H.K. Moffatt Local and Global Perspectives in Fluid Dynamics

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8/3/2019 H.K. Moffatt Local and Global Perspectives in Fluid Dynamics
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LOCAL AND GLOBAL PERSPECTIVES
IN FLUID DYNAMICS
H . Keith Moffatt
Isaac N ew ton Insti tute fo r Mathematical SciencesUniversity of Cambridge, United Kingdom
1. INTRODUCTION
It is a great honor to present this Closing Lecture at what has been an
outstanding Congress, ICTAM 2000. These Congresses have a proud tra
dition, going back to Delft 1924, when Prandt l, von KBrm&n,G . I. Taylor
and Burgers initiated the series and laid the ground rules. Since then,every four years, with a brief interlude during World War 11, the world
mechanics community has gathered together to share its knowledge and
to grapple with new emerging problems. This is truly a global enterprise,
and we can all take pride and pleasure in our involvement in it . The
first ICTAM that I was privileged to attend was the tenth in the series,
held at Stresa in 1960. And here we are now at the 20th ICTAM and
actually the 10th that I have been personally involved in.
The global participation at these Congresses is achieved in each caseonly through an effective local organization. Each Congress, whether at
Stresa or Stanford, Lyngby or Haifa, Moscow or Toronto, has had its
own distinctive local flavor that remains in the heart of each participant
long after the details of individual lectures have faded from the memory.
Chicago will be no different in this respect, and I would like to record
my personal thanks to Hassan Aref and his great team on the Local
Organizing Committee here, who have succeeded in making this 20th
and millennia1 Congress so uniquely memorable.Locality and globality: the two complementary perspectives whose
interplay is essential to the successful running of a World Congress. It
is a different kind of locality and globality that I propose to discuss in
this lecture; but certain parallels may emerge, and I shall return to the
Congress scenario in my closing remarks.
52 1
H. Aref and J.W. Philips (eds. ) ,Mechanics fo r a Ne w Millennium, 521540.
0 2001 Kluwer Academic Publishers. Printed in the Netherlands.
www.moffatt.tc
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522 UJCLOSING L E C T U R E :H. K E I T HMOFFATT
In fluid dynamics, the concept of locality emerges most clearly through
what we describe as local similarity solutions of the governing Navier
Stokes equations, and I shall give some examples of these. Although
such solutions are generally accurate only within a small subregion of
a fluid domain, nevertheless an understanding of them can be of crucialimportance for two reasons: first, they provide a check on the accuracy of
numerical solutions computed throughout the fluid domain; and second,
they can act as organizing centers for the whole flow field; and knowledge
of their structure can provide important clues concerning global features
of the flows.
But what are these global features? They are those features that
are essentially topological in character, structural properties that are
‘robust’, i.e. insensitive to small perturbations. Again, an understanding of such topological features provides a guide for computation and
a possible check on computational accuracy. By way of simple exam
ple, consider an incompressible twodimensional flow on the surface of a
sphere. At any instant there are a number of stagnation points, which
are of either elliptic or hyperbolic type. Topological considerations tell
us that if these stagnation points are ‘nondegenerate’ and if N is the
number of elliptic points and M the number of hyperbolic points then
NA4 = 2 . A numerical search for stagnation points for a given complexflow can be conducted, and their classification as elliptic or hyperbolic
can be achieved, at least in principle. If this process leads to values of N
and M with M < N  2 , then we know that we must have missed some
hyperbolic points. As a first priority, we should always seek to ensure
that computed flow properties are compatible with such fundamental
topological constraints.
2. LOCAL SIMILARITY SOLUTIONSI want to focus first on a fascinating problem in which I have been
intermittently involved since 1963this is the problem of flow near a
sharp corner. Our Chairman has made a vital distinction (Barenblatt
1979) between two types of similarity solution, which both crop up in this
context: similarity solutions of the first kind for which dimensional argu
ments play a key role; and of the second kind for which a key parameter
has to be determined through solution of an eigenvalue problem. The
two prototype situations are sketched in Figs. l (a ) and l (b) . The first
shows G . I. Taylor’s (1960) ‘paintscraper’ configuration in which one
plane boundary is scraped relative to the other with constant velocity
U . Sufficiently near the intersection, the flow is a Stokes flow and the
streamfunction $ (T , 0) is determined on dimensional grounds in the form
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Local and global perspect ives in fluid d y n a m i c s 523
$ = U r f ( O ) ,where
1a2 k e ) sin 8  8 sin2a CO S ea2  sin2a
, k = T ( 2 a  s in2a) . (1)( 8 )=
This is evidently, in Barenblatt’s terminology, a similarity solution of
the first kind.
