Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 9.5.2 - Closure approximations in particle dispersion coefficients derived from pdf kinetic equation
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Title: 9.5.2 - Closure approximations in particle dispersion coefficients derived from pdf kinetic equation Particle-Laden Flows
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Bragg, A.
Swailes, D.C.
Skartlien, R.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: disperse flows
boundary layers
inhomogeneous turbulence
pdf kinetic models
 Notes
Abstract: A comparison is made between two distinct formulations of the dispersion coefficients in pdf kinetic equations. By reference to some simple constraints imposed by the underlying particle equation of motion it is shown that the difference between these formulations can be non-trivial, and that one form of the dispersion coefficients provides a more consistent model than the other. A method for closing the averages appearing in these coefficients is presented. This takes into account turbulent inhomogeneities and the influence of particle-surface interactions. The approach makes use of Eulerian two-point, two-time fluctuating fluid velocity correlations, and illustrations of such correlations obtained from LES of turbulent channel flow are given. The effectiveness of the approach is assessed by reference to a simple one-dimensional system, making comparison with particle tracking simulations and the alternative, local-homogeneous closure approximation.
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7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4, 2010


Closure approximations in particle dispersion coefficients
derived from pdf kinetic equations


A. Bragg* D.C. Swailes* and R. Skartlient

Mechanical & Systems Engineering, Newcastle University, Newcastle upon Tyne NE1 7RU, UK
t Institute for Energy Technology, P.O. Box 40, N-2027 Kjeller, Norway
andrew.bragg@ncl.ac.uk, d.c.swailes@ncl.ac.uk, roar.skartlien@ife.no
Keywords: disperse flows, boundary layers, inhomogeneous turbulence, pdf kinetic models




Abstract

A comparison is made between two distinct formulations of the dispersion coefficients in pdf kinetic equations. By
reference to some simple constraints imposed by the underlying particle equation of motion it is shown that the
difference between these formulations can be non-trivial, and that one form of the dispersion coefficients provides a
more consistent model than the other. A method for closing the averages appearing in these coefficients is presented.
This takes into account turbulent inhomogeneities and the influence of particle-surface interactions. The approach
makes use of Eulerian two-point, two-time fluctuating fluid velocity correlations, and illustrations of such correlations
obtained from LES of turbulent channel flow are given. The effectiveness of the approach is assessed by reference
to a simple one-dimensional system, making comparison with particle tracking simulations and the alternative,
local-homogeneous closure approximation.


Introduction

The kinetic equation first introduced by Reeks (1991)
represents an important model for the transport of dis-
persed particulates in turbulent flows. In particular, this
type of pdf model provides a rational basis for the con-
struction of transport equations for mean-field variables
(mass, momentum and kinetic stresses) of the disperse
phase (Reeks (1992)). The coefficients that appear in
these mean-field equations, and that describe the influ-
ence of the fluid turbulence, follow directly from the dis-
persion coefficients in the kinetic equation. The detailed
specification of these dispersion coefficients is therefore
critical. However, in formulating expressions for these
coefficients some level of closure modeling is required,
and it is consideration of such closures that forms the
basis of the work presented in this paper. As originally
formulated by Reeks, using the approach of Lagrangian
History Direct Interactions (LHDI), the key quantities
that require closure are the correlations

S(X', t') j(x,t) t'
Here u denotes the fluctuating fluid velocity field, and
the subscripts x, v indicate a conditional ensemble av-
erage, based on those particle trajectories x' = x(t')


that satisfy x(t) = x and v(t) -= (t) = v.
Slightly different expressions for the dispersion coeffi-
cients were derived by Swailes & Darbyshire (1997) us-
ing the Furutsu-Novikov (FN) correlation-splitting for-
mula (see, for example, Klyatskin (2005)). In these ex-
pressions the correlations (1) are replaced by


