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Computers and Structures 83 (2005) 127–142
www.elsevier.com/locate/compstruc
A Newton method using exact jacobians for solvingfluid–structure coupling
Miguel Angel Fernandez a,*, Marwan Moubachir b
a Ecole Polytechnique Federale de Lausanne, IACS, 1015 Lausanne, Switzerlandb Institut National de Recherche en Informatique et en Automatique, OPALE, BP 93, 2004 route de Lucioles,
06902 Sophia-Antipolis, France
Accepted 6 April 2004
Available online 23 November 2004
Abstract
This paper aims at introducing a partitioned Newton based method for solving nonlinear coupled systems arising in
the numerical approximation of fluid–structure interaction problems. We provide a method which characteristic lies in
the use of exact cross jacobians evaluation involving the shape derivative of the fluid state with respect to solid motion
perturbations. Numerical tests based on an implementation inside a 3D fluid–structure interaction code show how the
exactness of the cross jacobians computation guarantee the overall convergence of the Newton�s loop.� 2004 Elsevier Ltd. All rights reserved.
Keywords: Fluid–structure interaction; Navier–Stokes equations; ALE formulation; Full coupled schemes; Newton methods; Shape
sensitivity analysis; Haemodynamics
1. Introduction
Nowadays, Computational Fluid-Structure Dynam-
ics (CFSD) spreads out in every engineering field, from
aeroelasticity to bio-mechanics problems (see, for in-
stance, [28,9,11,24,16,27,25,31,18,34,32,38]). One issue
arising in the numerical approximation of these nonlin-
ear coupled systems, is the definition of coupling algo-
rithms based on specific solvers involving efficient
discretization for each of the solid and fluid subsystems,
that may guarantee accurate and fast convergence of the
overall system. This issue is particularly difficult to face
0045-7949/$ - see front matter � 2004 Elsevier Ltd. All rights reserv
doi:10.1016/j.compstruc.2004.04.021
* Corresponding author. Present address: INRIA Project
REO, Rocquencourt BP 105, 78153 Le Chesnay Cedex, France.
Tel.: +33 1 3963 5470; fax: +33 1 3963 5882.
E-mail address: [email protected] (M.A.
Fernandez).
when the structure is light, namely, when the fluid and
the solid densities are of the same order, as it happens
in haemodynamics for example. Indeed, in this case,
numerical experiments show that only fully coupled
schemes can ensure stability of the resulting method
(see [24,30,7,18,25,26]). Thus, at each time step, the rule
is to solve a coupled highly nonlinear system using effi-
cient methods that may preserve, inside inner loops,
the fluid-structure subsystem splitting.
Standard and simple strategies to solve these nonlin-
ear problems are fixed-point based methods [4,1]. Unfor-
tunately, these methods are very expensive (even if
several acceleration techniques may improve their effi-
ciency [27,7]) and may fail to converge [25–27,7,18]. Re-
cent advances in this topic suggest the use of Newton
based methods for a fast convergence towards the solu-
tion of the nonlinear coupled system [1,25,26,34,18,
2,38,21]. These methods rely on the evaluation of the
ed.
Fig. 1. Geometric configurations.
128 M. �A. Fern�andez, M. Moubachir / Computers and Structures 83 (2005) 127–142
jacobians associated to the fluid-solid coupled state
equations. More precisely, the critical step consists in
the evaluation of the cross jacobians [34], e.g. the sensi-
tivity of the fluid state with respect to solid motions. Up
to now, this difficulties have been overcome either by
using finite difference approximations [25,26,34,21], or
by replacing the tangent operator of the coupled system
by a simpler operator [34,2,38,18,6,8,19]. In both cases,
such approximations may deteriorate or avoid the over-
all convergence [34]. In this paper, we provide an explicit
expression of these cross jacobians, using shape sensitiv-
ity calculus [33], in the case of an incompressible Newto-
nian fluid coupled with a nonlinear elastic solid under
large displacements. These expressions are then imple-
mented inside a 3D finite element (FE) library [12] and
we show on some model cases the superiority of using
exact jacobians against approximated versions of the
Newton�s method.
The remainder of the paper is organized as follows.
In Section 2, we introduce a general fluid-solid coupled
system and describe its associated mathematical model.
In Section 3, the resulting coupled set of equations is
used to build a coupled weak variational formulation.
The latter is discretized in time using a fully coupled
scheme in Section 4, where, the resulting nonlinear cou-
pled system is turned into an abstract form. In order to
solve this abstract coupled system, in Section 5 we de-
scribe the main steps of the Newton�s method in terms
of fluid and solid operators and its derivatives. The
expressions of such derivatives are obtained, in Section
6, using general shape derivative calculus results that
are recalled in Appendix A. These expressions have been
already announced in [14,15] as brief notes. In Section 7,
we detail the major steps of the Newton�s algorithm ap-
plied to the coupled fluid-solid problem. The numerical
results are presented in Section 8, showing the relevance
of our approach. Finally, we give some conclusions and
draw some lines for future works.
Fig. 2. The map x.
2. Mechanical problem
Let us first describe a general nonlinear fluid-struc-
ture system in large displacements. We consider a
mechanical system occupying a moving domain X(t). It
consists of a deformable structure Xs(t) (vessel wall,
pipe-line, . . .) surrounding a fluid under motion (blood,
oil, . . .) in the complement Xf(t) of Xs(t) in X(t) (see Fig.
1). The problem is to determinate the time evolution of
the configuration X(t), as well as the velocity and Cau-
chy stress tensor within the fluid and the structure.
The latter being governed by the classic conservation
laws of the continuum mechanics, endowed with appro-
priate constitutive laws.
The evolution of X(t) (see e.g. [9,22,24]), can be de-
scribed through a smooth and injective mapping
x : X0 � Rþ ! R3;
which maps any point x0 of a given fixed (reference) con-
figuration X0 ¼def Xf0 [ Xs
0 into its corresponding image
x(x0, t), inside the present configuration X(t) (see Fig.