1
ruV
+O. 1
U (> 0 )
Figure l ( a ) Solid curves are the streamlines for the Taylor similarity solution (1);
dashed curves show the firstorder inertial correction obtained by Hancock, Moffatt,
and Lewis (1981).
Figure l ( b )
$J N r’f(0) with X complex. [Courtesy C. P. Hills]
Corner eddies described by a similarity solution of the second kind,
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524 LJCLOSINGL E C T U R E :H . KEITHMO FFA T T
In Fig. l( b ) by contrast, both boundaries are at rest and fixed at
angle a , and a twodimensional flow is driven by some agency (e.g. a
rotating cylinder) far from the corner. In the Stokes approximation,
the streamfunction sufficiently near the corner satisfies the biharmonic
equation
with noslip boundary conditions
v*+= 0: (2)
1+!1=+0=0 on O=O:a. ( 3 )
Here similarity solutions of the form II,= ~ ' f ( 0 ) ay be found in which
the parameter X must be determined from the eigenvalue problem result
ing from ( a ) , ( 3 ) . This is evidently a similarity solution of the secondkind. The most interesting feature of the solution is that, for all acute
angles a (and actually over the wider range cy < 147"); all relevant eigen
values X i are complex, and the corresponding flows are 'eddying' flows
as T + 0 (Moffatt 1964a;b). The fundamental structure is precisely as
indicated in Fig. l(b).
Figure 2
plane boundary (Taneda 1979).
Corner eddies in a Stokes flow over a cylinder at a small distance from a
Although this was all well understood in 1964, it was not until 1979
that the first photographs of such corner eddies were published (Taneda
1979). Taneda's beautiful photographs, many of which are reproduced
in Van Dyke's Album of Fluid Motion (1982), show a great variety of
situations in which corner eddies can arise. For example, shear flow over
a cylinder resting on a plane boundary shows the eddy sequence in the
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Local and global perspec t ives in f l u i d d y n a m i c s 525
two cuspshaped regions. If the cylinder is separated from the plane by
a very small amount) then a finite number of these eddies survive as
attached ’walleddies’ on both the plane and the cylinder (Fig. 2 ) . If the
cylinder is rotated slowly, then these eddies still survive, and may in some
circumstances move into the interior of the flow (Jeffrey and Sherwood1980). For all such situations, it is evident that an understanding of the
basic prototype problem of Fig. l(b) is the prerequisite for an analysis,
whether analytical or numerical, of the more complicated configurations.
Fzgure 3similarity structure (Hills and Moffatt 2000) .A threedimensional configuration for which the Stokes flow admits a double
We have recently found an example of a steady threedimensional
Stokes flow for which both types of similarity solution play a part (Hills
and Moffatt 2000). The configuration is shown in Fig. 3: two fixed planes
c p = +,f~ re ‘honed’ by rotation of the plane boundary 8 = 7r/2 with
constant angular velocity R (using spherical polar coordinates ( r ,8,v)).The fluid domain is 0 < 8 < ./a, 1 ~ 1 p. In the ‘Stokes’ region near
T = 0 , on dimensional grounds, the velocity field has the form
U k ?8’ p ) = flTW?p)? (4)
where U ( 8 ’ p ) is dimensionless, a similarity structure of the first kind. I
use here the word ’structure’ rather than ‘solution’,because the function
u ( 8 : c p ) is as yet unknown. A solution can be sought in the form U =
X ( z T ( r , @ , p ) )here, consistent with (4)’ (r,8?cp) flrL?(e,cp).It
turns out tha t, near 8 = 0 , the solution has the form
W ’ )= exf(P) ( 5 )
where X is determined by precisely the same eigenvalue problem as in
the twodimensional case described previously. The flow again exhibits
corner eddies (when 2p < 1 4 7 O ) , which now scale with radius r (as well
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as with arigle B according to ( 5 ) ) . We thus see a similarity solution of
the second kind ( 5 ) imbedded wi th in one of the first kind ( 4 ) .
