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T Leray-Hopf and Continuity Properties for All Weak Solutions for F the 3D Navier-Stokes Equations 5 1 0 October12,2015 2 t c A O NataliiaV.Gorban1,PavloO.Kasyanov2,OlhaV.Khomenko3,andLuisaToscano4. 8 ] P Abstract A . Inthisnoteweprovethateachweaksolutionforthe3DNavier-StokessystemsatisfiesLeray-Hopf h property. Moreover,each weak solutionis rightlycontinuousin the standardphasespace H endowed t a withthestrongconvergencetopology. m [ 2 1 IntroductionRand Main Result v 9 7 LetΩ ⊂ R3 beabounded domainwithrathersmoothboundary Γ = ∂Ω,and[τ,T]beafixedtimeinterval 6 with−∞ < τ < T < +∞. Weconsider3DNavier-Stokes system inΩ×[τ,T] 0 0 . ∂y 1 −ν△y+(y·∇)y = −∇p+f, divy = 0, 0  ∂t (1.1) 5  y = 0, y = y , 1 (cid:12)Γ (cid:12)t=τ τ :  (cid:12) (cid:12) v where y(x,t) means the unknown velocity, p(x,t) the unknown pressure, f(x,t) the given exterior force, i D X andy (x)thegiveninitialvelocitywitht ∈[τ,T],x ∈ Ω,ν > 0meanstheviscosity constant. τ r ThroughoutthisnoteweconsidergeneralizedsettingofProblem(1.1). Forthispurposedefinetheusual a function spaces V = {u ∈ (C0∞(Ω))3 :divu= 0}, Vσ = cl(H0σ(Ω))3V, σ ≥ 0, wherecl denotestheclosureinthespaceX. SetH := V ,V := V . ItiswellknownthateachV ,σ > 0, X 0 1 σ ∗ ∗ is a separable Hilbert space and identifying H and its dual H we have V ⊂ H ⊂ V with dense and σ σ 1InstituteforAppliedSystemAnalysis,NationalTechnicalUniversityofUkraine“KyivPolytechnicInstitute”,Peremogyave., 37,build,35,03056,Kyiv,Ukraine,nata [email protected] 2InstituteforAppliedSystemAnalysis,NationalTechnicalUniversityofUkraine“KyivPolytechnicInstitute”,Peremogyave., 37,build,35,03056,Kyiv,Ukraine,[email protected]. 3InstituteforAppliedSystemAnalysis,NationalTechnicalUniversityofUkraine“KyivPolytechnicInstitute”,Peremogyave., 37,build,35,03056,Kyiv,Ukraine,[email protected] 4University of Naples “Federico II”, Dep. Math. and Appl. R.Caccioppoli, via Claudio 21, 80125 Naples, Italy, [email protected] 1 T compactembeddingforeachσ > 0. Wedenoteby(·,·), k·kand((·,·)), k·k theinnerproductandnorm V ∗ in H and V, respectively; h·,·i will denote pairing between V and V that coincides on H ×V with the innerproduct(·,·). LetH bethespaceH endowedwiththeweaktopology. Foru,v,w ∈ V weput w F3 ∂v j b(u,v,w) = u w dx. i j Z ∂x ΩiX,j=1 i It is known that b is a trilinear continuous form on V and b(u,v,v) = 0, if u,v ∈ V. Furthermore, there existsapositiveconstantC suchthat |b(u,v,w)| ≤ Ckuk kvk kwk , (1.2) V V V foreachu,v,w ∈ V;see,forexample,ASohr[17,LemmaV.1.2.1]andreferences therein. Let f ∈ L2(τ,T;V∗) + L1(τ,T;H) and y ∈ H. Recall that the function y ∈ L2(τ,T;V) with τ dy ∈ L1(τ,T;V∗)isaweaksolution