共查询到20条相似文献,搜索用时 31 毫秒
1.
Masaru Sumida 《Experiments in fluids》2007,43(6):949-958
This paper presents the results of an experimental study on the developing pulsatile flow in curved pipes with a long, straight
pipe upstream. In order to examine the dependence of flow-field development on the governing parameters, LDV measurements
were conducted systematically for six cases of flow, where the Womersley number α was varied from 5.5 to 18, the mean Dean number D
m
was 200 and 300, the flow rate ratio η was 0.5 and 1, and the curvature radius ratio Rc was 10 and 30. Peculiar flow phenomena, such as flow reversal for all values of α and a depression in the axial velocity profile for α = 10, were analyzed by decomposing the axial velocity into a time-mean and a varying component, as well as by obtaining the
bias of their profiles. The velocity distributions abruptly change with the phase at turn angles Ω of 15–30°, corresponding to the nondimensional axial length z′ ≅ 1–2 from the bend entrance, and their development along the pipe axis is the most complicated for the flow at a moderate
α of 10 and large η of 1. The entrance length in the case of pulsatile flow is shorter than that for steady flow with the same flow rate as the
maximum pulsatile flow rate. 相似文献
2.
A class of steady potential flows of an ideal fluid is considered in which the fluid flows between fixed boundaries and then
emerges as a jet with one free boundary. Gravity acts on the fluid perpendicularly to the direction of the jet at infinity
downstream. An inverse Froude number α is defined in terms of the flux Q and the depth d of the fluid at the separation point. It is proved that for each α>0 there is at most one flow which reaches to a supercritical uniform stream depth at infinity downstream. Monotonicity properties
are proved for various flow parameters, and the behaviour of the flow as α → 0 is described. 相似文献
3.
Drag Reduction of a Circular Cylinder Using an Upstream Rod 总被引:3,自引:0,他引:3
Experimental studies on the drag reduction of the circular cylinder were conducted by pressure measurement at a Reynolds number
of 82 000 (based on the cylinder diameter). A rod was placed upstream of and parallel to the cylinder to control the flow
around the cylinder. The upstream rod can reduce the resultant force of the cylinder at various spacing between the rod and
the cylinder for α < 5∘(α defined as the staggered angle of the rod and the cylinder). For α > 10∘, the resultant force coefficient has a large value, so the upstream rod cannot reduce the force on the cylinder any more.
For α = 0∘ and d/D = 0.5 (where d and D are the diameter of the rod and the cylinder, respectively), the maximum drag of the cylinder reduces to 2.34% that of the
single cylinder. The mechanism of the drag reduction of the cylinder with an upstream rod in tandem was presented by estimating
the local contributions to the drag reduction of the pressure variation. In the staggered arrangement, the flow structures
have five flow patterns (they are the cavity mode, the wake splitting mode, the wake merge mode, the weak boundary layer interaction
mode and the negligible interaction mode) according to the pressure distribution and the hydrogen bubble flow visualization.
The half plane upwind of the cylinder can be divided to four regions, from which one can easily estimates the force acting
on the circular cylinder with an upstream rod in staggered arrangement. 相似文献
4.
Summary We study the two-dimensional instantaneous Stokes flow driven by gravity in a viscous triangular prism supported by a horizontal
rigid substrate and a vertical wall. The oblique side of the prism, inclined at an angle α with respect to the substrate,
is a fluid-air interface, where the stresses are zero and surface tension is neglected. We develop the stream function ψ in
polar coordinates (r,θ) centered at the vertex of α and split it into an inhomogeneous part, which accounts for gravity effects, and a homogeneous
part, which is expressed as a series expansion. The inhomogeneous part and the first term of the expansion may be envisioned,
respectively, as self-similar solutions of the first kind and of the second kind for r→0, each one holding in complementary α domains with a crossover at α
c
=21.47∘, which we study in some detail. The coefficients of the series are calculated by truncating the expansion and using the method
of direct collocation with a suitable set of points at the wall. The solution strictly holds for t=0, because later the free surface ceases to be a plane; nevertheless, it provides a good approximation for the early time
evolution of the fluid profile, as shown by the comparison with numerical simulations. For 0<α<45∘, the vertex angle remains constant and the edge remains strictly at rest; the transition at α
c
manifests itself through a change in the rate of growth of the curvature. The time at which the edge starts to move (waiting
time) cannot be calculated since the instantaneous solution ceases to be valid. For α>45∘, the instantaneous local solution indicates that the surface near the vertex is launched against the substrate so that the
edge starts to move immediately with a power law dependence on time t. However, due to the high value of the exponent, the vertex may seem to be at rest for some finite time even in this case.
