Boundary-Layer and Interior Separations in the Taylor--Couette--Poiseuille Flow

0.37 MB·English

by  Tian Ma

BOUNDARY-LAYER AND INTERIOR SEPARATIONS IN
THE TAYLOR–COUETTE–POISEUILLE FLOW
TIAN MA ANDSHOUHONGWANG
7
0
0 Abstract. In this article, we derive a rigorous characterization of the
2 boundary-layerandinteriorseparationsintheTaylor-Couette-Poiseuille

n (TCP) flow. The results obtained provide a rigorous characterization
a on how, when and where the propagating Taylor vortices (PTV) are
J generated. In particular, contrary to what is commonly believed, we

0 show thatthePTVdonotappearafterthefirstdynamicalbifurcation,
3 and they appear only when the Taylor number is further increased to
crossanothercriticalvaluesothatastructuralbifurcationoccurs. This

1 structural bifurcation corresponds to the boundary-layer and interior
v
separations of theflow structure in the physicalspace.
3
7
0
1
0
1. Introduction
7
0
Consider a viscous fluid between two coaxial rotating cylinders. The base
/
h (Couette) flow becomes unstable as soon as the rotation speed of the inner
p
cylinder exceeds a critical value. This instability gives rise to a stationary
-
h axisymmetric counter-rotating vortices that fill the whole annular region.
t
a The associated flow is referred to as Taylor–Couette (TC) flow. When a
m through flow driven by a pressure gradient along the rotating axis is added,
: the resulting system can exhibit both convective and absolute instabilities.
v
i ThebaseflowconsistsofasuperpositionofcircularCouetteflowandannular
X
Poiseuille flow, called Couette-Poiseuille (CP) flow. The axial through–
r
flow suppresses the basic stationary instability, and as the axial pressure
a
gradient increases while the rotation speed of the inner cylinder is held
fixed, the first bifurcation gives rise to a traveling train of axisymmetric
Taylorvortices, commonlyreferredtoaspropagatingTaylorvortices (PTV).
Henceforth, the term Taylor–Couette–Poiseuille (TCP) flow is used to refer
to all hydrodynamic phenomena pertaining to the open system described
above; see among others Raguin and Georgiadis [13] and the references
therein.
In this article we rigorously characterize the stability and transitions of
the CP and TCP flows in both the physical and phase spaces. The main
focus is on a rigorous characterization on how, when and where the PTV
are generated.
The work was supported in part by the Office of Naval Research and by the National
ScienceFoundation.
1
2 MAANDWANG
We first examine the existence, explicit formula, and basin of attractions
of the secondary flows. The existence of a bifurcation to a steady state so-
lution is classical, and can be proved by the classical Krasnoselskii theorem.
The new ingredients here are the4 explicit formula and the basin of attrac-
tion of the bifurcated solutions. These enable us to determine, in the next
two sections, the asymptotic structure in the physical space of the solutions
oftheproblem,leadingtojustification oftheflow structureoftheTCPflow.
The main result of this article is on the rigorous characterization of the
boundary-layer and interior separations, associated with the TCP flows. In
particular, contrary to what is commonly believed, we show that the PTV
do not appear after the first dynamical bifurcation, and they appear only
whentheTaylornumberisfurtherincreasedtocrossanothercriticalvalueso
thatastructuralbifurcation occurs. Thisstructuralbifurcation corresponds
to the boundary layer and interior separations of the flow structure in the
physical space. This study gives another example linking the dynamics of
fluid flows to the structure and its transitions in the physical spaces.
The analysis is based on a geometric theory of two-dimensional (2D) in-
compressibleflows,and abifurcationtheory for nonlinearpartial differential
equations and both developed recently by the authors; see respectively [11]
and [12] and the references therein.
The geometric theory of 2D incompressible flows was initiated by the au-
thors to study the structure and its stability and transitions of 2-D incom-
pressible fluid flows in the physical spaces. This program of study consists
of research in two directions: 1) the study of the structure and its tran-
sitions/evolutions of divergence-free vector fields, and 2) the study of the
structure and its transitions of the flow fields of fluid flows governed by the
Navier-Stokes equations or the Euler equations. The study in the first di-
rection is more kinematic in nature, and the results and methods developed
can naturally be applied to other problems of mathematical physics involv-
ing divergence-free vector fields. In fluid dynamics context, the study in
the second direction involves specific connections between the solutions of
the Navier-Stokes or the Euler equations and flow structure in the physi-
cal space. In other words, this area of research links the kinematics to the
dynamics of fluid flows. This is unquestionably an important and difficult
problem. Progresses have been made in several directions. First, a new
rigorous characterization of boundary-layer separations for 2D viscous in-
compressible flows is developed recently by the authors, in collaboration in
partwithMichaelGhil; see[12]andthereferencestherein. Anotherexample
in this area is the structure(e.g. rolls) in the physical space in theRayleigh-
B´enard convection, usingthe structural stability theorem developed in Area
1) together with application of the aforementioned bifurcation theory; see
[9, 12]. We would like to mention that this article gives another example in
this program of research.
Thedynamicbifurcationtheoryiscenteredatanewnotionofbifurcation,
called attractor bifurcation for dynamical systems, both finite dimensional
SEPARATIONS IN TAYLOR-COUETTE-POISEUILLE FLOW 3
andinfinitedimensional,togetherwithnewnewstrategiesfortheLyapunov-
Schmidt reduction and the center manifold reduction procedures. The bi-
furcation theory has been applied to various problems from science and en-
gineering, including, in particular, the Kuramoto-Sivashinshy equation, the
Cahn-Hillard equation, the Ginzburg-Landau equation, Reaction-Diffusion
equations in Biology and Chemistry, the B´enard convection problem and
the Taylor problem in classical fluid dynamics, and the some geophysical
fluid dynamics problems. We mention the interested readers to a recent
monograph by the authors [11] and the references therein.
Thepaperis organized as follows. Section 2 introduces the physical prob-
lem, TCP problem. Section 3 proves an abstract dynamical bifurcation
theorem, which will be used in Section 4 to derive an explicit form and the
basinofattractions ofthebifurcatedsolutions. Sections5and6characterize
the boundary-layer and interior separations associated with the TCP flow.
2. Couette-Poiseuille flow
2.1. Governing equations. We consider the viscous flow between two ro-
tating coaxial cylinders with an axial pressure gradient. The instability
of this flow was first discussed by S. Goldstein in 1933 [5], and a further
investigation was made by S. Chandrasekhar [1].
Let r and r (r < r ) be the radii of two coaxial cylinders, Ω and Ω
1 2 1 2 1 2
the angular velocities with which the inner and the outer cylinders rotate
respectively, and
(2.1) µ = Ω /Ω , η = r /r .
2 1 1 2
The hydrodynamical equations governing an incompressible viscous fluid
between two coaxial cylinders are the Navier–Stokes equations in the cylin-
der polar coordinates (z,r,θ), given by
∂u 1 ∂p
z
+(u )u = ν∆u ,
z z
∂t ·∇ − ρ ∂z

