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1.
Large-Eddy simulations (LES) of spatially evolving turbulent buoyant round jets have been carried out with two different density
ratios. The numerical method used is based on a low-Mach-number version of the Navier–Stokes equations for weakly compressible
flow using a second-order centre-difference scheme for spatial discretization in Cartesian coordinates and an Adams–Bashforth
scheme for temporal discretization. The simulations reproduce the typical temporal and spatial development of turbulent buoyant
jets. The near-field dynamic phenomenon of puffing associated with the formation of large vortex structures near the plume
base with a varicose mode of instability and the far-field random motions of small-scale eddies are well captured. The pulsation
frequencies of the buoyant plumes compare reasonably well with the experimental results of Cetegen (1997) under different
density ratios, and the underlying mechanism of the pulsation instability is analysed by examining the vorticity transport
equation where it is found that the baroclinic torque, buoyancy force and volumetric expansion are the dominant terms. The
roll-up of the vortices is broken down by a secondary instability mechanism which leads to strong turbulent mixing and a subsequent
jet spreading. The transition from laminar to turbulence occurs at around four diameters when random disturbances with a 5%
level of forcing are imposed to a top-hat velocity profile at the inflow plane and the transition from jet-like to plume-like
behaviour occurs further downstream. The energy-spectrum for the temperature fluctuations show both −5/3 and −3 power laws,
characteristic of buoyancy-dominated flows. Comparisons are conducted between LES results and experimental measurements, and
good agreement has been achieved for the mean and turbulence quantities. The decay of the centreline mean velocity is proportional
to x
−1/3 in the plume-like region consistent with the experimental observation, but is different from the x
−1 law for a non-buoyant jet, where x is the streamwise location. The distributions of the mean velocity, temperature and their fluctuations in the near-field
strongly depend upon the ratio of the ambient density to plume density ρa/ρ0. The increase of ρa/ρ0 under buoyancy forcing causes an increase in the self-similar turbulent intensities and turbulent fluxes and an increase
in the spatial growth rate. Budgets of the mean momentum, energy, temperature variance and turbulent kinetic energy are analysed
and it is found that the production of turbulence kinetic energy by buoyancy relative to the production by shear is increased
with the increase of ρa/ρ0.
Received 16 June 2000 and accepted 26 June 2001 相似文献
2.
Hot-wire anemometry is a measuring technique that is widely employed in fluid mechanics research to study the velocity fields
of gas flows. It is general practice to calibrate hot-wire sensors against velocity. Calibrations are usually carried out
under atmospheric pressure conditions and these suggest that the wire is sensitive to the instantaneous local volume flow
rate. It is pointed out, however, that hot wires are sensitive to the instantaneous local mass flow rate and, of course, also
to the gas heat conductivity. To calibrate hot wires with respect to mass flow rates per unit area, i.e., with respect to
(ρU), requires special calibration test rigs. Such a device is described and its application is summarized within the (ρU) range 0.1–25 kg/m2 s. Calibrations are shown to yield the same hot-wire response curves for density variations in the range 1–7 kg/m3. The application of the calibrated wires to measure pulsating mass flows is demonstrated, and suggestions are made for carrying
out extensive calibrations to yield the (ρU) wire response as a basis for advanced fluid mechanics research on (ρU) data in density-varying flows. 相似文献
3.
The problem of heat convection from a vertically oscillating cylinder in a quiescent fluid is investigated. The governing
equations of motion and energy are solved numerically in a non-inertial frame of references to determine the flow field and
heat transfer characteristics under different conditions. The main dominating parameters are Keulegan–Carpenter number, KC,
frequency parameter, β, Grashof number, Gr and Prandtl number, Pr. The ranges considered for these parameters are KC ≤ 10,
β≤40 and Gr ≤ 105 while Prandtl number is kept constant. The study revealed that the effect of amplitude and frequency of oscillation on heat
transfer is strongly influenced by the Grashof number range. In the forced convection regime (Gr = 0), the increase of KC
creates extensive vortex motion at all cylinder positions that leads to a significant increase in heat transfer. A similar
trend, but with a lesser extent, is also observed for the increase of β. However, at high Grashof numbers, the effect of oscillation
on heat convection is only significant at large values of KC.
Received on 5 June 2000 / Published online: 29 November 2001 相似文献
4.
In this paper, we propose a novel method for evaluating the frequency response of shock accelerometers using Davies bar and
interferometry. The method adopts elastic wave pulses propagating in a thin circular bar for the generation of high accelerations.
