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1.
The turbulence structure near a wall is a very active subject of research and a key to the understanding and modeling of this
flow. Many researchers have worked on this subject since the fifties Hama et al. (J Appl Phys 28:388–394, 1957). One way to study this organization consists of computing the spatial two-point correlations. Stanislas et al. (C R Acad
Sci Paris 327(2b):55–61, 1999) and Kahler (Exp Fluids 36:114–130, 2004) showed that double spatial correlations can be computed from stereoscopic particle image velocimetry (SPIV) fields and can
lead to a better understanding of the turbulent flow organization. The limitation is that the correlation is only computed
in the PIV plane. The idea of the present paper is to propose a new method based on a specific stereoscopic PIV experiment
that allows the computation of the full 3D spatial correlation tensor. The results obtained are validated by comparison with
2D computation from SPIV. They are in very good agreement with the results of Ganapthisubramani et al. (J Fluid Mech 524:57–80,
2005a). 相似文献
2.
Drag correction factors are calculated for the creeping motion of spheres descending in various associative polymers of different
concentration with various sphere-container ratios and Weissenberg numbers. The simple-shear rheology and linear viscoelasticity
of these polymeric fluids have been previously presented and modeled with the BMP (Bautista–Manero–Puig) equation of state
(Mendoza-Fuentes et al., Phys Fluids 21:033104, 2009). The drag on the sphere is initially kept nearly constant for small Weissenberg numbers, We < 0.1. As the Weissenberg number
increases, We < 0.1, a reduction in drag is found. Experimental results show the presence of a critical Weissenberg number
at which a drag reduction occurs. The reduction in the drag correction factor is associated to the onset of extension-thinning,
which coincides with the formation of a negative wake. No increase in the drag correction factor was observed, due to the
simultaneous opposing effects of extension-thickening and shear-thinning viscosity. The shape of the drag correction factor
curve may be predicted considering the extensional properties of the solutions, as suggested elsewhere (Chen and Rothstein,
J Non-Newton Fluid Mech 116:205–215, 2004). 相似文献
3.
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
4.
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
5.
In the present paper, the interaction mechanisms of the vortices shed by a single-screw propeller with a rudder installed
in its wake are addressed; in particular, following the works by Felli et al. (Exp Fluids 6(1):1–11, 2006a, Exp Fluids 46(1):147–1641, 2009a, Proceedings of the 8th international symposium on particle image velocimetry: Piv09, Melbourne, 2009b), the attention is focused on the analysis of the evolution, instability, breakdown and recovering mechanisms of the propeller
tip and hub vortices during the interaction with the rudder. To investigate these mechanisms in detail, a wide experimental
activity consisting in time-resolved visualizations, velocity measurements by particle image velocimetry (PIV) and laser Doppler
velocimetry (LDV) along horizontal chordwise, vertical chordwise and transversal sections of the wake have been performed
in the Cavitation Tunnel of the Italian Navy. Collected data allows to investigate the major flow features that distinguish
the flow field around a rudder operating in the wake of a propeller, as, for example, the spiral breakdown of the vortex filaments,
the rejoining mechanism of the tip vortices behind the rudder and the mechanisms governing the different spanwise misalignment
of the vortex filaments in the pressure and suction sides of the appendage. 相似文献
6.
The full energy dissipation rate and enstrophy are measured simultaneously using a probe consisting of four X-wires in the
intermediate region of a cylinder wake for Taylor microscale Reynolds number in the range of 120–320. Longitudinal and transverse
velocity increments are also obtained temporally using Taylor’s hypothesis. The inertial range scaling exponents indicate
that the full enstrophy field has a stronger intermittency than does the full dissipation field for all the Reynolds numbers
considered. The approximations of the energy dissipation rate and enstrophy based on isotropy are more intermittent than their
corresponding true values. While the scaling exponents of the full energy dissipation rate remain approximately constant for
different Reynolds numbers, those of the enstrophy decrease slightly and consistently with the increase of Reynolds number.
It is conjectured that the scaling of the energy dissipation rate and the enstrophy may be the same when Reynolds number is
extremely high, a trend that is consistent with that suggested by Nelkin (Phys Fluids 11:2202–2204, 1999; Am J Phys 68:310–318, 2000). 相似文献
7.
