and transitional flows past a stationary/plunging SD7003 airfoil. To suppress the non-physical force oscillations in the plunging case, we use the smoothed discrete delta function for interpolation in the IB method. The results of the present work demonstrate that the IB method can be used to simulate turbulent flows and transitional flows with moving boundaries

Subgrid scale fluid velocity timescales seen by inertial particles in large-eddy simulation of particle-laden turbulence

Large-eddy simulation (LES) has emerged as a promising tool for simulating turbulent flows in general and, in recent years,has also been applied to the particle-laden turbulence with some success (Kassinos et al., 2007). The motion of inertial particles is much more complicated than fluid elements, and therefore, LES of turbulent flow laden with inertial particles encounters new challenges. In the conventional LES, only large-scale eddies are explicitly resolved and the effects of unresolved, small or subgrid scale (SGS) eddies on the large-scale eddies are modeled. The SGS turbulent flow field is not available. The effects of SGS turbulent velocity field on particle motion have been studied by Wang and Squires (1996), Armenio et al. (1999), Yamamoto et al. (2001), Shotorban and Mashayek (2006a,b), Fede and Simonin (2006), Berrouk et al. (2007), Bini and Jones (2008), and Pozorski and Apte (2009), amongst others. One contemporary method to include the effects of SGS eddies on inertial particle motions is to introduce a stochastic differential equation (SDE), that is, a Langevin stochastic equation to model the SGS fluid velocity seen by inertial particles (Fede et al., 2006; Shotorban and Mashayek, 2006a; Shotorban and Mashayek, 2006b; Berrouk et al., 2007; Bini and Jones, 2008; Pozorski and Apte, 2009).However, the accuracy of such a Langevin equation model depends primarily on the prescription of the SGS fluid velocity autocorrelation time seen by an inertial particle or the inertial particle–SGS eddy interaction timescale (denoted by \delt T_{Lp} and a second model constant in the diffusion term which controls the intensity of the random force received by an inertial particle (denoted by C_0, see Eq. (7)). From the theoretical point of view, dTLp differs significantly from the Lagrangian fluid velocity correlation time (Reeks, 1977; Wang and Stock, 1993), and this carries the essential nonlinearity in the statistical modeling of particle motion. dTLp and C0 may depend on the filter width and particle Stokes number even for a given turbulent flow. In previous studies, dTLp is modeled either by the fluid SGS Lagrangian timescale (Fede et al., 2006; Shotorban and Mashayek, 2006b; Pozorski and Apte, 2009; Bini and Jones, 2008) or by a simple extension of the timescale obtained from the full flow field (Berrouk et al., 2007). In this work, we shall study the subtle and on-monotonic dependence of \delt T_{Lp} on the filter width and particle Stokes number using a flow field obtained from Direct Numerical Simulation (DNS). We then propose an empirical closure model for $\delta T_{Lp}$. Finally, the model is validated against LES of particle-laden turbulence in predicting single-particle statistics such as particle kinetic energy. As a first step, we consider the particle motion under the one-way coupling assumption in isotropic turbulent flow and neglect the gravitational settling effect. The one-way coupling assumption is only valid for low particle mass loading

中国科学院力学研究所非线性力学国家重点实验室;

湍流的小尺度混合特性对湍流燃烧过程至关重要。在大涡模拟中,由于采用空间滤波操作,湍流的亚格子尺度运动被过滤掉,亚格子尺度的湍流混合特性需要进一步模化。假设的标量在亚格子尺度中的概率密度分布函数是一种主要的模化方法。作者之前的研究显示,使用标量的一阶和二阶滤波矩作为参数的假设双高斯滤波密度函数模型可以很好的描述标量在亚格子中的混合状态,用于预测三阶和四阶滤波矩时也得到了很好的结果。这预示着,标量的滤波密度函数可能是一阶和二阶标量滤波矩的某种函数。因此,以一阶和二阶标量滤波矩作为条件变量,从解析的角度推导出湍流亚格子混合的条件滤波密度函数表达式,并与湍流标量混合的DNS结果互相验证。更进一步地,将..

Self-propulsion of flapping bodies in viscous fluids: Recent advances and perspectives

Flapping-powered propulsion is used by many animals to locomote through air or water. Here we review recent experimental and numerical studies on self-propelled mechanical systems powered by a flapping motion. These studies improve our understanding of the mutual interaction between actively flapping bodies and surrounding fluids. The results obtained in these works provide not only new insights into biolocomotion but also useful information for the biomimeticdesign of artificial flyers and swimmers.</div

On applicability of the Kutta-Joukowski theorem to low-Reynolds-number unsteady flows