The accelerometer to be examined is attached to one end of the bar and experiences high accelerations of the order of 103∼105 m/s2. A laser interferometer system is newly designed for the absolute measurement of the bar end motion. It can measure the motion
of a diffuse surface specimen at a speed of 10−3 ∼100 m/s. Uncertainty of the velocity measurement is estimated to be±6×10−4 m/s, proving a high potential for use in the primary calibration of shock accelerometers. Frequency characteristics of the
accelerometer are determined by comparing the accelerometer's output with velocity data of the interferometry in the frequency
domain. Two piezoelectric-type accelerometers are tested in the experiment, and their frequency characteristics are obtained
over a wide frequency range up to several ten kilohertz. It is also shown that the results obtained using strain gages are
consistent with those by this new method.
Paper was presented at the 1992 SEM Spring Conference on Experimental Mechanics held in Las Vegas, NV on June 8–11. 相似文献
5.
A wedge subjected to tractions in proportion tor
n
(n≥0), is considered. The stresses in the solutions of the classical theory of elasticity become infinite when the angle of the
wedge is ρ or 2ρ. The paradox has been resolved by Dempsey[4] and T.C.T. Ting[5] whenn=0. The purpose of this paper is to resolve the paradox whenn>0. 相似文献
6.
Laure Saint-Raymond 《Archive for Rational Mechanics and Analysis》2000,155(3):171-199
Using the stability results of Bressan & Colombo [BC] for strictly hyperbolic 2 × 2 systems in one space dimension, we prove that the solutions of isentropic and non-isentropic
Euler equations in one space dimension with the respective initial data (ρ0, u
0) and (ρ0, u
0, &\theta;0=ρ0
γ− 1) remain close as soon as the total variation of (ρ0, u
0) is sufficiently small.
Accepted April 25, 2000?Published online November 24, 2000 相似文献
7.
Bernard Ducomet ?��rka Ne?asov�� Alexis Vasseur 《Journal of Mathematical Fluid Mechanics》2011,13(2):191-211
We consider the Cauchy problem for the equations of spherically symmetric motions in
\mathbb R3{\mathbb {R}^3}, of a selfgravitating barotropic gas, with possibly non monotone pressure law, in two different situations: in the first
one we suppose that the viscosities μ(ρ), and λ(ρ) are density-dependent and satisfy the Bresch–Desjardins condition, in the second one we consider constant densities. In
the two cases, we prove that the problem admits a global weak solution, provided that the polytropic index γ satisfy γ > 1. 相似文献
8.
High-speed tomographic PIV was used to investigate the coalescence of drops placed on a liquid/liquid interface; the coalescence
of a single drop and of a drop in the presence of an adjacent drop (side-by-side drops) was investigated. The viscosity ratio
between the drop and surrounding fluids was 0.14, the Ohnesorge number (Oh = μd/(ρdσD)1/2) was 0.011, and Bond numbers (Bo = (ρ
d
− ρ
s
)gD
2/σ) were 3.1–7.5. Evolving volumetric velocity fields of the full coalescence process allowed for quantification of the velocity
scales occurring over different time scales. For both single and side-by-side drops, the coalescence initiates with an off-axis
film rupture and film retraction speeds an order of magnitude larger than the collapse speed of the drop fluid. This is followed
by the formation and propagation of an outward surface wave along the coalescing interface with wavelength of approximately
2D. For side-by-side drops, the collapse of the first drop is asymmetric due to the presence of the second drop and associated
interface deformation. Overall, tomographic PIV provides insight into the flow physics and inherent three-dimensionalities
in the coalescence process that would not be achievable with flow visualization or planar PIV only. 相似文献
9.
Global Behavior of Compressible Navier-Stokes Equations with a Degenerate Viscosity Coefficient 总被引:2,自引:0,他引:2
In this paper, we study a free boundary problem for compressible Navier-Stokes equations with density-dependent viscosity.
Precisely, the viscosity coefficient μ is proportional to ρ
θ
with
, where ρ is the density, and γ > 1 is the physical constant of polytropic gas. Under certain assumptions imposed on the initial data, we obtain the global
existence and uniqueness of the weak solution, give the uniform bounds (with respect to time) of the solution and show that
it converges to a stationary one as time tends to infinity. Moreover, we estimate the stabilization rate in L
∞ norm, (weighted) L
2 norm and weighted H
1 norm of the solution as time tends to infinity. 相似文献
10.