Olivier Lafitte Mark Williams Kevin Zumbrun 《Archive for Rational Mechanics and Analysis》2012,204(1):141-187
The rigorous study of spectral stability for strong detonations was begun by Erpenbeck (Phys. Fluids 5:604–614 1962). Working with the Zeldovitch–von Neumann–D?ring (ZND) model (more precisely, Erpenbeck worked with an extension of ZND to
general chemistry and thermodynamics), which assumes a finite reaction rate but ignores effects such as viscosity corresponding
to second order derivatives, he used a normal mode analysis to define a stability function V(t,e){V(\tau,\epsilon)} whose zeros in ${\mathfrak{R}\tau > 0}${\mathfrak{R}\tau > 0} correspond to multidimensional perturbations of a steady detonation profile that grow exponentially in time. Later in a remarkable
paper (Erpenbeck in Phys. Fluids 9:1293–1306, 1966; Stability of detonations for disturbances of small transverse wavelength, 1965) he provided strong evidence, by a combination of formal and rigorous arguments, that for certain classes of steady ZND profiles,
unstable zeros of V exist for perturbations of sufficiently large transverse wavenumber e{\epsilon} , even when the von Neumann shock, regarded as a gas dynamical shock, is uniformly stable in the sense defined (nearly 20 years
later) by Majda. In spite of a great deal of later numerical work devoted to computing the zeros of V(t,e){V(\tau,\epsilon)} , the paper (Erpenbeck in Phys. Fluids 9:1293–1306, 1966) remains one of the few works we know of [another is Erpenbeck (Phys. Fluids 7:684–696, 1964), which considers perturbations for which the ratio of longitudinal over transverse components approaches ∞] that presents
a detailed and convincing theoretical argument for detecting them. The analysis in Erpenbeck (Phys. Fluids 9:1293–1306, 1966) points the way toward, but does not constitute, a mathematical proof that such unstable zeros exist. In this paper we identify
the mathematical issues left unresolved in Erpenbeck (Phys. Fluids 9:1293–1306, 1966) and provide proofs, together with certain simplifications and extensions, of the main conclusions about stability and instability
of detonations contained in that paper. The main mathematical problem, and our principal focus here, is to determine the precise
asymptotic behavior as e?¥{\epsilon\to\infty} of solutions to a linear system of ODEs in x, depending on e{\epsilon} and a complex frequency τ as parameters, with turning points x
* on the half-line [0,∞). 相似文献
8.
9.
Concerning to the non-stationary Navier–Stokes flow with a nonzero constant velocity at infinity, just a few results have
been obtained, while most of the results are for the flow with the zero velocity at infinity. The temporal stability of stationary
solutions for the Navier–Stokes flow with a nonzero constant velocity at infinity has been studied by Enomoto and Shibata
(J Math Fluid Mech 7:339–367, 2005), in L
p
spaces for p ≥ 3. In this article, we first extend their result to the case
\frac32 < p{\frac{3}{2} < p} by modifying the method in Bae and Jin (J Math Fluid Mech 10:423–433, 2008) that was used to obtain weighted estimates for the Navier–Stokes flow with the zero velocity at infinity. Then, by using
our generalized temporal estimates we obtain the weighted stability of stationary solutions for the Navier–Stokes flow with
a nonzero velocity at infinity. 相似文献
10.
In this paper we consider a fully developed turbulent flow in a round pipe with a small inner annulus. The diameter of the
inner annulus is less than 10% of the diameter of the outer pipe. As a consequence, the surface area of the inner pipe compared
to the outer pipe is small. The friction exerted by the wall on the flow is proportional to the surface area and the wall
shear stress. Due to the small surface area of the inner annulus the additional stress on the flow due to the presence of
the annulus may expected to be negligible. However, it will be shown that the inner annulus drastically changes the flow patterns
and gives rise to unexpected scaling properties. In previous studies (Chung et al., Int J Heat Fluid Flow 23:426–440, 2002; Churchill and Chan, AIChE J 41:2513–2521, 1995) it was argued that radial position of the point of zero shear stress does not coincide with the radial location of the point
of maximum axial velocity. In our direct numerical simulations we observe a coincidence of these points within the numerical
accuracy of our model. It is shown that the velocity profile close to the inner annulus is logarithmic. 相似文献
11.
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
12.
