共查询到20条相似文献,搜索用时 15 毫秒
1.
The optimal control of infinitesimal flow disturbances experiencing the largest transient gain over a fixed time span, commonly
termed “optimal perturbations,” is undertaken using a variational technique in two- and three-dimensional boundary layer flows.
The cost function employed includes various energy metrics which can be weighted according to their perceived importance,
simplifying the task of determining which terms are essential for a “good” control scheme. In the accelerated boundary layers
investigated, disturbance kinetic energy can be typically reduced by about one order of magnitude. However, it seems impossible
to suppress completely over the entire control interval; “good” control strategies still permit approximately an order magnitude
growth over the initial energy at some point in the interval. It is shown that the control effort efficiently targets the
physical mechanisms behind transient growth.
Received 5 February 2001 and accepted 15 June 2001 相似文献
2.
R. J. Gribben 《Applied Scientific Research》1979,35(1):43-58
The flow in a laminar boundary layer for an arbitrary periodic main stream is considered at high frequencies when the fluid
is incompressible. The analysis, incorporating length scales appropriate to the thin “Stokes” layer immediately adjacent to
the surface and to the outer “Prandtl” boundary layer, involves expressing the dependent variables as mean parts plus superimposed
periodic parts and expanding these in inverse powers of the frequency parameter in the two layers. Thus earlier approaches
based on physical arguments are placed in the context of a systematic mathematical expansion scheme which is itself formulated
for more general main stream velocities than hitherto. Expressions for skin friction and heat transfer are obtained and briefly
discussed. 相似文献
3.
The aim of this work is to propose design criteria, based on rheological characterisation for improving drill-in fluids performance.
In particular, it reports an example in which rheological approaches helped improve drill-in fluids resistance to temperature.
As a starting system a commercial drill-in fluid containing xanthan gum and calcium carbonate was chosen and evaluated. Different
samples were then prepared by changing the initial formulation in order to increase the system's stability to temperature.
Drill-in fluids' performance have been compared by considering their “damaging potential”, filtration properties and, “cakes”.
All drill-in fluids have been tested before and after aging at a given temperature with “hot rolling tests”. The systems'
gel structure was characterized by measuring dynamic moduli (G′ and G′′) in the linear viscoelastic range and all samples
were compared by evaluating their “melting” temperature and gel network strength during time cure tests. The results obtained
from this work suggest that the rheological tests carried out on the whole drill-in fluid can provide insights into fluids'
damaging potential and “cake” structure. In particular, rheology proved to be able to provide quantitative information about
gel strength and temperature stability that permitted one to improve drill-in fluids' formulation in order to preclude formation
damage and to meet industrial requirements.
Received: 6 February 2000 Accepted: 15 November 2000 相似文献
4.
P. A. Aswatha Narayana 《Applied Scientific Research》1977,33(5-6):427-435
Mean velocity measurements made for two-dimensional incompressible turbulent boundary layers, for 3 pressure gradients on
a smooth wall have been used to evaluate the pressure gradient parameter, the skin-friction and the roughness function by
the method of “curve-fit” to the entire profile. Local skin-friction coefficients so obtained do not agree with the experimental
results but the integral parameters are well represented. 相似文献
5.
The Darcy free convection boundary layer flow over a vertical flat plate is considered in the presence of volumetric heat
generation/absorption. In the present first part of the paper it is assumed that the heat generation/absorption takes place
in a self-consistent way, the source term q
′′′≡ S of the energy equation being an analytical function of the local temperature difference T − T
∞. In a forthcoming second part, the case of the externally controlled source terms S = S(x,y ) will be considered. It is shown that due to the presence of S, the physical equivalence of the up- and downflows gets in general broken, in the sense that the free convection flow over
the upward projecting hot plate (“upflow”) and over its downward projecting cold counterpart (“downflow”) in general become
physically distinct. The consequences of this circumstance are examined for different forms of S. Several analytical solutions are given. Some of them describe algebraically decaying boundary layers which can also be recovered as limiting cases of exponentially decayingones. This asymptotic phenomenon is discussed in some detail. 相似文献
6.
