Model reduction of nonlinear problems in structural mechanics · S. Herkt, K. Dreßler, R. Pinnau Model reduction of nonlinear problems in structural mechanics Berichte des Fraunhofer

S. Herkt, K. Dreßler, R. Pinnau

Model reduction of nonlinear problems in structural mechanics

Berichte des Fraunhofer ITWM, Nr. 175 (2009)

© Fraunhofer-Institut für Techno- und Wirtschaftsmathematik ITWM 2009

ISSN 1434-9973

Bericht 175 (2009)

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Prof. Dr. Dieter Prätzel-Wolters Institutsleiter

Kaiserslautern, im Juni 2001

7th EUROMECH Solid Mechanics ConferenceJ. Ambrosio et.al. (eds.)

Lisbon, Portugal, 7–11 September 2009

MODEL REDUCTION OF NONLINEAR PROBLEMS IN STRUCTURALMECHANICS

Sabrina Herkt1, Klaus Dreßler1, Rene Pinnau2

1Fraunhofer ITWMFraunhofer Platz 1

D-67663 [email protected]@itwm.fraunhofer.de

2 Universitat KaiserslauternErwin-Schrodinger-StraßeD-67663 Kaiserslautern

[email protected]

Keywords: Flexible Bodies, FEM, Nonlinear Model Reduction, POD

Abstract. This contribution presents a model reduction method for nonlinear problems instructural mechanics. Emanating from a Finite Element model of the structure, a subspaceand a lookup table are generated which do not require a linearisation of the equations.

The method is applied to a model created with commercial FEM software. In this case, theterms describing geometrical and material nonlinearities are not explicitly known.

1

Sabrina Herkt, Klaus Dreßler, Rene Pinnau

1 INTRODUCTION

In the context of full vehicle simulation, numerical effort is an important issue. Due to thelong simulation times when computing durability or comfort loading, the number of degrees offreedom needs to be kept small.

In this field, multibody simulation methods are extensively used and well established. Bymodelling most parts as rigid bodies, the behaviour of a vehicle can be described with veryfew degrees of freedom.However, some applications require the inclusion of flexible components. In industrialapplications, these are discretised by a Finite Element approach and treated by linear modalmodel reduction techniques like the Craig-Bampton method ([2]). In this procedure, thedeformation of the flexible body is projected onto a subspace of relevant eigenmodes, whichresults in a reduced system of equations in the variable of modal participation factors.

These classical modal methods can only be used for linear systems. This assumption failswhen large deformations or nonlinear materials are involved, which frequently occurs for rubbermaterials like bushings or tyres ([3]). The following sections present an approach to reduceFinite Element models with nonlinearities which consists of two parts: a projection subspacefor reduction of the dimension and a lookup strategy for the nonlinear equations.

2 THE METHOD OF PROPER ORTHOGONAL DECOMPOSITION

One possibility to construct a subspace for the projection of nonlinear systems is the method ofProper Orthogonal Decomposition (POD). Its flexibility in application is based on analysing agiven data set to provide the reduced model.

2.1 Construction of a Subspace for Data Reduction

The method of POD can be regarded as an approach to approximate a given data set with a lowdimensional subspace.

Let V be a Hilbert space with dimV = N and let Y = {y1, . . . , ym} be a data set ⊂ V withrank(Y) = d. Furthermore, let V l = span{ϕ1 . . . ϕl} be an l-dimensional subspace, l ≤ d,N ,with orthonormal basis {ϕi}i=1,...,l. Then the projection error of the data set onto the subspaceis given by

PE(Y , V l) =1

m

m∑k=1

∥∥∥∥∥yk −l∑

j=1

〈yk, ϕj〉X · ϕj

∥∥∥∥∥2

X

(1)

where 〈·, ·〉X denotes a scalar product in V .In the following, the projection of yk onto the subspace V l is denoted by

P l(yk) =l∑

j=1

〈yk, ϕj〉X · ϕj (2)

The crucial idea behind the POD method is the construction of the subspace based on data.In brief, we search for the basis {ϕi}i=1,...,l of given dimension l which minimises the projectionerror (1). Then, the resulting subspace V l can be seen as the best approximation to the set Y ina least squares sense.

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Mathematically, this yields the following constrained optimisation problem:

min J(ϕ1, . . . , ϕl) over ϕi ∈ V (3)with J(ϕ1, . . . , ϕl) = PE(Y , span{ϕ1, . . . , ϕl}) (4)subject to 〈ϕi, ϕj〉 = δij (5)

Setting up the Lagrange functional and the respective Karush-Kuhn-Tucker equations for thesystem, we get ([5]):

L(ϕ1, . . . , ϕl, λ11, . . . , λll) = J(ϕ1, . . . , ϕl) +l∑

i,j=1

λij (〈ϕi, ϕj〉 − δij)

∂L

∂ϕi

= 0 ⇐⇒m∑

j=1

yj〈yj, ϕi〉 = λiiϕi

and λij = 0 for i 6= j

∂L

∂λij

= 0 ⇐⇒ 〈ϕi, ϕj〉 = δij

Setting λi = λii, we identify the vectors ϕ1, . . . , ϕl as the solution of the eigenvalue problem:

m∑j=1

yj〈yj, ϕi〉 = λiϕi for i = 1, . . . , l. (6)

This problem possesses d = rank(Y) solutions λ1, . . . , λd.

Combining equations (6) and (1), we find ([5]):

PE(Y , {ϕ1, . . . , ϕl}) =1

m

m∑k=1

∥∥∥∥∥yk −l∑

j=1


∥∥∥∥∥2

X

=d∑

i=l+1

λi (7)

where d < m denotes the dimension of the set Y , i.e., d = rank(Y).

Remark 1 Equation (7) states that the projection error of the set Y onto the subspaceV l = span{ϕ1 . . . ϕl} can be expressed by the sum of eigenvalues λi corresponding to theeigenvectors ϕi, i = l + 1, . . . , d, that are not included in the basis of V l.

Including the eigenvectors ϕ of the l largest eigenvalues into the basis, we obtain the subspaceV l with the smallest projection error of all possible l-dimensional subspaces in V for the set Yin a least squares sense. This choice of basis vectors is called the Karhunen-Loeve basis ([4]).

2.2 The Snapshot POD Method for Model Reduction

If subspaces created by the POD method serve as the foundation for a Galerkin projection, thesystem can be used for the reduction of large models. In the following the procedure for thesnapshot POD method ([4]) is described.

Let y(t) ∈ V be defined as the solution of a dynamical system

∂y(t)

∂t= f (y(t), t) , t ∈ [0, T ] (8)

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and yi = y(ti) be snapshots of the solution at time instances ti, i = 1, . . . ,m, ti ∈ [0, T ]. If theprecision is sufficient for the corresponding application, these snapshots can also be taken frommeasurements or from computations with many degrees of freedom (e.g. large FEM models).

Defining the snapshot matrix

Y = [y1, . . . , ym] ∈ V m

the eigenvalue problem (6) to solve for the POD basis vectors ϕ1, . . . , ϕl ∈ V can be written as

Y Y ∗ϕi = λiϕi for i = 1, . . . , l (9)

where Y ∗ denotes the transpose of Y .

Clearly, we get the same results from

Y ∗Y vi = λivi for i = 1, . . . , l with ϕi = Y ∗vi. (10)

Depending on the dimension of Y , we solve (9) if N << m or (10) if m << N , whereN = dimV .

The matrix C = Y ∗Y with Cij = 〈yi, yj〉 is referred to as the correlation matrix of the snapshotset.

Using (10), each eigenvector vk of the correlation matrix defines a basis vector ϕk ofthe POD subspace. Depending on the number of basis vectors used for the subspaceV l = span{ϕ1, . . . , ϕl}, the projection error for

P ly :=l∑

j=1

〈y, ϕj〉X · ϕj (11)

is defined by:

1

m

m∑k=1

∥∥∥∥∥yk −l∑

j=1


∥∥∥∥∥2

X

=d∑

j=l+1

λj (12)

where d < n shall denote the dimension of the snapshot set Y and l < d the number of PODbasis vectors used for the projection.

Applying the subspace projection to (8), we get the reduced surrogate model

∂α(t)

∂t= Φf (Φ∗α(t), t) , t ∈ [0, T ] . (13)

where Φ denotes the POD projection matrix with columns ϕk, k = 1 . . . l.

