共查询到20条相似文献,搜索用时 15 毫秒
1.
《Experimental Thermal and Fluid Science》2009,33(1):169-172
Flow control using zero-net-mass-flow jets in an S-shaped diffusing duct was investigated. Experiments were conducted in a channel flow facility at a Reynolds number, Re = 4.1 × 104 with particle image velocimetry measurements in the symmetry plane of the duct. In the natural configuration, separation of the boundary layer occurs in a region of the duct with an high degree of curvature. A stability analysis of the wall normal base flow at the location of the applied control is presented and estimates the most effective frequency of the actuator. Time-averaged velocity fields show total reattachment of the boundary layer using active flow control. 相似文献
2.
D. Estruch D. G. MacManus J. L. Stollery N. J. Lawson K. P. Garry 《Experiments in fluids》2010,49(3):683-699
The understanding of the behaviour of the flow around surface protuberances in hypersonic vehicles is developed and an engineering
approach to predict the location and magnitude of the highest heat transfer rates in their vicinity is presented. To this
end, an experimental investigation was performed in a hypersonic facility at freestream Mach numbers of 8.2 and 12.3 and Reynolds
numbers ranging from Re
∞/m = 3.35 × 106 to Re
∞/m = 9.35 × 106. The effects of protuberance geometry, boundary layer state, freestream Reynolds number and freestream Mach numbers were
assessed based on thin-film heat transfer measurements. Further understanding of the flowfield was obtained through oil-dot
visualizations and high-speed schlieren videos. The local interference interaction was shown to be strongly 3-D and to be
dominated by the incipient separation angle induced by the protuberance. In interactions in which the incoming boundary layer
remains unseparated upstream of the protuberance, the highest heating occurs adjacent to the device. In interactions in which
the incoming boundary layer is fully separated ahead of the protuberance, the highest heating generally occurs on the surface
just upstream of it except for low-deflection protuberances under low Reynolds freestream flow conditions in which case the
heat flux to the side is greater. 相似文献
3.
Mean lift generation on cylinders induced with plasma actuators 总被引:1,自引:0,他引:1
Bluff body flow control based on plasma actuators requires suitable strategies to attain the desired objectives. The strategy
selection becomes more critical in situations where the free airstream velocity is much higher than the maximum velocity that
can be produced by the flow control device. In this work, we report recent efforts to produce on a circular cylinder forces
in direction transverse to the free flow. Free stream velocities considered in this work are as high as 45 m/s ((Re(O) 1 × 105), which result much higher values than the maximum velocities (about 5 m/s) usually induced by the kind of plasma actuator
here considered (dielectric barrier type). Our strategy consisted on promoting asymmetries on boundary layer separation with
a four electrode arrangement. In our experiments, we measured drag and lift forces and explored the effect of exciting the
flow with steady and non-steady actuations. The device demonstrated authority to induce significant transverse forces and
optimal frequencies resulted in all cases close to the vortex shedding frequency. 相似文献
4.
Experimental Investigation of Three-Dimensional Vortex Structures Downstream of Vortex Generators Over a Backward-Facing Step 总被引:1,自引:0,他引:1
An experimental investigation of vortex generators has been carried out in turbulent backward-facing step (BFS) flow. The Reynolds number, based on a freestream velocity U0 = 10 m/s and a step height h = 30 mm, was Reh = 2.0 × 104. Low-profile wedge-type vortex generators (VGs) were implemented on the horizontal surface upstream of the step. High-resolution planar particle image velocimetry (2D-2C PIV) was used to measure the separated shear layer, recirculation region and reattachment area downstream of the BFS in a single field of view. Besides, time-resolved tomographic particle image velocimetry (TR-Tomo-PIV) was also employed to measure the flow flied of the turbulent shear layer downstream of the BFS within a three-dimensional volume of 50 × 50 × 10 mm3 at a sampling frequency of 1 kHz. The flow control result shows that time-averaged reattachment length downstream of the BFS is reduced by 29.1 % due to the application of the VGs. Meanwhile, the Reynolds shear stress downstream of the VGs is considerably increased. Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) have been applied to the 3D velocity vector fields to analyze the complex vortex structures in the spatial and temporal approaches, respectively. A coherent bandwidth of Strouhal number 0.3 < Sth < 0.6 is found in the VG-induced vortices, and moreover, Λ-shaped three-dimensional vortex structures at Sth = 0.37 are revealed in the energy and dynamic approaches complementarily. 相似文献
5.