Ri(x', t'; x, t)
\ I x,v


t' < t (2)


where Rij(x',t';x,t) = (u(x',t') uj(x,t)). Super-
ficially the correlations given by (1) and (2) appear
similar. However the two-point, two-time fluid velocity
correlations defined by the tensor R are strictly Eu-
lerian and are based on all realizations of u. So that
while the ensemble in (2) is still conditioned by the
constraints x(t) x, v(t) v it contains, unlike (1),
information determined by all realizations of u. The
natural question then is whether or not this difference is
siilm.iiiI and, if so, which of the two approaches can
be considered to represent a more consistent model with
respect to the underlying particle equation of motion
used in the derivation of the kinetic equation. It should
also be noted that the question relates to Corrsin's
Ii\i'1,'lcii (Corrsin (1959)) which, in essence, invokes
the assumption that (1) and (2) can be considered
equivalent. In the next section it is shown that the







7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4, 2010


difference between (1) and (2) can be significant, and
can lead to crucially different predictions for important
correlations involving the dispersion coefficients. The
conclusion being that the kinetic equation formulation
involving (2) offers a more consistent model than that
using (1).

Notwithstanding the difference between (1) and (2)
some closure is necessary for these averages, and the
approximation that is normally invoked is to set x' x
and to introduce Lagrangian time scales Tij that charac-
terize the rates of decorrelation with respect to t t.
These time scales also require modeling. This closure
approach has proved satisfactory in turbulent regimes
that do not exhibit strong inhomogeneities. However the
effectiveness of such closures in the presence of large
turbulent gradients is less clear and, of course, the spec-
ification of suitable Lagrangian time scales Tij is less
straightforward. A stringent test case for such a closure
approach is therefore provided by particle transport in a
turbulent boundary layer. Further, in the near wall re-
gion of such flows any particle-surface interactions will
influence the values of the averages defined by (1) and
(2), and there is evidence to suggest that such effects will
further compromise the accuracy of this closure method-
ology (Skartlien (2007)). In view of these observations
this work considers the construction of alternative clo-
sures strategies for (1) and (2) that take into account the
effect of both turbulent inhomogeneities and particle-
surface interactions. The effectiveness of the closure
approach introduced here is assessed for a simple one-
dimensional system in which particles experience elastic
particle-surface interactions. Results are compared with
those obtained from corresponding local-homogeneous
closure approximations, and also with exact values of
the required averages as obtained from particle tracking
simulations.


in its simplest form, can be written


a
09
-vjp
8xI


(Fj + I j)p
vj


a L a, [ a"
+ r + r
dvj [axi avi j
This form is obtained from both the LHDI and the
FN approaches (Reeks (1991); Swailes & Darbyshire
(1997)). Moreover, the expressions for the dispersion
coefficients v, A, /p derived from the two approaches
appear, at least superficially, to be equivalent. Specifi-
cally, they can be written
t
KJ (umt;') j (x', t'; t) di
\ On \ / x,v

t I
Ai Him.(t; t')m ( x', t'; \ dt'
0 \ XV


Pj = f Him(t; t') Rm(x', t'; x, t)) dt'

(5)
The response tensor Him is defined the same in both
approaches


Kim(t; t')


6xj(t)
6fm(X',t') 6t'


however the specification of m,j in (5) differs: The
LHDI derivation gives


Rj (x',t';x,t) = fm(x',t')fj(X,t)


while the FN derivation gives


Rj (x', t'; x, t)


(fm(X', t')f (X, t))


Kinetic Equations LHDI or FN?


The starting point for the construction of kinetic equa-
tions is a stochastic model for particle motion, generally
of the form


x(t) = (t) = F(x(t), v(t)) + f(x(t), t) (3)


where F and f define the mean and fluctuating acceler-
ations of a particle with position x, velocity v at time t.
The pdf p(x, v, t) defining the joint distribution of x(t)
and v(t) is then governed by a kinetic equation which,


Note that (7) and (8) are intrinsically different, in as
much that (7) is a stochastic quantity whereas (8) is de-
terministic. It is this difference that leads to the reduced
forms given by (1) and (2), which result when H is taken
out of the conditional averages in (5). However, this re-
duction invokes a further approximation, namely that H
is independent of particle path (i.e. not stochastic). The
spatially dependent nature of f means that H must nec-
essarily be stochastic, and it is only by neglecting the
gradients in f that a deterministic approximation for H
can be formulated. This device lies at the heart of the
commonly used interpretation of H as a deterministicc)
Green's function for x(t). Such additional approxima-
tions for H must, of course, also be taken into account
in any analysis of the implications arising from the dif-
ferences between (1) and (2).







