共查询到20条相似文献,搜索用时 46 毫秒
1.
Rayleigh's classical transformation t=x/U connecting an unsteady parallel boundary flow to the steady plane Blasius-flow is generalized to t=C·x
c
and applied to the case of natural convection in the vicinity of a vertical flat plate. The parameters C and c of this “chronotopic transformation” (CTT) are determined self-consistently. It is shown that the analytic expressions obtained
in this way for temperature and for the main component of the steady velocity field reproduce the “numerically exact” patterns
to a good accuracy. Surprisingly, the transversal component of the steady flow (which in the unsteady flow is entirely missing)
can also be generated by CTT with a remarkable precision. Moreover, the CTT is also able to extrapolate unsteady buoyant flows
to steady ones with a good performance, even if these belong to a different set of boundary conditions (e.g. time-dependent
vs. coordinate-dependent wall temperatures).
Received on 6 May 1999 相似文献
2.
Vortex pairs are studied using a dye tracing visualisation technique and a particle tracking velocimetry system. The vortex
pairs are produced by gravity induced inlets of water issued through a uniform gap. The inlet Reynolds number is Re=Ud/ν≈875 in all tests (d being the gap width and U the cross sectional mean velocity), i.e. the flow is in the laminar regime. Initially, the dipolar vortex structure is two-dimensional,
but after travelling a distance of a few times its own width, the flow structure becomes unstable, breaks up and changes into
a three-dimensional flow structure. The breakup appears to be caused by an axial flow in the core centres of each vortex of
the dipolar structure. These axial flows are induced by boundary effects related to the von Karman viscous pump. After the
breakup, it is believed that a vortex ring is formed through reconnection of rudiments from the dipolar structure mediated
by the wall induced vorticity.
Received: 20 November 1995/Accepted: 14 November 1996 相似文献
3.
A new laser Doppler anemometer optimized for high spatial resolution near a wall is described. The instrument uses short
focal length optics, a mirror probe in the flow, and side-scatter collection to produce a measuring volume 35 μm in diameter
by 66 μm long. Data are presented for a two-dimensional boundary layer, demonstrating the instrument’s ability to measure
Reynolds shear stresses as close to the wall as 0.1 mm, or y
+≈3.
Received: 6 October 1995/Accepted: 9 April 1996 相似文献
4.
Tomographic and time resolved PIV measurements on a finite cylinder mounted on a flat plate 总被引:1,自引:0,他引:1
Tomographic and time resolved PIV measurements were performed to examine the 3D flow topology and the flow dynamic above the
upper surface of a low-aspect ratio cylinder at Re ≈ 1 × 105. This generic experiment is of fundamental interest because it represents flow features which are relevant to many applications
such as laminar separation bubbles and turbulent reattachment. At Re ≈ 1 × 105, laminar separation bubbles arise on the side of the cylinder. Furthermore, on the top of the cylinder a separation with
reattachment is of major interest. The tomographic PIV measurement, which allows to determine all three velocity components
in a volume instantaneously, was applied to examine the flow topology and interaction between the boundary layer and wake
structures on the top of the finite cylinder. In the instantaneous flow fields the tip vortices and the recirculation region
becomes visible. However, it is also observed that the flow is quite unsteady due to the large separation occurring on the
top of the cylinder. In order to study the temporal behaviour of the separation, time resolved PIV was applied. This technique
allows capturing the dynamic processes in detail. The development of vortices in the separated shear layer is observed and
in addition regions with different dominant frequencies are identified. 相似文献
5.
Hot-wire measurements have been carried out in the turbulent flow around a rotating circular cylinder in still air for Reynolds
numbers Re=∣U
w
∣D/ν=1.5×104 to 105. The experimental results confirm the analysis derived by asymptotic theory for high Reynolds numbers. Two different ways
of deriving the friction law from the experiments (via shear stress and via velocity distribution) resulted practically in
the same law. It is shown, that in spite of the curvature of the streamlines the universal logarithmic velocity distribution
is still valid near the wall.
Received: 8 August 1996/Accepted: 24 April 1998 相似文献
6.
