Defect and Diffusion Forum Vols. 283-286 (2009) pp 346-351 Online available since 2009/Mar/02 at www.scientific.net © (2009) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/DDF.283-286.346
Experimental and Numeric Study of Flow Around a Parallelepiped Obstacle Issued from a Bent Chimney Inès Bhouri Baouab1a, Nejla Mahjoub Said1b, Hatem Mhiri1c, Georges Le Palec2d, Philippe Bournot2e 1
Unité de Thermique et Environnement, Ecole Nationale d’Ingénieurs de Monastir, Route de Ouardanine 5000 Moastir, Tunisie
2
Institut de Mécanique de Marseille, 60 rue Juliot Curie Technopôle de Château-Gombert 13453 MARSEILLE Cedex 13, France a
[email protected],
[email protected],
[email protected], d
[email protected],
[email protected]
Keywords: Bent chimney, obstacle, PIV, distance.
Abstract. Our work consisted in carrying out an experimental investigation in order to study the structure of the flow field issuing from a bent chimney and deviated when meeting a parallelepiped obstacle. For the matter we used: the Particle Image Velocimetry (PIV). A parallel numeric simulation of the problem was also elaborated and compared with the above mentioned experimental results. A three-dimensional numerical model based on the RMS turbulence closure model was used. The adopted grid is not uniform, particularly refined near the chimney and around the obstacle. A good level of agreement was achieved between the experimental data and numerical calculations. Once the model validated, we studied the effects of the distance separating the chimney and the obstacle. The tested values are Dch-obs= 10 cm and Dch-obs= 20 cm. Introduction The interaction of continuous flumes released from point sources of buildings and other structures is the major factor affecting dispersion of atmospheric pollutants in urban areas. This problem has been the subject of research by experiments and also by using numerical simulation. Alan Huber et al. [1] studied the influence of building width and orientation on plume issued from a stack in presence of building. The stack height was at ground level or 1.5 times the height of the building. The velocity and turbulence data were obtained with constant-temperature hot wire anemometers. They showed that the influence of the flow end around the sides of the building had less effect on wider buildings, in deed for the ground-level source, an oblique angle resulted in a maximum ground-level concentration increase by a factor of 2-3 at three building heights downstream. Finally they proved that concentrations were increased by less than a factor of 1.5 at ten building heights downstream. I. Marvoidis et al. [2] examined in scaled field and wind tunnel experiments the dispersion of atmospheric pollutant in the vicinity of isolated obstacles of different shape (cube, cylinder and taller obstacle) and orientation with respect to the mean wind direction. The concentration measurements were supported by meteorological data collected by an ultrasonic anemometer positioned upwind of the obstacle. The result shows that concentrations are affected by obstacle shape. Mahjoub et al. [3] study the flow field structure around three dimensional circular cylinders. The Particle Image Velocimetry (PIV) technique is used to measure the velocity field in the vicinity of the circular cylinder obstacle. The results show the dependence of the flow structure around the obstacle on its Reynolds number, and the spacing between a pair of obstacles. They compared the experimental results with the numerical model. Overall, the agreement between the numerical results and laboratory velocity measurements is good. In another numerical work Mahjoub et al. [4] studied the dispersion of a pollutant around a twodimensional obstacle. The pollutant issued from the chimney is constituted by a mixture of sulfur – All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 41.229.65.11-12/04/10,17:24:47)
Defect and Diffusion Forum Vols. 283-286
347
dioxyde and air. A finite volume method is used for the resolution of the equation governing the problem. They treat the influence of the wind velocity orientation (α = 0°, α = 20°, α = 45°, α = 315° and α = 340°) on the dynamic and thermal characteristics of the plume, as well as the pollutant concentration around an obstacle. They showed that the wind direction has a great effect on the ejected pollutant evolution and consequently on its environmental impact near the obstacle. I. Marvoidis et al. [5] presented computational simulations of atmospheric dispersion around an isolated cubical obstacle using the code ADREA-HF that was compared with experimental data [2] they studied the effect of using different turbulence closure schemes on the computations. Furthermore, specific characteristics of dispersion were investigated using the computational tool, such as the effect of the lateral displacement of a source on the concentration fluctuations intensity, the effects of natural variability and the sensitivity of concentrations on wind direction fluctuations intensity, the effects of natural variability and the sensitivity of concentrations to wind direction fluctuations when ensemble averaged data were available from the field experiments. The objective of this paper is to perform an experimentally validated simulation of a flow field issuing from a bent chimney in presence of a parallelepiped obstacle. Our purpose is to examine the effect of the distance separating the bent chimney and the obstacle. Experimental set-up In order to understand the flow pattern of a plume an experimental technique has been used: the Particle Image Velocimetry (PIV) [3]. We consider a bent chimney (100 mm high, 10 mm diameter and 100 mm the length of a bend); a parallelepiped obstacle (150 mm high, 50 mm wide and 50 mm deep) is inserted at 10 cm upstream from the smokestack. y (mm) 150 100 90
U0 U∝
-5 0 5
100
200
x (mm) 250
Fig. 1. Model configuration.
