共查询到20条相似文献,搜索用时 31 毫秒
1.
N. V. Nikitin 《Fluid Dynamics》2000,35(2):185-190
The attenuation of turbulent pulsations in near-wall flows by means of spanwise periodic surface oscillation is examined.
A direct numerical simulation of the flow in a circular pipe with imposed rotational oscillations has shown that for Re=4000
and the optimal oscillation frequency, the degree of turbulence attenuation increases with increase in the oscillation amplitude
until the flow relaminarizes. The estimated optimal frequency ω+=0.06. The results of applying the theory of the development of near-wall coherent structures agree qualitatively with those
of numerical simulation. It is concluded that the intensity of the pulsations is reduced because the spanwise movements weaken
the longitudinal vortices which cause turbulent bursts in near-wall flows.
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 2, pp. 37–44, March–April, 2000.
The research was carried out with financial support from the Russian Foundation for Basic Research (project No. 99-01-01095). 相似文献
2.
Fujihiro Hamba 《Theoretical and Computational Fluid Dynamics》2001,14(5):323-336
Large eddy simulation (LES) is combined with the Reynolds-averaged Navier–Stokes (RANS) equation in a turbulent channel-flow
calculation. A one-equation subgrid-scale model is solved in a three-dimensional grid in the near-wall region whereas the
standard k–ε model is solved in a one-dimensional grid in the outer region away from the wall. The two grid systems are overlapped to
connect the two models smoothly. A turbulent channel flow is calculated at Reynolds numbers higher than typical LES and several
statistical quantities are examined. The mean velocity profile is in good agreement with the logarithmic law. The profile
of the turbulent kinetic energy in the near-wall region is smoothly connected with that of the turbulent energy for the k–ε model in the outer region. Turbulence statistics show that the solution in the near-wall region is as accurate as a usual
LES. The present approach is different from wall modeling in LES that uses a RANS model near the wall. The former is not as
efficient as the latter for calculating high-Reynolds-number flows. Nevertheless, the present method of combining the two
models is expected to pave the way for constructing a unified turbulence model that is useful for many purposes including
wall modeling.
Received 11 June 1999 and accepted 15 December 2000 相似文献
3.
In order to reduce the high computational effort of wall-resolved large-eddy simulations (LES), the present paper suggests
a hybrid LES–RANS approach which splits up the simulation into a near-wall RANS part and an outer LES part. Generally, RANS
is adequate for attached boundary layers requiring reasonable CPU-time and memory, where LES can also be applied but demands
extremely large resources. Contrarily, RANS often fails in flows with massive separation or large-scale vortical structures.
Here, LES is without a doubt the best choice. The basic concept of hybrid methods is to combine the advantages of both approaches
yielding a prediction method, which, on the one hand, assures reliable results for complex turbulent flows, including large-scale
flow phenomena and massive separation, but, on the other hand, consumes much fewer resources than LES, especially for high
Reynolds number flows encountered in technical applications. In the present study, a non-zonal hybrid technique is considered
(according to the signification retained by the authors concerning the terms zonal and non-zonal), which leads to an approach
where the suitable simulation technique is chosen more or less automatically. For this purpose the hybrid approach proposed
relies on a unique modeling concept. In the LES mode a subgrid-scale model based on a one-equation model for the subgrid-scale
turbulent kinetic energy is applied, where the length scale is defined by the filter width. For the viscosity-affected near-wall
RANS mode the one-equation model proposed by Rodi et al. (J Fluids Eng 115:196–205, 1993) is used, which is based on the wall-normal
velocity fluctuations as the velocity scale and algebraic relations for the length scales. Although the idea of combined LES–RANS
methods is not new, a variety of open questions still has to be answered. This includes, in particular, the demand for appropriate
coupling techniques between LES and RANS, adaptive control mechanisms, and proper subgrid-scale and RANS models. Here, in
addition to the study on the behavior of the suggested hybrid LES–RANS approach, special emphasis is put on the investigation
of suitable interface criteria and the adjustment of the RANS model. To investigate these issues, two different test cases
are considered. Besides the standard plane channel flow test case, the flow over a periodic arrangement of hills is studied
in detail. This test case includes a pressure-induced flow separation and subsequent reattachment. In comparison with a wall-resolved
LES prediction encouraging results are achieved.
相似文献
4.
