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1.
The wave instability of convective boundary layers in a horizontal cylindrical layer of ethanol under the action of vertical
hamonic high-frequency vibration is studied. A strong destabilizing effect of the vibration on the stability of the convective
boundary layers is detected. In the plane of the gravity and vibration Rayleigh numbers (Ra and R
V
), the excitation limit of the wave instability is determined. The periods of the temperature oscillations caused by the waves
in the boundary layers near the inner and outer cavity boundaries are studied as functions of the Rayleigh numbers.
Perm’. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 3, pp. 32–40, May–June, 1998. 相似文献
2.
Thomas Boeck Jie Li Enrique López-Pagés Philip Yecko Stéphane Zaleski 《Theoretical and Computational Fluid Dynamics》2007,21(1):59-76
We perform numerical simulations of two-phase liquid–gas sheared layers, with the objective of studying atomization. The Navier–Stokes equations for two-dimensional incompressible flow are solved in a periodic domain. A volume-of-fluid method is used to track the interface. The density ratio is kept around 10. The calculations show good agreement with a fully viscous Orr–Sommerfeld linear theory over several orders of magnitude of interface growth. The nonlinear development shows the growth of finger-like structures, or ligaments, and the detachment of droplets. The effect of the Weber and Reynolds numbers, the boundary layer width and the initial perturbation amplitude are discussed through a number of typical cases. Inversion of the liquid boundary layer is shown to yield more readily ligaments bending upwards and is thus more likely to produce droplets. 相似文献
3.
P. R. Spalart S. Deck M. L. Shur K. D. Squires M. Kh. Strelets A. Travin 《Theoretical and Computational Fluid Dynamics》2006,20(3):181-195
Detached-eddy simulation (DES) is well understood in thin boundary layers, with the turbulence model in its Reynolds-averaged Navier–Stokes (RANS) mode and flattened grid cells, and in regions of massive separation, with the turbulence model in its large-eddy simulation (LES) mode and grid cells close to isotropic. However its initial formulation, denoted DES97 from here on, can exhibit an incorrect behavior in thick boundary layers and shallow separation regions. This behavior begins when the grid spacing parallel to the wall Δ∥ becomes less than the boundary-layer thickness δ, either through grid refinement or boundary-layer thickening. The grid spacing is then fine enough for the DES length scale to follow the LES branch (and therefore lower the eddy viscosity below the RANS level), but resolved Reynolds stresses deriving from velocity fluctuations (“LES content”) have not replaced the modeled Reynolds stresses. LES content may be lacking because the resolution is not fine enough to fully support it, and/or because of delays in its generation by instabilities. The depleted stresses reduce the skin friction, which can lead to premature separation.For some research studies in small domains, Δ∥ is made much smaller than δ, and LES content is generated intentionally. However for natural DES applications in useful domains, it is preferable to over-ride the DES limiter and maintain RANS behavior in boundary layers, independent of Δ∥ relative to δ. For this purpose, a new version of the technique – referred to as DDES, for Delayed DES – is presented which is based on a simple modification to DES97, similar to one proposed by Menter and Kuntz for the shear–stress transport (SST) model, but applicable to other models. Tests in boundary layers, on a single and a multi-element airfoil, a cylinder, and a backward-facing step demonstrate that RANS function is indeed maintained in thick boundary layers, without preventing LES function after massive separation. The new formulation better fulfills the intent of DES. Two other issues are discussed: the use of DES as a wall model in LES of attached flows, in which the known log-layer mismatch is not resolved by DDES; and a correction that is helpful at low cell Reynolds numbers. 相似文献
4.
We investigate in this paper two numerical methods for solving low Mach number compressible flows and their application to
single-phase natural convection flow problems. The first method is based on an asymptotic model of the Navier–Stokes equations
valid for small Mach numbers, whereas the second is based on the full compressible Navier–Stokes equations with particular
care given to the discretization at low Mach numbers. These models are more general than the Boussinesq incompressible flow
model, in the sense that they are valid even for cases in which the fluid is subjected to large temperature differences, that
is when the compressibility of the fluid manifests itself through low Mach number effects. Numerical solutions are computed
for a series of test problems with fixed Rayleigh number and increasing temperature differences, as well as for varying Rayleigh
number for a given temperature difference. Numerical difficulties associated with low Mach number effects are discussed, as
well as the accuracy of the approximations.
Received on 17 January 2000 相似文献
5.
Fully developed forced convection inside a circular tube filled with saturated porous medium and with uniform heat flux at
the wall is investigated on the basis of a Brinkman–Forchheimer model. The matched asymptotic expansion method is applied
at small Darcy numbers. For large Darcy numbers, the solution for the Brinkman–Forchheimer momentum equation is found in terms
of an asymptotic expansion. Once the velocity distribution is determined, the energy equation is solved using the same asymptotic
technique. The results for the two limiting cases of clear fluid and Darcy flow conditions show good agreement with those
available in the literature. 相似文献
6.