Development of an X-ray computed tomography (CT) system with sparse sources: application to three-phase pipe flow visualization 总被引:1,自引:0,他引:1
Bin Hu Colin Stewart Colin P. Hale Christopher J. Lawrence Andrew R. W. Hall Holger Zwiens Geoffrey F. Hewitt 《Experiments in fluids》2005,39(4):667-678
This paper describes the application of a two-beam X-ray computed tomography (CT) system to multiphase (gas–oil–water) flow measurement. Two high-voltage (160 keV) X-ray sources are used to penetrate a 4-in. (101.6 mm ID) pipeline. A rotating filter wheel mechanism is employed to alternately “harden” and “soften” the X-ray spectra to provide discrimination between the three phases. Because this system offers only two projections, conventional back-projection algorithms are ineffective and thus a new reconstruction technique has been developed. A matrix equation is formed, to which additional “smoothing equations” are added to compensate for the lack of projection data. The tomographic result is obtained by computing an inverse matrix. This is a one-off computation and the inverse is stored for repeated use; reconstructed images from synthesized data demonstrate the effectiveness of this technique. Three-phase tomographic images of a horizontal slug flow are presented, which clearly show the mixing of oil and water layers within the slug body. The relevance of this work to the offshore oil and gas industry is summarized. 相似文献
7.
Vortex Shedding from a Hemisphere in a Turbulent Boundary Layer 总被引:1,自引:0,他引:1
Michael Manhart 《Theoretical and Computational Fluid Dynamics》1998,12(1):1-28
Supercritical turbulent boundary layer flow over a hemisphere with a rough surface (Re= 150000) has been simulated using Large Eddy Simulation (LES) and analyzed using the Karhunen--Loève expansion (“Proper Orthogonal
Decomposition,” POD). The time-dependent inflow condition is provided from a separate LES of a boundary layer developing behind
a barrier fence and a set of vorticity generators. LES results using significantly different grid resolutions are compared
with a corresponding wind tunnel experiment to demonstrate the reliability of the simulation. The separation processes are
analyzed by inspecting second-order moments, time spectra, and instantaneous velocity distributions. Applying POD, a detailed
study of the spatiotemporal structure of the separation processes has been carried out. From this analysis it can be concluded
that the major event in the separated flow behind the obstacle is the shedding of “von Kármán”-type vortices, which can be
represented by the first three energetically dominant modes.
Received 23 January 1997 and accepted 19 February 1998 相似文献
8.
This paper presents a finite-difference (FD) method with spatially non-rectangular irregular grids to simulate the elastic wave propagation. Staggered irregular grid finite difference operators with a second-order time and spatial accuracy are used to approximate the velocity-stress elastic wave equations. This method is very simple and the cost of computing time is not much. Complicated geometries like curved thin layers, cased borehole and nonplanar interfaces may be treated with nonrectangular irregular grids in a more flexible way. Unlike the multi-grid scheme, this method requires no interpolation between the fine and coarse grids and all grids are computed at the same spatial iteration. Compared with the rectangular irregular grid FD, the spurious diffractions from “staircase” interfaces can easily be eliminated without using finer grids. Dispersion and stability conditions of the proposed method can be established in a similar form as for the rectangular irregular grid scheme. The Higdon‘s absorbing boundary condition is adopted to eliminate boundary reflections. Numerical simulations show that this method has satisfactory stability and accuracy in simulating wave propagation near rough solid-fluid interfaces. The computation costs are less than those using a regular grid and rectangular grid FD method. 相似文献
9.
10.
In this paper, a novel decentralized adaptive neural control scheme is proposed for a class of uncertain multi-input and multi-output
(MIMO) nonlinear time-delay systems. RBF neural networks (NNs) are used to tackle unknown nonlinear functions, then the decentralized
adaptive NN tracking controller is constructed by combining Lyapunov–Krasovskii functions and the dynamic surface control
(DSC) technique along with the minimal-learning-parameters (MLP) algorithm. The proposed controller guarantees semi-global
uniform ultimate boundedness (SGUUB) of all the signals in the closed-loop large-scale system, while the tracking errors converge
to a small neighborhood of the origin. An advantage of the proposed control scheme lies in that the number of adaptive parameters
for each subsystem is reduced to one, and three problems of “computational explosion,” “dimension curse” and “controller singularity”
are solved, respectively. Finally, a numerical simulation is presented to demonstrate the effectiveness and performance of
the proposed scheme. 相似文献
11.