F i g w e 4rotating a horizontal plate under a fixed vertical plate (Hills and lloffatt 2000)
Closed streamlines (revealed by injection of dye) in the flow generated by
,4n exp erim enta l realization of th e si tua t ion w ith 2p = T is shown
in Fig. 4. The streamlines are closed. being the intersections of 'Taylor
surfaces' r f ( Q )= const and spheres r2+ z 2 = const . A ful l analyt ical
solution of th e Stokes problem has been found for this case.
3. FREE SURFACE CUSPS
The corner flows considered above are nonanalytic (or 'singular ' ) in
m y rieighborhood of the corner as a result of the imposed geometrical
s ingular i ty on the boundary . I now wish to briefly consider a different
type of singular behavior t h a t occurs at a free surface an d t h at is intr insic
to th e prope rt ies of the f luid. T his is the phenomenon of 'cusping ' th at
occurs w hen flow is induced to converge tow ards a line on a free surface
( Jo se p h e t al. 1991. Jeong a nd M offatt 199 2). T h e pro toty pe configuration is shown in Fig. 5: a flow driven by some stirring mechanism
in the interior of the fluid of characteristic scale L say, is supposed to
converge sym metrically an d th e surface typically dips downw ards form
ing a cus p as ind ica ted . An exa mp le in which th e flow is driven by
two co unte rrota t ing cyl inders im mersed in sy rup is shown in Fig . 6. i n
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Local a n d global perspec t i ves in f l u i d d y n a m i c s 527
which incidentai iy corner eddies can also be observed in the corners of
the containing vessel .
FIguie 5 here.
the floxv is generated b) a vortex dipole of strength C I placed at depth L below the
undisturbed freesurface level (Jeong and SIoffatt 1992)
Prototype configuration for the formation of a free surface cusp
F i g w e 6 Freesurface cusp formed i n go!den syrup by the counte rro tation of
immersed cylinders (the left cylinder rotating clockwise, the right anticlockw
The Reynolds number is small. Note the viscous eddies in the bottom corners of
ta nk . [Ph oto courtesy of 1'.Kimura]
tM'O
ise).
' the
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528 UJCLOSING L E C T U R E :H . K E I T HMOFFATT
Of course, one would expect surface tension y to ‘round off’ the appar
ent cusp singularity. The small radius of curvature R at the cusp may
be estimated by a purely local argument (first noted by E. J. Hinch) as
follows: the surface tension provides a virtual ‘line force’ 2y located at
the center of curvature, which we take at T = 0 (Fig. 7) . This drives aStokeslet flow with upward velocity on the plane of symmetry
where c is a constant of order unity. There is also a local downward
velocity U say, associated with the imposed stirring. These must equili
brate at the stagnation point on the free surface at T = R, i.e.
U =Y n ($)2TP
or equivalently,  = c exp (,>TPU .
L
Thus the radius of curvature decreases exponentially with local capillary
number (@IT).
Circle ofcurvature
radius R
Figure 7 Representation of the flow near the freesurface stagnation point as a super
position of a twodimensional Stokeslet and a locally uniform stream induced by
remote forcir,g.
The details have been worked out for the idealized Stokes flow in
which the stirring is induced by a vortex dipole of strength (U at depth L
below the (undisturbed) free surface (Jeong and Moffatt 1992). In this
case, the cusp forms at depth 2L/3 (i.e. at height L/3 above the vortex
dipole), and the exact solution yields U = 16a/L2 and c = 256/3. Hence
(6) becomes
R 256 = xp(  32~C ) ,L 3
(7 )
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Local a nd g lobal per spec t i ves in f l u i d d y n a m i c s 529
where C = p a / L 2 y , the capillary number based on the ‘given’ flow
parameters Q and L. This number represents a ratio of downward viscous
drag to restoring surface tension. If C = 1 , we might anticipate that R / L
should be of order unity, and so in a mathematical sense it is; however,
numerically, (7) then gives
which must surely be the smallest 0 ( 1 ) number ever encountered in a
fluid dynamical context! In reality therefore a cusp does form, and there
is an associated stress singularity that is presumably resolved only if
nonNewtonian and/or noncontinuum effects are taken into account.
In a region R (< r << L , the streamfunction of the ‘cusp’ flow within
the fluid is given in polar coordinates ( r , ) by
The second term shows a selfsimilar structure, the same as that associ
ated with a symmetric flow near the edge of a flat plate 0 = & T .