ofProblem(1.1)on[τ,T],ifforallv ∈ V dt d (y,v)+ν((y,v))+b(y,y,v) = hf,vi (1.3) dt inthesenseofdistributions, and y(τ)= y . (1.4) τ The weak solution y of Problem (1.1) on [τ,T] is called a Leray-Hopf solution of Problem (1.1)on [τ,T], R ify satisfiestheenergy inequality: V (y(t)) ≤ V (y(s)) forallt ∈ [s,T], a.e. s > τ ands = τ, (1.5) τ τ where ς ς 1 V (y(ς)) := ky(ς)k2 +ν ky(ξ)k2 dξ− hf(ξ),y(ξ)idξ, ς ∈[τ,T]. (1.6) τ 2 Z V Z τ τ For each f ∈ L2(τ,T;V∗) + L1(τ,T;H) and y ∈ H there exists at least one Leray-Hopf so- τ lution ofDProblem (1.1); see, for example, Temam [18, Chapter III] and references therein. Moreover, y ∈ C([τ,T],Hw) and dy ∈ L34(τ,T;V∗) + L1(τ,T;H). If f ∈ L2(τ,T;V∗), then, additionally, dt dy 4 ∗ ∈ L3(τ,T;V ). Inparticular, theinitialcondition (1.4)makessense. dt The following Theorem 1.1 implies that each weak solution of the 3D Navier-Stokes system is Leray- Hopf one and it is rightly strongly continuous in H at all the points t ∈ [τ,T). This theorem is the main resultofthisnote. Theorem 1.1. Let −∞ < τ < T < +∞, y ∈ H, f ∈ L2(τ,T;V∗)+ L1(τ,T;H), and y be a weak τ solution ofProblem(1.1)on[τ,T]. Thenthefollowing statementshold: (a) y ∈ C([τ,T],H )andthefollowingenergyinequality holds: w V (y(t)) ≤ V (y(s)) forallt,s ∈ [τ,T], t ≥ s, (1.7) τ τ whereV isdefinedinformula(1.6); τ 2 T (b) foreacht ∈ [τ,T)thefollowing convergence holds: y(s) → y(t)strongly inH ass → t+; (c) thefunction t → ky(t)k2 isofbounded variation on[τ,T]. F Remark 1.2. Since a real function of bounded variation has no more than countable set of discontinuity points, then statement (a) of Theorem 1.1, weak continuity in Hilbert space H of each weak solution of Problem (1.1) on [τ,T], yield that each weak solution of the 3D Navier-Stokes system has no more than countable set of discontinuity points in the phase space H endowed with the strong convergence topology. Theorem 1.1partially clarifiestheresults provided inBall[1];Balibrea etal. [2];Barbuetal. [3];Caoand Titi [4]; Chepyzhov and Vishik [5]; Cheskidov and Shvydkoy [6]; Kapustyan et al. [9, 10]; Kloeden et al. A [13];Sohr[17]andreferences therein. 2 Topological Properties of Solutions for Auxiliary Control Problem Let−∞ < τ < T < +∞. Weconsider thefollowingspaceofparameters: U := (L2(τ,T;V))× L2(τ,T;V∗)+L1(τ,T;H) ×H. τ,T (cid:0) (cid:1) Eachtriple(u,g,z ) ∈ U iscalledadmissible forthefollowingauxiliary controlproblem: τ τ,T dz Problem (C) on [τ,T] with (u,g,z ) ∈ U : find z ∈ L2(τ,T;V) with ∈ L1(τ,T;V∗) such that τ τ,T R dt z(τ) = z andforallv ∈ V τ d (z,v)+ν((z,v))+b(u,z,v) = hg,vi (2.1) dt inthesenseofdistributions; cf. Kapustyanetal. [9,10];Kasyanovetal. [11,12];MelnikandToscano[14]; Zgurovskyetal. [19,Chapter6]. ∗ As usual, let A : V → V be the linear operator associated with the bilinear form ((u,v)) = hAu,vi, ∗ u,v ∈ V. For u,v ∈ V we denote by B(u,v) the element of V defined by hB(u,v),wi = b(u,v,w), forallw ∈ V. ThenProblem(C)on[τ,T]with(u,g,z ) ∈ U canberewrittenas: findz ∈ L2(τ,T;V) τ τ,T dz D with ∈ L1(τ,T;V∗)suchthat dt dz ∗ +νAz+B(u,z) = g, inV , andz(τ) = z . (2.2) τ dt Thefollowingtheorem establishes theuniqueness properties forsolutions ofProblem(C). Theorem2.1. Let−∞ < τ < T < +∞andu∈ L2(τ,T;V). ThenProblem(C)on[τ,T]with(u,¯0,¯0)∈ U hastheuniquesolution z ≡ ¯0. τ,T We recall, that {w ,w ,...} ⊂ V is the special basis, if ((w ,v)) = λ (w ,v) for each v ∈ V and 1 2 j j j j = 1,2,...,where 0 < λ ≤ λ ≤ ...is thesequence ofeigenvalues. LetP bethe projection operator 1 2 m of H onto H := span{w ,...,w }, that is P v = m (v,w )w for each v ∈ H and m = 1,2,.... m 1 m m i=1 i i Of course we may consider P as a projection operatoPr that acts from V onto H for each σ > 0 and, m σ m sincePm∗ = Pm,wededuce thatkPmkL(Vσ∗;Vσ∗) ≤ 1. Notethat(wj,v)Vσ = λσj(wj,v)foreachv ∈ Vσ and j = 1,2,.... 3 T ProofofTheorem2.1. Let−∞ < τ < T < +∞,u ∈ L2(τ,T;V),and z beasolution ofProblem (C)on [τ,T]with(u,¯0,¯0)∈ U . Provethatz ≡ ¯0. τ,T Letusfixanarbitrary m = 1,2,....According tothedefinitionofasolution forProblem(C)on[τ,T] with(u,¯0,¯0) ∈ U ,thefollowingequality holds: τ,T F 1 d kP z(t)k2 +νkP z(t)k2 = b(u(t),P z(t),z(t)), (2.3) 2dt m m V m fora.e. t ∈ (τ,T).Sinceb(u(t),P z(t),P z(t)) = 0fora.e. t ∈ (τ,T),theninequality (1.2)yieldsthat m m b(u(t),P z(t),z(t)) ≤ Cku(t)k kP z(t)k kz(t)−P z(t)k , m V m V m V fora.e. t ∈ (τ,T).Therefore, equality(2.3)implythefollowinginequality A 1 d kP z(t)k2 +kP z(t)k (νkP z(t)k −Cku(t)k kz(t)−P z(t)k ) ≤ 0, (2.4) m m V m V V m V 2dt fora.e. t ∈ (τ,T). Letussetψ (t) := kP z(t)k (νkP z(t)k −Cku(t)k kz(t)−P z(t)k ),foreachm = 1,2,... m m V m V V m V anda.e. t ∈ (τ,T).Thefollowingstatements hold: (i) ψ ∈ L1(τ,T)foreachm = 1,2,...; m (ii) ψ (t) ≤ ψ (t)foreachm = 1,2,... anda.e. t ∈ (τ,T); m m+1 R (iii) ψ (t) → νkz(t)k2 asm → ∞,fora.e. t ∈ (τ,T). m V Indeed, statement (i) holds, because u,z ∈ L2(τ,T;V) and P z ∈ L∞(τ,T;V) for each m = 1,2,.... m Statement(ii)holds,becausekP z(t)k ≤ kP z(t)k and−kz(t)−P z(t)k ≤ −kz(t)−P z(t)k m V m+1 V m V m+1 V for each m = 1,2,... and a.e. t ∈ (τ,T). Statement (iii) holds, because P z(t) → z(t) strongly in V as m m → ∞,fora.e. t ∈ (τ,T). Since kz(·)k2 ∈ L1(τ,T), then statements (i)–(iii) and Lebesgue’s monotone convergence theorem V yield Dt t t lim ψ (s)ds = lim ψ (s)ds = kz(s)k2 ds, (2.5) m→∞Z m Z m→∞ m Z V τ τ τ foreacht ∈ [τ,T].Inequality (2.4)implies 1 t t 1 d t kP z(t)k2 +ν ψ (s)ds = kP z(t)k2 +ν