The effects of viscous dissipation on unsteady free convection from an isothermal vertical flat plate in a fluid saturated
porous medium are examined numerically. The Darcy–Brinkman–Forchheimer model is employed to describe the flow field. A new
model of viscous dissipation is used for the Darcy–Brinkman–Forchheimer model of porous media. The simultaneous development
of the momentum and thermal boundary layers are obtained by using a finite difference method. Boundary layer and Boussinesq
approximation have been incorporated. Numerical calculations are carried out for various parameters entering into the problem.
Velocity and temperature profiles as well as local friction factor and local Nusselt number are shown graphically. It is found
that as time approaches infinity, the values of friction factor and heat transfer coefficient approach steady state. 相似文献
7.
Xiao-Jun Gu David R. Emerson Gui-Hua Tang 《Continuum Mechanics and Thermodynamics》2009,21(5):345-360
A set of linearized 26 moment equations, along with their wall boundary conditions, are derived and used to study low-speed
gas flows dominated by Knudsen layers. Analytical solutions are obtained for Kramers’ defect velocity and the velocity-slip
coefficient. These results are compared to the numerical solution of the BGK kinetic equation. From the analysis, a new effective
viscosity model for the Navier–Stokes equations is proposed. In addition, an analytical expression for the velocity field
in planar pressure-driven Poiseuille flow is derived. The mass flow rate obtained from integrating the velocity profile shows
good agreement with the results from the numerical solution of the linearized Boltzmann equation. These results are good for
Knudsen numbers up to 3 and for a wide range of accommodation coefficients. The Knudsen minimum phenomenon is also well captured
by the present linearized 26-moment equations. 相似文献
8.
A technique for stability analysis of anisotropic cylindrical shells is developed. It permits us to examine the cases of reinforcement
where the elastic axes of layers do not coincide with the coordinate axes of the shell. The solution is obtained using the
mixed equations of the Donnell-Mushtari-Vlasov theory of shells. The deflection and force functions are approximated by trigonometric
series. Single-layer and multilayer cylindrical shells with fiber orientation of two types are analyzed for stability. It
is revealed that when layers are few, failure to incorporate the direction of fibers in layers into the design model results
in highly inaccurate values of critical loads
__________
Translated from Prikladnaya Mekhanika, Vol. 42, No. 3, pp. 80–88, March 2006. 相似文献
9.
The problem of the convective instability of a plane fluid layer bounded by rigid walls with heating in a narrow layer running
parallel to the walls inside the volume in question is solved. Instability criteria depending on the location of the heated
layer and the Rayleigh numbers of the upper and lower layers are found. The results are compared with those for a plane layer
with uniform energy release inside the volume.
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 8–15, January–February,
1997.
The work was carried out with partial support from the Russian Foundation for Fundamental Research (project No. 95-01-00354a). 相似文献
10.
Turbulent wall pressure fluctuation measurements on a towed model at high Reynolds numbers 总被引:1,自引:1,他引:0
Turbulent wall pressure fluctuation measurements were made in water on a towed model of length 129.8 (m) and diameter 3.8
(cm) for steady speeds from 6.2 (m/s) to 15.5 (m/s). The drag on the model was measured with a strut mounted load cell which
provided estimates of the momentum thickness and friction velocity. Momentum thickness Reynolds numbers Re
θ varied from 4.8 × 105 to 1.1 × 106. The ratio of momentum thickness to viscous length scale is significantly greater than for flat plate cases at comparable
Re
θ. The effectiveness of inner and outer velocity and length scales for collapsing the pressure spectra are discussed. The wavenumber–frequency
spectra show a convective ridge at higher frequencies similar to flat plate boundary layers. At low frequencies, energy broad
in wavenumber extends outside the convective ridge and acoustic cone, with no characteristic wave speed. Wall pressure cross-spectral
levels scaled with similarity variables are shown to increase with increasing tow speed, and to follow decay constants consistent
with flat plate cases. The convection velocities also display features similar to flat plate cases. 相似文献
11.
David E. Zeitoun Yves Burtschell Irina A. Graur Mikhail S. Ivanov Alexey N. Kudryavtsev Yevgeny A. Bondar 《Shock Waves》2009,19(4):307-316
Numerical simulations of shock wave propagation in microchannels and microtubes (viscous shock tube problem) have been performed
using three different approaches: the Navier–Stokes equations with the velocity slip and temperature jump boundary conditions,
the statistical Direct Simulation Monte Carlo method for the Boltzmann equation, and the model kinetic Bhatnagar–Gross–Krook
equation with the Shakhov equilibrium distribution function. Effects of flow rarefaction and dissipation are investigated
and the results obtained with different approaches are compared. A parametric study of the problem for different Knudsen numbers
and initial shock strengths is carried out using the Navier–Stokes computations.