7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4, 2010


As a simple illustration of instrinsic differences orginat-
ing from the alternative interpretations (7), (8) for R in
(5) consider the simple first-order scalar stochastic dif-
ferential equation


1(t) f(x,t) x(t) -(t),


be such that


(v) f3)
(Vi* h)


(O) x (9)


(X j + Aij)

(v*Kj + Pi)


where 7(t) is a zero-mean (and Gaussian) stochastic
process correlated in time. The response function for
x(t) is given by


6x(t)
6f(x', t')6t'


h(t t')


te


Consequently x(t) = x0h(t), and then


K K (t; t')R(x' t'; xt))x


x{ x (h(t


xx (h(t


t')h(t')7(t' )(t)


P)h(PI X \ /w t X


using (7)


using (8)
(11)


These alternative forms would be equivalent if h
independent of 7. That this is not the case is evi
from (10). It is clear, therefore, that care must be tak
the more general case when interpreting R in (5). (1
that the conditionality on the averages in (11) sh
not be omitted, since the realizations of 7 (and hen
must still be restricted to those that give x(t) x)
a further illustration of the consequences of the di
ences manifest in (7) and (8), and of the treatment
as a determinstic Green's function, consider the ev
tion equations for the second-order correlations (x*
(v*v*), where x* x (x) and v* v (v).
equation of motion (3) gives the following exact sy


d


(x (F + fj)) + KvvJ)
\' i jU


d (t4v) (- (F + (F )) + (v (Fi + f1))

while the kinetic equation (4) gives (also exact and i
pendent of the LHDI or FN interpretation)

d
d (x v 3)= (x (F + ) + Ai) + (v>v*)
d
d (v ) = (v* (F + K) + pij) + ( v(Fi + i)


In (12) and (13) F, f, v, A and p are evaluated alon
particle trajectories. Comparing (12) with (13) w
that, for consistency, the definitions of v, A and p 1


In Appendix it is shown that, based on the LHDI for-
mulation, (with H interpreted as the Green's function
for x(t))

() = ((x x)f) and (pj) = ((v, v) fj)
(15)
where x(t), v(t) satisfy x = v = F(x, v). Comparing
(15) with (14), it follows that, for consistency, the LHDI
formulation should also give (when x (x))

(4xK) 0, (v<*,) 0 (16)

However the same LHDI formulation for v also implies
(appendix)


were
ident
en in
Note
would


KV KJ) ( (x,
\v[n / ^ (-


ce h) Clearly there is no reason to expect that the correlations
.As given by (17) should be identically zero as required by
iffer- (16), and this suggests that this LHDI based interpreta-
of H tion of the coefficients (5) is not strictly consistent with
volu- the original equation of motion, and that the FN inter-
v*), pretation of (5) provides a more consistent pdf model.
The One benefit of the conclusion that R in (5) ought to be
stem interpreted as (8), as opposed to (7), is that this simpli-
fies the formulation closures for the coefficients (5) since
these are then reduced to forms involving (2) rather than
(12) (1). The correlation tensor R, which describes single-
phase Eulerian fluid statistics is presumed to be known
(can be calculated) ab initio and no further assumptions
S (e.g. Corrsin's hi ,lici,,i need be invoked.
nde-
By way of illustration figure 1 shows some spatial corre-
lation profiles computed from LES of a turbulent chan-
nel flow at Re, = 180. Specifically, the figure shows
profiles for


+ ji)
(13)

g the
e see
must


RSc (x',t;X, t)
R22 (x, t; x, t)


where the subscript 2 indicates the wall-normal channel
direction.


