Suggestions for potential research projects are summarized in the following. More projects are given than candidates will be able to join as a scholar, so candidates can choose among the suggestions. Students will elect their preferred research project after consultation with their supervisors. According to the guidelines of the Deutsche Forschungsgemeinschaft, only basic ideas and aspects of the suggested research projects are given.
Wind Excitation of Tall Structures
Supervisors: Peil, Matthies
Large-amplitude wind-induced excitation of tall structures can result from different mechanisms. Slender structures with low eigenfrequencies and low damping are excited mainly by the gustiness of the natural wind, leading to buffeting phenomena. Linear algorithms for the description of the system response usually only take into account longitudinal turbulence effects. However, the lateral turbulence spectrum is higher than the longitudinal turbulence spectrum and thus needs to be included in the analysis as well. Measurements taken within the last 6 years on a 344m transmission tower in Gartow, Germany, show considerable vibration effects perpendicular to the main wind direction. Because of the strong non-linear character of the aerodynamic admittance function, turbulence spectra cannot be directly transformed into pressure spectra. Major factors in this context are the proportionality of dynamic pressure and relative flow velocity squared, as well as the dependency of aerodynamic drag coefficient and lateral displacement coefficient on the angle of attack. Longitudinal and lateral excitation forces are coupled. Based on measurements of wind velocity and wind direction taken on the Gartow transmission tower on many spots with a small vertical spacing, an approximative model shall be developed which is able to describe the lateral effects within a linear model. The turbulence intensity as a function of the height is known from the measurements, and the effects of neglecting deviating terms of higher order can be estimated. Lateral vibration can also result from aeroelastic instabilities (galloping excitation), which are dependent on the shape of the cross-section, and from periodic vortex shedding. For these effects, turbulence intensity plays a major role as well and the existing measurements constitute a good starting point for further research.
[1] U. Peil, G. Telljohann: Dynamisches Verhalten hoher Bauwerke im böigen Wind. Stahlbau, März 1997, Heft 3, S. 99-109. Verlag Ernst & Sohn.
[2] U. Peil, G. Telljohann: Lateral turbulence and dynamic response. Structural dynamics, EURODYN 96, Vol.1, pp. 207-211. A.A.Balkema/Rotterdam/Brookfield.
[3] Q. Zhang, U. Peil: Random finite element analysis for stochastical response of structures. Computers&Structures Vol.62, No 4, 1997, pp. 611-616. Elsevier Science Ltd.
Wind Loading of Structures
Supervisors: Antes, Peil
Tall and slender structures subjected to strong wind loading may exhibit large-amplitude vibration phenomena, which are closely related to their eigenfrequencies. Soil-structure interaction may influence the system´s eigenfrequencies to a large extent. A realistic description of wind effects can only be achieved if the interaction phenomena are included in the model. In this project, an existing FEM-BEM model for soil-structure interaction will be extended in a way that effects of wind loads of different kinds, including stochastic properties, can be examined.
[1] H. Antes, K. Latz: Soil-Structure-Fluid Interaction, Chapt. 12 in Boundary Element Techniques in Geomechanics (Eds.: G. D. Manolis, T. G. Davis) Elsevier, London 1993.
[2] H. Antes, K. Latz, M. Mahlmann: Dynamic Interaction Analysis of Liquid Storage Tanks, pp. 2179-2194 in Proc. 10th European Conf. on Earthquake Engineering (Ed.: Duma) Balkema, Rotterdamm 1995.
Transition Conditions of Different Scales in Structure and Flow
Supervisors: Dinkler, Kossira, Matthies
In the solution process of coupled fluid-structure interaction problems, the numerical behavior depends to a large extent on the scales in time and space of the field problems involved. Simple problems such as flexible plates in a channel flow will be used to examine the influence of different discretization procedures with respect to the properties of the system. Different model equations will be used for the fluid flow for small velocities (potential equation, Euler equation, Navier-Stokes equation). It is intended to discretize both fluid and structure by space-time finite elements, because previous work has been done in this area.
[1] D. Dinkler, G. Wittum: Transonische Umströmung elastischer Tragflügel. Arbeitsbericht, Teilprojekt B2, SFB 404 - Mehrfeldprobleme, Universität Stuttgart, 1997
[2] M. Schön: Finite-Raum-Zeit-Elemente für die Modellierung der instationären Eulergleichung. Dissertation Universität Stuttgart, 1994
[3] R. Dornberger: Effiziente Berechnung des Einflusses nichtviskoser Strömungen auf Strukturen in der Aeroelastik, Dissertationsschrift, Universität Stuttgart, wird eingereicht 1997
Dynamics of Cylindric Liquid Storage Tanks under Wind Loading
Supervisors: Dinkler, Antes, Hummel
Liquids are commonly stored in very slender cylindric steel steel structures. Empty tanks are susceptible to buckling under dead load due to the small wall thickness. Radial wind loading contributes to the critical buckling behavior and determines the buckling mode shape. Niemann et al. [2] have examined the buckling behavior of cylindrical containments reinforced at the upper rim. They state that the wind pressure at the front ram point has a major influence on the buckling load and pattern. In stationary flow there is no measurable fluid-structure interaction, thus the problem can be solved by traditional means of static analysis. Non-stationary wind flow induces several different vibration modes in the wall of the containment, and it becomes very difficult to identify the buckling mode resulting in instability of the overall structure. Resonance phenomena and self-exciting mechanisms may be caused by specific frequency spectra of the excitation, which can only be predicted by an analysis accounting for the coupled character of the fluid-structure system. A more precise three-dimensional analysis including extensive parametric studies is not yet feasible. To circumvent this problem, the coupled system will be described by a plane model where the cylinder is represented by a beam with nonlinear boundary conditions for the circumferential stiffness. Description of the fluid flow will be on basis of the Navier-Stokes equations. In a first step, an existing program for planar Euler flow will be modified in order to include the full Navier-Stokes equations. In a second step, a closer look will be taken at the dynamics of the shell structure subjected to wind flow. A systematic approach in the examination of buckling and vibration behavior of shells becomes possible. Previous work has been done in the areas of non-linear shell dynamics [1], [3], as well as in the field of modeling of coupled systems [4]. Extensive research has been done in aerodynamics on vortex flow phenomena [5].
