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1.
It is now well know that a small addition of salt to a micellar solution often increases the zero-shear viscosity η0 of the solution, the understanding of the behaviour at high salt content is more questionable. In this situation, addition
of more salt induces a decrease of η0. In this experimental work we investigate the linear and non-linear rheological behaviour of a new micellar system: CPCl
(surfactant)/NaClO3 (salt). Studies of the evolution of η0 as well as G0 (the elastic modulus) or τR (the relaxation time) are in agreement with the hypothesis of a diminution of the mean micellar length when, after the maximum
η0, the salt content increases. In the non-linear behaviour (non-Newtonian viscosity) the evolution of γ˙
c, (which defines the occurrence of the shear thinning) with salt concentration CS is also in agreement with such a hypothesis.
Received: 29 March 1999/Accepted: 20 March 2000 相似文献
2.
The rheological behavior and morphology of carbon nanofiber/polystyrene (CNF/PS) composites in their melt phase have been characterized both through experimental measurements and modeling. Composites prepared in the two different processes of solvent casting and melt blending are contrasted; melt-blended and solvent-cast composites were each prepared with CNF loadings of 2, 5, and 10 wt%. A morphological study revealed that the melt blending process results in composites with shorter CNFs than in the solvent-cast composites, due to damage caused by the higher stresses the CNFs encounter in melt blending, and that both processes retain the diameter of the as-received CNFs. The addition of carbon nanofiber to the polystyrene through either melt blending or solvent casting increases the linear viscoelastic moduli, G′ and G″, and steady-state viscosity, η, in the melt phase monotonically with CNF concentration, more so in solvent cast composites with their longer CNFs. The melt phase of solvent-cast composites with higher CNF concentrations exhibit a plateau of the elastic modulus, G′, at low frequencies, an apparent yield stress, and large first normal stress difference, N
1, at low strain rates, which can be attributed to contact-based network nanostructure formed by the long CNFs. A nanostructurally-based model for CNF/PS composites in their melt phase is presented which considers the composite system as rigid rods in a viscoelastic fluid matrix. Except for two coupling parameters, all material constants in the model for the composite systems are deduced from morphological and shear flow measurements of its separate nanofiber and polymer melt constituents of the composite. These two coupling parameters are polymer–fiber interaction parameter, σ, and interfiber interaction parameter, C
I. Through comparison with our experimental measurements of the composite systems, we deduce that σ is effectively 1 (corresponding to no polymer–fiber interaction) for all CNF/PS nanocomposites studied. The dependence of CNF orientation on strain rate which we observe in our experiments is captured in the model by considering the interfiber interaction parameter, C
I, as a function of strain rate. Applied to shear flows, the model predicts the melt-phase, steady-state viscosities, and normal stress differences of the CNF/PS composites as functions of shear rate, polymer matrix properties, fiber length, and mass concentration consistent with our experimental measurements. 相似文献
3.
A. Ślęzak J. Jasik-Ślęzak S. Grzegorczyn I. Ślęzak-Prochazka 《Transport in Porous Media》2012,92(2):337-356
The results of experimental study of volume osmotic flows in a double-membrane system are presented in this article. The double-membrane
system consists of two membranes (M
u, M
d) oriented in horizontal planes and three identical compartments (u, m, d), containing unstirred binary or ternary ionic solutions.
In this system concentrations of the solutions fulfil the following conditions C
us
= C
ds
< C
ms
(s = 1 or 2). Solutions of aqueous potassium chloride or ammonia were used as binary solutions, whereas potassium chloride dissolved
in aqueous ammonia solution or ammonia dissolved in aqueous potassium chloride solution were used as ternary solutions. For
binary solutions, the dependencies of a volume flux (J
v) on potassium chloride or ammonia concentration (C
ms
) are linear, whereas for ternary solutions these dependencies are nonlinear. The volume flux amplification and the osmotic
conductivity coefficients were calculated on the basis of experimental data. The coefficient of the volume flux amplification
for ternary solutions in comparison to binary ones depends on solutes concentrations and has maximum values dependent on solutes
concentrations. Similarly, the osmotic conductivity coefficient has maximal values dependent on solutes concentrations. Moreover,
the thermodynamic model of the osmotic volume flux was developed and the results were interpreted within the gravitational
instability category. 相似文献
4.