For linear second order equations, we get

ΦT MΦ︸︷︷︸=:fM

p + ΦT CΦ︸︷︷︸=: eC

p + ΦT KΦ︸︷︷︸=: eK

p = ΦT β (14)

Note that the matrices M, C, K are, in general, fully populated. Thus, the POD methodtransforms a large sparse system into a small dense system.

4


Remark 2 The POD-reduced system is not an approximation of the original system itself, butof the system and its external excitation. Due to the subspace construction which is based onthe snapshot data set, the reduction scheme depends on the previously computed setup. In thestrict sense, only the computed solutions can be properly represented by the reduced system.

In addition to the overall computation setup, the position of snapshots within the time-span isan important issue. Especially at time instances when the dynamics of the system are changingrapidly, the sampling rate for snapshots shall be increased.

States and phenomena not represented by the snapshot set can not be represented by the reducedsystem as well.

The procedure of snapshot computation depends on the model’s later purpose of use andtherefore requires thorough considerations.

3 LOOKUP TABLE APPROACH

Like all model reduction methods that are based on subspace projection, POD reduces the effortrequired to solve the linearised system of equations in each iteration step. For linear models,the reduced model is set up in the first step and can be used unchanged throughout the wholecomputation.

In nonlinear problems, the model equations depend on the current state u:

Mu + Cu + R (u) = fext (15)

P T ·M · ¨(Pα) + P T · C · ˙(Pα) + P T ·R (Pα) = P T · fext (16)

The effort to set up the nonlinear term R (Pα) requires the transformation of the currentstate α to the full dimensional variable u = Pα. In general industrial problems - except inrare cases - the nonlinearity is not explicitly known or can not be computed in the reducedvariable. Obviously, model reduction by POD is only helpful and sensible when most of thecomputational cost lies in the solution of the linearised problem and not in the composition ofthe equation system.

Due to the black-box character of commercial FEM codes, communication with these programsrequires a cumbersome read- and write-procedure whenever the current equation system isneeded. Some FEM programs only allow exchange of information by text files. This leadsto computational costs which are far beyond any feasible time scale. Furthermore, the problemof licences can become a serious issue, as the solver of the FEM program is used in each timestep to set up the equations.

For full exploitation of the capabilities of the POD method, the computation of the reducedproblem needs to be decoupled from the commercial tool. In general, profound knowledgeof the full system is needed in order to formulate the nonlinear equations with respect to thereduced variables only. However, black box tools do not allow such approaches as only fewdetails of the underlying equations can be gained from commercial programs.

In the following sections, an approach is presented which uses the value of the nonlinear termand the derivative matrices of a full model to build up the decoupled reduced system.

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3.1 Construction of the Lookup Table

We assume we have a nonlinear differential equation of second order

F (u, u, u, t) = 0 (17)

where F is continuous and differentiable with respect to u, u and u. Furthermore, it may dependnonlinearly on u and u′, but linearly on u and u.

Applying a Finite Element discretisation in space we define the semi-discretised equations. Wesearch for a vector u(t) ∈ RN satisfying

M(u)u + C(u)u + R(u) = fext(t) (18)

where M(u) :=∂F

∂u

C(u) :=∂F

∂u

where the matrices M, C ∈ RN×N denote the mass and damping matrix, respectively. Bothmatrices may depend on the current deformation u = u(t).

Assuming the matrices M, C, the nonlinearity R(u) and its linearisation K(u) can be extractedfrom the Finite Element software in each time step, the following procedure is set up:

(1) on each time level ti, store the current state ui, the nonlinear deformation expres-sion R(ui) and its derivative K(ui) = ∂R

∂u

ui

(2) (if coupled to a POD reduction scheme) project the data onto the given POD subspace

(3) if necessary, select few relevant states which define the look-up table

(4) in each iteration of the reduced model, construct approximation of the nonlinear term andits linearisations from the given data stored in the table

Note that steps (1)− (3) are treated offline.

The computation steps of the reduced problem mainly consist of the solution of the projectedequations and the treatment of the lookup table. Depending on the deformation variety andthe nonlinearity of the system, the table can become rather large. Therefore, a simple searchalgorithm is needed.Furthermore, during application of the reduced model, states ui may occur which are notincluded in the table. The lookup algorithm should account for variations of external loadsand therefore variations of the deformations within a reasonable range.

This implies that lookup strategies should allow for interpolation and extrapolation of the entriesand still remain stable.

3.2 The Reduced Surrogate Model

Setting up a surrogate model, we have

Mu + Cu + R(u) = fext(t) (19)

6


where M, C and f can be identical to the corresponding quantities in the original model orsuitable subspace approximations. In the computational treatment of model (19), the nonlinearpart is approximated using the following lookup scheme.

Given the current state uτ , the approximation of R(uτ ) and K(uτ ) is computed by datalinearisation in the lookup table. Let the lookup data consist of states ui, i = 1, . . . ,m, theircorresponding deformation terms R(ui) and linearisations K(ui) = ∂R

∂u

ui

. In each iteration,we determine the approximated model by Taylor expansion in the neighbourhood of a givenstate ui:

R (uτ ) = R (ui) + K (ui) (uτ − ui) + O(‖uτ − ui‖2

)(20)

K (uτ ) = K (ui) + O (‖uτ − ui‖) (21)

The state ui is chosen as the nearest state to the current deformation uτ measured in the L2

norm.

In the following lookup method, Taylor expansion up to first order is used. When the nonlinearterm and its linearisation are required, they are approximated by

R (uτ ) ≈ R (ui) + K (ui) (uτ − ui) (22)K (uτ ) ≈ K (ui) . (23)

Additional considerations can be found in [3].

4 MODEL DESCRIPTION

(a) Undeformed configuration (b) Deformed configuration after static step (innerpressure)

Figure 1: Abaqus model geometry of an airspring, output node marked

As a moderately large industrially motivated example an airspring model was chosen (Figure1). The Finite Element mesh is composed of 450 linear 8-node continuum elements yielding960 nodes and therefore 2880 degrees of freedom. The entire spring cushion consisted of anonlinear Neo-Hooke rubber material with the parameters C10 = 2.9 · 105 and D1 = 3.5 · 10−7

7


(see, for example, [1]) and a density of ρ = 1.1 · 103kg/m3. Furthermore, mass proportionalRayleigh damping (α = 100) was included in the material.

The computation consists of two steps. In the first static step, an inner pressure of 4kPa isapplied to the inner surface. Notice that this type of load induces difficulties in the geometricallynonlinear case, as pressure forces have to be perpendicular to the inner surface. This leadsto deformation dependent external forces fext(u, t) which require additional iteration routineswithin the computations of the reduced model. For simplification, inner pressure was onlyapplied to six element rows in the middle of the spring cushion (see Figure 2), as these areassumed to not undergo excessive rotation.

Figure 2: Position and direction of inner pressure and boundary conditions

The second step consists of the dynamic computations. In this step a vertical load varyingin magnitude and sign is applied to the upper element ring (Figures 2, 3). The dashed linedenotes the run used for the setup of the reduced models, whereas the continuous line denotesthe computation used for comparison of the model performance. Like in the static step, thenodes of the lower boundary are fixed in all three directions. The nodes of the upper boundaryare allowed to move in vertical direction only (see Figure 2).

The computation is accomplished by an implicit transient analysis in Abaqus as described in [1]:

Mui + Cui + I (ui)− Pi + Lj = 0 (24)

with constant mass and damping matrices M and C = α · M . The vector I denotes the innerforces in the element, Pi the external excitation at time ti and Lj the Lagrangian forces inducedby boundary conditions at node j.

In general nonlinear computations, the inner forces term I (ui) as well as its linearisation, thestiffness matrix K (ui) = ∂I

∂u|ui

, depend on the current deformation u. The damping term may

8


Figure 3: Amplitude of the external load over time: training input and input for the reference computation

also include several nonlinear relations, depending on the underlying material law. For theairspring model, the simplest form of Rayleigh damping was used.

Unless otherwise stated, the following results refer to the solution at the output reference nodenumber 718 (Figure 1).

5 RESULTS OF THE REDUCED MODEL

In a first run with the full model, the lookup table is generated. The airspring is objectedto a large tensile force followed by a large compressive force to cover the relevant range ofdeformation. The step is finished at time t = 0.15 after a rather generous number of 150equally sized increments (∆t = 10−3s).

