摘要 :
In general, motion-induced vortex vibration is considered to be caused by the unification of the separated vortex from leading edge and the secondary vortex at trailing edge. However, the results of wind tunnel tests for rectangul...
展开
In general, motion-induced vortex vibration is considered to be caused by the unification of the separated vortex from leading edge and the secondary vortex at trailing edge. However, the results of wind tunnel tests for rectangular cylinders have revealed that the secondary vortex at trailing edge is not necessary for the generation of motion-induced vortex vibration. In this study, two kinds of cross sections where the secondary vortex at trailing edge is considered less likely to occur were selected as the target cross sections. Wind tunnel tests were conducted in order to give a further explanation of the effect of secondary vortex at trailing edge on the motion-induced vortex vibration at the wind velocity region where larger response amplitude occurs. As a result, the secondary vortex at trailing edge is not necessary not only for the onset of the motion-induced vortex vibration, but also at wind velocities where larger response amplitude occurs.
收起
摘要 :
In general, motion-induced vortex vibration is considered to be caused by the unification of separated vortex from leading edge and secondary vortex at trailing edge. However, the results of wind tunnel tests for the cross section...
展开
In general, motion-induced vortex vibration is considered to be caused by the unification of separated vortex from leading edge and secondary vortex at trailing edge. However, the results of wind tunnel tests for the cross sections with side ratios of 1.18 reveal that motion-induced vortex in heaving mode might occur without the formation of secondary vortex at trailing edge. Therefore, in order to clarify the relation between secondary vortex at trailing edge and the generation of motion- induced vortex vibration, spring-supported and flow visualization tests for rectangular cross sections with side ratios of 0.50-6.0 were conducted. It was found that the stable shedding of secondary vortex at trailing edge is not necessary condition of the generation of motion-induced vortex vibration in heaving mode.
收起
摘要 :
In previous research the present authors have shown that cross flow vibrations can be induced on a cylinder by setting another cylindrical body downstream in cruciform arrangement with a certain gap s between them. These vibration...
展开
In previous research the present authors have shown that cross flow vibrations can be induced on a cylinder by setting another cylindrical body downstream in cruciform arrangement with a certain gap s between them. These vibrations are excited by two longitudinal vortices (LVs), i.e. the trailing vortex (TV) and the necklace vortex (NV), shedding periodically near the cross of the system. In this study, LVs shedding from three cruciform systems, i.e. circular-cylinder/circular-cylinder (CC/CC), circular-cylinder/strip-plate (CC/SP) and square-cylinder/strip-plate (SC/SP) system, are observed from three directions using water tunnel and surface-die-streak technique to clarify their three-dimensional structure. The gap-to-diameter ratio s/d is set at 0.08 or 0.28 as the representative values of TV and NV, respectively. Formation of TV or NV depends on the system geometry s/d and Reynolds number Re. In the case of CC/CC, TV is formed at s/d = 0.08 and NV at s/d = 0.28 steadily when 1400 ≤ Re ≤ 2000, and they shed periodically when 3300 ≤ Re ≤ 5700. In the case of CC/SP, steady shedding of TV is observed at the both values of s/d in the lower Re range and periodic shedding is observed only for TV at s/d = 0.08 in the higher Re range, but with poorer periodicity. In the case of SC/SP, NV is formed at the both values of s/d and periodic shedding is not observed up to the highest value of Re = 5000 in this experiment.
收起
摘要 :
Trailing-edge vortex manipulation has been investigated using particle image velocimetry (PIV) for an airfoil undergoing harmonic plunging superimposed with a pitching motion near the bottom of the stroke. The so-called quick-pitc...
