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1.
The near-wall flow structures of a turbulent boundary layer over a riblet surface with semi-circular grooves were investigated
experimentally for the cases of drag decreasing (s
+=25.2) and drag increasing (s
+=40.6). One thousand instantaneous velocity fields over riblets were measured using the velocity field measurement technique
and compared with those above a smooth flat plate. The field of view was 6.75 × 6.75 mm2 in physical dimension, containing two grooves. Those instantaneous velocity fields were ensemble averaged to get turbulent
statistics including turbulent intensities and turbulent kinetic energy. To see the global flow structure qualitatively, flow
visualization was also carried out using the synchronized smoke-wire technique under the same experimental conditions. For
the case of drag decreasing (s
+=25.2), most of the streamwise vortices stay above the riblets, interacting with the riblet tips frequently. The riblet tips
impede the spanwise movement of the streamwise vortices and induce secondary vortices. The normalized rms velocity fluctuations
and turbulent kinetic energy are small near the riblet surface, compared with those over a smooth flat plate. Inside the riblet
valleys, these are sufficiently small that the increased wetted surface area of the riblets can be compensated. In addition,
in the outer region (y
+ > 30), these values are almost equal to or slightly smaller than those for the smooth plate. For the case of drag increasing
(s
+=40.6), however, most of the streamwise vortices stay inside the riblet valleys and contact directly with the riblet surface.
The high-speed down-wash flow penetrating into the riblet valley interacts actively with the wetted riblet surface and increases
the skin friction. The rms velocity fluctuations and turbulent kinetic energy have larger values compared with those over
a smooth flat plate.
Received: 24 March 1999/Accepted: 10 March 2000 相似文献
2.
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
3.
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
4.
A supersonic exhaust plume test rig and a Rayleigh scattering system were developed. Molecular number densities in the supersonic
high-temperature exhaust plume with and without an annular base flow were investigated. The physical meaning of the inferred
mean temperature from the number density measurement in turbulent flows is clarified. For both flows, the potential core extends
up to about six nozzle diameters, and self-similarity of the radial density distributions is observed at downstream sections
Z/d=10–50. The recovery of the flow density deficit (or the decay of temperature) with the annular flow is faster than that without
the annular flow at upstream sections Z/d ≤ 10.
Received: 16 August 2000 / Accepted: 20 November 2001 相似文献
5.
Influence of unsteady wake on a turbulent separation bubble 总被引:1,自引:0,他引:1
An experimental study was made of turbulent separated and reattaching flow over a blunt body, where unsteady wake was generated
by a spoked-wheel type of wake generator with cylindrical rods. The influence of unsteady wake was scrutinized by altering
the rotation direction (clockwise and counter-clockwise) and the normalized passing frequency (0 ≤ St
H
≤ 0.20). The Reynolds number based on the cylindrical rod was Re
d
=375. A phase-averaging technique was employed to characterize the unsteady wake. The effect of different rotation directions,
which gave a significant reduction of x
R
, was examined in detail. The wall pressure fluctuations on the blunt body were analyzed in terms of the spectrum and the
coherence.
Received: 15 January 2001 / Accepted: 17 July 2001 相似文献
6.
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
7.
The flow of a two-dimensional plane turbulent jet impinging on a porous screen has been studied experimentally. It is shown
how the overall flow structure depends on the porosity of the surface. For low screen porosity (β < 0.41, say), transverse wall jets can be formed on both sides of the screen and in extreme cases the axial momentum flux some way downstream of the screen falls to zero, so that
the screen has the same drag as would a solid wall. For high screen porosity (β > 0.57, say) the axial volume flux is largely preserved through the screen, but the dominant eddy structures present in the
upstream jet are largely destroyed, so that entrainment rates downstream of the screen can be very low. The relatively small,
intermediate range of porosities (0.41 < β < 0.57, where β is the screen open area ratio) is associated with dramatic changes in flow pattern and recirculating regions can exist on
the upstream side of the screen. These flows, although all geometrically very simple, provide a serious challenge for computational
modelling.
Received: 25 May 2000 / Accepted: 22 February 2001 相似文献
8.