Avner Friedman Bei Hu Juan J. L. Velazquez 《Archive for Rational Mechanics and Analysis》2000,152(2):103-139
When a crack Γ
s
propagates in an elastic medium the stress intensity factors evolve with the tip x(s) of Γ
s
. In this paper we derive formulae which describe the evolution of these stress intensity factors for a homogeneous isotropic
elastic medium under plane strain conditions. Denoting by ψ=ψ(x,s) the stress potential (ψ is biharmonic and has zero traction along the crack Γ
s
) and by κ(s) the curvature of the crack at the tip x(s), we prove that the stress intensity factors A
1(s), A
2(s), as functions of s, satisfy:
where , are stress intensity factors of the tangential derivative of in the polar coordinate system at x(s) with θ=0 in the direction of the crack at x(s). The case of antiplane shearing is also briefly considered; in this case ψ is harmonic. 相似文献
5.
Maria del Mar Ramos-Tejada Alfonso Ontiveros Rosario del Carmen Plaza Angel V. Delgado Juan D. G. Durán 《Rheologica Acta》2003,42(1-2):148-157
Steady-state, oscillatory, and transient rheological determinations were used to assess the stability of homoionic sodium
montmorillonite (NaMt) suspensions at constant ionic strength (10–2 mol/l NaCl) and different pH values, after adsorption of humic acid (HA) on the particles. The adsorption of the latter was
first spectrophotometrically determined, at pH 3 and 9. While at pH 9 adsorption saturation was observed, at pH 3 the adsorption
density continued to grow up to the maximum equilibrium HA concentration reached (∼200 mg/l). Considering the similarity between
the structure of edge surfaces of NaMt particles and the surfaces of silica and alumina, the adsorption of HA was also investigated
on the latter solids. The results suggest that at pH 3 humic acids adsorb preferentially on edge surfaces, mainly through
electrostatic attraction with positively charged aluminol groups. This hypothesis is indirectly confirmed by zeta potential,
ζ, values: while HA concentration has little effect on ζ for silica, the addition of HA yields the zeta potential of alumina
increasingly negative for all pH values. Using shear stress vs shear rate plots, the yield stress of NaMt was determined as
a function of particle concentration, C, for pH 3, 5, 7, and 9, with and without addition of 50 mg/l HA. The yield stress, σy, was fitted with a power law σy∝C
n
; it was found that n values as high as 12 are characteristic of NaMt suspensions at pH 9 in the presence of HA. This indicates a strong stabilizing
effect of humic acid. This stabilization was confirmed by oscillometric measurements, as the storage modulus G′ in the viscoelastic linear region also scales with C, displaying large n values at neutral and basic pHs in the presence of HA. The modulus (in the viscoelastic linear region, for a frequency ν=1 Hz)
was found to increase with time, but G′ was lower at any time when HA was added, a consequence of the stabilization provided by HA. Similarly, creep-recovery experiments
demonstrated that NaMt suspensions containing HA displayed a less elastic behavior, and a permanent deformation. Modeling
the results as a Kelvin-Voigt model allowed one to establish a new scaling law of the reciprocal instantaneous deformation
with C. As before, high values of n were found for suspensions at pH 9 in the presence of HA. 相似文献
6.
