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1.
Surface roughness effects in turbulent boundary layers 总被引:7,自引:7,他引:0
The effects of surface roughness on a turbulent boundary layer are investigated by comparing measurements over two rough
walls with measurements from a smooth wall boundary layer. The two rough surfaces have very different surface geometries although
designed to produce the same roughness function, i.e. to have nominally the same effect on the mean velocity profile. Different
turbulent transport characteristics are observed for the rough surfaces. Substantial effects on the stresses occur throughout
the layer showing that the roughness effects are not confined to the wall region. The turbulent energy production and the
turbulent diffusion are significantly different between the two rough surfaces, the diffusion having opposite sign in the
region γ/δ < 0.5. Although velocity spectra exhibit differences between the three surfaces, the mean energy dissipation rate
does not appear to be significantly affected by the roughness.
Received: 19 August 1998/Accepted: 16 February 1999 相似文献
2.
Velocity profile measurements in zero pressure gradient, turbulent boundary layer flow were made on a smooth wall and on two
types of rough walls with a wide range of roughness heights. The ratio of the boundary layer thickness (δ) to the roughness
height (k) was 16≤δ/k≤110 in the present study, while the ratio of δ to the equivalent sand roughness height (k
s) ranged from 6≤δ/k
s≤91. The results show that the mean velocity profiles for all the test surfaces agree within experimental uncertainty in velocity-defect
form in the overlap and outer layer when normalized by the friction velocity obtained using two different methods. The velocity-defect
profiles also agree when normalized with the velocity scale proposed by Zagarola and Smits (J Fluid Mech 373:33–70, 1998). The results provide evidence that roughness effects on the mean flow are confined to the inner layer, and outer layer similarity
of the mean velocity profile applies even for relatively large roughness. 相似文献
3.
Turbulence in rough-wall boundary layers: universality issues 总被引:1,自引:0,他引:1
Wind tunnel measurements of turbulent boundary layers over three-dimensional rough surfaces have been carried out to determine
the critical roughness height beyond which the roughness affects the turbulence characteristics of the entire boundary layer.
Experiments were performed on three types of surfaces, consisting of an urban type surface with square random height elements,
a diamond-pattern wire mesh and a sand-paper type grit. The measurements were carried out over a momentum thickness Reynolds
number (Re
θ) range of 1,300–28,000 using two-component Laser Doppler anemometry (LDA) and hot-wire anemometry (HWA). A wide range of
the ratio of roughness element height h to boundary layer thickness δ was covered (0.04 £ h/d £ 0.400.04 \leq h/\delta \leq 0.40). The results confirm that the mean profiles for all the surfaces collapse well in velocity defect form up to surprisingly
large values of h/δ, perhaps as large as 0.2, but with a somewhat larger outer layer wake strength than for smooth-wall flows, as previously
found. At lower h/δ, at least up to 0.15, the Reynolds stresses for all surfaces show good agreement throughout the boundary layer, collapsing
with smooth-wall results outside the near-wall region. With increasing h/δ, however, the turbulence above the near-wall region is gradually modified until the entire flow is affected. Quadrant analysis
confirms that changes in the rough-wall boundary layers certainly exist but are confined to the near-wall region at low h/δ; for h/δ beyond about 0.2 the quadrant events show that the structural changes extend throughout much of the boundary layer. Taken
together, the data suggest that above h/δ ≈ 0.15, the details of the roughness have a weak effect on how quickly (with rising h/δ) the turbulence structure in the outer flow ceases to conform to the classical boundary layer behaviour. The present results
provide support for Townsend’s wall similarity hypothesis at low h/δ and also suggest that a single critical roughness height beyond which it fails does not exist. For fully rough flows, the
data also confirm that mean flow and turbulence quantities are essentially independent of Re
θ; all the Reynolds stresses match those of smooth-wall flows at very high Re
θ. Nonetheless, there is a noticeable increase in stress contributions from strong sweep events in the near-wall region, even
at quite low h/δ. 相似文献
4.
