Lecture Note
Lecture 16 Notes: Particle Motions, Stokes’ Drift
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University:
Massachusetts Institute of Technology -
Course:
12.820 | Turbulence in the Ocean and Atmosphere Academic year:
2021
-
Views:
484
Pages:
15
Author:
waspitisw
Particle Dispersion
Random F l i g h t
-
Lagrangian dispersion
As an exanlple, we examine the randoin flight model. which assumes that the accelerations have a stochastic component and use Newton's equations
dX
dV
=Vdt
= Adt
+PdR
where A is the acceleration produced by deterministic (or large-scale) forces. We include
randoin accelerations with the random increment d R satisfying ( d R i d R j ) = Gijdt.
As examples, consider a drag law for the acceleration
with u being the water velocity. The dispersion is determined by
equations: we can show that
The latter corresponds to a diffusivity of
r; =
P
and
T;
from the
p2/2r2.
Area grows like 4 ~ (6r;t
t in 3-D)
Velocity variance is TK
Demos, Page 1:
Random flight
Taylor dispersion
In 1922, Taylor described the dispersion under the assumption that the Lagrangian
velocity had a known covariance structure. He considered just
We find that
3
-X,Xj
3t
= KXj
+ x,y
and, in the ensemble average,
If we substitute
X
= Xo
+
6
t
V(tl)dt'
and look a t the case where ( V )= 0 and the flow is statioilary, we have
where R& is the covariance of the Lagrangian velocities
For isotropic motions RL.(t)= U2RL(t)bijw ith R L ( t )being the autocorrelation function;
%J.
the change in x-variance is given by
From this formulal we see that
For short times,
( x 2=) U2t2
For long times, if the integral Tint=
JrRL(t)dtis finite and non-zero, Relation to diffiisivity
Consider the diffusion of a passive scalar
and define moments of the distribution
Integrating the diffusion equation gives conservation of the total scalar, under the assumption that the initial distribution is compact and the values decay rapidly at infinity
The first inoment gives
In the absence of flow and with a constant r;, &(x) = 0. Otherwise, the center of mass
migrates according to a weighted version of u+VK: it inoves with the flow and upgradient
in diffusivity.
The second inonlent
a
a
-(x2) = =(xu) + 2(- (xr;))
at
3x
implies that
3
at
ar;
[(x2)- ( x ) ~ =
] 2 [(xu) - (x)(u) + (xz)
-
"I
(x)(%)
+ 2(s)
For uniform flow and constant diffusivity, the blob spreads in x a t a rate 2 ~ Thus
.
we can
identify the effective diffusivity
K =U2~int
Strain in the flow and curvature in
K
will alter the rate of spread. Small amplitude motions
If we assume that the scale of a typical particle excursion over time Tint is small
compared to the scale over which the flow variesl we can relate the Lagrangian and Eulerian
statistics. The displacement ti = Xi(t) - Xi(0) satisfies
and we can substitute the lowest order solution
into the second term above to write
and average, recognizing that the mean Lagrangian velocity is just
(;ti):
For simplicity, we assuine that the turbulent velocities are large coinpared to the inean;
then this becomes
(u:) = (
+
~ i )
3
1"
~ i j ( xf > t i ) = (ui)+
Let us assuine that the integrals with respect to
its syininetric and antisyminetric parts
T
"[
3xj.o
d ~ R i(jx ,T )
exist and split the covariance into
with
We can write an arbitrary ailtisymmetric tensor in terins of the unit ailtisymmetric tensor
so that the contribution to the Lagrangian velocity is Note that the antisymmetric part of the contribution to the Lagrangian velocity is nondivergent:
Thus the Lagrangian mean velocity has contributions from the mean Eulerian flowl from
the Stokes' drift, and a term which tends to move into regions of higher diffusivity
We will discuss the meanings of these terms in more detail next.
Random Rossby Waves
Consider a randomly-forced R.ossby wave in a channel:
where
is randomly distributed on a disk of radius ro. This gives a streamfuilction
with
and w = - f l k / ( k 2
+ e2).
d T e - ( ~ - $ " ~ T ( t- T ) Stokes' drift
Consider first the steady wave case.