Linear and nonlinear viscoelastic properties were examined for aqueous suspensions of monodisperse poly(methyl methacrylate-co-styrene) (MS) particles having the radius a
0
=45 nm and the volume fractions φ=0.428−0.448. These particles had surface charges and the resulting electrostatic surface
layer (electric double layer) had a thickness of ts=5.7 nm. At low frequencies in the linear viscoelastic regime, the MS particles behaved approximately as the Brownian hard
particles having an effective radius a
eff=a
0 + ts, and the dependence of their zero-shear viscosity η0 on an effective volume fraction φeff (={a
eff/a
0}3φ) agreed with the φ dependence of η0 of ideal hard-core silica suspensions. In a range of φeff < 0.63, this φeff dependence was well described by the Brady theory. However, the φeff dependence of the high-frequency plateau modulus was weaker and the terminal relaxation mode distribution was narrower for
the MS suspensions than for the hard-core suspensions. This result suggested that the electrostatic surface layer of the MS
particles was soft and penetrable (at high frequencies). In fact, this “softness” was more clearly observed in the nonlinear
regime: the nonlinear damping against step strain was weaker and the thinning under steady shear was less significant for
the MS suspension than for the hard-core silica suspensions having the same φeff. These weaker nonlinearities of the concentrated MS particles with φeff∼ 0.63 (maximum volume fraction for random packing) suggested that the surface layers of those particles were mutually penetrating
to provide the particles with a rather large mobility.
Received: 10 July 2001 Accepted: 2 November 2001 相似文献
7.
Henning Storz Ulrich Zimmermann Heiko Zimmermann Werner-Michael Kulicke 《Rheologica Acta》2010,49(2):155-167
Ultra-high viscosity alginates were extracted from the brown seaweeds Lessonia nigrescens (UHVN, containing 61% mannuronate (M) and 2% guluronate (G)) and Lessonia trabeculata (UHVT, containing 22% M and 78% G). The viscoelastic behavior of the aqueous solutions of these alginates was determined in shear
flow in terms of the shear stress σ
21, the first normal stress difference N
1, and the shear viscosity η in isotonic NaCl solutions (0.154 mol/L) at T = 298 K in dependence of the shear rate [(g)\dot]\dot{\gamma} for solutions of varying concentrations and molar masses (3–10 × 105 g/mol, homologous series was prepared by ultrasonic degradation). Data obtained in small-amplitude oscillatory shear (SAOS)
experiments obey the Cox–Merz rule. For comparison, a commercial alginate with intermediate chemical composition was additionally
characterized. Particulate substances which are omnipresent in most alginates influenced the determination of the material
functions at low shear rates. We have calculated structure–property relationships for the prediction of the viscosity yield,
e.g., η–M
w–c–[(g)\dot]\dot{\gamma} for the Newtonian and non-Newtonian region. For the highest molar masses and concentrations, the elasticity yield in terms
of N
1 could be determined. In addition, the extensional flow behavior of the alginates was measured using capillary breakup extensional
rheometry. The results demonstrate that even samples with the same average molar mass but different molar mass distributions
can be differentiated in contrast to shear flow or SAOS experiments. 相似文献
8.
Chen Zhiying 《Acta Mechanica Sinica》1996,12(1):73-84
The gliding behavior of edge dislocation near a grain boundary (GB) in copper under pure shear stresses is simulated by using
molecular dynamics(MD) method. Many-body potential incorporating the embedded atom method (EAM) is used. The critical shear
stresses for a single disocation to pass across GB surface are obtained at values of σ
c
=23MPa ∼ 68 MPa and 137MPa∼274 MPa for Ω=165 small angle tilt GB at 300K and 20K, respectively. The first result agrees with
the experimental yield stress σ
y
(=42MPa) quite well. It suggests that there might be one of the reasons of initial plastic yielding caused by single dislocation
gliding across GB. In addition, there might be possibility to obtain yield strength from microscopic analysis. Moreover, the
experimental value of σ
y
at low temperature is generally higher than that at room temperature. So, these results are in conformity qualitatively with
experimental fact. On the other hand, the Ω=25 GB is too strong an obstacle to the dislocation. In this case, a dislocation
is able to pass across GB under relatively low stress only when it is driven by other dislocations. This is taken to mean
that dislocation pile-up must be built up in front of this kind of GB, if this GB may take effect on the process of plastic
deformation.