展开
Trailing-edge vortex manipulation has been investigated using particle image velocimetry (PIV) for an airfoil undergoing harmonic plunging superimposed with a pitching motion near the bottom of the stroke. The so-called quick-pitch motion has been evaluated through a comparison with a benchmark pure-sinusoidal plunge motion for Re = 30000 and k = 0.25. It has been shown that the trailing-edge vortex circulation can be reduced by more than 60% for all quick-pitch cases. The reduction in trailing-edge vortex circulation has been achieved without diminishing the strength of the leading-edge vortex, thus maintaining the lift augmentation achieved through dynamic stall. The improvement over the benchmark case is then confirmed through a statistical analysis. Finally, an analysis of the flow separation over the airfoil shows that the various quick-pitch motions facilitate earlier flow reattachment at the bottom of the stroke.
收起
摘要 :
Numerical investigation of the scaling intake duct has been carried out to analyze the generation mechanisms of ground vortex in crosswind condition and develop a mathematical model for predicting the vortex strength according to ...
展开
Numerical investigation of the scaling intake duct has been carried out to analyze the generation mechanisms of ground vortex in crosswind condition and develop a mathematical model for predicting the vortex strength according to the ground vortex aerodynamic characteristic. Combination of different ground clearances and different velocity ratios are considered. Results show that the trailing vortex and ambient vorticity are both the key aspects for the generation of ground vortex in crosswind condition. Ground vortex can be transformed from trailing vortex in crosswind condition even if there is no ambient vorticity which is obviously different from the generation of ground vortex in headwind or quiescent condition. In crosswind condition, ground vortex strength can be intensified by the ambient vorticity. The process of ground vortex formation in crosswind condition can be well illustrated by trailing vortex-ground vortex flow model proposed in this paper. Aerodynamic characteristic of ground vortex show that the vorticity of ground vortex increases to its maximum and then decreases sharply with the increasing of approaching flow velocity which is called aerodynamic self-similarity. Based on this aerodynamic self-similarity, a mathematical model for predicting the ground vortex strength is established and verified. The good agreement between the numerical results and the predicting results shows the accuracy and effectiveness of the mathematical model in some extent. (C) 2019 Elsevier Masson SAS. All rights reserved.
收起
摘要 :
Extensive experimental and numerical investigations on a highly swept generic unmanned combat aerial vehicle (UCAV) configuration of lambda type with a variable leading edge contour have been conducted. Within these investigations...
展开
Extensive experimental and numerical investigations on a highly swept generic unmanned combat aerial vehicle (UCAV) configuration of lambda type with a variable leading edge contour have been conducted. Within these investigations, it was shown that the flow field is dominated by complex vortex systems including vortex-to-vortex and vortex-to-boundary layer interactions. The vortex-dominated flow field has a strong nonlinear influence on the aerodynamic behavior of the configuration. Hence, controllability is demanding and poses a real challenge in the design of these kinds of configurations. Especially, the dimensioning of the control surfaces, for the lateral- and longitudinal stability of tailless configurations of low aspect ratio and high leading-edge sweep, poses a challenging task which is not yet solved. To understand the problem of lacking lateral- and longitudinal stability for these kinds of configurations, experiments in the subsonic and transonic flow regime have been conducted for the Stability and Control Configuration (SACCON), which has a leading edge sweep of 53°, to assess the control surface effectiveness of conventional trailing-edge control devices. The present study reviews the experimental investigations conducted with the highly swept generic UCAV configuration SACCON.
收起
摘要 :
Recent research(A.Ducci and M.Yianneskis M.,Vortex tracking and mixing enhancement in stirred processes,AIChE J 53(2)(2007),p.305-315)has shown that mixing times in reactors stirred by radial impellers can be reduced by 20-30% whe...
展开
Recent research(A.Ducci and M.Yianneskis M.,Vortex tracking and mixing enhancement in stirred processes,AIChE J 53(2)(2007),p.305-315)has shown that mixing times in reactors stirred by radial impellers can be reduced by 20-30% when feed insertion is made in the vortex core of precessional flow structures,denoted as macro-instabilities(Mis).The aim of the present work is to investigate the interaction between large-scale flow structures,such as Mis and trailing vortices,and assess to which extent local mixing might be affected by their combined activity.Proper orthogonal decomposition(POD)analysis was employed to identify and characterise particle image velocimetry(PIV)data of the different flow structures.It is shown that the combined presence of the trailing vortex and MI structures in the impeller vicinity results in energy levels that are substantially enhanced and thus locations showing further promise for feed insertion and mixing enhancement may be identified.