Although the cusping phenomenon has been analyzed only within theStokes approximation, I believe that it occurs quite generally in high
Reynolds number flow also, and that many observed phenomena can be
explained only through an understanding of the cusping mechanism. I
show one example in Fig. 8 , which shows a spherical cap bubble rising
in glycerine in the bubble tube at DAMTP, Cambridge. This bubble is
in the ‘wavyskirted regime’ characterized by the very thin film of air
entrained into the glycerine from the sharp edge of the bubble. The
flow is converging near this sharp edge, and it is therefore presumablycusped. The skirt forms because of the large air pressure generated in
this cusp region.
Similarly, when water runs smoothly into a deep bath (the ‘Japanese
bath problem’), it may be observed that, above a critical flow rate,
bubbles of air are entrained and the pouring suddenly becomes noisy as
a result of bubble oscillation. I believe that this entrainment of bubbles
must occur through the circular cusp that forms where the incoming jet
of water impinges on the free surface (Fig. 9) .
4. GLOBAL INVARIANTS
Let me pass now to the opposite situation, namely the unsteady
fully threedimensional flow of a strictly inviscid, incompressible fluid (of
course an idealization). This is the classical fluid dynamics of Helmholtz
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530 UJCLOSIAGL E C T U R EH K E I T H f O F T A T T
Figuie 8 The rise of a T $ ~ \ T . s k i r te d bubb le t h r ough t he c om m on r oom of D A L I T PC a m br i dge T he bubb l e t ube o f d i a m e t e r 17 c m c on ta i n s g l \ ce r i ne T he po r t r a i t 1s
of X S E dd i ng t on
Figuie 9 Th e Japanese ba t h prob lem a l amina r s t ream of w a t e r flows i n to a deep
ba th , above a c r it ica l flow ra t e Q c , bubbles a re en t ra ined th rough the c i rcu la r cusp
formed at the free surface
and Kelvin, for which we know t ha t t he c i rcu la t ion round eve ry ma te 
r ia l ( i .e . Lag rang ian) c i rcui t is conserved. A closely related result ( J .  J .
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Local and g lobal perspect ives in fluid dynamics 531
M oreau 1961, M offatt 1969) is th a t t h e helicity
Fl= 1 , u . w d V
for any m ater ial volume V . on whose surface w . n = 0. is also conserve d:here. w = c u r l u is th e vorticity field. T hi s conserv ation of helicity
ad m its topological in terpreta t ion : i t i s. in a sense that can be ref ined
(X rno l’d 19 74 ). th e degree of l inkage of co nsti tu ent vortex tub es in th e
flow. a linkage that. as recognized by Kelvin, is indeed conserved.
An ana logous resu lt ho lds in the mag ne tohydrodyn amics (M HD ) of
perfectly co nd uc tin g fluids, in which lines of m agne tic field B (or ‘B
l ines’) are f rozen in th e f lu id . T h e analogous conserved qu ant i ty is the
magnetic helici ty
X.11= l7 . B d V
where B = c u r l A . an d i t ad mi t s s imi la r topolog ical in te rp re ta t ion . I t
is in this MHD context that the concept of helici ty has proved to have
profound s ignif icance. in two quite complementary s i tuations:
4.1. The turbulent dynamo
I be l ieve tha t the mos t impor tan t fundamenta l b reak th rough o f the
last halfcentury in our understanding of turbulent processes relates to
the dynamo problem: under what c i rcumstances wi l l a magnet ic f ie ld
B ( z . ) systematically increase in intensity un der th e dis tort in g an d dif
fusive action of a field of s ta t i o n a ry h om og en eo us t u rb u le n c e u ( ~ , t ) ?
T h e ques tion was posed first in th is form by Batchelor (195 0) . T h e sim
ple answer is tha t a suff icient condit ion for such dy na m o acti on is tha t
the turbule nce should . lack reflectional sy m m etry ’ : th e s imples t mea
s u r e of such lack of reflectional sy m m et ry is th e m ean helicity of th e
tu rbu lence ( U w ) . an d if this mean helici ty is nonzero. the n dy na m o
action will in general occur (Steenbeck, Krause, and Radler 1966. Mof
fa t t 1970) . Th is result h as huge impl icat ions for curren t un ders tan ding
of the process by which magnetic f ields are generated in planets , s tars
an d galaxies: i t is in more senses th a n one, a t ru ly g lobal phen omen on .
