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1.
Experiments were conducted in water and wind tunnels on spheres in the Reynolds number range 6 × 103 to 6.5 × 105 to study the effect of natural ventilation on the boundary layer separation and near-wake vortex shedding characteristics.
In the subcritical range of Re (<2 × 105), ventilation caused a marginal downstream shift in the location of laminar boundary layer separation; there was only a small
change in the vortex shedding frequency. In the supercritical range (Re > 4 × 105), ventilation caused a downstream shift in the mean locations of boundary layer separation and reattachment; these lines
showed significant axisymmetry in the presence of venting. No distinct vortex shedding frequency was found. Instead, a dramatic
reduction occurred in the wake unsteadiness at all frequencies. The reduction of wake unsteadiness is consistent with the
reduction in total drag already reported. Based on the present results and those reported earlier, the effects of natural
ventilation on the flow past a sphere can be categorized in two broad regimes, viz., weak and strong interaction regimes.
In the weak interaction regime (subcritical Re), the broad features of the basic sphere are largely unaltered despite the large addition of mass in the near wake. Strong
interaction is promoted by the closer proximity of the inner and outer shear layers at supercritical Re. This results in a modified and steady near-wake flow, characterized by reduced unsteadiness and small drag.
Received: 8 September 1998 / Accepted: 1 January 2000 相似文献
2.
A thre-dimensional direct numerical simulation is combined with a laboratory study to describe the turbulent flow in an enclosed
annular rotor-stator cavity characterized by a large aspect ratio G = (b − a)/h = 18.32 and a small radius ratio a/b = 0.152, where a and b are the inner and outer radii of the rotating disk and h is the interdisk spacing. The rotation rate Ω considered is equivalent to the rotational Reynolds number Re = Ωb
2/ν= 9 .5 × 104 (ν the kinematic viscosity of water). This corresponds to a value at which experiment has revealed that the stator boundary
layer is turbulent, whereas the rotor boundary layer is still laminar. Comparisons of the computed solution with velocity
measurements have given good agreement for the mean and turbulent fields. The results enhance evidence of weak turbulence
by comparing the turbulence properties with available data in the literature (Lygren and Andersson, J Fluid Mech 426:297–326,
2001). An approximately self-similar boundary layer behavior is observed along the stator. The wall-normal variations of the structural
parameter and of characteristic angles confirm that this boundary layer is three-dimensional. A quadrant analysis (Kang et
al., Phys Fluids 10:2315–2322, 1998) of conditionally averaged velocities shows that the asymmetries obtained are dominated by Reynolds stress-producing events
in the stator boundary layer. Moreover, Case 1 vortices (with a positive wall induced velocity) are found to be the major
source of generation of special strong events, in agreement with the conclusions of Lygren and Andersson (J Fluid Mech 426:297–326,
2001). 相似文献
3.
M. D. Brodetsky A. M. Kharitonov E. Krause A. A. Pavlov S. B. Nikiforov A. M. Shevchenko 《Experiments in fluids》2000,29(6):592-604
The leeside vortex structures on delta wings with sharp leading edges were studied for supersonic flow at the Institute of
Theoretical and Applied Mechanics of the Russian Academy of Sciences in Novosibirsk. The experiments were carried out with
three wings with sweep angles of χ=68°, 73°, and 78° and parabolic profiles in the 0.6 × 0.6 m2 test section of the blow-down wind tunnel T-313 of the institute. The test conditions were varied from Mach numbers M=2 to 4, unit Reynolds numbers from Re
l=26 × 106 to 56 × 106 m−1, and angles of attack from α=0° to 22°. The results of the investigations revealed that for certain flow conditions shocks
are formed above, below, and between the primary vortices. The experimental data were accurate enough to detect the onset
of secondary and tertiary separation as well as other boundaries. The various flow regimes discussed in the literature were
extended in several cases. The major findings are reported.
Received: 6 September 1999/Accepted: 24 January 2000 相似文献
4.
Venkata Bharathi L. Boppana Zheng-Tong Xie Ian P. Castro 《Flow, Turbulence and Combustion》2012,88(3):311-342
Large-eddy simulations of the dispersion from scalar line sources at various locations within a fully developed turbulent
channel flow at Re = uh/ν = 10,400 are presented. Both mean and fluctuating scalar quantities are compared with those from the single available set
of experimental data (Lavertu and Mydlarski, J Fluid Mech 528:135–172, 2005) and differences are highlighted and discussed. The results are also discussed in the context of scalar dispersion in other
kinds of turbulent flows, e.g. homogeneous shear-flow. Initial computations at a much lower Reynolds number are also reported
and compared with the two available direct numerical simulation data sets. 相似文献
5.
