共查询到20条相似文献,搜索用时 15 毫秒
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Moshe Rosenfeld 《国际流体数值方法杂志》1995,21(4):323-335
To permit simplified analysis of complex time-dependent flows, possible relationship between the near-wall flow, flow separation and vortices are studied numerically for a flow in a constricted two-dimensional channel. The pulsating incoming wave-form consists of a steady flow, followed by a half-sinus flow superimposed on the steady component. One pair of vortices is created in each cycle, one vortex near each wall. The vortices propagate downstream in the next cycles, promoting flow separation as they move. Existing flow separation criteria were not found to be uniformly valid. A relation between the near-wall flow and the vortical system exists only during the steady incoming flow phase of the cycle. It seems that local criteria of flow separation cannot be found for complex internal pulsating flow fields. However, the vorticity field can be utilized, even in complex time-periodic flows, for identifying vortices that have been formed by the roll-up of shear layers. 相似文献
3.
Richard A. Truesdell J. W. Bartsch T. Buranda L. A. Sklar A. A. Mammoli 《Experiments in fluids》2005,39(5):819-827
Pulsatile action can be used to mix two streams entering a tube from two separate branches of a bifurcation at low Reynolds
numbers. The pulsatile action is provided by two pinch valves, which deform flexible tubing immediately upstream of the connection.
The pinch valve action is controlled using a master-slave pulse generator setup. The quality of mixing is evaluated directly
by measuring the fluorescence that results from the chemical reaction of species transported in the two streams, one containing
native biotin and the other, fluorescein biotin bound to streptavidin. The reaction kinetics are accounted for by normalization
using fluorescence measurements on well mixed solutions at the same residence time. The results show that the pulsatile micromixer
provides almost complete mixing. Furthermore, the present measurements match results obtained in a previous experiment where
flow visualization and image analysis were used to measure mixing quality in a scaled-up model. 相似文献
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Numerical simulation on micromixer based on synthetic jet 总被引:1,自引:0,他引:1
This paper studied a concept of micromixer with a synthetic jet placed at the bottom of a rectangular channel. Due to periodic ejections from and suctions into the channel, the fluids are mixed effectively. To study the effects of the inlet velocity, the jet intensity and frequency, and the jet location on the mixing efficiency, 3-D numerical simulations of the micromixer have been carried out. It has been found that when the jet intensity and the frequency are fixed, the mixing efficiency increases when Re 〈 50, and decreases when Re 〉 50 with the best mixing efficiency achieved at Re = 50. When the ratio of the jet velocity magnitude to the inlet velocity is taken as 10 and the jet frequency is 100 Hz, the mixing index reaches the highest value. It has also been found that to get better mixing efficiency, the orifice of the synthetic jet should be asymmetrically located away from the channel's centerline. 相似文献
6.
A wind-tunnel study of the influence of flow suction on laminar boundary-layer separation behind a two-dimensional step on
the surface is performed. Hot-wire measurements are carried out at low subsonic flow velocities. It is demonstrated that this
method of flow control allows suppressing the formation of large-scale vortices determined by global stability properties
of the separation region.
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Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 4, pp. 60–65, July–August, 2006. 相似文献
7.
A time-dependent local elliptic vortex in a differential two-dimensional incompressible fluid flow is considered. Nonlinear oscillations of vortices of the cyclonic and anticyclonic types are described. It is found that the evolutionary tracks can be both closed and unclosed. The former correspond to the azimuthal oscillations of the principal axis and the latter to the complete rotational state of the elliptic vortex. Steady-state solutions are also obtained; they are represented by ellipses elongated or compressed along the flow. Small oscillations of the vortex equilibrium figures are investigated and a general dispersion relation for arbitrary perturbations is derived. The stability criterion is found.__________Translated from Izvestiya Rossiiskoi Academii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, 2005, pp. 81–93. Original Russian Text Copyright © 2005 by Antonov, Baranov, and Kondratev. 相似文献
8.
