The Hopf Bifurcation And Its Applications

THE HOPF BIFURCATION AND ITS APPLICATIONS1SECTION 1INTRODUCTION TO STABILITY AND BIFURCATION INDYNAMICAL SYSTEMS AND FLUID MECHANICSSuppose we are studying a physical system whose stateis governed by an evolution equationunique integral curves.ofX; i.e., X(X )O O.upon the system at timein statexo.will remain atanswer to thisdxdt X(x)xwhich hasLetXbe a fixed point of the flowoImagine that we perform an experimentt 0and conclude that it is thenAre we justified in predicting that the systemXofor all future time?qu stionThe mathematicalis obviously yes, but unfortunatelyit is probably not the question we really wished to ask.Experiments in real life seldom yield exact answers to ouridealized models, so in most cases we will have to ask whetherthe system will remain nearXif it started near xo. Theoanswer to the revised question is not always yes, but even so,by examining the evolution equation at hand more minutely, onecan sometimes make predictions about the future behavior ofa system starting nearxo.A trivial example will illustratesome of the problems involved.Consider the following two

2THE HOPF BIFURCATION AND ITS APPLICATIONSdi erentialequations on the real line:X'(t) -x (t)(1.1)X(t) x(1. 2)andI(t) The solutions are respectively:(1.1')and(1.2')Note that0is a ixedpointbotho lows.In the firstE R, lim x(xo,t)case, for allX O.The whole real linet oomoves toward the origin, and the prediction that if Xisonearx(xo,t)0, thenois near0is obviously justified.On the other hand, suppose we are observing a system whosestatexat timeis governed by (1.2).t 0, x'( 0)An experiment telling us thatis approximately zero will certainly notpermit us to conclude thatx(t)all time, since all points exceptstays near the origin for0move rapidly away from O.Furthermore, our experiment is unlikely to allow us to makean accurate prediction aboutx(t)because ifmoves rapidly away from the origin towardx(O) 0, x(t)moves toward 00.alltbut ifThus, an observer watchingsuch a system would expect sometimes to observeand sometimesx(O) 0, x(t)x(t)t x(t) 00.The solution x(t)o fort oowould probably never be observed to occur because aslight perturbation of the system would destroy this solution.This sort of behavior is frequently observed in nature.is not due to any nonuniqueness in the solution to theItdif er-ential equation involved, but to the instability of thatsolution under small perturbations in initial data.

THE HOPF BIFURCATION AND ITS APPLICATIONS3Indeed, it is only stable mathematical models, orfeatures of models that can be relevant in "describing" nature. Consider the following example.*A rigid hoop hangsfrom the ceiling and a small ball rests in the bottom of thehoop.The hoop rotates with frequencywabout a verticalaxis through its center (Figure l.la).Figure l.laFor small values ofFigure l.lbw, the ball stays at the bottom of thehoop and that position is stable.some critical valueHowever, whenwreachesw ' the ball rolls up the side of thehoop to a new positionox(w), which is stable.The ball mayroll to the left or to the right, depending to which side ofthe vertical axis it was initially leaning (Figure l.lb).The position at the bottom of the hoop is still a fixed point,but it has become unstable, and, in practice, is never observed to occur.The solutions to the differential equationsgoverning the ball's motion are unique for all values of For further discussion, see the conclusion of AbrahamMarsden [1].* Thisexample was first pointed out to us by E. Calabi.w,

4THE HOPF BIFURCATION AND ITS APPLICATIONSbut foro'w wthis uniqueness is irrelevant to us, for wecannot predict which way the ball will roll.Mathematically,we say that the original stable fixed point has become unstable and has split into two stable fixed points.SeeFigure 1.2 and Exercise 1.16 below.wfixedpoints(0 )(b)Figure 1.2Since questions of stability are of overwhelming practical importance, we will want to define the concept ofstability precisely and develop criteria for determining it.(1.1)Definition.LetFtsemiflow)* on a topological spacevariant set; i. e. , Ft (A) C AcObe aflow (orM and letfor allt.Abe an in-We sayAisstable (resp. asymptotically stable or an attractor) i f for anyneighborhoodUofAthere is a neighborhoodVofAsuch*i.e.,F t : M ; M, F O identity, and. F t s FsoFt for allt, s ER. Cmeans Ft(x)is continuous in(t,x). Asemiflow is one defined only fort O.Consult, e.g.,Lang [1], Hartman [1], or Abraham-Marsden [1] for a discussionof flows of vector fields.See section BA, or Chernoff-Marsden [lJ for the infinite dimensional case.