2). We set xf ¼def xjXf0and xs ¼def xjXs
0. For x0 2 Xs
0, xs(x0, t)
represents the position at time tP 0 of the material
point x0 inside the solid domain. This corresponds to
the classical lagrangian flow. In particular, the solid dis-
placement ds(x0, t) reads
dsðx0; tÞ ¼def xsðx0; tÞ x0; x0 2 Xs0:
This implies that the configuration velocity matches
the solid velocity inside the solid domain. Conversely,
the map xf ¼ IXf0þ df can be defined from the interface
displacement dsjCw0as an arbitrary extension over the do-
main Xf0, namely,
df ¼ ExtðdsjCw0Þ:
This explains the use of the terminology Arbitrary
Lagrangian Eulerian (ALE) formulation for the result-
ing equations [9,22,24]. In practice, we can choose an
harmonic extension operator [24,30,7,18], which means
that df solves the following elliptic problem:
jDdf ¼ 0; in Xf0;
df ¼ ds; on Cw0 ;
(
where j > 0 is a given ‘‘diffusion’’ coefficient, that might
depend on ds. Other alternative extension approaches
can be found, for instance, in [3,37].
Therefore, the fluid domain can be parameterized as
follows:
M. �A. Fern�andez, M. Moubachir / Computers and Structures 83 (2005) 127–142 129
XfðtÞ ¼ Xfðdfð�; tÞÞ ¼ ðIXf0þ dfÞðXf
0Þ; ð1Þ
and its corresponding velocity (the so-called ALE veloc-
ity) by
wðdfÞ ¼def odf
ot:
We deal with a Newtonian viscous, homogeneous
fluid under incompressible flow with density q and
kinetic viscosity l. Its behavior is described by its
Eulerian velocity u and pressure p. The constitutive
law for the Cauchy stress tensor is given by the following
expression:
rðu; pÞ ¼ pI þ l½ruþ ðruÞT�:
The elastic solid under large displacements is de-
scribed by its displacement ds and its stress tensor S (sec-
ond Piola–Kirchhoff tensor). The field S is related to ds
through an appropriate constitutive law, S = S(ds) (see
[5,23,20]). Its boundary is divided into three disjoint
parts Cd0 [ Cn
0 [ Cw0 . We impose respectively a homoge-
neous Dirichlet boundary condition on Cd0 and a homo-
geneous Neumann boundary condition on Cn0 .
The coupling between the solid and the fluid is realized
through standard boundary conditions at the fluid-struc-
ture interface Cw0 , namely, the kinematic continuity of the
velocity and the kinetic continuity of the stress:
u ¼ wðdfÞ;F ðdsÞSðdsÞn0 ¼ JðdfÞrðu; pÞF ðdfÞTn0;
ð2Þ
with n0 being the unit outward normal vector to Xf0, and
F ðdsÞ ¼def I þr0ds; F ðdfÞ ¼def I þr0d
f ;
JðdsÞ ¼def det F ðdsÞ; JðdfÞ ¼def det F ðdfÞ;
where F(ds) and F(df) stand, respectively, for the defor-
mation gradient inside the solid and the fluid domains.
Finally, the fluid-structure coupled state (u,p,df;ds)
satisfies the following strong coupled system (involving
an ALE fluid formulation),
qoJðdf Þuot jx0
þ div ½qu� ðu wðdfÞÞ rðu; pÞ� ¼ 0;
in XfðtÞ;divu ¼ 0; in XfðtÞ;u ¼ wðdfÞ; on Cw
0 ;
rðu; pÞn ¼ g; on CinðtÞ [ CoutðtÞ;df ¼ ExtðdsjCw
0Þ; in Xf
0;
q0o2ds
ot2 div0ðF ðdsÞSðdsÞÞ ¼ 0; in Xs0;
F ðdsÞSðdsÞn0 ¼ JðdfÞrðu; pÞF ðdfÞTn0; on Cw0 ;
ds ¼ 0; on Cd0 ;
F ðdsÞSðdsÞn0 ¼ 0; on Cn0 ;
8>>>>>>>>>>>>>>>>>>>>><>>>>>>>>>>>>>>>>>>>>>:
ð3Þ
with q0 the solid density, n the unit outward normal vec-
tor to Xf(t), given by
n ¼ F TðdfÞn0kF TðdfÞn0k
;
and g standing for the external forces acting on the fluid.
For the sake of simpleness, no body forces are consid-
ered. In the sequel, we shall set Cinout ¼def Cin [ Cout.
Remark 1. Let notice that, in (3), the fluid domain Xf(t)
is directly related to the unknown df through expression
(1).
Remark 2. A variant to bypass the fluid ALE formu-
lation in (3) is to use a fully Eulerian fluid description
and then to discretise using espace-time finite elements,
as in [35,36,34].
3. Continuous weak formulation
Problem (3) can be reformulated in a weak varia-
tional form using appropriate test functions (see
[17,24]), performing integration by parts and taking into
account the boundary and interface conditions. This
leads to the following coupled weak formulation (see
[30,24]), involving a fluid and a solid weak-formulations:
Find df : Xf0 ! R3, u : XfðtÞ ! R3, p : XfðtÞ ! R and
ds : Xs0 ! R3, satisfying the following coupled nonlinear
subproblems:
(1) The fluid weak-formulation:
qd
dt
ZXf ðtÞ
u � vf dxþ qZ
Xf ðtÞdiv ½u� ðu wðdfÞÞ� � vf dx
þZ
Xf ðtÞrðu; pÞ : rvf dx
ZCinoutðtÞ
gðtÞ � vf da
Z
Xf ðtÞqdivudxþ
ZCwðtÞðu wðdfÞÞ � nda
þZ
Xf0
df Ext dsjCw0
� �� �� kdx0 ¼ 0;
8ðvf ; q; n; kÞ 2 V fðtÞ: ð4Þ
(2) The solid weak-formulation:ZXs0
q0
o2ds
ot2� vsdx0 þ
ZXs0
F ðdsÞSðdsÞ : rvs dx0
þ qd
dt
ZXf ðtÞ
u �RðvsjCw0Þdx
þ qZ
Xf ðtÞdiv ½u� ðu wðdfÞÞ� �RðvsjCw
0Þdx
þZ
Xf ðtÞrðu; pÞ : rRðvsjCw
0Þdx ¼ 0; 8vs 2 V s: ð5Þ
Here, the space of solid test functions is given by
V s ¼def vs 2 H 1ðXs0Þ
3jvs ¼ 0 on Cd0
n o;
130 M. �A. Fern�andez, M. Moubachir / Computers and Structures 83 (2005) 127–142
and the space of fluid test functions Vf(t) is defined using
the ALE parameterization xf (see [17]),
V fðtÞ ¼def vf ¼ v � ðxfÞ1 j v 2 H 1ðXf0Þ
3; v ¼ 0 on Cw0
n o� q ¼ q � ðxfÞ1 j q 2 L2ðXf
0Þn o� n ¼ n � ðxfÞ1jCwðtÞ j n 2 L2ðCw
0 Þ3
n o� L2ðXf
0Þ3:
In addition, we denote by R a given linear continu-
ous lift operator
R : H12ðCw
0 Þ
! vf ¼ v � ðxfÞ1 j v 2 H 1ðXf0Þ
3; v ¼ 0 on Cinout
n o:
Remark 3. Notice that the interface coupling condition
(2)2 is implicitly treated in (5). Indeed, the last three
terms of Eq. (5) represent nothing but the residual of
the fluid momentum equations in (4). This furnishes a
weak description of the fluid load at the interface
(see [24]).
