共查询到20条相似文献,搜索用时 15 毫秒
1.
The development of large-scale organised motions in a compressible mixing layer is studied experimentally using holographic
interferometry, pressure and turbulence measurements. The mixing layer was formed behind the base of a parallel strut with
a Mach 2 air main stream and a co-flowing two-dimensional slot jet (aspect ratio = 45) of helium at a Mach number of 1.2.
The mixing layer exhibited highly organised vortical structures which were elongated and inclined approximately 45–50° to
the flow direction. The mixing layer showed a linear growth and the mean velocity data indicated self-similar behaviour at
sufficiently downstream distances. The results have shown that the thickness of the primary boundary layer has a strong influence
on the growth and structure of the mixing layer.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
Results of experimental study on DDT in a smooth tube filled with sensitive mixtures having detonation cell size from 1 to
3 orders of magnitude smaller than the tube diameter are presented. Stoichiometric hydrogen–oxygen mixtures were used in the
tests with initial pressure ranging from 0.2 to 8 bar. A dependence of the run-up distance to DDT on the initial pressure
is studied. This dependence is found to be close to the inverse proportionality. It is suggested that the flow ahead of the
flame results in formation of the turbulent boundary layer. This boundary layer controls the scale of turbulent motions in
the flow. A simple model to estimate the maximum scale of the turbulent pulsations (boundary layer thickness) at flame positions
along the tube is presented. The largest scale of the turbulent motions at the location of the onset of detonation is shown
to be 1 order of magnitude greater than the detonation cell widths, λ, in all the tests. It is suggested that the onset of
detonation is triggered during flame acceleration as soon as the maximum scale of the turbulent pulsations increases up to
about 10 λ. The model to estimate the maximum size of turbulent motions, δ, and the correlation δ≈ 10λ, give a basis for estimations
of the run-up distances to DDT in tubes with internal diameter D > 20λ.
PACS 47.40.-x; 47.27.Nz
This paper was based on work that was presented at the 19th Inter-national Colloquium on the Dynamics of Explosions and Reactive
Systems, Hakone, Japan, July 27 - August 1, 2003 相似文献
3.
V. P. Reutov A. B. Ezersky G. V. Rybushkina V. V. Chernov 《Journal of Applied Mechanics and Technical Physics》2007,48(4):469-478
Evolution of convective structures in a thin layer of an evaporating liquid (ethanol) located under a turbulent boundary layer
of an airflow is studied experimentally and theoretically. Evolution of the structures is examined under conditions of an
increased flow velocity. A transition is found from convective cells formed in the absence of the flow to convective rolls
elongated in the streamwise direction. The theoretical analysis is performed within a two-dimensional model of the flow in
the liquid layer. The boundary conditions on the liquid surface are obtained with the use of self-similar solutions for mean
fields in the airflow. The onset and evolution of a periodic system of rolls are simulated numerically. Theoretical conclusions
are compared with experimental data.
__________
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 4, pp. 3–14, July–August, 2007. 相似文献
4.
The stability of the Ekman thermally stratified boundary layer simulating the atmospheric boundary layer is studied theoretically.
The system of thermohydrodynamic equations is solved in the Boussinesq approximation. Two mechanisms of development of the
vortex structures, namely, thermal and dynamic, as well as their interaction, are investigated. It is shown that a sharp boundary
on which the flow characteristics change qualitatively exists between various domains of the parameters corresponding to thermal
and dynamic instabilities.
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 3, pp. 71–76, May–June, 1998.
The work was carried out with financial support from the Russian Foundation for Basic Research (project No. 96-01-01118). 相似文献
5.
S. V. Meleshko V. V. Pukhnachev 《Journal of Applied Mechanics and Technical Physics》1999,40(2):208-216
A family of partially invariant solutions of the Navier-Stokes equations of rank 2 and defect 2 is considered. These solutions
describe the three-dimensional unsteady motions of a viscous incompressible fluid in which the vertical velocity component
and the pressure are independent of the horizontal coordinates. In particular, they can be interpreted as flows in a horizontal
layer, one boundary of which is the free surface.
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 2, pp. 24–33, March–April, 1999. 相似文献
6.
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
7.
Large-eddy simulation (LES) results for laminar-to-turbulent transition in a spatially developing boundary layer are presented.