 ∂∂utr +(u·∇)ur − ur2θ = ν ∆ur − r22 ∂∂uθθ − ur2r − 1ρ ∂∂pr
(2.2)  (cid:18) (cid:19)

 ∂uθ +(u )u + uruθ = ν ∆u + 2 ∂ur uθ 1 ∂p ,
∂t ·∇ θ r θ r2 ∂θ − r2 − rρ ∂θ
(cid:18) (cid:19)
 ∂(ruz) ∂(rur) ∂uθ
 + + = 0,

 ∂z ∂r ∂θ



whereνis thekinematic viscosity, ρthedensity, u= (ur,uθ,uz)thevelocity
field, p the pressure function, and
∂ ∂ u ∂
θ
u = u +u + ,
z r
·∇ ∂z ∂r r ∂θ
∂2 ∂2 1 ∂ 1 ∂2
∆ = + + + .
∂z2 ∂r2 r ∂r r2 ∂θ2
4 MAANDWANG
In this article, we consider the case where a constant pressure gradient
∂p
= p is applied in the z-direction, the cylinders rotate, and there are no
∂z 0
radialmotions. Undertheseconditions, (2.2)admitsasteady state solution:
(2.3) cp = (u ,u ,u ) = (W,0,V), p = p(r,z),
z r θ
U
where
1 dp
(2.4) = V2/r,
ρ dr
d d 1
(2.5) ν + V = 0,
dr dr r
(cid:18) (cid:19)
d 1 d p
0
(2.6) ν + W = ,
dr r dr p
(cid:18) (cid:19)
supplemented with the following boundary conditions
p
0
(u ,u )= 0, W at r = r ,r ,
r z 0 1 2
(2.7) −4ρν
 (cid:18) (cid:19)
uθ = riΩi at r = ri (i=1, 2).
Here W 0 is a constant.
0
≥
We derive then from (2.4)—(2.7) that
Ω 1
(2.8) V(r)= 1 r2(1 µ) (η2 µ)r ,
1 η2 1 − r − −
− (cid:18) (cid:19)
p 2r d+d2 r
(2.9) W(r)= 0 r2 r2+ 1 ln +W ,
−4ρν 1 − ln(1+d/r ) r 0
(cid:18) 1 (cid:18) 1(cid:19) (cid:19)
1
(2.10) p(r,z) = p z+ρ V2(r)dr,
0
r
Z
where d (= r r ) is the gap width, µ and η are as in (2.1).
2 1
−
Hereafter, we always assume that
(2.11) η2 > µ,
which is a necessary condition for the instability to occur.
Classically, the flow described by c = (0,0,V(r)) is called the Couette
U
flow, and the flow by p = (W(r),0,0) is called the Poiseuille flow. There-
U
fore the flow described by (2.3) is a super-position of the Couette flow and
the Poiseuille flow, and is usually called the Couette-Poiseuille (CP) flow.
The main objective of this article is to study the stability and transitions
of the CP flow in both the physical and the phase spaces.
For this purpose, we consider the perturbed state
1
(2.12) W +u , u , V +u , p+p z+ρ V2dr.
z r θ 0
r
Z
SEPARATIONS IN TAYLOR-COUETTE-POISEUILLE FLOW 5
Assume that the perturbation is axisymmetric and independent of θ, we
derive from (2.2) that
∂u ∂u dW 1 ∂p
z z
+(u )u = ν∆u W u ,
z z r
∂t ·∇ − ∂z − dr − ρ ∂z