$ = -t s.i n ( ~ [ x t]) s i n ( ~ y )
7r
We look a t the particle trajectories by solving the Lagrangian equations as above
For small t (which is the ratio of the flow speed to the phase speed, we can find an
approximate solution (as before) by iterating
The mean Lagrangian drift is therefore
Rij (x,T) d~
Treating the mean as a phase average gives
Rij (T)
=
22 (
cos TT cos2TY
sin 7rr sin TY cos TY
cos TT sin2 TY
- sin TT sin TY cos 7rv
the integral gives
1
t
Qj(t) =
R - ( T ) ~ T= -
0
so that the drift is
u,
(1 - cos ~ tsin
) ~y cos 7rv
sin ~t cos2 TY
sin TT sin2 xy
-(1 - cos xt) sin xy cos x?ry
a
2
= -El = - cos(27q)[l - cos(TIt)]
at
2
3
UL
= -[2
3t
Note that there is a time-averaged drift
t2
= - sin(2xv) sin7rt
2
prograde on the walls and retrograde in the center.
6 Note that we can split Dij as usual:
with the first term giving the up-diffusive-gradient transport associated with the symmetric
part of j' Rij and the second, llondivergent part: arising from the antisymmetric term; gives
the Stokes drift. For the primary wave,
K..
%"
-
c2 ( ~ i ~ 7r
r t2
lrv
2lr
sin lrt sin2lr?rl/
and has no time average, while
produces the nondivergeilt Stokes drift (and does have a mean).
drift
FINITE
AMPLITUDE
In the frame of reference of the wave ( X i = X
-
Demos, Page 7:
ct)
Thus particles simply move along the streamlines. At some Lagrailgian period T L , the
particle will have moved one period to the left so that
Stokes drifts occur when the Lagrallgian period differs from the Eulerian period. Trapped
particles have Back t o random wave
with
d T e - ( ~ - + " ~ r (t T)
we find
~ ( xY,, t)*(x1, Y', t l j
=
2
Uo - y ( t - t l ) cos[k(x - x')
2e2
R,,
- w(t - t')]
sin(@) sin(!yl)
$ sin w r sill& cos v!
-$ sill w r siillg cos e~ k 2 cos WT sin2v!
cos wT C O S ~ey
1 2 -7,
(7) = - UOe
2
,
This gives
Dm,
=
w $ sinV! cos y!
y cos2
-w $ sin Vg cos Vg
-
y k 2 sin28~
The diffusivities and Stokes' drift are given by
Q3 =
1 k
w
-Al2 = AZ1= --U 2 Oey2+
u
L
a
Demos, Page 8: s t r u c t u r e
vs a c t s d >
=U
L
sin
codel/
d
w
-u2k
cos 2ey
2 O y2+w2
' 7 1
=
1 2k2
=-Usin 2tg
2 Oey2+w2
Demos, Page 8:
stokes d r i f t
When we have a non-zero, the trajectories become less regular in the vicinity of the
stagnation points. A line of particles approachiilg the point begins to fold, with some fluid
crossing into the interior and some being ejected. Which way a parcel goes depends on the
phase of the perturbing wave as it nears the stagnation point.
Demos, Page 9: lobe dynamics
We can look a t Poincark sections (snapshots at the period of the perturbing wave)
at various amplitudes to see the mixing regions Demos, Page 9: poincare sections
The inixing across the channel is still blocked for a small enough < 0.05 so the inixing
is still diffusion-limited: although some gain is realized by enhanced flux out of the wall
and a decrease in the width of the blocked region.
Demos, Page 9 : Continuum
References
Flierl, G.R.. (1981) Particle motions in large amplitude wave fields. Geoph,ys. Astrophys.
Fluid D,yn., 18,39-74.
Pierrehuinbert. R..T. (1991) Chaotic mixing of tracer and vorticity by modulated travelling
R.ossby waves. Geoj,h,ys. Astrophys. Fluid D,yn., 58, 285-319.
Active Tracers
We review mixing length theory applied to a set of active scalars (think in terms of
biological properties):
Split the field into an eddy part which varies rapidly in space and time and a mean part
which changes over larger (order l / t ) horizontal distances and longer (order l / t 2 )times:
We must allow for short vertical scales in both means and fluctuations. Counterbalancing
x
We
this difficulty is the fact that vertical velocities tend to be weak (order
F). assume the mean flows are small E
NU' and the coefficients in the reaction terms vary
rapidly in the vertical but slowly horizontally and in time.
i
i
Vertical Strnctlire
1) We assume the case with no flow has a stable solution:
Demos, Page 10: bio dynamics
2) The eddy-induced perturbations satisfy
with W = ( 3 / 3 X , 3 / 3 Y , 3 / 8 2 ) .