4. Time semi-discretized weak formulation
The weak coupled formulation (4) and (5) is now
semi-discretized in time. Numerical experiments (see
[30,24–26,18,7]) show that only fully coupled schemes
(e.g. the fluid geometry and the interface coupling condi-
tions are implicitly treated) ensure stability and allow to
solve effectively problems in which both the fluid and
structural densities are of the same order. This arises,
for instance, in haemodynamics applications (see
[30,16,7,18,19]).
4.1. An implicit coupling scheme
We use an implicit Euler scheme for the ALE Na-
vier–Stokes equations, with a semi-implicit treatment
of the nonlinear convective term. Furthermore we use
a mid-point rule for the structural equation (see
[24,16,18,7]). Thus, for n = 0,1, . . . , the semi-discretized
in time problem writes: Given (un,df,n; ds,n,ws,n), find
ðunþ1; pnþ1; df;nþ1; ds;nþ1Þ;
satisfying the following coupled nonlinear subproblems:
(1) Fluid time semi-discretized weak-formulation:
qDt
ZXf ðtnþ1Þ
unþ1 � vfdx qDt
ZXf ðtnÞ
un � vf dx
þ qZ
Xf ðtnþ1Þdiv unþ1 � ðun wgðdf ;nþ1ÞÞ
� �� vfdx
þZ
Xf ðtnþ1Þrðunþ1; pnþ1Þ : rvfdx
Z
Cinoutðtnþ1Þgðtnþ1Þ � vfda
ZXf ðtnþ1Þ
qdivunþ1dx
þZ
Cwðtnþ1Þunþ1 wgðdf;nþ1Þ �
� nda
þZ
Xf0
df ;nþ1 Ext ds;nþ1jCw
0
�� �� kdx0 ¼ 0;
8ðvf ; q; n; kÞ 2 V fðtnþ1Þ; ð6Þ
with the notation wgðdf ;nþ1Þ ¼def 1Dt df ;nþ1 df ;n �
:(2) Solid time semi-discretized weak-formulation:
2
ðDtÞ2Z
Xs0
q0ds;nþ1 � vs dx0
2
ðDtÞ2Z
Xs0
q0ðds;n þ Dtws;nÞ � vs dx0
þ 1
2
ZXs0
F ðds;nþ1ÞSðds;nþ1Þ : rvs dx0
þ 1
2
ZXs0
F ðds;nÞSðds;nÞ : rvs dx0
þ qDt
ZXf ðtnþ1Þ
unþ1 �RðvsjCw0Þdx
qDt
ZXf ðtnÞ
un �RðvsjCw0Þdx
þ qZ
Xf ðtnþ1Þdiv unþ1 � ðun wgðdf;nþ1ÞÞ
� ��RðvsjCw
0Þdx
þZ
Xf ðtnþ1Þrðunþ1; pnþ1Þ : rRðvsjCw
0Þdx ¼ 0; 8vs 2 V s;
ð7Þwith ws;nþ1 ¼ 2
Dt ds;nþ1 ds;n �
ws;n:
4.2. Abstract formulation
In order to use general methods to solve the coupled-
system (6) and (7), it is useful to turn it into an abstract
form. Let Uf · Us be the space of fluid and solid states.
We introduce the following operators:
(1) Fluid operator:
: U f � U s ! ðV fÞ0
ðuf ; usÞ 7! ðuf ; usÞ;
with uf ¼def ðu; p; dfÞ and us ¼def ds, whose action on fluid
test functions (in Vf(tn+1)) is defined as follows,
h ðu; p; df ; dsÞ; ðvf ; q; n; kÞi
¼def qDt
ZXf ðdf Þ
u � vf dx qDt
ZXf ðtnÞ
un � vf dx
þ qZ
Xf ðdf Þdiv u� ðun wgðdfÞÞ
� �� vf dx
þZ
Xf ðdf Þrðu; pÞ : rvf dx
ZCinoutðdf Þ
gðtnþ1Þ � vf da
Z
Xf ðdf Þqdivudxþ
ZCwðdf Þ
u wgðdfÞ �
� nda
þZ
Xf0
df Ext dsjCw
0
�� �� kdx0; ð8Þ
with wgðdfÞ ¼ 1Dt df df ;n �
.