afi
09x,

a f)
09,





















Spatial Correlations in a turbulent boundary


7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4, 2010


are taken in account directly in the closure. In addition,
the influence of particle-surface interactions can be ac-
commodated within the formulation of approximations
to p(x',t'|x, t). In this work attention is confined to
elastic particle-surface interactions (specular reflection).
In this case the problem can be simplified by recasting
the boundary as a line of symmetry: For a 2D flow
restricted to x2 > 0, with x2 = 0 denoting the flow
boundary at which particle are reflected elastically
(v2 -v2), the domain can be extended to x2 < 0 by
defining (for x2 < 0)


F(x,v) -J.F(J.x,J.v), J- 0


0 ) (21)


As figure 1 demonstrates, the spatial correlations within
the turbulent boundary layer (along with the other
turbulence properties) exhibit strong spatial variation.
The characteristic distance that a particle moves within
the time for which the local fluid velocity field is
correlated determines how sensitive the particle is to the
inhomogeneity of the turbulence. Therefore, smaller
particles can be very sensitive to the inhomogeneity of
the turbulence making it essential to incorporate this
effect in any closure model for (Rij (', t';x, t))x for
particle transport in a turbulent boundary layer.

In the next section we consider how knowledge of Rij
can be utilized in the construction of closure approxima-
tion for conditional averages that involve evaluation of
this fluid correlation tensor along particle trajectories.

Closure Modeling

The dispersion coefficients (5) appear in the transport
equations for the particle phase mean-field variables in
velocity averaged forms, 9, A, p (see, for example,
Swailes et al. (1998)), where


and similarly for u and f. Trajectories governed by
(3) in this extended, symmetry-line model (i.e. with no
boundary at x2 0 and with a suitably symmetrized
initial distribution) then generate the required trajecto-
ries of the original reflecting boundary model through
the mapping (Xl, X2) (Xz,'' |). Moreover, in this
symmetry-line formulation we can replace (20) with the
equivalent exact representation


Ki *x' t'; Xt)


Rij(x',t';x,t) (p(x', t'x,t)
x!
... + p(J -x', t'x, t)) dx'
(22)


with the interpretation that p(x', t'|x, t) now represents
the conditional spatial distribution in the symmetry-line
model. The advantage of this representation is that it
is now unnecessary to consider particle-surface interac-
tions at X2 = 0: The influence of such interactions on
(20) is recovered via the use of (22). The distribution
p(x', t'lx, t) is approximated using

p(x', t'lx,t) = p(x', t' tx, 0)


S= Jp dv
V


etc. (19)


These forms of the coefficients involve the condi-
tioned averages (as determined from the FN approach)
(Rij(x', t'; x, t))x. By definition these can written as


(Ri (x', t'; x, t)
\ / -x


Rij(x', t'; x, t) p(x', t'lx, t) dx'


(20)
The aim is to close the conditional averages
(Rij(x',t';x,t))x by constructing approximations
for the conditional spatial distribution p(x', t' x, t) and
then evaluating explicitly the corresponding right-hand
side of (20). In this way inhomogeneities in the
underlying fluid turbulence, as characterized by Rij,


where p(x, v, s x) gives the distribution of x(s), v(s)
(s > 0) subject to imposed 'initial' (s 0) conditions;
x(0) x, and pdf y(v, ulx) defining the joint distri-
bution of v0 v(0) and uO u(x, t). The form of
the kinetic equation defining this pdf will be analogous
to (4), although the correlation between v and uO (as
defined by 4) will modify the form of the response ten-
sor for x (this is discussed in Hyland et al. (1999)). For
the closure of the dispersion tensors it is only necessary
to consider the behaviour of particle trajectories x over
times intervals 0 < s < t t' of the order of the correla-
tion times inherent in R and this prompts several differ-
ent closure strategies. The simplest approach would be
to introduce the approximation f(x(s), t s) z f(x, t).