The disturbances are ingested into a laminar flow through an unsteady suction-and-blowing strip. The filtered, three-dimensional
time-dependent Navier–Stokes equations are integrated numerically using spectral, high-order finite-differences, and a three-stage
low-storage Runge–Kutta/Crank–Nicolson time-advancement method. The buffer-domain technique is used for the outflow boundary
condition. The localized dynamic model used to parametrize the subgrid-scale (SGS) stresses begins to have a significant impact
at the beginning of the nonlinear transition (or intermittency) region. The flow structures commonly found in experiments
are also observed in the present simulation; the computed linear instability modes and secondary instability $\Lambda$-vortex
structures are in agreement with the experiments, and the streak-like structures and turbulent statistics compare with both
the experiments and the theory. The physics captured in the present LES are consistent with the experiments and the full Navier–Stokes
simulation (DNS), at a significant fraction of the DNS cost. A comparison of the results obtained with several SGS models
shows that the localized model gives accurate results both in a statistical sense and in terms of predicting the dynamics
of the energy-carrying eddies, while requiring fewer ad hoc adjustments than the other models.
Received: 5 April 1996 and accepted 27 March 相似文献
8.
C. B. Lee Z. X. Hong Y. S. Kachanov V. I. Borodulin V. V. Gaponenko 《Experiments in fluids》2000,28(3):243-251
This study is concerned with transition in flat plate boundary layer flow. Sets of results are obtained as follows: (1) Very
clear pictures of the formation and the development of the butterfly-like structures rather than ∧-structures in the K-regime of boundary layer transition are obtained. (2) A chain of ring like vortices, which generate the high-frequency spikes
on the time traces of velocity and still present periodical behaviour, at the tip of each ∧-vortex, which is the part of the
butterfly-like structure, are visualized for the first time. (3) A wave-like structure is observed to occupy the whole boundary
layer, extending from the near-wall region to the outer edge of the boundary layer.
Received: 24 September 1998/Accepted: 24 April 1999 相似文献
9.
Fluid flow at the interface of a porous medium and an open channel is the governing phenomenon in a number of processes of
industrial importance. Traditionally, this has been modeled by applying the Brinkman’s modification of Darcy’s law to obtain
the velocity profile in terms of an additional parameter known as the “apparent viscosity” or the “slip coefficient”. To test
this ad hoc approach, a detailed experimental investigation of the flow was conducted using Laser Doppler Anemometry (LDA) in the close
vicinity of the permeable boundary of a porous medium. The porous medium used in the experiments consisted of a network of
continuous glass strands woven together in a random fashion.
A Hele–Shaw cell was partially filled with a fibrous preform such that an open channel flow is coupled with the Darcy flow
inside the preform through the permeable interface of the preform. The open channel portion of the Hele–Shaw cell also acts
as an ideal porous medium of known in-plane permeability which is much higher than the permeability of the fibrous porous
medium. A viscous fluid is injected at a constant flow rate through the above arrangement and a saturated and steady flow
is established through the cell. Using LDA, steady state velocity profiles are accurately measured by traversing across the
cell in the direction perpendicular to the flow. A series of experiments were conducted in which fluid viscosity, flow rate,
solid volume fraction of the porous medium and depth of the Hele–Shaw cell were varied. For each and every case in which the
conditions for Hele–Shaw approximation were valid, the depth of the boundary layer zone or the screening length inside the
fibrous preform was found to be of the order of the channel depth. This is much larger as compared to the Brinkman’s prediction
of the screening length which is of the order of √K, where K is the permeability of the fibrous porous medium. Based on this finding, we modified the boundary condition in the Brinkman’s
solution and found that the velocity profile results compared well with the experimental data for the planar geometry and
the fibrous preforms for volume fractions of 7%, 14% and 21% for Hele–Shaw cell depths of 1.6 and 3.175 mm. For a cell depth
of 4.8 cm, in which the Hele–Shaw approximation was not valid, the boundary layer thickness or the screening length was found
to be less than the mold or channel depth but was still much larger than the Brinkman’s prediction.
Received: 10 May 1996 / Accepted: 26 August 1996 相似文献
10.
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
11.
Analysis of DNS and LES of Flow in a Low Pressure Turbine Cascade with Incoming Wakes and Comparison with Experiments 总被引:1,自引:0,他引:1
The flow around a low-pressure turbine rotor blade with incoming periodic wakes is computed by means of DNS and LES. The latter
adopts a dynamic sub-grid-scale model. The computed results are compared with time-averaged and instantaneous measured quantities.