The standard backward‐facing step flow problem is solved for steady state laminar case using stream function‐vorticity method. The steady state results are obtained as the asymptotic solution of the transient formulation. The primary reattachment length is studied and the discrepancy in the v velocity is reported. A method for determining appropriate locations for comparison is proposed. The energy equation is solved and found to be in good agreement with benchmark results. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
7.
The boundary layers developing on a single and two tandem circular cylinders were examined using multiple hot-film sensor
arrays for Re=2.4–5.1×104. Hot-wire and surface pressure measurements, and smoke-wire flow visualization were also made to better understand the flow
pattern and the evolution of the vortex street. The results show that, by use of the sensor arrays in conjunction with a bank
of constant-temperature anemometers, (i) the effects of the upstream cylinder on the boundary layer developing on the downstream
cylinder, (ii) the frequency of the vortex shedding, and (iii) the locations of flow separation and reattachment can be determined
non-intrusively and simultaneously. These measurement capabilities will provide a practical means for the characterization
and manipulation of unsteady flow phenomena.
Received: 27 October 1996 / Accepted: 13 February 1997 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.
We study the resolvent equation associated with a linear operator L{\mathcal{L}} arising from the linearized equation for perturbations of a steady Navier–Stokes flow U*{\mathbf{U^*}}. We derive estimates which, together with a stability criterion from [33], show that the stability of U*{\mathbf{U^*}} (in the L2-norm) depends only on the position of the eigenvalues of L{\mathcal{L}}, regardless the presence of the essential spectrum. 相似文献
10.
Non-Darcy mixed convection in a porous medium from horizontal surfaces with variable surface heat flux of the power-law distribution
is analyzed. The entire mixed convection regime is divided into two regions. The first region covers the forced convection
dominated regime where the dimensionless parameter ζ
f
=Ra*
x
/Pe2
x
is found to characterize the effect of buoyancy forces on the forced convection with K
′
U
∞/ν characterizing the effect of inertia resistance. The second region covers the natural convection dominated regime where
the dimensionless parameter ζ
n
=Pe
x
/Ra*1/2
x
is found to characterize the effect of the forced flow on the natural convection, with (K
′
U
∞/ν)Ra*1/2
x
/Pe
x
characterizing the effect of inertia resistance. To obtain the solution that covers the entire mixed convection regime the
solution of the first regime is carried out for ζ
f
=0, the pure forced convection limit, to ζ
f
=1 and the solution of the second is carried out for ζ
n
=0, the pure natural convection limit, to ζ
n
=1. The two solutions meet and match at ζ
f
=ζ
n
=1, and R
*
h
=G
*
h
.
Also a non-Darcy model was used to analyze mixed convection in a porous medium from horizontal surfaces with variable wall
temperature of the power-law form. The entire mixed convection regime is divided into two regions. The first region covers
the forced convection dominated regime where the dimensionless parameter ξ
f
=Ra
x
/Pe
x
3/2 is found to measure the buoyancy effects on mixed convection with Da
x
Pe
x
/ɛ as the wall effects. The second region covers the natural convection dominated region where ξ
n
=Pe
x
/Ra
x
2/3 is found to measure the force effects on mixed convection with Da
x
Ra
x
2/3/ɛ as the wall effects. Numerical results for different inertia, wall, variable surface heat flux and variable wall temperature
exponents are presented.
Received on 8 July 1996 相似文献
11.
Results are presented of an experimental investigation of vortex ring formation by a fluid drop contacting a free surface
with negligible velocity. The pool fluid is mixed with fluorescein dye, and a laser sheet is used to illuminate a plane of
the flow. A series of representative images is recorded by a CCD camera and speculation is made regarding specific sources
of vorticity flux through the free surface. Two scaling analyses previously presented by other investigators are demonstrated
to be equivalent under the assumptions of this experiment, and they provide the motivation for a series of test runs in which
the duration of the coalescence process, τ*, is related to variations in drop diameter L and fluid surface tension σ. Experimental results are in agreement with the analyses, showing τ*∼σ-1/2 and τ*∼L
3/2.