For the lookup table, it is not necessary to include all snapshots, as the deformation states arelikely to repeat themselves. If possible, the deformation range should be checked and doublestates be dismissed. For the airspring, this task is relatively simple as the deformation of thenodes in the upper ring can only move in one direction. Yet, the displacement of the nodes inthe inner part of the airspring can be different depending on the preceding load history due todynamic effects.

In this example, the lookup table consists of the states 55 − 151 of the dynamic computationstep, as this range covers the transition from the largest tension to the largest compressiondeformation of the upper ring (see Figure 3). The computation of the POD basis is done withall deformation snapshots u(ti) using the L2 norm. The reduced mass and damping matricesM , C = αM and the lookup table {pi, Ri, Ki}i=1,...,97 are constructed by projection of therespective quantities onto the POD subspace.

This setup defines the input for the reduced computations.

The static step is not reproduced by the POD model. From the full model, the final state of theinflation step is projected onto the POD subspace to provide the initial conditions p0, p0 for thedynamic computations with the reduced one.

In the following result presentation, three different setups are compared: the full solutioncomputed with Abaqus, the POD reduced lookup table approach and a Craig-Bampton method.For the latter one, the model was linearised at the beginning of the dynamic step, i.e., afterinflation of the airspring. The Abaqus simulation serves as the reference solution for the reducedmodels. For the reduced bases, POD and Craig-Bampton, 15 basis vectors were chosen.

To study the extrapolation abilities of the reduced approaches, the external excitation is adjustedin order to provoke larger deformations than the lookup table includes.

9


(a) Vertical displacement (b) Radial displacement

Figure 4: Vertical displacement of upper ring and radial displacement of node 718, computed with Abaqus andCraig-Bampton method, using 15 basis vectors


Figure 5: Vertical displacement at upper ring and radial displacement at node 718, computed with Abaqus andPOD Lookup, using 15 basis vectorsThe horizontal dashed lines border the range covered by the lookup table

Figure 4 shows the vertical displacement of the upper ring and the radial displacement ofthe reference node 718 for the Craig Bampton method. The resulting deformation is entirelydifferent from the full solution. Obviously, a linear approach is not justified in this model setup.

Compared to that, the POD Lookup method captures the nonlinear behaviour in a large portionof the time span. However, both the vertical and radial displacements show that the method failsto reproduce the deformations correctly, when the lookup table range is exceeded (Figure 5).This effect is seen in the periods around t = 0.18s, t = 0.6s and t = 0.85s.

In other computations, where the deformations remained within the lookup table range, theresults of the reduced models were nearly identical to the full solution for all three computedPOD and lookup setups.

The following section takes a closer look at the deformation of the full system at time t = 0.18s,where the lookup methods do not reproduce the displacements correctly. Around this timeperiod, the airspring undergoes a large compression followed by a rapid change of forcedirection. Due to inertia effects, the central belt rows of the airspring bulge downwards anddevelop a deformation that is not included in the training setup (see Figure 6). Therefore,neither the POD basis nor the lookup table contain information on this state.

10


Figure 6: Training deformation states ui, i = 61, 71, . . . , 151 (blue lines) and deformation state of the full solutionat time t = 0.18s (red line) in a vertical line along the outer surface of the airspring

To deal with this shortcoming, an additional training input is set up. The new load case coversa time span of 0.2s in 200 time steps and contains a short period of tensile loading aftercompression. With this, the structure deforms similarly as the full solution at time t = 0.18s ofthe dynamic step. A new reduced model is defined using 201 snapshots and a lookup table of147 entries {pi, Ri, Ki}i=1,...,147 for the states 55− 201.


Figure 7: Radial displacement at upper ring and vertical displacement at node 718, computed with Abaqus, PODLookup with original input and POD Lookup with additional input, using 15 basis vectors

Figure 7 compares the vertical and radial displacements of the full solution with the reducedone for both the original and the additional training input. The original lookup table consists of97 entries and the one with the additional input of 147 states. In both cases, the POD Lookupmethod was used with a subspace dimension of 15. Both figures show a significant improvementnot only at position t = 0.18s, but also around the critical regions t = 0.6 and t = 0.85. Thelisting of errors in Table 1 confirms this observation.

11


Method Number of Number of relative error, relative error,

DOF lookup entries L2 norm L∞ norm

POD Lookup 15 97 22.1% 25.7%

POD Lookup 15 147 4.7% 13.3%

Table 1: Comparison of errors: The reduced models are based on 15 POD basis vectors with 151 or 201 snapshotsand 97 or 147 Lookup table entries, respectively

Method Number of DOF CPU Time

Abaqus 2880 810.4s

POD Lookup 15 1.7s

Craig-Bampton 15 0.6s

Table 2: Comparison of CPU time

Table 2 shows a comparison of CPU time for the different approaches. All tested reductionmethods run at least two orders of magnitude faster in Matlab than the original model in Abaqus.

Despite the computational savings obtained by the reduced methods, the CPU time for themodel setup needs to be taken into account (see Table 3). The effort basically consists ofthe communication with Abaqus and the computation of the POD basis. Note that thesecomputations can be done offline and need to be undertaken only once for the model setupand not for each computation.

Due to the slow communication with Abaqus via text files, the read steps require most effort- especially the reading and assembling procedure for the full stiffness matrices. This costcan be reduced if an efficient positioning of the lookup table states within the time interval isperformed. Then only the relevant matrices need to be output from Abaqus and imported inMatlab. Furthermore, if the POD basis is determined in advance from a preceding run, theelement matrices can be read in and projected onto the POD subspace within the same step.Assembly of the matrices is done by adding the projected element matrices.

Procedure CPU Time

Read and assemble stiffness matrices and mass matrix 1015.4s

Read deformations and inner forces 70.9s

Compute POD basis 4.7s

Table 3: CPU time for the Abaqus communication and offline computations

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6 CONCLUSION

This work shows a method to reduce the computational effort of nonlinear flexible bodies.

The structure is described by a Finite Element Method combined with nonlinear modelreduction. Unlike most model reduction methods - as the frequently used Craig-Bamptonapproach - the method of Proper Orthogonal Decomposition (POD) offers a projection basissuitable for nonlinear models.

The reduction of nonlinear problems introduces additional difficulties. The projection-basedmethod of POD reduces the effort needed for solution of the model equations, but not for thefunction evaluations required for the equation setup. Especially in the case of collaboration withblack-box software, the effort for equation setup exceeds the savings obtained from reductionof the degrees of freedom.

To decouple the reduced surrogate system from the full model, a lookup table approach ispresented. It makes use of the preceding computation step with the full model necessary to setup the POD basis. The nonlinear term of inner forces and the stiffness matrix are output andstored in a lookup table for the reduced system.

The method is applied to an airspring computed in Abaqus. It is shown that effort reductions oftwo orders of magnitude are possible within a reasonable error tolerance. The classical Craig-Bampton method is unable to reproduce any nonlinear effects and yields completely wrongresults.

Furthermore, the example illustrates the influence of training excitation on the quality of thereduced model. The reduced solution strongly deviates from the full one when dynamical effectsoccur that were not included in the training. An adapted training input accounting for themissing states yields considerable improvements.

Overall, the computations show that the Lookup method combined with a POD subspaceprojection constitutes a good method to massively reduce the computational effort of largenonlinear structures.

REFERENCES

[1] Abaqus Theory Manual, Version 6.6

[2] R. R. Craig, Jr.: Coupling of Substructures for Dynamic Analyses: an Overview, AIAA-2000-1573, 2000

[3] S. Herkt: Model Reduction of Nonlinear Problems in Structural Mechanics: Towards aFinite Element Tyre Model for Multibody Simulation, PhD Thesis, TU Kaiserslautern,2009

[4] W.H.A. Schilders, H. van der Vorst: Model Order Reduction: Theory, Research Aspectsand Applications, Springer, 2008

[5] S. Volkwein: Interpretation of proper orthogonal decomposition as singular value de-composition and HJB-based feedback design, Proceedings of the Sixteenth InternationalSymposium on Mathematical Theory of Networks and Systems (MTNS), Leuven, Belgium,July 5-9, 2004

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7. I. ChoquetHeterogeneous catalysis modelling and numerical simulation in rarified gas flows Part I: Coverage locally at equilibrium (24 pages, 1998)

8. J. Ohser, B. Steinbach, C. LangEfficient Texture Analysis of Binary Images(17 pages, 1998)

9. J. OrlikHomogenization for viscoelasticity of the integral type with aging and shrinkage(20 pages, 1998)

10. J. MohringHelmholtz Resonators with Large Aperture(21 pages, 1998)

11. H. W. Hamacher, A. SchöbelOn Center Cycles in Grid Graphs(15 pages, 1998)

12. H. W. Hamacher, K.-H. KüferInverse radiation therapy planning - a multiple objective optimisation approach(14 pages, 1999)