Hot-wire and oil-film interferometry measurements are taken for 3D rough wall boundary layers at very high Reynolds numbers
(61,000 < Re θ < 120,000) with low blockage ratios, 10 < δ/H < 135, and high roughness, 100 < H
+ < 4,900. The results cover flows over both rough walls and over obstacles and are compared with and provide extension to
recent lower Reynolds number results. The validity of the Townsend ‘wall similarity hypothesis’ in relation to consistently
increasing 3D roughness is interrogated. In agreement with recent work, Schultz and Flack (J Fluid Mech 580:381–405, 2007) and Castro (J Fluid Mech 585:469–485, 2007) found that, for relatively low roughness, Townsend’s hypothesis holds for the mean velocity field. With increasing roughness,
the equilibrium layer diminishes and gradually vanishes. The viscous component of the wall shear stress decreases, while the
turbulent component increases as the roughness effects extend across the boundary layer. 相似文献
5.
Fujihiro Hamba 《Theoretical and Computational Fluid Dynamics》2001,14(5):323-336
Large eddy simulation (LES) is combined with the Reynolds-averaged Navier–Stokes (RANS) equation in a turbulent channel-flow
calculation. A one-equation subgrid-scale model is solved in a three-dimensional grid in the near-wall region whereas the
standard k–ε model is solved in a one-dimensional grid in the outer region away from the wall. The two grid systems are overlapped to
connect the two models smoothly. A turbulent channel flow is calculated at Reynolds numbers higher than typical LES and several
statistical quantities are examined. The mean velocity profile is in good agreement with the logarithmic law. The profile
of the turbulent kinetic energy in the near-wall region is smoothly connected with that of the turbulent energy for the k–ε model in the outer region. Turbulence statistics show that the solution in the near-wall region is as accurate as a usual
LES. The present approach is different from wall modeling in LES that uses a RANS model near the wall. The former is not as
efficient as the latter for calculating high-Reynolds-number flows. Nevertheless, the present method of combining the two
models is expected to pave the way for constructing a unified turbulence model that is useful for many purposes including
wall modeling.
Received 11 June 1999 and accepted 15 December 2000 相似文献
6.
Wall-resolved Large Eddy Simulation of fully developed turbulent channel flows over two different rough surfaces is performed to investigate on the effects of irregular 2D and 3D roughness on the turbulence. The two geometries are obtained through the superimposition of sinusoidal functions having random amplitudes and different wave lengths. In the 2D configuration the irregular shape in the longitudinal direction is replicated in the transverse one, while in the 3D case the sinusoidal functions are generated both in streamwise and spanwise directions. Both channel walls are roughened in such a way as to obtain surfaces with statistically equivalent roughness height, but different shapes. In order to compare the turbulence properties over the two rough walls and to analyse the differences with a smooth wall, the simulations are performed at the same Reynolds number Reτ = 395. The same mean roughness height h = 0.05δ (δ the half channel height) is used for the rough walls.The roughness function obtained with the 3D roughness is larger than in the 2D case, although the two walls share the same mean height. Thus, the considered irregular 3D roughness is more effective in reducing the flow velocity with respect to the 2D roughness, coherently with the literature results that identified a clear dependence of the roughness function on the effective slope (see Napoli et al. (2008)), higher in the generated 3D rough wall. The analysis of higher-order statistics shows that the effects of the roughness, independently on its two- or three-dimensional shape, are mainly confined in the inner region, supporting the Townsend’s wall similarity hypothesis. The tendency towards the isotropization is investigated through the ratio between the resolved Reynolds stress components, putting in light that the 3D irregular rough wall induces an higher reduction of the anisotropy, with respect to the 2D case. 相似文献
7.
Efficient Generation of Inflow Conditions for Large Eddy Simulation of Street-Scale Flows 总被引:2,自引:0,他引:2
Using a numerical weather forecasting code to provide the dynamic large-scale inlet boundary conditions for the computation
of small-scale urban canopy flows requires a continuous specification of appropriate inlet turbulence. For such computations
to be practical, a very efficient method of generating such turbulence is needed. Correlation functions of typical turbulent
shear flows have forms not too dissimilar to decaying exponentials. A digital-filter-based generation of turbulent inflow
conditions exploiting this fact is presented as a suitable technique for large eddy simulations computation of spatially developing
flows. The artificially generated turbulent inflows satisfy the prescribed integral length scales and Reynolds-stress-tensor.