This paper reports results of DPIV measurements on a two-dimensional elliptic airfoil rotating about its own axis of symmetry
in a fluid at rest and in a parallel freestream. In the former case, we examined three rotating speeds (Re
c,Ω = 400, 1,000 and 2,000), and in the later case, four rotating speeds (Ro
c,Ω = 2.4, 1.2, 0.6 and 0.4), together with two freestream velocities (Re
c,u
= 200 and 1,000) and two starting configurations of the airfoil (i.e., chord parallel to (α
0 = 0°) or normal (α
0 = 90°) to the freestream). Results show that a rotating airfoil in a stationary fluid produces two distinct types of vortex
structures depending on the Reynolds number. The first type occurs at the lowest Reynolds number (Re
c,Ω = 400), where vortices shed from the two edges or tips of the airfoil dissipated quickly, resulting in the airfoil rotating
in a layer of diffused vorticity. The second type occurs at higher Reynolds numbers (i.e., Re
c,Ω = 1,000 and 2,000), where the corresponding vortices rotated together with the airfoil. Due to the vortex suction effect,
the torque characteristics are likely to be heavily damped for the first type because of the rapidly subsiding vortex shedding,
and more oscillatory for the second type due to persistent presence of tip vortices. In a parallel freestream, increasing
the tip-speed ratio (V/U) of the airfoil (i.e., decreasing the Rossby number, Ro
c,Ω) transformed the flow topology from periodic vortex shedding at Ro
c,Ω = 2.4 to the generation of a “hovering vortex” at Ro
c,Ω = 0.6 and 0.4. The presence of the hovering vortex, which has not been reported in literature before, is likely to enhance
the lift characteristics of the airfoil. Freestream Reynolds number is found to have minimal effect on the vortex formation
and shedding process, although it enhances shear layer instability and produces more small-scale flow structures that affect
the dynamics of the hovering vortex. Likewise, initial starting configuration of the airfoil, while affecting the flow transient
during the initial phase of rotation, has insignificant effect on the overall flow topology. Unfortunately, technical constraint
of our apparatus prevented us from carrying out complimentary force measurements; nevertheless, the results presented herein,
which are more extensive than those computed by Lugt and Ohring (1977), will provide useful benchmark data, from which more advanced numerical calculations can be carried out to ascertain the
corresponding force characteristics, particularly for those conditions with the presence of hovering vortex. 相似文献
6.
The impact of Gurney flaps, of different heights and perforations, on the growth and development of a tip vortex, both along
the tip and in the near field of a finite NACA 0012 wing, at Re = 1.05 × 105 was investigated by using particle image velocimetry (PIV). Wind-tunnel force balance measurements were also made to supplement
the PIV results. This study is a continuation of the work of Lee and Ko (Exp Fluids 46(6):1005–1019, 2009) on the near-wake measurements behind perforated Gurney flaps. The present results show that along the tip, the overall behavior
of the secondary vortices and their interaction with the primary, or tip, vortex remained basically unchanged, regardless
of flap height and perforation. The peak vorticity of the tip vortex, however, increased with flap height and always exhibited
a local maximum at x/c = 0.8 (from the leading edge). In the near field, the strength and structure of the near-field tip vortex were found to vary
greatly with the flap height and perforation. The small flaps produced a more concentrated tip vortex with an increased circulation,
while the large Gurney flaps caused a disruption of the tip vortex. The disrupted vortex can, however, be re-established by
the addition of flap perforation. The larger the flap perforation the more organized the tip vortex. The Gurney flaps have
the potential to serve as an alternative off-design wake vortex control device. 相似文献
7.
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
8.