4.2. R,elaxation to equilibrium states
The relaxation process is the converse of the dynamo process and is
bes t i l lus t r a ted by s imple example . S uppose th a t a t some in i t i a l ins tan t
t = 0 , a magnetic field Bo(z )having some nontr ivial knotted or l inked
s t ruc tu re i s imbedded in a perfectly con du ctin g. bu t viscous. incom
pressible fluid a t res t . T h e pr ot oty pe consis t ing of two l inked, un kn ot
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532 UJCLOSING L E C T U R E :H. EITHNOFFATT
ted flux tubes, is shown in Fig. 10. The current corresponding to Bo is
j o = curlBo, the Lorentz force distribution Fo = j , x Bo is in general
rotational, and the fluid must flow in response to this force. Thus mag
netic energy is converted to kinetic energy as in an electric motor, and
this is dissipated by viscosity. However, the topology of the field B ( z , )is conserved, since the lines of force are frozen in the fluid and deform
continuously with the flow. In particular , the magnetic helicity, which
in this special case is XM = 2QlQ.2 where Q1 and cP2 are the fluxes in
the two tubes, is conserved.
Fegure 10 Pro toty pe conf igurat ion for the problem of magne t ic re laxat io n: two l inked
f lux tubes c ontrac t in a perfectly cond uct ing f lu id , wi th f luxes an d volumes conserved,
unt i l th e l inkage prevents fu r ther c ontrac t ion (Moffat t 1985) .
We are thus faced with the problem (Arnol’d 1974, Moffatt 1985) of
minimizing magnetic energy subject to conservation of field topology;
more precisely, of finding minimum energy states that are topologically
accessible from the initial state. In these states, the Lorentz force is
irrotational, since the fluid must come to rest, i.e.
j x B = V p , j = c u r l B ,
for some scalar field p (evidently the pressure field). This process of
magnetic relaxation is brought about in physical terms through relax
ation of the Maxwell stress in each flux tube, as illustrated in Fig. 10.
The tubes contract in axial length, and since volume is conserved, their
cross sections increase in area. The topological constraint arrests the
relaxation process when the tubes ultimately make contact: a singular
ity (here a surface discontinuity) inevitably appears as t +cc (it may
occur at finite time, but this seems unlikely).
Equation (8) has precisely the same structure as the Euler equation
for steady flow of an inviscid incompressible fluid:
u x w = V h , w = c u r l u , ( 9 )
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Local and global perspect ives in f l u i d d y n a m i c s 533
where h = p / p + i u 2 a n d p is the f luid densi ty (assumed co ns tan t) . To
every so lution of (8) , the re corresponds a solution of (9) via th e analogy
B + u , j + w , p + h o  h ,
where ho is an arb i t rary con s tant . Th us , magnet ic relaxation provides anindirect means of proving the existence of steady Euler flows (solutions
of (9)) of arb itra rily complex streamline topology; consideration of their
s tabi l i ty is quite another matter (Moffat t 1986; see also Vladimirov,
M offatt , and I lin 1999).
5. EXISTENCE OF SMOOTH SOLUTIONS
OF THE NAVIERSTOKES EQUATIONS
FOR ALL T > 0
T h e problem of existence an d smoothn ess for th e NavierStokes (N S)
equa tions was addressed by Leray (1934) an d has att ra ct ed intense effort
in the mathematical community s ince then. Despite these effor ts , the
problem remains open; it has recently achieved millennia1 status as one
of th e seven problems identified by th e Clay In sti tu te for which a prize
of one million d ollars is offered. Cha rles Fefferman has given d etails for
the NS problem on the websi te of the Clay Inst i tute.I do n’t propose t o solve thi s problem toda y! I do wish however to
make some observat ions about i t , because i t seems to me that both
local and global considerations are likely to play a part in its solution.