[1] D. Dinkler: Phenomena in nonlinear Dynamic Buckling Behaviour of Elastic Structures. AIAA-Proceedings, SDM-Conference, 1991
[2] H.-J. Niemann, M. Kasperski, V. Görnandt: Thin-Walled Shells Subjected to Wind Loading. Ruhr-Universität Bochum, Institut für Konstruktiven Ingenieurbau, 1996
[3] U. Hänle, D. Dinkler, B. Kröplin: "Interaction of Local and Global Nonlinearities of Elastic Rotating Structures" in: AIAA Journal, Vol. 33 No. 5, 1995
[4] D. Dinkler, G. Wittum: Transonische Umströmung elastischer Tragflügel. Arbeitsbericht, Teilprojekt B2, SFB 404 - Mehrfeldprobleme, Universität Stuttgart, 1997
[5] D. Strohmeyer, M. Orlowski, J. M A. Longo, D. Hummel, A. Bergmann: An analysis of vortex breakdown predicted by the Euler equations. 20th ICAS-Kongreß Sorrent 1996
Aeroelastic Interaction Effects of Wind Energy Converters
Supervisors: Matthies, Dinkler
Modern wind energy converters are very flexible structures and exhibit strong aeroelastic interaction effects. Specifically, during fast deceleration of the rotating parts, complex effects can be observed which are very difficult to describe mechanically. Rotating structures pose certain difficulties to the analyst such as different grids used for discretization of the air flow (fixed and rotating), and of the rotor (rotating). In this project, common space-time discretizations will be used.
[1] H. G. Matthies & C. Nath: Dynamische Stabilität nichtlinearer Systeme mit periodischer Erregung am Beispiel der Großen Windenergieanlage GROWIAN, ZAMM 64 (1984)
[2] H. G. Matthies & C. Nath: Dynamic stability of periodic solutions of large scale nonlinear systems, Comp. Math. Appl. Mech. Engrng. 48 (1985) 191-202
[3] H. G. Matthies & C. Nath: Detection of inadmissible states of operation - a basic safety concept; in: Proc. of the European Wind Energy Conference Hamburg 1984 (EWEC ´84), W. Palz ed., H. S. Stephens & Assoc., London, 1985
[4] H. G. Matthies & D. Sideris: A rational basis for safety systems of wind turbines, in: Proc. of. the European Wind Energy Conference Glasgow 1989 (EWEC ´89), Peter Peregrinus Ltd., London, 1989
[5] D. Dinkler, F. Döngi: Robust Vibration Control of Rotor Blades in Forward Flight. Proc. of the Intn. Forum on Aeroelasticity and Structural Dynamics, Manchester (1995)
Reduction Methods for Fluid-Structure Systems
Supervisors: Dinkler, Matthies
The number of discretized equations necessary for the solution of fluid-structure interaction problems can easily exceed the order of n = 105 - 106. In case only structural forces and pressure distribution on the interface are of interest, direct integration of the nonlinear equations with respect to time becomes rather expensive. Modal methods can reduce the degrees of freedom of the overall system to a few important displacement modes without reducing the accuracy of the solution to a large extent. Reduction methods are well-known in structural dynamics and have been successfully applied to the analysis of shell structures [1]. In [2], reduction methods are used to describe transition effects between laminar and turbulent flow around a plate. Objective of the project consists in studying different reduction techniques for the coupled fluid-structure problem, neglecting the transition phenomenon.
[1] D. Dinkler: Nonlinear Dynamic Deformation Behaviour of Elastic Sthells. EUROMECH 292, München, 1992
[2] D. Rempfer: Kohärente Strukturen und Chaos beim laminar-turbulenten Grenzschicht-umschlag. Dissertation, Universität Stuttgart, 1991
[3] H. G. Matthies: A subspace Lanczos method for the generalized symmetric eigenproblem, Computers and Structures 21 (1985) 319 - 325
Thermal-Mechanical Vibration and Stability of Shell Structures in Supersonic Flow
Supervisors: Kossira, Dinkler
Shell structures exhibit a strong geometrically nonlinear deformation behavior under purely thermal or mechanical loading conditions [1]. With an additional loading from supersonic flow [2], load bearing capacitiy and stability issues become even more complex. This project deals with the modeling of shell structures using suitable theories of higher order that can describe the behavior of composite materials commonly used in aircraft design today. Thermal and mechanical loading conditions through supersonic flow phenomena need to be covered by the theory used. Loading from supersonic flow will be examined by employing a finite element computer program using Navier-Stokes equations for the flow field [3]. Previous research [4] on the static and dynamic behaviour of structures including stability, vibration, and flutter will be continued and extended systematically. Objective of research will be the classification of different phenomena and the determination of the major parameters that have an important effect on structural behavior.