In this paper, we describe measurements of a three-dimensional (3D) flow in a T-shaped micromixer by means of digital holographic
microscopy. Imaging tracer particles in a microscopic flow with conventional microscopy is accompanied by a small depth-of-field,
which hinders true volumetric flow measurements. In holographic microscopy, the depth of the measurement domain does not have
this limitation because any desired image plane can be reconstructed after recording. Our digital holographic microscope (DHM)
consists of a conventional in-line recording system with an added magnifying optical element. The measured flow velocity and
the calculated vorticity illustrate four streamwise vortices in the micromixer outflow channel. Because the investigated flow
is stationary and strongly 3D, the DHM performance (i.e. accuracy and resolution) can be precisely investigated. The obtained
Dynamic spatial range and Dynamic velocity range are larger than 20 and 30, respectively. High-speed multiple-frame measurements illustrate the capability to simultaneously
track about 80 particles in a volumetric measurement domain. 相似文献
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J. Lepicovsky 《Experiments in fluids》2008,44(6):939-949
An extensive experimental study into the nature of the separated flows on the blade suction surface of modern transonic fans
is described in this paper. The study was a subtask of a larger experimental effort focused on blade flutter excited by flow
separation in the blade tip region. The tip sections of airfoils on transonic fan blades are designed for precompression and
consequently they differ from sections on the rest of the blade. The blade tip section was modeled by a low aspect ratio blade
and therefore most of the blade tested was exposed to the secondary flow effects. The aim of this work was to supply reliable
data on flow separation on transonic fan blades for validation of future analytical studies. The experimental study focused
on two visualization techniques: surface flow visualization using dye oils and schlieren (and shadowgraph) flow visualization.
The following key observations were made during the study. For subsonic inlet flow, the flow on the suction surface of the
blade was separated over a large portion of the blade, and the separated area increased with increasing inlet Mach number.
For the supersonic inlet flow condition, the flow was attached from the leading edge up to the point where a bow shock from
the upper neighboring blade imposed on the blade surface. Downstream, there was a separated flow region in which air flowed
in the direction opposite the inlet flow. Finally, past the separated flow region, the flow reattached to the blade surface.
For subsonic inlet flow, the low cascade solidity resulted in an increased area of separated flow. For supersonic flow conditions,
the low solidity resulted in an improvement in flow over the suction surface. 相似文献
11.
The advection properties of monopolar vortices subjected to background strain were investigated both experimentally and numerically. Dye-visualization studies in a stratified fluid demonstrated the deformation of the vortex core and the shedding of passive tracers from the edge of the vortex. The main kinematic characteristics of the vortex evolution could be well captured by a simple model, in which the monopole was represented by a (strength-varying) point vortex surrounded by a contour of passive tracers. Full numerical simulations of the vortex evolution showed an excellent agreement with the observed tracer distributions and also revealed that the spatial distribution of vorticity must be taken into account in order to explain the final tearing of the laboratory vortex. 相似文献
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I. V. Kolin V. G. Markov V. L. Sukhanov T. I. Trifonova D. V. Shukhovtsov 《Fluid Dynamics》2009,44(5):680-686
The results of an experimental wind-tunnel investigation of the flow patterns on the swept wing of a model aircraft realized
for pitching oscillations with an amplitude A
α = 5° with respect to setup angles of attack α0 = 10 and 16° are presented. 相似文献
14.
Bio-inspired corrugated airfoils show favourable aerodynamic characteristics such as high coefficient of lift and delayed stall at low Reynolds numbers. Two-dimensional (2D) direct numerical simulation has been performed here on a corrugated airfoil at various angles of attack (0°, +5°, -5°) and Reynolds number of 280 to 6700. The objective is to analyse the pressure variation inside the corrugations and correlate it to the vortex movement across the corrugations and the overall aerodynamic characteristics of the corrugated airfoil. The flow characteristics have been examined based on the local Strouhal numbers in the corrugations of the airfoil. It is observed that the pressure variation in each corrugation is the result of vortex merging and separation in the corrugation which plays a major role in changing the flow characteristics. The Strouhal number of the flow is dictated by the most dominant local Strouhal number. The numerical results are further compared with experimental results obtained using particle image velocimetry, and the two set of results are found to match well. These results are significant because they elucidate the effect of corrugation, angle of attack, and Reynolds number on flow over a corrugated airfoil. 相似文献
15.