The simulation sreveal the presence of elongated flow structures, stemming from the incoming wake vorticity, which interact
with the pressure side boundary layer. As the wake approaches the upstream half of the suction side, its vortical structures
are stretched and align with the main flow, resulting in an impingement at virtually zero angle of attack. Periodically, in
the absence of impinging wakes, the laminar suction side boundary layer separates in the adverse pressure gradient region.
Flow in the laminar separation bubble is found to undergo transition via a Kelvin–Helmholtz instability. Subsequent impingement
of the wake inhibits separation and thus promotes boundary layer reattachment. LES provides a fair reproduction of the DNS
results both in terms of instantaneous, phase-averaged, and time-averaged flow fields with a considerable reduction in computational
effort.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
12.
D. V. Lyubimov 《Fluid Dynamics》2000,35(2):177-184
Thermal vibrational convection is considered for an acoustic wavelength at the vibration frequency comparable with the dimensions
of the cavity. Equations of the pulsatory and average motions of a medium that generalize the well-known equations of thermovibrational
convection are obtained. Effective boundary conditions for average fields on rigid boundaries are formulated. The quasi-equilibrium
stability problem for a plane horizontal layer heated from below and executing high-frequency oscillations is solved. It is
shown that the compressibility effects can be significant even when the acoustic wavelength substantially exceeds the length
of the layer. A destabilizing compressibility effect which can lead to instability of the layer even under conditions of weightlessness
is established.
Perm’, e-mail: lyubimov@psu.ru. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 2, pp.
28–36, March–April, 2000.
The work was carried out with partial financial support from the Program of State Support for Leading Science Schools (project
No. 96-015-96084). 相似文献
13.
Wind turbines operate in the surface layer of the atmospheric boundary layer, where they are subjected to strong wind shear
and relatively high turbulence levels. These incoming boundary layer flow characteristics are expected to affect the structure
of wind turbine wakes. The near-wake region is characterized by a complex coupled vortex system (including helicoidal tip
vortices), unsteadiness and strong turbulence heterogeneity. Limited information about the spatial distribution of turbulence
in the near wake, the vortex behavior and their influence on the downwind development of the far wake hinders our capability
to predict wind turbine power production and fatigue loads in wind farms. This calls for a better understanding of the spatial
distribution of the 3D flow and coherent turbulence structures in the near wake. Systematic wind-tunnel experiments were designed
and carried out to characterize the structure of the near-wake flow downwind of a model wind turbine placed in a neutral boundary
layer flow. A horizontal-axis, three-blade wind turbine model, with a rotor diameter of 13 cm and the hub height at 10.5 cm,
occupied the lowest one-third of the boundary layer. High-resolution particle image velocimetry (PIV) was used to measure
velocities in multiple vertical stream-wise planes (x–z) and vertical span-wise planes (y–z). In particular, we identified localized regions of strong vorticity and swirling strength, which are the signature of helicoidal
tip vortices. These vortices are most pronounced at the top-tip level and persist up to a distance of two to three rotor diameters
downwind. The measurements also reveal strong flow rotation and a highly non-axisymmetric distribution of the mean flow and
turbulence structure in the near wake. The results provide new insight into the physical mechanisms that govern the development
of the near wake of a wind turbine immersed in a neutral boundary layer. They also serve as important data for the development
and validation of numerical models. 相似文献
14.
Highly porous two-dimensional (2D) cellular metals have multifunctional attributes, with tailorable structures to achieve
multifunctional performance. The focus of this study is to explore the optimal cellular topology of 2D cellular metals for
heat dissipation, and to investigate the eligibility of different heat enhancement techniques for more efficient heat dissipation.
An analytical approach for the optimal design of metallic 2D cellular materials, cooled by single-phase laminar forced convection
in various flow configurations, is proposed and validated by comparison with full numerical simulations. The optimal design
is characterized by two subsidiary dimensionless parameters: one reflecting the trade-off between convection and fluid friction,
and the other reflecting the optimal balance between conduction and convection. A heat transfer enhancement technique––boundary
layer redevelopment––is subsequently introduced and its feasibility examined experimentally. Future research directions in
specific areas are discussed. 相似文献
15.