The paper reports on experiments carried out over a wide range of Reynolds numbers in a high pressure wind tunnel. The model was a sharp-edged rectangular cylinder with aspect ratio height/width 1:5 (width/span ratio 1:10.8), which was investigated in both basic orientations, lengthwise (4×103<Re<4×105) and perpendicular to the flow (2.7×104<Re<6.4×105). The Reynolds number is based on the height of the model normal to the flow. Steady and unsteady forces were measured with a piezoelectric balance. Thus along with steady (i.e. time averaged values) including the base pressure coefficient, also power spectra and probability density functions were measured yielding for example Strouhal numbers, higher statistical moments, etc. A response diagram for the vortex resonance phenomenon was taken for the natural bending motion of the slender model. If lift coefficient for constant angle of attack is plotted against Reynolds number, a significant Reynolds number effect is seen. For α=4°, the curve shows an inflection point and the lift varies between 0.3 and 0.6. For α=6° and 2° there are similar variations shifted to lower and higher values of Re, respectively. Probably the shapes of separation bubbles that depend on the Reynolds number are responsible for these effects. No Reynolds number effects were observed when the long side was normal to the flow, an orientation where reattachment at the side walls is not possible. Comparing both basic cases (α=0° and 90°), the interpretation of the probability distributions of lift force leads to the conclusion that the possibility of reattachment (α=0°) seems to enhance the degree of order in the vortex shedding process. 相似文献
6.
High-lift airfoil trailing edge separation control using a single dielectric barrier discharge plasma actuator 总被引:3,自引:0,他引:3
Control of flow separation from the deflected flap of a high-lift airfoil up to Reynolds numbers of 240,000 (15 m/s) is explored
using a single dielectric barrier discharge (DBD) plasma actuator near the flap shoulder. Results show that the plasma discharge
can increase or reduce the size of the time-averaged separated region over the flap depending on the frequency of actuation.
High-frequency actuation, referred to here as quasi-steady forcing, slightly delays separation while lengthening and flattening
the separated region without drastically increasing the measured lift. The actuator is found to be most effective for increasing
lift when operated in an unsteady fashion at the natural oscillation frequency of the trailing edge flow field. Results indicate
that the primary control mechanism in this configuration is an enhancement of the natural vortex shedding that promotes further
momentum transfer between the freestream and separated region. Based on these results, different modulation waveforms for
creating unsteady DBD plasma-induced flows are investigated in an effort to improve control authority. Subsequent measurements
show that modulation using duty cycles of 50–70% generates stronger velocity perturbations than sinusoidal modulation in quiescent
conditions at the expense of an increased power requirement. Investigation of these modulation waveforms for trailing edge
separation control similarly shows that additional increases in lift can be obtained. The dependence of these results on the
actuator carrier and modulation frequencies is discussed in detail. 相似文献
7.
E. Krause W. Limberg A. M. Kharitonov M. D. Brodetsky A. Henze 《Experiments in fluids》1999,26(5):423-436
Supersonic flight of aerospace planes is of marked interest since several flow regimes characterized by different local flow structures have to be flown through. This problem was investigated experimentally for the hypersonic research configuration ELAC 1. The aim of the study was to detect the influence of the rounded leading edge, of the thickness distribution prescribed, and of the Reynolds number, especially on the flow on the leeward side of the configuration. The experiments were carried out in the transonic wind tunnel of Aerodynamisches Institut of RWTH Aachen, at a freestream Mach number Ma ∞=2, a unit Reynolds number of Re ∞=13×106, angles of attack between ?3°?α?10°, and in a wind tunnel of the Institute for Theoretical and Applied Mechanics of the Russian Academy of Sciences in Novosibirsk. The freestream Mach numbers covered in these experiments were varied between 2?Ma ∞?4, freestream Reynolds numbers per unit length between 25×106?Re ∞?56×106 and angles of attack between ?3°?α?10°. Flow visualization studies, measurements of surface pressure distributions and of aerodynamic forces were used to analyze the flow. The results, which will also be compared with numerical data, clearly indicate marked differences in the location of the separation and reattachment lines, and the formation of the primary, secondary and tertiary vortices, for the flow regimes investigated. 相似文献
8.