Received: 22 December 1995 / Accepted: 15 October 1996 相似文献
12.
Accurate high-order asymptotic analyses were carried out for Mode II plane strain crack in power hardening materials. The
second-order crack tip fields have been obtained. It is found that the amplitude coefficientk
2 of the second term of the asymptotic field is correlated to the first order field as the hardening exponentn<n
* (n
*≈5), but asn≥n
*,k
2 turns to become an independent parameter. Our results also indicated that, the second term of the asymptotic field has little
influence on the near-crack-tip field and can be neglected whenn<n
*. In fact,k
2 directly reflects the effects of triaxiality near the crack tip, the crack geometry and the loading mode, so that besidesJ-integral it can be used as another characteristic parameter in the two-parameter criterion.
The project supported by National Natural Science Foundation of China 相似文献
13.
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
14.
Mixing by secondary flow is studied by particle image velocimetry (PIV) in a developing laminar pulsating flow through a circular curved pipe. The pipe curvature ratio is η = r
0/r
c
= 0.09, and the curvature angle is 90°. Different secondary flow patterns are formed during an oscillation period due to
competition among the centrifugal, inertial, and viscous forces. These different secondary-flow structures lead to different
transverse-mixing schemes in the flow. Here, transverse mixing enhancement is investigated by imposing different pulsating
conditions (Dean number, velocity ratio, and frequency parameter); favorable pulsating conditions for mixing are introduced.
To obviate light-refraction effects during PIV measurements, a T-shaped structure is installed downstream of the curved pipe.
Experiments are carried out for the Reynolds numbers range 420 ≤ Rest ≤ 1,000 (Dean numbers 126.6 ≤ Dn ≤ 301.5) corresponding to non-oscillating flow, velocity component ratios 1 ≤ (β = U
max,osc/U
m,st) ≤ 4 (the ratio of velocity amplitude of oscillations to the mean velocity without oscillations), and frequency parameters
8.37 < (α = r
0(ω/ν)0.5) < 24.5, where α2 is the ratio of viscous diffusion time over the pipe radius to the characteristic oscillation time. The variations in cross-sectional
average values of absolute axial vorticity (|ζ|) and transverse strain rate (|ε|) are analyzed in order to quantify mixing.
The effects of each parameter (Rest, β, and α) on transverse mixing are discussed by comparing the dimensionless vorticities (|ζ
P
|/|ζ
S
|) and dimensionless transverse strain rates (|ε
P
|/|ε
S
|) during a complete oscillation period. 相似文献
15.
16.
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
17.
A. Salhi 《Theoretical and Computational Fluid Dynamics》2002,15(6):339-358
Linear theory is applied to examine rotation and buoyancy effects on homogeneous turbulent shear flows with given vertical
velocity shear, S=d/dx
3. In the rotating shear case (where the rotation vector is perpendicular to the plane of the mean flow, Ω
i
=Ωδ
i
2), general solutions for the Fourier components of the fluctuating velocity are proposed. These solutions are compared with
those proposed in the literature for the Fourier components of the fluctuating velocity and density in the case of a homogeneous
stratified shear flow with vertical density gradient, S
ρ=d/dx
3. It is shown that from the normal mode stability stand point the Bradshaw parameter B=2Ω/S(1+2Ω/S) (in the rotating shear case) and the Richardson number R
i
(in the statified shear case) play similar roles in identifying the stability for all the wave components except in the case
where Ω·k=0, for which rotation has no effects on the flow.
Analysis of the long-time behavior of the non-dimensional spectral density of energy, e
g
, is carried out. In the stable case, e
g
has decaying oscillations or undergoes a power law decay in time. Analytical solutions for the streamwise two-dimensional
energy ℰ
ii
1/2 (i.e. the limit at k
1=0 of the one-dimensional energy spectra) are proposed. At large time, ℰ
ii
1(t)/ℰ
ii
1(0) oscillates around the value (3R
i
+1)/(4R
i
) except at R
i
=1 it stays constant in time. Similar behavior for ℰ
ii
1(t)/ℰ
ii
1(0) is also observed in the rotating shear case (ℰ
ii
1(t)/ℰ
ii
1(0) oscillates around the value (1+4B)/(4B)).
