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1.
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
2.
Javad Rostami 《Heat and Mass Transfer》2008,44(9):1079-1087
In this paper, unsteady heat transfer and fluid flow characteristics in an enclosure are investigated. The enclosure consists
of two vertical wavy and two horizontal straight walls. The top and the bottom walls are considered adiabatic. Two wavy walls
are kept isothermal and their boundaries are approximated by a cosine function. Governing equations including continuity,
momentum and energy were discretized using the finite-volume method and solved by SIMPLE method in curvilinear coordinate.
Simulation was carried out for a range of Grashof number Gr = 103–106, Prandtl number Pr = 0.5–4.0, wave ratio A (defined by amplitude/wavelength) 0.0–0.35 and aspect ratio W (defined by average width/wavelength) 0.5–1.0. Streamlines and isothermal lines are presented to corresponding flow and thermal
fields. Local and average Nusselt number distributions are presented. The obtained results are in good agreement with available
numerical and experimental data. 相似文献
3.
A. Marshall P. Venkateswaran D. Noble J. Seitzman T. Lieuwen 《Experiments in fluids》2011,51(3):611-620
Experimental turbulent combustion studies require systems that can simulate the turbulence intensities [u′/U
0 ~ 20–30% (Koutmos and McGuirk in Exp Fluids 7(5):344–354, 1989)] and operating conditions of real systems. Furthermore, it is important to have systems where turbulence intensity can be
varied independently of mean flow velocity, as quantities such as turbulent flame speed and turbulent flame brush thickness
exhibit complex and not yet fully understood dependencies upon both U
0 and u′. Finally, high pressure operation in a highly pre-heated environment requires systems that can be sealed, withstand high
gas temperatures, and have remotely variable turbulence intensity that does not require system shut down and disassembly.
This paper describes the development and characterization of a variable turbulence generation system for turbulent combustion
studies. The system is capable of a wide range of turbulence intensities (10–30%) and turbulent Reynolds numbers (140–2,200)
over a range of flow velocities. An important aspect of this system is the ability to vary the turbulence intensity remotely,
without changing the mean flow velocity. This system is similar to the turbulence generators described by Videto and Santavicca
(Combust Sci Technol 76(1):159–164, 1991) and Coppola and Gomez (Exp Therm Fluid Sci 33(7):1037–1048, 2009), where variable blockage ratio slots are located upstream of a contoured nozzle. Vortical structures from the slots impinge
on the walls of the contoured nozzle to produce fine-scale turbulence. The flow field was characterized for two nozzle diameters
using three-component Laser Doppler velocimetry (LDV) and hotwire anemometry for mean flow velocities from 4 to 50 m/s. This
paper describes the key design features of the system, as well as the variation of mean and RMS velocity, integral length
scales, and spectra with nozzle diameter, flow velocity, and turbulence generator blockage ratio. 相似文献
4.
The flow-induced microstructure of a mesophase pitch was studied within custom-made dies for changing wall shear rates from
20 to 1,100 s − 1, a flow scenario that is typically encountered during fiber spinning. The apparent viscosity values, measured at the nominal
wall shear rates ranging from 500 to 2,500 s − 1 using these dies, remain fairly constant. The microstructure was studied in two orthogonal sections: r–θ (cross section) and r–z (longitudinal mid plane). In these dies, the size of the microstructure gradually decreases toward the wall (to as low as
a few micrometers), where shear rate is highest. Furthermore, as observed in the r–θ plane of the capillary, for a significant fraction of the cross section, discotic mesophase has a radial orientation. Thus,
the directors of disc-like molecules were aligned in the vorticity (θ) direction. As confirmed from the microstructure in the r–z plane, most of the discotic molecules remain nominally in the flow plane. Orientation of the pitch molecules in the shear
flow conditions is consistent with that observed in controlled low-shear rheometric experiments reported earlier. Microstructral
investigation suggests that the radial orientation of carbon fibers obtained from a mesophase pitch originates during flow
of pitch through the die. 相似文献
5.
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
6.
CFD Calculation of Turbulent Flow with Arbitrary Wall Roughness 总被引:1,自引:0,他引:1
David Apsley 《Flow, Turbulence and Combustion》2007,78(2):153-175
7.