A. T. Zinov'ev S. N. Yakovenko 《Journal of Applied Mechanics and Technical Physics》1998,39(6):868-874
A modified model of turbulence is proposed to describe the processes of vertical transport in inhomogeneous turbulent flows.
This model includes algebraic relations for the Reynolds stresses and turbulent-exchange coefficients. Using this model, the
growth of the depth of a mixed layer under the action of the wind load in neutral and stable stratified near-wall flows has
been predicted. The calculation results for a stable stratified flow that were obtained using the modified and standard two-parametric
models of turbulence are compared with experimental data.
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 39, No. 6, pp. 57–64, November–December, 1998. 相似文献
5.
V. N. Vetlutskii V. L. Ganimedov M. I. Muchnaya 《Journal of Applied Mechanics and Technical Physics》2005,46(6):824-834
A two-phase flow with high Reynolds numbers in the subsonic, transonic, and supersonic parts of the nozzle is considered within
the framework of the Prandtl model, i.e., the flow is divided into an inviscid core and a thin boundary layer. Mutual influence
of the gas and solid particles is taken into account. The Euler equations are solved for the gas in the flow core, and the
boundary-layer equations are used in the near-wall region. The particle motion in the inviscid region is described by the
Lagrangian approach, and trajectories and temperatures of particle packets are tracked. The behavior of particles in the boundary
layer is described by the Euler equations for volume-averaged parameters of particles. The computed particle-velocity distributions
are compared with experiments in a plane nozzle. It is noted that particles inserted in the subsonic part of the nozzle are
focused at the nozzle centerline, which leads to substantial flow deceleration in the supersonic part of the nozzle. The effect
of various boundary conditions for the flow of particles in the inviscid region is considered. For an axisymmetric nozzle,
the influence of the contour of the subsonic part of the nozzle, the loading ratio, and the particle diameter on the particle-flow
parameters in the inviscid region and in the boundary layer is studied.
__________
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 6, pp. 65–77, November–December, 2005. 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.
Comparative evaluation of three heat transfer enhancement strategies in a grooved channel 总被引:1,自引:0,他引:1
Results of a comparative evaluation of three heat transfer enhancement strategies for forced convection cooling of a parallel
plate channel populated with heated blocks, representing electronic components mounted on printed circuit boards, are reported.
Heat transfer in the reference geometry, the asymmetrically heated parallel plate channel, is compared with that for the basic
grooved channel, and the same geometry enhanced by cylinders and vanes placed above the downstream edge of each heated block.
In addition to conventional heat transfer and pressure drop measurements, holographic interferometry combined with high-speed
cinematography was used to visualize the unsteady temperature fields in the self-sustained oscillatory flow. The locations
of increased heat transfer within one channel periodicity depend on the enhancement technique applied, and were identified
by analyzing the unsteady temperature distributions visualized by holographic interferometry. This approach allowed gaining
insight into the mechanisms responsible for heat transfer enhancement. Experiments were conducted at moderate flow velocities
in the laminar, transitional and turbulent flow regimes. Reynolds numbers were varied in the range Re = 200–6500, corresponding
to flow velocities from 0.076 to 2.36 m/s. Flow oscillations were first observed between Re = 1050 and 1320 for the basic
grooved channel, and around Re = 350 and 450 for the grooved channels equipped with cylinders and vanes, respectively. At
Reynolds numbers above the onset of oscillations and in the transitional flow regime, heat transfer rates in the investigated
grooved channels exceeded the performance of the reference geometry, the asymmetrically heated parallel plate channel. Heat
transfer in the grooved channels enhanced with cylinders and vanes showed an increase by a factor of 1.2–1.8 and 1.5–3.5,
respectively, when compared to data obtained for the basic grooved channel; however, the accompanying pressure drop penalties
also increased significantly.
Received on 5 April 2001 相似文献
8.
G. V. Kuznetsov M. A. Sheremet 《Journal of Applied Mechanics and Technical Physics》2008,49(6):946-956
Conjugate convective-conductive heat transfer in a rectangular region with forced flow and a heat source is simulated numerically.
Distributions of the thermal and hydrodynamic characteristics of the flow regimes studied are obtained. The evolution of the
process analyzed is shown.
__________
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 6, pp. 69–81, November–December, 2008 相似文献
9.
