查看更多>>摘要：Experimental studies dealing with the wake of isolated stationary bluff bodies are reviewed. After briefly recalling the pioneering works in this domain, the paper focuses on recent research conducted with the latest experimental methods and techniques. The review encompasses a range of topics, including the effects of bluff body geometry (non-circular cross sections and non-uniformity in spanwise direction), steady and unsteady (periodic and non-periodic) inflow conditions, surface proximity (rigid wall, confinement, and water free surface) and non- Newtonian fluids. Focus is brought to the flow physics of the wakes, especially the complex three-dimensional and oscillatory behaviors induced by the periodic vortex shedding phenomenon. The paper aims to offer a critical and systematic review of new knowledge and findings on the subject area, as well as the most frequently adopted experimental techniques. The review also helps identifying knowledge gaps in the literature that need to be addressed in future investigations.
查看更多>>摘要：In this paper, we conducted a selective review on the recent progress in physics insight and modeling of flexible cylinder flow-induced vibrations (FIVs). FIVs of circular cylinders include vortex-induced vibrations (VIVs) and wake-induced vibrations (WIVs), and they have been the center of the fluid-structure interaction (FSI) research in the past several decades due to the rich physics and the engineering significance. First, we summarized the new understanding of the structural response, hydrodynamics, and the impact of key structural properties for both the isolated and multiple circular cylinders. The complex FSI phenomena observed in experiments and numerical simulations are explained carefully via the analysis of the vortical wake topology. Following up with several critical future questions to address, we discussed the advancement of the artificial intelligent and machine learning (AI/ML) techniques in improving both the understanding and modeling of flexible cylinder FIVs. Though in the early stages, several AL/ML techniques have shown success, including auto-identification of key VIV features, physics-informed neural network in solving inverse problems, Gaussian process regression for automatic and adaptive VIV experiments, and multi-fidelity modeling in improving the prediction accuracy and quantifying the prediction uncertainties. These preliminary yet promising results have demonstrated both the opportunities and challenges for understanding and modeling of flexible cylinder FIVs in today's big data era.
查看更多>>摘要：Here, we show that the fundamental Fickian process of diffusion can be equivalently described by a pressureless, compressible, and irrotational flow dynamics, whose effective force is governed by the divergence of the viscous stress tensor. This brings together the seemingly unrelated processes of mass and momentum transports.
查看更多>>摘要：Self-excited combustion instabilities of transverse modes were experimentally investigated in a rectangular multi-injector model combustor, operating with the bipropellants O_2/CH_4. The propellants were injected through a linear array of five oxidizer-centered shear coaxial injectors into the combustor. High-amplitude limit cycles obtained in hot-fire tests were analyzed in detail. Different combustion instability modes, including first and second width modes, were observed in cases with three different injection distribution schemes. Hence, the injection distribution strongly determined the combustion dynamics. One insight can be gained that the stable combustion could be achieved by properly designing the propellants' injection distributions.
查看更多>>摘要：Acoustic waves existing in hypersonic boundary layers act as a heat pump that transfers energy from the sonic line to the wall causing the wall temperature to rise, which explains the newly identified aerodynamic heating related to Mack's second mode from the perspective of thermoacoustic effects. The analysis of data from direct numerical simulations shows that Mack's second mode, belonging to the family of trapped acoustic waves, is highly amplified in a Mach 6 boundary layer and becomes sufficiently strong to affect the mean wall-normal temperature gradient, and the energy transport in the wall-normal direction due to the thermoacoustic effect balancing the thermal conduction brought by the additional temperature gradient.
J. Lira-EscobedoJ. R. Vélez-CorderoPedro E. Ramírez-González
7页
查看更多>>摘要：Dynamical heterogeneities in glass-forming liquids subjected to cooling processes are studied by a theoretical framework based on the non-equilibrium self-consistent generalized Langevin equation theory. This theory predicts that slow cooling rates permit the relaxation to the equilibrium state distinguished by a homogeneous local density. In contrast, fast cooling rates provoke dynamically arrested densityfluctuations and the establishment of permanent spatial heterogeneities even in the presence of density gradients. We further show that the dynamics toward the arrested state has two steps: a truncated relaxation followed by a second relaxation of the diluted part of the system.