[1] H. Kossira, M. Haupt: Buckling of Laminated Plates and Cylindrical Shells Subjected to Combined Thermal and Mechanical Loads; Procs. of ECCS int. Coll., Lyon, 1991; in: 'Buckling of Shellstructures on Land, in the Sea and in the Air', Ed. J. F. Jullien, S. 201 - 212, Elsevier Applied Science, 1991
[2] M. Haupt, H. Kossira, M. Kracht, J. Pleitner: A Very Efficient Tool for the Structural Analysis of Hypersonic Vehicles under High Temperature Aspects; Proc. of 18th Congress of the International Council of the Aeronautical Sciences, Paper ICAS-92-5.2.2, Beijing (China), September 1992
[3] M. Haupt, H. Kossira, R. Radespiel: Analyse von aerothermodynamisch belasteten Flügelvorderkanten mit einer Methode der Fluid-Struktur-Kopplung; Deutsche Gesellschaft für Luft- und Raumfahrt, Jahrbuch 1995 III, 1996
[4] M. Haupt: Ein Beitrag zur thermischen und mechanischen Analyse von heißen Flugzeugstrukturen, Dissertation, TU Braunschweig, 1996
Aeroelasticity of High-Lift Devices in Slow Flight
Supervisors: Kossira, Hummel
During take-off, initial climb, and descent, aircraft utilize high-lift devices. The efficiency of high-lift devices depends largely on the elasticity of the wing. This project covers the modeling of different types of flap systems under stationary conditions. A formulation of the respective interface problem will be developed for a simulation in the time-domain. The formulation will be implemented in existing standard codes for analysis (finite element and finite volumes). For a simple wing structure, such a model has previously been developed and described in [1]. Coupling of programs will comprise mainly the mesh generation for the different flap system types under aerostatic deformations. For this problem, safe and stable algorithms and methods will be developed. Objective of the studies is providing a tool for simulating and optimizing flap systems with respect to the structural components, i.e. weight minimization by variation of the geometric parameters of the flaps such as length, bearings, structural system, and material. The study is expec-ted to show whether flap systems retain their aerodynamic effectiveness if structu-ral elasticity is considered in the analysis.
[1] A. Beckert: Beitrag zur Strömungs-Struktur-Kopplung für die Berechnung des aeroelastischen Gleichgewichtszustandes, Eingereichte Dissertation, TU Braunschweig, 1997
auch
DLR FB-97/42, Deutsches Zentrum für Luft- und Raumfahrt e.V., 1997.
[2] M. Haupt, H. Kossira: Thermal Fluid Structure Interaction - A Numerical Concept and an Application to Structures in Hypersonic Fluid Flow; Proc. of Euromech-Colloquium 349, 1996
Structural Optimization for New Aircraft Generations Considering Aeroelastic Interaction
Supervisors: Kossira, Dinkler
Structural optimization by iterative structural analysis necessitates models that can reflect the major effects with sufficient accuracy. In aircraft design simple finite element models are used for structural analysis, whereas the aerodynamical analysis is done using lattice or panel methods [1]. Those simple models can be integrated into the optimization cycle [2]. This cycle is designed to minimize structural mass under given stiffness and strength parameters [3]. If aeroelastic aspects are considered, the optimization problem becomes much more difficult. Dynamic systems in aeroelasticity are more complex to analyze, because aerodynamic nonlinearities have a significant effect when Mach numbers of about 0.85 are present. In the non-stationary case, transient analysis becomes rather demanding due to flutter phenomena. Methods developed in the past cannot be used any longer because of their high demand in computing time and because of the simplifications commonly used. In this project, methods for the modeling and solution of such aeroelastic systems will be developed with the use of reduction methods. The resulting algorithms will be implemented into the optimization cycle, e.g. by means of evolutionary algorithms. Objective will be to develop an integrative concept for the optimization of aircraft structures which comprises analytical methods with sufficient physical and numerical accuracy for the modeling process (Euler and Navier-Stokes code and finite element code). In this way it may be achieved to analyze and optimize new generation aircraft structures in early phases of the design process.
