bluuebunny/crossref_metadata_2025_split huggingface dataset:
Turbulent drag reduction at moderate Reynolds numbers via spanwise velocity waves
Authors: Gatti Davide , Quadrio Maurizio 
Published: December 2012

AbstractThe question whether large turbulent drag reduction can be achieved at the high values of Re typical of applications is addressed. Answering such question, either by experiments or DNS, is obviously challenging. For DNS, the problem lies in the tremendous increase of the computational cost with Re, that has to be appreciated in view of the need of carrying out an entire parametric study at every Re, owing to the unknown location of the optimal forcing parameters.In this paper we limit ourselves to considering an open‐loop technique based on spanwise forcing, the streamwise‐traveling waves introduced by [1], and explore via Direct Numerical Simulations (DNS) how the drag reduction varies when the friction Reynolds number is increased from Reτ = 200 to Reτ = 2000. To achieve high Re while keeping the computational cost affordable, computational domains of reduced size are employed. We adopted special care to interpret results that are indeed still box‐size dependent, as well as strategies to compute the random errors and give the results an error bar.Our results indicate that still R = 0.29 can be obtained at Reτ = 2000 in the partial region of the parameter space studied. The maximum R is found to decrease as R ˜ Reτ−0.22 in the Reynolds range investigated. As most important outcome, we find that the sensitivity of R to Re becomes smaller when far from the low‐Re optimum parameters: in this region, we suggest R ˜ Reτ−0.08. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Non‐sinusoidal wall oscillation for drag reduction
Authors: Cimarelli Andrea , Frohnapfel Bettina , Hasegawa Yosuke , De Angelis Elisabetta , Quadrio Maurizio 
Published: December 2012

AbstractThe drag reducing properties of wall flows under non‐sinusoidal oscillations are analyzed in the present work. The dependency of the drag reduction rate, R, and of the power spent to move the walls, Pin, with respect the shape of the wall motion are investigated. This preliminary work highlights a significant role of the oscillation shape for turbulence control suggesting more research efforts in the field with possible repercussions for practical applications. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Distortion of liquid metal flow in a square duct due to the influence of a magnetic point dipole
Authors: Tympel Saskia , Krasnov Dmitry , Boeck Thomas , Schumacher Jörg 
Published: December 2012

AbstractWe consider liquid metal flow in a square duct with electrically insulating walls under the influence of a magnetic point dipole using three‐dimensional direct numerical simulations with a finite‐difference method. The dipole acts as a magnetic obstacle. The Lorentz force on the magnet is sensitive to the velocity distribution that is influenced by the magnetic field. The flow transformation by an inhomogeneous local magnetic field is essential for obtaining velocity information from the measured forces. In this paper we present a numerical simulation of a spatially developing flow in a duct with laminar inflow and periodic boundary conditions. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Electromagnetic interaction of a conducting cylinder with a magnetic dipole caused by steady rotation
Authors: Engert Sonja , Boeck Thomas , Thess André 
Published: December 2012

AbstractThe motion of a conductor in a magnetic field induces eddy currents whose interaction with the field produces Lorentz forces opposing the motion. One can determine the velocity of the conductor from the force on the magnet system since the latter is equal but opposite to the Lorentz force on the conductor. This contactless method is known as Lorentz force velocimetry (LFV). We study an idealized configuration of LFV, i.e. a rotating solid cylinder interacting with a point dipole. The understanding of parameter influences in this setup can be helpful for more realistic configurations. We use a purely kinematic approach appropriate for low magnetic Reynolds numbers. Numerical results for small and large distances between dipole and cylinder have been obtained with the commercial software COMSOL Multiphysics. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Covering Surfaces with Noncommutative Monodromy Groups and their Industrial Applications
Authors: Dmitrieva Irina 
Published: December 2012

AbstractAn explicit construction of the algebraic equations of the covering surfaces with the noncommutative monodromy groups is done by means of the corresponding homogeneous vector boundary Riemann‐Hilbert problem solution. The direct applications concern soliton theory and Landau‐Lifshitz equation, in particular. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The exterior Dirichlet and the interior Neumann boundary value problems for the scalar Oseen equation
Authors: Skopin Emma 
Published: December 2012

AbstractThe scalar Oseen equation represents a linearized form of the Navier Stokes equations. We present an explicit potential theory for this equation and solve the exterior Dirichlet and interior Neumann boundary value problems via a boundary integral equations method in spaces of continuous functions on a C2‐boundary, extending the classical approach for the isotropic selfadjoint Laplace operator to the anisotropic non‐selfadjoint scalar Oseen operator. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Maximum Modulus Estimates for the Linear Steady Stokes System
Authors: Varnhorn Werner 
Published: December 2012