Figure 1:
layer


1 = p dv,
P J
V


p (x', v, t t'l x) dv











This is essentially the approach adopted by Oesterl6 &
Zaichik (2006). An extension of this idea, and the one
that is implemented here, is to model the Lagrangian
fluctuations f(x(s), t s) f(s; x) as a prescribed
stochastic process, with


7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4, 2010


(2005))
32 ((K/3a 0(0 UJja i)
=7- (/ ) + 6i 1+3) (u2 )
a+ (( + P) )
(27)


(f(x, t) f(xt))
x exp [-a s s'l]


This, of course, presumes that f is statistically sta-
tionary in time and, more problematically, introduces
the same Lagrangian decorrelation rates a = (x)
which, if known, would provide the direct, local-
homogeneous closure for (2). However, by introducing
these rate-scales into the modeling approach developed
here it becomes possible to treat these parameters as
variables, and to identify their precise values via an
iterative process. In addition, it is questionable whether
direct inclusion of the same Lagrangian scales into
the local-homogeneous closure approximation would
necessarily produce the same level of resolution as
that offered by the use of (20) and (22) which take
explicit account of spatial variations in the Rij and
of the influence of boundary effects. Note that the
simpler model f(x(s),t s) z f(x,t), which has
proved satisfactory in homogeneous systems (Oesterl6
& Zaichik (2006)), is a special case obtained by setting
a 0.

With model (24) for f (x (s), t s) (and with F linear in x
and v) the kinetic equation admits an exact closed form
Gaussian solution p. The distribution p, as defined by
(23), is then also Gaussian, with mean and covariances
that are easily evaluated (Swailes & Darbyshire (1997,
1999)). These parameters, defining the distribution of p,
will depend on the model for y, and the simplest first-
approximation to this can be obtained from asymptotic
(t oc) expressions: Specifically, if


(x) ( vv vu )
C(x) = ouv ouu


denotes the covariances of (v un) then take Huu
')(u" u- (u))and


21 +;


( 1 O)i Pn,


With a Stokes drag law for particle motion in a simple
linear shear, d(ui)/oxj -= 7Yi1j2, we obtain (Reeks


1 D Test Case


A simple 1D model serves as a useful test case to evalu-
ate the above approach for constructing closure approx-
imations. The particle equation of motion is based on a
simple drag law;

(t) = i(t) = 3(u(x(t,) t)) (28)

with trajectories confined to the interval 0 < x < X
by reflecting boundaries at x 0 and x X. The
fluctuating velocity field u(x, t) is modelled in the form
u(x, t) -= (x)w(t) where y(x) is a deterministic 'pro-
filing' factor and the velocity w(t) a zero-mean Gaussian
process with (w(t')w(t)) = exp[-,|t t']. The
two-point, two-time Eulerian correlation function is then

R(x', t';x, t) -32(x')q(x)2 exp[-,,., I| t|]
(29)
Although artificial this closed form expression for R
does provide a basis for evaluating closure integrals
of the type (22) and, via y, also enables large gradi-
ents in the profile of the mean-square velocity (u2)
r y2(x): A simple model for y is


y(x) A (1


exp [-(kx)2])


The coefficient A is just the normalization factor to
give y(X) 1 With k > X the profile for (u2)
then exhibits large gradients in the neighborhood of
x = 1/ 2k < X. Applying the approach outlined in
the previous section leads to the following approxima-
tion for the conditional spatial distribution p(x', t' x, t)
(treating x 0 as a line of symmetry)

p(x',t'|x,t) (27) 1 exp [ (x' -x)2
(31)
Here 0, 0- 6(s;x), s = t t' > 0, takes the form
(appendix)


B (s; x) = G O, ,, 0,, + G,,,, (32)
where O (vv0), O .".,") and O, = ', "").
Details of the functions G (s; x) are presented in ap-
pendix 2. The relation of 08 and O,, to 0u (u2)
follows from the one-dimensional form of (26), (27).
Specifically


3 +/ u
0 c%


VU a3 O (33)
Ov o p


Sf(s'; x)f(s; )







7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4, 2010


Using these results the closure approximation for the
conditional average (R(x', t'; x, t))x (x < X) becomes

J R(x', t'; x, t) ((x', t'x, t) + p(-x', t'|x, )) dx'
x'>0
S32(a exp[-a s] (x) (s; x)
(34)

where


+0s; x) p t
IF(S;x)- j 1 \)xTp(x',t'x,t) dx'