Zhuo Li Xue-Feng Yuan Simon J. Haward Jeffrey A. Odell Stephen Yeates 《Rheologica Acta》2011,50(3):277-290
The non-linear dynamics of a semi-dilute (c/c* = 15) polydisperse polyethylene oxide (PEO) solution in microfluidics are studied experimentally using benchmark contraction–expansion
flow geometries with three contraction–expansion ratios (4:1:4, 8:1:8 and 16:1:16) and two narrow channel lengths (L
c/D
h = 53 and 5.3, where L
c is the length of the narrow channel and D
h is its hydraulic diameter). Complex flows over a range of elasticity numbers (El), Weissenberg numbers (Wi) and Reynolds numbers (Re) are characterized using micro-particle image velocimetry (
\upmu\upmu-PIV) and pressure drop measurements. The evolution of vortex formation and dynamics has been visualized through a step-flow-rate
experiment. Various flow dynamics regimes have been quantified and are presented in a Wi–Re diagram. The experimental results reveal that the contraction ratio can result in qualitatively different vortex dynamics
of semi-dilute polymer solutions in microfluidics, whereas the length of the narrow channel merely affects the dynamics at
a quantitative level. A single elasticity number, if defined by the size of the narrow channel, is not sufficient to account
for the effects of contraction ratio on the non-linear vortex dynamics. 相似文献
9.
Effect of local forcing on a turbulent boundary layer 总被引:6,自引:0,他引:6
An experimental study is performed to analyze flow structures behind local suction and blowing in a flat-plate turbulent
boundary layer. The local forcing is given to the boundary layer flow by means of a sinusoidally oscillating jet issuing from
a thin spanwise slot at the wall. The Reynolds number based on the momentum thickness is about Re
θ
=1700. The effects of local forcing are scrutinized by altering the forcing frequency (0.011 ≤ f +≤ 0.044). The forcing amplitude is fixed at A
0=0.4. It is found that a small local forcing reduces the skin friction and the skin friction reduction increases with the
forcing frequency. A phase-averaging technique is employed to capture the large-scale vortex evolution. An organized spanwise
vortical structure is generated by the local forcing. The cross-sectional area of vortex and the time fraction of vortex are
examined by changing the forcing frequency. An investigation of the random fluctuation components reveals that turbulent energy
is concentrated near the center of vortical structures.
Received: 17 March 2000/Accepted: 3 April 2001 相似文献
10.
11.
Large-eddy simulations (LES) of a planar, asymmetric diffuser flow have been performed. The diverging angle of the inclined
wall of the diffuser is chosen as 8.5°, a case for which recent experimental data are available. Reasonable agreement between
the LES and the experiments is obtained. The numerical method is further validated for diffuser flow with the diffuser wall
inclined at a diverging angle of 10°, which has served as a test case for a number of experimental as well as numerical studies
in the literature (LES, RANS). For the present results, the subgrid-scale stresses have been closed using the dynamic Smagorinsky
model. A resolution study has been performed, highlighting the disparity of the relevant temporal and spatial scales and thus
the sensitivity of the simulation results to the specific numerical grids used. The effect of different Reynolds numbers of
the inflowing, fully turbulent channel flow has been studied, in particular, Re
b
= 4,500, Re
b
= 9,000 and Re
b
= 20,000 with Re
b
being the Reynolds number based on the bulk velocity and channel half width. The results consistently show that by increasing
the Reynolds number a clear trend towards a larger separated region is evident; at least for the studied, comparably low Reynolds-number
regime. It is further shown that the small separated region occurring at the diffuser throat shows the opposite behaviour
as the main separation region, i.e. the flow is separating less with higher Re
b
. Moreover, the influence of the Reynolds number on the internal layer occurring at the non-inclined wall described in a recent
study has also been assessed. It can be concluded that this region close to the upper, straight wall, is more distinct for
larger Re
b
. Additionally, the influence of temporal correlations arising from the commonly used periodic turbulent channel flow as inflow
condition (similar to a precursor simulation) for the diffuser is assessed. 相似文献
12.
V. M. Kulik 《Experiments in fluids》2001,31(5):558-566
Change of drag reduction (DR) along a tube (D=2 mm, L=4 m) was experimentally investigated. To attain turbulent flow with Re=8 × 104, a tank operated under high pressure up to 16 MPa. Solutions of different brands of polyethyleneoxide (PEO) with concentrations
from 1 ppm to 100 ppm were tested. The results indicate that DR is not a constant value but depends on the time and intensity
of interaction between the polymer and the turbulent flow. There are three regions with different behaviors of DR: growth,
maximum, and slope down. Maximum DR coincides with the Virk ultimate DR and can be described by the suggested simple formula
. A decrease in the DR maximum has not been found even for high shear stresses τ
p < 800 Pa. DR dynamics for four brands of PEO with different molecular weight was studied. Direct experimentally determined
DR may be greater than the Virk ultimate value if the change in velocity profile is not taken into account. The corrected
DR never exceeds the ultimate DR.
Received: 10 April 2000/Accepted: 24 May 2001 相似文献
13.