13. C. Lang, J. Ohser, R. HilferOn the Analysis of Spatial Binary Images(20 pages, 1999)

14. M. JunkOn the Construction of Discrete Equilibrium Distributions for Kinetic Schemes(24 pages, 1999)

15. M. Junk, S. V. Raghurame RaoA new discrete velocity method for Navier-Stokes equations(20 pages, 1999)

16. H. NeunzertMathematics as a Key to Key Technologies(39 pages (4 PDF-Files), 1999)

17. J. Ohser, K. SandauConsiderations about the Estimation of the Size Distribution in Wicksell’s Corpuscle Problem(18 pages, 1999)

18. E. Carrizosa, H. W. Hamacher, R. Klein, S. Nickel

Solving nonconvex planar location prob-lems by finite dominating setsKeywords: Continuous Location, Polyhedral Gauges, Finite Dominating Sets, Approximation, Sandwich Algo-rithm, Greedy Algorithm(19 pages, 2000)

19. A. BeckerA Review on Image Distortion MeasuresKeywords: Distortion measure, human visual system(26 pages, 2000)

20. H. W. Hamacher, M. Labbé, S. Nickel, T. Sonneborn

Polyhedral Properties of the Uncapacitated Multiple Allocation Hub Location Problem Keywords: integer programming, hub location, facility location, valid inequalities, facets, branch and cut(21 pages, 2000)

21. H. W. Hamacher, A. SchöbelDesign of Zone Tariff Systems in Public Transportation(30 pages, 2001)

22. D. Hietel, M. Junk, R. Keck, D. TeleagaThe Finite-Volume-Particle Method for Conservation Laws(16 pages, 2001)

23. T. Bender, H. Hennes, J. Kalcsics, M. T. Melo, S. Nickel

Location Software and Interface with GIS and Supply Chain ManagementKeywords: facility location, software development, geographical information systems, supply chain man-agement(48 pages, 2001)

24. H. W. Hamacher, S. A. TjandraMathematical Modelling of Evacuation Problems: A State of Art(44 pages, 2001)

25. J. Kuhnert, S. TiwariGrid free method for solving the Poisson equationKeywords: Poisson equation, Least squares method, Grid free method(19 pages, 2001)

26. T. Götz, H. Rave, D. Reinel-Bitzer, K. Steiner, H. Tiemeier

Simulation of the fiber spinning processKeywords: Melt spinning, fiber model, Lattice Boltz-mann, CFD(19 pages, 2001)

27. A. Zemitis On interaction of a liquid film with an obstacleKeywords: impinging jets, liquid film, models, numeri-cal solution, shape(22 pages, 2001)

28. I. Ginzburg, K. SteinerFree surface lattice-Boltzmann method to model the filling of expanding cavities by Bingham FluidsKeywords: Generalized LBE, free-surface phenomena, interface boundary conditions, filling processes, Bing-ham viscoplastic model, regularized models(22 pages, 2001)

29. H. Neunzert»Denn nichts ist für den Menschen als Men-schen etwas wert, was er nicht mit Leiden-schaft tun kann« Vortrag anlässlich der Verleihung des Akademie preises des Landes Rheinland-Pfalz am 21.11.2001Keywords: Lehre, Forschung, angewandte Mathematik, Mehrskalenanalyse, Strömungsmechanik(18 pages, 2001)

30. J. Kuhnert, S. TiwariFinite pointset method based on the projec-tion method for simulations of the incom-pressible Navier-Stokes equationsKeywords: Incompressible Navier-Stokes equations, Meshfree method, Projection method, Particle scheme, Least squares approximation AMS subject classification: 76D05, 76M28(25 pages, 2001)

31. R. Korn, M. KrekelOptimal Portfolios with Fixed Consumption or Income StreamsKeywords: Portfolio optimisation, stochastic control, HJB equation, discretisation of control problems(23 pages, 2002)

32. M. KrekelOptimal portfolios with a loan dependent credit spreadKeywords: Portfolio optimisation, stochastic control, HJB equation, credit spread, log utility, power utility, non-linear wealth dynamics(25 pages, 2002)

33. J. Ohser, W. Nagel, K. SchladitzThe Euler number of discretized sets – on the choice of adjacency in homogeneous lattices Keywords: image analysis, Euler number, neighborhod relationships, cuboidal lattice(32 pages, 2002)

34. I. Ginzburg, K. Steiner Lattice Boltzmann Model for Free-Surface flow and Its Application to Filling Process in Casting Keywords: Lattice Boltzmann models; free-surface phe-nomena; interface boundary conditions; filling pro-cesses; injection molding; volume of fluid method; in-terface boundary conditions; advection-schemes; up-wind-schemes(54 pages, 2002)

35. M. Günther, A. Klar, T. Materne, R. Wegener

Multivalued fundamental diagrams and stop and go waves for continuum traffic equationsKeywords: traffic flow, macroscopic equations, kinetic derivation, multivalued fundamental diagram, stop and go waves, phase transitions(25 pages, 2002)

36. S. Feldmann, P. Lang, D. Prätzel-WoltersParameter influence on the zeros of net-work determinantsKeywords: Networks, Equicofactor matrix polynomials, Realization theory, Matrix perturbation theory(30 pages, 2002)

37. K. Koch, J. Ohser, K. Schladitz Spectral theory for random closed sets and es timating the covariance via frequency spaceKeywords: Random set, Bartlett spectrum, fast Fourier transform, power spectrum(28 pages, 2002)

38. D. d’Humières, I. GinzburgMulti-reflection boundary conditions for lattice Boltzmann modelsKeywords: lattice Boltzmann equation, boudary condis-tions, bounce-back rule, Navier-Stokes equation(72 pages, 2002)

39. R. KornElementare FinanzmathematikKeywords: Finanzmathematik, Aktien, Optionen, Port-folio-Optimierung, Börse, Lehrerweiterbildung, Mathe-matikunterricht(98 pages, 2002)

40. J. Kallrath, M. C. Müller, S. NickelBatch Presorting Problems: Models and Complexity ResultsKeywords: Complexity theory, Integer programming, Assigment, Logistics(19 pages, 2002)

41. J. LinnOn the frame-invariant description of the phase space of the Folgar-Tucker equation Key words: fiber orientation, Folgar-Tucker equation, in-jection molding(5 pages, 2003)

42. T. Hanne, S. Nickel A Multi-Objective Evolutionary Algorithm for Scheduling and Inspection Planning in Software Development Projects Key words: multiple objective programming, project management and scheduling, software development, evolutionary algorithms, efficient set(29 pages, 2003)

43. T. Bortfeld , K.-H. Küfer, M. Monz, A. Scherrer, C. Thieke, H. Trinkaus

Intensity-Modulated Radiotherapy - A Large Scale Multi-Criteria Programming Problem

Keywords: multiple criteria optimization, representa-tive systems of Pareto solutions, adaptive triangulation, clustering and disaggregation techniques, visualization of Pareto solutions, medical physics, external beam ra-diotherapy planning, intensity modulated radiotherapy(31 pages, 2003)

44. T. Halfmann, T. WichmannOverview of Symbolic Methods in Industrial Analog Circuit Design Keywords: CAD, automated analog circuit design, sym-bolic analysis, computer algebra, behavioral modeling, system simulation, circuit sizing, macro modeling, dif-ferential-algebraic equations, index(17 pages, 2003)

45. S. E. Mikhailov, J. OrlikAsymptotic Homogenisation in Strength and Fatigue Durability Analysis of Compos-itesKeywords: multiscale structures, asymptotic homoge-nization, strength, fatigue, singularity, non-local con-ditions(14 pages, 2003)

46. P. Domínguez-Marín, P. Hansen, N. Mladenovi c , S. Nickel

Heuristic Procedures for Solving the Discrete Ordered Median ProblemKeywords: genetic algorithms, variable neighborhood search, discrete facility location(31 pages, 2003)

47. N. Boland, P. Domínguez-Marín, S. Nickel, J. Puerto

Exact Procedures for Solving the Discrete Ordered Median ProblemKeywords: discrete location, Integer programming(41 pages, 2003)