1 F 2]
1 -exp (kx/a)
a


a(s;x) = 1 + 2k2 (s;x)

is the closure approximation for (y(x'))x, x < X. For
this model the Lagrangian rate scale a(x) is given by


a1 1
/32/2(x)\


R(x', t'; x, t) xdt'


t)
j V j' (X)) ep[
-P0(x


the results presented here have been generated by simply
taking a = w. Further, the results are presented for the
normalized form of the model, scaled such that a 1
and X 1.
To evaluate how results from the closure model proposed
here differ from those obtained from the corresponding
local-homogeneous approach we first compare values of
the two approximations to (R(x', t';x, t))x: For s > 0
we have, respectively,

R(x',t';xt) exp[ ] 7p.(x)(s;xx)
C(s; x)


R(x',t'; ,t))x


/32 ,2 exp[-as02 (x)


Co(s; x)
Figures 2 to 4 illustrate the differences between these as
measured by Q(s; x) C/Co, x > 0, for St 0.5,
4 and 10. With a = aw this measure reduces to Q
'I(s;x)/y(x). The results were produced with k 10
and ac 10 1, and the figures show Q in the region
0 < x < 0.2 over the time interval 0 < s < 2.

St=0.5


a (t t')]dt'


(36)

so that, replacing ((x'))x in (36) with t(s;x), pro-
vides an implicit equation for a. It is anticipated that
a will only differ markedly from aw (which is spatially
uniform in this simple model) in the near-wall region -
due to the influence of particle-surface interactions and
the gradients in y.

Results
From (36) it follows that if ( (x'))x y (x) for t -t' ~
(aw)1 then a(x) ~ a,. This, of course will be the
case for large Stokes number (St a/3) particles.
With decreasing St this approximation becomes less ap-
propriate and would certainly be expected to produce
unsatisfactory results if used in the formulation of lo-
cal homogeneous closure approximations. Of course,
for St < 1 the usual model for a would be the fluid
Lagrangian rate scale (often used for St ~ 1 as well). It
is of interest, therefore, to test whether or not the same
level of approximation performs any better when applied
to the closure approach developed here, and whether the
two closure strategies would then produce significantly
different values for the dispersion coefficients appearing
in the mean-field transport equations. With this in mind


0.05


0.2 0


Figure 2: Comparison of closure model predictions:
Q C/Co


St=4


0.05


.0.1
0.15


0.2 0


Figure 3: Comparison of closure model predictions:
Q C/Co







7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4, 2010


and the results were produced with k = 10, = 10 1
and show Q in the region 0 < x < 0.2 over the time
interval 0 < s < 2. The results are shown Figures 5 to
7.

St=0.5


Figure 4: Comparison
Q C/Co


of closure model predictions:


As expected, for St > 10 the two models produce
essentially the same approximations over this time-span.
However there are clear and icgnilik.iim differences for
St = 4 and less: Very close to the wall (x < 0.05) it is
seen that Q > 1. This is a consequence, in part, of the
fact that the closure model used here takes into account
the influence of the elastic collisions at x 0 which will
result in (x')x > x in this near-wall region; particles
in this region will, on average, sample fluctuating
velocities significantly larger than those at x. Further
away from the wall (0.05 < x < 0.2) it is observed
that Q attains values significantly less that 1. This can
be attributed to the gradients (actually the curvature) in
y which again bias the values of u sampled along the
trajectories of particles co-incident with x at a given
time. The deviation between the two models is seen to
occur more rapidly in the case smaller Stokes number
particles (St =0.5) and it is clear that it is the inclusion
of temporal variations in F that capture the coupled
influences of particle-surface interactions and spatial
variations in (u2).