3) The equation for the mean is
Summary:
Eddies generate fluctuations by horizoiltal and vertical advection of large-scale gradients; but the strength and structure depends on the biologically-induced perturbation
decay rates.
Perturbatioils generate eddy fluxes and alter the average values of the nonlinear biological terms. NPZ
A simple biological model (mixed layer):
Mean-field approach
We call get a very similar picture using the mean-field approximation: take
-u' . Vb, +
+ b', X,t) - &((b + bb',X,t )
or (dropping the quadratic and higher terms)
The differences are subtle: the MFA does not presume that the scale of
linearizes in a way which may not be consisteilt.
Zi
is large but Separable Problems
The inesoscale eddy field has horizontal velocities in the near-surface layer which
are nearly independent of z; and the vertical velocity increases linearly with depth w' =
s(x; t)z. The stretching satisfies
For linear (or linearized perturbation) problems in the near-surface layers, we can separate
the physics and the biology using Greens' functions.
We define the Greens function for the horizontal flow problem:
(4+
u ( x , t) V
-
V
1
G(x, x'; t
- t') = 6(x - x1)6(t- t')
The perturbation equations can now be solved:
-
1 1
dx'
dt1G(x;tlx', t')s'(xl, tl)pi(z,t
- tl)
The two functions representing the biological dynamics both satisfy
with Bij = 3Bi/3bj. These give the diffusive/ biological decay of standardized initial
perturbations
@m,i(z,0) = v,&
,
a-
pi (z, 0) = z-bi
3% Simple Example
If we ignore vertical diffusion and advectioil and consider only one component with
all = -A, we have
4m:i= e-X7Vm&
so that
a,I
=-
[Jdxf J
i t l c * ( t - t ~ c (t~l x
, ~
tl).h(xI,
,
I
The eddy flux takes the form
If we split the right-hand side into symmetric and ailtisymmetric parts, we find
The last term has no divergence and can be dropped. Thus the eddy flux is a mix of
diffusion and Stokes' drift:
= -K:,V&
+
a
vA~,
Both coefficients depend on the biological time scale Apl.
For the random R.ossby wave case: the Stokes drift term is
while the diffusivity tensor is
Demos, Page 13:
effective coeff
Not so simple example
"Mixing length;' models
Flux(b) = - K , V ~
even if appropriate for passive tracers are not suitable for biological properties whose time
scales may be comparable to those in the physics. Instead, we find
where R,, is the equivalent of Taylor's Lagrangian covariance (but including K ) .
We divide the coefficient into symmetric ( K ) and antisymmetric terms related to the
Stokes drift (V)
Note that
Eddy diffusivities and wave drifts mix different components (flux of P depends on
gradient of 2).
If R has a negative lobe, the biological diffusivities can be larger than that of a passive
scalar
The quasi-equilibrium approximation
works reasonably well in the upper water column. In particular
so that
P
I
1
= -ul
.V 7
unlike
C
I
= -5.
Demos, Page 14: complex diffusion
up Z grad flux of Ptl.ps downgradient Kpp,KZZt2.ps Eulerian-Lagrangian
If
K
= 0:
we can relate the relevant form of the Eulerian covariance
to Taylor's form. The Greens' function equation
has a solution
G(x, t x ' , t') = 6 ( x - X ( t x l , t'))
where
3
-X(tx',tl)=u(X,t)
, X(tlx',t')=x I
at
gives the Lagrangian position of the particle initially at x' at time t'.
convenient to back up along the trajectory and let
But it is more
c
where the particle at at time t' passes x at time t (and takes a time T for this tranistion).
Thus the c's give the starting position: which, for stochastic flows varies from realization
to realization. We can solve
c.
0 to T = t - t' to find
We can now define the generalization of the Lagrangian correlation function used by
Taylor
for
T =
R,,(t
- t',
x) =
dx'ul(x, t)G(x, t x', t1)u'(x', t')
= &(x;
t)u:,([(t
-t'x,
t); t
-
(t - t'))
For homogeneous, stationary turbulence (on the scales intermediate between the eddies
and the mean), this will be equivalent to Taylor's
R,,(T)
= uk(X(tl
+ T X ' , t'), t' + T)u&(x',t')
but we include inhomogeileity and (forl general G, diffusion).
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