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1.
We have developed a modular rheo-optical apparatus to study the flow properties of liquid crystals. Its main components are shearing device, strong magnetic field, and optical microscope. We performed experiments on well defined initial morphologies with uniform molecular alignment. The monodomains were achieved with strong magnetic fields (4.7T). Time-resolved conoscopy is the primary optical technique in our investigation. We propose a simple relation between the distribution of alignment angles over the sample thickness and the conoscopically measured angle, to quantitatively measure the alignment angle in shear flow.We followed the relaxation of a shear-induced splay deformation in small molecule model systems (N-(p-methoxybenzylidene) p-butylaniline (MBBA), pentyl-cyano-biphenyl (5 CB) and a commercially available mixture OM14244). We define a rotational director diffusivity
(K
s splay elastic constant.i7s splay viscosity) from the relaxation process and devised a model, based on the diffusion equation to determine their values.The director alignment behavior of the small molecule liquid crystals (SMLC's) in shear flow is well described by the two-dimensional Leslie-Ericksen model. The effect of director elasticity can clearly be seen in our experiments, resulting in a decrease of the steady state alignment angle at smaller Ericksen numbers.We found that there is no strain rate dependence of the director vorticity from 0.002/s to 2/s for poly-(-benzyl-D/L-glutamate) (PBG). We determined 2/3 = –44 for a 2007o solution of 280000 molecular weight PBG in m-cresol at 20°C. The conoscopic interference pattern vanished after 8 strain units from an initially planar alignment and shearing could be reversed up to 10 strain units to completely recover the initial monodomain.Presented at 4th Meeting of European Rheologists, Sept. 4–9, 1994, Seville, SpainDedicated to Prof. H. Janeschitz-Kriegl at the occasion of his 70th birthday. 相似文献
2.
The time-dependent transformation of an ionically charged lamellar phase (L
α-phase) into a vesicle phase under the influence of shear is investigated using rheological and conductivity measurements.
The L
α-phase consists of the zwitterionic surfactant tetradecyldimethylaminoxide (C14DMAO), hexanol, oxalic acid and water. The experiments were carried out on the L
α-phase in a well defined state. It was prepared by a special route from the neighbouring L
3-phase that consists of 100 mM C14DMAO, 250 mM hexanol and 5 mM oxalicdiethylester (OEE). The OEE hydrolyses in the L
3
-phase to oxalic acid and ethanol. The result is a virgin L
α-phase which consists of stacked bilayers and which has not been exposed to shear. When this low-viscous phase is subjected
to shear it is transformed into a highly viscous vesicle phase. The transformation of the L
α-phase into vesicles under constant shear was monitored by recording the viscosity and conductivity with time. It is observed
that at least three different time constants can be distinguished in the transformation process. The conductivity passes through
a minimum (τ1) in the direction of shear. The viscosity first passes through a minimum (τ2) and then over a maximum (τ3). It is concluded that τ1 belongs to the complete alignment of the bilayer parallel to the wall, τ2 to the beginning of the break-up of the bilayers to the vesicles and τ3 to the complete transformation of the L
α- to the vesicle phase. When the shear rate was varied, it was noted that the product of the time constants and shear is constant.
Received: 30 June 1999/Accepted: 30 August 1999 相似文献
3.
This study investigates the influence on the oscillating characteristics of a cavity shear layer by introducing either a
sloped bottom or a flow path modifier at the bottom of the cavity. All the experiments are performed in a recirculating water
channel. The laser Doppler velocimetry system and the laser sheet technique are employed to perform the quantitative velocity
measurements and the qualitative flow visualization, respectively. The Reynolds number, based on the momentum thickness at
the upstream edge of the cavity, is kept at about Re
θ
0=194 ± 3.4. It is found that, in addition to the feedback effect, the upstream moving part of the recirculating flow inside
the cavity also plays an important role in changing the oscillating characteristics of the unstable shear layer. As the bottom
of the cavity is either negatively or positively sloped, the oscillating characteristics of the cavity shear layer are modified
to different extents. Significant reduction of the oscillating amplitude within the cavity is found while the bottom slope
increases up to d/L=± 2/5. As the bottom slope further increases up to d/L=± 1/2, the self-excited oscillation is completely suppressed. In addition, the ability to suppress the self-excited oscillation
by the negative bottom slopes is superior to that in the case of a positive bottom slope. Depending upon the fence locations,
the upstream moving part of the recirculating flow will perturb the unstable shear layer at different x/L locations, leading to different oscillating amplitudes. The ability to promote the enlarged oscillating amplitude of the
unstable shear layer is better for a fence inclined at a positive angle than for one at a negative angle.