M. �A. Fern�andez, M. Moubachir / Computers and Structures 83 (2005) 127–142 131
(2) Solid operator:
: U f � U s ! ðV sÞ0
ðuf ; usÞ 7! ðuf ; usÞ;
whose action on solid test functions is defined as follows,
h ðu; p; df ; dsÞ; vsi
¼def 2
ðDtÞ2Z
Xs0
q0ds � vs dx0
2
ðDtÞ2Z
Xs0
q0ðds;n þ Dtws;nÞ � vs dx0
þ 1
2
ZXs0
F ðdsÞSðdsÞ : rvsdx0
þ 1
2
ZXs0
F ðds;nÞSðds;nÞ : rvsdx0 þ h ðu; p; dfÞ; vsi;
ð9Þ
where the fluid load transfer operator : U f ! ðV sÞ0 isdefined by
h ðu; p; dfÞ; vsi ¼def h ðu; p; df ; dsÞ; ðRðvsjCw0Þ; 0; 0; 0Þi
¼ qDt
ZXf ðdf Þ
u �RðvsjCw0Þdx q
Dt
ZXf ðtnÞ
un �RðvsjCw0Þdx
þ qZ
Xf ðdf Þdiv ½u� ðun wgðdfÞÞ� �RðvsjCw
0Þdx
þZ
Xf ðdf Þrðu; pÞ : rRðvsjCw
0Þdx: ð10Þ
Therefore, using the above definitions, the coupled
problem (6) and (7) reads: For n = 0,1, . . . , find
ðunþ1; pnþ1; df ;nþ1; ds;nþ1Þ;
solution of the following nonlinear coupled system:
h ðunþ1; pnþ1; df ;nþ1; ds;nþ1Þ; ðvf ; q; n; kÞi ¼ 0;
8ðvf ; q; n; kÞ 2 V fðtnþ1Þ;h ðunþ1; pnþ1; df ;nþ1; ds;nþ1Þ; vsi ¼ 0; 8vs 2 V s;
that is,
ðunþ1; pnþ1; df ;nþ1; ds;nþ1Þ ¼ 0;
ðunþ1; pnþ1; df ;nþ1; ds;nþ1Þ ¼ 0:
To simplify the notation, we shall only consider one
time step so that we drop the upper index n + l. Thus,
the above coupled problem writes: find uf ¼def ðu; p; dfÞand us ¼def ds such that
ðuf ; usÞ ¼ 0;
ðuf ; usÞ ¼ 0:ð11Þ
In the sequel, we shall assume that the methods and
software which have been developed to solve separately
each subsystem (the fluid and the structure) will con-
tinue to be used, Hence, it is useful to turn problem
(11) into an abstract form in terms of the fluid and struc-
ture solvers. To this purpose, we introduce the fluid sol-
ver operator F : U s ! U f , defined by
ðFðusÞ; usÞ ¼ 0; 8us 2 U s; ð12Þ
and the solid solver operator S : U f ! U s, defined by
ðuf ;SðufÞÞ ¼ 0; 8uf 2 U f : ð13Þ
Therefore, using the above definitions, the coupled
nonlinear problem (11) can be reduced to: find us 2 Us
solution of the following nonlinear equation
ðusÞ ¼def us S �FðusÞ ¼ 0; ð14Þ
see [18]. If (uf,us) solves (11) then us satisfies (14) and,
conversely, if us is a solution of (14) then ðuf ¼FðusÞ; usÞ solves (11).
In the next section, we describe the main steps of the
Newton�s algorithm when applied to the above nonlin-
ear problem.
5. The Newton�s algorithm
Standard forms for solving the nonlinear problem
(14) are fixed-points based iterations, see [30,27,
38,18,7]. Unfortunately, these methods usually show
poor convergence properties and may fail to converge
(see [25–27,7,18]). In order to speed up the convergence,
it is useful to use Newton-Raphson based methods (see
[25,26,2,38,34,18,21]).
The Newton�s method applied to the nonlinear Eq.
(14) reads:
(1) Choose �us 2 U s
(2) Do until convergence
(a) Evaluate the fluid operator �uf ¼Fð�usÞ
(b) Evaluate the solid operator us ¼Sð�ufÞ
(c) Evaluate the residual ð�usÞ ¼ �us us
(d) Solve
Dus ð�usÞ½ �dus ¼ ð�usÞ
(e) Update rule: �us �us þ dus.
Step 2 (b) of this algorithm can be carried out by
using an iterative free matrix method as GMRES (see
[25,26,34,18]). In this case, we only need to evaluate sev-
eral times the operator ½Dus ð�usÞ� against solid state per-
turbations z. In particular, we have
132 M. �A. Fern�andez, M. Moubachir / Computers and Structures 83 (2005) 127–142
By differentiating the Eqs. (12) and (13), we are able
to split the above expression in terms of the derivatives
of the fluid an solid operators (8) and (9), defined in
the previous Section:
(i) Solve the fluid tangent sub-pro-
blem:
Duf ð�uf ; �usÞ� �
duf ¼ Dus ð�uf ; �usÞ� �
z; ð15Þ
(ii) Solve the solid tangent sub-pro-
blem:
Dus ð�uf ; usÞ� �
dz ¼ Duf ð�uf ; usÞ� �
duf ;
(iii) Evaluate
Dus ð�usÞ½ �z ¼ z dz:
The main difficulty here (see [25,26,34,18,21]) relies
on the fluid tangent subproblem (15). Indeed, as we shall
see in the next section, this problem involves the evalu-
ation of the following cross-derivative of the fluid
operator:
Ddf ð�u; �p; �df; �d
sÞddf ;
which corresponds to the directional derivative with re-
spect to fluid-domain perturbations. In previous works,
the evaluations of these cross-jacobians were performed
using finite difference approximations, that only require
state operators evaluations [25,26,34,21]. However, the
lack of a priori rules for selecting optimal finite differ-
ence infinitesimal steps, leads to nonconsistent jacobians
and a reduction of the overall convergence speed [18]. In
the following section, we will show how to avoid these
approximations by establishing the exact expression of
the above linearized sub-systems.
6. Weak state operators derivatives
In this section, we present the differentiation of the
fluid and solid operators with respect to the fluid and
solid state variables. We will use shape derivative calcu-
lus results in order to perform the differentiation of inte-
gral terms with respect to their supports.
6.1. Fluid operator derivatives
We are able to obtain the expressions of the action of
the derivative of the fluid weak state operator with re-
spect to the state variables uf = (u,p,df) and us ¼def ds; atpoint ð�u; �p; �df ; �dsÞ in the direction (du,dp,ddf,dds). We
can split the operator derivative in two terms:
Dðu;p;df Þ ð�u; �p; �df; �d
sÞðdu; dp; ddfÞ¼ Dðu;pÞ ð�u; �p; �d
f; �d
sÞðdu; dpÞ þ Ddf ð�u; �p; �df; �d
sÞddf :ð16Þ
The first term Dðu;pÞ ð�u; �p; �df; �d
sÞðdu; dpÞ is a classical
Frechet derivative, in the sense we do not need to per-
turb the support of the integrals inside the operator.
Thus, we have
hDðu;pÞ ð�u; �p; �df; �d
sÞðdu; dpÞ; ðvf ; q; n; kÞi
¼ qDt
ZXf ð�df Þ
du � vf dx
þ qZ
Xf ð�df Þdiv ½du� ðun wgð�d
fÞÞ� � vf dx
þZ
Xf ð�df Þrðdu; dpÞ : rvf dx
ZXf ð�df Þ
qdivdudx
þZ
Cwð�df Þdu � nda: ð17Þ
The second term, Ddf ð�u; �p; �df; �d
sÞddf , requires the
derivation with respect to df of Eulerian integrals over
Xf(df) and its boundary. Actually, this kind of derivatives
can be viewed as shape derivatives, see for instance [33].