[2] A. Lubis: Zur Optimierung des multidisziplinären Entwurfs von Verkehrsflugzeugen in der parallelen Rechenumgebung, Dissertation, TU Braunschweig, ZLR-FB 94-02, 1994
[3] A. Bardenhagen, H. Kossira, W. Heinze: Weight estimation of hypersonic waveriders within the integrated design program PrADO-Hy; AIAA-Paper 96-4546, 1996
[4] A. Axmann: Raumflugtechnische Optimierungen mit adaptiven Evolutionären Algorithmen auf Parallelrechnern; Deutsche Gesellschaft für Luft- und Raumfahrt, Jahrbuch 1995, S. 455 - 463, 1994
Thrust Generation by Flapping Airfoils
Supervisors: Hummel, Matthies
The mechanism of flapping wing propulsion for two dimensional and three dimensional flow will be studied throughout the project. The investigations emphasize on the flow phenomena resulting from unsteady motion. This is achieved by simulating a complete flapping cycle for a given wing in forward motion with the help of a numerical flow solver. In contrast to nature, where the kinematics of the wing is governed either actively by muscular forces or by aerodynamic forces, the motion and deformation of the wing is regarded as given. The thrust efficiency of such given motion is then determined as a function of flapping frequency, amplitude and torsion of the wing. Special emphasis will be put on the question whether the drag of the wing in unsteady flow can be less than in case of a steady state forward motion.
[1] D. Hummel, W. Möllenstädt: On the calculations of the aerodynamic forces acting on a House Sparrow (Passer domesticus L.) during downstroke by means of aerodynamic theory. Fortschritt Zoologie 24, 1977
[2] D. Hummel: Aerodynamic investigatons on the tail effect in birds. Z. Flugwiss. Weltraumforsch. 16, 1992
[3] D. Hummel: Formation flight as an energy-saving mechanism. Israel J. of Zoology 41, 1995 [4] D. Hummel: The use of aircraft wakes to achieve power reductions in formation flight. AGARD-CP-584, 1996
Non-Stationary Flow around Slender Airfoils
Supervisor: Hummel, Kossira
For large angles of attack the stationary vortex flow across an airfoil changes into a non-stationary flow. The resulting vortex breakdown has been investigated for some time and it now amenable to numerical analysis. Non-stationary vortex flow will be examined throughout this project for different boundary conditions. Numerical results will be compared to Laser-Doppler measurements obtained from wind tunnel experiments.
[1] D. Hummel, S. Kommallein: LDA-investigations of the separated flow over slender wings. In: K. Gersten (Ed.): Physics of separated flows. NNFM 40, 1993
[2] D. Hummel, H.-Chr. Oelker: Low-speed characteristics for the wing-canard configuration of the International Vortex Flow Experiment. J. Aircraft 31, 1994
[3] D. Hummel, A. Brümmer: Aerodynamics of a slender wing with vertical fins at low speed. 19th ICAS-Kongreß Anaheim, 1994
[4] D. Strohmeyer, M. Orlowski, J. M A. Longo, D. Hummel, A. Bergmann: An analysis of vortex breakdown predicted by the Euler equations. 20th ICAS-Congress Sorrent 1996
Design of Airfoils Including Structural Response
Supervisors: Rossow, Kossira
Currently airfoils are designed without accounting for aeroelastic effects. This may result in the wing design being unusable due to large deformations which have previously not been included in the design analysis. A method for the design of airfoils for a given pressure distribution has been developed in [1]. The method uses the postprocessor of the FLOWer code, which has been extended as part of the research program MEGAFLOW of the BMBF [2]. As a result of the design process according to [1] a geometry is available which produces the given pressure distribution. However, deformations resulting from aerodynamic forces are not included in this analysis. Structural behavior can be approximated by simple models like spring-damper systems, flexural-torsional beam systems, or by use of the finite element method. Throughout the research program of the BMBF "`Dynamics of Flexible Aircraft"', an interface between numerical methods for fluid flow analysis on the one hand and finite element methods for structural analysis on the other hand will be developed which uses the FLOWer code coupled the structural model as a postprocessor.
[1] W. Bartelheimer: Ein Entwurfsverfahren für Tragflügel in transsonischer Strömung, Dissertation an der TU Braunschweig, 1996
[2] W. Bartelheimer: Projektbeschreibung MEGAFLOW, DLR-IB 129-96/8, 1996
[3] Dynamik des flexiblen Flugzeuges; Antrag auf Förderung eines Verbundvorhabens im Rahmen des BMBF Luftfahrtförderungsprogramms, 1996
Development of a Method for Strong Coupling of Aerodynamics and Structures
Supervisors: Rossow, Hummel, Dinkler
When coupling aerodynamic and structural analysis, the two resulting systems of equations are commonly treated separately. Integration is carried out in an alternating way, with a solution per time step which has not fully converged. [1] presents a method which in contrast to the described procedure does accomplish a strong coupling of aerodynamics and structure by integrating both systems of equations simultaneously. The research in [1] covers non-friction profile flow with a spring-damper system representing the structural side of the problem. In [1] a time-stepping algorithm is used for the integration of the implicit system of fluid flow equations. The FLOWer code contains an algorithm for this dual time-stepping method and can be applied successfully to the solution of three-dimensional, non-stationary systems including or excluding friction effects [2]. In this project the method [1] will be extended to include strong coupling of aerodynamics and structural analysis. In addition, fluid flow with friction effects will be studied, because flow separation has a large impact on the aeroelastic behavior. An extension to three-dimensional problems is planned, where the spring-damper system [1] will be replaced by a flexural-torsional beam model.