The paper discusses localized measurements that may be used to validate individual sub-models in mechanistic models of nucleate
boiling on inclined surfaces with sliding bubbles. A previous study of wall temperature variations near isolated sliding bubbles
by Kenning et al. (Multiphase Sci Technol 14:75–94, 2002), employing liquid crystal thermography, has been expanded in range and compared with recent studies by Qiu and Dhir (Exp
Therm Fluid Sci 26:605–616, 2002) and Bayazit et al. (J Heat Transfer 125:503–509, 2003). The implications for modelling nucleate boiling are discussed. 相似文献
13.
The velocity field and skin friction distribution around a row of five jets issuing into a crossflow from short (L/D ≃ 1) pipes inclined by 35° with respect to the streamwise direction, (i.e., “short holes”) are presented for two different
jet supply flow directions. Velocity was measured using PIV, while the skin friction was measured with oil-film interferometry.
The flow features are compared with previously published data for jets issuing through holes oriented normal to the crossflow
and with numerical simulations of similar geometries. The distinguishing features of the flow field include a reduced recirculation
region in comparison to the 90° case and markedly different in-hole flow physics. The jetting process caused by in-hole separations
force the bulk of the jet fluid to issue from the leading half of the streamwise-angled injection hole, as previously reported
by Brundage et al. (Tech Rep ASME 99-GT-35, 1999) and predicted by Walters and Leylek (ASME J Turbomach 122:101–112, 2000). The flow structure impacts the skin friction distribution around the holes, resulting in higher near-hole shear stress
for a counter-flow supply plenum (jet fluid supplied by a high speed plenum flowing opposite to the free stream direction).
In contrast, the counter-flow supply plenum was previously found to have the lowest near-hole wall shear stress for normal
injection holes (Peterson and Plesniak in Exp Fluids 37:497–503, 2004b). Streamwise-angled injection generally reduces the near-hole skin friction due to the reduced jet trajectory resulting from
the lower wall-normal jet momentum. Far downstream, the skin friction distributions are similar for the two injection angle
cases. 相似文献
14.
T. Astarita 《Experiments in fluids》2006,40(6):977-987
As testified by a previous article (Astarita and Cardone in Exp Fluids 38:233–243, 2005), a critical point that can influence significantly the accuracy of image deformation methods (IDM) for particle image velocimetry (PIV) is the interpolation scheme (IS) used in the reconstruction of deformed images. In the cited paper the effect of noise has been neglected and for this reason in this follow-up paper the influence of the IS, in the presence of noise, on both accuracy and spatial resolution is studied. Performance assessment is conducted using synthetic images with particles of Gaussian shape and with constant and sinusoidal displacement fields. Both the local and the top hat moving average approaches are investigated and the modulation transfer function, the total and bias errors have been used to evaluate the performances of IDMs for PIV applications. The results show that, when a high noise level is present in the images, the influence of the IS is less relevant than what was shown by Astarita and Cardone (Exp Fluids 38:233–243, 2005). 相似文献
15.
B. D. Reddy 《Continuum Mechanics and Thermodynamics》2011,23(6):551-572
Variational formulations are constructed for rate-independent problems in small-deformation single-crystal strain-gradient
plasticity. The framework, based on that of Gurtin (J Mech Phys Solids 50: 5–32, 2002), makes use of the flow rule expressed in terms of the dissipation function. Provision is made for energetic and dissipative
microstresses. Both recoverable and non-recoverable defect energies are incorporated into the variational framework. The recoverable
energies include those that depend smoothly on the slip gradients, the Burgers tensor, or on the dislocation densities (Gurtin
et al. J Mech Phys Solids 55:1853–1878, 2007), as well as an energy proposed by Ohno and Okumura (J Mech Phys Solids 55:1879–1898, 2007), which leads to excellent agreement with experimental results, and which is positively homogeneous and therefore not differentiable
at zero slip gradient. Furthermore, the variational formulation accommodates a non-recoverable energy due to Ohno et al. (Int
J Mod Phys B 22:5937–5942, 2008), which is also positively homogeneous, and a function of the accumulated dislocation density. Conditions for the existence
and uniqueness of solutions are established for the various examples of defect energy, with or without the presence of hardening
or slip resistance. 相似文献
16.
The structure of turbulence in a drag-reduced flat-plate boundary layer flow has been studied with particle image velocimetry (PIV). Drag reduction was achieved by injection of a concentrated polymer solution through a spanwise slot along the test wall at a location upstream of the PIV measurement station. Planes of velocity were measured parallel to the wall (x–z plane), for a total of 30 planes across the thickness of the boundary layer. For increasing drag reduction, we found a significant modification of the near-wall structure of turbulence with a coarsening of the low-speed velocity streaks and a reduction in the number and strength of near-wall vortical structures. 相似文献
17.
