共查询到20条相似文献,搜索用时 15 毫秒
1.
Mouloud Haddad Larbi Labraga Laurent Keirsbulck 《Experimental Thermal and Fluid Science》2007,31(8):1021-1032
An experimental investigation is performed on a fully developed turbulent channel flow with local injection through a porous strip. The Reynolds number based on the channel half-width was set to 5000. In addition to the no blowing case, measurements are made for three different blowing rates σ = 0.22, 0.36 and 0.58 (where σ is the ratio of momentum flux gain due to the blowing and momentum flux of the incoming channel flow). Measurements carried out with hot-wire anemometry reveal that injection strongly affects both the velocity profiles and the turbulence characteristics. The injection decreases the skin friction coefficient and increases all the Reynolds stresses downstream the blowing strip. The turbulence structure and the bursting phenomena were examined using space-time correlations measurements and conditional analysis. It is found that the injection increases the frequency of occurrence of the bursts. 相似文献
2.
Transpiration cooling using ceramic matrix composite materials is an innovative concept for cooling rocket thrust chambers. The coolant (air) is driven through the porous material by a pressure difference between the coolant reservoir and the turbulent hot gas flow. The effectiveness of such cooling strategies relies on a proper choice of the involved process parameters such as injection pressure, blowing ratios, and material structure parameters, to name only a few. In view of the limited experimental access to the subtle processes occurring at the interface between hot gas flow and porous medium, reliable and accurate simulations become an increasingly important design tool. In order to facilitate such numerical simulations for a carbon/carbon material mounted in the side wall of a hot gas channel that are able to capture a spatially varying interplay between the hot gas flow and the coolant at the interface, we formulate a model for the porous medium flow of Darcy–Forchheimer type. A finite‐element solver for the corresponding porous medium flow is presented and coupled with a finite‐volume solver for the compressible Reynolds‐averaged Navier–Stokes equations. The two‐dimensional and three‐dimensional results at Mach number Ma = 0.5 and hot gas temperature THG=540 K for different blowing ratios are compared with experimental data. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
3.
U. K. Zhapbasbaev G. Z. Isakhanova 《Journal of Applied Mechanics and Technical Physics》1998,39(1):53-59
Results of a numerical-theoretical study of a developed turbulent flow of an incompressible fluid in a plane channel with
simultaneous injection of mass through one porous wall and suction of the same mass through the other are presented. The system
of equations of averaged motion is closed using a turbulent-stress model. The calculated data of the mean and fluctuational
characteristics are in reasonable agreement with experimental results for two values of the Reynolds number of the main flow
(Re=10,400 and34,000).
Al-Farabi Kazakh National State University, Almaty 480121. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika,
Vol. 39, No. 1, pp. 61–68, January–February, 1998. 相似文献
4.
The onset of buoyancy-driven convection in an initially quiescent ferrofluid saturated horizontal porous layer in the presence
of a uniform vertical magnetic field is investigated. The Brinkman-Lapwood extended Darcy equation with fluid viscosity different
from effective viscosity is used to describe the flow in the porous medium. The lower boundary of the porous layer is assumed
to be rigid-paramagnetic, while the upper paramagnetic boundary is considered to be either rigid or stress-free. The thermal
conditions include fixed heat flux at the lower boundary, and a general convective–radiative exchange at the upper boundary,
which encompasses fixed temperature and fixed heat flux as particular cases. The resulting eigenvalue problem is solved numerically
using the Galerkin technique. It is found that increase in the Biot number Bi, porous parameter σ, viscosity ratio Λ, magnetic susceptibility χ, and decrease in the magnetic number M
1 and non-linearity of magnetization M
3 is to delay the onset of ferroconvection in a porous medium. Further, increase in M
1, M
3, and decrease in χ, Λ, σ and Bi is to decrease the size of convection cells. 相似文献
5.
A computational study is performed on three-dimensional turbulent flow and heat transfer in a rotating rectangular channel
with aspect ratio (AR) of 10:1, oriented 120° from the direction of rotation. The Focus is on high rotation and high-density
ratios effects on the heat transfer characteristics of the 120° orientation. The Reynolds stress model (RSM), which accounts
for rotational effects are used to compute the turbulent flow and heat transfer in the rotating channel. The effects of rotation
and coolant-to-wall density ratio on the fluid flow and heat transfer characteristics is reported on a range of rotation numbers
and density ratios (0 < Ro < 0.25 and 0.07 < Δρ/ρ < 0.4). The computational results are in good agreement with experimental data within ±15%. The results show that the density
ratio, rotation number and channel orientation significantly affect the flow field and heat transfer characteristics in the
rotating rectangular channel. Flow reversal occurs at high rotation number and density ratio. 相似文献
6.