Leray himself was unable to prove the existence of smooth solutions
of the N S equations in 3 dimensions for all t > 0, nd recognized the
al ter nat iv e possibi li ty th a t a s ingular i ty may develop a t f inite time, t*
say. If such a singularity develops at a point that we may take to be
z =0, then i t seems plausible that the approach to this s ingular i ty (ast + *) hou ld be a t least locally selfsimilar. Leray n oted a remarkable
similar ity t ransfo rm ation of th e NS equations, or equivalently of the
vorticity equation
dW
at v x ( U x w ) + v v 2 w
Th is t r ans format ion is
where I? is a constant with the dimensions of circulation (velocity x
length ) . Th e vor tici ty the n t ransforms as
w = c u r l u = Cl((X) . (11)1
t* 0 = cur1,U
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534 UJCLOSISG L E C T U R E :H . KEITH IOFFATT
and equation (10) transforms to
0= v x (U + ;x) f2 + E V f  2 , (12)
whereE= u/r, dimensionless parameter that we may assume to besmall. The (x,) problem of (10) is thus converted to a 'steady' problem
( 1 2 ) in the scaled space variable X . The term dw/d2 in (10) transforms
to the term i V x ( X x f2) in (12). which represents outward transport of
the vorticity field f2 by the steady spherically symmetric 'compressible
velocity field' i X . The question now is whether there exists a distribu
tion of vorticity such that the corresponding velocity U in conjunction
with diffusion can compensate this outward transport.
For reasons developed in a recent paper (Moffatt 2000a), it is relevantto seek solutions of (12) in an inner region (in the sense of matched
asymptotic expansions) for which O ( X )matches with w ( x , ) (the outer
solution) in an overlap region X + m, x + 0. At leading order, this
seems to require that
1 11
. R ( X )  L a s X + x ,IN2
( X )1x1
with corresponding outer behavior
(This, incidentally, is suggestive of Stokeslet behavior, which might be
thought to require a source of momentum in the inner region. and thus
to be incompatible with unforced evolution!)
It has been shown by NeEas, R6iiEka and SverAk (1996) that no
smooth nontrivial solution of (12) exists (with E > 0) for which IUI E
L 3 ( X 3 ) . he behavior (13) is jus t at the boundary of this function space,
and is not excluded by the theorem.
A vorticity singularity as represented by (11) can occur only through
stretching of vortex lines at a rate that also becomes infinite as t + t * .Such a process can conceivably occur through the nonlinear interaction
of vortex tubes each one of which provides the strain field acting on the
other. Some mechanism must be present to cause all length scales in the
inner region t o decrease to zero (like (t* ) ' I 2 ) as t + t * . Scenariosinvolving the collision of nonparallel vortex pairs seem plausible candi
dates (Pelz 1997, Moffatt 2000a), the collision zone being then the inner
region in which the Leray transformation is relevant. An alternative
scenario is sketched in Fig. 11, which shows a vortex pair knotted in
the internal zone and arranged in the external zone in such a way as to
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Locul u n d global perspec t i ves in fluid dy numic s 535
.pul l th e knot t ig ht ‘ . Th is is cer ta in ly suggest ive of how a finitetime
s ingula r i ty may ar ise if th e ini tia l condit,ions are sufficiently ingeniously
contr ived.
Figuie 11 Conceptual sketch of a vortex pair configuration fo r which propagation of
the pair in the external region leads to tightening of any knot that may be contained
within t he inner region (here represented simply by a ’black box‘).
Topology evidently plays a pa r t in th i s . Note tha t th e con t r ibu t ion to
helicity from any region V ( t ) :1x1 < R ( w i th R co ns ta nt) of the inner
zone is given by
?i/ c . . u d 3 x = r l R U , 2 d 3 X ,
an d th a t th is is ind epe nd ent of t im e. Th is suggests conservation of
topology of vortex tubes in the inner region d e s p i t e the znfluence of
viscoszty. I t i s pe rha ps here th at one may f ind a clue for t h e relevance of
global ( i .e . topological) considerations for t h e NS s ingularity p roblem.
6. A TOY MODEL OF A FINITETIMESINGULARITY
I t may be app ropr ia te to conc lude th i s lec tu re wi th a real finitetimes ingu la r i ty ! Th is is exh ibite d by t h e toy ’Euler’s disk’ , a heavy disk that
ro lls on i t s edge (F ig . 1 2 ) ’ and exh ib i t s a paradoxical increase of rolling
speed as i ts energy decreases (Bendik 2000) . T he energy E is equal to
:Mgact. (M = m as s , a = r ad ius ) when th e ang le Q between th e disk and
th e table is sma l l ( i . e . du r ing th e f inal ‘ shuddering’ phase of m ot ion) .