AbstractA maximum modulus estimate for the Stokes system in bounded domains of ℝn (n ≥ 2) is established via methods of hydrodynamical potential theory. The method is based on the unique solvability of the boundary integral equations' system resulting from the double layer potential ansatz together with a projection onto the normal field on the boundary. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Estimations on Thermo‐mechanical Dynamics of Vibration Elastomeric Isolators
Authors: Nastac Silviu , Debeleac Carmen 
Published: December 2012

AbstractThis analysis deals with one of the basic problem category of vibratory systems, means the complete and complex characterization of elastic and viscous isolators behaviour under dynamic loads such as vibrations, seismic waves, shocks, etc. Usually, the dynamic characteristics of vibration isolators made by elastomeric materials are considered to have a constant shape for a certain practical case. It is ignored the thermal phenomenon inside the isolator block during the exploitation cycles and its influences on the proper characteristic parameters. This usual approximation leads to more or less significant differences between simulation and practical evolution of a vibration isolator subjected to the same dynamic load. Continuous changes of rigidity modulus and/or dissipative characteristics due to internal thermal effects imply aleatory evolution of the isolated system, unstable movements and resonance imminence danger. The partial results of this analysis dignify the linkage between thermal effects into the elastomeric isolator and its essential dynamic parameters. Using of these correlations frames the seismic shock and vibration protective devices designing and deployment areas. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Geometry Optimization of Branched Sheet Metal Products
Authors: Göllner Thea , Hess Wolfgang , Ulbrich Stefan 
Published: December 2012

AbstractWe consider the geometry optimization of branched sheet metal products, which can be produced continuously and in integral style using the new technologies of linear flow splitting and linear bend splitting. These are explored within the framework of the Collaborative Research Centre (CRC) 666. The geometry of such products can be parameterized by means of free form surfaces whereas their mechanical behaviour is described by the three dimensional linear elasticity equations. The associated PDE‐constrained problem for optimizing the stiffness of the structure is formulated and an algorithm for solving this problem using exact constraints and a globalization strategy based on adaptive cubic regularization is presented. Numerical results for an example are given. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Sonofusion: EA optimisation of acoustic resonator
Authors: Stokmaier Markus J. , Class Andreas G. , Schulenberg Thomas , Lahey Richard T. 
Published: December 2012

AbstractWe present a novel recipe combining evolutionary algorithm (EA) concepts, robust against local optima, for high‐dimensional multimodal optimisation problems, which we developed for the purpose of optimising piezo‐driven liquid‐filled acoustic resonators for future sonofusion experiments. In contrast to other resonators (e.g. mirrors for laser, flute), the structure bounding the oscillating medium cannot be assumed to be static. Instead, the vibration response of the whole fluid‐structure setup to a periodic driving force is considered. Manufacturing and reproducibility criteria exclude trivial spherical or cylindrical shapes leading to many design parameters to be optimised. With maximising the sound pressure amplitude in the center of the fluid volume being the primary goal, one faces a competition of an increased number of resonances varying in amplitude and frequency, and interacting with other resonances. In this smooth, yet high‐dimensional search space scattered with many local optima, an EA approach is efficient. Evaluating a solution candidate corresponds to calculating a harmonic response over a frequency band of a 2D‐axisymmetric FE model of the piezo‐driven structure filled with acoustic fluid elements (modeled in Ansys). Thus, more than 104 evaluation calls per run cannot be afforded. Serving those needs we developed an EA combining elements from genetic algorithms (GA), evolution strategies (ES), differential evolution (DE), and simulated annealing (SA). (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Improvement of classical first‐order adjoint sensitivity relations
Authors: Materna Daniel , Barthold Franz‐Joseph 
Published: December 2012

AbstractThis contribution is concerned with a novel approach for the improvement of classical first‐order adjoint sensitivity relations of general objective functionals or quantities of interest. The improvement approach is based on an exact variational formulation for the change in the quantity of interest due to finite design perturbations. The exact change in the quantity of interest can be expressed in a closed variational form. This formulation is used in order to predict the change in the quantity of interest and yields an improved solution in comparison to the results of classical first‐order approximations. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
An Exploratory Line Search for Piecewise Differentiable Objective Functions based on Algorithmic Differentiation
Authors: Fiege Sabrina , Griewank Andreas , Walther Andrea 
Published: December 2012