The project supported by KM85-33 of Academia Sinica and the National Natural Science Foundation of China 相似文献
9.
Saturated flow boiling of environmentally acceptable nonionic surfactant solutions of Alkyl (8–16) was compared to that of pure water. The concentration of surfactant solutions was in the range of 100–1000 ppm. The liquid flowed in an annular gap of 2.5 and 4.4 mm between two vertical tubes. The heat was transferred from the inner heated tube to two-phase flow in the range of mass flux from 5 to 18 kg/m2 s and heat flux from 40 to 200 kW/m2. Boiling curves of water were found to be heat flux and channel gap size dependent but essentially mass flux independent. An addition of surfactant to the water produced a large number of bubbles of small diameter, which, at high heat fluxes, tend to cover the entire heater surface with a vapor blanket. It was found that the heat transfer increased at low values of relative surfactant concentration C/C0, reaches a maximum close to the value of C/C0 = 1 (where C0 = 300 ppm is the critical micelle concentration) and decreased with further increase in the amount of additive. The dependence of the maximal values of the relative heat transfer enhancement, obtained at the value of relative concentration of C/C0 = 1, on the boiling number Bo may be presented as single curve for both gap sizes and the whole range of considered concentrations. 相似文献
10.
In this paper an experimental investigation of a starting water flow downstream of a backward-facing step is described. The
Reynolds number of the asymptotic steady flow is Re≈4300 based on the step height of s=2 cm and the free stream velocity of U=21.4 cm/s. Velocity measurements were performed with video-based DPIV (Digital Particle Image Velocimetry) at a sampling
frequency of 25 Hz. The main purpose of this study is to reveal the temporal development of global structures which could
not be analyzed with single-point probes. It was found that at initialization of the flow a regular vorticity street is formed,
which collapses at a normalized time t
*
=U t/s≈17 due to vorticity interactions. After this the flow is dominated by complicated vorticity roll-up and shedding dynamics
in the recirculation region. The starting phase is terminated for t
*
>40. Prior to the collapse of the vorticity street values of 9 times the steady state asymptotic wall normal stress and of
twice the steady state negative wall shear stress were observed. The early increasing slope of the reattachment length is
constant over a time of approximately t
*
=8. The collapse of the vorticity street and the vorticity interactions thereafter cause fluctuations both in the velocity
in the free shear layer and in the reattachment length. The fully developed flow has a dominant frequency corresponding to
a Strouhal number St=fs/U≈0.04.
Received: 20 September 1996/Accepted: 1 April 1997 相似文献
11.
Maik Nowak 《Rheologica Acta》2001,40(4):366-372
The first normal stress differences N
1 of a highly dilute cationic surfactant solution are investigated in a cone-and-plate rheometer. In continuation of a previous
paper (Nowak 1998), where the buildup of a shear induced structure in such a solution was attained after a reduced deformation,
the N
1 turned out to be in proportion to the square of the shear rate γ˙ reduced by a critical value γ˙
c
in a first range above γ˙
c
. At higher shear rates the N
1 tend to lower values than predicted by this relation.
Relaxation experiments were performed in the same geometry to determine the characteristic time scales of the shear induced
state's decay. In the lower range above &γdot;
c
the stress decay is a monoexponential process, while a second time constant has to be introduced to describe the relaxation
in that range, where the N
1 deviate from the parabolic dependence of the reduced shear rate.
Received: 10 May 1999 Accepted: 15 November 2000 相似文献
12.