Effect of local forcing on a turbulent boundary layer 总被引:6,自引:0,他引:6
An experimental study is performed to analyze flow structures behind local suction and blowing in a flat-plate turbulent
boundary layer. The local forcing is given to the boundary layer flow by means of a sinusoidally oscillating jet issuing from
a thin spanwise slot at the wall. The Reynolds number based on the momentum thickness is about Re
θ
=1700. The effects of local forcing are scrutinized by altering the forcing frequency (0.011 ≤ f +≤ 0.044). The forcing amplitude is fixed at A
0=0.4. It is found that a small local forcing reduces the skin friction and the skin friction reduction increases with the
forcing frequency. A phase-averaging technique is employed to capture the large-scale vortex evolution. An organized spanwise
vortical structure is generated by the local forcing. The cross-sectional area of vortex and the time fraction of vortex are
examined by changing the forcing frequency. An investigation of the random fluctuation components reveals that turbulent energy
is concentrated near the center of vortical structures.
Received: 17 March 2000/Accepted: 3 April 2001 相似文献
7.
Venkata Bharathi L. Boppana Zheng-Tong Xie Ian P. Castro 《Flow, Turbulence and Combustion》2012,88(3):311-342
Large-eddy simulations of the dispersion from scalar line sources at various locations within a fully developed turbulent
channel flow at Re = uh/ν = 10,400 are presented. Both mean and fluctuating scalar quantities are compared with those from the single available set
of experimental data (Lavertu and Mydlarski, J Fluid Mech 528:135–172, 2005) and differences are highlighted and discussed. The results are also discussed in the context of scalar dispersion in other
kinds of turbulent flows, e.g. homogeneous shear-flow. Initial computations at a much lower Reynolds number are also reported
and compared with the two available direct numerical simulation data sets. 相似文献
8.
An experimental study on unsteady turbulent near wake of a rectangular cylinder in channel flow 总被引:1,自引:0,他引:1
Unsteady turbulent near wake of a rectangular cylinder in channel flow has been studied experimentally with a laser Doppler
velocimetry (LDV). The time-averaged and phase-averaged statistics were measured for the cylinders having various width-to-height
ratios, b/h. It is shown that the turbulent intensities on the centerline of the channel have their maxima near the rear stagnation point
of a recirculation region. The contours of coherent vorticity and streamline reproduce clearly the shed vortices from the
cylinder observed by the flow visualization. The characteristics of the flow field, which depends on b/h, are discussed and the significant contribution of the coherent structure to the flow field is clarified. Moreover, the turbulent
kinetic energy budget has been examined.
Received: 19 January 1998/Accepted: 21 July 1998 相似文献
9.
Giancarlo Alfonsi 《Flow, Turbulence and Combustion》2001,67(2):137-142
Data collected from several studies of experimental and numerical nature in wall-bounded turbulent flows and in particular
in internal flows (channel and pipe flows, Mochizuki and Nieuwstadt [1]) at different Reynolds numbers R
+(Ru
*/ν), indicate that: (i) the peak of the rms-value (normalized by u
*) of the streamwise velocity fluctuations (σ
u
+|peak) is essentially independent of the Reynolds number, (ii) the position of the rms peak value (y
+|peak) is weakly dependent of the Reynolds number, (iii) the skewness of the streamwise velocity fluctuations (S
u
) is close to zero at the position in which the variance has its peak. A series of measurements of streamwise velocity fluctuations
has been performed in turbulent pipe flow with the use of an Ultrasonic Doppler Velocimeter and our results support those
reported in [1].
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
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
11.
Reynolds-number-dependence of the maximum in the streamwise velocity fluctuations in wall turbulence
A survey is made of the standard deviation of the streamwise velocity fluctuations in near-wall turbulence and in particular
of the Reynolds-number-dependency of its peak value. The following canonical flow geometries are considered: an incompressible
turbulent boundary layer under zero pressure gradient, a fully developed two-dimensional channel and a cylindrical pipe flow.