umerical Study The mean flow field surrounding the chimney and obstacle attached to a wall under a turbulent flow is obtained by varying u∞. Consideration is given to an unsteady, three-dimensional and turbulent flow. The Cartesian coordinate system used here is centred at the base of the chimney, with x in the streamwise (flow) direction, y in the transverse (vertical) direction and z in the spanwise direction (perpendicular to the obstacle axis). The equations governing this problem are obtained using the Favre decomposition [3] and are thus written in the following form: Continuity ∂ ρ ∂ ρ u~i + = 0 (1) ∂t ∂ xi
( )
Momentum equations
( )
(
~
)
~ ∂ ρ T ∂ ρ u~ j T ∂ + = ∂t ∂ xj ∂ xj
λ ∂ T~ − ρ u "j T " (2) C ∂ x j p
348
Diffusion in Solids and Liquids IV
Energy
( ) (
)
~ ~ ∂ ρ~ ui ∂ ρ u j ui ∂p ∂ '' '' + =− + τij − ρui u j + ρ∞ - ρ g δij (3) ∂t ∂ xj ∂ xi ∂ x j
(
)
The introduction of the fluctuating sizes makes this system open. Its closing requires the use of a turbulence model which makes it possible to obtain a number of equations equal to the number of Unknown parameters. We choose this second-order closure model (also called Reynolds Stress Model) in this work [3]. The boundary conditions associated with the above system of differential equations are summarised in Table 1. Boundaries
Velocity
Temperature
Mass fraction Kinetic energy
Rate of dissipation
Chimney
~ u~ = U 0 , v = 0
~ T = T0
~ f m = f 0m
k 0 = 10 −3 v 02
ε = k 30 2 0.5d
~ T = T∞
~ fm = 0
2 k ∞ = 5 10 -3 u ∞
ε = k 3∞ 2 0.2H T
~ ∂ T =0 ∂ n
~ ∂ fm =0 ∂ n
k =0
∂ε/∂y = 0
~ ∂ T =0 ∂ n
~ ∂ fm =0 ∂ n
∂k ∂n=0
∂ε ∂n =0
~ =0 w
Crossflow
~ u = u∞ ,
~ v=0
~ =0 w
Obstacle and ground
~ v=0 u =0,~
Other boundaries of the domain
∂ u~ ∂ n = 0
~ =0 w ∂ ~v ∂ n = 0
~ ∂n=0 ∂w
Table 1. Boundary conditions.
Resultants and discussion In Figure 2, both flow visualization and velocity measurements using PIV are conducted to have a closer look at the structures in the near wake region. We examined the influence of the presence of a parallelepiped obstacle. We present the streamlines field corresponding to an ejection velocity of Y (mm) U0 = 8 m/s and a wind velocity U∞= 8 m/s.
(a) without obstacle
100 90 X (mm) Y (mm)
100
150
200
250
150 (b) parallelipiped obstacle 100 90 100 150 140 190 -1 -1 Figure 2. Effect of the presence of the obstacle u∞ = 8 m s , u0 = 8 m s .
X (mm) 200
250
290
Defect and Diffusion Forum Vols. 283-286
349
We examine in Figure 2 the flow patterns of a plume issued from a bent chimney in absence of any obstacle. We obtain a jet like flow. In the near field, we find three different zones: the initial region, the intermediate region and the development jet region. Just downstream the chimney exit appears a potential core that extends from 2 to 3 times the diameter of the chimney. In this zone, the inertia forces are important and the air velocity is practically equal to the ejection velocity. Immediately downstream the intermediate region and the developed region, where the shear stresses are more important. Figure 2 presents the cartography of the longitudinal velocity in the presence of a parallelepiped obstacle. If we assume that the incoming wind is blowing at a velocity of 8 m/s, the Reynolds number is 5334 and the air is in state of turbulent motion, we end up having a large clockwise recirculation region behind the building. The presence of the building has a marked effect on the flow structure. The pattern of the flow downwind the building can be reasonably explained by enhanced turbulence near the obstacle. In Figure 3 we analyse the efficiency of the closure model in predicting the evolution of the flow as functions of the distance y by comparing the longitudinal evolution of the mean velocity u and the mean vertical velocity v with that given from experiments by PIV. Y (mm)
Experimental
X=160mm 0.14
Y (mm)
X=160mm
0.4
Numerical 0.3
0.12 0.2
0.10
0.1
V (m/s)
U (m/s) 0.08 2
3.
4.
5.
6
0. -0.4
0
0.4
0.8
1.2
Fig. 3. Mean u velocity profile for U∞ = 5 m/s, U0 = 8 m/s at x = 160 mm.