48. S. Feldmann, P. LangPadé-like reduction of stable discrete linear systems preserving their stability Keywords: Discrete linear systems, model reduction, stability, Hankel matrix, Stein equation(16 pages, 2003)

49. J. Kallrath, S. NickelA Polynomial Case of the Batch Presorting Problem Keywords: batch presorting problem, online optimization, competetive analysis, polynomial algorithms, logistics(17 pages, 2003)

50. T. Hanne, H. L. TrinkausknowCube for MCDM – Visual and Interactive Support for Multicriteria Decision MakingKey words: Multicriteria decision making, knowledge management, decision support systems, visual interfac-es, interactive navigation, real-life applications.(26 pages, 2003)

51. O. Iliev, V. LaptevOn Numerical Simulation of Flow Through Oil FiltersKeywords: oil filters, coupled flow in plain and porous media, Navier-Stokes, Brinkman, numerical simulation(8 pages, 2003)

52. W. Dörfler, O. Iliev, D. Stoyanov, D. VassilevaOn a Multigrid Adaptive Refinement Solver for Saturated Non-Newtonian Flow in Porous MediaKeywords: Nonlinear multigrid, adaptive refinement, non-Newtonian flow in porous media(17 pages, 2003)

53. S. KruseOn the Pricing of Forward Starting Options under Stochastic VolatilityKeywords: Option pricing, forward starting options, Heston model, stochastic volatility, cliquet options(11 pages, 2003)

54. O. Iliev, D. StoyanovMultigrid – adaptive local refinement solver for incompressible flowsKeywords: Navier-Stokes equations, incompressible flow, projection-type splitting, SIMPLE, multigrid methods, adaptive local refinement, lid-driven flow in a cavity (37 pages, 2003)

55. V. Starikovicius The multiphase flow and heat transfer in porous media Keywords: Two-phase flow in porous media, various formulations, global pressure, multiphase mixture mod-el, numerical simulation(30 pages, 2003)

56. P. Lang, A. Sarishvili, A. WirsenBlocked neural networks for knowledge ex-traction in the software development processKeywords: Blocked Neural Networks, Nonlinear Regres-sion, Knowledge Extraction, Code Inspection(21 pages, 2003)

57. H. Knaf, P. Lang, S. Zeiser Diagnosis aiding in Regulation Thermography using Fuzzy Logic Keywords: fuzzy logic,knowledge representation, expert system(22 pages, 2003)

58. M. T. Melo, S. Nickel, F. Saldanha da GamaLarge scale models for dynamic multi-commodity capacitated facility location Keywords: supply chain management, strategic planning, dynamic location, modeling(40 pages, 2003)

59. J. Orlik Homogenization for contact problems with periodically rough surfacesKeywords: asymptotic homogenization, contact problems(28 pages, 2004)

60. A. Scherrer, K.-H. Küfer, M. Monz, F. Alonso, T. Bortfeld

IMRT planning on adaptive volume struc-tures – a significant advance of computa-tional complexityKeywords: Intensity-modulated radiation therapy (IMRT), inverse treatment planning, adaptive volume structures, hierarchical clustering, local refinement, adaptive clustering, convex programming, mesh gener-ation, multi-grid methods(24 pages, 2004)

61. D. KehrwaldParallel lattice Boltzmann simulation of complex flowsKeywords: Lattice Boltzmann methods, parallel com-puting, microstructure simulation, virtual material de-sign, pseudo-plastic fluids, liquid composite moulding(12 pages, 2004)

62. O. Iliev, J. Linn, M. Moog, D. Niedziela, V. Starikovicius

On the Performance of Certain Iterative Solvers for Coupled Systems Arising in Dis-cretization of Non-Newtonian Flow Equa-tions

Keywords: Performance of iterative solvers, Precondi-tioners, Non-Newtonian flow(17 pages, 2004)

63. R. Ciegis, O. Iliev, S. Rief, K. Steiner On Modelling and Simulation of Different Regimes for Liquid Polymer Moulding Keywords: Liquid Polymer Moulding, Modelling, Simu-lation, Infiltration, Front Propagation, non-Newtonian flow in porous media (43 pages, 2004)

64. T. Hanne, H. NeuSimulating Human Resources in Software Development ProcessesKeywords: Human resource modeling, software process, productivity, human factors, learning curve(14 pages, 2004)

65. O. Iliev, A. Mikelic, P. PopovFluid structure interaction problems in de-formable porous media: Toward permeabil-ity of deformable porous mediaKeywords: fluid-structure interaction, deformable po-rous media, upscaling, linear elasticity, stokes, finite el-ements(28 pages, 2004)

66. F. Gaspar, O. Iliev, F. Lisbona, A. Naumovich, P. Vabishchevich

On numerical solution of 1-D poroelasticity equations in a multilayered domainKeywords: poroelasticity, multilayered material, finite volume discretization, MAC type grid(41 pages, 2004)

67. J. Ohser, K. Schladitz, K. Koch, M. NötheDiffraction by image processing and its ap-plication in materials scienceKeywords: porous microstructure, image analysis, ran-dom set, fast Fourier transform, power spectrum, Bar-tlett spectrum(13 pages, 2004)

68. H. NeunzertMathematics as a Technology: Challenges for the next 10 YearsKeywords: applied mathematics, technology, modelling, simulation, visualization, optimization, glass processing, spinning processes, fiber-fluid interaction, trubulence effects, topological optimization, multicriteria optimiza-tion, Uncertainty and Risk, financial mathematics, Mal-liavin calculus, Monte-Carlo methods, virtual material design, filtration, bio-informatics, system biology(29 pages, 2004)

69. R. Ewing, O. Iliev, R. Lazarov, A. NaumovichOn convergence of certain finite difference discretizations for 1 D poroelasticity inter-face problems Keywords: poroelasticity, multilayered material, finite volume discretizations, MAC type grid, error estimates (26 pages,2004)

70. W. Dörfler, O. Iliev, D. Stoyanov, D. Vassileva On Efficient Simulation of Non-Newto-nian Flow in Saturated Porous Media with a Multigrid Adaptive Refinement Solver Keywords: Nonlinear multigrid, adaptive renement, non-Newtonian in porous media(25 pages, 2004)

71. J. Kalcsics, S. Nickel, M. Schröder Towards a Unified Territory Design Approach – Applications, Algorithms and GIS IntegrationKeywords: territory desgin, political districting, sales territory alignment, optimization algorithms, Geo-graphical Information Systems(40 pages, 2005)

72. K. Schladitz, S. Peters, D. Reinel-Bitzer, A. Wiegmann, J. Ohser

Design of acoustic trim based on geometric modeling and flow simulation for non-woven Keywords: random system of fibers, Poisson line process, flow resistivity, acoustic absorption, Lattice-Boltzmann method, non-woven(21 pages, 2005)

73. V. Rutka, A. WiegmannExplicit Jump Immersed Interface Method for virtual material design of the effective elastic moduli of composite materials Keywords: virtual material design, explicit jump im-mersed interface method, effective elastic moduli, composite materials(22 pages, 2005)

74. T. HanneEine Übersicht zum Scheduling von BaustellenKeywords: Projektplanung, Scheduling, Bauplanung, Bauindustrie(32 pages, 2005)

75. J. LinnThe Folgar-Tucker Model as a Differetial Algebraic System for Fiber Orientation Calculation Keywords: fiber orientation, Folgar–Tucker model, in-variants, algebraic constraints, phase space, trace sta-bility(15 pages, 2005)

76. M. Speckert, K. Dreßler, H. Mauch, A. Lion, G. J. Wierda

Simulation eines neuartigen Prüf systems für Achserprobungen durch MKS-Model-lierung einschließlich RegelungKeywords: virtual test rig, suspension testing, multibody simulation, modeling hexapod test rig, opti-mization of test rig configuration(20 pages, 2005)

77. K.-H. Küfer, M. Monz, A. Scherrer, P. Süss, F. Alonso, A. S. A. Sultan, Th. Bortfeld, D. Craft, Chr. Thieke

Multicriteria optimization in intensity modulated radiotherapy planning Keywords: multicriteria optimization, extreme solu-tions, real-time decision making, adaptive approxima-tion schemes, clustering methods, IMRT planning, re-verse engineering (51 pages, 2005)

78. S. Amstutz, H. Andrä A new algorithm for topology optimization using a level-set methodKeywords: shape optimization, topology optimization, topological sensitivity, level-set(22 pages, 2005)

79. N. EttrichGeneration of surface elevation models for urban drainage simulationKeywords: Flooding, simulation, urban elevation models, laser scanning(22 pages, 2005)