Some elements on shape derivative calculus are given in
Appendix A and allow to establish the following identity,
hDdf ðu; p; �df; �d
sÞddf ; ðvf ; q; n; kÞi
¼ qDt
ZXf ðdf ÞðdivddfÞu � vf dx
þZ
Xf ðdf Þqdivfu� ðun wgðd
fÞÞ½Idivddf
ðrddfÞT�g � vfdx qDt
ZXf ðdf Þ
div ðu� ddfÞ � vf dx
þZ
Xf ðdf Þrðu; pÞ½Idivddf ðrddfÞT� : rvf dx
Z
Xf ðdf Þl½rurddf þ ðrddfÞTðruÞT� : rvf dx
Z
Cinoutðdf ÞðgðddfÞ � nÞgðtnþ1Þ � vfda
Z
Xf ðdf Þqdivfu½Idivddf ðrddfÞT�gdx
þZ
Xf0
ddf � kdx0 þZ
Cwðdf Þðu wgðd
fÞÞðgðddfÞ � nÞ�
ddf
Dt
�� nda: ð18Þ
Here, we used the notation gðddfÞ ¼def ½IdivddfðrddfÞT�n, which represents the first order variation of
the surface vector nda (see, for instance, [13,10]). The deriv-
ative of with respect to the solid variable is given by,
hDds ðu; p; df; d
sÞdds; ðvf ; q; n; kÞi
¼ Z
Xf0
Ext0ðdsÞdds � kdx0: ð19Þ
with the notation Ext0ðdsÞdds ¼def DdsExtðdsÞdds.
M. �A. Fern�andez, M. Moubachir / Computers and Structures 83 (2005) 127–142 133
6.2. Solid operator derivatives
We proceed in the same fashion for the solid operator
differentiation. We first get the classical Frechet
derivatives,
hDds ðu; p; df; dsÞdds; vsi ¼ 2
ðDtÞ2Z
Xs0
q0dds � vsdx0
þ 1
2
ZXs0
F ðdsÞdS þrddsSðdsÞ� �
: rvsdx0; ð20Þ
where dS ¼def DdsSðdsÞdds.
Again we can split the derivative with respect to the
fluid state in two terms:
Dðu;p;df Þ ðu; p; df; d
sÞðdu; dp; ddfÞ
¼ Dðu;pÞ ðu; p; df; d
sÞðdu; dpÞ
þ Ddf ðu; p; df; d
sÞddf : ð21Þ
The first term Dðu;pÞ ðu; p; df; d
sÞðdu; dpÞ is a classical
Frechet derivative. Thus, we have
hDðu;pÞ ðu; p; df; d
sÞðdu; dpÞ; vsi
¼ qDt
ZXf ðdf Þ
du �R vsjCw0
� �dx
þ qZ
Xf ðdf Þdiv du� ðun wgðd
fÞh i
�RðvsjCw0Þ
þZ
Xf ðdf Þrðdu; dpÞ : rRðvsjCw
0Þdx: ð22Þ
Using the definition (9), the derivative of the solid
operator with respect to df corresponds to the derivative
of the fluid loads operator with respect to df, i.e.,
hDdf ðu; p; df ; dsÞddf ; vsi ¼ hDdf ðu; p; dfÞddf ; vsi
¼ hDdf ðu; p; df; d
sÞddf ;ðRðvsjCw
0Þ; 0; 0; 0Þi:
Then using (18), we get
hDdf ðu; p; df; d
sÞddf ; vsi
¼ qDt
ZXf ðdf ÞðdivddfÞu �R vsjCw
0
� �dx
þZ
Xf ðdf Þqdivfu� ðun wgðd
fÞÞ½I divddf
ðrddfÞT�g �R vsjCw0
� �dx
qDt
ZXf ðdf Þ
div ðu� ddfÞ �R vsjCw0
� �dx
þZ
Xf ðdf Þrðu; pÞ I divddf ðrddfÞT
h i: rR vsjCw
0
� �dx
Z
Xf ðdf Þl rurddf þ ðrddfÞTðruÞTh i
: rR vsjCw0
� �dx
ð23Þ
7. Detailed Newton�s algorithm sub-steps
Let us recall the following notations:
z ¼def z; uf ¼def ðu; p; dfÞ; us ¼def ds;
us ¼def ds; duf ¼def ðdu; dp; ddfÞ; dz ¼def dz:
Thus, from (16), it follows that step 2(d) (i) consists
in solving for (du,dp,ddf) the following linear variational
problem
Dðu;pÞ ðu; p; df; d
sÞðdu; dpÞ þ Ddf ðu; p; df; d
sÞddf
¼ DdS ðu; p; df; dsÞz: ð24Þ
Using the expressions of the fluid operator deriva-
tives provided in (17)–(19) and by taking (0, 0, 0, k) as
test function in (24) we obtain that
ddf ¼ Ext0ðdsÞz; in Xf0:
On the other hand, taking the quadruplet (0, 0, n, 0)
as test functions in (24), we obtain the following bound-
ary condition,
du ¼ ddf
Dt ðu wgðd
fÞÞðgðddfÞ � nÞ on CwðdfÞ: ð25Þ
Moreover, since by definition uf ¼FðusÞ, it followsthat u wgðd
fÞ ¼ 0 on CwðdfÞ so that (25) reduces to
du ¼ ddf
Dt; on CwðdfÞ: ð26Þ
Finally, using (17)–(19) and choosing the quadruplet
(0, 0, n, 0) as test functions in (24), we obtain the follow-
ing variational expression satisfied by (du,dp),
hDðu;pÞ ðu; p; df; d
sÞðdu; dpÞ; ðvf ; q; 0; 0Þi
¼ hDdf ðu; p; df; d
sÞddf ; ðvf ; q; 0; 0Þi; ð27Þ
Actually, the last expression corresponds to the weak
formulation associated to the following PDE involving
the linearized ALE Navier–Stokes equations:
qDt duþ div qdu� ðun wgðd
fÞÞ rðdu; dpÞh i
¼ qDt ðdivddfÞu div qu� ðun wgðd
fÞÞhn
rðu; pÞiIdivddf ðrddfÞTh io
þ qDt divðu� ddfÞ div l
hrurddf
nþðrddfÞTðruÞT
io; in XfðdÞ;
divdu ¼ div u I divddf ðrddfÞTh in o
; in XfðdfÞ;
8>>>>>>>>>>>>>>>>>><>>>>>>>>>>>>>>>>>>:endowed with the following inflow – outflow condition
on CinoutðdfÞ:
134 M. �A. Fern�andez, M. Moubachir / Computers and Structures 83 (2005) 127–142
rðdu; dpÞn ¼ gðtnþ1ÞðgðddfÞ � nÞ rðu; pÞgðddfÞ