[1] J. J. Alonso, A. Jameson: Fully Implicit Time-Marching Aeroelastic Solutions, AIAA Paper No. 94-0056, 1994
[2] R. Heinrich, H. Bleecke: Simulation instationärer, dreidimensionaler, viskoser Strömungen unter Verwendung einer "Dualen Zeitschritt Methode", 10. DGLR-STAB Fachsymposium, 1996
Dynamics of Ice in Open Water
Supervisors: Matthies, Oumeraci
Object of research in this project is the large-scale dynamics of ice floes in still and moving water. Calculations will take into account the interaction between the ice cover and the water below by means of a mechanical and thermodynamic model. Moving forces are wind and thermal flow near the surface. Modeling of these processes is the prerequisite for a prediction of type and thickness of ice covers on open waters. Due to the high demand in computing time the development and use of parallel computing procedures is necessary in this project.
[1] H. Matthies: Existence theorems in thermo-plasticity, J. de Mécanique 18 (1979) 695-712
[2] F. U. Häusler, H. G. Matthies & C. S. Moore: Sea-ice under complex stress states - constitutive modeling and test results; in: Proc. of IUTAM/IAHR Symposion on Ice-Structure Interaction St. Johns 1969, I. Jordan ed., Springer-Verlag, Berlin, 1990
[3] F U. Häusler, H. G. Matthies: Elastic-plastic deformation of floating columnar grained ice - computer implementation and ice tank test results; in: Computational Plasticity, D. R. J. Owen, E. Hinton & E. Oñate eds. Pineridge Press, Swansea, 1987
Stability of Partially Filled Centrifuges for Liquids
Supervisors: Brommund, Hempel
Partially filled centrifuges are used for separating solid matter from liquids. When less favorable operation parameters are present, the stationary rotation without unbalance can become unstable and the liquid may start swashing. In the laboratory of the Institut für Technische Mechanik experiments on the rotation stability can be conducted. In several papers and two dissertations [1][2], stability charts have been compiled for centrifuges on anisotropic bearings. Deviations of calculations and experimental results are generally associated with small unbalance. In this project, the influence of a small unbalance on the stability of a hanging centrifuge with a rigid cylindrical barrel and isotropic bearing will be investigated by means of perturbation calculus. Mathematically, it is more demanding to examine the stability of an anisotropically suspended centrifuge with a small eccentricity by means of perturbation calculus. This results in partial differential equations with time-dependent periodic coefficients.
[1] B. Schmolck: Laufstabilität einer Maschine, deren Rotor Flüssigkeiten verschiedener Dichten enthält. VDI-Fortschrittberichte, VDI-Verlag, Düsseldorf 1989
[2] U. Riedel: Laufstabilität flüssigkeitsgefüllter Zentrifugen. VDI-Fortschrittberichte, VDI-Verlag, Düsseldorf 1992
[3] U. Simon, E. Brommundt: Periodische Bewegungen einer Pendelzentrifuge in einem mehreckigen Fanglager, S. 181 bis S. 188 in H. Irretier, R. Nordmann, H. Springer (Hrsg.): Schwingungen in rotierenden Maschinen IV. Vieweg, Braunschweig/ Wiesbaden 1997
Pressure and Volume Fluctuations in Drilling Shafts
Supervisors: Brommundt, Matthies
The scavenging liquid used in oil drilling shafts bring out the drillings from the bottom of the borehole and is used for propelling underground motors and transmitting signals between the chisel and the drive coupling. It is planned to use displacement pumps and motors underground. Both transmitting devices and displacement motors possiblibly cause deviations in pressure and volume of the fluid that need to be understood and controlled in order to avoid destruction of the drilling shaft. Objective of this project is the development of a model for a drilling shaft surrounded by a pulsating liquid. This model for the fluid-solid vibrations will take into account interaction effects of the descending liquid, the flexible wall of the guide tube, and the ascending liquid outside of the shaft guide tube. Probably, energy dissipation within the surrounding rock formation has to be considered in the model as well. Previous research has been done in the course of a diploma thesis [1]. The major difference between the proposed project and the previous research documented in literature [2] is the consideration of energy dissipation into shear and compressive viscosity and the absorption in the surrounding rock. In [3], a simple tube model is used.
[1] U. Gippner: Dynamische Belastung eines Öl-Bohrstranges durch Schwingungen der Spülflüssigkeit. Diplomarbeit am Institut für Technische Mechanik, TU Braunschweig, Frühjahr 1997
[2] A. S. Tijsseling: Fluid-structure Interaction in Liquid-Filled Pipe Systems. A Review. J. of Fluids and Structures 10, 1996, p. 109-146
[3] E. B. Wylie; V. L. Streeter: Fluid Transients in Systems. Prentice Hall, Englewood Cliffs 1993 [4] A. Baumgart; E. Brommundt: Berechnung der Koppelschwingungen von Bohrgestänge und Spülflüssigkeit bei Tiefbohrungen, S. 219 bis 226 in H. Irretier, R. Nordmann, H. Springer (Hrsg.): Schwingungen in rotierenden Maschinen IV. Vieweg, Braunschweig/ Wiesbaden 1997
Simulation of Pressure Impact of Waves on Structures
Supervisors: Oumeraci, Matthies
Impact effects of waves on structures can be found in a great number of cases in engineering, e.g. in water and coastal engineering. Depending on the specific problem, either the stiffness of the structure or the compressibility of the liquid can be the major factor of the behavior of the coupled system. Numerical simulation of breaking waves is a very difficult task and will be dealt with in this project by means of a Volume of Fluid approach [1]. A further challenge is the coupling of wave impact and flexible structures. Due to the high cost of computations the use of effective parallel algorithms on a suitable computer system is advisable.