Received 29 August 1997; accepted for publication 21 January 1998 相似文献
5.
We develop a continuum-mechanical formulation and generalization of the Navier–Stokes-α equation based on a recently developed framework for fluid-dynamical theories involving higher-order gradient dependencies.
Our flow equation involves two length scales α and β. The first of these enters the theory through the specific free-energy α
2|D|2, where D is the symmetric part of the gradient of the filtered velocity, and contributes a dispersive term to the flow equation. The
remaining scale is associated with a dissipative hyperstress which depends linearly on the gradient of the filtered vorticity
and which contributes a viscous term, with coefficient proportional to β
2, to the flow equation. In contrast to Lagrangian averaging, our formulation delivers boundary conditions and a complete structure
based on thermodynamics applied to an isothermal system. For a fixed surface without slip, the standard no-slip condition
is augmented by a wall-eddy condition involving another length scale ℓ characteristic of eddies shed at the boundary and referred to as the wall-eddy length. As an application, we consider the
classical problem of turbulent flow in a plane, rectangular channel of gap 2h with fixed, impermeable, slip-free walls and make comparisons with results obtained from direct numerical simulations. We
find that α/β ~ Re
0.470 and ℓ/h ~ Re
−0.772, where Re is the Reynolds number. The first result, which arises as a consequence of identifying the specific free-energy with the
specific turbulent kinetic energy, indicates that the choice β = α required to reduce our flow equation to the Navier–Stokes-α equation is likely to be problematic. The second result evinces the classical scaling relation η/L ~ Re
−3/4 for the ratio of the Kolmogorov microscale η to the integral length scale L.
相似文献
6.
We describe how outer flow turbulence phenomena depend on the interaction with the wall. We investigate coherent structures
in turbulent flows over different wavy surfaces and specify the influence of the different surface geometries on the coherent
structures. The most important contribution to the turbulent momentum transport is attributed to these structures, therefore
this flow configuration is of large engineering interest. In order to achieve a homogeneous and inhomogeneous reference flow
situation two different types of surface geometries are considered: (1) three sinusoidal bottom wall profiles with different
amplitude-to-wavelength ratios of α = 2a/Λ = 0.2 (Λ = 30 mm), α = 0.2 (Λ = 15 mm), and α = 0.1 (Λ = 30 mm); and (2) a profile consisting of two superimposed sinusoidal
waves with α = 0.1 (Λ = 30 mm). Measurements are carried out in a wide water channel facility (aspect ratio 12:1). Digital
particle image velocimetry (PIV) is performed to examine the spatial variation of the streamwise, spanwise and wall-normal
velocity components in three measurement planes. Measurements are performed at a Reynolds number of 11,200, defined with the
half channel height h and the bulk velocity U
B. We apply the method of snapshots and perform a proper orthogonal decomposition (POD) of the streamwise, spanwise, and wall-normal
velocity components to extract the most dominant flow structures. The structure of the most dominant eigenmode is related
to counter-rotating, streamwise-oriented vortices. A qualitative comparison of the eigenfunctions for different sinusoidal
wall profiles shows similar structures and comparable characteristic spanwise scales Λ
z
= 1.5 H in the spanwise direction for each mode. The scale is observed to be slightly smaller for α = 0.2 (Λ = 15 mm) and slightly
larger for α = 0.2 (Λ = 30 mm). This scaling for the flow over the basic wave geometries indicates that the size of the largest
structures is neither directly linked to the solid wave amplitude, nor to the wavelength. The characteristic spanwise scale
of the dominant eigenmode for the developed flow over the surface consisting of two superimposed waves reduces to 0.85 H. However, a scale in the order of 1.3 H is identified for the second mode. The eigenvalue spectra for the superimposed waves is much broader, more modes contribute
to the energy-containing range. The turbulent flow with increased complexity of the bottom surface is characterized by an
increased number of dominant large-scale structures with different spanwise scales. 相似文献
7.