ψ (s)ds ≤ 0, (2.6) m m m m 2 Z Z 2dt Z τ τ τ foreachm = 1,2,... andt ∈[τ,T]. Wenotethattheequalityin(2.6)holds,because z(τ) = ¯0. Equality(2.5)andinequality (2.6)yieldthat 1 t kz(t)k2 +ν kz(s)k2 ds ≤ 0, 2 Z V τ for a.e. t ∈ (τ,T), because P z(t) → z(t) strongly in H for a.e. t ∈ (τ,T). Thus, z(t) = ¯0 for a.e. m t ∈ (τ,T). Sincez ∈ C([τ,T];V∗),thenz ≡ ¯0,thatis,Problem(C)on[τ,T]with(u,¯0,¯0) ∈ U hasthe τ,T uniquesolution z ≡ ¯0. 4 T ThefollowingtheoremestablishessufficientconditionsfortheexistenceofanuniquesolutionforProb- lem(C).Thisisthemainresultofthissection. Theorem 2.2. Let −∞ < τ < T < +∞, y ∈ H, f ∈ L2(τ,T;V∗)+ L1(τ,T;H), and y be a weak τ solutionofProblem(1.1)on[τ,T]. Then(y,f,y )F∈ U andProblem(C)on[τ,T]with(y,f,y ) ∈ U τ τ,T τ τ,T hastheuniquesolution z = y. Moreover,y satisfies inequality (1.5). BeforetheproofofTheorem2.2weremarkthatAC([τ,T];H ),m = 1,2,...,willdenotethefamily m ofabsolutely continuous functions actingfrom[τ,T]intoH ,m = 1,2,.... m ProofofTheorem2.2. Prove that z = y is the unique solution of Problem (C) on [τ,T] with (y,f,y ) ∈ τ U . Indeed,yisthesolutionofProblem(C)on[τ,T]with(y,f,y ) ∈U ,becausey isaweaksolution τ,T τ τ,T A of Problem (1.1) on [τ,T]. Uniqueness holds, because if z is a solution of Problem (C) on [τ,T] with (y,f,y ) ∈ U ,thenz−y ≡ ¯0istheunique solution ofProblem (C)on[τ,T]with(y,¯0,¯0) ∈ U (see τ τ,T τ,T Theorem2.1). The rest of the proof establishes that y satisfies inequality (1.5). We note that y can be obtained via d standard Galerkin arguments, that is, if y ∈ AC([τ,T];H ) with y ∈ L1(τ,T;H ), m = 1,2,..., m m m m dt istheapproximate solution suchthat dy m +νAy +P B(y,y ) = P f, inH , y (τ) = P y(τ), (2.7) m m m m m m m dt R thenthefollowingstatements hold: (i) y satisfythefollowingenergyequality: m 1 t1 t1 ky (t )k2 +ν ky (ξ)k2 dξ − hf(ξ),y (ξ)idξ 2 m 1 Z m V Z m s s (2.8) 1 t2 t2 = ky (t )k2+ν ky (ξ)k2 dξ− hf(ξ),y (ξ)idξ, 2 m 2 Z m V Z m s s foreacht ,t ∈ [τ,T],foreachm = 1,2,...; 1 2 D (ii) there exists a subsequence {y } ⊆ {y } such that the following convergence (as mk k=1,2,... m m=1,2,... m → ∞)hold: (ii) y → y weaklyinL2(τ,T;V); 1 mk ∞ (ii) y → y weaklystarinL (τ,T;H); 2 mk (ii) P B(u,y ) → B(u,y)weaklyinL2(τ,T;V∗); 3 mk mk 3 2 (ii) P f → f stronglyinL2(τ,T;V∗)+L1(τ,T;H); 4 mk dy dy (ii) mk → weaklyinL2(τ,T;V∗)+L1(τ,T;H). 5 dt dt 3 2 Indeed, convergence (ii) and (ii) follow from (2.8) (see also Temam [18, Remark III.3.1, pp. 264, 282]) 1 2 1 1 and Banach-Alaoglu theorem. Since there exists C1 > 0 such that |b(u,v,w)| ≤ CkukVkwkVkvkV2kvk2, for each u,v,w ∈ V (see, for example, Sohr [17, Lemma V.1.2.1]), then (ii) , (ii) and Banach-Alaoglu 1 2 5 T theorem imply (ii) . Convergence (ii) holds, because of the basic properties of the projection operators 3 4 {P } . Convergence (ii) directly follows from (ii) , (ii) and (2.7). We note that we may not to m m=1,2,... 