This study investigates the flow past a confined circular cylinder built into a narrow rectangular duct with a Reynolds number
range of 1,500 ≤ Re
d
≤ 6,150, by employing the particle image velocimetry technique. In order to better explain the 3-D flow behaviour in the
juncture regions of the lower and upper plates and the cylinder, respectively, as well as the dynamics of the horseshoe vortex
system, both time-averaged and instantaneous flow data are presented for regions upstream and downstream of the cylinder.
The size, intensity and interaction of the vortex systems vary substantially with the Reynolds number. Although the narrow
rectangular duct with a single built-in cylinder is a geometrically symmetrical arrrangement, instantaneous flow data have
revealed that the flow structures in both the lower and upper plate–cylinder junction regions are not symmetrical with respect
to the centreline of the flow passage. The vortical flow structures obtained in side-view planes become dominant sometimes
in the lower juncture region and sometimes in the upper juncture region in unsteady mode. 相似文献
14.
In order to evaluate characteristics of the liquid film flow and their influences on heat and mass transfer, measurements
of the instantaneous film thickness using a capacitance method and observation of film breakdown are performed. Experimental
results are reported in the paper. Experiments are carried out at Re = 250–10000, T
in = 20–50°C and three axial positions of vertically falling liquid films for film thickness measurements. Instantaneous surface
waveshapes are given by the interpretation of the test data using the cubic spline method. The correlation of the mean film
thickness versus the film Reynolds number is also given by fitting the test data. It is revealed that the surface wave has
nonlinear behavior. Observation of film breakdown is performed at Re = 1.40 × 103–1.75 × 104 and T
in = 85–95°C. From experimental results, the correlation of the film breakdown criterion can be obtained as follows: Bd = 1.567 × 10−6
Re
1.183 相似文献
15.
Philip Schaefer Markus Gampert Jens Henrik Goebbert Lipo Wang Norbert Peters 《Flow, Turbulence and Combustion》2010,85(2):225-243
Direct Numerical Simulations (DNS) of Kolmogorov flows are performed at three different Reynolds numbers Re
λ
between 110 and 190 by imposing a mean velocity profile in y-direction of the form U(y) = F sin(y) in a periodic box of volume (2π)3. After a few integral times the turbulent flow turns out to be statistically steady. Profiles of mean quantities are then
obtained by averaging over planes at constant y. Based on these profiles two different model equations for the mean dissipation
ε in the context of two-equation RANS (Reynolds Averaged Navier–Stokes) modelling of turbulence are compared to each other.
The high Reynolds number version of the k-ε-model (Jones and Launder, Int J Heat Mass Transfer 15:301–314, 1972), to be called the standard model and a new model by Menter et al. (2006), to be called the Menter–Egorov model, are tested against the DNS results. Both models are solved numerically and it is
found that the standard model does not provide a steady solution for the present case, while the Menter–Egorov model does.
In addition a fairly good quantitative agreement of the model solution and the DNS data is found for the averaged profiles
of the kinetic energy k and the dissipation ε. Furthermore, an analysis based on flow-inherent geometries, called dissipation elements (Wang and Peters, J Fluid Mech 608:113–138,
2008), is used to examine the Menter–Egorov ε model equation. An expression for the evolution of ε is derived by taking appropriate moments of the equation for the evolution of the probability density function (pdf) of the
length of dissipation elements. A term-by-term comparison with the model equation allows a prediction of the constants, which
with increasing Reynolds number approach the empirical values. 相似文献
16.
The flow and heat transfer in an inclined and horizontal rectangular duct with a heated plate longitudinally mounted in the
middle of cross section was experimentally investigated. The heated plate and rectangular duct were both made of highly conductive
materials, and the heated plate was subjected to a uniform heat flux. The heat transfer processes through the test section
were under various operating conditions: Pr ≈ 0.7, inclination angle ϕ = −60° to +60°, Reynolds number Re = 334–1,911, Grashof number Gr = 5.26 × 102–5.78 × 106. The experimental results showed that the average Nusselt number in the entrance region was 1.6–2 times as large as that
in the fully developed region. The average Nusselt numbers and pressure drops increased with the Reynolds number. The average
Nusselt numbers and pressure drops decreased with an increase in the inclination angle from −60° to +60° when the Reynolds
number was less than 1,500. But when the Reynolds number increased to over about 1,800, the heat transfer coefficients and
pressure drops were independent of inclination angles. 相似文献
17.