The method is much more efficient than, for example, Klein’s (J Comp Phys 186:652–665, 2003) or Kempf et al.’s (Flow Turbulence Combust, 74:67–84, 2005) methods because at every time step only one set of two-dimensional (rather than three-dimensional) random data is filtered
to generate a set of two-dimensional data with the appropriate spatial correlations. These data are correlated with the data
from the previous time step by using an exponential function based on two weight factors. The method is validated by simulating
plane channel flows with smooth walls and flows over arrays of staggered cubes (a generic urban-type flow). Mean velocities,
the Reynolds-stress-tensor and spectra are all shown to be comparable with those obtained using classical inlet-outlet periodic
boundary conditions. Confidence has been gained in using this method to couple weather scale flows and street scale computations. 相似文献
8.
We report on velocity fluctuations and the fluctuation-driven radial transport of angular momentum in turbulent circular Couette
flow. Our apparatus is short (cylinder height to gap width ratio Γ ~ 2) and of relatively high wall curvature (ratio of cylinder
radii η ~ 0.35). Fluctuation levels and the mean specific angular momentum are found to be roughly constant over radius, in
accordance with previous studies featuring narrower gaps. Synchronized dual beam Laser Doppler Velocimetry (2D LDV) is used
to directly measure the r − θ Reynolds stress component as a function of Reynolds number (Re), revealing approximate scalings in the non-dimensional angular momentum transport that confirm previous measurements of
torque in similar flows. 2D LDV further allows for a decomposition of the turbulent transport to assess the relative roles
of fluctuation intensity and r − θ cross-correlation. We find that the increasing angular momentum transport with Re is due to intensifying absolute fluctuation levels accompanied by a slightly weakening cross-correlation. 相似文献
9.
Large eddy simulation (LES) is carried out to investigate the turbulent boundary-layer flows over a hill-shaped model with
a steep or relatively moderate slope at moderately high Reynolds numbers (Re = O(103)) defined by the hill height and the velocity at the hill height. The study focuses on the effects of surface roughness and
curvature. For Sub-grid Scale (SGS) modeling of LES, both the dynamic Smagorinsky model (DSM) and the dynamic mixed model
(DMM) are applied. The behavior of the separated shear layer and the vortex motion are affected by the oncoming turbulence,
such that the shear layer comes close to the ground surface, or the size of a separation region becomes small because of the
earlier instability of the separated shear layer. Appropriate measures are required to generate the inflow turbulence. The
methods of Lund et al. (J. Comput. Phys., 140:233–258, 1998) and Nozawa and Tamura (J. Wind Eng. Ind. Aerodyn., 90:1151–1162, 2002; The 4th European and African Conference on Wind Engineering, 1–6, 2005) are employed to simulate the smooth- and rough-wall turbulent boundary layers in order to generate time-sequential
data of inflow turbulence. This paper discusses the unsteady phenomena of the wake flows over the smooth and rough 2D hill-shaped
obstacles and aims to clarify the roughness effects on the flow patterns and the turbulence statistics distorted by the hill.
Numerical validation is conducted by comparing the simulation results with wind tunnel experiment data for the same hill shape
at almost the same Re. The applicability of DSM and DMM are discussed, focusing on the recirculation region behind a steep hill. 相似文献
10.
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
11.
An experimental study of a fully developed turbulent channel flow and an adverse pressure gradient (APG) turbulent channel flow over smooth and rough walls has been performed using a particle image velocimetry (PIV) technique. The rough walls comprised two-dimensional square ribs of nominal height, k = 3 mm and pitch, p = 2k, 4k and 8k. It was observed that rib roughness enhanced the drag characteristics, and the degree of enhancement increased with increasing pitch. Similarly, rib roughness significantly increased the level of turbulence production, Reynolds stresses and wall-normal transport of turbulence kinetic energy and Reynolds shear stress well beyond the roughness sublayer. On the contrary, the distributions of the eddy viscosity, mixing length and streamwise transport of turbulence kinetic energy and Reynolds shear stress were reduced by wall roughness, especially in the outer layer. Adverse pressure gradient produced a further reduction in the mean velocity (in comparison to the results obtained in the parallel section) but increased the wall-normal extent across which the mean flow above the ribs is spatially inhomogeneous in the streamwise direction. APG also reinforced wall roughness in augmenting the equivalent sand grain roughness height. The combination of wall roughness and APG significantly increased turbulence production and Reynolds stresses except in the immediate vicinity of the rough walls. The transport velocities of the turbulence kinetic energy and Reynolds shear stress were also augmented by APG across most part of the rough-wall boundary layer. Further, APG enhanced the distributions of the eddy viscosity across most of the boundary layer but reduced the mixing length outside the roughness sublayer. 相似文献
12.