∂∂utr +(ue·∇)ur − ur2θ = ν ∆ur − ur2r − ρ1 ∂∂pr

 (cid:16) (cid:17)
 ∂u 2V
(2.13)  e −W ∂zr + r rθ,

∂∂utθ +(u·∇)uθ + uθrur = ν ∆uθ − ur2θ
 e V′+ 1(cid:16)V ur W(cid:17) ∂uθ ,
 − r − ∂z
 (cid:18) (cid:19)
∂(ruz) ∂(rur)
 + = 0,

 ∂z ∂r





where u = (u ,u ), and
z r
e ∂2 1 ∂ ∂2
∆ = + + ,
∂r2 r ∂r ∂z2
∂ ∂
(u ) = u +u .
z r
·∇ ∂z ∂r
The spatial domain for (2e.13) is M = (0,L) (r ,r ), where L is the
1 2
×
height of the cylinders. The physically sound boundary conditions are as
follows
u= (u ,u ,u ) = 0 at r = r , r ,
z r θ 1 2
(2.14) ∂u ∂u
u = 0, r = θ = 0 at z = 0,L.
z
 ∂z ∂z

2.2. Narrow-gap approximation. Consider the narrow-gap approxima-
tion with µ 0. Namely, we assume that the gap d = r r is small in
2 1
≥ −
comparison to the mean radius:
1
(2.15) d = r r (r +r ).
2 1 2 1
− ≪ 2
Under this assumption, we can neglect the terms having the factors r−n
(n 1) in the equations. Let
≥
η2 µ
(2.16) α= − .
1 η2
−
6 MAANDWANG
By (2.11), α > 0. We derive then from (2.8) and (2.9) that
1
(2.17) V′+ V = 2αΩ ,
1
r −
2V 1 µ r2 r2 (1 µ)(r r )
(2.18) = 2Ω 1 − − 1 2Ω 1 − − 1 ,
r 1 − 1 η2 r2 ≃ 1 − d
(cid:18) − (cid:19) (cid:18) (cid:19)
p
0
(2.19) W [(r r )(r r)+W ].
1 2 0
≃ −4ρν − −
Replacing u by √αu in (2.13), we obtain the following approximation
θ θ
equations describing the flow between two cylinders with a narrow gap:
(2.20)
∂u p
z 0
+(u )u =ν∆u + (d 2r)u
z z r
∂t ·∇ 4ρν −

p ∂u 1 ∂p
 e + 4ρ0ν(W0+r(d−r)) ∂zz − ρ ∂z ,



 ∂u (1 µ)r
 ∂tr +(u·∇)ur =ν∆ur +2√αΩ1 1− −d uθ
 (cid:18) (cid:19)

 p ∂u 1 ∂p
 0 r
e + (W +r(d r)) ,
0
4ρν − ∂z − p ∂r

 ∂u p ∂u
 ∂tθ +(u·∇)uθ =ν∆uθ +2√αΩ1ur + 4ρ0ν(W0+r(d−r)) ∂zθ ,


 divu= 0,

 e




e
where r = r r , ∆ = ∂2/∂z2+∂2/∂r2.
1
−
We need to consider the nondimensionalized form of (2.20). To this end,
let
x = x′d (x= (z,r,rθ)),
t = t′d2/ν,
u = u′ν/d (u = (u ,u ,u )),
z r θ
p = p′ρν2/d2,
W = W′d2.
0 0
SEPARATIONS IN TAYLOR-COUETTE-POISEUILLE FLOW 7
Omitting the primes, the equations (2.20) can be rewritten as
∂u
z
=∆u +γ(1 2r)u
z r
∂t −