Carlo A. Borghi Andrea Cristofolini Gabriele Neretti Paolo Seri Alessandro Rossetti Alessandro Talamelli 《Meccanica》2017,52(15):3661-3674
This paper reports on the effects of a series of fluid-dynamic dielectric barrier discharge plasma actuators on a NACA0015 airfoil at high angle of attack. A set of jet actuators able to produce plasma jets with different directions (vectoring effect) and operated at different on/off duty cycle frequencies are used. The experiments are performed in a wind tunnel facility. The vectorized jet and the transient of the flow induced by unsteady duty cycle operation of each actuator are examined and the effectiveness of the actuator to recover stall condition in the range of Reynolds numbers between 1.0 × 105 and 5.0 × 105 (based on airfoil chord), is investigated. The actuator placed on the leading edge of the airfoil presents the most effective stall recovery. No significant effects can be observed for different orientations of the jet. An increase of the stall recovery is detected when the actuator is operated in unsteady operation mode. Moreover, the frequency of the on/off duty cycle that maximizes the stall recovery is found to be a function of the free stream velocity. This frequency seems to scale with the boundary layer thickness at the position of the actuator. A lift coefficient increase at low free stream velocities appears to linearly depend on the supply voltage. 相似文献
9.
Tomographic and time resolved PIV measurements on a finite cylinder mounted on a flat plate 总被引:1,自引:0,他引:1
Tomographic and time resolved PIV measurements were performed to examine the 3D flow topology and the flow dynamic above the
upper surface of a low-aspect ratio cylinder at Re ≈ 1 × 105. This generic experiment is of fundamental interest because it represents flow features which are relevant to many applications
such as laminar separation bubbles and turbulent reattachment. At Re ≈ 1 × 105, laminar separation bubbles arise on the side of the cylinder. Furthermore, on the top of the cylinder a separation with
reattachment is of major interest. The tomographic PIV measurement, which allows to determine all three velocity components
in a volume instantaneously, was applied to examine the flow topology and interaction between the boundary layer and wake
structures on the top of the finite cylinder. In the instantaneous flow fields the tip vortices and the recirculation region
becomes visible. However, it is also observed that the flow is quite unsteady due to the large separation occurring on the
top of the cylinder. In order to study the temporal behaviour of the separation, time resolved PIV was applied. This technique
allows capturing the dynamic processes in detail. The development of vortices in the separated shear layer is observed and
in addition regions with different dominant frequencies are identified. 相似文献
10.
G. Blois G. H. Sambrook Smith J. L. Best R. J. Hardy J. R. Lead 《Experiments in fluids》2012,53(1):51-76
This paper presents results of an experimental study investigating the mean and temporal evolution of flow within the pore space of a packed bed overlain by a free-surface flow. Data were collected by an endoscopic PIV (EPIV) technique. EPIV allows the instantaneous velocity field within the pore space to be quantified at a high spatio-temporal resolution, thus permitting investigation of the structure of turbulent subsurface flow produced by a high Reynolds number freestream flow (Re s in the range 9.8?×?103?C9.7?×?104). Evolution of coherent flow structures within the pore space is shown to be driven by jet flow, with the interaction of this jet with the pore flow generating distinct coherent flow structures. The effects of freestream water depth, Reynolds and Froude numbers are investigated. 相似文献
11.
The flow over a smooth sphere is examined in the Reynolds number range of 5.0 × 104 < Re < 5.0 × 105 via measurements of the fluctuating forces and particle image velocimetry measurements in a planar cut of the velocity field.