Jae Hwa Lee Seung-Hyun Lee Kyoungyoun Kim Hyung Jin Sung 《International Journal of Heat and Fluid Flow》2009,30(6):1087-1098
Turbulent coherent structures near a rod-roughened wall are scrutinized by analyzing instantaneous flow fields obtained from direct numerical simulations (DNSs) of a turbulent boundary layer (TBL). The roughness elements used are periodically arranged two-dimensional spanwise rods, and the roughness height is k/δ = 0.05 where δ is the boundary layer thickness. The Reynolds number based on the momentum thickness is varied in the range Reθ = 300–1400. The effect of surface roughness is examined by comparing the characteristics of the TBLs over smooth and rough walls. Although introduction of roughness elements onto the smooth wall affects the Reynolds stresses throughout the entire boundary layer when scaled by the friction velocity, the roughness has little effect on the vorticity fluctuations in the outer layer. Pressure-strain tensors of the transport equation for the Reynolds stresses and quadrant analysis disclose that the redistribution of turbulent kinetic energy of the rough wall is similar to that of the smooth wall, and that the roughness has little effect on the relative contributions of ejection and sweep motions in the outer layer. To elucidate the modifications of the near-wall vortical structure induced by surface roughness, we used two-point correlations, joint weighted probability density function, and linear stochastic estimation. Finally, we demonstrate the existence of coherent structures in the instantaneous flow field over the rod-roughened surface. 相似文献
13.
Lyazid Djenidi Robert A. Antonia Muriel Amielh Fabien Anselmet 《Experiments in fluids》2008,44(1):37-47
Particle image velocimetry (PIV) measurements and planar laser induced fluorescence (PLIF) visualizations have been made in
a turbulent boundary layer over a rough wall. The wall roughness consisted of square bars placed transversely to the flow
at a pitch to height ratio of λ/k = 11 for the PLIF experiments and λ/k = 8 and 16 for the PIV measurements. The ratio between the boundary layer thickness and the roughness height k/δ was about 20 for the PLIF and 38 for the PIV. Both the PLIF and PIV data showed that the near-wall region of the flow was
populated by unstable quasi-coherent structures which could be associated to shear layers originating at the trailing edge
of the roughness elements. The streamwise mean velocity profile presented a downward shift which varied marginally between
the two cases of λ/k, in agreement with previous measurements and DNS results. The data indicated that the Reynolds stresses normalized by the
wall units are higher for the case λ/k = 16 than those for λ/k = 8 in the outer region of the flow, suggesting that the roughness density effects could be felt well beyond the near-wall
region of the flow. As expected the roughness disturbed dramatically the sublayer which in turn altered the turbulence production
mechanism. The turbulence production is maximum at a distance of about 0.5k above the roughness elements. When normalized by the wall units, the turbulence production is found to be smaller than that
of a smooth wall. It is argued that the production of turbulence is correlated with the form drag. 相似文献
14.
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
15.
Understanding turbulence kinetic energy (TKE) budget in gas–liquid two-phase bubbly flows is indispensable to develop and
improve turbulence models for the bubbly flows. In this study, a molecular tagging velocimetry based on photobleaching reaction
was applied to turbulent bubbly flows with sub-millimeter bubbles in a vertical square duct to examine the applicability of
the k–ε models to the bubbly flows. Effects of bubbles on TKE budget are discussed and a priori tests of the standard and low Reynolds
number k–ε models are carried out to examine the applicability of these models to the bubbly flows. The conclusions obtained are as
follows: (1) The photobleaching molecular tagging velocimetry is of use for validating turbulence models. (2) The bubbles
increase the liquid velocity gradient in the near wall region, and therefore, enhance the production and dissipation rates
of TKE. (3) The k–ε models can reasonably evaluate the production rate of TKE in the bubbly flows. (4) The modulations of diffusion due to the
bubbles have different characteristics from the diffusion enhancement due to shear-induced turbulence. Hence, the k–ε models fail in evaluating the diffusion rate in the near wall region in the bubbly flows. (5) The k–ε models represent the trends of the production, dissipation, and diffusion rates of ε in the bubbly flow, although more accurate
experimental data are required for quantitative validation of the ε equation. 相似文献
16.
Scalar transport from a point source in flows over wavy walls 总被引:1,自引:0,他引:1
Simultaneous measurements of the velocity and concentration field in fully developed turbulent flows over a wavy wall are
described. The concentration field originates from a low-momentum plume of a passive tracer. PLIF and digital particle image
velocimetry are used to make spatially resolved measurements of the structure of the scalar distribution and the velocity.