þ l rurddf þ ðrddfÞTðruÞTh i
n:
In short, taking into account the above consider-
ations, step 2(d) (i) can be carried out by computing
ddf ¼ Ext0ðdsÞz and by solving the following linearized
Navier–Stokes problem for (du,dp):
qDt duþ div qdu� ðun wgðd
fÞÞ rðdu; dpÞh i
¼ qDt ðdivddfÞu div qu� ðun wgðd
fÞÞhn
rðu; pÞiIdivddf ðrddfÞTh io
þ qDt divðu� ddfÞ div l
hrurddf
nþðrddfÞTðruÞT
io; in XfðdÞ;
divdu ¼ div u Idivddf ðrddfÞTh in o
;
in XfðdfÞ;du ¼ ddf
Dt ; on CwðdfÞ;rðdu; dpÞn ¼ gðtnþ1ÞðgðddfÞ � nÞ rðu; pÞgðddfÞ
þl rurddf þ ðrddfÞTðruÞTh i
n;
on CinoutðdfÞ:
8>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>><>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>:
ð28Þ
Finally, using the expressions of the solid operator
derivatives provided in (20)–(23), we are able to identify
the structure of step 2(d) (ii). It consists in solving for the
perturbed solid state dz the following problem:
2
ðDtÞ2Z
Xs0
q0dz:vsdx0 þ
1
2
ZXs0
ðF ðdsÞdSðzÞ
þ rdzSðdsÞÞ : rvsdx0
¼ qDt
ZXf ð�df Þ
du �RðvsjCw0Þdx
þ qZ
Xf ð�df Þdiv½du� ðun wgðd
fÞ� �RðvsjCw0Þ
þZ
Xf ð �df Þrðdu; dpÞ : rRðvsjCw
0Þdx
þ qDt
ZXf ð �df ÞðdivddfÞ�u �RðvsjCw
0Þdx
þZ
Xf ð �df Þqdivf�u� ðun wgð �dfÞÞ½I divddf
ðrdd fÞT�g �RðvsjCw0Þdx
qDt
ZXf ð�df Þ
divð�u� ddfÞ �RðvsjCw0Þdx
þZ
Xf ð �df Þrð�u; �pÞ½I divddf ðrddfÞT� : rRðvsjCw
0Þdx
Z
Xf ð �df Þl½r�urddf þ ðrddfÞTðr�uÞT� : DRðvsjCw
0Þdx;
8vs 2 V s:
Let notice, that the right hand side of this problem is
nothing but the residual of the linearized fluid momen-
tum Eq. (28)1 involved in step 2(d) (i).
An approximate fluid tangent problem can be simply
derived from exact expression (28) by neglecting the
shape derivative terms (see [2,38,34,6,8]), yielding
qDt duþ div qdu� ðun wgðd
fÞÞ rðdu; dpÞh i
¼ 0;
in XfðdfÞ;
divdu ¼ 0; in XfðdfÞ;
du ¼ ddf
Dt ; on CwðdfÞ;
rðdu; dpÞn ¼ 0; on CinoutðdfÞ;
8>>>>>>>>>>><>>>>>>>>>>>:
ð29Þ
Still, by neglecting here the convective and diffusive
terms we get the following approximate fluid tangent
problem:
qDt duþrdp ¼ 0; in XfðdfÞ;
divdu ¼ 0; in XfðdfÞ;
du ¼ ddf
Dt ; on CwðdfÞ;
dpn ¼ 0; on CinoutðdfÞ;
8>>>>>>><>>>>>>>:that was proposed in [18] (see also [8,19]).
8. Numerical results
In this section we compare the exact Newton�s algo-rithm, whose jacobian evaluations have been described
in Section 7, with two former approaches:
• fixed-point iterations combined with Aitken�s acceler-ation techniques (FP-Aitken) [27,18],
• a quasi-Newton�s algorithm, whose approximate jac-
obians evaluations are carried out using the simpli-
fied fluid tangent problem (29), i.e., without taking
into account the shape derivatives of the fluid opera-
tor (see [2,38,34,6,8]).
As we shall see, in the sequel, the expected superiority
of the exact jacobian approach is validated by the
numerical experiments, mainly when using moderate
time steps.
We consider a fluid-structure problem arising in the
modelling of the blood flow in large arteries: it consists
of a thin elastic vessel conveying an incompressible vis-
cous fluid (see [7,6,30,16,18]). The solid is described by
the linear elasticity equations (Saint Venant–Kirchhof
material), while the fluid is described by the incompress-
ible Navier–Stokes equations. We consider two different
geometries:
M. �A. Fern�andez, M. Moubachir / Computers and Structures 83 (2005) 127–142 135
(1) a straight cylinder of radius R0 = 0.5cm and length
L = 5cm,
(2) a curved cylinder of radius R0 = 0.5cm with curva-
ture ratio 0.25cm1.
The surrounding structure has a thickness of
h = 0.1cm. The physical parameters are the following
(see [16,18]):
Fig. 3. Pressure wave propaga
Fig. 4. Pressure wave propag
• Fluid: viscosity l = 0.03Poise, density q = 1g/cm3,
• Solid: density q0 = 1.2g/cm3, Young modulus
E = 3 · 106dynes/cm2 and Poisson ratio m = 0.3.
Both systems, the fluid and the structure, are initially
at rest. The structure is clamped at the inlet and the out-
let. An over pressure of 1.3332 · 104 dynes/cm2
(10mmHg) is imposed on the inlet boundary Cin during
tion (straight cylinder).
ation (curved cylinder).