Received: 31 May 2000/Accepted: 11 January 2001 相似文献
4.
5.
Measurements and scaling of wall shear stress fluctuations 总被引:2,自引:0,他引:2
Measurements of velocity and wall shear stress fluctuations were made in an external turbulent boundary layer developed over
a towed surface-piercing flat plate. An array of eight flush-mounted wall shear stress sensors was used to compute the space-time
correlation function. A methodology for in situ calibration of the sensors for ship hydrodynamic applications is presented.
The intensity of the wall shear stress fluctuations, τ
rms/τ
avg was measured as 0.25 and 0.36 for R
θ
=3,150 and 2,160 respectively. The probability density is shown to exhibit positive skewness, and lack of flow reversals at
the wall. Correlations between velocity and wall shear stress fluctuations are shown to collapse with outer boundary layer
length and velocity scales, verifying the existence of large-scale coherent structures which convect and decay along the wall
at an angle of inclination varying from 10 to 13° over the range of Reynolds numbers investigated. The wall shear stress convection
velocity determined from narrow band correlation measurements is shown to scale with outer variables. The space-time correlation
of the wall shear is shown to exhibit a well-defined convective ridge, and to decay 80% over approximately 3δ for R
θ
=3,150.
Published online: 7 November 2002 相似文献
6.
H. Watanabe Tomohiro Sato Motoyuki Hirose Kunihiro Osaki Ming-Long Yao 《Rheologica Acta》1998,37(6):519-527
Dielectric relaxation behavior was examined for 4-4′-n-pentyl-cyanobiphenyl (5CB) and 4-4′-n-heptyl-cyanobiphenyl (7CB) under flow. In quiescent states at all temperatures examined, both 5CB and 7CB exhibited dispersions
in their complex dielectric constant ε*(ω) at characteristic frequencies ω
c
above 106 rad s–1. This dispersion reflected orientational fluctuation of individual 5CB and 7CB molecules having large dipoles parallel to
their principal axis (in the direction of C≡N bond). In the isotropic state at high temperatures, these molecules exhibited no detectable changes of ε*(ω) under flow at
shear rates . In contrast, in the nematic state at lower temperatures the terminal relaxation intensity of ε*(ω) as well as the static
dielectric constant ε′(0) decreased under flow at . This rheo-dielectric change was discussed in relation to the flow effects on the nematic texture (director distribution)
and anisotropy in motion of individual molecules with respect to the director.
Received: 14 April 1998 Accepted: 29 July 1998 相似文献
7.
If a pair of material line elements, passing through a typical particle P in a body, subtend an angle Θ before deformation, and Θ+γ after deformation, the pair of material elements is said to be
sheared by the amount γ. Here all pairs of material elements at P are considered for arbitrary deformations. Two main problems are addressed and solved. The first is the determination of
all pairs of material line elements at P which are unsheared. The second is the determination of that pair of material line elements at P which suffers the maximum shear.
All unsheared pairs of material elements in a given plane π(S) with normal S passing through P are considered. Provided π(S) is not a plane of central circular section of the C-ellipsoid at P (where C is the right Cauchy-Green strain tensor), it is seen that corresponding to any material element in π(S) there is, in general, one companion material element in π(S) such that the element and its companion are unsheared.
There are, however, two elements in π(S) which have no companions. We call their corresponding directions \textit{limiting directions.} Equally inclined to the direction
of least stretch in the plane π(S), the limiting directions play a central role. It is seen that, in a given plane π(S), the pair of material line elements which suffer the maximum shear lie along the limiting directions in π(S). If Θ
L
is the acute angle subtended by the limitig directions in π(S) before deformation, then this angle is sheared into its supplement π−Θ
L
so that the maximum shear γ*;(S) is γ*=π− 2 Θ
L
. If S is given and C is known, then Θ
L
may be determined immediately. Its calculation does not involve knowing the eigenvectors or eigenvalues of C.