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536 U J C L O S I N G L E C T U R E :H. K E I T HMOFFATT
The rolling (precessional) speed R is related to Q during this phase by
the equation
(14)R 2 a = 49,
a
a consequence of Euler’s angular momentum equation, coupled with the
rolling condition. As a tends to zero, tnen for so long as (14) holds (the
‘adiabatic approximation’), R increases like ciP1l2, apparently without
limit. The interesting feature of this toy is that the phenomenon can
indeed be observed at finite time (of the order of 102s) after the disk is
set in motion; I demonstrate this here with the aid of a microphone to
amplify the increasing frequency R and the very abrupt stop. Reflection
of a laser from the surfa,ce of the disk onto a screen can be used to
visualize and measure the decrease of a as a function of time.
\ I
\ I
\ I
\ I
7/
F z g w e 12 Euler’s disk: a heavy c i rcular d isk rol ls on i t s r im wi th precessional angu lar
velocity fl. T h e sy s t e m e x h i b it s a f in ite time s ingu lar i ty induced by weak diss ipat iveprocesses (Moffat t 2000b).
The energy equation is   @ ( E ) ,E
dt
where @ is the rate of dissipation of energy, whatever the dominant
dissipative mechanism may be. If @(E)( E X wkere X < 1, (15) implies
that E goes to zero at finite time, t* say, and hence that, within the
adiabatic approximation, R + cx) as t + t*. If X < 0, then obviously
d E / d t + cc also as t + t*, and the settling process may reasonably
be described as ‘dramatic’. The behavior is familiar to anyone who has
spun a coin upon a table. (It is to be contrasted with, for example,
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Local and g loba l perspect ives in f l u i d d y n a m i c s 537
linear damping of a simple pendulum, for which X = 1, and the decay is
exponential.)
The viscosity of the air in the thin layer between the disk and the table
in the final shuddering phase provides one obvious dissipative mecha
nism for which @ ( E )may be calculated (Moffatt 2000b). A lubricationapproximation gives X = 2; an improved analysis allowing for Stokes
layers on the disk and table (L . Bildsten, private communication) gives
X = 5/4. Either way, the finitetime singularity is dramatic.
There is also a contribution to dissipation from rolling friction, which
is apparently much more difficult to quantify. On the simple assumption
that for this mechanism @ c( R , (14) with E 0: (Y gives X = 3 , and again
the behavior is dramatic. The difficulty here is to obtain the coefficient
of proportionality between @ and R in terms of the physical propertiesof the disk and the tablea challenging problem that perhaps someone
in this large audience knows how to solve.
It is likely that different dissipative mechanisms dominate at differ
ent stages of the settling process, and that the effect of air viscosity is
dominant only in the very final stage before the sudden stop. The pre
dicted singularity is resolved during the final few microseconds, when the
downward vertical acceleration of the centerofmass of the disk becomes
comparable with the acceleration of gravity (Moffatt 2000b).
7. CONCLUSION
The smoothness, or alternatively the finitetime singularity, of the
NavierStokes equations offers a challenge that will continue to make
great demands on both analytical ingenuity and computational power.
If, as computer simulations continue to indicate (Kerr 1997), a finitetime
singularity does occur and if this is generic behavior, then of course we
shall have to understand by what mechanism these putative singularities
are resolved. Since the pressure gradient must also become unbounded as
a singularity is approached, the incompressibility assumption, on which
most analyses of this phenomenon are based, becomes no longer tenable.
The infinite stress at a singularity can be relieved by cavitation in liq
uids, and by acoustic radiation in gases. The Japanese bath provides a
congenial environment for the contemplation of such problems!
As I mentioned in my introductory remarks, this Congress exhibits a
most fruitful interplay between local and global characteristics. I would
maintain now that it also has some features of a finitetime singular
ity: there was certainly a decreasing lengthscale as we all converged on
Chicago; and the Organizers were acutely aware of a decreasing time
scale in the last few weeks of hectic preparations. We should be relieved
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tha t the Narr io t t Hote l has ne i ther imploded nor exploded under the
extrenic pressure of activity th a t i t has experienced this week. Now
we are at the stage where the singularity must be resolved: we shall
returri to our homes aro und th e world like acoust ic pulses radia t ing from
this source. and th e local ly acquired imp act of th is C ongress will surelyinform and inspire future research in theoretical and applied mechanics
on a global scale . O n th at note , an d l ike a finitet ime singularity, I mu s t
bring this ta lk t o a sudden end .
Acknowledgments
I t han k S arah Ki rkup an d Jon a than Chin fo r he lp in process ing th e t ex t a nd
f iguies , and SIu stap ha Xm rani for help in prepar in g the pow erpoint version of t h e
lec tu re
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