Based on parameter identification, a parameter observer is designed for a class of chaotic systems. The digital signals modulated
in the parameter will be recovered by the observer. By choosing different frequency signals as “0” and “1,” a practical digital
secure communication scheme is proposed by this parameter modulation method. Numerical simulations show the effectiveness
of this communication scheme. 相似文献
12.
A. Nakkasyan 《Theoretical and Computational Fluid Dynamics》2003,16(5):319-348
One-point time-averaged velocity correlations and joint probability density functions (p.d.f.s) are analyzed with a multi-step
method for steady three-dimensional turbulent boundary layers (3DTBLs). The data are derived from a time series of velocity
fluctuations measured along the measurement axes (
1,
2,
3). The method includes a Monte Carlo (MC) technique in which, firstly, the 3×3 Reynolds stress tensors are diagonalized locally
in order to obtain the experimental eigenvalues or principal values and the experimental eigenvectors or principal axes (
1,
2,
3). Secondly, trial independent p.d.f.s are MC-generated along these are projected from system into and the built-in hypotheses are tested for validity, using stringent self-consistency tests. All p.d.f. investigations are
made with “perturbed centered-Gaussians” hypotheses, in which the “centered-Gaussians” are experimentally defined and the
“perturbations” are trial drift velocities. Our MC-analysis method [1-3] is applied to the first [4] and second [5] generation “S-duct” experiments
performed at the école Polytechnique Fédérale de Lausanne (EPFL). Additionally, two complementary algebraic self-consistency
tests are developed for the double and the triple correlations separately. New results in the p.d.f. properties of 3DTBLs
are presented, using Kinetic Theory as background.
Received 6 November 2001 and accepted 27 August 2002 Published online 28 February 2003
Communicated by J.R. Blake 相似文献
13.
The rheological behaviour of Ca(OH)2 suspensions is investigated, predominantly at a solid volume fraction of 0.25. The influence of standing without being subject
to shear (“contact time”) is distinguished from that of being sheared (“shearing time”). The results are interpreted on the
basis of the “elastic floc” model of energy dissipation during flow, with a view to the problem whether, in addition to an
energy dissipation term related to the viscous drag experienced by particles moving within flocs, there should be an independent
energy dissipation term related to fluid movement in the flocs when they change volume or shape. It appears that this additional
energy dissipation term is not necessary, if the increase in viscous friction, experienced by two particles which are close
together, is taken into account.
Paper, presented at the First Conference of European Rheologists at Graz, April 14–16, 1982. A short version has been published
in [18]. 相似文献
14.
Anastassia N. Rissanou Marianna Yiannourakou Ioannis G. Economou Ioannis A. Bitsanis 《Rheologica Acta》2007,46(5):755-764
In this work, we study the temperature-induced development of “dynamically arrested” states in dense suspensions of “soft
colloids” (multi-arm star polymers and/or block-copolymers micelles) by means of molecular dynamics (MD) simulations. Temperature
increase in marginal solvents results in “soft sphere” swelling, dynamical arrest, and eventually crystallization. However,
two distinct “dynamically arrested” states were found, one almost amorphous (“glassy”) and one with a considerable degree
of crystallinity, yet lower than that of the fully equilibrated crystal. It is remarkable that even that latter state permitted
self-diffusion in the timescale of the simulations, an effect that underlies the importance of the “ultra-soft” nature of
inter-particle potential. The “number of connections” criterion for crystallinity proved to be very successful in identifying
the ultimate thermodynamic trend from the very early stages of the α-relaxation.
This paper was presented at the Third Annual Rheology Conference, AERC 2006, April 27–29, 2006, Crete, Greece. 相似文献
15.
We investigate nonlinear stability for equilibrium of a pendulum with viscoelastic components. The tracking force is chosen
so that the matrix of the linearized part of the perturbed motion has two purely imaginary roots or one zero and one negative
root. The other two roots are complex with negative real part. The boundary of the domain of stability is divided into “dangerous”
and “safe” (in the sense of Bautin) zones.
Translated from Prikladnaya Mekhanika, Vol. 35, No. 9, pp. 100–105, September, 1999. 相似文献
16.