THE HOPF BIFURCATION AND ITS APPLICATIONSx(xo,t) - Ft(x )Othat the flow lines (integral curves)long toUThusifAXoE V(resp.nt O(resp. tends towardsAbe-F (V) A).tis stable (resp. attracting) when an initialcondition slightly perturbed fromIf5A).Aremains nearA(See Figure 1.3).is not stable it is called unstable.stablefixedpointas ymptot ieo Ilysto bl efixed pointstableclosedor bitFigure 1.3(1.2)Exercise.Show that in the ball in the hoopexample, the bottom of the hoop is an attracting fixed pointw W Ig/R and that for W oing fixed points determined by cos eforWo there are attract2g/w R, wherethe angle with the negative vertical axis, Rof the hoop andgeisis the radiusis the acceleration due to gravity.The simplest case for which we can determine thestability of a fixed point Xo is the finite dimensional,nlinear case. Let X: R Rn be a linear map. The flow of

THE HOPF BIFURCATION AND ITS APPLICATIONS6xis X(Xo,t)etX(x ). Clearly, the origin is a fixedOAjtpoint. Let {A } be the eigenvalues of X. Then {e}jtXare the eigenvalues of eSuppose Re A. 0 for all j JRe A.tJThen- - 0as t - - 00. One can check, usingI/jtl ethe Jordan canonical form, that in this caseA.totica11y stable and that if there is areal part, 0(1.3)is unstable.Theorem.Letmap on a Banach spaceE.fixed point of the flow ofz E o(X)withJpos tiveMore generally, we have:X: E- -Ebe a continuous, linearThe origin is a stable attractingXif the spectrumis in the open left-half plane.there existsis asymp-0o(X)ofXThe origin is unstable ifsuch thatRe(z) O.This will be proved in Section 2A, along with a reviewof some relevant spectral theory.Consider now the nonlinear case. Let P be a Banach1manifo1d* and let X be a cvector field on P. LetThendX(PO): T (P)POlinear map on a Banach space.- -is a continuousT (P)PoAlso in Section 2A we shalldemonstrate the following basic theorem of Liapunov [1].(1.4)Theorem.Banach manifoldX(PO) O.PLetX(F t (x», FO(X)F If the spectrum ofO(dX(Po»*Wetx.Letand letXbe aPovector field on abe a fixed point ofbe the flow of(Note thatdX(PO)c1XFt(P O)i. e. , PoaatX, i. e. ,Ft(X)for allt. )lies in the left-half plane; i.e.,C {z EQ::IRe z O}, thenPois asymptoticallyshall use only the most elementary facts about manifoldtheory, mostly because of the convenient geometricallanguage. See Lang [1] or Marsden [4] for the basic ideas.

THE HOPF BIFURCATION AND ITS APPLICATIONS7stable.If there exists an isolatedRe z a, Pathere is ais unstable.Ifz E o(dX(Pa))z Eo(dX(Pa))O(dX(Pa))such thatcRe zsuch that{zlRe z S a}anda, then stability cannot be determined from the linearized equation.(1.5) 2Exercise.2: X(x,y) (y, (l-xConsider the following vector field on)y-x).Decide whether the origin is un-stable, stable, or attracting for a, a, and a.Many interesting physical problems are governed by differential equations depending on a parameter such as thewangular velocityX : P TP in the ball in the hoop example.be a (smooth) vector field on a Banach manifoldAssume that there is a continuous curvethatofX X .(p( ))a, i.e., Suppose that unstable for a.p( )Xinp( )Psuch ais attracting for(p( a)' a)The pointX .Forandis then called athe flowcan be described (at least in a neighborhood of by saying that points tend towardtrue for a' a.abruptly at a' a.P.is a fixed point of the flowp( )a bifurcation point of the flow ofofLetp( ).p( ))However, this is notand so the character of the flow may changeSince the fixed point is unstable forwe will be interested in finding stable behavior forThat is, we are interested in finding bifurcationabove criticality to stable behavior.For example, several curves of fixed points may corne to(A curve of fixed points is agether at a bifurcation point.curvea: I Psuch thatX (a( ))such curve is obviously p( ).)stable fixed points for a. afor all .OneThere may be curves ofIn the case of the ball in

THE HOPF BIFURCATION AND ITS APPLICATIONS8the hoop, there are two curves of stable fixed points forW w ' one moving up the left side of the hoop and one movingOup the right side (Figure 1.2).Another type of behavior that may occur is bifurcationto periodic orbits.forma: I . Pclosed orbitThis means that there are curves of thesuch thatY]1a(]10)of the flow of p(]10)X]1.Hopf bifurcation is of this type.anda(]1)is on a(See Figure 1.4).ThePhysical examples in fluidmechanics will be given shortly.-----t -f--unstable fixed point-sta b Ie closed orbity-stable fixed pointxunstable fixedpoint - - stable fixed pointx(a) Supercritica I Bifurcation(b) Subcritical Bifurcation(Stable Closed Orbits)(Unstable Closed Orbits)Figure 1.4The General Nature of the Hopf Bifurcation