相似文献
12.
Shahid Hussain Arshad Muhammad Nawaz Naeem Nazra Sultana Abdul Ghafar Shah Zafar Iqbal 《Archive of Applied Mechanics (Ingenieur Archiv)》2011,81(3):319-343
In the present study, a vibration frequency analysis of a bi-layered cylindrical shell composed of two independent functionally
graded layers is presented. The thickness of the shell layers is assumed to be equal and constant. Material properties of
the constituents of bi-layered functionally graded cylindrical shell are assumed to vary smoothly and continuously through
the thickness of the layers of the shell and are controlled by volume fraction power law distribution. The expressions for
strain–displacement and curvature–displacement relationships are utilized from Love’s first approximation linear thin shell
theory. The versatile Rayleigh–Ritz approach is employed to formulate the frequency equations in the form of eigenvalue problem.
Influence of material distribution in the two functionally graded layers of the cylindrical shells is investigated on shell
natural frequencies for various shell parameters with simply supported end conditions. To check the validity, accuracy and
efficiency of the present methodology, results obtained are compared with those available in the literature. 相似文献
13.
The stability problem of two-dimensional compressible flat-plate boundary layers is handled using the linear stability theory.
The stability equations obtained from three-dimensional compressible Navier–Stokes equations are solved simultaneously with
two-dimensional mean flow equations, using an efficient shoot-search technique for adiabatic wall condition. In the analysis,
a wide range of Mach numbers extending well into the hypersonic range are considered for the mean flow, whereas both two-
and three-dimensional disturbances are taken into account for the perturbation flow. All fluid properties, including the Prandtl
number, are taken as temperature-dependent. The results of the analysis ascertain the presence of the second mode of instability
(Mack mode), in addition to the first mode related to the Tollmien–Schlichting mode present in incompressible flows. The effect
of reference temperature on stability characteristics is also studied. The results of the analysis reveal that the stability
characteristics remain almost unchanged for the most unstable wave direction for Mach numbers above 4.0. The obtained results
are compared with existing numerical and experimental data in the literature, yielding encouraging agreement both qualitatively
and quantitatively.
相似文献
14.
The flow field developed behind a shock wave propagating inside a constant cross-section conduit is solved numerically for
two different cases. First, when the density of the ambient gas into which the shock propagates has a logarithmic change with
distance. In the second, and the more practical case, the ambient gas is composed of pairs of air–helium layers having a continually
decreasing width. It is shown that in both cases meaningful pressure amplification can be reached behind the transmitted shock
wave. It is especially so in the second case. By proper choice of the number of air–helium layers and their width reduction
ratio, pressure amplification as high as 7.5 can be obtained.
相似文献
15.
In this study, we use the method of homogenization to develop a filtration law in porous media that includes the effects of
inertia at finite Reynolds numbers. The result is much different than the empirically observed quadratic Forchheimer equation.
First, the correction to Darcy’s law is initially cubic (not quadratic) for isotropic media. This is consistent with several
other authors (Mei and Auriault, J Fluid Mech 222:647–663, 1991; Wodié and Levy, CR Acad Sci Paris t.312:157–161, 1991; Couland
et al. J Fluid Mech 190:393–407, 1988; Rojas and Koplik, Phys Rev 58:4776–4782, 1988) who have solved the Navier–Stokes equations
analytically and numerically. Second, the resulting filtration model is an infinite series polynomial in velocity, instead
of a single corrective term to Darcy’s law. Although the model is only valid up to the local Reynolds number, at the most,
of order 1, the findings are important from a fundamental perspective because it shows that the often-used quadratic Forchheimer
equation is not a universal law for laminar flow, but rather an empirical one that is useful in a limited range of velocities.
Moreover, as stated by Mei and Auriault (J Fluid Mech 222:647–663, 1991) and Barree and Conway (SPE Annual technical conference
and exhibition, 2004), even if the quadratic model were valid at moderate Reynolds numbers in the laminar flow regime, then
the permeability extrapolated on a Forchheimer plot would not be the intrinsic Darcy permeability. A major contribution of
this study is that the coefficients of the polynomial law can be derived a priori, by solving sequential Stokes problems.
In each case, the solution to the Stokes problem is used to calculate a coefficient in the polynomial, and the velocity field
is an input of the forcing function, F, to subsequent problems. While numerical solutions must be utilized to compute each coefficient in the polynomial, these
problems are much simpler and robust than solving the full Navier–Stokes equations. 相似文献
16.