To investigate these effects further a particle tracking
simulation of trajectories governed by (28) was per-
formed in order to compare results for (R(x', t'; x, t))x
generated by particle tracking and the closure model.
For s > 0 we have, respectively


Figure 5: Comparison of closure model and particle
tracking results for: Q = C/Cpr




St=4


1
0.8
Q 06
0.4
0.2
0.
2


0.15


0 0


Figure 6: Comparison of closure model and particle
tracking results for: Q = C/Cpr


St=10


R(x', t'; x,t))



( ',t';Xt))X


32T2 exp[-a)s]u(x)'(s;x)

C(s; x)

'2,o exp[-as]y(x) ((x'))x

CpT (S; X)


The particle tracking and closure model results are com-
pared by the measure Q(s; x) C/CpT, x > 0, for
St = 0.5, 4 and 10. Again a = aw in the closure model


Figure 7: Comparison of closure model and particle
tracking results for: Q = C/OCp


St=10

^^^LSfZPI


0.05
x 0.1


0.15


0.2 0


0.15


0.1
x


0.05


C'0?
o C' :
2' .1
CI -
0-
2


0 0


0.05











The results show that in the near-wall region
(0 < x < 0.2), where particle-surface interactions
and large gradients in y((x) are important, C can
provide satisfactory approximations to (R(x', '; x, t))x.
In the region 0.05 < x < 0.2, where there are large
gradients in y(x) the closure model predictions are in
good agreement with the particle tracking results across
the range of Stokes numbers tested. This demonstrates
that the closure model is able to describe the effect of
the spatial variations in u on (R(x', t'; x, t))x.

However very near the wall (x < 0.05) there is clearly
a difference. In this region, the particle tracking results
showed a very strong spatial variation in a(x), varying
from a, by a considerable degree for all of the Stokes
numbers tested. In the closure model, a was set equal to
a, which, in light of the particle tracking data, is clearly
not appropriate. Also, the closure model prediction very
close to the wall is worse for smaller Stokes numbers
(for which a deviates the most from a,) which suggests
that setting a equal to a, in the closure model is, to
some extent, a cause of the poor prediction in the very
near wall region. For future work, when using the
closure model for particle dispersion in a 3D turbulent
boundary layer, a 1 could be approximated by the
fluid Lagrangian timescale, for which data is readily
available. It is expected that this will produce more
satisfactory results close to the wall because the fluid
Lagrangian timescale is qualitatively more appropriate
than the Eulerian timescale as an approximation to a 1.

A second explanation for the poor performance of
the closure model in the very near wall region is the
specification of q(v, u x). In this work ~, is specified
by using a locally homogeneous approximation (26).
The implication of this is that 0, is always predicted
to be less than 0~, which, for larger particles, is not
true in the very near wall region. The effect of this
in the closure model, for larger particles, would be to
underpredict Ox(s;x). Clearly then more appropriate
specifications for 8, must be developed in order to
improve the closure model, and this will be considered
in future work.

A third possible cause of the discrepancy between the
simulation and closure model results is the approxima-
tion that p(x')/p(x) 1 (see appendix 2). In the very
near wall region where there are steep gradients in p(x)
this approximation is clearly not valid. As a next step,
this will be addressed by solving the equation for p(x)
which can be obtained from the the continuum equations
for the particle phase (see Skartlien (2007)). With this
solution, p(x')/p(x) may be more appropriately speci-
fied and implemented in the closure model.


7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4, 2010


Conclusions

The results from this preliminary study are encouraging
and suggest that, with further development, the method-
ology outlined here has the potential to provide good ap-
proximations to (R(x', t'; x, t))x for particle transport in
a regime exhibiting strong spatial variations in the flow
properties. The intention is to next test the model on a
more complex system, based on LES data, in which the
Eulerian fluid time-scales exhibit strong spatial varia-
tion. A key issue that needs to be addressed is the formu-
lation of more precise estimates for the joint one-point
pdf q(v, ulx), going beyond the simple Gaussian forms
suggested here. Also, it will be of interest to see what
scope there is to extend the approach and incorporate
other types of particle-surface interactions; the obvious
next case being particle adhesion. Clearly this presents
further challenges in the formulation of suitable approx-
imations to p(x', t' x, t).

Acknowledgements

This work was performed as part of the FACE centre; a
research co-operation between IFE, NTNU and SINTEF.
The authors gratefully acknowledge funding through this
centre from the Research Council of Norway and by
the following industrial partners: StatoilHydro ASA,
Norske ConocoPhillips AS, Vetco Gray Scandinavia
AS, Scandpower Petroleum Technology AS, FMC, CD-
adapco, ENI Norge AS, Shell Technology Norway AS.