The influence of the shear number on the turbulence evolution in a stably stratified fluid is investigated using direct numerical
simulations on grids with up to 512 × 256 × 256 points. The shear number SK/ε is the ratio of a turbulence time scale K/ε to the shear time scale 1/S. Simulations are performed at two initial values of the Reynolds number Re
Λ= 44.72 and Re
Λ= 89.44. When the shear number is increased from small to moderate values, the nondimensional growth rate γ= (1/SK)dK/dt of the turbulent kinetic energy K increases since the shear forcing and its associated turbulence production is larger. However, a further increase of the
shear number from moderate to large values results in a reduction of the growth rate γ and the turbulent kinetic energy K shows long-time decay for sufficiently large values of the shear number. The inhibition of turbulence growth at large shear
numbers occurs for both initial values of the Reynolds number and can be explained with the predominance of linear effects
over nonlinear effects when the shear number is sufficiently high. It is found that, at the higher initial value of the Reynolds
number, the reduction of the growth rate occurs at a higher value of the shear number. The shear number is found to affect
spectral space dynamics. Turbulent transport coefficients decrease with increasing shear number.
Received 23 June 1998 and accepted 25 February 1999 相似文献
13.
Rheological analysis of highly concentrated w/o emulsions 总被引:2,自引:0,他引:2
Nathalie Jager-Lézer Jean-Francois Tranchant Valérie Alard Can Vu Pierre Cyrill Tchoreloff J.-L. Grossiord 《Rheologica Acta》1998,37(2):129-138
A series of highly concentrated lipophilic cosmetic emulsions were analysed, in order to determine their rheological and
textural properties, as a function of their microstructure. The originality of this study lies in the methodology used, especially
the shear-stress scanning analysis. The results of a very powerful and comprehensive dynamic rheological analysis suggest
the existence of two critical volume fraction values: besides the “close-packed” value φ
c
, a “slack-packed” value φ0, close to 0.60 could be demonstrated. It has been shown that the close-packed structure is stable under shear; in constrast,
the slack-packed configuration, defined as φ0<φ<φ
c
is unstable under shear. A comparison with theoretical models, especially that of Princen, showed good agreement and allowed
the close-packed value φ
c
to be defined more precisely as 0.67. The gap between 0.67 and 0.74 is probably indicative of a highly polydisperse distribution,
as confirmed by microscopic analysis. Flow experiments confirmed the validity of Princen‘s model.
Received: 20 February 1997 Accepted: 20 January 1998 相似文献
14.
Pulsatile flows in a lateral aneurysm anchored on a stented and curved parent vessel 总被引:5,自引:0,他引:5
We present particle tracking velocimetry measurements and flow visualization of pulsatile flow fields in a stented cerebrovascular
lateral aneurysm model with a wide ostium anchored on a curved parent vessel. Among the stent parameters, the blocking ratioC
α ranging from 0% to 75% was selected to study its effect on the changes of intra-aneurysmal hemodynamics for the reference
of minimally invasive endovascular aneurysm treatment. The Womersley number was 3.9 and the mean, peak, and minimal Reynolds
numbers based on the bulk average velocity and diameter of the parent vessel were 600, 850, and 300, respectively. The results
are characterized in terms of velocity vector field, coded streak images, region averaged velocity, vorticity, and wall shear
stress. A critical range ofC
α related to the inflow location as well as the shape and number of intra-aneurysmal vortices is identified. The intra-aneurysmal
flow activity, vortex strength, and wall shear stress are found to decrease with increasingC
α. Among theC
α examined,C
α=75% is the most favorable in attenuating the risk of aneurysmal rupture and promoting intra-aneurysmal thrombus. 相似文献
15.