∂u ∂p
 +γ(W0+r(1−r)) ∂zz − ∂z −(u·∇)uz,


(2.21)  ∂∂utr =∆ur +λ(1−(1−µ)r)uθ e


 γ(W +r(1 r))∂ur ∂p (u )u ,
0 r
− − ∂z − ∂r − ·∇
 ∂∂utθ =∆uθ +λur +γ(W0+r(1−r))∂∂uzθe−(u·∇)uθ,

divu=0,

 e




where λ = √T. Heere the Taylor number T and the nondimensional param-
eter γ are given by
4αΩ2d4 p d3
(2.22) T = 1 , γ = 0 .
ν2 4ρν2
We remark here that the nondimensional parameter γ, proportional to the
Reynolds number Re squared times the small gap d, is a small parameter.
Whenthegap d= r r issmallincomparisonwiththemeanradius,we
2 1
−
need not to distinguish the two equations (2.13) and (2.21) to discuss their
dynamicbifurcation. Therefore, we always consider theproblem (2.21) with
(2.14) instead of (2.13) with (2.14). The equations (2.21) are supplemented
with the following initial condition
(2.23) u= ϕ at t = 0.
3. Dynamic Bifurcation for Perturbed Systems
3.1. Preliminaries. Let H and H be two Hilbert spaces, and H H
1 1
→
be a dense and compact inclusion. We consider the following nonlinear
evolution equation:
du
= Au+λBu+ εu+G(u,λ),
(3.1) dt Lλ

u(0) = ϕ,

where u : [0, ) H is the unknown function, λ R and ε Rm (m
∞ → ∈ ∈ ≥
1) are the system parameters, and A,B : H H are linear operators,
1
→
ε :H H are parameterized linear operators continuously dependingon
Lλ 1 →
8 MAANDWANG
λ R, ε Rm, which satisfy
∈ ∈
A: H H a sectorial operator,
1
→
(3.2) Aα :Hα → H the fractional power operators for α ∈ R,
Hα = D(Aα) the domain of Aα with H0 = H;
(3.3) Lελ,B : Hθ → H bounded for some θ < 1,
ε 0 if ε 0, λ R.
( λ
kL k → → ∀ ∈
We know that H H are compact inclusions for all α > β, and the
α β
⊂
operators A + λB + ε : H H are sectorial operators. Furthermore,
Lλ 1 →
we assume that the nonlinear terms G(,λ) : H H for some α < 1
α
· →
are a family of parameterized Cr bounded operators (r 1), depending
continuously on the parameter λ R, such that ≥
∈
(3.4) G(u,λ) = o( u ), λ R.
k kHα ∀ ∈
Let S(t) be an operator semigroup generated by (3.1), which enjoys
t≥0
{ }
the semigroup properties. Then the solution of (3.1) can be expressed as
u(t,ϕ) = S(t)ϕ for any t 0.
≥
Definition3.1. A setΣ H iscalled an invariant setof (3.1)ifS(t)Σ = Σ
⊂
for any t 0. An invariant set Σ H of (3.1) is called an attractor if Σ
≥ ⊂
is compact, and there exists a neighborhood U H of Σ such that for any
⊂
ϕ U we have
∈
lim dist(S(t)ϕ,Σ) = 0 in H norm,
t→∞
and we say that Σ attracts U. Furthermore, if U = H, then Σ is called a
global attractor of (3.1).
A number β = β +iβ C is called an eigenvalue of a linear operator
1
∈
L :H H if there exist x,y H with x= 0 such that
1 1
→ ∈ 6
Lz =βz, (z = x+iy).
The space
E = x,y H (L β)nz = 0, z = x+iy
β 1
{ ∈ | − }
n∈N
[
is called the eigenspace of L, and x,y E are called eigenvectors of L.
β
∈
Definition 3.2. A linear mapping L∗ : H H is called the conjugate
1
→
operator of L : H H, if
1
→
Lx,y = x,L∗y , x,y H .
H H 1
h i h i ∀ ∈
A linear operator L : H H is called symmetric if L = L∗.
1
→
SEPARATIONS IN TAYLOR-COUETTE-POISEUILLE FLOW 9
3.2. Eigenvalue problem. Here we consider the eigenvalue problem for
the perturbed linear operators
Lε = L + ε,
λ λ λ
L
L = A+λB.
λ
Let the eigenvalues (counting multiplicities) of L and Lε be given respec-
λ λ
tively by
β (λ) k = 1,2,... C,
k
{ | } ⊂
βε(λ) k = 1,2,... C.
k
{ | } ⊂
Suppose that the first eigenvalue β (λ) of L is simple near λ = λ , and
1 λ 0
the eigenvalues β (λ) (k = 1,2,...) satisfy the principle of exchange of
k
stabilities (PES):
< 0 if λ< λ ,
0
(3.5) β1(λ) = = 0 if λ = λ0,
> 0 if λ> λ0,
(3.6) Reβ (λ ) < 0 j 2.
j 0
 ∀ ≥
Let e ,e∗ H be the eigenvectors of L and L∗ corresponding to an eigen-
λ λ ∈ 1 λ λ
value β(λ) respectively, i.e.,
Ae +λBe = β(λ)e ,
λ λ λ
(3.7)
A∗e∗ +λB∗e∗ = β(λ)e∗.
( λ λ λ
We also assume that
(3.8) e ,e∗ = 1.
λ λ H
h i
The following theorem is important for our discussion later, which pro-
vides a differential formula for simple eigenvalues of linear operators.
Theorem 3.3. Let the eigenvalue β(λ) in (3.7) be simple. Then β(λ) is
differentiable at λ, and with the assumption (3.8) we have
(3.9) β′(λ) = Be ,e∗ .
λ λ H
h i
Proof. By the genericity of simple eigenvalues of linear operators (see Kato
[6] and Ma and Wang [11]), there is a number δ > 0 such that as δ < δ ,
0 0
| |
β(λ+δ) is also a simple eigenvalue of L . Let
λ+δ
Au+(λ+δ)Bu = β(λ+δ)u,
(3.10) u= eλ+vδ,
 vδ H 0 if δ 0.
k k −→ →
We infer from (3.7), (3.8) and (3.10) that