The confrontation of the calculated and measured results relative to the longitudinal velocity U at x = 160 mm gives a satisfying matching along all the variation of the features, the results coincide again which comforts the obtained validation. We precede the same as the vertical velocity component (V) by superposing the experimental and the numerical results. The results are not totally matching like in the longitudinal velocity but they share the same evolution. That may originate from a non uniformity in the jet seeding. Elsewhere we can presume that we have a qualitatively acceptable agreement. umerical study The composition of smoke ejected by the bent chimney is: 20.9% CO2, 76.9%N2, 1.8% O2, and 0.4% SO2. The ejection velocity U0 = 8 m/s, the temperature of ejection T0 = 120° and a wind velocity U∞ = 8 m/s the cross flow temperature T∞ = 18°. It is assumed that all species have the same mass behaviour in this way; we will consider the CO2 as a reference. The aim of this part is to study the distance that separates the chimney and the obstacle for the ratio of velocity R = 1. We consider two configurations (Dch-obs = 10 cm and Dch_obs = 20 cm).
350
Diffusion in Solids and Liquids IV
Figure 4 present the vectors of longitudinal velocity corresponding to an ejection velocity of U0 = 8 m/s and a wind velocity U∞ = 8 m/s (R = 1). We notice in this figure that the flow is more homogeneous if the obstacle is inserted at 20 cm upstream the smokestack. We may also notice that the distribution of the velocity is more regular and the reverse flow region is wider in the same case. 8.33 5.82
(a)
(b)
4.31 1.79 0.785 0
Figure 4. Vectors of the longitudinal velocity for R = 1 (a): Dch-obs = 10 cm, (b): Dch-obs = 20 cm.
We determine in table 2. The mass fraction of CO2 present at the ground and at different faces of the building. Dch-obst = 10 cm
Dch-obst = 20 cm
9.05 10-5
2.11 10-5
11.4 10-3
7.3 10-3
Mass fraction of CO2 present at the ground Mass fraction of CO2 present at different faces of the building
Table 2. Evolution of mass fraction of CO2 relatively to Dch-obst.
We deduct that the rates of pollutants present at the ground and at different faces of the building are more important in the case of Dch_obs = 10 cm. In Figure 5 we present for two configurations the evolution of mass fraction of CO2 and the temperature when the velocity ratio is equal to 1. Fco2
T (K)
0.025
373.5
Dch-obst = 10cm Dch-obst = 20cm
0.02 0.015
373. 372.5
0.01
372.
0.005
371.5
X (m) 0
0.1 0.2
X (m)
371
0.3
0.4
0.5
0.6
0.10 0.2
0.3
0.4
0.5
0.6
Figure 5. Distribution of mass fraction and temperature U0 = 8 m/s, U∞ = 8 m/s.
Figure 5 confirms what we stated already Table 2, in fact the evolution of the mass fraction of CO2 is more attenuated in the case of Dch_obs= 20 cm. The temperature attains a higher maximum for Dch_obs= 10 cm than in the other case.
Defect and Diffusion Forum Vols. 283-286
351
Fathering the obstacle delays the attaining of the temperature maximum. The twin maxima are reported by a distance of 10 cm. Conclusion The present paper described the investigation on the flow issued from a bent chimney around a parallelepiped obstacle. The Particle Image Velocimetry (PIV) technique is used to measure the velocity field in the vicinity of the chimney and to describe the structure of the flow. A three-dimensional numerical model which employs a RMS turbulence closure scheme and a nonuniform grid system was used. Overall, the agreement between the numerical results and laboratory velocity measurements is good. In this numerical study we examine the effects of the distance separating the chimney and the obstacle on the dynamic, thermal and mass characteristics. We deduct that this parameter play an important role to reduce the rate of the pollutant present at the building and at the ground. Nomenclature ~ ~ u, ~ v, w x, y, z n HT R
velocity components along x, y and z direction (m.s-1) coordinates (m) The normal on the considered surface wind tunnel height Ratio of velocity
u"iu"j
Reynolds stress
ε Dch-obs
Rate of dissipation of turbulence kinetic energy (m2.s-3) Distance separating the chimney and the obstacle
References [1] A. Huber: Atmos. Environ. Vol. 23 (1988), p. 2109. [2] I. Marvoidis, I. Griffithis, R.F. Hall, D.J: Atmos. Environ. Vol. 37 (2003), p. 2903. [3] N. Mahjoub Said, S. Habli, H. Mhiri: J. Wind Eng. Ind. Aerod. (2006) [4] Mahjoub Said, N., Mhiri, H, El Golli, S., Le Palec, G., Bournot, P: Rev. Energ. Ren. Vol. 4 (2001), p. 107. [5] I. Marvoidis, S. Andronopoulos: Atmos. Environ. Vol. 41 (2006), p. 2740.
Diffusion in Solids and Liquids IV doi:10.4028/www.scientific.net/DDF.283-286 Experimental and Numeric Study of Flow Around a Parallelepiped Obstacle Issued from a Bent Chimney doi:10.4028/www.scientific.net/DDF.283-286.346