The adverse pressure gradient induced by a surface-mounted obstacle in a turbulent boundary layer causes the approaching flow
to separate and form a dynamically rich horseshoe vortex system (HSV) in the junction of the obstacle with the wall. The Reynolds
number of the flow (Re) is one of the important parameters that control the rich coherent dynamics of the vortex, which are known to give rise to
low-frequency, bimodal fluctuations of the velocity field (Devenport and Simpson, J Fluid Mech 210:23–55, 1990; Paik et al., Phys Fluids 19:045107, 2007). We carry out detached eddy simulations (DES) of the flow past a circular cylinder mounted on a rectangular channel for
Re = 2.0 × 104 and 3.9 × 104 (Dargahi, Exp Fluids 8:1–12, 1989) in order to systematically investigate the effect of the Reynolds number on the HSV dynamics. The computed results are compared
with each other and with previous experimental and computational results for a related junction flow at a much higher Reynolds
number (Re = 1.15 × 105) (Devenport and Simpson, J Fluid Mech 210:23–55, 1990; Paik et al., Phys Fluids 19:045107, 2007). The computed results reveal significant variations with Re in terms of the mean-flow quantities, turbulence statistics, and the coherent dynamics of the turbulent HSV. For Re = 2.0 × 104 the HSV system consists of a large number of necklace-type vortices that are shed periodically at higher frequencies than
those observed in the Re = 3.9 × 104 case. For this latter case the number of large-scale vortical structures that comprise the instantaneous HSV system is reduced
significantly and the flow dynamics becomes quasi-periodic. For both cases, we show that the instantaneous flowfields are
dominated by eruptions of wall-generated vorticity associated with the growth of hairpin vortices that wrap around and disorganize
the primary HSV system. The intensity and frequency of these eruptions, however, appears to diminish rapidly with decreasing
Re. In the high Re case the HSV system consists of a single, highly energetic, large-scale necklace vortex that is aperiodically disorganized
by the growth of the hairpin mode. Regardless of the Re, we find pockets in the junction region within which the histograms of velocity fluctuations are bimodal as has also been
observed in several previous experimental studies. 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.
We have experimentally and analytically studied transport of a passive scalar in the wake of a thin flat plate located at
the centerline of a planar contraction with flat walls. The constant Launder parameter in the contraction, K = 6.25 ×10 − 6, was twice the value required for a turbulent boundary layer to relaminarize. In addition to the mixing analysis inside the
contraction, layer mixing is also investigated downstream, where the flow continues inside a constant cross-section channel.
In order to generate the passive scalar, the airflow above the plate was heated and the temperature stratification in the
wake was traced by measuring the temperature field using constant current anemometry. Using different plate lengths, we found
that the degree of mixing, obtained at a given position in the straight channel, is a function of the distance from the plate
trailing edge to the contraction outlet. For a plate which does not protrude into the straight channel, we demonstrate the
existence of an optimal trailing edge-contraction outlet distance that results in the lowest possible degree of mixing at
a given downstream position in the straight channel. This finding is also supported by a semi-empirical relationship based
on our developed self-similar solution for mixing layers in planar contractions. 相似文献
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.
Large-eddy simulation of a turbulent reactive jet with and without evaporating droplets is performed to investigate the interactions
among turbulence, combustion, heat transfer and evaporation. A hybrid Eulerian–Lagrangian approach is used for the gas–liquid
flow system. Arrhenius-type finite-rate chemistry is employed for the chemical reaction. To capture the highly local interactions,
dynamic procedures are used for all the subgrid-scale models, except that the filtered reaction rate is modelled by a scale
similarity model. Various representative cases with different initial droplet sizes (St
0) and mass loading ratios (MLR) have been simulated, along with a case without droplets. It is found that compared with the bigger, slow responding droplets
(St
0 = 16), smaller droplets (St
0 = 1) are more efficient in suppressing combustion due to their preferential concentration in the reaction zones. The peak
temperature and intensity of temperature fluctuations are found to be reduced in all the droplet cases, to a varying extent
depending on the droplet properties. Detailed analysis on the contributions of respective terms in a transport equation for
grid-scale kinetic energy (GSKE) shows that the droplet evaporation effect on GSKE is small, while the droplet momentum effect
depends on St
0. When the MLR is sufficiently high, the bigger (St
0 = 16) droplets can have profound influence on GSKE, and consequently on the formation and evolution of large-scale flow structures.