Radiation-conduction interaction on mixed convection from a horizontal circular cylinder 总被引:1,自引:0,他引:1
A mixed convection flow of an optically dense viscous incompressible fluid along a horizontal circular cylinder has been
studied with the effect of radiation when the surface temperature is uniform. Using appropriate transformations, the boundary
layer equations governing the flow are reduced to local nonsimilarity form. Solutions of the governing equations are obtained
employing the implicit finite difference method. Effects of varying the pertinent parameters, such as, the Planck number,
R
w the surface temperature parameter, θw and the buoyancy parameter, α on the local skin-friction and local heat transfer coefficients are shown graphically as well
as in tabular form against the curvature parameter ξ, while taking Prandtl number Pr = 1.0. It is found that an increase of
R
d,θw or α leads to increases in the values of the local skin-friction and the local rate of heat transfer coefficients. At the
stagnation point asymptotic solutions for large value of α are also obtained and the effect of the other pertinent parameters
on the formation of the flow separation are studied.
Received on 28 July 1998 相似文献
8.
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
9.
A new approach on MHD natural convection boundary layer flow from a finite flat plate of arbitrary inclination in a rotating
environment, is presented. This problem plays a significant role on boundary layer flow control. It is shown that taking into
account the pressure rise region at the leading edge of the plate leads to avoid separation and the back flow is reduced by
the strong magnetic field. It is also shown that the frictional drag at the leading edge of the plate is reduced when the
inclination angle α=π/4. In the case of isothermal flat plate, the bulk temperature becomes identical for any value of Gr (Grashof number) when the value of M
2 (Hartmann number) and K
2 (rotation parameter) are kept fixed. 相似文献
10.
Kyung Min Kim Hyun Lee Beom Seok Kim Sangwoo Shin Dong Hyun Lee Hyung Hee Cho 《Heat and Mass Transfer》2009,45(12):1617-1625
In the present study, an optimal design for enhancement of heat transfer and thermal performance for a stationary channel
with angled rib turbulators was investigated to find the most suitable rib geometry. Among various design parameters, two
design variables, rib angle of attack (α) and pitch-to-rib height (p/e), were chosen. The ranges of two design variables were set as 30° ≤ α ≤ 80° and 3.0 ≤ p/e ≤ 15.0. Approximations for design of the best rib turbulators were obtained using the advanced response surface method with
functional variables. The second-order response surfaces (or correlations) within the ranges of two design variables were
completed by this method. As for the optimized results, maximum averaged heat transfer value was obtained at α = 53.31° and p/e = 6.50, while the highest thermal performance value was presented at α = 54.67° and p/e = 6.80. 相似文献
11.
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
12.
Flow characteristics in the interdisk midplane between two shrouded co-rotating disks were experimentally studied. A laser-assisted
particle-laden flow-visualization method was used to identify the qualitative flow behaviors. Particle image velocimetry was
employed to measure the instantaneous flow velocities. The flow visualization revealed rotating polygonal flow structures
(hexagon, pentagon, quadrangle, triangle, and oval) existing in the core region of the interdisk spacing. There existed a difference between the rotating frequencies of the
polygon and the disks. The rotating frequency ratio between the polygonal flow structure and the disks depended on the mode
shapes of the polygonal core flow structures—0.8 for pentagon, 0.75 for quadrangle, 0.69 for triangle, and 0.6 for oval. The
phase-resolved flow velocities relative to the bulk rotation speed of the polygonal core flow structure were calculated, and
the streamline patterns were delineated. It was found that outside the polygonal core flow structure, there existed a cluster
of vortex rings—each side of the polygon was associated with a vortex ring. The radial distributions of the time-averaged
and phase-resolved ensemble-averaged circumferential and radial velocities were presented. Five characteristic regions (solid-body
rotation region, hub-influenced region, buffer region, vortex region, and shroud-influenced region) were identified according
to the prominent physical features of the flow velocity distributions in the interdisk midplane. In the solid-body rotation
region, the fluid rotated at the angular velocity of the disks and hub. In the hub-influenced region, the circumferential
flow velocity departed slightly from the disks’ angular velocity. The circumferential velocities in the hub-influenced and
vortex regions varied linearly with variation of radial coordinates. The phase-resolved ensemble-averaged relative radial
velocity profiles in the interdisk midplane at various phase angles exhibited grouping behaviors in three ranges of polygon
phase angles (θ = 0 and α/2, 0 < θ < α/2, and α/2 < θ < α) because three-dimensional flow induced similar flow patterns to
appear in the same range of polygon phase angles. 相似文献
13.