[1] N. T. Wu, H. Oumeraci, H.-W. Partenschky: Numerical Modelling of Breaking Wave Impacts on a Vertical Wall. ASCE, 24th ICCE'94, Kobe/Japan, Vol. II, 1994, p. 1672-1686
[2] H. Oumeraci, T. Bruce, P. Easson Klammer: PIV-measurements on breaking wave kinematics and impact loading. Proc. 4th Intern. Conf. Port Eng. (COPEDEC), Rio de Janeiro, 1995
[3] H. Oumeraci: Vertical breakwaters - invited paper by the Task Committee of ASCE, 1995. Book publ. by ASCE, Vicksburg unter Titel: "Wave forces on inclined and vertical structures".
Sediment Transport in a Fluid Flow
Supervisors: Matthies, Oumeraci, Helmig
The problem of sediment transport plays an important role in many coastal engineering problems. If in a resting position, sand or gravel can be regarded as a porous solid, if in motion, as a suspended, granular material. A changing boundary between ground and liquid resulting from the sediment transport in turn influences the fluid flow. In this project, suitable algorithms will be developed, examined and applied to the problem of modeling sand in resting position and in motion. A starting point will be a combination of classical discretization methods and particle methods. The dynamics of a single particle will be embedded in the global computations. This allows and demands the use of parallel computing strategies. Validation of the computational models developed throughout this project will be achieved using data from experiments on sediment transport under swell action conducted at the Leichtweiß-Institut and funded by the European Union (MAST III/SAFE Project), and from basic investigations on scouring in the laboratory [1]. Previous research on numerical simulation of sediment transport and the resulting morphodynamic changes has been done at the department [2]. However, interaction effects of swell, sediment and morphodynamics has not been taken into account in that work.
[1] H. Oumeraci: Scour in front of vertical break-waters. PHRI, Proc. Intern. Workshop on wave barriers, Yokosuka/Japan, 1994
[2] Y. Wu: Simulation of cross-shore beach profile evolution under random waves. Diss. TU Braunschweig, Leichtweiß-Institut, 1993
Elastic Structures in Free-Surface Fluid Flow
Supervisors: Helmig, Dinkler
The influence of arbitrarily complex structures on the behavior of free-surface fluid flow will be examined throughout this project. Two categories of problems will be dealt with: first, liquid-filled containers or tanks under dynamic excitation (earthquake, tank trucks, etc.); second, objects surrounded by fluid flow (structures in rivers, etc.). In the beginning of the investigations, structures will be modeled as rigid solids and the fluid flow will be described by the Navier-Stokes equation for incompressible, non-stationary flow in an arbitrary Langrangian Eulerian formulation (ALE). The free surface will be approximated by a Langrangian formulation or by a volume of fluid method. Using both finite element and finite volume methods for discretization purposes, methods can be compared and evaluated in terms of stability and conservative character for different approaches (coupled, uncoupled, operator splitting). During the realization process, experience gathered in recent years can be applied. Previous research has been done in the field of finite element and finite volume methods for scalar balance equations, which produce a better behavior of the front when transiting from the parabolic into the hyperbolic region [1][2][3], and comply with the limiting principle (monotonous solution). An operator-splitting technique has been developed for the solution of the Navier-Stokes equation. It is thus possible to include free surfaces into the formulation.
[1] R. Helmig: Multiphase flow and transport processes in the subsurface, Springer, 1997
[2] R. Helmig, R. Huber: Comparison of Galerkin-type discretization techniques für two-phase flow in heterogeneous porous media, eingereicht bei Advances in Water Resources. Preprint: Forschungsbericht, SFB 404. Mehrfeldprobleme in der Kontinuumsmechanik, Universität Stuttgart
[3] R. Huber, R. Helmig: Multiphase Flow in heterogeneous Porous Media: a classical Finite Element method versus an IMPES-based mixed FE/FV Approach, eingereicht bei International Journal for Numerical Methods in Fluids. Preprint: Forschungsbericht, SFB 404, Mehrfeldprobleme in der Kontinuumsmechanik, Universität Stuttgart
Simulation of a Swimming Fish
Supervisors: Antes, Hempel
For the example of the "`Lamprey"' fish, the motion of a fish in water will be simulated including neural control, muscular movements and interaction of muscular forces with the surrounding fluid flow. Models have been previously developed showing the interaction of neurons and muscles. This type of simulation becomes necessary if the proper functioning of the neural network is depending on the feedback from sensors, or if a higher efficiency can be achieved through such a feedback and successive adaptation of the body shape of the fish. In this project, a simulation will be done to model the major effects needed for the realistic simulation of the control a fish exerts while swimming in water (solid fish - water - control by neural network). This will be done in an increasingly detailed way, resulting in an evolutionary loop for the optimization of the swimming behavior of the fish.