Mataz Alcoutlabi S. G. Baek J. J. Magda Xiangfu Shi S. A. Hutcheson G. B. McKenna 《Rheologica Acta》2009,48(2):191-200
A novel pressure sensor plate (normal stress sensor (NSS) from RheoSense, Inc.) was adapted to an Advanced Rheometrics Expansion
System rheometer in order to measure the radial pressure profile for a standard viscoelastic fluid, a poly(isobutylene) solution,
during cone–plate and parallel-plate shearing flows at room temperature. We observed in our previous experimental work that
use of the NSS in cone-and-plate shearing flow is suitable for determining the first and second normal stress differences
N
1 and N
2 of various complex fluids. This is true, in part, because the uniformity of the shear rate at small cone angles ensures the
existence of a simple linear relationship between the pressure [i.e., the vertical diagonal component of the total stress
tensor (Π22)] and the logarithm of the radial position r (Christiansen and coworkers, Magda et al.). However, both normal stress differences can also be calculated from the radial
pressure distribution measured in parallel-plate torsional flows. This approach has rarely been attempted, perhaps because
of the additional complication that the shear rate value increases linearly with radial position. In this work, three different
methods are used to investigate N
1 and N
2 as a function of shear rate in steady shear flow. These methods are: (1) pressure distribution cone–plate (PDCP) method,
(2) pressure distribution parallel-plate (PDPP) method, and (3) total force cone–plate parallel-plate (TFCPPP) method. Good
agreement was obtained between N
1 and N
2 values obtained from the PDCP and PDPP methods. However, the measured N
1 values were 10–15% below the certified values for the standard poly(isobutylene) solution at higher shear rates. The TFCPPP
method yielded N
1 values that were in better agreement with the certified values but gave positive N
2 values at most shear rates, in striking disagreement with published results for the standard poly(isobutylene) solution.
相似文献
J. J. MagdaEmail: |
16.
T.C.T. Ting 《Journal of Elasticity》1997,47(1):23-50
The three Barnett-Lothe tensors H, L, S appear often in the Stroth formalism of two-dimensional deformations of anisotropic
elastic materials [1–3]. They also appear in certain three-dimensional problems [4, 5]. The algebraic representation of H,
L, S requires computation of the eigenvalues pv(v=1,2,3) and the normalized eigenvectors (a, b). The integral representation of H, L, S circumvents the need for computing
p
v(v=1,2,3) and (a, b), but it is not simple to integrate the integrals except for special materials. Ting and Lee [6] have recently
obtained an explicit expression of H for general anisotropic materials. We present here the remaining tensors L, S using the
algebraic representation. They key to our success is the obtaining of the normalization factor for (a, b) in a simple form.
The derivation of L and S then makes use of (a, b) but the final result does not require computation of (a, b), which makes
the result attractive to numerical computation. Even though the tensor H given in [6] is in terms of the elastic stiffnesses
Cμ v while the tensors L, S presented here are in terms of the reduced elastic compliances s′
μv
, the structure of L, S is similar to that of H. Following the derivation of H, we also present alternate expressions of
L, S that remain valid for the degenerate cases p
1
p
2 and p1=p2 = p
3. One may want to compute H, L, S using either C
μv
or s′
μv
v, but not both. We show how an expression in Cμ v can be converted to an expression in s′
μv
v, and vice versa. As an application of the conversion, we present explicit expressions of the extic equation for p in Cμ v and s′
μv
v.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
17.
José Inés Escalante Daniel Escobar Emma Rebeca Macias Juan Humberto Pérez-López Fernando Bautista Eduardo Mendizábal Jorge E. Puig Octavio Manero 《Rheologica Acta》2007,46(5):685-691
Low-viscosity micellar aqueous solutions of cetyltrimethylammonium bromide (CTAB) undergo a major change in the presence of
the hydrotrope, potassium 1-phenylmethylsulfate (KPhMS), producing a highly viscoelastic entanglement network of polymer-like
micelles. The system studied here shows typical shear banding flow behavior, which tends to disappear with increasing the
hydrotrope-to-surfactant concentration ratio (C
H / C
S). The linear rheological response was analyzed with the model of Granek–Cates, whereas the nonlinear behavior was reproduced
with the Bautista–Manero–Puig (BMP) model. Both models introduce a kinetic equation to account for the breaking and reformation
of the micelles, and they predict the linear and nonlinear rheological data very well.
This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27–29, 2006. 相似文献
18.