Appendix 1

To test the equivalence of the two systems (12) and
(13) it is necessary to formulate expressions for the La-
grangian averages (Aij), (cj) and (xzKj) appearing in
(14). In this appendix expressions are derived for these
averages when the forms of v, A and p are those given
by the LHDI interpretation, (5) and (7). The analysis fol-
lows the approach given in Hyland et al. (1999); Reeks
(1991, 1992) and treats 7- as the Green's function for
the particle equation of motion (3). This, in essence, ne-
glects gradients in f and requires F to be linear in x and
v. With these caveats, we may write


Xi(t)


vi(t)


t
x(t) + JHm(t;t')fm (x',t') dt'
t
o

v(t) + J im(; t')fm (x', t') dt'
o


where x, v satisfy x = v
that, in general, (f(x, t))


(39)
F(x, v). It should be noted
= 0 -= (f(x,t)) 0, and






7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4, 2010


consequently, in general, x (x), and v 0 (v). Now,
with the LHDI interpretation of A
(AifAj.(,v,t))


where


J Aij (x, v, t) p(x, v, t) dx dv
X V
x v


x v 0dv dx
...p(x, v, t) dv dx


t) )X dt'


g(s) = /3- l(1 e )

Then Ox(s; x) = (x'x') is given by


Ox(s;x) g2(s)(vov) + .g(s sl)g(s
00

...(s) f(s2))ds ds2 + 2g(s) g(s s
0
.(f ))ds'


t

=r ix O I [ (- t; t') f-{x',t')fj (X, t)6(x
x v 0

...6(v v)) dt'dv dx

t
= -Hi t;') m(x',t'x)f ,t) dt'
0
K (x,- 3 xi)0


From (44)

f(s11)f(s2))


/32 (uoo)exp


a|sl 21] (46)


and, using the Furutsu-Novikov formula,

(40) (fs')v0 =- Pu(s')vo) = uovo) exp [ as] (47)


In a similar fashion the LHDI forms of p and v together
with (39) imply that


{Pij(x, v,t))


((K


S* (41)
xm (m,


Xi (Xm


Appendix 2


Using Bayes' theorem we may write

p(x')
p(x', t'lx, t) = p(x, tx', t') (42)
where p(x') represents the particle phase concentration
at x at t'. The implication of this is that the backward
in time conditional spatial distribution (p(x',t'lx, t))
may be modeled via the forward in time conditional
spatial distribution (p(x, tlx', t')). In this work, as a first
approximation, the ratio p(x')/p(x) is set to unity.

The model for p(x, tlx',t') is constructed using (28)
with 'initial' conditions x(0) x (fixed) and v(0) = vo
a random variable correlated with u(x,t) .",-..
Since (in this 1D model) vo and u are both zero-mean
variables it follows that (x(s))x = x and then that

x'(s) = (s) x g(s)v0 + g(s s')f(s') ds'
Jo


Substituting these into (45) gives


where (vov), = ".'), 0 = "") and

G, g 2(s)
G 2 (r(1 r2)s (1 + r)g(rs)
r2(1 r2)
+ r(1 + r)(2r 1)g(s) + rg((1 + r)s)- (49)
r 2(1 + r)g(2s))
2
Gu g((1 ) (g(rs)- rg(s)
r(1 r)

with r = a/3 and T = 31. The formula for the de-
generate case r 1 are not presented here, but results
based on expressions (49) converge to those for this spe-
cial case as r 1.

References

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research. Adv. Geophys., 6:161-184, 1959.

KE Hyland, S McKee, and MW Reeks. Derivation of
a pdf kinetic equation for the transport of particles in
turbulent flows. J. Phys. A: Math. Gen., 32:6169-6190,
1999.


u(x, t s)


Ox(s;x) = GOv + G0, + Gv,, O


i)fj (xt)







7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4, 2010


VI Klyatskin. Dynamics Of Stochastic Systems. Elsevier,
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B Oesterl6 and LI Zaichik. Time scales for predicting
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