When all possible planes through P are considered, it is seen that the global maximum shear γ*
G
occurs for material elements lying along the limiting directions in the plane spanned by the eigenvectors of C corresponding to the greatest principal stretch λ3 and the least λ1. The limiting directions in this principal plane of C subtend the angle and . Generally the maximum shear does not occur for a pair of material elements which are originally orthogonal.
For a given material element along the unit vector N, there is, in general, in each plane π(S passing through N at P, a companion vector M such that material elements along N and M are unsheared. A formula, originally due to Joly (1905), is presented for M in terms of N and S.
Given an unsheared pair π(S), the limiting directions in π(S) are seen to be easily determined, either analytically or geometrically.
Planar shear, the change in the angle between the normals of a pair of material planar elements at X, is also considered. The theory of planar shear runs parallel to the theory of shear of material line elements. Corresponding
results are presented.
Finally, another concept of shear used in the geology literature, and apparently due to Jaeger, is considered. The connection
is shown between Cauchy shear, the change in the angle of a pair of material elements, and the Jaeger shear, the change in
the angle between the normal N to a planar element and a material element along the normal N. Although Jaeger's shear is described in terms of one direction N, it is seen to implicitly include a second material line element orthogonal to N.
Accepted: May 25, 1999 相似文献
8.
Density Maximum Effect on Double-diffusive Natural Convection in a Porous Cavity with Variable Wall Temperature 总被引:1,自引:0,他引:1
The effect of density maximum of water on double-diffusive natural convection in a two-dimensioned cavity filled with a water
saturated isotropic porous medium is studied numerically. The horizontal walls of the cavity are insulated. The opposing vertical
walls are kept at different temperatures θ
h
(linearly varies with height) and θ
c
(θ
c
≤ θ
h
). The concentration levels at cold wall and hot wall are, respectively, c
1 and c
2 with c
1 > c
2. Brinkman-Forchheimer extended Darcy model is used to investigate the average heat and mass transfer rates. The non-dimensional
equations for momentum, energy, and concentration are solved by finite volume method with power law scheme for convection
and diffusion terms. The results are presented in the form of streamlines, isotherms, and isoconcentration lines for various
values of Grashof numbers, Schmidt number, porosity, and Darcy numbers. It is observed that the density maximum of water has
profound effect on the thermosolutal convection. The effects of different parameters on the velocity, temperature, and species
concentrations are also shown graphically. 相似文献
9.
The failure strengths of laminated composite shafts subjected to static bending load and/or torque are studied using both
theoretical and experimental approaches. We investigate optimal angle-ply orientations of laminated composite shafts with
the design of symmetric main fiber lay-ups [θ/−θ/...]
S
and anti-symmetric reinforcing fiber lay-ups [α/−α/...]−S
for maximum stiffness. Different analytical methods, together with various failure criteria, are used to predict the first-ply
failure strengths of the composite shafts. A hand lay-up and pressure bag technique was used to manufacture the composite
shafts, which includes the stacking of pieces of prepreg in the desired orientations and the curing process. The first-ply
failure loads of laminated composite shafts were determined using acoustic emission. The failure modes of the shafts are studied
and experimental results are used to verify the theoretical predictions. The experimental results have been proven to be efficient
and effective in the theoretical prediction of first-ply failure loads of laminated composite shafts. 相似文献
10.
This work is the continuation of a previous study [Van Horn BL, Winter HH (2000) Dynamics of shear aligning nematic liquid crystal monodomains. Rheol Acta 39:294–300] on the shear dynamics of monodomains of shear aligning nematic liquid crystals [NLC]. The strain dependence of director orientation has been experimentally investigated for monodomains of a NLC with various initial orientations. Comparison of experimental results to predictions using Ericksen's transversly isotropic fluid [TIF] model supports the validity of the TIF model for systems of low molecular weight NLCs. The TIF model has been used to examine the effect of pre-tilt on the dynamics of flow-aligning NLC monodomains. It is shown that small deviations from planar alignment (no pre-tilt) have a large effect on orientation dynamics. 相似文献
11.
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
12.