Data were collected from 47 independent experimental and numerical studies, which cover a Reynolds number range of R
θ=U
∞
θ/v=300−20,920 for the boundary layer with θ the momentum thickness and R
+=u
*R/v=100-4,300 for the internal flows with R the pipe radius or the channel half-width. It is found that the peak value of the rms-value normalised by the friction velocity,
u
*, is within statistical errors independent of the Reynolds number. The most probable value for this parameter was found to
be 2.71±0.14 and 2.70±0.09 for the case of a boundary layer and an internal flow, respectively. The present survey also includes
some data of the streamwise velocity fluctuations measured over a riblet surface. We find no significant difference in magnitude
of the normalised peak value between the riblet and smooth surfaces and this property of the normalised peak value may for
instance be exploited to estimate the wall shear stress from the streamwise velocity fluctuations. We also consider the skewness
of the streamwise velocity fluctuations and find its value to be close to zero at the position where the variance has its
peak value. This is explained with help of the equations of the third-order moment of velocity fluctuations. These results
for the peak value of the rms of the streamwise velocity fluctuations and also the coincidence of this peak with the zero
value of the third moment can be interpreted as confirmation of local equilibrium in the near-wall layer, which is the basis
of inner-layer scaling. Furthermore, these results can be also used as a requirement which turbulence models for the second
and triple velocity correlations should satisfy.
The authors are indebted to Prof. P. Bradshaw for making available his list of references on this topic and for his remarks
on “active” and “inactive” motions. We also gratefully acknowledge discussions with Prof. I. Castro regarding the value of
σ
u
+
above rough walls. 相似文献
12.
The effects of material, geometry, length and position of the porous channels on energy transfer in air-filled enclosures
carrying a compressible pulsating wave are investigated. The pulsating fluid motion is created by an acoustic driver in a
resonant chamber. Three different porous materials (Corning Celcor, Reticulated Vitreous Carbon (RVC), and Mylar plastic),
three different geometries (square, open foam, and circular cross-section), six different lengths, “L” (varying between 1 and 6.5 cm, L = 0.01–0.068 λ, where λ is the wavelength of the fundamental acoustic mode), and eight different positions (hot end of the
channel, varying between 0.5 and 8 cm) of the channels from the pressure anti-node is experimentally measured. The surface
temperature distribution on the channel wall and temperature difference generated across the channel walls are measured while
energy flow along the channel walls is calculated analytically. The experimental results are compared with a 1-D numerical
code and found excellent agreement. The material, geometry, length, and position of the porous channel strongly affect the
energy interactions between the porous channel and the working fluid. The temperature difference generated across the porous
RVC channel increases as the porosity increases form 20 to 80 PPI; but decreases if the porosity increases further. Corning
Celcor shows improved temperature difference generated across the channel as the length of the channel increases; but then
decreases if the length is further increased. The results of this study are applicable to the design of thermoacoustic devices. 相似文献
13.
Zouhaier Mehrez Mourad Bouterra Afif El Cafsi Ali Belghith Patrick Le Quere 《Heat and Mass Transfer》2009,46(1):107-112
A numerical study based on the large eddy simulation methodology was made of heat transfer in locally disturbed turbulent
separated and reattached flow over a backward facing step. The local disturbance was given to the flow by a sinusoidally blowing/suction
of the fluid into a separated shear layer. The Reynolds number was fixed at 33,000 and Richardson number at 0.5. The disturbance
frequency was varied in the range 0 ≤ St ≤ 2, where St is the Strouhal number of disturbance. The obtained results revealed the existence of an optimum perturbation frequency value,
St = 0.25, in terms of the reduced reattachment length. At this frequency the heat transfer is significantly enhanced in the
recirculation zone. The influence of the frequency and the amplitude of disturbance, in the maximum heat transfer positions
and the maximum local Nusselt number, is analysed. 相似文献
14.