G. N. Dudin 《Fluid Dynamics》2000,35(1):101-107
The flow formed as a result of gas injection through the permeable surface of a triangular plate is investigated in the regime
of strong viscous-inviscid interaction between the hypersonic flow and the laminar boundary layer. The features of the flow
past strongly cooled surfaces with the formation of supercritical and subcritical flow regions in the boundary layer are studied.
The injected gas distribution ensuring the existence of self-similar solutions in the supercritical flow regions is obtained.
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 125–133, January–February,
2000.
The study was carried out with the support of the Russian Foundation for Basic Research (project No. 96-01-01391). 相似文献
16.
The stability of a laminar boundary layer flow under natural convection on a vertical isothermally heated wall is studied
analytically. The analysis is performed by using two different two-dimensional linear models: (1) The non-parallel flow model
in which the steady mean flow as well as the disturbance amplitude functions can change in the streamwise direction; (2) The
parallel flow model in which the effects of the mean flow and disturbance changes in the streamwise direction are neglected.
The linear non-parallel stability analysis is based on the so-called parabolised stability equations (PSEs) which have been
successfully applied to the stability analysis of forced convection boundary layers. In this study the PSE equations are applied
to natural convection boundary layers in order to show the difference between parallel and non-parallel stability analysis.
A second part of this study deals with the effects of variable properties, which are always present in natural convection
flows. They are analysed by an extended version of the Orr–Sommerfeld equation (EOSE).
Received on 31 May 2000 相似文献
17.
An experimental study has been performed to improve the understanding of the initial air–liquid interaction in the near field
of an air-blasted breaking water sheet. For the first time, planar laser-induced fluorescence (PLIF) has been used to visualize
the air-flow field, seeding the air streams with acetone vapor. Mie scattering from the liquid sheet, together with the acetone
fluorescence signal has enabled simultaneous determination of the instantaneous water sheet location and the air-flow structures.
The two-phase flow visualization has revealed detachment of the air boundary layer over the air–water interface behind the
zones of strong curvature. The pressure field induced by these vortices has been identified as a cause of the enhanced sheet
flapping and the instability growth.
Received: 30 October 2000/Accepted: 29 March 2001 相似文献
18.
M. V. Ustinov 《Journal of Applied Mechanics and Technical Physics》2000,41(4):658-665
The flow past a flat plate with a blunted leading edge by a flow of a viscous incompressible fluid with a small spanwise-periodic,
steady nonuniformity of the velocity profile is considered. Such a flow simulates the interaction of one type of vortex disturbances
of a turbulent external flow with the boundary layer. The solution obtained predicts generation of strong disturbances in
the boundary layer, which are similar to the streaky structure observed in the case of high free-stream turbulence. It is
shown that the boundary-layer flow on blunted bodies is more sensitive to vortex disturbances than on a plate with a sharp
leading edge.
Central Aerohydrodynamic Institute, Zhukovskii, 140160. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol.
41, No. 4, pp. 93–100, July–August, 2000. 相似文献
19.
I. B. Semenova 《Journal of Applied Mechanics and Technical Physics》1998,39(3):447-453
The velocity field in a neighborhood of the point of contact between the free and solid boundaries is studied numerically
for the problem of noncrucible zone melting in a two-dimensional model formulation. A distinct Prandtl boundary layer on the
solid boundary and a Marangoni boundary layer on the free boundary and high gradients of the longitudinal velocity along the
free boundary in the immediate vicinity of the “cold corner” are observed. It is found for the first time that with distance
from the solid boundary, the velocity curve has a maximum, which is not typical of the ordinary flow near the solid boundary.
Lavrent'ev Institute of Hydrodynamics, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from
Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 39, No. 3, pp. 141–148, May–June, 1998. 相似文献
20.
Studies of the logarithmic layer structure in turbulent boundary layers are presented that span three orders of magnitude change in Reynolds number. The experiments considered used two separate laboratory scale facilities, as well as the atmospheric surface layer at the SLTEST facility in Utah. Several experimental techniques were used in order to probe the three-dimensional nature of the flow structures. The main focus is on two-point correlation statistics at a given z/δ, which are found to agree well over all Reynolds numbers when scaled with an outer length-scale. Large-scale coherence recently noted in the logarithmic region of laboratory-scale boundary layers is also found to be present in the atmospheric surface layer flow. Recent findings regarding the influence of these large scale motions on the near-wall region are also presented. 相似文献