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Control and optimization of structures with fluid interaction
KTH, Superseded Departments, Aeronautical Engineering.
2000 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Various problems on the optimal design of elastic structures subject to nonconservative fluid-dynamic forces are considered. The optimal design problem istypically posed as minimizing structural weight subject to constraints on structural stability. Traditionally, structural dimensions and orientations of fibercomposite materials are common design variables. It is demonstrated that the structural weight can be reduced further by including the design of a stabilizingcontrol system in the structural design optimization, giving an integrated optimization problem where both structural and control system parameters are used as design variables. The integrated approach may result in a design with significantly improved performance compared to traditional methods, both in terms of reduced structural weight and control system performance. Using optimization for design of mechanical systems with nonconservative external load tends to increase the likelihood of obtaining a design which is very sensitive to imperfections. As a result, the predicted performance of the optimal design may not be achieved in practice. The importance of this fundamental difficulty is emphasized throughout the thesis by comparing numerically obtained results to experiments.

The first part of the thesis is concerned with the stability and optimal design of a beam subject to forces induced by fluid flow through attached pipes. A nozzle control system deflecting the fluid jet at the beam tip is used to improve the stability of the system. The simultaneous design of the control system and the beam shape minimizing structural mass is performed using numerical optimization. The inclusion of the control system in the optimization gives a considerable reduction of the beam weight but results in an optimal design which is very sensitive to imperfections. An optimal design with improved robustness is obtained by solving a modified optimization problem. The stability of a flexible wing structure with a controllable trailing edge flap is investigated. Due to uncertainties in the numerical stability analysis, the wing is predicted to become unstable at a significantly higher speed than what is observed in wind tunnel tests. Two different approaches to stabilize the wing in flutter is demonstrated. First, numerical optimization is used to design a controller which at each flow speed maximizes the damping of the flutter mode observed in the wind tunnel experiment. Second, an integrated approach is adopted, where a simultaneous mass balancing and control law design is performed. It is argued that a two-step procedure may be required to obtain a design with minimum weight and a control law that is well-defined for all operating conditions.

Place, publisher, year, edition, pages
Stockholm: KTH , 2000. , 8 p.
Series
Report. Department of Aeronautics, 2000:1
Identifiers
URN: urn:nbn:se:kth:diva-2956ISBN: 993-261067-4 OAI: oai:DiVA.org:kth-2956DiVA: diva2:8696
Public defence
2000-05-12, 00:00 (English)
Note
QC 20100421 NR 20140805Available from: 2000-05-11 Created: 2000-05-11 Last updated: 2010-04-28Bibliographically approved
List of papers
1. On the optimal design of pipes conveying fluid.
Open this publication in new window or tab >>On the optimal design of pipes conveying fluid.
1998 (English)In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 12, no 3, 353-365 p.Article in journal (Refereed) Published
Abstract [en]

The stability and optimal design of a beam subject to forces induced by fluid flow through attached pipes is investigated. The structure is assumed to have the same dynamics as a fluid-conveying pipe, and the dynamic stability is analysed using a finite element formulation of the linear equation of motion. The optimal design problem of minimizing the structural mass at fixed critical flow speed is solved. The numerical results are compared to experiments with satisfactory agreement, provided that the lower bounds of the beam dimensions are properly chosen. The influence of structural damping on the critical flow speed is significant, and is found to be strongly design-dependent.

Identifiers
urn:nbn:se:kth:diva-12425 (URN)10.1006/jfls.1997.0135 (DOI)
Note
QC 20100421Available from: 2010-04-21 Created: 2010-04-21 Last updated: 2017-12-12Bibliographically approved
2. Active nozzle control and integrated design optimization of a beam subject to fluid-dynamic forces.
Open this publication in new window or tab >>Active nozzle control and integrated design optimization of a beam subject to fluid-dynamic forces.
1999 (English)In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 13, no 2, 269-287 p.Article in journal (Refereed) Published
Abstract [en]

Active nozzle control is used to improve the stability of a beam subject to forces induced by fluid flow through attached pipes. The control system has a significant effect on the structural stability, making both flutter and divergence type of instabilities possible. The stability analysis is carried out using a state-variable approach based on a finite element formulation of the structural dynamics. The simultaneous design of the control system and the beam shape minimizing structural mass is performed using numerical optimization. The inclusion of the control system in the optimization gives a considerable reduction of the structural mass but results in an optimal design which is very sensitive to imperfections. Using a simple model of the control system uncertainties, a more robust design is obtained by solving a modified optimization problem. Throughout the study, the theoretical findings are verified by experiments.