Laser-Doppler measurements were conducted in a plane turbulent wall jet at a Reynolds number based on inlet velocity, Re
0, of 9600. The initial development as well as the fully developed flow was studied. Special attention was given to the near-wall
region, including the use of small measuring volumes and the application of specific near-wall data corrections, so that wall
shear stresses were determined directly from the mean velocity gradient at the wall using only data below y
+=4. It was possible to resolve the inner peak in the streamwise turbulence intensity as well as the inner (negative) peak
in the shear stress. Limiting values of (u′)+ and uv
+ were determined. Turbulence data from the outer region of the flow were compared to earlier hot wire measurements and large
differences in the normal turbulence intensity and the shear stress were found. These differences can be attributed to high
turbulence intensity effects on the hot-wires.
Received: 17 October 1996 / Accepted: 8 December 1997 相似文献
19.
Flap-bounding is a common flight style in small birds in which flapping phases alternate with flexed-wing bounds. Body lift
is predicted to be essential to making this flight style an aerodynamically attractive flight strategy. To elucidate the contributions
of the body and tail to lift and drag during the flexed-wing bound phase, we used particle image velocimetry (PIV) and measured
properties of the wake of zebra finch (Taeniopygia guttata, N = 5), flying at 6–10 m s−1 in a variable speed wind tunnel as well as flow around taxidermically prepared specimens (N = 4) mounted on a sting instrumented with force transducers. For the specimens, we varied air velocity from 2 to 12 m s−1 and body angle from −15° to 50°. The wake of bounding birds and mounted specimens consisted of a pair of counter-rotating
vortices shed into the wake from the tail, with induced downwash in the sagittal plane and upwash in parasagittal planes lateral
to the bird. This wake structure was present even when the tail was entirely removed. We observed good agreement between force
measures derived from PIV and force transducers over the range of body angles typically used by zebra finch during forward
flight. Body lift:drag (L:D) ratios averaged 1.4 in live birds and varied between 1 and 1.5 in specimens at body angles from 10° to 30°. Peak (L:D) ratio was the same in live birds and specimens (1.5) and was exhibited in specimens at body angles of 15° or 20°, consistent
with the lower end of body angles utilized during bounds. Increasing flight velocity in live birds caused a decrease in C
L
and C
D
from maximum values of 1.19 and 0.95 during flight at 6 m s−1 to minimum values of 0.70 and 0.54 during flight at 10 m s−1. Consistent with delta-wing theory as applied to birds with a graduated-tail shape, trimming the tail to 0 and 50% of normal
length reduced L:D ratios and extending tail length to 150% of normal increased L:D ratio. As downward induced velocity is present in the sagittal plane during upstroke of flapping flight, we hypothesize that
body lift is produced during flapping phases. Future efforts to model the mechanics of intermittent flight should take into
account that flap-bounding birds may support up to 20% of their weight even with their wings fully flexed. 相似文献
20.
Takamasa Hashimoto Koji Kido Shintaro Kaki Takehiro Yamamoto Noriyasu Mori 《Rheologica Acta》2006,45(6):841-852
Dynamic viscoelasticities and flow properties were measured for aqueous solutions of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) to examine the effects of surfactant (C
D) and salt (C
S). The relaxation time λ of a single mode Maxwell model was obtained, and the relationship between λ and free NaSal concentration was discussed. The relation between λ and was applied to the classification of flow curves, which were obtained using a capillary rheometer. In the flow curves, a shear rate jump occurred at low shear rates for the solutions with low , while bending was seen at high shear rates for all the flow curves. On the other hand, vortex growth at the salient corner in the entrance region of the capillary was also investigated. Four different flow patterns were identified: Newtonian-like flow (A), steady vortex flow (B), periodically oscillated flow (C), and perfectly unstable flow (D). In the steady vortex of the flow pattern B, the vortex length increased with increasing shear rate. In the flow patterns C and D, white turbidity was observed. Furthermore, the relation between λ and was also applied to the discussion on the development of the vortex. 相似文献