Simultaneous velocity and concentration fields in a confined liquid-phase rectangular jet with a Reynolds number based on
the hydraulic diameter of 50,000 (or 10,000 based on the velocity difference between streams and the jet exit dimension) and
a Schmidt number of 1,250 were obtained by means of a combined particle image velocimetry (PIV) and planar laser-induced fluorescence
(PLIF) system. Data were collected at the jet exit and six further downstream locations. The velocity and concentration field
data were analyzed for flow statistics such as turbulent fluxes, turbulent viscosity and diffusivity, and turbulent Schmidt
number (Sc
T
). The streamwise turbulent flux was found to be larger than the transverse turbulent flux, and the mean concentration gradient
was not aligned with the turbulent flux vector. The average Sc
T
was found to vary both in streamwise and in cross stream directions and had a mean value around 0.8, a value consistent with
the literature. Spatial correlation fields of turbulent fluxes and concentration were then determined. The R
u′ϕ′ correlation was elliptical in shape with a major axis tilted downward with respect to the streamwise axis, whereas the R
v′ϕ′ correlation was an ellipse with a major axis aligned with the cross-stream direction. Negative regions of R
u′ϕ′ were observed in the outer streams, and these negatively correlated regions decayed with downstream distance and finally
disappeared altogether. The R
ϕ′ϕ′ correlation field was found to be an ellipse with the major axis inclined at about 45° with respect to the streamwise direction.
Linear stochastic estimation was used to interpret spatial correlation data and to determine conditional flow structures.
It is believed that a vortex street formed near the splitter plate is responsible for the negatively correlated region observed
in the R
u′ϕ′ spatial correlations of turbulent fluxes. A positive concentration fluctuation event was observed to correspond to a finger
of nearly uniform concentration fluid reaching out into the outer stream, whereas a negative event corresponds to a pocket
of nearly uniform fluid being entrained from the outer stream into the center jet region. Large-scale vortical structures
were observed in the conditional velocity fields with an elliptical shape and a streamwise major axis. The growth of the structure
size increased linearly initially but then grew more slowly as the flow transitioned toward channel flow.
Support of this work was provided by the National Science Foundation through grants CTS-9985678 and CTS-0336435 and by the
Dow Chemical Company. The author greatly acknowledge Charles Lipp at Dow Chemical and Ken Junk at Emerson Fisher for their
valuable assistance in the design and construction of the flow system. 相似文献
15.
In high-velocity open channel flows, the measurements of air–water flow properties are complicated by the strong interactions
between the flow turbulence and the entrained air. In the present study, an advanced signal processing of traditional single-
and dual-tip conductivity probe signals is developed to provide further details on the air–water turbulent level, time and
length scales. The technique is applied to turbulent open channel flows on a stepped chute conducted in a large-size facility
with flow Reynolds numbers ranging from 3.8E+5 to 7.1E+5. The air water flow properties presented some basic characteristics
that were qualitatively and quantitatively similar to previous skimming flow studies. Some self-similar relationships were
observed systematically at both macroscopic and microscopic levels. These included the distributions of void fraction, bubble
count rate, interfacial velocity and turbulence level at a macroscopic scale, and the auto- and cross-correlation functions
at the microscopic level. New correlation analyses yielded a characterisation of the large eddies advecting the bubbles. Basic
results included the integral turbulent length and time scales. The turbulent length scales characterised some measure of
the size of large vortical structures advecting air bubbles in the skimming flows, and the data were closely related to the
characteristic air–water depth Y
90. In the spray region, present results highlighted the existence of an upper spray region for C > 0.95–0.97 in which the distributions of droplet chord sizes and integral advection scales presented some marked differences
with the rest of the flow. 相似文献
16.
Particle tracking velocimetry (PTV) is applied to a bubbly two-phase turbulent flow in a horizontal channel at Re = 2 × 104 to investigate the turbulent shear stress profile which had been altered by the presence of bubbles. Streamwise and vertical velocity components of liquid phase are obtained using a shallow focus imaging method under backlight photography. The size of bubbles injected through a porous plate in the channel ranged from 0.3 to 1.5 mm diameter, and the bubbles show a significant backward slip velocity relative to liquid flow. After bubbles and tracer particles are identified by binarizing the image, velocity of each phase and void fraction are profiled in a downstream region. The turbulent shear stress, which consists of three components in the bubbly two-phase flow, is computed by analysis of PTV data. The result shows that the fluctuation correlation between local void fraction and vertical liquid velocity provides a negative shear stress component which promotes frictional drag reduction in the bubbly two-phase layer. The paper also deals with the source of the negative shear stress considering bubble’s relative motion to liquid. 相似文献
17.