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-12426 (URN)10.1006/jfls.1998.0196 (DOI)000079725500006 ()
Note
QC 20100421Available from: 2010-04-21 Created: 2010-04-21 Last updated: 2017-12-12Bibliographically approved
3. Aeroservoelastic design optimization with experimental verification
Open this publication in new window or tab >>Aeroservoelastic design optimization with experimental verification
2001 (English)In: Journal of Aircraft, ISSN 0021-8669, E-ISSN 1533-3868, Vol. 38, no 5, 958-961 p.Article in journal (Refereed) Published
Abstract [en]

A demonstration of integrated design optimization of an aeroservoelastic system was presented. It was shown that the simultaneous design of structural and control system required a two-step procedure to result in a minimum weight design and a control law that is well-designed for all operating conditions. The essential dynamics was predicted using a aeroservoelastic model, and final design with 42% less weight penalty was obtained using the integrated approach.

National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-12429 (URN)000171526200025 ()
Note
QC 20100421Available from: 2010-04-21 Created: 2010-04-21 Last updated: 2017-12-12Bibliographically approved
4. Active wing flutter suppression using a trailing edge flap
Open this publication in new window or tab >>Active wing flutter suppression using a trailing edge flap
2002 (English)In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 16, no 3, 271-294 p.Article in journal (Refereed) Published
Abstract [en]

The aeroservoelastic behaviour of a thin rectangular wing with a controllable trailing edge flap is investigated. A rather high aspect ratio motivates a numerical model based on linear beam theory for the structural dynamics and strip theory for the unsteady aerodynamic loads. Experimental flutter testing shows good agreement with the numerical stability analysis, and the impact of the trailing edge flap on the dynamics is verified by open-loop testing. The problem of stabilizing the wing utilizing the trailing edge flap is posed, and the design of a fixed-structure feedback controller is performed using numerical optimization. The problem of maximizing closed-loop modal damping with constraints on actuator performance is solved for a sequence of flow speeds and the obtained controller is synthesized using gain scheduling. The fairly large predicted increase in critical speed is experimentally verified with satisfactory accuracy.

National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-12428 (URN)10.1006/jfls.2001.0426 (DOI)000176948100001 ()
Note
QC 20100421Available from: 2010-04-21 Created: 2010-04-21 Last updated: 2017-12-12Bibliographically approved
5. Experiments on the Onset of Impacting Motion Using a Pipe Conveying Fluid
Open this publication in new window or tab >>Experiments on the Onset of Impacting Motion Using a Pipe Conveying Fluid
1999 (English)In: Nonlinear dynamics, ISSN 0924-090X, E-ISSN 1573-269X, Vol. 19, no 3, 261-271 p.Article in journal (Refereed) Published
Abstract [en]

The transition from stable periodic nonimpacting motion to impacting motion, due to variations of parameters, is observable in a wide range of vibro-impact systems. Recent theoretical studies suggest a possible scenario for this type of transition. A key element in the proposed scenario is fulfilled if the oscillatory motion involved in the transition is born in a supercritical Hopf bifurcation. If the onset of impacting motion is close to the Hopf bifurcation, the impacting motion is likely to be chaotic. A numerical simulation of a system of articulated pipes conveying fluid is used to illuminate the theory. An experimental setup is presented, where a cantilevered pipe conveying fluid is unilaterally constrained. Results from experiments are found to be in good qualitative agreement with the theory.

Keyword
grazing bifurcation, impact oscillations, pipe conveying fluid, Hopf bifurcation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-12427 (URN)10.1023/A:1008322725617 (DOI)000081414500003 ()
Note
QC 20100421Available from: 2010-04-21 Created: 2010-04-21 Last updated: 2017-12-12Bibliographically approved

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