[1] Örjan Ekeberg, Anders Lansner, Sten Grillner: The neural control of fish swimming studied through numerical simulations, Adaptive Behavior Vol. 4 No. 4, 363-384, 1995
[2] Ö. Ekeberg et al.: The neural control of fish swimming, Scientific American, 1996
Transmission of Sound in Multilayer Glass
Supervisors: Antes, Hempel
Efficiency of sound attenuation in multilayer glass panes in windows is dependent on several parameters, e.g. type of support, thickness, material, distance between layers, and pressure in the space between the glass layers. Previous research has been done in the field of boundary-element simulations of sound transmission in acoustic media, and the finite-element method has been applied to thin, elastic plates. Within this project, a coupled simulation for the system air--plate--air--plate--air will be developed. Objective is the optimization of the sound attenuating effect for multilayer glass panes.
[1] H. Antes, K. Volk: on Parallel Processing in 3-D Acoustic BEM, Proc. GAMM Seminar on Numerical Techniques for Boundary Element (Ed.: W. Hackbusch), pp. 1-12, Notes on Numerical Fluid Mechanics 33, Vieweg, Braunschweig, 1992.
[2] H. Antes, G. Tröndle, M. Jäger: Efficient Calculation of Acoustic Fields by Boundary Element Methods, in: Boundary Element Methods 1989-1995 (Ed.: W. Wendland) Springer, 1997.
Optimization of Noise Guards
Supervisors: Antes, Dinkler
For the reduction of noise pollution along major highways, noise guarding walls are erected more and more often. In the planning stage, designers usually do not account for the topographical situation in the area around the noise guard, resulting in an even higher noise level for some residents after the erection. Objective of this project is the development of effective numerical simulation tools which enable the designer to use optimal shapes and materials for noise guards in a given topography.
Noise Propagation around Roads
Supervisors: Antes, Dinkler
Guidelines for noise abatement procedures on roads are given today as two-dimensional models. They can only describe a situation where a route and its surrounding topography does not change over a long distance. Often this does not reflect the actual situation. Therefore, a model for the three-dimensional simulation will be developped in this project, where noise propagation for arbitrary routes within an arbitrary terrain situation can be calculated. The model will be capable of describing the high frequency sound propagation typical for noise pollution from traffic, and cover a terrain situation with dimensions up to several kilometers.
[1] H. Antes, K. Volk: On Parallel Processing in 3-D Acoustic BEM, Proc. GAMM Seminar on Numerical Techniques for Boundary Element (Ed.: W. Hackbusch), pp. 1-12, Notes on Numerical Fluid Mechanics 33, Vieweg, Braunschweig, 1992
[2] H. Antes, G. Tröndle, Experience with Multigrid in Three-Dimensional Acoustic Boundary Element Method, Proc. 14th DGLR/AiAA Conference, Aachen, 1992
Influence of Wind on Sound Propagation
Supervisors: Antes, Matthies
Observation shows that noise pollution caused by road traffic is influenced to a high degree by wind direction and wind velocity. These important influencing factors are not yet included in most numerical models for the simulation of sound propagation. An existing boundary element formulation will be extended as to include the propagation of sound under the influence of laminar air flow within this project. This formulation is a prerequisite for a more realistic investigation of problems like noise propagation along roads in a specific topography.
[1] H. Antes, Th. Meise: 3-D Sound Generated by Moving Sources, Proc. IABEM-90 Symp., Rome (Eds.: T. Cruse, L. Morino) Springer, Berlin, 1991
[2] H. Antes: Applications in Environmental Noise, Chapt. 12 in Boundary Element Methods in Acoustics (Eds.: R. D. Ciskowski, C. A. Brebbia) Elsevier, London 1991
Aerothermodynamische Analysen und Sensitivitäten der Strömung bei heißen Strukturbauteilen von Wiedereintrittsfahrzeugen
Betreuer: Radespiel, Horst
Thermische und mechanische Lasten beim Wiedereintritt führen bei heißen Strukturbauteilen wie zum Beispiel der Nasenkappe zu Unstetigkeiten im Konturverlauf und zu Veränderungen der Spaltgeometrie. Die Antwort der Strömung auf diese Konturveränderungen ist nichtlinear wegen lokal komplexer Strömungstopologien mit Ablösungen, Strahlungskühlung, Rückstrahlung und Hochtemperatureffekten in der Strömung.
Thema des Projektes ist die aerothermodynamische Simulation und Analyse der Strömungsvorgänge als Folge der Strömungs-Struktur-Kopplung. Sensitivitätsbetrachtungen der Strömungsvorgänge sollen Entwurfsgrundlagen für zukünftige Auslegungen erbringen. Die Strömungssimulationen sollen mit existierenden Navier-Stokes-Verfahren [1, 2, 3] durchgeführt werden. Die Kopplungen zwischen Strömung und Struktur sollen mit Hilfe der am Institut für Flugzeugbau und Leichtbau entwickelten Methoden [3] berücksichtigt werden.