Due to the behavior of the dimensionless spectral density of energy in both flow cases, the turbulent kinetic energy, /2, the production rate, ?, and the rate due to the buoyancy forces, ℬ, are split into two parts, , ?=?1+?2, ℬ=ℬ1+ℬ2 (in the stratified shear case, both ?1 and ℬ1 vanish when R
i
>?, while in the rotating shear case one has ℬ=0). It is shown that when rotation is “cyclonic” (i.e. Ω/S>0), part reaches maximum magnitudes at St
≈2, independent of the B value, and the first time to a zero crossing of ?2 occurs at this particular value. When rotation is “anticyclonic” (i.e. Ω/S<0) one finds St
≈1.6 instead of St
≈2. In the stratified shear case, both ?2 and ℬ2 cross zero at Nt=St
≈2, and part reaches maximum magnitudes at this particular value. These results and in particular those for the turbulent kinetic energy
are compared with previous direct numerical simulation (DNS) results in homogeneous stratified shear flows.
Received 30 July 2001 and accepted 19 February 2002 相似文献
18.
The evolution of wake structures and variation of the forces on a flat plate in harmonic oscillatory and in-line combined
flows are obtained numerically by improved discrete vortex method. For the oscillatory oncoming flow cases, wyenKc number varies from 2 to 40, the vortex pattern changes from a “harmonic wave” shaped (in a range of smallKc number) to a slight inclined “harmonic wave” shaped (in a range of moderateKc numbers), then to inclined vortex clusters with an angle of 50° to the oncoming flow direction (atKc=20), at last, asKc number becomes large, the vortex pattern is like a normal Karman vortex street. The well predicted drag and inertia force
coefficients are obtained, which are more close to the results of Keulegan & Carpenter's experiment as compared with previous
vortex simulation by other authors. The existence of minimum point of inertia force coefficientC
m
nearKc=20 is also well predicted and this phenomenon can be interpreted according to the vortex structure. For steady-oscillatory
in-line combined flow cases, the vortex modes behave like a vortex street, exhibit a “longitudinal wave” structure, and a
vortex cluster shape corresponding to the ratios ofU
m
toU
0 which are ofO (10−1)O(1) andO(10), respectively. The effect on the prediction of forces on the flat plate from the disturbance component in a combined
flow has been demonstrated qualitatively. In addition to this, the lock in phenomenon of vortex shedding has been checked.
The project supported by National Natural Science Foundation of China & LNM, Institute of Mechanics, CAS 相似文献
19.
The profile and excitation mechanism of vacuum-ultraviolet radiation emitted from shock wave is investigated in a shock tube.
For shock wave in argon, the rdiation is due to resonant transition excited by argon-argon collision in the shock front with
excitation cross section coefficientS
*=1.0×10−17 cm2·ev−1 and activation energyE
*=11.4 ev. For shock wave in air the radition is emitted from a very thin shock layer in which the mechanism ofX
1∑→b
1∑ of N2 is excited with excitation cross sectionQ=2×10−16cm2 and activation energyE
*=12.1 ev.
Institute of Mechanics, Academia Sinica 相似文献
20.
Hermano Frid 《Archive for Rational Mechanics and Analysis》2006,181(1):177-199
We prove the asymptotic stability of two-state nonplanar Riemann solutions for a class of multidimensional hyperbolic systems
of conservation laws when the initial data are perturbed and viscosity is added. The class considered here is those systems
whose flux functions in different directions share a common complete system of Riemann invariants, the level surfaces of which
are hyperplanes. In particular, we obtain the uniqueness of the self-similar L∞ entropy solution of the two-state nonplanar Riemann problem. The asymptotic stability to which the main result refers is
in the sense of the convergence as t→∞ in Lloc1 of the space of directions ξ = x/t. That is, the solution u(t, x) of the perturbed problem satisfies u(t, tξ)→R(ξ) as t→∞, in Lloc1(ℝn), where R(ξ) is the self-similar entropy solution of the corresponding two-state nonplanar Riemann problem. 相似文献