A laminar separation bubble occurs on the suction side of the SD7003 airfoil at an angle of attack α = 4–8° and a low Reynolds
number less than 100,000, which brings about a significant adverse aerodynamic effect. The spatial and temporal structure
of the laminar separation bubble was studied using the scanning PIV method at α = 4° and Re = 60,000 and 20,000. Of particular interest are the dynamic vortex behavior in transition process and the subsequent vortex
evolution in the turbulent boundary layer. The flow was continuously sampled in a stack of parallel illuminated planes from
two orthogonal views with a frequency of hundreds Hz, and PIV cross-correlation was performed to obtain the 2D velocity field
in each plane. Results of both the single-sliced and the volumetric presentations of the laminar separation bubble reveal
vortex shedding in transition near the reattachment region at Re = 60,000. In a relatively long distance vortices characterized by paired wall-normal vorticity packets retain their identities
in the reattached turbulent boundary layer, though vortices interact through tearing, stretching and tilting. Compared with
the restricted LSB at Re = 60,000, the flow at Re = 20,000 presents an earlier separation and a significantly increased reversed flow region followed by “huge” vortical structures. 相似文献
18.
Y. J. Jan 《国际流体数值方法杂志》2002,39(3):189-211
An operator splitting and element‐by‐element conjugated gradient solver, and equal order interpolations are applied for solving time dependent Navier–Stokes (NS) equations to simulate flow induced vortex shedding in the present study. In addition, the convection term is corrected by balanced tensor diffusivity, which can stabilize the numerical simulation and overcome the numerical oscillations. The evolution of the interested flowing properties with time is analyzed by using spectral analysis. The developed code has been validated by the application of two examples: a driven cavity flow and a flow induced vortex vibration. Results from the first example for Reynolds number Re=103 and Re=104 are compared with other numerical simulations. Results from the second example, uniform flow past a square rod over a wide range of high Reynolds numbers from Re=103~105, are compared with experimental data and other numerical studies. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
19.
Miguel R. Visbal Raymond E. Gordnier Marshall C. Galbraith 《Experiments in fluids》2009,46(5):903-922
The present paper highlights results derived from the application of a high-fidelity simulation technique to the analysis
of low-Reynolds-number transitional flows over moving and flexible canonical configurations motivated by small natural and
man-made flyers. This effort addresses three separate fluid dynamic phenomena relevant to small fliers, including: laminar
separation and transition over a stationary airfoil, transition effects on the dynamic stall vortex generated by a plunging
airfoil, and the effect of flexibility on the flow structure above a membrane airfoil. The specific cases were also selected
to permit comparison with available experimental measurements. First, the process of transition on a stationary SD7003 airfoil
section over a range of Reynolds numbers and angles of attack is considered. Prior to stall, the flow exhibits a separated
shear layer which rolls up into spanwise vortices. These vortices subsequently undergo spanwise instabilities, and ultimately
breakdown into fine-scale turbulent structures as the boundary layer reattaches to the airfoil surface. In a time-averaged
sense, the flow displays a closed laminar separation bubble which moves upstream and contracts in size with increasing angle
of attack for a fixed Reynolds number. For a fixed angle of attack, as the Reynolds number decreases, the laminar separation
bubble grows in vertical extent producing a significant increase in drag. For the lowest Reynolds number considered (Re
c
= 104), transition does not occur over the airfoil at moderate angles of attack prior to stall. Next, the impact of a prescribed
high-frequency small-amplitude plunging motion on the transitional flow over the SD7003 airfoil is investigated. The motion-induced
high angle of attack results in unsteady separation in the leading edge and in the formation of dynamic-stall-like vortices
which convect downstream close to the airfoil. At the lowest value of Reynolds number (Re
c
= 104), transition effects are observed to be minor and the dynamic stall vortex system remains fairly coherent. For Re
c
= 4 × 104, the dynamic-stall vortex system is laminar at is inception, however shortly afterwards, it experiences an abrupt breakdown
associated with the onset of spanwise instability effects. The computed phased-averaged structures for both values of Reynolds
number are found to be in good agreement with the experimental data. Finally, the effect of structural compliance on the unsteady
flow past a membrane airfoil is investigated. The membrane deformation results in mean camber and large fluctuations which
improve aerodynamic performance. Larger values of lift and a delay in stall are achieved relative to a rigid airfoil configuration.
For Re
c
= 4.85 × 104, it is shown that correct prediction of the transitional process is critical to capturing the proper membrane structural
response. 相似文献
20.
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