In this paper we investigate a subgrid model based on an anisotropic version of the NS-α model using a lid-driven cavity flow at a Reynolds number of 10,000. Previously the NS-α model has only been used numerically in the isotropic form. The subgrid model is developed from the Eulerian-averaged anisotropic
equations (Holm, Physica D 133:215, 1999). It was found that when α
2 was based on the mesh numerical oscillations developed which manifested themselves in the appearance of streamwise vortices
and a ‘mixing out’ of the velocity profile. This is analogous to the Craik–Leibovich mechanism, with the difference being
that the oscillations here are not physical but numerical. The problem could be traced back to the discontinuity in α
2 encountered when α
2 = 0 on the endwalls. A definition of α
2 based on velocity gradients, rather than mesh spacing, is proposed and tested. Using this definition the results with the
model show a significant improvement. The splitting of the downstream wall jet, rms and shear stress profiles are correctly
captured a coarse mesh. The model is shown to predict both positive and negative energy transfer in the jet impingement region,
in qualitative agreement with DNS results. 相似文献
14.
H.-J. Kaltenbach 《Theoretical and Computational Fluid Dynamics》2003,16(3):187-210
A three-dimensional separated flow behind a swept, backward-facing step is investigated by means of DNS for Re
H
= C
∞
H/ν = 3000 with the purpose to identify changes in the statistical turbulence structure due to a variation of the sweep angle
α from 0° up to 60°. With increasing sweep angle, the near-wall turbulence structure inside the separation bubble and downstream
of reattachment changes due to the presence of an edge-parallel mean flow component W. Turbulence production due to the spanwise shear ∂W/∂y at the wall becomes significant and competes with the processes caused by impingement of the separated shear-layer. Changes
due to a sweep angle variation can be interpreted in terms of two competing velocity scales which control the global budget
of turbulent kinetic energy: the step-normal component U
∞ = C
∞cosα throughout the separated flow region and the velocity difference C
∞ across the entire shear-layer downstream of reattachment. As a consequence, the significance of history effects for the development
into a two-dimensional boundary layer decreases with increasing sweep angle. For α ≥50°, near-wall streaks tend to form inside
the separated flow region.
Received 7 November 2000 and accepted 9 July 2002 Published online 3 December 2002
RID="*"
ID="*" Part of this work was funded by the Deutsche Forschungsgemeinschaft within Sfb 557. Computer time was provided by the
Konrad-Zuse Zentrum (ZIB), Berlin.
Communicated by R.D. Moser 相似文献
15.
This paper reports the results of an experimental investigation on a two-dimensional (2-D) wing undergoing symmetric simple
harmonic flapping motion. The purpose of this investigation is to study how flapping frequency (or Reynolds number) and angular
amplitude affect aerodynamic force generation and the associated flow field during flapping for Reynolds number (Re) ranging from 663 to 2652, and angular amplitudes (α
A) of 30°, 45° and 60°. Our results support the findings of earlier studies that fluid inertia and leading edge vortices play
dominant roles in the generation of aerodynamic forces. More importantly, time-resolved force coefficients during flapping
are found to be more sensitive to changes in α
A than in Re. In fact, a subtle change in α
A may lead to considerable changes in the lift and drag coefficients, and there appears to be an optimal mean lift coefficient
around α
A = 45°, at least for the range of flow parameters considered here. This optimal condition coincides with the development a
reverse Karman Vortex street in the wake, which has a higher jet stream than a vortex dipole at α
A = 30° and a neutral wake structure at α
A = 60°. Although Re has less effect on temporal force coefficients and the associated wake structures, increasing Re tends to equalize mean lift coefficients (and also mean drag coefficients) during downstroke and upstroke, thus suggesting
an increasing symmetry in the mean force generation between these strokes. Although the current study deals with a 2-D hovering
motion only, the unique force characteristics observed here, particularly their strong dependence on α
A, may also occur in a three-dimensional hovering motion, and flying insects may well have taken advantage of these characteristics
to help them to stay aloft and maneuver.
An erratum to this article can be found at 相似文献
16.
The flow of an incompressible couple stress fluid in an annulus with local constriction at the outer wall is considered. This
configuration is intended as a simple model for studying blood flow in a stenosed artery when a catheter is inserted into
it. The effects couple stress fluid parameters α and σ, height of the constriction (ε), and ratio of radii (k) on the impedance and wall shear stresses are studied graphically. Graphical results show that the resistance to the flow
as well as the wall shear stress increases as the ratio of the radii increases and decreases as the couple stress fluid parameters
increases. 相似文献
17.