THE HOPF BIFURCATION AND ITS APPLICATIONS9The appearance of the stable closed orbits ( periodicsolutions) is interpreted as a "shift of stability" from theoriginal stationary solution to the periodic one, i.e., apoint near the original fixed point now is attracted to andbecomes indistinguishable from the closed orbit.{SeeFigures 1.4 and 1.5).furtherbifurco tionsstable pointappearance ofthe closed orbita closed orbitgrows in amplitudeFigure 1.5The Hopf BifurcationOther kinds of bifurcation can occur; for example, aswe shall see later, the stable closed orbit in Figure 1.4 maybifurcate to a stable 2-torus.In the presence of symmetries,the situation is also more complicated.This will be treatedin some detail in Section 7, but for now we illustrate whatcan happen via an example.(1.6)Example:The Ball in the Sphere.A rigid,hollow sphere with a small ball inside it hangs from the

10THE HOPF BIFURCATION AND ITS APPLICATIONSceiling and rotates with frequency wthrough its centerabout a vertical axis(Figure 1.6).w woFigure 1. 6For smallbut forw Wow, the bottom of the sphere is a stable point,the ball moves up the side of the sphere toa new fixed point.For eachw wo ' there is a stable, in-variant circle of fixed points (Figure 1.7).We get a circleof fixed points rather than isolated ones because of thesymmetries present in the problem.stable circleFigure 1. 7Before we discuss methods of determining what kind ofbifurcation will take place and associated stability questions,

THE HOPF BIFURCATION AND ITS APPLICATIONS11we shall briefly describe the general basin bifurcationpicture of R. Abraham [1,2].In this picture one imagines a rolling landscape onwhich water is flowing.We picture an attractor as a basininto which water flows.Precisely, ifandAis an attractor, the basin ofx E M which tend toAast 00.FtAis a flow onMis the set of all(The less picturesquephrase "stable manifold" is more commonly used.)As parameters are tuned, the landscape, undulates andthe flow changes.Basins may merge, new ones may form, oldones may disappear, complicated attractors may develop, etc.The Hopf bifurcation may be pictured as follows.Webegin with a simple basin of parabolic shape; Le., a pointattractor.As our parameter is tuned, a small hillock formsand grows at the center of the basin.The new attractor is,therefore, circular (viz the periodic orbit in the Hopftheorem) and its basin is the original one minus the top pointof the hillock.Notice that complicated attractors can spontaneouslyappear or dissappear as mesas are lowered to basins or basinsare raised into mesas.Many examples of bifurcations occur in nature, as aglance at the rest of the text and the bibliography shows.The Hopf bifurcation is behind oscillations in chemical andIbiological systems (see e.g. Kopell-Howard [1-6], Abraham [1,2]and Sections 10, 11), including such 'things as "heart flutter".One of the most studied examples comes from fluid mechanics,so we now pause briefly to consider the basic ideas of* That "heart flutter" is a Hopf bifurcation is a conjecturetold to us by A. Fischer;cf. Zeeman [2].*

12THE HOPF BIFURCATION AND ITS APPLICATIONSthe subject.The Navier-Stokes EquationsLetDC R3be an open, bounded set with smooth boundary.We will considerDto be filled with an incompressiblehomogeneous (constant density) fluid.Letuandvelocity and pressure of the fluid, respectively.p,be theIf thefluid is viscous and if changes in temperature can beneglected, the equations governing its motion are:au (u.V)u - v u -grad pat( external forces)(L3)div u 0The boundary condition isif the boundary ofthatu(x,t)u(x,O)andDuloaD(orulaDprescribed,is moving) and the initial condition isis some givenp(x,t)(1. 4)fortuO(x). O.The problem is to findThe first equation (1.3) isanalogous to Newton's Second LawFrna; the second (1.4)isequivalent to the incompressibility of the fluid.*Think of the evolution equation (1.3) as a vector fieldand so defines a flow, on the spacevector fields onD.Iof all divergence free(There are major technical difficultieshere, but we ignore them for now-see Section 8. )The Reynolds number of the flow is defined bywhereUandLwith the flow, andR ULv,are a typical speed and a length associatedvis the fluid's viscosity.For example,if we are considering the flow near a sphere.toward which fluid is projected with constant velocityU00 i*see any fluid mechanics text for a discussion of thesepoints. For example, see Serrin [1], Shinbrot [1] or HughesMarsden [3].