Kalabin et al. (Numer. Heat Transfer A 47, 621-631, 2005) studied the unsteady natural convection for the sinusoidal oscillating wall temperature on one side wall
and constant average temperature on the opposing side wall. The present article is on the unsteady natural convective heat
transfer in an inclined porous cavity with similar temperature boundary conditions as those of Kalabin et al. The inclined
angle of the cavity is varied from 0° to 80°. The flow field is modeled with the Brinkman-extended Darcy model. The combined effects
of inclination angle of the enclosure and oscillation frequency of wall temperature are studied for Ra* = 103, Da = 10−3, , and Pr=1. Some results are also obtained with the Darcy–Brinkman–Forchheimer model and Darcy’s law and are compared with the present
Brinkman-extended Darcy model. The maximal heat transfer rate is attained at the oscillating frequency f = 46.7π and the inclined angle . 相似文献
11.
Laminar mixed convection from a continuously moving vertical surface with suction or injection 总被引:1,自引:0,他引:1
The boundary layer flow over a uniformly moving vertical surface with suction or injection is studied when the buoyancy forces
assist or oppose the flow. Similarity solutions are obtained for the boundary layer equations subject to power law temperature
and velocity boundary conditions. The effect is of various governing parameters, such as Prandtl number Pr, temperature exponent
n, injection parameter d, and the mixed convection parameter λ=Gr/Re2, which determine the velocity and temperature distributions and the heat transfer coefficient, are studied. The heat transfer
coefficient increases as λ assisting the flow for all d at Pr=0.72 however, for n=−1 it decreases sharply with λ. On the other hand, increasing λ has no effect on heat transfer coefficient for Pr=10 at n=0, and 1 for almost all values of d studied. However, for n=−1 it has similar effect as for Pr=0.72. It is also found that Nusselt number increases as n increases for fixed λ and d.
Received on 26 March 1997 相似文献
12.
In this paper, an artificial neural network (ANN) for predicting critical heat flux (CHF) of concentric-tube open thermosiphon
has been trained successfully based on the experimental data from the literature. The dimensionless input parameters of the
ANN are density ratio, ρ
l/ρ
v; the ratio of the heated tube length to the inner diameter of the outer tube, L/D
i; the ratio of frictional area, d
i/(D
i + d
o); and the ratio of equivalent heated diameter to characteristic bubble size, D
he/[σ/g(ρ
l−ρ
v)]0.5, the output is Kutateladze number, Ku. The predicted values of ANN are found to be in reasonable agreement with the actual values from the experiments with a mean
relative error (MRE) of 8.46%. New correlations for predicting CHF were also proposed by using genetic algorithm (GA) and
succeeded to correlate the existing CHF data with better accuracy than the existing empirical correlations. 相似文献
13.
Critical heat flux (CHF) has been measured in saturated forced convective boiling with a wall jet on a rectangular heated
surface of 40 and 80 mm in length and 20 mm in width. The jet velocity is varied from 3 to 15 m/s, and the system pressure
is 0.1, 0.2, and 0.4 MPa for R113. It is found that the existing correlation for saturation condition can be applied to the
CHF at high and low ρl/ρg values (e.g. water and R22), but hardly to the CHF at medium ρl/ρg values (e.g. R113 at 0.2 and 0.4 MPa). A revised correlation is proposed to predict most of the CHF data within an accuracy
of ±25%.
Received on 26 April 1999 相似文献
14.
We prove a blow-up criterion in terms of the upper bound of (ρ, ρ
−1, θ) for a strong solution to three dimensional compressible viscous heat-conductive flows. The main ingredient of the proof
is an a priori estimate for a quantity independently introduced in Haspot (Regularity of weak solutions of the compressible isentropic Navier–Stokes equation, arXiv:1001.1581, 2010) and Sun et al. (J Math Pure Appl 95:36–47, 2011), whose divergence can be viewed as the effective viscous flux. 相似文献
15.
The onset of convective instability in an initially quiescent, stably stratified fluid layer between two horizontal plates
is analyzed with linear theory. The bottom boundary is heated suddenly from below, subjected to a step change in surface temperature.
The critical time t
c to mark the onset of Rayleigh-Bénard convection is predicted by propagation theory. This theory uses the length scaled by
, where α denotes thermal diffusivity. Under the normal mode analysis the dimensionless disturbance equations are obtained
as a function of τ(=αt/d
2) and ζ(=Z/), where d is the fluid layer depth and Z is the vertical distance. The resulting equations are transformed to self-similar ones by using scaling and finally fixing
τ as τc under the frame of coordinates τ and ζ. For a given γ, Pr and τc, the minimum value of Ra is obtained from the marginal stability curve. Here γ denotes the temperature ratio to represent the degree of stabilizing
effect, Pr is the Prandtl number and Ra is the Rayleigh number. With γ=0, the minimum Ra value approaches the well-known value of 1708 as τc increases. However, it is inversely proportional to τc
3/2 as τc decreases. With increasing γ, the system becomes more stable. It is interesting that in the present system, propagation theory
produces the stability criteria to bound the available experimental data over the whole domain of time.