Carlos B. da Silva Pedro Neto José C. F. Pereira 《Theoretical and Computational Fluid Dynamics》2009,23(4):287-296
Direct and large-eddy simulations (DNS/LES) of accelerating round jets are used to analyze the effects of acceleration on
the kinematics of vortex rings in the near field of the jet (x/D < 12). The acceleration is obtained by increasing the nozzle jet velocity with time, in a previously established (steady)
jet, and ends once the inlet jet velocity is equal to twice its initial value. Several acceleration rates (α = 0.02–0.6) and Reynolds numbers (Re
D
= 500–20000) were simulated. Acceleration maps were used to make a detailed study of the kinematics of vortex rings in accelerating
jets. One of the effects of the acceleration is to cause a number of new primary and secondary vortex merging events that
are absent from steady jets. As the acceleration rate α increases, both the number of primary merging events between rings
and the axial position where these take place decreases. The statistics for the speed of the starting ring that forms at the start of the acceleration phase for each simulation, agree well with the statistics for the “front” speed
observed by Zhang and Johari (Phys Fluids 8:2185–2195, 1996). Acceleration maps and flow visualizations show that during the
acceleration phase the near field coherent vortices become smaller and are formed at an higher frequency than in the steady
jet, and their (mean) shedding frequency increases linearly with the acceleration rate. Finally, it was observed that the
acceleration decreases the spreading rate of the jet, in agreement with previous experimental works.
相似文献
18.
The combined effect of rotation and magnetic field is investigated for the axisymmetric flow due to the motion of a sphere
in an inviscid, incompressible electrically conducting fluid having uniform rotation far upstream. The steady-state linearized
equations contain a single parameter α=1/2βR
m, β being the magnetic pressure number and R
m the magnetic Reynolds number. The complete solution for the flow field and magnetic field is obtained and the distribution
of vorticity and current density is found. The induced vorticity is O(α4) and the current density is O(R
m) on the sphere. 相似文献
19.
This paper presents an exact asymptotic analysis on the interfacial crack between two dissimilar elastic-plastic materials.
These two materials have identical hardening exponent (n
1=n
2) but different hardening coefficient (α1 ≠ α2). Two groups of the near-crack-tip fields have been obtained, which not only satisfy the continuity of both tractions (σθ, τrθ) and displacements (u
r
,u
θ) on the interface, but also meet the traction free conditions on the crack faces. The first group of fields have the mode
mixityM
P
quite close toM
P
=1 (MODE I) within the whole range 0 ≤ α1/α2 < ∞. As for the second group of fields, which is only obtained within the narrow range 0.9 ≤ α1/α2 ≤ 1, it is found that the mode mixity changes sharply with the ratio value α1/α2.
The project supported by National Natural Science Foundation of China 相似文献
20.
The streamwise evolution of an inclined circular cylinder wake was investigated by measuring all three velocity and vorticity
components using an eight-hotwire vorticity probe in a wind tunnel at a Reynolds number Red of 7,200 based on free stream velocity (U
∞) and cylinder diameter (d). The measurements were conducted at four different inclination angles (α), namely 0°, 15°, 30°, and 45° and at three downstream
locations, i.e., x/d = 10, 20, and 40 from the cylinder. At x/d = 10, the effects of α on the three coherent vorticity components are negligibly small for α ≤ 15°. When α increases further
to 45°, the maximum of coherent spanwise vorticity reduces by about 50%, while that of the streamwise vorticity increases
by about 70%. Similar results are found at x/d = 20, indicating the impaired spanwise vortices and the enhancement of the three-dimensionality of the wake with increasing
α. The streamwise decay rate of the coherent spanwise vorticity is smaller for a larger α. This is because the streamwise
spacing between the spanwise vortices is bigger for a larger α, resulting in a weak interaction between the vortices and hence
slower decaying rate in the streamwise direction. For all tested α, the coherent contribution to [`(v2)] \overline{{v^{2}}} is remarkable at x/d = 10 and 20 and significantly larger than that to [`(u2)] \overline{{u^{2}}} and [`(w2)]. \overline{{w^{2}}}. This contribution to all three Reynolds normal stresses becomes negligibly small at x/d = 40. The coherent contribution to [`(u2)] \overline{{u^{2}}} and [`(v2)] \overline{{v^{2}}} decays slower as moving downstream for a larger α, consistent with the slow decay of the coherent spanwise vorticity for
a larger α. 相似文献