The limitations of the Kutta-Joukowski (K-J) theorem in prediction of the time-averaged and instantaneous lift of an airfoil and a wing in low-Reynolds-number unsteady flows are examined. A general lift formula for a rectangular control volume is given in a very simple form in the framework of viscous flow theory, which provides a rational foundation for a direct comparison with the K-J theorem considered as a reduced case. Direct numerical simulations on the stationary and flapping flat plate and rectangular wing are conducted to assess the accuracy of both the K-J theorem and the general lift formula. In particular, the Lamb vector integral for the vortex force and the acceleration term of fluid for the unsteady inertial effect are evaluated as the main contributions to the unsteady lift generation of a flapping wing

丙型肝炎患者核酸定量检测与抗体水平相关性研究

目的分析丙型肝炎患者核酸定量检测(HCV-RNA)与抗体水平的相关性。方法研究共纳入104例丙肝患者,对HCV抗体水平与HCV病毒载量进行相关性分析,根据HCV病毒载量水平升高与否分组,比较两组间HCV抗体水平差异有无统计学意义。双侧P值<0.05认为差异有统计学意义。结果HCV抗体水平与HCV病毒载量呈正相关(r=0.466),HCV病毒载量正常组与升高组HCV抗体水平(S/CO值)分别为5.70±1.96 vs. 12. 84±2.76(t=-8. 350,P=0.001),当cut-off值为9. 88时,ROC曲线下面积达到最大值(AUC=0. 868),其灵敏度与特异性分别为94. 12%及73.56%。结论HCV抗体水平与HCV病毒载量呈正相关,在临床工作中,当HCV抗体水平处于较高水平时,HCV病毒载量往往也会升高,HCV抗体水平可赋予更多的临床应用价值

How flexibility affects the wake symmetry properties of a self-propelled plunging foil

The wake symmetry properties of a flapping-foil system are closely associated with its propulsive performance. In the present work, the effect of the foil flexibility on the wake symmetry properties of a self-propelled plunging foil is studied numerically. We compare the wakes of a flexible foil and a rigid foil at a low flapping Reynolds number of 200. The two foils are of the same dimensions, flapping frequency, leading-edge amplitude and cruising velocity but different bending rigidities. The results indicate that flexibility can either inhibit or trigger the symmetry breaking of the wake. We find that there exists a threshold value of vortex circulation above which symmetry breaking occurs. The modification of vortex circulation is found to be the pivotal factor in the influence of the foil flexibility on the wake symmetry properties. An increase in flexibility can result in a reduction in the vorticity production at the leading edge because ofthe decrease in the effective angle of attack, but it also enhances vorticity production at the trailing edge because of the increase in the trailing-edge flapping velocity. The competition between these two opposing effects eventually determines the strength of vortex circulation, which, in turn, governs the wake symmetry properties. Further investigation indicates that the former effect is related to the streamlined shape of the deformed foil while the latter effect is associated with structural resonance. The results of this work provide new insights into the functional role of passive flexibility inflapping-based biolocomotion

Molecular Dynamics Simulation of Thermal and Mechanical Properties of Polyimide-Carbon-Nanotube Composites

An aromatic polyimide and its mixture with randomly distributed carbon nanotubes (NTs) are simulated by using molecular dynamics, repeated energy minimization and cooling processes. The glass transition temperatures are identified through volume-temperature curves. Stress-strain curves, Young's moduli, densities and Poisson ratios are computed at different temperatures. It is demonstrated that the carbon NT reduces the softening effects of temperature on mechanical properties and increases the ability to resist deformation

A dynamic coupling model for hybrid atomistic-continuum computations

A dynamic coupling model is developed for a hybrid atomistic-continuum computation in micro- and nano-fluidics. In the hybrid atomistic-continuum computation, a molecular dynamics (MD) simulation is utilized in one region where the continuum assumption breaks down and the Navier-Stokes (NS) equations are used in another region where the continuum assumption holds. In the overlapping part of these two regions, a constrained particle dynamics is needed to couple the MD simulation and the NS equations. The currently existing coupling models for the constrained particle dynamics have a coupling parameter, which has to be empirically determined. In the present work, a novel dynamic coupling model is introduced where the coupling parameter can be calculated as the computation progresses rather than inputing a priori. The dynamic coupling model is based on the momentum constraint and exhibits a correct relaxation rate. The results from the hybrid simulation on the Couette flow and the Stokes flow are in good agreement with the data from the full MD simulation and the solutions of the NS equations, respectively. (c) 2007 Elsevier Ltd. All rights reserved

Mapping Closure Approximation for Reactive Scalars in Random Flows

The Mapping Closure Approximation (MCA) approach is developed to describe the statistics of both conserved and reactive scalars in random flows. The statistics include Probability Density Function (PDF), Conditional Dissipation Rate (CDR) and Conditional Laplacian (CL). The statistical quantities are calculated using the MCA and compared with the results of the Direct Numerical Simulation (DNS). The results obtained from the MCA are in agreement with those from the DNS. It is shown that the MCA approach can predict the statistics of reactive scalars in random flows