On the other hand, the turbulence level is found to be lower in the droplet cases than in the pure flame case, due to the
dissipative droplet dynamic effect. 相似文献
13.
Temperatures, velocities, and droplet sizes are measured in turbulent condensing steam jets produced by a facial sauna, for
varying nozzle diameters and varying initial velocities (Re=3,600–9,200). The release of latent heat due to droplet condensation causes the temperature in the two-phase jet to be significantly
higher than in a single-phase jet. At some distance from the nozzle, droplets reach a maximum size and start to evaporate
again, which results in a change in sign of latent heat release. The distance of maximum size is determined from droplet size
measurements. The experimental results are compared with semi-analytical expressions and with a fully coupled numerical model
of the turbulent condensing steam jet. The increase in centreline temperature due to droplet condensation is successfully
predicted.
Received: 5 April 2000 / Accepted: 15 November 2000 相似文献
14.
The accuracy of large-eddy simulation (LES) of a turbulent premixed Bunsen flame is investigated in this paper. To distinguish
between discretization and modeling errors, multiple LES, using different grid sizes h but the same filterwidth Δ, are compared with the direct numerical simulation (DNS). In addition, LES using various values
of Δ but the same ratio Δ/h are compared. The chemistry in the LES and DNS is parametrized with the standard steady premixed flamelet for stochiometric
methane-air combustion. The subgrid terms are closed with an eddy-viscosity or eddy-diffusivity approach, with an exception
of the dominant subgrid term, which is the subgrid part of the chemical source term. The latter subgrid contribution is modeled
by a similarity model based upon 2Δ, which is found to be superior to such a model based upon Δ. Using the 2Δ similarity model
for the subgrid chemistry the LES produces good results, certainly in view of the fact that the LES is completely wrong if
the subgrid chemistry model is omitted. The grid refinements of the LES show that the results for Δ = h do depend on the numerical scheme, much more than for h = Δ/2 and h = Δ/4. Nevertheless, modeling errors and discretization error may partially cancel each other; occasionally the Δ = h results were more accurate than the h ≤ Δ results. Finally, for this flame LES results obtained with the present similarity model are shown to be slightly better
than those obtained with standard β-pdf closure for the subgrid chemistry. 相似文献
15.
Among many presumed-shape pdf approaches for modeling non-premixed turbulent combustion, the presumed β-function pdf is widely used in the literature. However, numerical integration of the β-function pdf may encounter singularity difficulties at mixture fraction values of Z = 0 or 1. To date, this issue has been addressed by few publications. The present study proposes the Piecewise Integration
Method (PIM), an efficient, robust and accurate algorithm to overcome these numerical difficulties with the added benefit
of improving computational efficiency. Comparison of this method to the existing numerical integration methods shows that
the PIM exhibits better accuracy and greatly increases computational efficiency. The PIM treatment of the β-function pdf integration is first applied to the Burke–Schumann solution in conjunction with the k − ε turbulence model to simulate a CH4/H2 bluff-body turbulent flame. The proposed new method is then applied to the same flow using a more complex combustion model,
the laminar flamelet model. Numerical predictions obtained by using the proposed β-function pdf integration method are compared to experimental values of the velocity field, temperature and species mass fractions
to illustrate the efficiency and accuracy of the present method. 相似文献
16.
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
17.