(A+λB)e ,e∗ + (A+λB)v ,e∗ +δ Bu,e∗
λ λ H δ λ H λ H
h i h i h i
= β(λ)+β(λ) v ,e∗ +δ Bu,e∗
δ λ H λ H
h i h i
= β(λ+δ)+β(λ+δ) v ,e∗ .
δ λ H
h i
10 MAANDWANG
Thus the last equality implies that
β(λ+δ) β(λ)
− (1+ v ,e∗ ) = B(e +v ),e∗ .
δ h δ λiH h λ δ λiH
Then the theorem follows from (3.10). (cid:3)
Remark 3.4. For general linear operators L : H H, if L are differ-
λ 1 λ
entiable on λ with = d L : H H bounded, th→en Theorem 3.3 holds
Lλ dλ λ 1 →
true as well, and
(3.11) β′(λ) = e ,e∗ .
λ λ λ H
hL i
(cid:3)
By Theorem 3.3 or (3.11), for the perturbed linear operators Lε we im-
λ
mediately obtain the following corollary.
Corollary 3.5. Assume the PES (3.5) and (3.6), and d ε exists. Then
dλ Lλ
for each ε < δ for some δ > 0, there exists λε, with lim λε = λ , such
| | 0 ε→0 0 0
that the eigenvalues βε(λ) of Lε at λε satisfy the following PES:
k λ 0
< 0 if λ< λε,
0
(3.12) βε(λ) = 0 if λ = λε,
1  0
> 0 if λ> λε,
0
(3.13) Reβε(λε) < 0, 2 j.
j 0 ∀ ≤
Moreover, βε(λ) has the following expansion at λ = λε:
1 0
βε(λ) = α (λ λε)+o(λ λε ),
1 ε − 0 | − 0|
(3.14)
(αε = h(B + ddλ Lελε0)eε,e∗εiH,
where e and e∗ satisfy
ε ε
(3.15) Lελεeε = 0, Lελ∗εe∗ε = 0.
0 0
3.3. Bifurcation for perturbed equations. We study now the dynamic
bifurcation for the nonlinear evolution equation (3.1), which is a perturba-
tion of the following equation
du
(3.16) = Au+λBu+G(u,λ).
dt
Under the conditions (3.7) and (3.8), let e H be an eigenvector of L
0 1 λ
∈
at λ = λ :
0
(3.17) Ae +λ Be = 0, e = 1.
0 0 0 0
k k
We assume that
L = A+λB are symmetric,
λ
(3.18) G(u,λ) are bilinear,

 G(u,v),v H = 0 u,v Hα.
h i ∀ ∈