Ola Lögdberg Kristian Angele P. Henrik Alfredsson 《European Journal of Mechanics - B/Fluids》2010,29(1):9-17
The robustness of vane-type vortex generators (VGs) for separation flow control was studied in a separating turbulent boundary layer on a flat plate. VG arrays of different sizes and streamwise positions were positioned upstream of the separation bubble and their effect on the flow field was studied with the help of particle image velocimetry (PIV). The extent of the separated region was varied by changing the pressure gradient. Three different separation bubbles were produced and their extent was approximately doubled for each increase in pressure gradient. It was found that the sensitivity of the control effect to changes in the size of the separation bubble is small within the applied range of pressure gradients. Furthermore, the importance of the relative position of the VGs with respect to the separated region is small. 相似文献
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Sound generation and flow interaction of vortices with an airfoil and a flat plate in transonic flow
The mechanisms of sound generation and the kind of interaction of vortices with airfoils in an airflow are investigated. Experiments have been performed in stationary flow with vortices of a Kármán vortex street and in a shock tube flow with a starting vortex of a lifting airfoil. Depending on the dimensions of vortices and airfoils, their distance, and the flow Mach numbers, different kinds and amplitudes of upstream propagating steep sound waves occur. 相似文献
18.
A new type of flow visualization method utilizing a smoke-wire, a high-speed camera with high framing rates and a laser light
sheet was employed to delineate the unsteady processes of large-scale vortices in the separated shear layer about a blunt-faced
flat plate at Re
H
= 560. The sequential images showed that the unsteady behavior of large-scale vortices in the separated shear layer varies
as the shedding phases of large-scale vortices alter. Particularly, at a certain phase, a vortex-merging process between the
two neighboring large-scale vortices took place.
Received: 17 November 1998/Accepted: 1 November 1999 相似文献
19.
This paper presents a numerical prediction of the formation of Goertler vortices on a curved surface with effect of rotation. The criterion of flow visualization marking the onset position of Goertler vortices is employed in the present paper. For facilitating the numerical study, the computation is carried out in the transformed x and ηplane. The results show that the onset position characterized by the Goertler number, depends on the rotation number Ro, the Prandtl number and the wave number. The value of critical Goertler number increases with the increase in negative rotation, while the value of Goertler number decreases with the increase in positive rotation on a concave surface. On the contrary, the value of critical Goertler number decreases with the increase in negative rotation on a convex surface. The obtained critical Goertler number and wave number are compared with the previous theoretical and experimental data. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
20.
To better understand mixing by hairpin vortices, time-series particle image velocimetry (PIV) was applied to the wake of
a trapezoidal-shaped passive mixing tab mounted at the bottom of a square turbulent channel (Re
h
=2,080 based on the tab height). Instantaneous velocity/vorticity fields were obtained in sequences of 10 Hz in the tab wake
in the center plane (x–y) and in a plane (x–z) parallel to the wall. Periodically-shed hairpin vortices were clearly identified and seen to rise as they advected downstream.
Experimental evidence shows that the vortex-induced ejection of the near-wall viscous fluid to the immediate upstream is important
to the dynamics of hairpin vortices. It can increase the strength of the hairpin vortices in the near tab region and cause
generation of secondary hairpin vortices further downstream when the hairpin heads are farther away from the wall. Measurements
also reveal the existence of a type of new secondary vortice with the opposite-sign spanwise vorticity. The distribution of
vortex loci in the x–y plane shows that the hairpin vortices and the reverse vortices are spatially segregated in distinct layers. Turbulence statistics,
including mean velocity profiles, Reynolds stresses, and turbulent kinetic energy dissipation rate distributions, were obtained
from the PIV data. These statistical quantities clearly reveal imprints of the identified vortex structures and provide insight
into mixing effectiveness.
Received: 24 February 2000/Accepted: 24 October 2000 相似文献