Discrete dipoles located near the crack tip play an important role in nonlinear electric field induced fracture of piezoelectric
ceramics. A physico-mathematical model of dipole is constructed of two generalized concentrated piezoelectric forces with
equal density and opposite sign. The interaction between crack and electric dipole in piezoelectricity is analyzed. The closed
form solutions, including those for stress and electric displacement, crack opening displacement and electric potential, are
obtained. The function of piezoelectric anisotropic direction,p
a
(θ)=cosθ+p
a
sinθ, can be used to express the influence of a dipole's direction. In the case that a dipole locates near crack tip, the
piezoelectric stress intensity factor is a power function with −3/2 index of the distance between dipole and crack tip.
Supported by National Natural Science Foundation of China(No. 10072033) 相似文献
13.
S. V. Yagudin 《Fluid Dynamics》2012,47(2):214-222
Inviscid gas flows in nozzles with a uniform exit flow and contours profiled starting from the lower point of a steeply converging
region with an angle θ = −90° are analyzed. It is shown that there exists a class of convergent-divergent contours, within which the flow is characterized
by the fact that the line θ = 0 of zero angle of the velocity vector inclination to the x axis consists of two oppositely-directed regions located partially or even completely ahead of the minimum section, while
near the minimum sections their regions convex inward the gas stream are in decelerated flow. The minimum sections of the
nozzles with M
e
→ 1 approach the center of the nozzle from the right. 相似文献
14.
Steady-state conjugate natural convection in a square cavity filled with a porous medium is studied numerically in this paper.
The enclosure consists of two horizontal conductive walls of finite thickness and two vertical walls at different uniform
temperatures. The focus is on the role of solid-fluid conductivity ratio, k, on the flow and heat transfer characteristics and the average Nusselt number, , over the vertical hot and cold walls of the cavity for a limited set of particular parameters. It was shown that the interface
temperature, θw, along the top of the solid wall decreases with the increase in the wall conductivity k. Also, the values of decreases with the increase of the values of the parameter k. Comparison with known results from the open literature when the wall thickness of the horizontal solid walls is neglected
(non-conjugate problem) is excellent.
Received on 4 April 2000 相似文献
15.
Separating oscillating flows in an internal, adverse pressure gradient geometry are studied experimentally. Simultaneous velocity
and pressure measurements demonstrate that the minor losses associated with oscillating flow in an adverse pressure gradient
geometry can be smaller or larger than those for steady flow. Separation is found to begin high in the diffuser and propagate
downward. The flow is able to remain attached further into the diffuser with larger Reynolds numbers, small displacement amplitudes,
and smaller diffuser angles. The extent of separation grows with L
0/h. The minor losses grow with increasing displacement amplitude in the measured range 10 < L
0/h < 40. Losses decrease with increasing Re
δ in the measured range of 380 < Re
δ < 740. It is found that the losses increase with increasing diffuser angle over the measured range of 12° < θ < 30°. The
nondimensional acoustic power dissipation increases with Reynolds number in the measured range and decreases with displacement
amplitude. 相似文献
16.
IntroductionInordertodeveloptheapparelCADsystemthebucklingofwovenfabrichasreceivedmuchattentioninrecentyears[1,2 ].Thefabricsheethasalwaysbeenregardedaselasticthinplateinthefabricmechanics,however,therearesomepeculiarbucklingphenomenawecanseeinthefabricb… 相似文献
17.
A theory is developed for the vorticity and its substantial derivative just downstream of a curved shock wave, the resulting
formulas are exact, algebraic, and explicit. Analysis is for a cylinder-wedge or sphere-cone body, at zero incidence, whose
downstream half-angle is θb. Derived formulas directly depend only on the ratio of specific heats, γ, the freestream Mach number, M
1, the local slope and curvature of the shock, and the dimensionality parameter, σ, which is zero for a two-dimensional shock
and unity for an axisymmetric shock. In turn, the slope and curvature depend on γ, M
1, and θb. Numerical results are provided for a bow shock in which θb is 5°, 10°, or 15°, M
1 is 2, 4, or 6, and γ = 1.4. There is little dependence on the half angle but a strong dependence on the freestream Mach number
and on dimensionality. For vorticity and its substantial derivative, the dimensionality dependence gradually decreases with
increasing Mach number. In comparison to the two-dimensional case, an axisymmetric shock generates considerable vorticity
in a region relatively close to the symmetry axis. Moreover, the magnitude of the vorticity, in this region, is further enhanced
in the flow downstream of the shock. This dimensionality difference in vorticity and its substantial derivative is attributed
to the three-dimensional relief effect in an axisymmetric flow.