80. H. Andrä, J. Linn, I. Matei, I. Shklyar, K. Steiner, E. Teichmann

OPTCAST – Entwicklung adäquater Struk-turoptimierungsverfahren für Gießereien Technischer Bericht (KURZFASSUNG)Keywords: Topologieoptimierung, Level-Set-Methode, Gießprozesssimulation, Gießtechnische Restriktionen, CAE-Kette zur Strukturoptimierung(77 pages, 2005)

81. N. Marheineke, R. WegenerFiber Dynamics in Turbulent Flows Part I: General Modeling Framework Keywords: fiber-fluid interaction; Cosserat rod; turbu-lence modeling; Kolmogorov’s energy spectrum; dou-ble-velocity correlations; differentiable Gaussian fields(20 pages, 2005)

Part II: Specific Taylor Drag Keywords: flexible fibers; k-e turbulence model; fi-ber-turbulence interaction scales; air drag; random Gaussian aerodynamic force; white noise; stochastic differential equations; ARMA process (18 pages, 2005)

82. C. H. Lampert, O. Wirjadi An Optimal Non-Orthogonal Separation of the Anisotropic Gaussian Convolution FilterKeywords: Anisotropic Gaussian filter, linear filtering, ori-entation space, nD image processing, separable filters(25 pages, 2005)

83. H. Andrä, D. StoyanovError indicators in the parallel finite ele-ment solver for linear elasticity DDFEM Keywords: linear elasticity, finite element method, hier-archical shape functions, domain decom-position, par-allel implementation, a posteriori error estimates(21 pages, 2006)

84. M. Schröder, I. SolchenbachOptimization of Transfer Quality in Regional Public TransitKeywords: public transit, transfer quality, quadratic assignment problem(16 pages, 2006)

85. A. Naumovich, F. J. Gaspar On a multigrid solver for the three-dimen-sional Biot poroelasticity system in multi-layered domains Keywords: poroelasticity, interface problem, multigrid, operator-dependent prolongation(11 pages, 2006)

86. S. Panda, R. Wegener, N. MarheinekeSlender Body Theory for the Dynamics of Curved Viscous Fibers Keywords: curved viscous fibers; fluid dynamics; Navier-Stokes equations; free boundary value problem; asymp-totic expansions; slender body theory(14 pages, 2006)

87. E. Ivanov, H. Andrä, A. KudryavtsevDomain Decomposition Approach for Auto-matic Parallel Generation of Tetrahedral GridsKey words: Grid Generation, Unstructured Grid, Delau-nay Triangulation, Parallel Programming, Domain De-composition, Load Balancing(18 pages, 2006)

88. S. Tiwari, S. Antonov, D. Hietel, J. Kuhnert, R. Wegener

A Meshfree Method for Simulations of In-teractions between Fluids and Flexible StructuresKey words: Meshfree Method, FPM, Fluid Structure Interaction, Sheet of Paper, Dynamical Coupling(16 pages, 2006)

89. R. Ciegis , O. Iliev, V. Starikovicius, K. SteinerNumerical Algorithms for Solving Problems of Multiphase Flows in Porous MediaKeywords: nonlinear algorithms, finite-volume method, software tools, porous media, flows(16 pages, 2006)

90. D. Niedziela, O. Iliev, A. LatzOn 3D Numerical Simulations of Viscoelastic FluidsKeywords: non-Newtonian fluids, anisotropic viscosity, integral constitutive equation (18 pages, 2006)

91. A. WinterfeldApplication of general semi-infinite Pro-gramming to Lapidary Cutting ProblemsKeywords: large scale optimization, nonlinear program-ming, general semi-infinite optimization, design center-ing, clustering(26 pages, 2006)

92. J. Orlik, A. OstrovskaSpace-Time Finite Element Approximation and Numerical Solution of Hereditary Linear Viscoelasticity ProblemsKeywords: hereditary viscoelasticity; kern approxima-tion by interpolation; space-time finite element approxi-mation, stability and a priori estimate(24 pages, 2006)

93. V. Rutka, A. Wiegmann, H. AndräEJIIM for Calculation of effective Elastic Moduli in 3D Linear ElasticityKeywords: Elliptic PDE, linear elasticity, irregular do-main, finite differences, fast solvers, effective elas-tic moduli(24 pages, 2006)

94. A. Wiegmann, A. ZemitisEJ-HEAT: A Fast Explicit Jump Harmonic Averaging Solver for the Effective Heat Conductivity of Composite MaterialsKeywords: Stationary heat equation, effective ther-mal conductivity, explicit jump, discontinuous coeffi-cients, virtual material design, microstructure simula-tion, EJ-HEAT(21 pages, 2006)

95. A. NaumovichOn a finite volume discretization of the three-dimensional Biot poroelasticity sys-tem in multilayered domainsKeywords: Biot poroelasticity system, interface problems, finite volume discretization, finite difference method(21 pages, 2006)

96. M. Krekel, J. WenzelA unified approach to Credit Default Swap-tion and Constant Maturity Credit Default Swap valuationKeywords: LIBOR market model, credit risk, Credit De-fault Swaption, Constant Maturity Credit Default Swap-method(43 pages, 2006)

97. A. DreyerInterval Methods for Analog CirciutsKeywords: interval arithmetic, analog circuits, tolerance analysis, parametric linear systems, frequency response, symbolic analysis, CAD, computer algebra(36 pages, 2006)

98. N. Weigel, S. Weihe, G. Bitsch, K. DreßlerUsage of Simulation for Design and Optimi-zation of TestingKeywords: Vehicle test rigs, MBS, control, hydraulics, testing philosophy(14 pages, 2006)

99. H. Lang, G. Bitsch, K. Dreßler, M. SpeckertComparison of the solutions of the elastic and elastoplastic boundary value problems

Keywords: Elastic BVP, elastoplastic BVP, variational inequalities, rate-independency, hysteresis, linear kine-matic hardening, stop- and play-operator(21 pages, 2006)

100. M. Speckert, K. Dreßler, H. MauchMBS Simulation of a hexapod based sus-pension test rigKeywords: Test rig, MBS simulation, suspension, hydraulics, controlling, design optimization(12 pages, 2006)

101. S. Azizi Sultan, K.-H. KüferA dynamic algorithm for beam orientations in multicriteria IMRT planningKeywords: radiotherapy planning, beam orientation optimization, dynamic approach, evolutionary algo-rithm, global optimization(14 pages, 2006)

102. T. Götz, A. Klar, N. Marheineke, R. WegenerA Stochastic Model for the Fiber Lay-down Process in the Nonwoven ProductionKeywords: fiber dynamics, stochastic Hamiltonian sys-tem, stochastic averaging(17 pages, 2006)

103. Ph. Süss, K.-H. KüferBalancing control and simplicity: a variable aggregation method in intensity modulated radiation therapy planningKeywords: IMRT planning, variable aggregation, clus-tering methods (22 pages, 2006)

104. A. Beaudry, G. Laporte, T. Melo, S. NickelDynamic transportation of patients in hos-pitalsKeywords: in-house hospital transportation, dial-a-ride, dynamic mode, tabu search (37 pages, 2006)

105. Th. HanneApplying multiobjective evolutionary algo-rithms in industrial projectsKeywords: multiobjective evolutionary algorithms, dis-crete optimization, continuous optimization, electronic circuit design, semi-infinite programming, scheduling(18 pages, 2006)

106. J. Franke, S. HalimWild bootstrap tests for comparing signals and imagesKeywords: wild bootstrap test, texture classification, textile quality control, defect detection, kernel estimate, nonparametric regression(13 pages, 2007)

107. Z. Drezner, S. NickelSolving the ordered one-median problem in the planeKeywords: planar location, global optimization, ordered median, big triangle small triangle method, bounds, numerical experiments(21 pages, 2007)

108. Th. Götz, A. Klar, A. Unterreiter, R. Wegener

Numerical evidance for the non- existing of solutions of the equations desribing rota-tional fiber spinningKeywords: rotational fiber spinning, viscous fibers, boundary value problem, existence of solutions(11 pages, 2007)

109. Ph. Süss, K.-H. KüferSmooth intensity maps and the Bortfeld-Boyer sequencerKeywords: probabilistic analysis, intensity modulated radiotherapy treatment (IMRT), IMRT plan application, step-and-shoot sequencing(8 pages, 2007)