The near-ground flow structure of tornadoes is of utmost interest because it determines how and to what extent civil structures
could get damaged in tornado events. We simulated tornado-like vortex flow at the swirl ratios of S = 0.03–0.3 (vane angle θv = 15°–60°), using a laboratory tornado simulator and investigated the near-ground-vortex structure by particle imaging velocimetry.
Complicated near-ground flow was measured in two orthogonal views: horizontal planes at various elevations (z = 11, 26 and 53 mm above the ground) and the meridian plane. We observed two distinct vortex structures: a single-celled
vortex at the lowest swirl ratio (S = 0.03, θv = 15°) and multiple suction vortices rotating around the primary vortex (two-celled vortex) at higher swirl ratios (S = 0.1–0.3, θv = 30°–60°). We quantified the effects of vortex wandering on the mean flow and found that vortex wandering was important
and should be taken into account in the low swirl ratio case. The tangential velocity, as the dominant velocity component,
has the peak value about three times that of the maximum radial velocity regardless of the swirl ratio. The maximum velocity
variance is about twice at the high swirl ratio (θv = 45°) that at the low swirl ratio (θv = 15°), which is contributed significantly by the multiple small-scale secondary vortices. Here, the results show that not
only the intensified mean flow but greatly enhanced turbulence occurs near the surface in the tornado-like vortex flow. The
intensified mean flow and enhanced turbulence at the ground level, correlated with the ground-vortex interaction, may cause
dramatic damage of the civil structures in tornadoes. This work provides detailed characterization of the tornado-like vortex
structure, which has not been fully revealed in previous field studies and laboratory simulations. It would be helpful in
improving the understanding of the interaction between the tornado-like vortex structure and the ground surface, ultimately
leading to better predictions of tornado-induced wind loads on civil structures. 相似文献
8.
9.
An experimental study was conducted to examine the effects of surface roughness and adverse pressure gradient (APG) on the
development of a turbulent boundary layer. Hot-wire anemometry measurements were carried out using single and X-wire probes in all regions of a developing APG flow in an open return wind tunnel test section. The same experimental conditions
(i.e., T
∞, U
ref, and C
p) were maintained for smooth, k
+ = 0, and rough, k
+ = 41–60, surfaces with Reynolds number based on momentum thickness, 3,000 < Re
θ < 40,000. The experiment was carefully designed such that the x-dependence in the flow field was known. Despite this fact, only a very small region of the boundary layer showed a balance
of the various terms in the integrated boundary layer equation. The skin friction computed from this technique showed up to
a 58% increase due to the surface roughness. Various equilibrium parameters were studied and the effect of roughness was investigated.
The generated flow was not in equilibrium according to the Clauser (J Aero Sci 21:91–108, 1954) definition due to its developing nature. After a development region, the flow reached the equilibrium condition as defined
by Castillo and George (2001), where Λ = const, is the pressure gradient parameter. Moreover, it was found that this equilibrium condition can be used
to classify developing APG flows. Furthermore, the Zagarola and Smits (J Fluid Mech 373:33–79, 1998a) scaling of the mean velocity deficit, U
∞δ*/δ, can also be used as a criteria to classify developing APG flows which supports the equilibrium condition of Castillo
and George (2001). With this information a ‘full APG region’ was defined. 相似文献
10.
Absorber is an important component in absorption machines and its characteristics have significant effects on the overall
efficiency of absorption machines. This article reports a model of simultaneous heat and mass transfer process in absorption
of refrigerant vapor into a lithium bromide solution of water––cooled vertical plate absorber in the Reynolds number range
of 5 < Re < 150. The boundary layer assumptions were used for the transport of mass, momentum and energy equations and the fully implicit
finite difference method was employed to solve the governing equations in the film flow. Dependence of lithium bromide aqueous
properties to the temperature and concentration and film thickness to vapor absorption was employed. This model can predict
temperature, concentration and properties of aqueous profiles as well as the absorption heat and mass fluxes, heat and mass
transfer coefficients, Nusslet and Sherwood number of absorber. An analysis for linear distribution of wall temperature condition
carries out to investigation the reliability of the present numerical method through comparing with previous investigation. 相似文献
11.