Astigmatism or wavefront deformation, microscopic particle tracking velocimetry (A-μPTV) (Chen et al. in Exp Fluids 47:849–863,
2009; Cierpka et al. in Meas Sci Technol 21:045401, 2010b) is a method to determine the complete 3D3C velocity field in micro-fluidic devices with a single camera. By using an intrinsic
calibration procedure that enables a robust and precise calibration on the basis of the measured data itself (Cierpka et al.
in Meas Sci Technol 22:015401, doi:, 2011), accurate results without errors due to spatial averaging or bias due to the depth of correlation can be obtained. This
method takes all image aberrations into account, allows for the use of the whole CCD sensor, and is easy to apply without
expert knowledge. In this paper, a comparative study is presented to assess the uncertainties of two state-of-the-art methods
for 3C3D velocity field measurements in microscopic flows: stereoscopic micro-particle image velocimetry (S-μPIV) and astigmatism
micro-particle tracking velocimetry (A-μPTV). First, the main parameters affecting all methods’ measurement uncertainty are
identified, described, and quantified. Second, the test case of the flow over a backward-facing step is analyzed using all
methods. For comparison, standard 2D2C μPIV measurements and numerical flow simulations are shown as well. Advantages and
disadvantages of both methods are discussed. 相似文献
18.
Spectral estimation of irregularly sampled velocity data issued from Laser Doppler Anemometry measurements is considered in
this paper. A new method is proposed based on linear interpolation followed by a deconvolution procedure. In this method,
the analytic expression of the autocorrelation function of the interpolated data is expressed as a linear function of the
autocorrelation function of the data to be estimated. For the analysis of both simulated and experimental data, the results
of the proposed method is compared with the one of the reference methods in LDA: refinement of autocorrelation function of
sample-and-hold interpolated signal method given by Nobach et al. (Exp Fluids 24:499–509, 1998), refinement of power spectral density of sample-and-hold interpolated signal method given by Simon and Fitzpatrick (Exp
Fluids 37:272–280, 2004) and fuzzy slotting technique with local normalization and weighting algorithm given by Nobach (Exp Fluids 32:337–345, 2002). Based on these results, it is concluded that the performances of the proposed method are better than the one of the other
methods, especially for what concerns bias and variance. 相似文献
19.
The purpose of this study is to implement a new analytical method which is a combination of the homotopy analysis method (HAM)
and the Padé approximant for solving magnetohydrodynamic boundary-layer flow. The solution is compared with the numerical
solution. Comparisons between the HAM–Padé and the numerical solution reveal that the new technique is a promising tool for
solving MHD boundary-layer equations. The effects of the various parameters on the velocity and temperature profiles are presented
graphically form. Favorable comparisons with previously published works (Crane, J. Appl. Math. Phys. 21:645–647, 1970, and Vajravelu and Hadjinicolaou, Int. J. Eng. Sci. 35:1237–1244, 1997) are obtained. It is predicted that HAM–Padé can have wide application in engineering problems (especially for boundary-layer
and natural convection problems). 相似文献
20.
S. Vantieghem X. Albets-Chico B. Knaepen 《Theoretical and Computational Fluid Dynamics》2009,23(6):525-533
We present numerical simulations without modeling of an incompressible, laminar, unidirectional circular pipe flow of an electrically
conducting fluid under the influence of a uniform transverse magnetic field. Our computations are performed using a finite-volume
code that uses a charge-conserving formulation [called current-conservative formulation in references (Ni et al J Comput Phys
221(1):174–204, 2007, Ni et al J Comput Phys 227(1):205–228, 2007)]. Using high resolution unstructured meshes, we consider
Hartmann numbers up to 3000 and various values of the wall conductance ratio c. In the limit c << Ha-1{c{\ll}{\rm Ha}^{-1}} (insulating wall), our results are in excellent agreement with the so-called asymptotic solution (Shercliff J Fluid Mech
1:644–666, 1956). For higher values of the wall conductance ratio, a discrepancy with the asymptotic solution is observed
and we exhibit regions of velocity overspeed in the Roberts layers. We characterise these overspeed regions as a function
of the wall conductance ratio and the Hartmann number; a set of scaling laws is derived that is coherent with existing asymptotic
analysis. 相似文献