The problem of the self-similar boundary flow of a “Darcy-Boussinesq fluid” on a vertical plate with temperature distribution
T
w(x) = T
∞+A·x
λ and lateral mass flux v
w(x) = a·x
(λ−1)/2, embedded in a saturated porous medium is revisited. For the parameter values λ = 1,−1/3 and −1/2 exact analytic solutions
are written down and the characteristics of the corresponding boundary layers are discussed as functions of the suction/ injection
parameter in detail. The results are compared with the numerical findings of previous authors.
Received on 8 March 1999 相似文献
17.
A. A. Pan’kov 《Mechanics of Solids》2010,45(2):247-256
We consider statistically homogeneous two-phase random piezoactive structures with deterministic properties of inclusions
and the matrix and with random mutual location of inclusions. We present the solution of a coupled stochastic boundary value
problem of electroelasticity for the representative domain of a matrix piezocomposite with a random structure in the generalized
singular approximation of the method of periodic components; the singular approximation is based on taking into account only
the singular component of the second derivative of the Green function for the comparison media. We obtain an analytic solution
for the tensor of effective properties of the piezocomposite in terms of the solution for the tensors of effective properties
of a composite with an ideal periodic structure or with the “statistical mixture” structure and with the periodicity coefficient
calculated for a given random structure with its specific characteristics taken into account. The effective properties of
composites with auxiliary structures (periodic and “statistical mixture”) are also determined in the generalized singular
approximation by varying the properties of the comparisonmedium. We perform numerical computations and analyze the effective
properties of a quasiperiodic piezocomposite with reciprocal polarization of oriented ellipsoidal inclusions and the matrix,
the layered structures with reciprocal polarization of the layers [1] of a polymer piezoelectric PVF, and find their unique
properties such as a significant increase in the Young modulus along the normal to the layers and in dielectric permittivities,
the appearance of negative values of the Poisson ratio under extension along the normal, and an increase in the absolute values
of the basic piezomoduli. 相似文献
18.
Eugen Magyari 《Transport in Porous Media》2010,84(3):711-715
It is shown that the governing equation for the stream function of the Darcy free convection boundary layer flows past a vertical
surface is invariant under arbitrary translations of the transverse coordinate y. The consequences of this basic symmetry property on the solutions corresponding to a prescribed surface temperature distribution
T
w
(x) are investigated. It is found that starting with a “primary solution” which describes the temperature boundary layer on an impermeable surface, infinitely many “translated solutions” can be generated which form a continuous group, the “translation group” of the given primary solution. The elements of this group describe free convection boundary layer flows from permeable counterparts of the original surface with a transformed temperature distribution [(T)\tilde]w ( x ){\tilde {T}_w \left( x \right)}, when simultaneously a suitable lateral suction/injection of the fluid is applied. It turns out in this way that several
exact solutions discovered during the latter few decades are in fact not basically new solutions, but translated counterparts of some formerly reported primary solutions. A few specific examples are discussed in detail. 相似文献
19.
Measurements of mean and fluctuating wall shear stress beneath spanwise-invariant separation bubbles
P. E. Hancock 《Experiments in fluids》1999,27(1):53-59
Pulsed-wire measurements of wall shear stress have been made beneath two separation bubbles. In one a cross flow was generated
by means of a (25°) swept separation line. Fluctuating stresses in orthogonal “streamwise” and cross-flow directions are very
nearly equal and independent of at least moderate cross flow velocity. These fluctuations are largely determined by large-scale
motions in the outer flow, whereas the mean shear stresses are not. The pdf of the “streamwise” fluctuations is unchanged
by the cross flow. When a cross flow is present the pdf of the cross-flow stresses is similar to the “streamwise” pdf. Dependence
on Reynolds number is the same in both flows.
Received: 10 April 1998/Accepted: 17 July 1998 相似文献
20.
Yu. M. Volchkov L. A. Dergileva 《Journal of Applied Mechanics and Technical Physics》1999,40(2):354-359
The edge effects in the stress state of an interlayer in stretching and shearing by rigid slabs are studied. On the basis
of the equations of momentless and moment elastic layers, we solve problems modeling qualitatively the stress-strain state
in the “soft” layer between two “rigid” layers.
Lavrent’ev Institute of Hydrodynamics, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from
Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 2, pp. 189–195, March–April, 1999. 相似文献