收起
摘要 :
This paper presents the results of an experimental investigation on the near field of a tip vortex generated by a blade at moderate incidence. The experiments were conducted at Xe = 15 000 and the boundary layer over the blade sep...
展开
This paper presents the results of an experimental investigation on the near field of a tip vortex generated by a blade at moderate incidence. The experiments were conducted at Xe = 15 000 and the boundary layer over the blade separated around midchord on the upper surface. Laser-Doppler measurements of the turbulent flow (Tu = 1.5%) were performed at various stations downstream of the blade. The three components of the mean velocity field and turbulent attributes were quantified at cross-planes, characterizing both the blade wake and the tip vortex structure. This allowed the analysis of the rollup and initial stages of decay of the lip vortex in the light of known theories and models. The axial velocity defect at the center of the vortex core evolved as x(-1) log x, without displaying any significant outgrowth imposed by the separated flow upstream. Momentum balances were also carried out at a station downstream to the conclusion of vortex rollup. The approximate axisymmetry of the now field in the trailing vortex was used to formulate the balances in a cylindrical coordinate system. Among other observations, it was seen that an adverse axial pressure gradient developed in the vortex core, which reinforced the tenacity of the axial velocity defect. In contrast, an area influenced by a favorable pressure gradient was found outside the core. (C) 2000 Elsevier Science Inc. All rights reserved. [References: 31]
收起
摘要 :
Stable columnar vortices subject to hydrodynamic noise (e.g. turbulence) present recurrent behaviours, such as the systematic development of vortex rings at the periphery of the vortex core. This phenomenon lacks a comprehensive e...
展开
Stable columnar vortices subject to hydrodynamic noise (e.g. turbulence) present recurrent behaviours, such as the systematic development of vortex rings at the periphery of the vortex core. This phenomenon lacks a comprehensive explanation, partly because it is not associated with an instability stricto sensu. The aim of the present paper is to identify the physical mechanism triggering this intrinsic feature of vortices using an optimal perturbation analysis as a tool of investigation. We find that the generation of vortex rings is linked to the intense and rapid amplification of specific disturbances in the form of azimuthal velocity streaks that eventually evolve into azimuthal vorticity rolls generated by the rotational part of the local Coriolis force. This evolution thus appears to follow a scenario opposite to the classical lift-up view, where rolls give rise to streaks.
收起
摘要 :
In general, motion-induced vortex vibration is considered to be caused by the unification of the separated vortex from leading edge and the secondary vortex at trailing edge. However, the results of previous wind tunnel tests for ...
展开
In general, motion-induced vortex vibration is considered to be caused by the unification of the separated vortex from leading edge and the secondary vortex at trailing edge. However, the results of previous wind tunnel tests for rectangular cross sections with side ratios of B/D = 2, 4, 6 (B: along-wind length, D: cross-wind length) reveal that in the heaving mode, the secondary vortex at trailing edge is not always essential for the generation of motion-induced vibration, but can be relevant to the magnitude of the exciting force acting on the model. In this study, flow visualization tests by smoke wire method under forced vibration conditions and spring-supported tests by free vibration method were conducted. The emphasis of this study is placed on clarifying the effect of the secondary vortex at trailing edge on response characteristics of motion-induced vortex vibration in the torsional mode for rectangular cross sections. As a result, the stable shedding of the secondary vortex at trailing edge is not a necessary condition for the generation of motion-induced vortex vibration in the torsional mode, but the secondary vortex at trailing edge can be relevant to the magnitude of the exciting moment acting on the model.
收起
原文获取