The measurements are performed at three different Reynolds numbers of Re
b = 5,600, Re
b = 11,200 and Re
b = 22,400, respectively, based on the bulk velocity u
b and the total channel height 2h. The velocity field and the scalar field are investigated in a water channel with an aspect ratio of 12:1, where the bottom
wall of the test section consists of a train of sinusoidal waves. The wavy wall is characterized by the amplitude to wavelength
ratio α = 0.05 and the ratio β between the wave amplitude and the half channel height where β = 0.1. The scalar is released
from a point source at the wave crest. For the concentration measurements, Rhodamine B is used as tracer dye. At low to moderate
Reynolds number, the flow field is characterized through a recirculation zone which develops after the wave crest. The recirculation
zone induces high intensities of the fluctuations of the streamwise velocity and wall-normal velocity. Furthermore, large-scale
structures are apparent in the flow field. In previous investigations it has been shown that these large-scale structures
meander laterally in flows over wavy bottom walls. The investigations show a strong effect of the wavy bottom wall on the
scalar mixing. In the vicinity of the source, the scalar is transported by packets of fluid with a high scalar concentration.
As they move downstream, these packets disintegrate into filament-like structures which are subject to strong gradients between
the filaments and the surrounding fluid. The lateral scale of the turbulent plume is smaller than the lateral scale of the
large-scale structures in the flow field and the plume dispersion is dominated by the structures in the flow field. Due to
the lateral meandering of the large-scale structures of the flow field, also the scalar plume meanders laterally. Compared
to turbulent plumes in plane channel flows, the wavy bottom wall enhances the mixing effect of the turbulent flow and the
spreading rate of the scalar plume is increased. 相似文献
17.
Jorge Bailon-Cuba Stefano Leonardi Luciano Castillo 《International Journal of Heat and Fluid Flow》2009,30(5):1007-1015
Direct numerical simulations (DNS) of a turbulent channel flow with 2D wedges of random height on the bottom wall have been performed. In addition, two other simulations have been carried out to assess the effect of the geometry on the overlying flow. In the first simulation, the four smallest elements were removed while in the other, a uniform distribution of wedges with the same area was used. Two Reynolds numbers were studied, Reb=2500 and Reb=5000 which correspond in case of smooth walls to Reτ=180 and 300, respectively. Roughness on the wall induces separated regions, the reattachment occurring on the walls of the wedges or on the bottom wall. The pressure gradients on the walls increase the ejections and inrushes towards the wall. As a consequence the flow is more isotropic. The mechanism inducing an improved isotropy has been explained in term of the spectra and budgets of Reynolds stress. The comparison of the 3 surfaces has shown that near the wall, the uniformly distributed roughness represents only a poor approximation of the surface with wedges of random height. The Reynolds stresses, pressure distribution and spectra on the modified wall agree well with those on the random surface. Energy spectra show the pitch to height ratio of the largest elements to be the more appropriate geometrical parameter to describe the geometry. 相似文献
18.
Instantaneous concentration profiles have been measured in turbulent water channel flows at 5 axial locations immediately
downstream of a line, wall injection of a dyed 700 ppm polymer solution and for comparison, dyed water. Concentration was
deduced from a line of fluoresced radiation that was stimulated by a laser beam directed through the dyed injectant and normal
to the channel wall. Both statistical and time-resolved results show how the turbulent mixing is modified and damped when
the injectant is a polymer solution.
A version of this paper was presented at the 11th Symposium on Turbulence, University of Missouri-Rolla, Oct. 17–19, 1988 相似文献
19.
An intermittency transport equation for RANS modeling, formulated in local variables, is extended for roughness-induced transition. To predict roughness effects in the fully turbulent boundary layer, published boundary conditions for k and ω are used. They depend on the equivalent sand-grain roughness height, and account for the effective displacement of wall distance origin. Similarly in our approach, wall distance in the transition model for smooth surfaces is modified by an effective origin, which depends on equivalent sand-grain roughness. Flat plate test cases are computed to show that the proposed model is able to predict transition onset in agreement with a data correlation of transition location versus roughness height, Reynolds number, and inlet turbulence intensity. Experimental data for turbine cascades are compared to the predicted results to validate the proposed model. 相似文献