AbstractNonsmoothness is a typical characteristic of numerous objective functions in optimisation that arises from applications. The standard approach in algorithmic differentiation (AD) is to consider only differentiable functions that are defined by an evaluation program. We extend this functionality by allowing also the functions abs(), min() and max() during the function evaluation yielding piecewise differential nonlinear functions. We will define an evaluation procedure for these functions and employ ADOL‐C in an adapted gradient based optimisation method that was adjusted to the special properties of the objective functions considered here. First numerical results will be presented. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Parallel Computing for Long‐Time Simulations of Calcium Waves in a Heart Cell
Authors: Wang Yu , Olano Marc , Gobbert Matthias , Griffin Wesley 
Published: December 2012

AbstractCalcium waves are modeled by parabolic partial differential equations, whose simulation codes contain Krylov subspace methods as computational kernels. This paper presents GPU‐based parallel computations for the conjugate gradient method applied to the finite difference discretization of a Poisson equation as prototype problem for the computational kernel. The CUDA algorithm tests the three memory systems of global memory, texture memory, and shared memory of a CUDA‐enabled GPU. Due to the caching mechanism and coalesced read/write operations, the CUDA algorithm using global memory and single precision floating point numbers outperforms algorithms accessing texture memory and the shared memory. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Adaptive Stochastic Collocation on Sparse Grids
Authors: Schieche Bettina 
Published: December 2012

AbstractWe consider Partial Differential Equations (PDEs) with random parameters, solved by adaptive stochastic collocation. Our research studies common error indicators compared to adjoint based error estimates extended to the stochastic framework. We show that such estimates are able to detect the true error very accurately and suggest to replace the expensive adjoint problem by a reduced model. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Generalized Park‐Sheen Finite Elements for Adaptivity
Authors: Altmann Robert , Carstensen Carsten 
Published: December 2012

AbstractThe P1‐nonconforming finite element method on simply connected Lipschitz domains partitioned into quadrilaterals was introduced by Park and Sheen in 2003. In order to apply mesh‐adaptive refinement strategies, the concept is generalized to arbitrary triangulations into quadrilaterals and triangles of multiple connected Lipschitz domains. An explicit a priori analysis is given for second‐order elliptic PDEs with inhomogeneous Dirichlet boundary conditions. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the error estimate for calculating some singular integrals
Authors: Vasilyev Alexander , Vasilyev Vladimir 
Published: December 2012

AbstractWe consider some classes of multi‐dimensional singular integral operators with Calderon‐Zygmund kernels, construct their discrete analogues and give the error estimate for approximation of continual singular integral operator by the discrete ones. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A comparison of the efficiency of Rosenbrock and DIRK variants
Authors: Bijl Hester , Birken Philipp , Meister Andreas , van Zuijlen Alexander 
Published: December 2012

AbstractWe consider the use of DIRK and Rosenbrock schemes for the time integration of unsteady flow problems. Thereby, two variants of solving the linear systems are compared regarding their efficiency, namely a Jacobian‐free method versus computing an approximate Jacobian. The Rosenbrock schemes are slow in the Jacobian‐free variant, but may be competitive in the other. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optimal dopant doping profiling with TV penalty
Authors: Fouego Marcisse , Burger Martin 
Published: December 2012

AbstractWe investigate the optimal control of stationary drift‐diffusion models for semiconductors. Our main focus is the control of the current by optimizing the doping profile, which is the natural control variables, wita total variation penalty. We investigate the augmented Lagrangian method for the numerical solution, with an efficient Gummel iteration for the subproblem. Numerical results are presented for the ballistic diode. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Dynamic PET Reconstruction based on a Reaction‐Diffusion Model
Authors: Reips Louise , Engbers Ralf , Burger Martin 
Published: December 2012

AbstractPositron‐Emission Tomography (PET) is an imaging technique in nuclear medicine used to image physiological processes. A major obstacle is the need for dynamic image reconstruction from low quality PET‐data, which applies in particular for tracers (radioactive water) with fast decay like H215O when looking for improved spatial resolution.Here we present a model‐based approach to overcome those difficulties. We derive a set of differential equations able to represent the kinetic behavior of H215O PET tracers during cardiac perfusion. In this model one takes into account the exchange of materials between artery, tissue and vein, which predicts the tracer activity if the reaction rates, velocities, and diffusion coefficients are known. One then interprets the computation of these distributed parameters (spatially dependent only) as a nonlinear inverse problem, which we solve using variational regularization approaches. For the minimization we use the gradient‐based methods and Forward‐Backward Splitting. The main advantage is the reduction of the degrees of freedom, which makes the problem overdetermined and thus allows to proceed to low quality data. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
LQR Control for Vibration Suppression of Piezoelectric Integrated Smart Structures
Authors: Zhang S. Q. , Schmidt R. 
Published: December 2012

