共查询到20条相似文献,搜索用时 31 毫秒
1.
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
2.
A thre-dimensional direct numerical simulation is combined with a laboratory study to describe the turbulent flow in an enclosed
annular rotor-stator cavity characterized by a large aspect ratio G = (b − a)/h = 18.32 and a small radius ratio a/b = 0.152, where a and b are the inner and outer radii of the rotating disk and h is the interdisk spacing. The rotation rate Ω considered is equivalent to the rotational Reynolds number Re = Ωb
2/ν= 9 .5 × 104 (ν the kinematic viscosity of water). This corresponds to a value at which experiment has revealed that the stator boundary
layer is turbulent, whereas the rotor boundary layer is still laminar. Comparisons of the computed solution with velocity
measurements have given good agreement for the mean and turbulent fields. The results enhance evidence of weak turbulence
by comparing the turbulence properties with available data in the literature (Lygren and Andersson, J Fluid Mech 426:297–326,
2001). An approximately self-similar boundary layer behavior is observed along the stator. The wall-normal variations of the structural
parameter and of characteristic angles confirm that this boundary layer is three-dimensional. A quadrant analysis (Kang et
al., Phys Fluids 10:2315–2322, 1998) of conditionally averaged velocities shows that the asymmetries obtained are dominated by Reynolds stress-producing events
in the stator boundary layer. Moreover, Case 1 vortices (with a positive wall induced velocity) are found to be the major
source of generation of special strong events, in agreement with the conclusions of Lygren and Andersson (J Fluid Mech 426:297–326,
2001). 相似文献
3.
Thomas Schweizer 《Rheologica Acta》2007,46(5):629-637
Creep experiments with a solution of polystyrene (M
w = 2.6 MDa, 16 vol.%, 25 °C) in diethyl phthalate are reported for stresses between 100 and 2,500 Pa (≈ 3G
N
0/4). The aim was to look for a flow transition as reported for strongly entangled poly(isobutylene) solutions. The experiments
with the polystyrene solution were repeated for cone angles of 2, 4, and 6° (radius 15 mm) and showed no dependence on cone
angle. The Cox–Merz rule was not fulfilled for stresses beyond about 800 Pa. The tangential observation with a CCD camera
showed that the edge took a concave shape because of the second normal stress difference. Beyond 1,000 Pa, the concave edge
develops into a crevice, thus substantially reducing the effective cross-section. This leads to runaway in a constant torque
experiment. At p
21 = 800 Pa, head-on particle tracking confirms that the originally linear velocity profile takes a gooseneck shape, thus revealing
shear banding. When the creep stress is stepped down to 100 Pa, this velocity profile evolves back to a linear one. The conclusion
from this work is that even if nonlinear creep experiments are reproducible and a steady state is reached, this does not mean
that the flow field is homogeneous.
This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27–29, 2006. 相似文献
4.
The problem of the self-similar boundary flow of a “Darcy-Boussinesq fluid” on a vertical plate with temperature distribution
T
w(x) = T
∞+A·x
λ and lateral mass flux v
w(x) = a·x
(λ−1)/2, embedded in a saturated porous medium is revisited. For the parameter values λ = 1,−1/3 and −1/2 exact analytic solutions
are written down and the characteristics of the corresponding boundary layers are discussed as functions of the suction/ injection
parameter in detail. The results are compared with the numerical findings of previous authors.
Received on 8 March 1999 相似文献
5.
Single exponential decay relationships, which define the molecular weight distribution (MWD) of a polymer as a function of the polymer’s relaxation
time spectrum (RTS), have been derived by Wu (Polym Eng Sci 28:538–543, 1988) and Thimm et al. (J Rheol 43:1663–1672, 1999). Experimental validation studies with monodisperse polymers, with quite precisely known MWDs, have been used to test their
reliability. It has been established that neither formula is always able to accurately recover the MWDs of monodisperse polymers
from their experimentally determined RTS. In this paper, different and more general relationships, based on theoretical results
of Anderssen and Loy (Bull Aust Math Soc 65:449–460, 2002a) for decays of the form , where the derivative of θ(t) is a completely monotone function, are derived, analyzed, and applied. It is shown how to transform these general relationships
to equivalent single exponential decay relationships for which Laplace transform solutions are derived. In order to illustrate
the interrelationship between an RTS and its corresponding MWD, an explicit analytic solution is given. The paper concludes
with a discussion of the rheological implications for the BSW model. 相似文献
6.