THE HOPF BIFURCATION AND ITS APPLICATIONS(Figure 1.8), thensphere andUL13may be taken to be the radius of theUoo -Figure 1.8If the fluid is not viscous(v0), thenR00, andthe fluid satisfies Euler's equations:auat (u·V)udivuThe boundary condition becomes:ul laDfrom 0on(1.5)o.(1.6)is parallel to aD, oraDThis sudden change of boundary conditionfor short.u-grad paDtoul laDulis of fundamental significanceand is responsible for many of the difficulties in fluidmechanics forRvery large (see footnote below).The Reynolds number of the flow has the property that,if we rescale as follows:u*x*t*P*U*UL*LT*uxT tu *) 2prlU

14THE HOPF BIFURCATION AND ITS APPLICATIONSthen ifR andT L/U, T*UL/v, u*t* L*/U*and providedR* U*L*/V* satisfies the same equations with respect tothatusatisfies with respect to* (u*·V*)u* dt*div u*with the same boundary conditionxandx*t; i.e.,-grad p*(1. 7)a(1. 8) au*1as before.(ThisdDis easy to check and is called Reynolds' law of similarity.)Thus, the nature of these two solutions of the Navier-Stokesequations is the same.The fact that this rescaling can bedone is essential in practical problems.For example, itallows engineers to testa scale model of an airplane at lowspeeds to determine whether the real airplane will be able tofly at high speeds.(1.7)Example.Consider the flow in Figure 1.8.Ifthe fluid is not viscous, the boundary condition is that thevelocity at the surface of the sphere is parallel to thesphere, and the fluid slips smoothly past the sphere(Figure 1.9).Figure 1. 9Now consider the same situation, but in the viscous case.Assume thatRstarts off small and isgradual yincreased.(In the laboratory this is usually accomplished by increasing

THE HOPF BIFURCATION AND ITS APPLICATIONSthe velocity- Uooi, but we may wish to think of it asi.e., molasses changing to water.)15v- 0,Because of the no-slipcondition at the surface of the sphere, asthe velocity gradient increases there.Uoogets larger,This causes the flowto become more and more complicated (Figure 1.10).*For small values of the Reynolds number, the velocityfield behind the sphere is observed to be stationary, orapproximately so, but when a critical value of the Reynoldsnumber is reached, it becomes periodic.For even highervalues of the Reynolds number, the periodic solution losesstability and further bifurcations take place.The furtherbifurcation illustrated in Figure 1.10 is believed to represent a bifurcation from an attracting periodic orbit to aperiodic orbit on an attracting 2-torus inI.bifurcations may eventually lead to turbulence.These furtherSee Remark1.15 and Section 9 below.--. 0::R 50 (a periodic solution)IIfurther bifurcation as R increases t -ct "-.Y"---./R 75 (a slightly altered periodic solution)Figure 1.10*Theselarge velocity gradients mean that in numericalstudies, finite difference techniques become useless forinteresting flows.Recently A. Chorin [1] has introduceda brilliant technique for overcoming these difficultiesand is able to simulate numerically for the first time,the "Karmen vortex sheet", illustrated in Figure 1.10.See also Marsden [5] and Marsden-McCracken [2].

16THE HOPF BIFURCATION AND ITS APPLICATIONS(1.8)Example.Couette Flow.A viscous,*incompressi-ble, homogeneous fluid fills the space between two long,coaxial cylinders which are rotating.For example, they mayrotate in opposite directions with frequencyFor small values ofw(Figure 1.11).w, the flow is horizontal, laminar andfluidstationary.IIIwfH-WIFigure 1.11If the frequency is increased beyond some valuewo' thefluid breaks up into what are called Taylor cells (Figure 1.12).top viewFigure 1.12*couetteflow is studied extensively in the literature (seeSerrin [1], Coles [1]) and is a stationary flow of the Eulerequations as well as of the Navier-Stokes equations (see thefollowing exercise).

THE HOPF BIFURCATION AND ITS APPLICATIONS17Taylor cells are also a stationary solution of the NavierStokes equations.For larger values ofw, bifurcations toperiodic, doubly periodic and more complicated solutions maytake place (Figure 1.13).doubly periodic structurehe Iicc I stru cture}o'igure 1.13For still larger values ofw, the structure of the Taylorcells becomes more complex and eventually breaks downcompletely and the flow becomes turbule

Department of Mathematics University of California at Berkeley M. McCracken Department of Mathematics University of California at Santa Cruz AMS Classifications: 34C15, 58F1 0, 35G25, 76E30 Library of Congress Cataloging in Publication Data Marsden, Jerrold E. The Hopf bifurcation and its applications. (Applied mathematical sciences; v. 19 .