Large eddy simulation (LES) is carried out to investigate the turbulent boundary-layer flows over a hill-shaped model with
a steep or relatively moderate slope at moderately high Reynolds numbers (Re = O(103)) defined by the hill height and the velocity at the hill height. The study focuses on the effects of surface roughness and
curvature. For Sub-grid Scale (SGS) modeling of LES, both the dynamic Smagorinsky model (DSM) and the dynamic mixed model
(DMM) are applied. The behavior of the separated shear layer and the vortex motion are affected by the oncoming turbulence,
such that the shear layer comes close to the ground surface, or the size of a separation region becomes small because of the
earlier instability of the separated shear layer. Appropriate measures are required to generate the inflow turbulence. The
methods of Lund et al. (J. Comput. Phys., 140:233–258, 1998) and Nozawa and Tamura (J. Wind Eng. Ind. Aerodyn., 90:1151–1162, 2002; The 4th European and African Conference on Wind Engineering, 1–6, 2005) are employed to simulate the smooth- and rough-wall turbulent boundary layers in order to generate time-sequential
data of inflow turbulence. This paper discusses the unsteady phenomena of the wake flows over the smooth and rough 2D hill-shaped
obstacles and aims to clarify the roughness effects on the flow patterns and the turbulence statistics distorted by the hill.
Numerical validation is conducted by comparing the simulation results with wind tunnel experiment data for the same hill shape
at almost the same Re. The applicability of DSM and DMM are discussed, focusing on the recirculation region behind a steep hill. 相似文献
17.
G. A. Khabakhpashev 《Journal of Applied Mechanics and Technical Physics》2005,46(6):807-817
Dynamics of three-dimensional disturbances of the interface between two fluid layers of different densities is considered
analytically and numerically. An evolutionary integrodifferential equation is derived, which takes into account long-wave
contributions of inertia of the layers and surface tension, small but finite amplitude of disturbances of the interface between
two incompressible immiscible fluids, gentle slopes of the lid and bottom, and nonstationary shear stresses at all boundaries.
Numerical solutions of this model equation for several (most typical) nonlinear problems of transformation of two- and three-dimensional
waves are obtained.
__________
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 6, pp. 45–57, November–December, 2005. 相似文献
18.
The transient heat conduction equation in a finite slab undergoing phase change (two-phase problem of melting and solidification),
with isothermal, adiabatic or convective boundary conduction is studied by the network simulation method; solid phase conductivity
and specific heat are assumed to be dependent on temperature. Ablation, as a particular case, is also analysed. A network
model is established for a cell and boundary conditions are added to complete the whole network model. No restrictions exist,
as to the kinds of linear and non-linear boundary conditions, Stefan number values or the initial conditions (when hypotheses
concern of the Stefan problem, numerical and exact solutions are compared for a large interval of Stefan numbers; simulation
values show good agreement). Movement of the solid–liquid boundary and thermal fields are determined in all cases.
Received on 10 May 2000 / Published online: 29 November 2001 相似文献
19.
P. I. Kolykhalov 《Fluid Dynamics》1986,21(6):854-860
The viscous dissipative instability of two flows with continuous spectrum of neutrally-stable perturbations in the absence
of dissipation is investigated. Ranges of wave numbers in which viscosity leads to flow destabilization are determined for
a shear discontinuity in a smoothly-stratified fluid. A shear flow with a velocity in the transition layer that depends linearly
on the coordinate has a continuum of neutral modes even in the case of an unstratified fluid. When viscosity is present in
one of the layers with constant velocity, one of the branches of the spectrum becomes unstable. When the viscosity is the
same above and below the shear layer, dissipation only leads to the damping of the perturbations.
Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 14–19, November–December, 1986. 相似文献
20.
Souzanna Sofou Edward B. Muliawan Savvas G. Hatzikiriakos Evan Mitsoulis 《Rheologica Acta》2008,47(4):369-381
Bread dough (a flour–water system) has been rheologically characterized using a parallel-plate, an extensional, and a capillary
rheometer at room temperature. Based on the linear and nonlinear viscoelastic and viscoplastic data, two constitutive equations
have been applied, namely a viscoplastic Herschel–Bulkley model and a viscoelastoplastic K–BKZ model with a yield stress.
For cases where time effects are unimportant, the viscoplastic Herschel–Bulkley model can be used. For cases where transient
effects are important, it is more appropriate to use the K-BKZ model with the addition of a yield stress. Finally, the wall
slip behavior of dough was studied in capillary flow, and an appropriate slip law was formulated. These models characterize
the rheological behavior of bread dough and constitute the basic ingredients for flow simulation of dough processing, such
as extrusion, calendering, and rolling. 相似文献