相似文献
相似文献
18.
We have measured the shear-induced rotation of the nematic director in a liquid crystalline polymer using poly benzyl glutamate (PBG) as model system. PBG is a well characterized synthetic poly ( amino acid) with rigid chain architecture and well defined conformations. For the experiments it is important to start out with a sample in which the molecules are highly aligned with a uniform director. This so-called monodomain morphology is obtained by use of strong magnetic fields and surface modifications of the sample holders. When shearing the monodomain at a constant rate, the macromolecules rotate initially homogeneously until a periodic director pattern develops. These spatially periodic structures emerge in a narrow range of shear strain and, as shearing continues, disintegrate into a chaotic texture. By varying the initial monodomain director with respect to the flow direction (but within the shear planes) we could show that the periodic patterns do not depend on the shear direction; they are governed by the director of the initial monodomain. We observe conoscopically that at high shear rates the texture becomes uniformly aligned. The molecules are aligned preferentially with an angle of about 4° to the shear direction (against vorticity direction). Interestingly, this agrees very well with predictions made by Larson (1990).Dedicated to Prof. Dr. J. Meissner on the occasion of his retirement from the chair of Polymer Physics at the Eidgenössische Technische Hochschule (ETH) Zürich, Switzerland 相似文献
19.
Reynolds-number-dependence of the maximum in the streamwise velocity fluctuations in wall turbulence
A survey is made of the standard deviation of the streamwise velocity fluctuations in near-wall turbulence and in particular
of the Reynolds-number-dependency of its peak value. The following canonical flow geometries are considered: an incompressible
turbulent boundary layer under zero pressure gradient, a fully developed two-dimensional channel and a cylindrical pipe flow.
Data were collected from 47 independent experimental and numerical studies, which cover a Reynolds number range of R
θ=U
∞
θ/v=300−20,920 for the boundary layer with θ the momentum thickness and R
+=u
*R/v=100-4,300 for the internal flows with R the pipe radius or the channel half-width. It is found that the peak value of the rms-value normalised by the friction velocity,
u
*, is within statistical errors independent of the Reynolds number. The most probable value for this parameter was found to
be 2.71±0.14 and 2.70±0.09 for the case of a boundary layer and an internal flow, respectively. The present survey also includes
some data of the streamwise velocity fluctuations measured over a riblet surface. We find no significant difference in magnitude
of the normalised peak value between the riblet and smooth surfaces and this property of the normalised peak value may for
instance be exploited to estimate the wall shear stress from the streamwise velocity fluctuations. We also consider the skewness
of the streamwise velocity fluctuations and find its value to be close to zero at the position where the variance has its
peak value. This is explained with help of the equations of the third-order moment of velocity fluctuations. These results
for the peak value of the rms of the streamwise velocity fluctuations and also the coincidence of this peak with the zero
value of the third moment can be interpreted as confirmation of local equilibrium in the near-wall layer, which is the basis
of inner-layer scaling. Furthermore, these results can be also used as a requirement which turbulence models for the second
and triple velocity correlations should satisfy.
The authors are indebted to Prof. P. Bradshaw for making available his list of references on this topic and for his remarks
on “active” and “inactive” motions. We also gratefully acknowledge discussions with Prof. I. Castro regarding the value of
σ
u
+
above rough walls. 相似文献
20.
PIV observations in a shear layer have been used to identify and characterize the discrete large-scale coherent motions (LSCMs)
in the nominally self-preserving region: x/θo ≈ 450–610, of a shear layer. The LSCMs are given an objective definition wherein their centers are the (swirling flow pattern)
nodes of the velocity-vector field as seen by an observer in the Galilean reference frame translating at an appropriately
defined reference velocity. The statistical attributes of size, lateral location, and separation between these coherent motions
(that exist in a single image) as well as their characteristic vorticity magnitude 〈ωmax〉 are reported. 相似文献