We study the problem of flow permeability of fracture joints using Lattice-Gas Automata simulations. We model the fracture as a rough channel bounded by a self-affine surface. Changing the surface roughness exponent, rough walls having different microstructures are obtained. Different relative roughnesses — defined as the height of the largest surface asperity divided by the mean aperture — are obtained pulling apart the two surfaces that constitute the rough walls of the channel. We calculate the macroscopic variables volume flow rate and pressure difference using microscopic balances. In the low Reynolds number regime the pressure difference and the flow rate are linearly related (the behavior is described by Darcy's law). In this regime, we study the effect of geometry on the permeability. We have found that permeability is independent of the surface roughness exponentH and it is fully determined in terms of the relative roughness and mean aperture of the fracture joint. For larger Reynolds numbers a transition to a regime in which pressure difference and flow rate are not longer linearly related is observed. This transition is observed in a domain of Reynolds numbers for which the behavior in a smooth channel remains linear. We discuss this transition. 相似文献
12.
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
13.
The paper reports on the application of the Time-dependent Reynolds-Averaged Navier–Stokes (T-RANS) approach to analysing
the effects of magnetic force and bottom-wall configuration on the reorganisation of a large coherent structure and its role
in the transport processes in Rayleigh–Bénard convection. The large-scale deterministic motion is fully resolved in time and
space, whereas the unresolved stochastic motion is modelled by a `subscale' model for which the conventional algebraic stress/flux
expressions were used, closed with the low-Re number (k)-(ε)-(θ2) three-equation model. The applied method reproduces long-term averaged mean flow properties, turbulence second moments,
and all major features of the coherent roll/cell structure in classic Rayleigh–Bénard convection in excellent agreement with
the available DNS and experimental results. Application of the T-RANS approach to Rayleigh–Bénard convection with wavy bottom
walls and a superimposed magnetic field yielded the expected effects on there organisation of the eddy structure and consequent
modifications of the mean and turbulence parameters and wall heat transfer.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
14.
The bioluminescence images of unstirred cultures show that lux reporter E. coli (0.10 mg biomass per ml of the broth medium) in 6.4–10 mm diameter circular containers induce center-fluid-rising toroidal
convection of ≤1 mm/min. The bioconvective torus is stable in a Teflon vessel and is deformed by 3.2–4.4 mm wavelength azimuthal
waves in polystyrene or glass vessels. 相似文献
15.
Adding surfactant into the displacing aqueous phase during surfactant-enhanced aquifer remediation of NAPL contamination and
in chemical flooding oil recovery significantly changes interfacial tension (IFT) (σ) on water–oil interfaces within porous
media. The change in IFT may have a large impact on relative permeability for the two-phase flow system. In most subsurface
flow investigations, however, the influence of IFT on relative permeability has been ignored. In this article, we present
an experimental study of two-phase relative- permeability behavior in the low and more realistic ranges of IFT for water–oil
systems. The experimental work overcomes the limitations of the existing laboratory measurements of relative permeability
(which are applicable only for high ranges of IFT (e.g., σ > 10−2 mN/m). In particular, we have (1) developed an improved steady-state method of measuring complete water–oil relative permeability
curves; (2) proven that a certain critical range of IFT exists such that IFT has little impact on relative permeability for
σ greater than this range, while within the range, relative permeabilities to both water and oil phases will increase with
decreasing IFT; and (3) shown that a functional correlation exists between water–oil two-phase relative permeability and IFT.
In addition, this work presents such correlation formula between water–oil two-phase relative permeability and IFT. The experimental
results and proposed conceptual models will be useful for quantitative studies of surfactant-enhanced aquifer remediation
and chemical flooding operations in reservoirs. 相似文献
16.
We consider a particular in-plane elastic orthotropy observed experimentally for various types of paper, namely: S
1111+S
2222−2S
1122=S
1212, where S
ijkm
are components of the in-plane compliance tensor. This is a statement of the invariance of in-plane shear compliance S
1212, which has been observed in some studies but questioned in others. We present a possible explanation of this “special orthotropy”
of paper, using an analysis in which paper is modeled as a quasi-planar random microstructure of interacting fiber-beams –
a model especially well suited for low basis weight papers. First, it is shown analytically that without disorder a periodic
fiber network fails the special orthotropy. Next, using a computational mechanics model, we demonstrate that two-scale geometric
disorder in a fiber network is necessary to explain this orthotropy. Indeed, disordered networks with weak flocculation best
satisfy this relationship. It is shown that no special angular distribution function of fibers is required, and that the uniform
strain assumption should not be used. Finally, it follows from an analogy to the thermal conductivity problem that the kinematic
boundary conditions, rather than the traction ones, lead quite rapidly to relatively scale-independent effective constitutive
responses.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
17.