AbstractIn the present paper, the equations of motion for piezoelectric integrated smart beams are derived by applying Hamilton's principle and the Finite Element (FE) method, based on the First‐Order Shear Deformation (FOSD) theory. Then, a state space model is constructed from the equations of motion for control design. A Linear Quadratic Regulator (LQR) control for vibration suppression is implemented on the mathematical model of the piezoelectric bonded beam. Finally, a numerical application is performed to testify the applicability and effectiveness of the present method. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Passivity based stabilization of linear mechanical systems with dissipation in unactuated degrees of freedom
Authors: Kotyczka Paul , Delgado‐L. Sergio 
Published: December 2012

AbstractA well‐known problem in nonlinear controller design for underactuated mechanical systems using the Interconnection and Damping Assignment (IDA‐PBC) technique is physical dissipationf in unactuated degrees of freedom. For certain equilibria the definiteness requirements on the virtual energy of the port‐Hamiltonian (pH) target system and the closed loop dissipation matrix can not be satisfied simultaneously. At the GAMM Meeting 2012 we presented a modification of the nonlinear pH target system, where particularly the total energy function is augmented by a cross term between coordinates and momenta. For the sake of brevity in this short communication we focus on the case of linear mechanical systems. In particular, it is shown how the generalized pH target structure is derived from the solution of a Lyapunov equation for the closed loop system. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Model order reduction methods and their possible application in compliant mechanisms
Authors: Rösner Malte , Lammering Rolf 
Published: December 2012

AbstractModel order reduction (MOR) is commonly used to approximate large‐scale linear time‐invariant dynamical systems. A new feed unit based on a compliant mechanism consisting of flexure hinges can be described by a discrete system of n ordinary differential equations. A projection framework using modal and Krylov subspace techniques is applied to reduce the order of the system to lower computational cost and make the model feasible for control, analysis and optimization. Single flexure hinges are investigated numerical, analytical and experimental and compared to reduced models via modal and tangential Krylov subspace methods regarding the first eigenfrequency. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Model reduction for optimal control problems in field‐flow fractionation
Authors: Stykel Tatjana , Willbold Carina 
Published: December 2012

AbstractWe discuss the application of model order reduction to optimal control problems governed by coupled systems of the Stokes‐Brinkman and advection‐diffusion equations. Such problems arise in field‐flow fractionation processes for the efficient and fast separation of particles of different size in microfluidic flows. Our approach is based on a combination of balanced truncation and tangential interpolation for model reduction of the semidiscretized optimality system. Numerical results demonstrate the properties of this approach. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Symmetric Positivity Preserving Balanced Truncation
Authors: Grußler Christian , Damm Tobias 
Published: December 2012

AbstractWe consider positivity preserving model order reduction of SISO linear systems. Whereas well‐established model reduction methods usually do not result in a positive approximation, we show that a symmetry characterization of balanced truncation can be used to preserve positivity after performing balanced truncation. As a consequence, the method is independent of the initial realization and always returns a symmetric reduced model. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
End‐Effector Trajectory Tracking of Serial Flexible Manipulators
Authors: Seifried Robert , Wörner Mark , Gorius Thomas 
Published: December 2012

AbstractLight weight manipulator design results in low energy consumption and allow often high working speeds. However, due to the increased system flexibility undesired vibrations occur. Thus, in the control design these flexibilities must be taken into account. In order to obtain a good performance in end‐effector trajectory tracking an efficient feedforward control is used, which is then supplemented by additional feedback control to account for small disturbances and uncertainties. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optimal Control on Stable Manifolds for a Double Pendulum
Authors: Flaßkamp Kathrin , Timmermann Julia , Ober‐Blöbaum Sina , Dellnitz Michael , Trächtler Ansgar 
Published: December 2012

AbstractOptimal control problems for mechanical systems often arise in technical applications, e.g. in the computation of energy efficient or time optimal steering maneuvers between operation points. In this contribution, we propose a control strategy for mechanical systems that includes uncontrolled trajectories on (un)stable manifolds in a sequence together with short control maneuvers to design a feedforward control. In particular, we present optimal swing‐up solutions for a double pendulum that are based on trajectories on the stable manifold of the pendulum's upper equilibrium. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Flatness‐based feedforward control design of a system of parabolic PDEs based on finite difference semi‐discretization
Authors: Utz Tilman , Kugi Andreas 
Published: December 2012

AbstractThis contribution is concerned with feedforward control design for setpoint transitions of systems of semilinear boundary‐controlled parabolic partial differential equations (PDEs). For this, the originally infinite‐dimensional system is semi‐discretized using finite differences on the spatial derivatives. The resulting system of ordinary differential equations (ODEs) is differentially flat, such that a nominal feedforward control can be determined using the flat output and its time derivatives. The method is applied to control the ignition of a nonlinear tubular reactor model. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)