N. H. Abu-Hamdeh 《Heat and Mass Transfer》2002,38(7-8):687-693
The analytical solutions of equations describing the dynamics of distributed parameter systems are usually complicated in
form and derivations and inconvenient to use for simulation and control system design. The liquid–liquid counter flow heat
exchanger is an example of these disturbed parameter systems. An analytical solution of the dynamics of a symmetrically operated
counter flow heat exchanger in the form of transfer function matrix is investigated in open-loop and close-loop conditions.
The resulting non-linear model was linearized using perturbation approach. A feed-forward path controller to counteract at
any disturbances in the boundary temperature and a non-interactive controller to decouple the outputs were implemented.
Received on 22 February 2001 / Published online: 29 November 2001 相似文献
10.
A new method of visualizing the coherent structures in the boundary layer is used to develop insight into how these structures
form and to provide information on the relative frequency of typical shapes noticed in the near-wall flow. These results were
achieved in a water channel using a recently developed tracer which remains as a moving dye streak while conforming to the
convoluted motions in the boundary layer. The tracer is formulated from a surfactant–polymer–emulsion mixture which retains
its capabilities as a marker of evolving flow motions in the boundary layer for a meter or more before eventually dispersing.
Three-dimensional, continuous visualization of the structures can be obtained as they move along a flat plate. Photos and
video frames demonstrate the evolution and properties of the most widely discussed boundary-layer structure, the Theodorsen
(horseshoe) vortex.
Received: 16 November 1999/Accepted: 24 May 2000 相似文献
11.
In the present study, we employed stereoscopic particle image velocimetry (PIV) to investigate the characteristics of turbulence
structures in a drag-reduced turbulent channel flow with addition of surfactant. The tested drag-reducing fluid was a CTAC/NaSal/Water
(CTAC: cetyltrimethyl ammonium chloride; NaSal: sodium salicylate) system at 25°C. The weight concentration of CTAC was 30 ppm.
Stereoscopic PIV measurement was performed for a water flow (Re=1.1×104) and a CTAC solution flow (Re=1.5×104 with 54% drag reduction) in both the streamwise–spanwise and wall-normal-spanwise planes, respectively. The three-dimensionality
of hairpin vortex structures in the near-wall region for wall-bounded turbulent flow was reproduced by conditionally averaging
the stereoscopic two-dimensional-three-component velocity fields. A series of wall-normal vortex cores were found to align
with the near-wall low-speed streaks with opposite vorticity signals at both sides of the streaks and with the vorticity decreased
on average by about one order of magnitude in CTAC solution flow compared with water flow; the spanwise spacing between the
near-wall low-speed streaks in the solution flow is increased by about 46%. The streamwise vorticity of the vortex cores appearing
in the wall-normal-spanwise plane was also decreased by the use of drag-reducing surfactant additives. 相似文献
12.
M. Yu. Plotnikov 《Journal of Applied Mechanics and Technical Physics》2008,49(3):400-406
A supersonic flow around a cylinder is studied by the direct statistical Monte Carlo method in a wide range of rarefaction:
from regimes close to continuum to free-molecular flow. The effect of the accommodation coefficient on the flow near the cylinder
and on heat transfer between the gas and the cylinder is examined.
__________
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 3, pp. 64–72, May–June, 2008. 相似文献
13.
A numerical study of fluid flow and heat transfer in a two-dimensional channel under fully developed turbulent conditions is reported. A computer program which is capable of treating both forced and natural convection problems under turbulent conditions has been developed. The code uses the high-Reynolds-number form of the two equation turbulent model(k-?) in which a turbulent kinetic energy near-wall model is incorporated in order to accurately represent the behavior of the flow near the wall, particularly in the viscous sublayer where the turbulent Reynolds number is small. A near-wall temperature model has been developed and incorporated into the energy equation to allow accurate prediction of the temperature distribution near the wall and, therefore, accurate calculation of heat transfer coefficients. The sensitivity of the prediction of flow and heat transfer to variations in the coefficients used in the turbulence model is investigated. The predictions of the model are compared to available experimental and theoretical results; good agreement is obtained. The inclusion of the near-wall temperature model has further improved the predictions of the temperature profile and heat transfer coefficient. The results indicate that the turbulent kinetic energy Prandtl number should be a function of Reynolds number. 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.