[1] Kossira, H.; Haupt, M.; Radespiel, R.: Analysen von aerothermodynamisch belasteten Flügelvorderkanten mit einer Methode der Fluid-Struktur-Kopplung. DGLR-Jahrbuch 1995, Band III
[2] Brück, S.; Radespiel, R.; Longo, J. M. A.: Comparison of Nonequilibrium Flows past a Simplified Space-Shuttle Configuration. AIAA Paper 97-0275, 1997
[3] Galle, M.; Gerhold, T.; Evans, L.: Parallel computation of turbulent flows around complex geometries on hybrid grids with the DLR TAU code. In: Parallel Computational Fluid Dynamics. Eds.: D. Kayes, A. Ecr, N. Satofuka, P. Fox, L. Periaux. North Holland (2000), pp 223-230
[4] Poppe, U.; Haupt, M.; Kossira, H.: Kopplung von Fluid-Struktur-Analyseprogrammen zur Untersuchung des aeroelastischen Verhaltens von Flügeln. Tagungsband Aeroelastik-Tagung des DGLR, 29.- 30. Juni 1998, DLR Göttingen, 1998
Simulation des Schlagfluges bei kleinen Reynoldszahlen
Betreuer: Radespiel, Horst
An der Modellierung und Simulation des Schlagfluges im Reynoldszahlbereich zwischen 104 und 105 besteht ein großes Interesse, da in diesem Bereich auch zukünftige Mikroflugzeuge operieren werden. Ein wesentliches Hindernis für effiziente und verlässliche Strömungssimulationen in diesem Bereich sind die laminaren Ablöseblasen.
Das Projekt soll verschiedene Ansätze der Modellierung der Transition laminar-turbulent in Ablöseblasen untersuchen und für die Verwendung in Verfahren für die Reynolds-gemittelten Navier-Stokes-Gleichungen (RANS) bewerten. Mit Anwendungen der Modelle auf den Schlagflug sollen die aerodynamischen Sensitivitäten des Schlagfluges bei kleinen Reynoldszahlen erforscht werden. Das RANS Verfahren hierfür ist vorhanden [1]. Die aerodynamische Modellierung für kleine Reynoldszahlen ist auch für die Verwendung in einer interdisziplinären Analyseumgebung am Institut für Flugzeugbau und Leichtbau [3] vorgesehen.
[1] N. Kroll; C.-C. Rossow; K. Becker; F. Thiele: The Megaflow-Project. Aerosp. Sci. Techn., Vol. 4 (2000), pp 223-237
[2] M. F. Neef; D. Hummel: Euler solutions for a finite-span flapping wing. In: Th. J. Mueller (Ed.): Proceedings of the conference ?Fixed, flapping and rotary wing vehicles at very low Reynolds numbers?. University of Notre Dame, Indiana, USA, June 5-7, 2000, pp 75-99
[3] Poppe, U.; Haupt, M.; Kossira, H.: Kopplung von Fluid-Struktur-Analyseprogrammen zur Untersuchung des aeroelastischen Verhaltens von Flügeln. Tagungsband Aeroelastik-Tagung des DGLR, 29.- 30. Juni 1998, DLR Göttingen, 1998
Nachlaufmodellierung für die Simulation der aeroelastischen Wechselwirkungen bei Windturbinen
Betreuer: Matthies
Bei der Ermittlung der Ermüdungslasten von Windturbinen müssen eine Vielzahl von Langzeitsimulationen für die verschiedenen Operationsbedingungen durchgeführt werden. Die Modellierung der gesamten Strömung durch Lösung der instationären Reynolds-gemittelten Navier-Stokes Gleichungen ist zwar für das Verständnis der zugrunde liegenden Strömungsverhältnisse erforderlich, ist jedoch viel zu aufwendig für die Berechnung der Ermüdungslasten. Aus diesem Grund wird die Aerodynamik der Windturbine vereinfacht modelliert: Die lokalen Blattlasten werden mittels instationärer 2-dimensionaler Profiltheorie berechnet und die globale Strömung mittels eines Nachlauf-Modells. Die bisher verwendeten Nachlaufmodelle sind von sehr einfacher Form und geben sowohl das dynamische Verhalten der Rotorströmung als auch die Verteilung der induzierten Geschwindigkeitskomponenten über der Rotorscheibe nur sehr grob wieder. Um eine Verbesserung der Genauigkeit der aerodynamischen Lasten zu erzielen, bietet die Verbesserung des Nachlaufmodells ein hohes Potential. Im Rahmen dieses Projekts soll die Modellierung des Nachlaufs verbessert werden: In letzter Zeit sind verschiedene neuartige Ansätze zur Modellierung des Nachlaufs verfolgt worden: Ein Ansatz verwendet einen 3D Navier-Stokes Löser, in den die lokalen Blattlasten als Volumenkräfte eingebracht werden. Als weiterer Ansatz ist die Verwendung der Wirbelpartikel-Methode zur Diskretisierung der Navier-Stokes Gleichungen für den Nachlauf denkbar.
[1] A.P. Schaffarczyk, J.T. Conway: Comparison of a Nonlinear Actuator Disk Theory with Numerical Integration Including Viscous Effects, Canadian aeronautics and space journal. Bd. 46 (2000), 4, S. 209
[2] R.Mikkelsen, J.N. Soerensen, W.Z.Shen: Yaw Analysis Using a 3D Actuator Line Model, Proceedings, EWEC 2001, Kopenhagen
[3] A. Leonard: Vortex Methods for flow simulation, Journal of Computational Physics, 1980; 37; 289-335