Comprehensive studies of the statistics and spectra of the forces are presented for a range of subcritical and supercritical
Reynolds numbers. While the subcritical lateral force spectra are dominated by activity corresponding to the large-scale vortex
shedding frequency at a Strouhal number of approximately 0.18, there is no such peak apparent in the supercritical spectra,
although resolution effects may become important in this region. Nor does the large-scale vortex shedding appear to have a
significant effect on the drag force fluctuations at either sub- or super-critical Reynolds numbers. A simple double spring
model is shown to capture the main features of the lateral force spectra. The low-frequency force fluctuations observed in
earlier computational studies are shown to have important implications for statistical convergence, and in particular, the
apparent mean side force observed in earlier studies. At least one thousand dimensionless time units are required for reasonable
estimates of the second and higher moments below the critical Reynolds number and even more for supercritical flow, stringent
conditions for computational studies. Lastly, investigation of the relationship between the motion of the instantaneous wake
shape, defined via the local position where the streamwise velocity is equal to half the freestream value, and the in-plane
lateral force for subcritical flow reveals a significant negative correlation throughout the near wake, which is shown to
be related to a structure inferred to arise from the large-scale vortex shedding convecting downstream at 61% of the freestream
velocity. In addition to its utility in understanding basic sphere flow, the apparatus is also a testbed that will be used
in future studies, examining the effect of both static and dynamic changes to the surface morphology. 相似文献
12.
Niloofar Moradian David S.-K. Ting Shaohong Cheng 《Experimental Thermal and Fluid Science》2009,33(3):460-471
The effects of freestream turbulence intensity and integral length scale as freestream turbulent parameters on the drag coefficient of a sphere were experimentally investigated in a closed circuit wind tunnel. The Reynolds number, Re = Ud/ν, was varied from 2.2 × 104 to 8 × 104 by using spheres with diameter d of 20, 51 and 102 mm in addition to altering the freestream velocity, U. The freestream turbulence intensity Tu and flow integral length scale Λ were manipulated by the utilization of orifice perforated plates. The proper combination of orifice perforated plate hole diameter, sphere size, and sphere location along the center line of the wind tunnel enabled the independent alterations of turbulence intensity and relative integral length scale (Λ/d) from 1.8% to 10.7% and from 0.1 to 2.6, respectively, at each studied Reynolds number. Results show that over the range of conditions studied, the drag always decreases with increasing Tu and, the critical Reynolds number at which the drag coefficient is dramatically reduced is decreased by increasing Tu. Most interestingly, the drag at any particular Re and Tu may be significantly lowered by reducing Λ/d; this is particularly the case at high Re and Tu. 相似文献
13.
《Journal of Fluids and Structures》2008,24(7):1006-1020
A localised synthetic jet offers promise of an optimum and cost-effective practical method of delaying separation and promoting reattachment in fluids with solid body interactions. The asymmetric flow that may result from its use may also be beneficial in improving the aerodynamic performance of a lifting body. There are insufficient studies of synthetic jets, particularly on three-dimensional bluff bodies that are more representative of complex flows in real situations. A comprehensive study on an 80 mm diameter sphere designed with localised synthetic jet orifices was, therefore, conducted in an 18 in×18 in open circuit closed test-section wind tunnel at a Reynolds number of 5×104. The coefficient of pressure distribution was measured by continuously varying the location of the synthetic jet and compared with the no synthetic jet condition. The three-dimensional effects on the flow over the sphere body are particularly made apparent through the growth and the effects of the boundary layer and the deviation from potential flow. Overall, the synthetic jet had the effect of delaying the separation point and extending it further downstream on the sphere surface concomitantly producing a significant reduction in drag, providing solid support to the viability of strategically located synthetic jet when higher lift or lower drag is desired. A surprising discovery was the ability of the synthetic jet to improve the flow at the junction of the sting support and sphere. This has promising implications in devising methods to reduce interference drag that are common in many practical applications such as near junctions between wing and the fuselage. 相似文献
14.
The ultra-low Reynolds number airfoil wake 总被引:1,自引:0,他引:1
Lift force and the near wake of an NACA 0012 airfoil were measured over the angle (α) of attack of 0°–90° and the chord Reynolds
number (Re
c
), 5.3 × 103–5.1 × 104, with a view to understand thoroughly the near wake of the airfoil at low- to ultra-low Re
c
. While the lift force is measured using a load cell, the detailed flow structure is captured using laser-Doppler anemometry,
particle image velocimetry, and laser-induced fluorescence flow visualization. It has been found that the stall of an airfoil,
characterized by a drop in the lift force, occurs at Re
c
≥ 1.05 × 104 but is absent at Re
c
= 5.3 × 103. The observation is connected to the presence of the separation bubble at high Re
c
but absence of the bubble at ultra-low Re
c
, as evidenced in our wake measurements. The near-wake characteristics are examined and discussed in detail, including the
vortex formation length, wake width, spanwise vorticity, wake bubble size, wavelength of K–H vortices, Strouhal numbers, and
their dependence on α and Re
c
. 相似文献
15.