The laws of heat transfer associated with the interaction of underexpanded supersonic gas jets and obstacles or blunt bodies
have been investigated, for example, in [1–3]. Similar problems of nonuniform flow occur when bodies move in the wake behind
other bodies; however, in this case the laws of heat transfer have so far received little attention [4–8]. It has been established
that for a certain Reynolds number and flow nonuniformity parameters a zone of reverse-circulatory flow develops near the
front of the blunt body. However, the conditions of transition to separated flow have not been determined. This paper presents
a self-similar solution of the equations of the viscous shock layer near the stagnation line in supersonic flow past an axisymmetric
blunt body located behind another body. On the basis of this solution a separationless flow criterion is proposed. The effect
of the nonuniformity and the Reynolds number on the shock standoff distance, the convective heat flux and the friction drag
of the blunt body is investigated.
Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 120–125, November–December, 1986.
In conclusion the authors wish to thank I. G. Eremeitsev for useful suggestions and G. A. Tirskii for discussing their work. 相似文献
16.
V. P. Lebedev V. V. Lemanov S. Ya. Misyura V. I. Terekhov 《Journal of Applied Mechanics and Technical Physics》1998,39(3):428-433
Heat transfer in a jet propagating in a cocurrent flow has been studied over wide ranges of the injection ratio (m=Us/U0<1 and m>1) and flow turbulence (Tu0=0.2–25%). It is shown experimentally that for m<1, a 1% increase in turbulence leads to a 1% increase in heat transfer, and
the wall adiabatic temperature and the relative heat-transfer function should be taken into account in heat-transfer calculations.
For m>1, the flow turbulence does not affect the heat transfer and the heat production can be calculated using the laws typical
of jet flows.
Kutateladze Institute of Thermal Physics, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from
Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 39, No. 3, pp. 119–125, May–June, 1998. 相似文献
17.
The results of a systematic experimental study of the flow turbulence level effect on the heat transfer and Reynolds analogy
coefficients over a wide range of the relevant parameters (the turbulence intensity and scale and the Reynolds number) are
presented. The notion of the equivalent flow turbulence, which unifies the above-mentioned parameters, is introduced. It is
established that the skin friction and heat transfer coefficients increase with the equivalent turbulence, while the Reynolds
analogy coefficient remains unchanged.
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 61–71, January–February,
2000. 相似文献
18.
A. V. Kashevarov 《Fluid Dynamics》1994,29(1):33-37
The problem of the heat exchange of a circular cylinder in an incompressible flow at small Prandtl numbers Pr ≪ 1 is solved.
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 43–48, January–February,
1994. 相似文献
19.
The structure of turbulence in a drag-reduced flat-plate boundary layer flow has been studied with particle image velocimetry (PIV). Drag reduction was achieved by injection of a concentrated polymer solution through a spanwise slot along the test wall at a location upstream of the PIV measurement station. Planes of velocity were measured parallel to the wall (x–z plane), for a total of 30 planes across the thickness of the boundary layer. For increasing drag reduction, we found a significant modification of the near-wall structure of turbulence with a coarsening of the low-speed velocity streaks and a reduction in the number and strength of near-wall vortical structures. 相似文献
20.
Experiments of flow boiling heat transfer and two-phase flow frictional pressure drop in a spirally internally ribbed tube
(φ22×5.5 mm) and a smooth tube (φ19×2 mm) were conducted, respectively, under the condition of 6×105 Pa (absolute atmosphere pressure). The available heated length of the test sections was 2500 mm. The mass fluxes were selected,
respectively, at 410, 610 and 810 kg/m2 s. The maximum heat flux was controlled according to exit quality, which was no more than 0.3 in each test run. The experimental
results in the spirally internally ribbed tube were compared with that in the smooth tube. It shows that flow boiling heat
transfer coefficients in the spirally internally ribbed tube are 1.4–2 times that in the smooth tube, and the flow boiling
heat transfer under the condition of smaller temperature differences can be achieved in the spirally internally ribbed tube.
Also, the two-phase flow frictional pressure drop in the spirally internally ribbed tube increases a factor of 1.6–2 as compared
with that in the smooth tube. The effects of mass flux and pressure on the flow boiling heat transfer were presented. The
effect of diameters on flow boiling heat transfer in smooth tubes was analyzed. Based on the fits of the experimental data,
correlations of flow boiling heat transfer coefficient and two-phase flow frictional factor were proposed, respectively. The
mechanisms of enhanced flow boiling heat transfer in the spirally internally ribbed tube were analyzed.
Received on 1 December 1999 相似文献