136 M. �A. Fern�andez, M. Moubachir / Compute
3 · 103 s. The fluid equations are discretized using
P1bubble=P1 finite elements, whereas for the solid equa-
tions we use P1 finite elements.
A pressure wave propagation is observed in both
configurations. Figs. 3 and 4 show the fluid pressure at
time t = 0.0025, 0.005, 0.0075, 0.01s with time step
Dt = 104 s. These results are similar to those provided
Fig. 5. Solid domain deformed con
Fig. 6. Solid domain deformed con
in [18,16]. In Figs. 5 and 6 the corresponding solid de-
formed configurations (half section) are displayed.
Remark 4. The boundary data imposed on the inlet
and outlet boundaries [16,18,19] do not have any
physiological meaning. Let us notice that the typical
period of a heart beat is about 1 second [29]. Providing
rs and Structures 83 (2005) 127–142
figuration (straight cylinder).
figuration (curved cylinder).
0
5
10
15
20
25
30
35
40
45
50
0 0.005 0.01 0.015 0.02 0.025 0.03
num
ber
of it
erat
ions
time
Newtonquasi-Newton
FP-Aitken
Fig. 7. Straight cylinder Dt = 104 s.
0
10
20
30
40
50
60
0 0.005 0.01 0.015 0.02 0.025 0.03
num
ber
of it
erat
ions
time
Newtonquasi-Newton
FP-Aitken
Fig. 8. Curved cylinder Dt = 104 s.
Table 1
Dimensionless CPU time (straight cylinder)
Algorithm CPU time
FP-Aitken 1.00
Quasi-Newton 0.56
Newton 0.52
Table 2
Dimensionless CPU time (curved cylinder)
Algorithm CPU time
FP-Aitken 1.00
Quasi-Newton 0.55
Newton 0.60
M. �A. Fern�andez, M. Moubachir / Computers and Structures 83 (2005) 127–142 137
realistic physiological simulations of the interaction
between the blood and the arterial wall, lies outside
the scope of this paper. This will be the purpose of a
forthcoming work. In such cases, the use of moderate
time steps will be crucial to perform over several cardiac
beats.
138 M. �A. Fern�andez, M. Moubachir / Computers and Structures 83 (2005) 127–142
For comparison purposes, we give in Figs. 7 and 8
the number of iterations of either method at each time
level, for Dt = 104 s. The superior convergence proper-
ties of both Newton�s algorithms are evident. However,
the cost of each Newton iteration is higher than the cost
of a fixed-point iteration. Therefore, the previous prom-
ising convergence behavior does not necessarily imply an
overall reduction of the computational cost. Hence, in
order to compare the computational cost of these algo-
0
5
10
15
20
25
0 0.005 0.01 0.015
num
ber
of it
erat
ions
tim
Fig. 9. Straight cylin
0
5
10
15
20
25
0 0.005 0.01 0.01
num
ber
of it
erat
ions
ti
Fig. 10. Curved cylin
rithms, we furnish in Tables 1 and 2 the elapsed CPU
time (dimensionless) for either algorithm after 300 time
steps of length Dt = 104 s. We can notice that both
Newton�s algorithms are (in our implementation) about
2 times faster than the fixed-point algorithm. This com-
putational gain can be also recovered (from Figs. 7 and
8) by simply estimating the cost of each Newton itera-
tion in terms of the number of fluid and solid solvers
calls. Each Newton iteration requires:
0.02 0.025 0.03
e
Newtonquasi-Newton
FP-Aitken
der Dt = 103 s.
5 0.02 0.025 0.03
me
Newtonquasi-Newton
FP-Aitken
der Dt = 103 s.
M. �A. Fern�andez, M. Moubachir / Computers and Structures 83 (2005) 127–142 139
• a fluid solver evaluation,
• a solid solver evaluation,
• about 9 inner GMRES iterations (with a tolerance of
104).
At this point, it is important to remark that each call
of the fluid solver updates the fluid mesh and the corre-
sponding FE fluid matrices, while the tangent fluid sol-
ver keeps the same FE matrix during the GMRES
process. As a result, and since we are dealing with linear
solvers, the cost of each Newton iteration is approxi-
mately 7 times the cost of a fluid solver evaluation, plus
1e-08
1e-07
1e-06
1e-05
0.0001
0.001
0.01
0 20 40
resi
dual
numbe
Fig. 11. Straight cylin
1e-08
1e-07
1e-06
1e-05
0.0001
0.001
0.01
0 20 40
resi
dual
numbe
Fig. 12. Curved cylin
10 times the cost of a solid solver evaluation. This ex-
plains the observed overall computational cost reduc-
tion. Obviously, this performance rises much more
when the fluid and solid solvers are nonlinear.
Now, let us address in detail the comparison between
the exact Newton and quasi-Newton algorithms. Obvi-
ously, the GMRES iterations in the exact version are
more expensive than in the quasi version. Indeed, the ex-
act Newton�s algorithm involves the computation of a
shape derivative which is required by the fluid tangent
problem (28). However, this is usually compensated by
a reduced number of Newton iterations. For instance,
60 80 100
r of iterations
Newtonquasi-Newton
der Dt = 103 s.
60 80 100
r of iterations
Newtonquasi-Newton
der Dt = 103 s.
140 M. �A. Fern�andez, M. Moubachir / Computers and Structures 83 (2005) 127–142
Figs. 7 and 8 and Tables 1 and 2 show that, for
Dt = 104 s, these algorithms exhibit a comparable
behavior.
The superiority of the exact Newton�s algorithm can
be simply illustrated by increasing the time step. Indeed,
in Figs. 9 and 10, we specify the number of iterations of
either method at each time level, with time step
Dt = 103 s. The fixed-point and quasi-Newton algo-
rithms fail to converge after two time steps: the allowed
maximum number of iterations is in both cases reached,
whereas the exact Newton�s method converges and re-
quires a low (almost constant) number of iterations.
This illustrates the expected good convergence proper-
ties of the Newton�s method using exact jacobians eval-
uations. Figs. 11 and 12 show the evolution of the
residual during the iteration process in both Newton�salgorithms at the third time step. We may observe that
the quasi-Newton�s algorithm is unable to reduce the
residual after 100 iterations, while the exact Newton
only requires 3 iterations to reach the convergence
threshold.