Received 5 November 2001 and accepted 29 March 2002 Published online: 2 October 2002
RID="*"
ID="*" This work has been supported by both SK Chemicals Co. Ltd. and LG Chemical Ltd., Seoul under the Brain Korea 21 Project
of the Ministry of Education.
Communicated by H.J.S. Fernando 相似文献
16.
A. Pantokratoras 《Heat and Mass Transfer》2000,36(4):351-360
In most studies concerning laminar natural convection along a vertical isothermal cylinder a linear relationship between
fluid density and temperature has been used and kinematic viscosity and thermal diffusivity have been considered constant
calculated at ambient temperature. However, it is known that the density–temperature relationship for water is non-linear
at low temperatures and kinematic viscosity and thermal diffusivity are functions of temperature. In this study the problem
of laminar natural convection of pure and saline water along a vertical isothermal cylinder has been investigated in the temperature
range between 20 and 0 ∘C taking into account the temperature dependence of ν, α and ρ. The results are obtained with the numerical solution of the
boundary layer equations. The variation of ν, α and ρ with temperature has a strong influence on free convection characteristics.
Received on 17 May 1999 相似文献
17.
Andreas Acrivos 《Applied Scientific Research》1980,36(1):35-40
The temperature profile in a circular tube of infinite extent through which a fluid is moving under conditions of small Péclet
numbersε is determined by means of an asymptotic analysis inε. The walls of the tube are heated forx>0 and are insulated whenx<0. It is shown that the heated region extends anO(ε
−1) distance — relative to the radius of the tube — upstream of the pointx=0, and that convective effects remain important even whenε→0. These results apply to a wider class of problems in which the Péclet number is small. 相似文献
18.
19.
This work aimed at improving fine-scale measurements using cold-wire anemometry. The dissipation ɛ
θ
of the temperature variance was measured on the axis of a heated turbulent round jet. The measurements were performed with a constant current anemometer
(CCA) operating fine Pt–10%Rh wires at very low overheat. The CCA developed for this purpose allowed the use of the current
injection method in order to estimate the time constant of the wire. In the first part of the paper, it is shown that the
time constants obtained for two wire diameters −d=1.2 and d=0.58 μm – compare well with those measured at the same time using two other methods (laser excitation and pulsed wire). Moreover,
for these two wires, the estimated time constants were in good agreement with those obtained from a semi-empirical relation.
In the second part of the paper, a compensation procedure – post-processing filtering – was developed in order to improved
the frequency response of the cold-wire probes. The measurements carried out on the axis of the jet (Re
D
=16 500, Re
λ
≃ 167) showed that the frequency response of the 1.2 μm wire was significantly improved. In fact, the spectral characteristics
of the compensated signal obtained with the 1.2 μm wire compared fairly well with those from the 0.58 μm wire. Moreover, the
results indicated that the compensation procedure must be applied when the cut-off frequency of the cold-wire f
c is lower than two times the Kolmogorov frequency f
K. In the case where f
c ≃ 0.6f
K, the compensation procedure can reduce the error in the estimate of ɛ
θ
by more than 20%. When f
c ≃ 2f
K, the effect of the compensation is reduced to about 5%.
Received: 3 November 2000/Accepted: 23 March 2001 相似文献
20.
Three-dimensional vorticity in the wake of an inclined stationary circular cylinder was measured simultaneously using a multi-hot
wire vorticity probe over a streamwise range of x/d = 10–40. The study aimed to examine the dependence of the wake characteristics on cylinder inclination angle α (=0°–45°).
The validity of the independence principle (IP) for vortex shedding was also examined. It was found that the spanwise mean
velocity which represents the three-dimensionality of the wake flow, increases monotonically with α. The root-mean-square (rms) values
of the streamwise (u) and spanwise (w) velocities and the three vorticity components decrease significantly with the increase of α, whereas the transverse velocity
(v) does not follow the same trend. The vortex shedding frequency decreases with the increase of α. The Strouhal number (St
N), obtained by using the velocity component normal to the cylinder axis, remains approximately a constant within the experimental
uncertainty (±8%) when α is smaller than about 40°. The autocorrelation coefficients ρ
u
and ρ
v
of the u and v velocity signals show apparent periodicity for all inclination angles. With increasing α, ρ
u
and ρ
v
decrease and approach zero quickly. In contrast, the autocorrelation coefficient ρ
w
of w increases with α in the near wake, implying an enhanced three-dimensionality of the wake. 相似文献