In this study, convective heat transfer and pressure drop in a cross-flow heat exchanger with hexagonal, square and circular
(HSC) pin–fin arrays were studied experimentally. The pin–fins were arranged in an in-line manner. For the applied conditions,
the optimal spacing of the pin–fin in the span-wise and stream-wise directions has been determined. The variable parameters
are the relative longitudinal pitch (S
L
/D = 2, 2.8, 3.5), and the relative transverse pitch was kept constant at S
T
/D = 2. The performances of all pin–fins were compared with each other. The experimental results showed that the use of hexagonal
pin–fins, compared to the square and circular pin–fins, can lead to an advantage in terms of heat transfer enhancement. The
optimal inter-fin pitches are provided based on the largest Nusselt number under the same pumping power, while the optimal
inter-fin pitches of hexagonal pin–fins are S
T
/D = 2 and S
L
/D = 2.8. Empirical equations are derived to correlate the mean Nusselt number and friction coefficient as a function of the
Reynolds number, pin–fin frontal surface area, total surface area, and total number. Consequently, the general empirical formula
is given in the present form.
NuD = a(ReD )b ( \fracNt Af A\texttotal )c \textand f = a(ReD )b Nu_{D} = a(Re_{D} )^{b} \left( {{\frac{{N_{t} A_{f} }}{{A_{\text{total}} }}}} \right)^{c} \quad {\text{and}}\quad f = a(Re_{D} )^{b} 相似文献
7.
Yoshihiro Ueda Tohru Nakamura Shuichi Kawashima 《Archive for Rational Mechanics and Analysis》2010,198(3):735-762
This paper is concerned with the asymptotic stability of degenerate stationary waves for viscous gases in the half space.
We discuss the following two cases: (1) viscous conservation laws and (2) damped wave equations with nonlinear convection.
In each case, we prove that the solution converges to the corresponding degenerate stationary wave at the rate t
−α/4 as t → ∞, provided that the initial perturbation is in the weighted space
L2a=L2(\mathbb R+; (1+x)a dx){L^2_\alpha=L^2({\mathbb R}_+;\,(1+x)^\alpha dx)} . This convergence rate t
−α/4 is weaker than the one for the non-degenerate case and requires the restriction α < α*(q), where α*(q) is the critical value depending only on the degeneracy exponent q. Such a restriction is reasonable because the corresponding linearized operator for viscous conservation laws cannot be dissipative
in L2a{L^2_\alpha} for α > α*(q) with another critical value α*(q). Our stability analysis is based on the space–time weighted energy method in which the spatial weight is chosen as a function
of the degenerate stationary wave. 相似文献
8.
We present convergence results for an adaptive algorithm to compute free energies, namely the adaptive biasing force (ABF)
method (Darve and Pohorille in J Chem Phys 115(20):9169–9183, 2001; Hénin and Chipot in J Chem Phys 121:2904, 2004). The free energy is the effective potential associated to a so-called reaction coordinate ξ(q), where q = (q
1, … , q
3N
) is the position vector of an N-particle system. Computing free energy differences remains an important challenge in molecular dynamics due to the presence
of metastable regions in the potential energy surface. The ABF method uses an on-the-fly estimate of the free energy to bias
dynamics and overcome metastability. Using entropy arguments and logarithmic Sobolev inequalities, previous results have shown
that the rate of convergence of the ABF method is limited by the metastable features of the canonical measures conditioned
to being at fixed values of ξ (Lelièvre et al. in Nonlinearity 21(6):1155–1181, 2008). In this paper, we present an improvement on the existing results in the presence of such metastabilities, which is a generic
case encountered in practice. More precisely, we study the so-called bi-channel case, where two channels along the reaction
coordinate direction exist between an initial and final state, the channels being separated from each other by a region of
very low probability. With hypotheses made on ‘channel-dependent’ conditional measures, we show on a bi-channel model, which
we introduce, that the convergence of the ABF method is, in fact, not limited by metastabilities in directions orthogonal
to ξ under two crucial assumptions: (i) exchange between the two channels is possible for some values of ξ and (ii) the free energy is a good bias in each channel. This theoretical result supports recent numerical experiments (Minoukadeh et al. in J Chem Theory Comput 6:1008–1017, 2010), where the efficiency of the ABF approach is demonstrated for such a multiple-channel situation. 相似文献
9.
Zdeněk Skalák 《Journal of Mathematical Fluid Mechanics》2010,12(4):503-535
In the first part of the paper we study decays of solutions of the Navier–Stokes equations on short time intervals. We show,
for example, that if w is a global strong nonzero solution of homogeneous Navier–Stokes equations in a sufficiently smooth (unbounded) domain Ω
⊆ R3 and β ∈[1/2, 1) , then there exist C0 > 1 and δ0 ∈ (0, 1) such that
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