Let us notice that the initial guess used in our compu-
tations, for starting each Newton�s loop, is based on a
extrapolation of the displacement of the previous time
steps (see, for instance, [24,30,18]). Consequently, larger
time steps lead to worse initial guess and then to a higher
number of iterations. The variations of the fluid domain
become thus more relevant and, therefore, the shape
derivatives (18) cannot be neglected in (28).
9. Conclusion
Using Newton based methods for solving the nonlin-
ear coupled systems arising in the numerical approxima-
tion of fluid-structure interaction problems, requires
evaluations of the jacobian associated to coupled non-
linear problem. Up to now, these evaluations were
approximated either using finite differences [25,26,
34,21], or by introducing simplified expressions [34,18,
6,8,19]. In any case, with a loss of some of the conver-
gence features of the Newton�s method. In this article,
we have proposed a new strategy consisting in imple-
menting linearized solvers in order to evaluate, in a con-
sistent way, the different jacobians involved in the
Newton�s algorithm. The main contribution of this
paper consists in establishing the expressions of these
jacobians, in the case of a coupled system consisting of
an incompressible Newtonian fluid interacting with a
nonlinear elastic solid under large displacements. On
the numerical view point, the Newton�s method has been
implemented in a 3D research code [12] in the case of a
linear elastic solid. Performing several numerical exper-
iments allowed us to show how the exactness level of
the jacobian evaluations influences drastically the con-
vergence properties of the Newton�s loop.
There exist several ways of improving the results pre-
sented in this paper. First we shall move from a linear to
a nonlinear elastic solid. Then we may address the tech-
nical point consisting in preconditioning the GMRES
iterations during the Newton�s loop [21,8]. We shall also
investigate the application of our approach in the case of
discretisations involving space-time finite elements [34].
Our underlying motivation is, in any case, to be able
to provide fast and reliable simulations of the mechani-
cal interaction between the blood and the arterial wall
[16,30,7,6,18,19], over several cardiac beats.
Acknowledgments
First author was supported by the Swiss National
Science Foundation (contract number 20-65110.01).
Second author was supported by the Semester Program
‘‘Mathematical Modelling of the Cardiovascular Sys-
tem’’ of the Bernoulli Center at the EPFL. This work
was partially supported by the Research Training Net-
work ‘‘Mathematical Modelling of the Cardiovascular
System’’ (Hae Model), contract HPRN-CT-2002-00270
of the European Community.
Appendix A. Shape derivative calculus
In this section we shall give some elements of the
shape derivative calculus used in Section 6 (see also
[13]), for establishing the expressions of the cross-deriv-
ative (18) of the fluid operator.
We consider a smooth domain X0 2 R3 and a one-to-
one mapping x 2 k;1 where
k;1 ¼def fxjx I 2 W k;1ðR3;R3Þ;x1 I 2 W k;1ðR3;R3Þg
with kP 1.
We introduce the transported domain X(x) = x(X0).
We introduce a scalar parameter s P 0 and a perturba-
tion mapping dx 2 sk,1 and we set Fs = $(x + sdx)and Js = det Fs. The following identities are easy to
verify:
Dx½J �dx ¼defd
dsJ sjs¼0 ¼ J divdx;
Dx½F 1�dx ¼ F 1rdx:
We introduce the unit outward normal to
CðxÞ ¼def oXðxÞ by
nðxÞ ¼ F Tn0jjF Tn0jj
;
where n0 is unit outward normal to C0 ¼def oX0. We have
the following identities
M. �A. Fern�andez, M. Moubachir / Compute
Dx
ZCðxÞ
da
" #dx ¼
ZCðxÞ
gðdxÞ � n da;
Dx
ZCðxÞ
n da
" #dx ¼
ZCðxÞ
gðdxÞ � da;
with gðdxÞ ¼def ½Idiv dx ðrdxÞT�n. We also have the fol-
lowing linearized Piola identity:
div½Idivx ðrxÞT� ¼ 0: ðA:1Þ
Now we consider a function uðxÞ : XðxÞ ! R3
depending on the mapping x. We set uðsÞ ¼defuðxþ sxÞ : Xðxþ sdxÞ ! R3:
Definition 1. We say that the function u(x) admits a
shape material derivative _uðx; dxÞ in the direction dx if
the following limit exists,
_uðx; dxÞ ¼def lims!0
uðsÞ � ðxþ sdxÞ uðxÞ � xs
:
In the sequel we shall need the following result,
whose proof can be found in [33, Section 2.31]
Dx
ZXðxÞ
u � vdx" #
dx ¼Z
XðxÞ½ _uðx; dxÞ � vþ u � _vðx; dxÞ
þ ðdivdxÞu � v�dx: ðA:2Þ
Let U(X0) be a given abstract functional space of
functions defined on X0, we define its transported image
through x as follows,
UðXðxÞÞ ¼def fu � x1; u 2 UðX0Þg:
Then we have the following result:
Lemma 2. For ((u, p), (v, q)) 2 U(X(x)) · V(X(x)) the
following identities hold true
Dx
ZXðxÞ
u � vdx" #
dx ¼Z
XðxÞðdivdxÞu � vdx; ðA:3Þ
Dx
ZXðxÞ
qdivudx
" #dx ¼
ZXðxÞ
qdivfu½Idivdx
ðrdxÞT�gdx; ðA:4Þ
Dx
ZXðxÞ
rðu; pÞ : rvdx
" #dx
¼Z
XðxÞrðu; pÞ½Idivdx ðrdxÞT� : rvdx
Z
XðxÞl½rurdxþ ðrdxÞTðruÞT� : rvdx: ðA:5Þ
Proof. In the special case where ((u,p), (v,q)) 2
U(X(x)) · V(X(x)), obviously we have_uðx; dxÞ; _pðx; dxÞð Þ; _vðx; dxÞ; _qðx; dxÞð Þ ¼ ð0; 0Þ; ð0; 0Þð Þ;
then, using (A.2), we get
Dx
ZXðxÞ
u � vdx" #
dx ¼Z
XðxÞ½ðdivdxÞu � v�dx:
The next results follows using the following identities,ZXðxÞ
qdivudx ¼Z
X0
qdivfJ uF Tgdx0;
ZXðxÞru : rvdx ¼
ZX0
JruF 1 : rvF 1dx0;
and the definition r(u,p) = pI + l[$u + ($u)T]. h
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