110. E. Ivanov, O. Gluchshenko, H. Andrä, A. Kudryavtsev

Parallel software tool for decomposing and meshing of 3d structuresKeywords: a-priori domain decomposition, unstruc-tured grid, Delaunay mesh generation(14 pages, 2007)

111. O. Iliev, R. Lazarov, J. WillemsNumerical study of two-grid precondition-ers for 1d elliptic problems with highly oscillating discontinuous coefficientsKeywords: two-grid algorithm, oscillating coefficients, preconditioner (20 pages, 2007)

112. L. Bonilla, T. Götz, A. Klar, N. Marheineke, R. Wegener

Hydrodynamic limit of the Fokker-Planck-equation describing fiber lay-down pro-cessesKeywords: stochastic dierential equations, Fokker-Planck equation, asymptotic expansion, Ornstein-Uhlenbeck process(17 pages, 2007)

113. S. RiefModeling and simulation of the pressing section of a paper machineKeywords: paper machine, computational fluid dynam-ics, porous media(41 pages, 2007)

114. R. Ciegis, O. Iliev, Z. LakdawalaOn parallel numerical algorithms for simu-lating industrial filtration problemsKeywords: Navier-Stokes-Brinkmann equations, finite volume discretization method, SIMPLE, parallel comput-ing, data decomposition method (24 pages, 2007)

115. N. Marheineke, R. WegenerDynamics of curved viscous fibers with sur-face tensionKeywords: Slender body theory, curved viscous bers with surface tension, free boundary value problem(25 pages, 2007)

116. S. Feth, J. Franke, M. SpeckertResampling-Methoden zur mse-Korrektur und Anwendungen in der BetriebsfestigkeitKeywords: Weibull, Bootstrap, Maximum-Likelihood, Betriebsfestigkeit(16 pages, 2007)

117. H. KnafKernel Fisher discriminant functions – a con-cise and rigorous introductionKeywords: wild bootstrap test, texture classification, textile quality control, defect detection, kernel estimate, nonparametric regression(30 pages, 2007)

118. O. Iliev, I. RybakOn numerical upscaling for flows in hetero-geneous porous media

Keywords: numerical upscaling, heterogeneous porous media, single phase flow, Darcy‘s law, multiscale prob-lem, effective permeability, multipoint flux approxima-tion, anisotropy(17 pages, 2007)

119. O. Iliev, I. RybakOn approximation property of multipoint flux approximation methodKeywords: Multipoint flux approximation, finite volume method, elliptic equation, discontinuous tensor coeffi-cients, anisotropy(15 pages, 2007)

120. O. Iliev, I. Rybak, J. WillemsOn upscaling heat conductivity for a class of industrial problemsKeywords: Multiscale problems, effective heat conduc-tivity, numerical upscaling, domain decomposition(21 pages, 2007)

121. R. Ewing, O. Iliev, R. Lazarov, I. RybakOn two-level preconditioners for flow in porous mediaKeywords: Multiscale problem, Darcy‘s law, single phase flow, anisotropic heterogeneous porous media, numerical upscaling, multigrid, domain decomposition, efficient preconditioner(18 pages, 2007)

122. M. Brickenstein, A. DreyerPOLYBORI: A Gröbner basis framework for Boolean polynomialsKeywords: Gröbner basis, formal verification, Boolean polynomials, algebraic cryptoanalysis, satisfiability(23 pages, 2007)

123. O. WirjadiSurvey of 3d image segmentation methodsKeywords: image processing, 3d, image segmentation, binarization(20 pages, 2007)

124. S. Zeytun, A. GuptaA Comparative Study of the Vasicek and the CIR Model of the Short RateKeywords: interest rates, Vasicek model, CIR-model, calibration, parameter estimation(17 pages, 2007)

125. G. Hanselmann, A. Sarishvili Heterogeneous redundancy in software quality prediction using a hybrid Bayesian approachKeywords: reliability prediction, fault prediction, non-homogeneous poisson process, Bayesian model aver-aging(17 pages, 2007)

126. V. Maag, M. Berger, A. Winterfeld, K.-H. Küfer

A novel non-linear approach to minimal area rectangular packingKeywords: rectangular packing, non-overlapping con-straints, non-linear optimization, regularization, relax-ation (18 pages, 2007)

127. M. Monz, K.-H. Küfer, T. Bortfeld, C. Thieke Pareto navigation – systematic multi-crite-ria-based IMRT treatment plan determina-tionKeywords: convex, interactive multi-objective optimiza-tion, intensity modulated radiotherapy planning(15 pages, 2007)

128. M. Krause, A. ScherrerOn the role of modeling parameters in IMRT plan optimizationKeywords: intensity-modulated radiotherapy (IMRT), inverse IMRT planning, convex optimization, sensitiv-ity analysis, elasticity, modeling parameters, equivalent uniform dose (EUD)(18 pages, 2007)

129. A. WiegmannComputation of the permeability of porous materials from their microstructure by FFF-StokesKeywords: permeability, numerical homogenization, fast Stokes solver(24 pages, 2007)

130. T. Melo, S. Nickel, F. Saldanha da GamaFacility Location and Supply Chain Manage-ment – A comprehensive reviewKeywords: facility location, supply chain management, network design(54 pages, 2007)

131. T. Hanne, T. Melo, S. NickelBringing robustness to patient flow manage ment through optimized patient transports in hospitalsKeywords: Dial-a-Ride problem, online problem, case study, tabu search, hospital logistics (23 pages, 2007)

132. R. Ewing, O. Iliev, R. Lazarov, I. Rybak, J. Willems

An efficient approach for upscaling proper-ties of composite materials with high con-trast of coefficientsKeywords: effective heat conductivity, permeability of fractured porous media, numerical upscaling, fibrous insulation materials, metal foams(16 pages, 2008)

133. S. Gelareh, S. NickelNew approaches to hub location problems in public transport planningKeywords: integer programming, hub location, trans-portation, decomposition, heuristic(25 pages, 2008)

134. G. Thömmes, J. Becker, M. Junk, A. K. Vai-kuntam, D. Kehrwald, A. Klar, K. Steiner, A. Wiegmann

A Lattice Boltzmann Method for immiscible multiphase flow simulations using the Level Set MethodKeywords: Lattice Boltzmann method, Level Set method, free surface, multiphase flow(28 pages, 2008)

135. J. OrlikHomogenization in elasto-plasticityKeywords: multiscale structures, asymptotic homogeni-zation, nonlinear energy (40 pages, 2008)

136. J. Almquist, H. Schmidt, P. Lang, J. Deitmer, M. Jirstrand, D. Prätzel-Wolters, H. Becker

Determination of interaction between MCT1 and CAII via a mathematical and physiological approachKeywords: mathematical modeling; model reduction; electrophysiology; pH-sensitive microelectrodes; pro-ton antenna (20 pages, 2008)

137. E. Savenkov, H. Andrä, O. Iliev∗An analysis of one regularization approach for solution of pure Neumann problemKeywords: pure Neumann problem, elasticity, regular-ization, finite element method, condition number(27 pages, 2008)

138. O. Berman, J. Kalcsics, D. Krass, S. NickelThe ordered gradual covering location problem on a networkKeywords: gradual covering, ordered median function, network location(32 pages, 2008)

139. S. Gelareh, S. NickelMulti-period public transport design: A novel model and solution approachesKeywords: Integer programming, hub location, public transport, multi-period planning, heuristics(31 pages, 2008)

140. T. Melo, S. Nickel, F. Saldanha-da-GamaNetwork design decisions in supply chainplanningKeywords: supply chain design, integer programming models, location models, heuristics(20 pages, 2008)

141. C. Lautensack, A. Särkkä, J. Freitag, K. Schladitz

Anisotropy analysis of pressed point pro-cessesKeywords: estimation of compression, isotropy test, nearest neighbour distance, orientation analysis, polar ice, Ripley’s K function(35 pages, 2008)

142. O. Iliev, R. Lazarov, J. WillemsA Graph-Laplacian approach for calculating the effective thermal conductivity of com-plicated fiber geometriesKeywords: graph laplacian, effective heat conductivity, numerical upscaling, fibrous materials(14 pages, 2008)

143. J. Linn, T. Stephan, J. Carlsson, R. BohlinFast simulation of quasistatic rod deforma-tions for VR applicationsKeywords: quasistatic deformations, geometrically exact rod models, variational formulation, energy min-imization, finite differences, nonlinear conjugate gra-dients(7 pages, 2008)