Commercial and military aircrafts or miniature aerial vehicles can suffer from massive flow separation when high angles of
attack are required. Single dielectric barrier discharge (DBD) actuators have demonstrated their capability of controlling
such a separated flow at low external velocity. However, the processes resulting in the improvement of the flight performances
remain unclear. In the present study, the reattachment process along the suction side of a NACA 0015 placed at an angle of
attack of 16° is experimentally investigated for an external velocity of 20 m/s (Re = 260,000). A single DBD actuator is mounted
at the leading edge of the model. The velocity fields above the suction side of the airfoil are measured by a high-speed acquisition
system (3 kHz). The results indicate that the baseline flow presents shed vortices that form at the leading edge and linearly
grow along the free shear layer axis. This vortex shedding is organized and exhibits a specific frequency of about 90 Hz.
The continuous actuation produces a partial flow reattachment up to 70% of the chord length. Temporal cross-correlation function
indicates the presence of a vortex shedding at the trailing edge of the controlled flow. Finally, the temporal analysis demonstrates
that the reattachment process requires 50 ms to reach a stabilized attached flow. The time-resolved analysis of the reattachment
suggests that the actuation by plasma discharge acts as a catalyser by reinforcing one of the coherent flow structures already
existing in the natural flow. 相似文献
16.
Control of diffuser jet flow: turbulent kinetic energy and jet spreading enhancements assisted by a non-thermal plasma discharge 总被引:2,自引:0,他引:2
An axisymmetric air jet exhausting from a 22-degree-angle diffuser is investigated experimentally by particle image velocimetry
(PIV) and stereo-PIV measurements. Two opposite dielectric barrier discharge (DBD) actuators are placed along the lips of
the diffuser in order to force the mixing by a co-flow actuation. The electrohydrodynamic forces generated by both actuators
modify and excite the turbulent shear layer at the diffuser jet exit. Primary air jet velocities from 10 to 40 m/s are studied
(Reynolds numbers ranging from 3.2 to 12.8 × 104), and baseline and forced flows are compared by analysing streamwise and cross-stream PIV fields. The mixing enhancement
in the near field region is characterized by the potential core length, the centreline turbulent kinetic energy (TKE), the
integrated value of the TKE over various slices along the jet, the turbulent Reynolds stresses and the vorticity fields. The
time-averaged fields demonstrate that an effective increase in mixing is achieved by a forced flow reattachment along the
wall of the diffuser at 10 m/s, whereas mixing enhancement is realized by excitation of the coherent structures for a primary
velocity of 20 and 30 m/s. The actuation introduces two pairs of contra-rotating vortices above each actuator. These structures
entrain the higher speed core fluid toward the ambient air. Unsteady actuations over Strouhal numbers ranging from 0.08 to
1 are also studied. The results suggest that the excitation at a Strouhal number around 0.3 is more effective to enhance the
turbulence kinetic energy in the near-field region for primary jet velocity up to 30 m/s. 相似文献
17.
J. J. Miau H. W. Tsai Y. J. Lin J. K. Tu C. H. Fang M. C. Chen 《Experiments in fluids》2011,51(4):949-967
Experiments were conducted for 2D circular cylinders at Reynolds numbers in the range of 1.73 × 105–5.86 × 105. In the experiment, two circular cylinder models made of acrylic and stainless steel, respectively, were employed, which
have similar dimensions but different surface roughness. Particular attention was paid to the unsteady flow behaviors inferred
by the signals obtained from the pressure taps on the cylinder models and by a hot-wire probe in the near-wake region. At
Reynolds numbers pertaining to the initial transition from the subcritical to the critical regimes, pronounced pressure fluctuations
were measured on the surfaces of both cylinder models, which were attributed to the excursion of unsteady flow separation
over a large circumferential region. At the Reynolds numbers almost reaching the one-bubble state, it was noted that the development
of separation bubble might switch from one side to the other with time. Wavelet analysis of the pressure signals measured
simultaneously at θ = ±90° further revealed that when no separation bubble was developed, the instantaneous vortex-shedding frequencies could
be clearly resolved, about 0.2, in terms of the Strouhal number. The results of oil-film flow visualization on the stainless
steel cylinder of the one-bubble and two-bubble states showed that the flow reattachment region downstream of a separation
bubble appeared not uniform along the span of the model. Thus, the three dimensionality was quite evident. 相似文献
18.