查看更多>>摘要：Since the onset of the COVID-19 pandemic, a large number of flow visualization procedures have been proposed to assess the effect of personal protective equipment on respiratory flows. This study suggests infrared thermography as a beneficial visualization technique because it is completely noninvasive and safe and, thus, can be used on live individuals rather than mannequins or lung simulators. Here, we examine the effect of wearing either of three popular face coverings (a surgical mask, a cloth mask, or an N95 respirator with an exhalation valve) on thermal signatures of exhaled airflows near a human face while coughing, talking, or breathing. The flow visualization using a mid-wave infrared camera captures the dynamics of thermal inhomogeneities induced by increased concentrations of carbon dioxide in the exhaled air. Thermal images demonstrate that both surgical and cloth face masks allow air leakage through the edges and the fabric itself, but they decrease the initial forward velocity of a cough jet by a factor of four. The N95 respirator, on the other hand, reduces the infrared emission of carbon dioxide near the person's face almost completely. This confirms that the N95-type mask may indeed lead to excessive inhalation of carbon dioxide as suggested by some recent studies.
查看更多>>摘要：Flexible tail fins are commonly found in undulatory swimmers which can propel freely in omni-direction with flapping-wing-based propulsion. In this work, the hydrodynamic performance of an unconstrained flapping foil equipped with a flexible tail fin at different length is investigated numerically. As the fin length L_(fin) changes from 0.2c to c with c being the cord length, the propelling speed of the system first increases and then decreases after maximum propelling speed is achieved when the fin length is 0.8c. There are two kinds of wake vortical structures observed with bending stiffness k_b = 2.0: (i) the regular reverse Bénard-von Kármán vortex configuration for foil with short fin and (ii) the aligned vortices with two-layered street at downstream for foil with long fin (L_(fin) ≥ 0.6c). Control volume analysis reveals that for both types of vortical structures, the time-averaged thrust force is mainly related to the momentum flux contribution from the downstream face. Besides, the wake symmetry of a pitching foil with flexible tail fin is sensitive to the vertical phase velocity of vortices, where it can be used to predict whether the wake symmetry of the unconstrained system is preserved. Moreover, the bending stiffness effectively affects the hydrodynamic performance, and the breaking of wake symmetry greatly reduces the propulsive efficiency. The results obtained shed some new light on the role of flexible structures in the self-propulsive biological system and furthered our understanding of flexible selfpropulsion system.
查看更多>>摘要：The influences of serrated trailing edge on the aerodynamic and aeroacoustic performance of a flapping wing during hovering flight are investigated using a hybrid framework of an immersed boundary Navier-Stokes solver for the flow field and the Ffowcs Williams-Hawkings (FW-H) analogy for the sound field. A rigid rectangular wing with an aspect ratio of 2 undergoes pitching and stroke motions at a Reynolds number (Re) of 310 and a Mach number (M) of 0.012. Simulations are conducted by varying the dimensionless wavenumber k* from 2p to 10p and wave amplitude 2h* from 0.25 to 1.0. We find that at k* = 8p and 2h* = 1:5 (D4), the average sound power level is reduced by up to 6.8 dB within the Strouhal number (St) between 2.0 and 4.0 compared to that of a plain trailing edge while the lift coefficient is maintained. The directivity at St=0.2, St=0.4, St=2.2, and St=2.4 is discussed. It is found that the serrations of D4 do not affect the directivity for the first two frequencies and significantly reduce the magnitude of the directivity for the last two frequencies. The serrations of D4 considerably alter the flow field near the wing surface and reduced the surface pressure fluctuations near the wing tip, leading to the noise reduction. The lift coefficient of D4 is not significantly changed, because the reduction in the pressure-contributed lift is compensated by an increase in the shear stress-contributed lift. The serrations with higher 2h* and k* have larger shear stress-contributed lift.
查看更多>>摘要：The swimming of a dumbbell and of a three-sphere chain in a viscous incompressible fluid is studied on the basis of simplified equations of motion, which take into account both friction and inertial effects. The study is focused on the question of whether a particular swimmer will swim slower or faster in syrup than in water. The answer to this question for the dumbbell turns out to be complex. For the three-sphere chain, there is a small increase in swimming velocity in syrup.
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