144. J. Linn, T. StephanSimulation of quasistatic deformations us-ing discrete rod modelsKeywords: quasistatic deformations, geometrically exact rod models, variational formulation, energy min-imization, finite differences, nonlinear conjugate gra-dients(9 pages, 2008)

145. J. Marburger, N. Marheineke, R. PinnauAdjoint based optimal control using mesh-less discretizationsKeywords: Mesh-less methods, particle methods, Eul-erian-Lagrangian formulation, optimization strategies, adjoint method, hyperbolic equations(14 pages, 2008

146. S. Desmettre, J. Gould, A. SzimayerOwn-company stockholding and work effort preferences of an unconstrained executiveKeywords: optimal portfolio choice, executive compen-sation(33 pages, 2008)

147. M. Berger, M. Schröder, K.-H. KüferA constraint programming approach for the two-dimensional rectangular packing prob-lem with orthogonal orientationsKeywords: rectangular packing, orthogonal orienta-tions non-overlapping constraints, constraint propa-gation(13 pages, 2008)

148. K. Schladitz, C. Redenbach, T. Sych, M. Godehardt

Microstructural characterisation of open foams using 3d imagesKeywords: virtual material design, image analysis, open foams(30 pages, 2008)

149. E. Fernández, J. Kalcsics, S. Nickel, R. Ríos-Mercado

A novel territory design model arising in the implementation of the WEEE-DirectiveKeywords: heuristics, optimization, logistics, recycling(28 pages, 2008)

150. H. Lang, J. LinnLagrangian field theory in space-time for geometrically exact Cosserat rodsKeywords: Cosserat rods, geometrically exact rods, small strain, large deformation, deformable bodies, Lagrangian field theory, variational calculus(19 pages, 2009)

151. K. Dreßler, M. Speckert, R. Müller, Ch. Weber

Customer loads correlation in truck engi-neeringKeywords: Customer distribution, safety critical compo-nents, quantile estimation, Monte-Carlo methods(11 pages, 2009)

152. H. Lang, K. DreßlerAn improved multiaxial stress-strain correc-tion model for elastic FE postprocessingKeywords: Jiang’s model of elastoplasticity, stress-strain correction, parameter identification, automatic differ-entiation, least-squares optimization, Coleman-Li algo-rithm(6 pages, 2009)

153. J. Kalcsics, S. Nickel, M. SchröderA generic geometric approach to territory design and districtingKeywords: Territory design, districting, combinatorial optimization, heuristics, computational geometry(32 pages, 2009)

154. Th. Fütterer, A. Klar, R. WegenerAn energy conserving numerical scheme for the dynamics of hyper elastic rodsKeywords: Cosserat rod, hyperealstic, energy conserva-tion, finite differences(16 pages, 2009)

155. A. Wiegmann, L. Cheng, E. Glatt, O. Iliev, S. Rief

Design of pleated filters by computer sim-ulationsKeywords: Solid-gas separation, solid-liquid separation, pleated filter, design, simulation(21 pages, 2009)

156. A. Klar, N. Marheineke, R. WegenerHierarchy of mathematical models for pro-duction processes of technical textiles

166. J. I. Serna, M. Monz, K.-H. Küfer, C. ThiekeTrade-off bounds and their effect in multi-criteria IMRT planningKeywords: trade-off bounds, multi-criteria optimization, IMRT, Pareto surface(15 pages, 2009)

167. W. Arne, N. Marheineke, A. Meister, R. We-gener

Numerical analysis of Cosserat rod and string models for viscous jets in rotational spinning processesKeywords: Rotational spinning process, curved viscous fibers, asymptotic Cosserat models, boundary value problem, existence of numerical solutions(18 pages, 2009)

168. T. Melo, S. Nickel, F. Saldanha-da-GamaAn LP-rounding heuristic to solve a multi-period facility relocation problemKeywords: supply chain design, heuristic, linear pro-gramming, rounding(37 pages, 2009)

169. I. Correia, S. Nickel, F. Saldanha-da-GamaSingle-allocation hub location problems with capacity choicesKeywords: hub location, capacity decisions, MILP for-mulations(27 pages, 2009)

170. S. Acar, K. Natcheva-AcarA guide on the implementation of the Heath-Jarrow-Morton Two-Factor Gaussian Short Rate Model (HJM-G2++)Keywords: short rate model, two factor Gaussian, G2++, option pricing, calibration(30 pages, 2009)

171. A. Szimayer, G. Dimitroff, S. LorenzA parsimonious multi-asset Heston model: calibration and derivative pricingKeywords: Heston model, multi-asset, option pricing, calibration, correlation(28 pages, 2009)

172. N. Marheineke, R. WegenerModeling and validation of a stochastic drag for fibers in turbulent flowsKeywords: fiber-fluid interactions, long slender fibers, turbulence modelling, aerodynamic drag, dimensional analysis, data interpolation, stochastic partial differen-tial algebraic equation, numerical simulations, experi-mental validations(19 pages, 2009)

173. S. Nickel, M. Schröder, J. SteegPlanning for home health care servicesKeywords: home health care, route planning, meta-heuristics, constraint programming(23 pages, 2009)

174. G. Dimitroff, A. Szimayer, A. WagnerQuanto option pricing in the parsimonious Heston modelKeywords: Heston model, multi asset, quanto options, option pricing(14 pages, 2009) 174. G. Dimitroff, A. Szimayer, A. Wagner

175. S. Herkt, K. Dreßler, R. PinnauModel reduction of nonlinear problems in structural mechanicsKeywords: flexible bodies, FEM, nonlinear model reduc-tion, POD(13 pages, 2009)

Status quo: December 2009

Keywords: Fiber-fluid interaction, slender-body theory, turbulence modeling, model reduction, stochastic dif-ferential equations, Fokker-Planck equation, asymptotic expansions, parameter identification(21 pages, 2009)

157. E. Glatt, S. Rief, A. Wiegmann, M. Knefel, E. Wegenke

Structure and pressure drop of real and vir-tual metal wire meshesKeywords: metal wire mesh, structure simulation, model calibration, CFD simulation, pressure loss(7 pages, 2009)

158. S. Kruse, M. MüllerPricing American call options under the as-sumption of stochastic dividends – An ap-plication of the Korn-Rogers modelKeywords: option pricing, American options, dividends, dividend discount model, Black-Scholes model(22 pages, 2009)

159. H. Lang, J. Linn, M. ArnoldMultibody dynamics simulation of geomet-rically exact Cosserat rodsKeywords: flexible multibody dynamics, large deforma-tions, finite rotations, constrained mechanical systems, structural dynamics(20 pages, 2009)

160. P. Jung, S. Leyendecker, J. Linn, M. OrtizDiscrete Lagrangian mechanics and geo-metrically exact Cosserat rodsKeywords: special Cosserat rods, Lagrangian mechanics, Noether’s theorem, discrete mechanics, frame-indiffer-ence, holonomic constraints(14 pages, 2009)

161. M. Burger, K. Dreßler, A. Marquardt, M. Speckert

Calculating invariant loads for system simu-lation in vehicle engineeringKeywords: iterative learning control, optimal control theory, differential algebraic equations(DAEs)(18 pages, 2009)

162. M. Speckert, N. Ruf, K. DreßlerUndesired drift of multibody models excit-ed by measured accelerations or forcesKeywords: multibody simulation, full vehicle model, force-based simulation, drift due to noise(19 pages, 2009)

163. A. Streit, K. Dreßler, M. Speckert, J. Lichter, T. Zenner, P. Bach

Anwendung statistischer Methoden zur Erstellung von Nutzungsprofilen für die Auslegung von MobilbaggernKeywords: Nutzungsvielfalt, Kundenbeanspruchung,Bemessungsgrundlagen(13 pages, 2009)

164. I. Correia, S. Nickel, F. Saldanha-da-GamaAnwendung statistischer Methoden zur Er-stellung von Nutzungsprofilen für die Auslegung von MobilbaggernKeywords: Capacitated Hub Location, MIP formulations(10 pages, 2009)

165. F. Yaneva, T. Grebe, A. ScherrerAn alternative view on global radiotherapy optimization problemsKeywords: radiotherapy planning, path-connected sub-levelsets, modified gradient projection method, improv-ing and feasible directions(14 pages, 2009)

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Model reduction of nonlinear problems in structural mechanics · S. Herkt, K. Dreßler, R. Pinnau Model reduction of nonlinear problems in structural mechanics Berichte des Fraunhofer