The flow around the Ahmed body at varying Reynolds numbers under yawing conditions is investigated experimentally. The body geometry belongs to a regime subject to spanwise flow instability identified in symmetric flow by Cadot and co-workers (Grandemange et al., 2013b). Our experiments cover the two slant angles 25° and 35° and Reynolds numbers up to 2.784 × 106. Special emphasis lies on the aerodynamics under side wind influence. For the 35° slant angle, forces and moments change significantly with the yawing angle in the range 10° ≤ |β| ≤ 15°. The lift and the pitching moment exhibit strong fluctuations due to bi-stable flow around a critical angle β of ±12.5°, where the pitching moment changes sign. Time series of the forces and moments are studied and explained by PIV measurements in the flow field near the rear of the body. 相似文献
19.
The present study describes the application of particle image velocimetry (PIV) to investigate the compressible flow in the wake of a two-dimensional blunt base at a freestream Mach number MX=2. The first part of the study addresses specific issues related to the application of PIV to supersonic wind tunnel flows, such as the seeding particle flow-tracing fidelity and the measurement spatial resolution. The seeding particle response is assessed through a planar oblique shock wave experiment. The measurement spatial resolution is enhanced by means of an advanced image-interrogation algorithm. In the second part, the experimental results are presented. The PIV measurements yield the spatial distribution of mean velocity and turbulence. The mean velocity distribution clearly reveals the main flow features such as expansion fans, separated shear layers, flow recirculation, reattachment, recompression and wake development. The turbulence distribution shows the growth of turbulent fluctuations in the separated shear layers up to the reattachment location. Increased velocity fluctuations are also present downstream of reattachment outside of the wake due to unsteady flow reattachment and recompression. The instantaneous velocity field is analyzed seeking coherent flow structures in the redeveloping wake. The instantaneous planar velocity and vorticity measurements return evidence of large-scale turbulent structures detected as spatially coherent vorticity fluctuations. The velocity pattern consistently shows large masses of fluid in vortical motion. The overall instantaneous wake flow is organized as a double row of counter-rotating structures. The single structures show vorticity contours of roughly elliptical shape in agreement with previous studies based on spatial correlation of planar light scattering. Peak vorticity is found to be five times higher than the mean vorticity value, suggesting that wake turbulence is dominated by the activity of large-scale structures. The unsteady behavior of the reattachment phenomenon is studied. Based on the instantaneous flow topology, the reattachment is observed to fluctuate mostly in the streamwise direction suggesting that the unsteady separation is dominated by a pumping-like motion. 相似文献
20.
The impact of Gurney flaps (GF), of different heights and perforations, on the aerodynamic and wake characteristics of a NACA
0015 airfoil equipped with a trailing-edge flap (TEF) was investigated experimentally at Re = 2.54 × 105. The addition of the Gurney flap to the TEF produced a further increase in the downward turning of the mean flow (increased
aft camber), leading to a significant increase in the lift, drag, and pitching moment compared to that produced by independently
deployed TEF or GF. The maximum lift increased with flap height, with the maximum lift-enhancement effectiveness exhibited
at the smallest flap height. The near wake behind the joint TEF and GF became wider and had a larger velocity deficit and
fluctuations compared to independent GF and TEF deployment. The Gurney flap perforation had only a minor impact on the wake
and aerodynamics characteristics compared to TEF with a solid GF. The rapid rise in lift generation of the joint TEF and GF
application, compared to conventional TEF deployment, could provide an improved off-design high-lift device during landing
and takeoff. 相似文献