5 3 4 pass to a subsequence in (ii) –(ii) , because z = y is the unique solution of Problem (C) on [τ,T] with 1 5 (y,f,y ) ∈U . τ τ,T F Moreover, thereexistsasubsequence {y } ⊆ {y } suchthat kj j=1,2,... mk k=1,2,... y (t) → y(t)stronglyinH fora.e. t ∈(τ,T)andt = τ, j → ∞. (2.9) kj t Indeed,accordingto(2.7),(2.8)and(ii) ,thesequence{y −F } ,whereF (t) := P f(s)ds, 3 mk mk k=1,2,... mk τ mk m = 1,2,..., t ∈ [τ,T], is bounded in a reflexive Banach space W := {w ∈ L2(τ,T;VR) : dw ∈ τ,T dt L2(τ,T;V∗)}. Compactness lemma yields that W ⊂ L2(τ,T;H) with compact embedding. There- 3 τ,T 2 A fore, (ii) –(ii) imply that y → y strongly inL2(τ,T;H) asm → ∞.Thus, there exists asubsequence 1 5 mk {y } ⊆ {y } suchthat(2.9)holds. kj j=1,2,... mk k=1,2,... Due to convergence (ii) –(ii) and (2.9), if we pass to the limit in (2.8) as m → ∞, then we obtain 1 5 kj thaty satisfiestheinequality 1 t t 1 ky(t)k2 +ν ky(ξ)k2 dξ− hf(ξ),y(ξ)idξ ≤ ky(τ)k2, (2.10) 2 Z V Z 2 s s fora.e. t ∈ (s,T),a.e. s ∈ (τ,T)ands= τ. ∞ ∗ ∗ Sincey ∈ L (τ,T;H)∩C([τ,T];V )andH ⊂ V withcontinuousembedding,theny ∈ C([τ,T];H ). w R Thus,equality (2.10)yields 1 t t 1 ky(t)k2 +ν ky(ξ)k2 dξ− hf(ξ),y(ξ)idξ ≤ ky(τ)k2, 2 Z V Z 2 s s foreacht ∈ [τ,T],a.e. s ∈ (τ,T)ands = τ. Therefore, y satisfiesinequality (1.5). 3 Proof Theorem 1.1 D Inthissectionweestablish theproofofTheorem1.1. LetΠ betherestriction operator tothefinitetime t1,t2 subinterval [t ,t ]⊆ [τ,T];ChepyzhovandVishik[5]. 1 2 ProofofTheorem1.1. Let −∞ < τ < T < +∞, y ∈ H, f ∈ L2(τ,T;V∗)+L1(τ,T;H), and y be a τ weaksolutionofProblem(1.1)on[τ,T]. Let us prove statement (a). Fix an arbitrary s ∈ [τ,T). Since (Π y,Π f,y(s)) ∈ U , then s,T s,T s,T ∞ Theorem2.2yieldsthatΠ y ∈ L (s,T;H)anditsatisfiesthefollowinginequality: s,T V (y(t)) ≤ V (y(s)) forallt ∈ [s,T], τ τ whereV isdefinedinformula(1.6). Sinces ∈ [τ,T)beanarbitrary, thenstatement (a)holds. τ Letusprovestatement(b). Statement(a)yields 1 t t 1 ky(t)k2 +ν ky(ξ)k2 dξ− hf(ξ),y(ξ)idξ ≤ ky(s)k2, (3.1) 2 Z V Z 2 s s 6 T foreacht ∈ [s,T],foreachs ∈[τ,T). Inparticular, limsupt→s+ky(t)k ≤ ky(s)kforalls ∈ [τ,T),and y(t)→ y(s)strongly inH ast → s+ foreachs ∈ [τ,T), (3.2) becausey ∈C([τ,T];H ). w F Letusprovestatement(c). Sincey ∈ L2(τ,T;V)∩L∞(τ,T;H)andf ∈ L2(τ,T;V∗)+L1(τ,T;H), thenstatements (a)and(b)implythatthemappingt → ky(t)k2 isofbounded variation on[τ,T]. References [1] Ball,J.M.:ContinuitypropertiesandglobalattractorsofgeneralizedsemiflowsandtheNavier-Stokesequations. 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