A surface grooved with microscopic riblets aligned parallel to the flow is an effective means to reduce the turbulent skin
friction up to 10% compared to a smooth surface. The maximum drag reduction is found for a dimensionless rib spacing s
+ in the range of 15–17. For s
+ < 10, a linear behaviour of the drag reduction curve is predicted by viscous theory. This linear slope of the drag reduction
curve is in contradiction to Schlichting’s postulation of a hydraulically smooth behaviour of small-scale roughness in a turbulent
flow. This regime of evanescent dimensionless rib spacings is investigated experimentally by direct wall shear stress measurements
in a fully developed channel flow. Additionally, a numerical calculation of the viscous flow over riblets was carried out
to predict the drag reducing behaviour. The experimental results show a linear drag reducing behaviour down to s
+ = 0.3, which is in good agreement with the numerical results of the viscous simulation. The postulation of Schlichting’s
hydraulically smooth regime of a rough surface was not confirmed, neither for a riblet surface nor for a surface geometry
with grooves oriented perpendicular to the flow. In the latter case, the drag increases as a quadratic function of the roughness
height. 相似文献
18.
This paper reports the first large eddy simulation (LES) of a self-excited oscillating triangular jet (OTJ) issuing from a
fluidic nozzle that consists of a small triangular orifice inlet followed by a large circular chamber and an orifice outlet.
The case simulated is identical to that measured experimentally by England et al. (Exp Fluids 48(1):69–80, 2010). The present prediction agrees well with the previous measurement. The simulation reveals that the central oscillating jet
exhibits axis-switching in the cross-section and rotates by 60° approximately over a downstream distance of x = 0.5D (chamber diameter). Three strong longitudinal vortices occur associated with the three vertices of the inlet triangle. These
vortices strongly interact with the central jet and also the surroundings, in the region at x/D ≤ 1, and appear to merge finally with the outer secondary swirling flow. These observations are consistent with the deduction
from previous experiments. 相似文献
19.
The present paper investigates the impact of the velocity and density ratio on the turbulent mixing process in gas turbine
blade film cooling. A cooling fluid is injected from an inclined pipe at α=30° into a turbulent boundary layer profile at a freestream Reynolds number of Re ∞ = 400,000. This jet-in-a-crossflow (JICF) problem is investigated using large-eddy simulations (LES). The governing equations
comprise the Navier–Stokes equations plus additional transport equations for several species to simulate a non-reacting gas
mixture. A variation of the density ratio is simulated by the heat-mass transfer analogy, i.e., gases of different density
are effused into an air crossflow at a constant temperature. An efficient large-eddy simulation method for low subsonic flows
based on an implicit dual time-stepping scheme combined with low Mach number preconditioning is applied. The numerical results
and experimental velocity data measured using two-component particle-image velocimetry (PIV) are in excellent agreement. The
results show the dynamics of the flow field in the vicinity of the jet hole, i.e., the recirculation region and the inclination
of the shear layers, to be mainly determined by the velocity ratio. However, evaluating the cooling efficiency downstream
of the jet hole the mass flux ratio proves to be the dominant similarity parameter, i.e., the density ratio between the fluids
and the velocity ratio have to be considered. 相似文献
20.
Buoyant flows often contain regions with unstable and stable thermal stratification from which counter gradient turbulent
fluxes are resulting, e.g. fluxes of heat or of any turbulence quantity. Basing on investigations in meteorology an improvement
in the standard gradient-diffusion model for turbulent diffusion of turbulent kinetic energy is discussed. The two closure
terms of the turbulent diffusion, the velocity-fluctuation triple correlation and the velocity-pressure fluctuation correlation,
are investigated based on Direct Numerical Simulation (DNS) data for an internally heated fluid layer and for Rayleigh–Bénard
convection. As a result it is decided to extend the standard gradient-diffusion model for the turbulent energy diffusion by
modeling its closure terms separately. Coupling of two models leads to an extended RANS model for the turbulent energy diffusion.
The involved closure term, the turbulent diffusion of heat flux, is studied based on its transport equation. This results
in a buoyancy-extended version of the Daly and Harlow model. The models for all closure terms and for the turbulent energy
diffusion are validated with the help of DNS data for internally heated fluid layers with Prandtl number Pr = 7 and for Rayleigh–Bénard convection with Pr = 0.71. It is found that the buoyancy-extended diffusion model which involves also a transport equation for the variance
of the vertical velocity fluctuation gives improved turbulent energy diffusion data for the combined case with local stable
and unstable stratification and that it allows for the required counter gradient energy flux. 相似文献