Scalar transport from a point source in flows over wavy walls 总被引:1,自引:0,他引:1
Simultaneous measurements of the velocity and concentration field in fully developed turbulent flows over a wavy wall are
described. The concentration field originates from a low-momentum plume of a passive tracer. PLIF and digital particle image
velocimetry are used to make spatially resolved measurements of the structure of the scalar distribution and the velocity.
The measurements are performed at three different Reynolds numbers of Re
b = 5,600, Re
b = 11,200 and Re
b = 22,400, respectively, based on the bulk velocity u
b and the total channel height 2h. The velocity field and the scalar field are investigated in a water channel with an aspect ratio of 12:1, where the bottom
wall of the test section consists of a train of sinusoidal waves. The wavy wall is characterized by the amplitude to wavelength
ratio α = 0.05 and the ratio β between the wave amplitude and the half channel height where β = 0.1. The scalar is released
from a point source at the wave crest. For the concentration measurements, Rhodamine B is used as tracer dye. At low to moderate
Reynolds number, the flow field is characterized through a recirculation zone which develops after the wave crest. The recirculation
zone induces high intensities of the fluctuations of the streamwise velocity and wall-normal velocity. Furthermore, large-scale
structures are apparent in the flow field. In previous investigations it has been shown that these large-scale structures
meander laterally in flows over wavy bottom walls. The investigations show a strong effect of the wavy bottom wall on the
scalar mixing. In the vicinity of the source, the scalar is transported by packets of fluid with a high scalar concentration.
As they move downstream, these packets disintegrate into filament-like structures which are subject to strong gradients between
the filaments and the surrounding fluid. The lateral scale of the turbulent plume is smaller than the lateral scale of the
large-scale structures in the flow field and the plume dispersion is dominated by the structures in the flow field. Due to
the lateral meandering of the large-scale structures of the flow field, also the scalar plume meanders laterally. Compared
to turbulent plumes in plane channel flows, the wavy bottom wall enhances the mixing effect of the turbulent flow and the
spreading rate of the scalar plume is increased. 相似文献
18.
Rafael Cortell 《Meccanica》2012,47(3):769-781
An analysis is presented for the steady non-linear viscous flow of an incompressible viscous fluid over a horizontal surface
of variable temperature with a power-law velocity under the influences of suction/blowing, viscous dissipation and thermal
radiation. Numerical results are illustrated by means of tables and graphs. The governing partial differential equations are
converted into nonlinear ordinary differential equations by a similarity transformation. The effects of the stretching parameter
n, suction/blowing parameter b, Prandtl number σ, Eckert number Ec(Ec * )E_{c}(E_{c}^{ *} ) and radiation parameter N
R
are discussed. Two cases are studied, namely, (i) Prescribed surface temperature (PST case) and, (ii) Prescribed heat flux
at the sheet (PHF case). 相似文献
19.
Measurement of the gradient field of a turbulent free surface 总被引:1,自引:1,他引:0
We study the free surface above a turbulent channel flow. We describe a laser scanning technique that can be used to measure the space–time turbulent surface gradient field along a line. A harmonically swiveling laser beam is focused on the surface and its angle of refraction is measured using a position sensing device. The registered signals can be converted easily to the desired gradient field, and spectra and correlations can be measured. Examples of measured spectra and correlation functions of the surface above a turbulent channel flow (Reynolds number R
λ ≈ 250) demonstrate the viability of the technique. We further assess the validity of Taylor’s frozen turbulence hypothesis that implies that time-dependent signals measured along a line that is oriented perpendicularly to the mean channel velocity can be interpreted as 2D measurements of the surface slope. While Taylor’s hypothesis works for a turbulent velocity field, it does not work for its free surface.
相似文献
Willem van de WaterEmail: |
20.
Experiments on the modulation characteristics of the wall shear stress τ′-longitudinal velocity u′ and u′−u′ space–time correlations are reported in a forced turbulent channel flow in a wide range of imposed frequencies. The resulting
integral and Taylor scale properties are discussed in detail in the low buffer layer under steady and unsteady flow conditions.
It is shown that the small-scale turbulence is sensitive to the imposed unsteadiness since the amplitude and phase of the
Taylor length scale vary considerably in the imposed frequency range investigated here. The Taylor hypothesis is acceptably
valid in steady and unsteady wall layers just above the low buffer layer. Production and instantaneous pressure gradients
are mostly responsible for the deviation of the frozen turbulence-state in the viscous and low buffer sublayers. 相似文献