Dibenzylidene sorbitol (DBS) is known to gel organic liquids and polymers such as poly(propylene oxide) (PPO) by forming long
fibers and fiber networks. Potential applications of these networks depend on their ability to withstand large deformations
without significant morphological changes. Therefore, we studied the nonlinear rheological properties of the DBS fiber network
in PPO for different DBS concentrations. We found that the concentration dependence of critical deformation (transition from
linear to nonlinear viscoelastic region) and gel strength (G′ plateau in the linear region) can be explained on the basis of a model for densely cross-linked fiber gels (MacKintosh et al.,
Phys Rev Lett 75:4425–4428, 1995). Performing periodic strain sweeps, we found that the decrease in gel strength during the deformation cycles can be ascribed
to reversible fiber coarsening. Additionally, start-up experiments showed a strong shear thinning behavior, which is in quantitative
agreement with the SGM model (Sollich, Phys Rev E 58:738–759, 1998). 相似文献
18.
The onset of air entrainment for curtain coating onto a pre-wetted substrate was studied experimentally in similar parameter
regimes to commercial coating (Re = ρQ/μ = O(1), We = ρQ
u
c
/σ = O(10), Ca = μU/σ = O(1)). Impingement speed and viscosity were previously shown to be critical parameters in correlating air entrainment data
with three qualitatively different regimes of hydrodynamic assist identified (Marston et al. in Exp Fluids 42(3):483–488, 2007a). The interaction of the impinging curtain with the pre-existing film also led to a significant hysteretic effect throughout
the flow rate-substrate speed parameter space. For the first time, results considering the influence of surfactants are presented
in attempt to elucidate the relative importance of surface tension in this inertia-dominated system. The results show quantitative
and qualitative differences to previous results with much more complex hysteretic behaviour which has only been reported previously
for rough surfaces. 相似文献
19.
Dynamic NMR microscopy measurement of the dynamics and flow partitioning of colloidal particles in a bifurcation 总被引:1,自引:0,他引:1
Einar O. Fridjonsson Joseph D. Seymour Giles R. Cokelet Sarah L. Codd 《Experiments in fluids》2011,50(5):1335-1347
The flow and distribution of Newtonian, polymeric and colloid suspension fluids at low Reynolds numbers in bifurcations has
importance in a wide range of disciplines, including microvascular physiology and microfluidic devices. A bifurcation consisting
of circular capillaries laser etched into a hard polymer with inlet diameter 2.50 ± 0.01 mm, bifurcating to a small diameter
outlet of 0.76 ± 0.01 mm and a large diameter outlet of 1.25 ± 0.01 mm is examined. Four distinct fluids (water, 0.25%wt xanthan
gum, 8 and 22%vol hard-sphere colloidal suspensions) are flowed at flow rates from 10 to 30 ml/h corresponding to Reynolds
numbers based on the entry flow from 0.001 to 8. PGSE NMR techniques are applied to obtain dynamic images of the fluids inside
the bifurcation with spatial resolution of 59 × 59 μm/pixel in plane over a 200-μm-thick slice. Velocity in all three spatial
directions is examined to determine the impact of secondary flows and characterize the transport in the bifurcation. The velocity
data provide direct measurement of the volumetric distribution of the flow between the two channels as a function of flow
rate. Water and the 8% colloidal suspension show a constant distribution with increasing flow rate, the xanthan gum shows
an increase in fluid going into the larger outlet with higher flow rate, and the 22% colloidal suspension shows a decrease
in fluid entering the larger channel with higher flow rate. For the colloidal particle flow, the distribution of colloid particles
down the capillary is determined by examining the spectrally resolved propagator for the oil inside the core–shell particles
in a direction perpendicular to the axial flow. Using dynamic magnetic resonance microscopy, the potential for using magnetic
resonance for “particle counting” in a microscale bifurcation is thus demonstrated. 相似文献
20.
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